100 Complex posterior spinal fusion cases performed with robotic instrumentation

Robotic navigation has been shown to increase precision, accuracy, and safety during spinal reconstructive procedures. There is a paucity of literature describing the best techniques for robotic-assisted spine surgery for complex, multilevel cases or in cases of significant deformity correction. We present a case series of 100 consecutive multilevel posterior spinal fusion procedures performed for multilevel spinal disease and/or deformity correction. 100 consecutive posterior spinal fusions were performed for multilevel disease and/or deformity correction utilizing robotic-assisted placement of pedicle screws. The primary outcome was surgery-related failure, which was defined as hardware breakage or reoperation with removal of hardware. A total of 100 consecutive patients met inclusion criteria. Among cases included, 31 were revision surgeries with existing hardware in place. The mean number of levels fused was 5.6, the mean operative time was 303 min, and the mean estimated blood loss was 469 mL. 28 cases included robotic-assisted placement of S2 alar-iliac (S2AI) screws. In total, 1043 pedicle screws and 53 S2AI screws were placed with robotic-assistance. The failure rate using survivorship analysis was 18/1043 (1.7%) and the failure rate of S2AI screws using survivorship analysis was 3/53 (5.7%). Four patients developed postoperative wound infections requiring irrigation and debridement procedures. None of the 1043 pedicle screws nor the 53 S2AI screws required reoperation due to malpositioning or suboptimal placement. This case series of 100 multilevel posterior spinal fusion procedures demonstrates promising results with low failure rates. With 1043 pedicle screws and 53 S2AI screws, we report low failure rates of 1.7% and 5.7%, respectively with zero cases of screw malpositioning. Robotic screw placement allows for accurate screw placement with no increased rate of postoperative infection compared to historical controls. Level of evidence: IV, Retrospective review.

Computer Simulated Enhancement and Planning, Robotics and Navigation With Patient Specific Implants and 3-D Printed Cages

STUDY DESIGN

This is a retrospective cohort study.

OBJECTIVES

Pre and postop Measurement Testing. This is a retrospective study of 33 consecutive interbody spacers in 21 patients who underwent pre, intra, and postoperative measurement of the middle column to determine if this would lead to more precise restoration of middle column height and spacer fit. Scaled transparencies of the pre-operative simulation of angular correction and spacer geometry could be overlayed on the post-operative imaging studies.

METHODS

Multiple Observers Measurement Testing. 33 consecutive vertebral levels requiring interbody spacers for multilevel deformities had middle column height pre and post operatively measured by 3 blinded observers. The preoperative and postoperative measurements were compared using a linear regression analysis and Pearson product-moment correlation.

RESULTS

Pre and postop Measurement Testing: Thirty-three interbody devices in 21 patients had pre-operative planning, simulation of cage dimensions to determine the proper cage fit which would provide for the desired correction of foraminal height and sagittal balance parameters. The simulated preoperative plan overlayed the final post-operative radiograph and was a near-perfect match in 20 of 21 patients (95.2%). Multiple Observers Measurement Testing: A Pearson product-moment correlation was run between each individual's pre-op and post-op middle column measurements. There was a strong, positive correlation between pre-operative and post-operative measurements, which was statistically significant (r = 0.903, n = 33, P < 0.001).

CONCLUSIONS

This consecutive series of 33 cases demonstrated the utility of measuring the preoperative middle column length in predicting the optimal height of the spacers, intervertebral disks, and posterior vertebral body height simultaneously restoring sagittal and coronal plane alignment.

Robot for Ligament Tensioning and Assessment of Spinal Stability

STUDY DESIGN

An in vitro human cadaveric biomechanics study.

OBJECTIVE

A proof-of-concept study to quantify whether or not differences in segmental mobility associated with spinal instability could be detected by a robotic distraction system.

METHODS

Testing was performed in fresh human cadaveric tissue. A prototype Robotic Middle Column Distractor was attached unilaterally to the pedicles of L3-4. Distraction forces up to 150 N were applied first in the intact state, and following discectomy of L3-4. Motions were recorded by time-indexed visual and fluoroscopic images, and analyzed to measure actual motions achieved. Functions of the robot unit were monitored during the procedure and evaluated qualitatively.

RESULTS

A difference of 2.5 mm in z-axis motion was detected at 150 N load between the intact and post-discectomy states. The robot coupled with the image analysis method was able to clearly detect the difference between the intact ("stable") and post-discectomy ("unstable") spine. Data analysis of fluoroscopic images taken during the procedure showed greater motion than perceived by the investigators from qualitative review of visual data. All monitored robot functions performed within design parameters without error.

CONCLUSION

The study demonstrates the feasibility and utility of utilizing an intraoperative robotic distractor to measure the amount of spinal mobility present at a level. This could lead to an important clinical tool for both diagnostic functions as well as operative assist functions.

Accuracy of Robotic-Assisted Spinal Surgery-Comparison to TJR Robotics, da Vinci Robotics, and Optoelectronic Laboratory Robotics

BACKGROUND

The optoelectronic camera source and data interpolation serve as the foundation for navigational integrity in the robotic-assisted surgical platform. The objective of the current systematic review serves to provide a basis for the numerical disparity that exists when comparing the intrinsic accuracy of optoelectronic cameras: accuracy observed in the laboratory setting versus accuracy in the clinical operative environment. It is postulated that there exists a greater number of connections in the optoelectronic kinematic chain when analyzing the clinical operative environment to the laboratory setting. This increase in data interpolation, coupled with intraoperative workflow challenges, reduces the degree of accuracy based on surgical application and to that observed in controlled musculoskeletal kinematic laboratory investigations.

METHODS

Review of the PubMed and Cochrane Library research databases was performed. The exhaustive literature compilation obtained was then vetted to reduce redundancies and categorized into topics of intrinsic optoelectronic accuracy, registration accuracy, musculoskeletal kinematic platforms, and clinical operative platforms.

RESULTS

A total of 147 references make up the basis for the current analysis. Regardless of application, the common denominators affecting overall optoelectronic accuracy are intrinsic accuracy, registration accuracy, and application accuracy. Intrinsic accuracy of optoelectronic tracking equaled or was less than 0.1 mm of translation and 0.1° of rotation per fiducial. Controlled laboratory platforms reported 0.1 to 0.5 mm of translation and 0.1°-1.0° of rotation per array. There is a huge falloff in clinical applications: accuracy in robotic-assisted spinal surgery reported 1.5 to 6.0 mm of translation and 1.5° to 5.0° of rotation when comparing planned to final implant position. Total Joint Robotics and da Vinci urologic robotics computed accuracy, as predicted, lies between these two extremes-1.02 mm for da Vinci and 2 mm for MAKO.

CONCLUSIONS

Navigational integrity and maintenance of fidelity of optoelectronic data is the cornerstone of robotic-assisted spinal surgery. Transitioning from controlled laboratory to clinical operative environments requires an increased number of steps in the optoelectronic kinematic chain and error potential. Diligence in planning, fiducial positioning, system registration, and intraoperative workflow have the potential to improve accuracy and decrease disparity between planned and final implant position. The key determining factors limiting navigation resolution accuracy are highlighted by this Cochrane research analysis.

Biomechanical and anatomical considerations in lumbar spinous process fixation--an in vitro human cadaveric model

BACKGROUND CONTEXT

Although multiple mechanisms of device attachment to the spinous processes exist, there is a paucity of data regarding lumbar spinous process morphology and peak failure loads.

PURPOSE

Using an in vitro human cadaveric spine model, the primary objective of the present study was to compare the peak load and mechanisms of lumbar spinous process failure with variation in spinous process hole location and pullout direction. A secondary objective was to provide an in-depth characterization of spinous process morphology.

STUDY DESIGN

Biomechanical and anatomical considerations in lumbar spinous process fixation using an in vitro human cadaveric model.

METHODS

A total of 12 intact lumbar spines were used in the current investigation. The vertebral segments (L1-L5) were randomly assigned to one of five treatment groups with variation in spinous process hole placement and pullout direction: (1) central hole placement with superior pullout (n=10), (2) central hole placement with inferior pullout (n=10), (3) inferior hole placement with inferior pullout (n=10), (4) superior hole placement with superior pullout (n=10), and (5) intact spinous process with superior pullout (n=14). A 4-mm diameter pin was placed through the hole followed by pullout testing using a material testing system. As well, the bone mineral density (BMD) (g/cm(3)) was measured for each segment. Data were quantified in terms of anatomical dimensions (mm), peak failure loads (newtons [N]), and fracture mechanisms, with linear regression analysis to identify relationships between anatomical and biomechanical data.

RESULTS

Based on anatomical comparisons, there were significant differences between the anteroposterior and cephalocaudal dimensions of the L5 spinous process versus L1-L4 (p<.05). Statistical analysis of peak load at failure of the four reconstruction treatments and intact condition demonstrated no significant differences between treatments (range, 350-500 N) (p>.05). However, a significant linear correlation was observed between peak failure load and anteroposterior and cephalocaudal dimensions (p<.05). Correlation between BMD and peak spinous processes failure load was approaching statistical significance (p=.08). 30 of 54 specimens failed via direct pullout (plow through), whereas 8 of 54 specimens demonstrated spinous process fracture. The remaining cases failed via plow through followed by fracture of the spinous process (16 of 54; 29%).

CONCLUSIONS

The present study demonstrated that variation in spinous process hole placement did not significantly influence failure load. However, there was a strong linear correlation between peak failure load and the anteroposterior and cephalocaudal anatomical dimensions. From a clinical standpoint, the findings of the present study indicate that attachment through the spinous process provides a viable alternative to attachment around the spinous processes. In addition, the anatomical dimensions of the lumbar spinous processes have a greater influence on biomechanical fixation than either hole location or BMD.

Multilevel extreme lateral interbody fusion (XLIF) and osteotomies for 3-dimensional severe deformity: 25 consecutive cases

Background

This is a retrospective review of 25 patients with severe lumbar nerve root compression undergoing multilevel anterior retroperitoneal lumbar interbody fusion and posterior instrumentation for deformity. The objective is to analyze the outcomes and clinical results from anterior interbody fusions performed through a lateral approach and compare these with traditional surgical procedures.

Methods

A consecutive series of 25 patients (78 extreme lateral interbody fusion [XLIF] levels) was identified to illustrate the primary advantages of XLIF in correcting the most extreme of the 3-dimensional deformities that fulfilled the following criteria: (1) a minimum of 40° of scoliosis; (2) 2 or more levels of translation, anterior spondylolisthesis, and lateral subluxation (subluxation in 2 planes), causing symptomatic neurogenic claudication and severe spinal stenosis; and (3) lumbar hypokyphosis or flat-back syndrome. In addition, the majority had trunks that were out of balance (central sacral vertical line ≥2 cm from vertical plumb line) or had sagittal imbalance, defined by a distance between the sagittal vertical line and S1 of greater than 3 cm. There were 25 patients who had severe enough deformities fulfilling these criteria that required supplementation of the lateral XLIF with posterior osteotomies and pedicle screw instrumentation.

Results

In our database, with a mean follow-up of 24 months, 85% of patients showed evidence of solid arthrodesis and no subsidence on computed tomography and flexion/extension radiographs. The complication rate remained low, with a perioperative rate of 2.4% and postoperative rate of 12.2%. The lateral listhesis and anterior spondylolisthetic subluxation were anatomically reduced with minimally invasive XLIF. The main finding in these 25 cases was our isolation of the major indication for supplemental posterior surgery: truncal decompensation in patients who are out of balance by 2 cm or more, in whom posterior spinal osteotomies and segmental pedicle screw instrumentation were required at follow up. No patients were out of sagittal balance (sagittal vertical line <3 cm from S1) postoperatively. Segmental instrumentation with osteotomies was also more effective for restoration of physiologic lumbar lordosis compared with anterior stand-alone procedures.

Conclusions

This retrospective study supports the finding that clinical outcomes (coronal/sagittal alignment) improve postoperatively after minimally invasive surgery with multilevel XLIF procedures and are improved compared with larger extensile thoracoabdominal anterior scoliosis procedures.

Epidural application of spinal instrumentation particulate wear debris: a comprehensive evaluation of neurotoxicity using an in vivo animal model

Object

The introduction and utilization of motion-preserving implant systems for spinal reconstruction served as the impetus for this basic scientific investigation. The effect of unintended wear particulate debris resulting from micromotion at spinal implant interconnections and bearing surfaces remains a clinical concern. Using an in vivo rabbit model, the current study quantified the neural and systemic histopathological responses following epidural application of 11 different types of medical-grade particulate wear debris produced from spinal instrumentation.

Methods

A total of 120 New Zealand White rabbits were equally randomized into 12 groups based on implant treatment: 1) sham (control), 2) stainless steel, 3) titanium alloy, 4) cobalt chromium alloy, 5) ultra–high molecular weight polyethylene (UHMWPe), 6) ceramic, 7) polytetrafluoroethylene, 8) polycarbonate urethane, 9) silicone, 10) polyethylene terephthalate, 11) polyester, and 12) polyetheretherketone. The surgical procedure consisted of a midline posterior approach followed by resection of the L-6 spinous process and L5–6 ligamentum flavum, permitting interlaminar exposure of the dural sac. Four milligrams of the appropriate treatment material (Groups 2–12) was then implanted onto the dura in a dry, sterile format. All particles (average size range 0.1–50 μm in diameter) were verified to be endotoxin free prior to implantation. Five animals from each treatment group were sacrificed at 3 months and 5 were sacrificed at 6 months postoperatively. Postmortem analysis included epidural cultures and histopathological assessment of local and systemic tissue samples. Immunocytochemical analysis of the spinal cord and overlying epidural fibrosis quantified the extent of proinflammatory cytokines (tumor necrosis factor–α, tumor necrosis factor–β, interleukin [IL]–1α, IL-1β, and IL-6) and activated macrophages.

Results

Epidural cultures were negative for nearly all cases, and there was no evidence of particulate debris or significant histopathological changes in the systemic tissues. Gross histopathological examination demonstrated increased levels of epidural fibrosis in the experimental treatment groups compared with the control group. Histopathological evaluation of the epidural fibrous tissues showed evidence of a histiocytic reaction containing phagocytized inert particles and foci of local inflammatory reactions. At 3 months, immunohistochemical examination of the spinal cord and epidural tissues demonstrated upregulation of IL-6 in the groups in which metallic and UHMWPe debris were implanted (p < 0.05), while macrophage activity levels were greatest in the stainless-steel and UHMWPe groups (p < 0.05). By 6 months, the levels of activated cytokines and macrophages in nearly all experimental cases were downregulated and not significantly different from those of the operative controls (p > 0.05). The spinal cord had no evidence of lesions or neuropathology. However, multiple treatments in the metallic groups exhibited a mild, chronic macrophage response to particulate debris, which had diffused intrathecally.

Conclusions

Epidural application of spinal instrumentation particulate wear debris elicits a chronic histiocytic reaction localized primarily within the epidural fibrosis. Particles have the capacity to diffuse intrathecally, eliciting a transient upregulation in macrophage/cytokine activity response within the epidural fibrosis. Overall, based on the time periods evaluated, there was no evidence of an acute neural or systemic histopathological response to the materials included in the current project.

Cervical total disk replacement: complications and avoidance

Anterior cervical diskectomy and fusion for neurologic deficits, radicular arm pain, and neck pain refractory to conservative management are successful. The approach and procedure were first described in 1955 and have become the anterior cervical standard of care for orthopedic surgeons and neurosurgeons. Advancements and innovations have addressed disease processes of the cervical spine with motion-preserving technology. The possibility of obtaining anterior cervical decompression while maintaining adjacent segment motion led to the advent of cervical total disk replacement. The Food and Drug Administration has approved 3 cervical devices with other investigational device exemption trials under way.

An attempt at clinically defining and assessing minimally invasive surgery compared with traditional "open" spinal surgery

Background

The goal of this editorial and literature review is to define the term “minimally invasive surgery” (MIS) as it relates to the spine and characterize methods of measuring parameters of a spine MIS technique.

Methods

This report is an analysis of 105,845 cases of spinal surgery in unmatched series and 95,161 cases in paired series of open compared with MIS procedures performed by the same surgeons to develop quantitative criteria to analyze the success of MIS.

Results

A lower rate of deep infection proved to be a key differentiator of spinal MIS. In unmatched series the infection rate for 105,845 open traditional procedures ranged from 2.9% to 4.3%, whereas for MIS, the incidence of infection ranged from 0% to 0.22%. For matched paired series with the open and MIS procedures performed by the same surgeons, the rate of infection in open procedures ranged from 1.5% to 10%, but for spine MIS, the rate of deep infection was much lower, at 0% to 0.2%. The published ranges for open versus MIS infection rates do not overlap or even intersect, which is a clear indication of the superiority of MIS for one specific clinical outcome measure (MIS proves superior to open spine procedures in terms of lower infection rate).

Conclusions

It is difficult, if not impossible, to validate that an operative procedure is “less invasive” or “more minimally invasive” than traditional surgical procedures unless one can establish a commonly accepted definition of MIS. Once a consensus definition or precise definition of MIS is agreed upon, the comparison shows a higher infection rate with traditional spinal exposures versus MIS spine procedures.

A computed tomography-based morphometric study of thoracic pedicle anatomy in a random United States trauma population

OBJECT

The objective of this study was to establish normative data for thoracic pedicle anatomy in the US adult population. To this end, CT scans chosen at random from an adult database were evaluated to determine the ideal pedicle screw (PS) length, diameter, trajectory, and starting point in the thoracic spine. The role of patient sex and side of screw placement were also assessed. The authors postulated that this information would be of value in guiding safe implant size and placement for surgeons in training.

METHODS

One hundred patients (50 males and 50 females) were selected via retrospective review of a hospital trauma registry database over a 6-month period. Patients included in the study were older than 18 years of age, had axial bone-window CT images of the thoracic spine, and had no evidence of spinal trauma. For each pedicle, the pedicle width, pedicle-rib width, estimated screw length, trajectory, and ideal entry point were measured using eFilm Lite software. Statistical analysis was performed using the Student t-test.

RESULTS

The shortest mean estimated PS length was at T-1 (33.9 ± 3.3 mm), and the longest was at T-9 (44.9 ± 4.4 mm). Pedicle screw length was significantly affected by patient sex; men could accommodate a PS from T1-12 a mean of 4.0 ± 1.0 mm longer than in women (p < 0.001). Pedicle width showed marked variation by spinal level, with T-4 (4.4 ± 1.1 mm) having the narrowest width and T-12 (8.3 ± 1.7 mm) having the widest. Pedicle width had an obvious affect on potential screw diameter; 65% of patients had a least 1 pedicle at T-4 that was < 5 mm in diameter and therefore would not accept a 4.0-mm screw with 1.0 mm of clearance, as compared with only 2% of patients with a similar status at T-12. Sex variation was also apparent, as thoracic pedicles from T-1 to T-12 were a mean of 1.4 ± 0.2 mm wider in men than in women (p < 0.001). The PS trajectory in the axial plane was measured, showing a marked decrease from T-1 to T-4, stabilization from T-5 to T-10, followed by a decrease at T11-12. When screw trajectory was stratified by side of placement, a mean of 1.7° ± 0.5° of increased medialization was required for ideal pedicle cannulation from T-3 to T-12 on the left as compared with the right side, presumably because of developmental changes in the vertebral body caused by the aorta (p < 0.05 for T3-12, except for T-5, where p = 0.051). The junction of the superior articular process, lamina, and the superior ridge of the transverse process was shown to be a conserved surface landmark for PS placement.

CONCLUSIONS

Preoperative CT evaluation is important in choosing PS length, diameter, trajectory, and entry point due to variation based on spinal level, patient sex, and side of placement. These data are valuable for resident and fellow training to guide the safe use of thoracic PSs.

The effect of spinal instrumentation on kinematics at the cervicothoracic junction: emphasis on soft-tissue response in an in vitro human cadaveric model

Object

Thoracic pedicle screw instrumentation is often indicated in the treatment of trauma, deformity, degenerative disease, and oncological processes. Although classic teaching for cervical spine constructs is to bridge the cervicothoracic junction (CTJ) when instrumenting in the lower cervical region, the indications for extending thoracic constructs into the cervical spine remain unclear. The goal of this study was to determine the role of ligamentous and facet capsule (FC) structures at the CTJ as they relate to stability above thoracic pedicle screw constructs.

Methods

A 6-degree-of-freedom spine simulator was used to test multidirectional range of motion (ROM) in 8 human cadaveric specimens at the C7–T1 segment. Flexion-extension, lateral bending, and axial rotation at the CTJ were tested in the intact condition, followed by T1–6 pedicle screw fixation to create a long lever arm inferior to the C7–T1 level. Multidirectional flexibility testing of the T1–6 pedicle screw construct was then sequentially performed after sectioning the C7–T1 supraspinous ligament/interspinous ligament (SSL/ISL) complex, followed by unilateral and bilateral FC disruption at C7–T1. Finally, each specimen was reconstructed using C5–T6 instrumented fixation and ROM testing at the CTJ performed as previously described.

Results

Whereas the application of a long-segment thoracic construct stopping at T-1 did not significantly increase flexion-extension peak total ROM at the supra-adjacent level, sectioning the SSL/ISL significantly increased flexibility at C7–T1, producing 35% more motion than in the intact condition (p < 0.05). Subsequent FC sectioning had little additional effect on ROM in flexion-extension. Surprisingly, the application of thoracic instrumentation had a stabilizing effect on the supra-adjacent C7–T1 segment in axial rotation, leading to a decrease in peak total ROM to 83% of the intact condition (p < 0.05). This is presumably due to interaction between the T-1 screw heads and titanium rods with the C7–T1 facet joints, thereby limiting axial rotation. Incremental destabilization served only to restore peak total ROM near the intact condition for this loading mode. In lateral bending, the application of thoracic instrumentation stopping at T-1, as well as SSL/ISL and FC disruption, demonstrated trends toward increased supraadjacent ROM; however, these trends did not reach statistical significance (p > 0.05).

Conclusions

When stopping thoracic constructs at T-1, care should be taken to preserve the SSL/ISL complex to avoid destabilization of the supra-adjacent CTJ, which may manifest clinically as proximal-junction kyphosis. In an analogous fashion, if a T-1 laminectomy is required for neural decompression or surgical access, consideration should be given to extending instrumentation into the cervical spine. Facet capsule disruption, as might be encountered during T-1 pedicle screw placement, may not be an acutely destabilizing event, due to the interaction of the C7–T1 facet joints with T-1 instrumentation.

Preclinical evaluation of the Dynesys posterior spinal stabilization system: a nonhuman primate model

BACKGROUND CONTEXT

Posterior dynamic spinal stabilization systems are intended to restore near-normal biomechanical function of the spine without inducing unnatural stresses to the spinal elements or eliciting a histopathological response. These devices must resist loosening within the challenging biomechanical environment of the lumbar spine.

PURPOSE

To determine the biomechanical effects of the Dynesys dynamic stabilization system (Zimmer, Inc., Warsaw, IN, USA) in the acute postoperative period and after 6 and 12 months in vivo; to examine the facet joints at the same postoperative intervals for signs of degeneration; and to measure the incidence of screw loosening after in vivo loading.

STUDY DESIGN/SETTING

This was an in vitro and in vivo animal survival study.

METHODS

Fourteen baboons were used. Eight animals underwent survival surgery to implant a posterior dynamic stabilization system spanning two lumbar levels. Six animals were sacrificed acutely, and their spines were biomechanically tested in the intact condition and with instrumentation implanted as described above. Six animals in the survival group were sacrificed at 6 months postoperatively and two animals at 12 months postoperatively. Their spines were biomechanically tested with instrumentation in situ and explanted. The facets were then processed using undecalcified technique. Microradiographs of the facets were examined for signs of arthrosis, inflammation, and degenerative changes.

RESULTS

The range of flexion-extension motion for the acute group of instrumented spines was 27% of the intact condition. After 6 months with instrumentation in situ, flexion-extension was 56% of the intact condition. After 12 months with instrumentation in situ, flexion-extension was 70% of the intact condition. With instrumentation explanted, flexion-extension at 6 and 12 months was not different from the intact condition (p>.05). Similar results were observed for lateral bending. There were no significant differences in axial rotation between any groups at any time point (p>.05). The facet joints at the operative and adjacent levels exhibited normal articular cartilage at both the 6- and 12-month postoperative time points. There was no evidence of facet arthrosis in any animal. At 6 months postoperatively, 0 of 36 screws exhibited radiolucency at the bone-metal interface. At 12 months postoperatively, 3 of 12 screws exhibited radiolucency.

CONCLUSIONS

After 12 months in vivo, spinal motions were stabilized by the dynamic instrumentation system. No facet arthrosis was observed at 6 and 12 months postoperatively. Explantation of the instrumentation restored motion to intact levels. A 25% rate of screw loosening (3 of 12 screws) was observed at the 12-month postoperative time point.

Autologous growth factors versus autogenous graft for anterior cervical interbody fusion: an in vivo caprine model

OBJECT

Using an in vivo caprine model, authors in this study compared the efficacy of autologous growth factors (AGFs) with autogenous graft for anterior cervical interbody arthrodesis.

METHODS

Fourteen skeletally mature Nubian goats were used in this study and followed up for a period of 16 weeks postoperatively. Anterior cervical interbody arthrodesis was performed at the C3-4 and C5-6 vertebral levels. Four interbody treatment groups (7 animals in each group) were equally randomized among the 28 arthrodesis sites: Group 1, autograft alone; Group 2, autograft + cervical cage; Group 3, AGFs + cervical cage; and Group 4, autograft + anterior cervical plate. Groups 1 and 4 served as operative controls. Autologous growth factors were obtained preoperatively from venous blood and were ultra-concentrated. Following the 16-week survival period, interbody fusion success was evaluated based on radiographic, biomechanical, and histological analyses.

RESULTS

All goats survived surgery without incidence of vascular or infectious complications. Radiographic analysis by 3 independent observers indicated fusion rates ranging from 9 (43%) of 21 in the autograft-alone and autograft + cage groups to 12 (57%) of 21 in the autograft + anterior plate group. The sample size was not large enough to detect any statistical significance in these observed differences. Biomechanical testing revealed statistical differences (p < 0.05) between all treatments and the nonoperative controls under axial rotation and flexion and extension loading. Although the AGF + cage and autograft-alone treatments appeared to be statistically different from the intact spine during lateral bending, larger variances and smaller relative differences precluded a determination of statistical significance. Histomorphometric analysis of bone formation within the predefined fusion zone indicated quantities of bone within the interbody cage ranging from 21.3 +/- 14.7% for the AGF + cage group to 34.5 +/- 9.9% for the autograft-alone group.

CONCLUSIONS

The results indicated no differences in biomechanical findings among the treatment groups and comparable levels of trabecular bone formation within the fusion site between specimens treated with autogenous bone and those filled with the ultra-concentrated AGF extract. In addition, interbody cage treatments appeared to maintain disc space height better than autograft-alone treatments.

A computed tomography-based feasibility study of translaminar screw fixation in the upper thoracic spine

OBJECT

Translaminar screws (TLSs) offer an alternative to pedicle screw (PS) fixation in the upper thoracic spine. Although cadaveric studies have described the anatomy of the laminae and pedicles at T1-2, CT imaging is the modality of choice for presurgical planning. In this study, the goal was to determine the diameter, maximal screw length, and optimal screw trajectory for TLS placement at T1-2, and to compare this information to PS placement in the upper thoracic spine as determined by CT evaluation.

METHODS

One hundred patients (50 men and 50 women), whose average age was 41.7 +/- 19.6 years, were selected by retrospective review of a trauma registry database over a 6-month period. Patients were included in the study if they were over the age of 18, had standardized axial bone-window CT imaging at T1-2, and had no evidence of spinal trauma. For each lamina and pedicle, width (outer cortical and cancellous), maximal screw length, and optimal screw trajectory were measured using eFilm Lite software. Statistical analysis was performed using the Student t-test.

RESULTS

The T-1 lamina was estimated to accommodate, on average, a 5.8-mm longer screw than the T-2 lamina (p < 0.001). At T-1, the maximal TLS length was similar to PS length (TLS: 33.4 +/- 3.6 mm, PS: 33.9 +/- 3.3 mm [p = 0.148]), whereas at T-2, the maximal PS length was significantly greater than the TLS length (TLS: 27.6 +/- 3.1 mm, PS: 35.3 +/- 3.5 mm [p < 0.001]). When the lamina outer cortical and cancellous width was compared between T-1 and T-2, the lamina at T-2 was, on average, 0.3 mm wider than at T-1 (p = 0.007 and p = 0.003, respectively). In comparison with the corresponding pedicle, the mean outer cortical pedicle width at T-1 was wider than the lamina by an average of 1.0 mm (lamina: 6.6 +/- 1.1 mm, pedicle: 7.6 +/- 1.3 mm [p < 0.001]). At T-2, however, outer cortical lamina width was wider than the corresponding pedicle by an average of 0.6 mm (lamina: 6.9 +/- 1.1 mm, pedicle: 6.3 +/- 1.2 mm [p < 0.001]). At T-1, 97.5% of laminae measured could accept a 4.0-mm screw with 1.0 mm of clearance, compared with 99.5% of T-1 pedicles; whereas at T-2, 99% of laminae met this requirement, compared with 94.5% of pedicles. The ideal screw trajectory was also measured (T-1: 49.2 +/- 3.7 degrees for TLS and 32.8 +/- 3.8 degrees for PS; T-2: 51.1 +/- 3.5 degrees for TLS and 20.5 +/- 4.4 degrees for PS).

CONCLUSIONS

Based on CT evaluation, there are no anatomical limitations to the placement of TLSs compared with PSs at T1-2. Differences were noted, however, in lamina length and width between T-1 and T-2 that must be considered when placing TLS at these levels.

Biomechanical comparison of iliac screws versus interbody femoral ring allograft on lumbosacral kinematics and sacral screw strain

STUDY DESIGN

This study evaluates the effect of iliac screw fixation versus interbody femoral ring allograft (FRA) on lumbosacral kinematics and sacral screw strain in long segment instrumentations.

OBJECTIVE

(1) Quantify kinematic properties of 3 lumbosacral fixation techniques; (2) Evaluate sacral screw strain as instrumented levels extend cephalad; and (3) Determine whether iliac screws or FRA biomechanically protect sacral screws.

SUMMARY OF BACKGROUND DATA

High failure rates at the lumbosacral junction have been reported with long posterior instrumentation ending with S1 pedicle screws. Achieving lumbosacral arthrodesis remains a clinical challenge.

METHODS

Seven human cadavaric lumbosacral spines were biomechanically evaluated intact and in 3 instrumented conditions: pedicle screw fixation alone (pedicle screw group), pedicle screw fixation supplemented with iliac screws (iliac screw group), and pedicle screw fixation supplemented with FRA (allograft group). Each condition was tested spanning L5-S1, L4-S1, L3-S1, L2-S1, and L1-S1. Testing included pure unconstrained moments (±10 Nm) in axial rotation, flexion/extension, and lateral bending, with quantification of S1 screw strain and lumbosacral range of motion (ROM).

RESULTS

Testing revealed decreasing lumbosacral ROM as instrumentation extended cephalad (P < 0.05). In axial rotation, ROM was markedly higher for the allograft group compared to pedicle screw and iliac screw groups with instrumentation to L4 (P < 0.05). In flexion/extension, length of instrumentation in each group correlated with ROM. As length of instrumentation increased, ROM decreased, particularly for the iliac screw group. In lateral bending, ROM decreased in all groups as instrumentation lengthened (P < 0.05). Strain on unprotected sacral screws increased in flexion, extension, and lateral bending as instrumentation extended to L3 (P < 0.05). Iliac screws reduced strain in constructs to L3 and above (P < 0.05). Allograft reduced strain when fixation reached L2, but was not as effective as iliac screws overall. Neither iliac screws nor allograft reduced strain in constructs terminating at L5 or L4. (P > 0.05) CONCLUSION.: For instrumented fusions extending above L3, sacral screws should be protected with supplemental iliac screws or FRA at L5-S1. Of the two, iliac screws appear more effective.

Comparative fixation methods of cervical disc arthroplasty versus conventional methods of anterior cervical arthrodesis: serration, teeth, keels, or screws?

Object

Using a synthetic vertebral model, the authors quantified the comparative fixation strengths and failure mechanisms of 6 cervical disc arthroplasty devices versus 2 conventional methods of cervical arthrodesis, highlighting biomechanical advantages of prosthetic endplate fixation properties.

Methods

Eight cervical implant configurations were evaluated in the current investigation: 1) PCM Low Profile; 2) PCM V-Teeth; 3) PCM Modular Flange; 4) PCM Fixed Flange; 5) Prestige LP; 6) Kineflex/C disc; 7) anterior cervical plate + interbody cage; and 8) tricortical iliac crest. All PCM treatments contained a serrated implant surface (0.4 mm). The PCM V-Teeth and Prestige contained 2 additional rows of teeth, which were 1 mm and 2 mm high, respectively. The PCM Modular and Fixed Flanged devices and anterior cervical plate were augmented with 4 vertebral screws. Eight pullout tests were performed for each of the 8 conditions by using a synthetic fixation model consisting of solid rigid polyurethane foam blocks. Biomechanical testing was conducted using an 858 Bionix test system configured with an unconstrained testing platform. Implants were positioned between testing blocks, using a compressive preload of −267 N. Tensile load-to-failure testing was performed at 2.5 mm/second, with quantification of peak load at failure (in Newtons), implant surface area (in square millimeters), and failure mechanisms.

Results

The mean loads at failure for the 8 implants were as follows: 257.4 ± 28.54 for the PCM Low Profile; 308.8 ± 15.31 for PCM V-Teeth; 496.36 ± 40.01 for PCM Modular Flange; 528.03± 127.8 for PCM Fixed Flange; 306.4 ± 31.3 for Prestige LP; 286.9 ± 18.4 for Kineflex/C disc; 635.53 ± 112.62 for anterior cervical plate + interbody cage; and 161.61 ± 16.58 for tricortical iliac crest. The anterior plate exhibited the highest load at failure compared with all other treatments (p < 0.05). The PCM Modular and Fixed Flange PCM constructs in which screw fixation was used exhibited higher pullout loads than all other treatments except the anterior plate (p < 0.05). The PCM VTeeth and Prestige and Kineflex/C implants exhibited higher pullout loads than the PCM Low Profile and tricortical iliac crest (p < 0.05). Tricortical iliac crest exhibited the lowest pullout strength, which was different from all other treatments (p < 0.05). The surface area of endplate contact, measuring 300 mm2 (PCM treatments), 275 mm2 (Prestige LP), 250 mm2 (Kineflex/C disc), 180 mm2 (plate + cage), and 235 mm2 (tricortical iliac crest), did not correlate with pullout strength (p > 0.05). The PCM, Prestige, and Kineflex constructs, which did not use screw fixation, all failed by direct pullout. Screw fixation devices, including anterior plates, led to test block fracture, and tricortical iliac crest failed by direct pullout.

Conclusions

These results demonstrate a continuum of fixation strength based on prosthetic endplate design. Disc arthroplasty constructs implanted using vertebral body screw fixation exhibited the highest pullout strength. Prosthetic endplates containing toothed ridges (≥ 1 mm) or keels placed second in fixation strength, whereas endplates containing serrated edges exhibited the lowest fixation strength. All treatments exhibited greater fixation strength than conventional tricortical iliac crest. The current study offers insights into the benefits of various prosthetic endplate designs, which may potentially improve acute fixation following cervical disc arthroplasty.

Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation

BACKGROUND CONTEXT

From a biomechanical perspective, the successful outcome of total disc replacement is largely based on the mechanisms of acute fixation obtained at the index procedure and the extent of porous biological osseointegration at the prosthesis-bone interface, ensuring long-term device fixation.

PURPOSE

The present retrospective investigation quantifies the extent of porous osseointegration in cervical and lumbar disc arthroplasty implants containing a bioactive titanium/calcium phosphate coating.

STUDY DESIGN

Based on radiographic analysis and quantitative histomorphometry, the study was designed to determine the extent of porous osseointegration and whether osseointegration was affected by arthroplasty implant position.

OUTCOME MEASURES

Quantitative histomorphometric analysis of trabecular apposition in metallic backed cervical and lumbar arthroplasty devices.

METHODS

Twenty-nine disc arthroplasty devices underwent radiographic and histomorphometric analysis after 6- to 12-month implantation. The specimens included 12 cervical porous-coated motion devices implanted in a caprine model, and 17 lumbar Charité devices implanted in a non-human primate baboon. The two prosthetic-bone surfaces (superior and inferior) of each implant were examined for a total of 58 vertebral end plates. The operative motion segments were processed using undecalcified histologic technique with production of high-resolution light photomicrographs and microradiographs used for histomorphometric quantification of trabecular bone area at the implant interface. Based on plain film radiographs and histologic microradiographs, the technical accuracy of implant placement was classified as Ideal, Suboptimal, or Poor, with alignment referenced to the sagittal and coronal planes.

RESULTS

The technical accuracy of implant placement in the cervical spine based on histologic microradiographs ranged from poor=8% (2 out of 24), suboptimal=17% (4 out of 24), to ideal=75% (18 out of 24), whereas accuracy of lumbar disc arthroplasty ranged from poor=20% (7 out of 34), suboptimal=52% (18 out of 34), and ideal=26% (9 out of 34). Based on histomorphometric analysis of the inferior and superior end plate surfaces, the trabecular apposition ranged from poor placement 21%+/-30% ingrowth, suboptimal 26%+/-33%, to ideal=44%+/-23% (p>.05). Similar findings were observed for the lumbar region; however, the suboptimal and ideal positions were closer in values with regard to trabecular apposition. Poor placement was 34%+/-29%, suboptimal 49%+/-19%, and ideal 51%+/-13%, but this was not statistically significant (p>.05).

CONCLUSIONS

The present study represents the largest analysis to date of any retrieved porous ingrowth disc replacement prostheses. A trend was observed of increase porous osseointegration with improved implant positioning; however, the small sample size and high standard deviations account for lack of statistical significance. Although osseointegration occurs despite nonideal intraoperative positioning, it remains imperative that surgeons strive for Ideal implant position.

Revision strategies for single- and two-level total disc arthroplasty procedures: a biomechanical perspective

BACKGROUND CONTEXT

The utilization of motion-preserving implants versus conventional instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties and optimal biomechanical method for surgical revision.

PURPOSE

Using an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties of single- versus two-level total disc arthroplasty procedures and determine the acute in vitro biomechanical characteristics of two methods of surgical revision-posterior transpedicular instrumentation alone or circumferential spinal arthrodesis.

STUDY DESIGN

This in vitro biomechanical study was undertaken to compare the multidirectional flexibility kinematics of single- versus two-level lumbar total disc arthroplasty reconstructions using an in vitro model.

METHODS

A total of seven human cadaveric lumbosacral spines (L1-sacrum) were biomechanically evaluated under the following L4-L5 reconstruction conditions: intact spine; discectomy alone; Charité total disc replacement; Charité with pedicle screws; two-level Charité (L4-S1); two-level Charité with pedicle screws (L4-S1); Charité L4-L5 with pedicle screws and femoral ring allograft (FRA) (L5-S1); and pedicle screws with FRA (L4-S1). Multidirectional flexibility testing used the Panjabi Hybrid Testing protocol, which includes pure moments for the intact condition with the overall spinal motion replicated under displacement control for subsequent reconstructions. Hence, changes in adjacent level kinematics can be obtained compared with pure moment testing strategies. Unconstrained intact moments of +/-7.5Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM). All data were normalized to the intact spine condition (intact=100%).

RESULTS

In axial rotation, single- and two-level Charité reconstructions produced significantly more motion than pedicle screw constructs combined with the Charité or FRA (p<.05). There were no differences between the Charité augmented with pedicle screws or pedicle screws with FRA (p>.05). The two-level annulus lumbar resection required for multilevel Charité implantation had an added destabilizing effect, resulting in a 140% to 160% ROM increase over the intact condition. Under two-level reconstructions, rotational motion at the L4-L5 level increased from 160+/-26% to 263+/-65% with the implantation of the second Charité at L5-S1. Flexion-extension and lateral bending conditions with the Charité reconstructions in this group of seven spines demonstrated no significant differences compared with the intact spine (p>.05). The Charité combined with pedicle screws or pedicle screws with FRA significantly reduced motion at the operative level compared with the Charité reconstruction (p<.05). The most pronounced changes in adjacent level kinematics and intradiscal pressures were observed under flexion-extension loading. The addition of pedicle screw fixation increased segmental motion and intradiscal pressures at the proximal and distal adjacent levels compared with the intact and Charité reconstruction groups (p<.05).

CONCLUSIONS

The findings highlight a variety of important trends at the operative and adjacent levels. In terms of revision strategies, posterior pedicle screw reconstruction combined with an existing Charité was not found acutely to be statistically different from pedicle screws combined with FRA.

Ceramic granules enhanced with B2A peptide for lumbar interbody spine fusion: an experimental study using an instrumented model in sheep

OBJECT

New generations of devices for spinal interbody fusion are expected to arise from the combined use of bioactive peptides and porous implants. The purpose of this dose-ranging study was to evaluate the fusion characteristics of porous ceramic granules (CGs) coated with the bioactive peptide B2A2-K-NS (B2A) by using a model of instrumented lumbar interbody spinal fusion in sheep.

METHODS

Instrumented spinal arthrodesis was performed in 40 operative sites in 20 adult sheep. In each animal, posterior instrumentation (pedicle screw and rod) and a polyetheretherketone cage were placed in 2 single-level procedures (L2-3 and L4-5). All cages were packed with graft material prior to implantation. The graft materials were prepared by mixing (1:1 vol/vol) CGs with or without a B2A coating and morselized autograft. Ceramic granules were coated with B2A at 50, 100, 300, and 600 microg/ml granules (50-B2A/CG, 100-B2A/CG, 300-B2A/CG, and 600-B2A/CG, respectively), resulting in 4 B2A-coated groups plus a control group (uncoated CGs). Graft material from each of these groups was implanted in 8 operative sites. Four months after arthrodesis, interbody fusion status was assessed with CT, and the interbody site was further evaluated with quantitative histomorphometry.

RESULTS

All B2A/CG groups had higher CT-confirmed interbody fusion rates compared with those in controls (CGs only). Seven of 8 sites were fused in the 50-B2A/CG, 100-B2A/CG, and 300-B2A/CG groups, whereas 5 of 8 sites were fused in the group that had received uncoated CGs. New woven and lamellar bone spanned the fusion sites with excellent osseointegration. There was no heterotopic ossification or other untoward events attributed to the use of B2A/CG in any group. Each B2A/CG treatment produced more new bone than that in the CG group.

CONCLUSIONS

Bioactive treatment with B2A effectively enhanced the fusion capacity of porous CGs. These findings suggest that B2A/CG may well represent a new generation of biomaterials for lumbar interbody fusion and indicate that additional studies are warranted.

Prospective, randomized, multicenter Food and Drug Administration investigational device exemption study of lumbar total disc replacement with the CHARITE artificial disc versus lumbar fusion: five-year follow-up

BACKGROUND CONTEXT

The CHARITE artificial disc, a lumbar spinal arthroplasty device, was approved by the United States Food and Drug Administration in 2004 based on two-year safety and effectiveness data from a multicenter, prospective, randomized investigational device exemption (IDE) study. No long-term, randomized, prospective study on the CHARITE disc or any other artificial disc has been published to date.

PURPOSE

The purpose of this study was to compare the safety and effectiveness at the five-year follow-up time point of lumbar total disc replacement using the CHARITE artificial disc (DePuy Spine, Raynham, MA) with that of anterior lumbar interbody fusion (ALIF) with BAK cages and iliac crest autograft, for the treatment of single-level degenerative disc disease from L4 to S1, unresponsive to nonoperative treatment.

STUDY DESIGN/SETTING

Randomized controlled trial-five-year follow-up.

PATIENT SAMPLE

Ninety CHARITE patients and 43 BAK patients.

OUTCOME MEASURES

Self-reported measures: visual analog scale (VAS); validated Oswestry disability index (ODI version 1.0); Short-Form 36 Questionnaire, and patient satisfaction. Physiologic measures: radiographic range of motion, disc height, and segmental translation. Functional measures: work status.

METHODS

Of the 375 subjects enrolled in the CHARITE IDE trial, 277 were eligible for the five-year study and 160 patients thereof completed the five-year follow-up. The completers included 133 randomized patients. Overall success was defined as improvement> or =15 pts in ODI vs. baseline, no device failure, absence of major complications, and maintenance or improvement of neurological status. Additional clinical outcomes included an ODI questionnaire as well as VAS, SF-36, and patient satisfaction surveys. Work status was tracked for all patients. Safety assessments included occurrence and severity of adverse events and device failures. Radiographic analyses such as index- and adjacent-level range of motion, segmental translation, disc height, and longitudinal ossification were also carried out.

RESULTS

Overall success was 57.8% in the CHARITE group vs. 51.2% in the BAK group (Blackwelder's test: p=0.0359, Delta=0.10). In addition, mean changes from baseline for ODI (CHARITE: -24.0 pts vs. BAK: -27.5 pts), VAS pain scores (CHARITE: -38.7 vs. BAK: -40.0), and SF-36 questionnaires (SF-36 Physical Component Scores [PCS]: CHARITE: 12.6 pts vs. BAK: 12.3 pts) were similar across groups. In patient satisfaction surveys, 78% of CHARITE patients were satisfied vs. 72% of BAK patients. A total of 65.6% patients in the CHARITE group vs. 46.5% patients in the BAK group were employed full-time. This difference was statistically significant (p=0.0403). Long-term disability was recorded for 8.0% of CHARITE patients and 20.9% of BAK patients, a difference that was also statistically significant (p=0.0441). Additional index-level surgery was performed in 7.7% of CHARITE patients and 16.3% of BAK patients. Radiographic findings included operative and adjacent-level range of motion (ROM), intervertebral disc height and segmental translation. At the five-year follow-up, the mean ROM at the index level was 6.0 degrees for CHARITE patients and 1.0 degrees for BAK patients. Changes in disc height were also similar for both CHARITE and BAK patients (0.7 mm for both groups, p=0.9827). Segmental translation was 0.4 and 0.8mm in patients implanted with CHARITE at L4-L5 vs. L5-S1, respectively, and 0.1mm in BAK patients.

CONCLUSIONS

The results of this five-year, prospective, randomized multicenter study are consistent with the two-year reports of noninferiority of CHARITE artificial disc vs. ALIF with BAK and iliac crest autograft. No statistical differences were found in clinical outcomes between groups. In addition, CHARITE patients reached a statistically greater rate of part- and full-time employment and a statistically lower rate of long-term disability, compared with BAK patients. Radiographically, the ROMs at index- and adjacent levels were not statistically different from those observed at two-years postsurgery.

Regeneration of a spinal ligament after total lumbar disk arthroplasty in primates

Total disk arthroplasty (TDA) is a new procedure that replaces the intervertebral disk space with an artificial motion segment and necessitates the resection of the anterior longitudinal ligament (ALL). We assessed whether a collagen-based graft made from porcine small-intestine submucosa (SIS) can be used as a regenerative scaffold to restore the function and structure of the ALL in the lumbar spine. A total of 10 mature male baboons underwent TDA at L5-L6 using one of two treatments: (1) TDA only (n = 5) or (2) TDA combined with SIS (n = 5). Six months postoperatively, mock revision surgery was performed to assess tissue adhesions followed by non-destructive multidirectional flexibility testing of the spinal segment. The vertebral segments were then processed for histology. The tissue adhesion score was 2.8 +/- 0.8 in the TDA only group and 1.8 +/- 1.4 in the TDA-SIS group (p = 0.2). Segmental range of motion and the length of the neutral zone were similar in both groups. Histology showed that the SIS scaffold led to an organized ligamentous structure with a significantly (p = 0.027) higher thickness (2.18 +/- 0.25 mm) compared to the connective tissue structure in the TDA-only group (1.66 +/- 0.33 mm). We concluded that using a SIS bioscaffold after TDA did not lead to increased great vessel adhesion while its use facilitated the formation of highly organized ligamentous tissues. However, the SIS- induced and newly formed ligamentous tissue anterior to the spinal segment did not lead to a measurable limitation of spinal extension.

Cervical disc replacement in patients with and without previous adjacent level fusion surgery: a prospective study

STUDY DESIGN

Prospective 6-center study. OBJECTIVE.: To evaluate outcomes of cervical disc replacement performed adjacent to a prior cervical fusion.

SUMMARY OF BACKGROUND DATA

The use of disc replacement adjacent to a prior anterior cervical decompression and fusion (ACDF) is an attractive reconstructive option, obviating the need for a multilevel fusion. This study reports outcomes from patients with and without previous ACDF receiving the porous coated motion (PCM) artificial cervical disc in a United States Federal Drug Administration Investigational Device Exemption trials.

METHODS

Patients between ages of 18 and 65 with single-level cervical radiculopathy and/or myelopathy, unresponsive to at least 6 weeks of nonsurgical therapy, or experiencing progressive neurologic symptoms were enrolled. Clinical outcomes are compared for patients receiving a PCM disc at a level adjacent to a prior ACDF ("adjacent") and those without having previously had fusion ("primary").

RESULTS

126 PCM patients were primary (mean age: 44.4 years.) and 26 patients had previous "adjacent level" fusion surgery (mean age: 46.4 years). Surgery time was similar in both groups (96 minutes and 98 minutes, respectively; P = 0.761), and mean blood loss was 76 mL and 66 mL in the 2 groups, respectively (P = 0.491). Clinical outcomes using Neck Disability Index and Visual Analog Scores neck and arm scores showed significant improvement after surgery and were similar between groups at all time points. Revision surgery occurred in 2 of 126 primary patients, and in 2 of 26 patients in the adjacent-to-fusion group.

CONCLUSION

Although the level adjacent to a prior cervical fusion is subject to increased biomechanical forces, potentially leading to a higher risk of failure, the PCM disc was well tolerated in the short term. The early clinical results of disc replacement adjacent to a prior fusion are good and comparable to the outcomes after primary disc replacement surgery. However, in view of the small study population and short-term follow-up, continued study is mandatory.

Prospective, Randomized, Multicenter FDA IDE Study of CHARITÉ Artificial Disc versus Lumbar Fusion: Effect at 5-year Follow-up of Prior Surgery and Prior Discectomy on Clinical Outcomes Following Lumbar Arthroplasty

Background

Candidates for spinal arthrodesis or arthroplasty often present with a history of prior surgery such as laminectomy, laminotomy or discectomy. In this study, lumbar arthroplasty patients with prior surgery, and in particular patients with prior discectomy, were evaluated for their clinical outcomes at the 5-year time point.

Methods

Randomized patients from the 5-year CHARITÉ investigational device exemption (IDE) study were divided as follows: 1) fusion prior surgery (excluding prior decompression with fusion) group (FSG); 2) fusion prior discectomy group (FDG); 3) fusion no prior surgery group (FNG); 4) arthroplasty prior surgery group (ASG); 5) arthroplasty prior discectomy group (ADG); and 6) arthroplasty no prior surgery group (ANG). The 5-year clinical outcomes included visual analog scale (VAS), Oswestry Disability Index 2.0 (ODI), patient satisfaction, and work status.

Results

In the arthroplasty group, all subgroups had statistically significant VAS improvements from baseline (VAS change from baseline: ASG = -36.6 ± 29.6, P < 0.0001; ADG = -40.2 ± 30.9, P = 0.0002; ANG = -36.5 ± 34.6, P < 0.0001). There was no statistical difference between subgroups (P = 0.5587). In the fusion group, VAS changes from baseline were statistically significant for the FNG and FSG subgroups, but not for the FDG patients (FNG = -46.3 ± 28.8, P < 0.0001; FSG = -24.2 ± 36.4, P = 0.0444; FDG = -26.7 ± 38.7, P = 0.2188). A trend of decreased VAS improvements was observed for FSG versus FNG (P = 0.0703) subgroups. Similar findings and trends were observed in ODI scores (Changes in ODI from baseline: ASG = -20.4 ± 23.8, P < 0.0001; ANG = -26.6±21.1, P < 0.0001; ADG= -17.6 ± 28.6, P = 0.0116; FSG = -14.5 ± 21.2, P = 0.0303; FNG= -32.5 ± 22.6, P < 0.0001; FDG = -10.7 ± 9.4, P = 0.0938). The greatest improvement in work status from preoperative to postoperative was seen in the ADG subgroup (28% increase in part- and full-time employment), while the FDG subgroup showed the greatest reduction in work status (17% decrease).

Conclusions

Arthroplasty patients with prior surgery or prior discectomy had similar clinical outcomes as arthroplasty patients without prior surgery, while fusion patients with prior surgery or prior discectomy showed trends of lowered clinical outcomes compared to fusion patients without prior surgery or discectomy.

Effect of previous surgery on clinical outcome following 1-level lumbar arthroplasty

OBJECT

A secondary lumbar surgery at a previously surgically treated level is believed to result in minimal clinical improvement. The clinical results of the CHARITE Investigational Device Exemption (IDE) study were analyzed to assess the effect of previous surgery on clinical outcomes following either total disc replacement with the CHARITE device or anterior lumbar interbody fusion with a BAK cage and iliac crest autograft.

METHODS

Patients with prior microdiscectomy, laminectomy, or minimal medial facetectomy were not excluded from enrollment in the CHARITE IDE study. Thus, the following 3 groups were analyzed: all patients treated with the CHARITE Artificial Disc, whether randomized or nonrandomized; only patients treated with CHARITE devices randomized against patients with BAK devices; and control patients with BAK devices. Each group was further subdivided based on the patients' medical history, whether they had undergone prior surgery (prior surgery group) or had not (no prior surgery group). For all groups, baseline demographics were collected and compared for any potential recruitment bias. Postoperative improvements based on Oswestry Disability Index (ODI), visual analog scale (VAS), and patient satisfaction scores were further collected and statistically analyzed.

RESULTS

For all 3 groups, there were no statistical differences in clinical improvement from 3 months to 2 years postoperatively as measured using ODI and VAS scores between the subgroups (those who had prior surgery and those who did not).

CONCLUSIONS

Patients indicated for 1-level lumbar arthroplasty with previous lumbar decompressive surgery can be expected to have similar clinical outcomes to patients undergoing arthroplasty without prior lumbar decompressive surgery. Similarly, candidates for anterior lumbar fusion with prior decompressive surgery may experience similar benefits from the surgical procedure as those without.

Distribution of in vivo and in vitro range of motion following 1-level arthroplasty with the CHARITE artificial disc compared with fusion

OBJECT

One of the goals of lumbar arthroplasty is to restore and maintain range of motion (ROM) and to protect adjacent levels from abnormal motion, which may be a factor in transition syndrome following arthrodesis. In this study, in vitro ROM results were compared with in vivo, 2-year postoperative radiographic ROM evaluations.

METHODS

Radiographs of patients enrolled in the CHARITE investigational device exemption study were analyzed at baseline and at 2 years postoperatively. The ROM in flexion/extension at the implanted and adjacent levels was measured, normalized, and compared with ROM results obtained using cadaver (in vitro) evaluations.

RESULTS

Preoperative ROM distributions in patients enrolled for arthroplasty or fusion at the L4-5 level was as follows: 28% motion was observed at L3-4, 35% at L4-5 and 37% at L5-S1. Following a one-level arthroplasty at L4-5, the in vivo ROM distribution from L-3 to S-1 at the 2-year time point was 36% at L3-4, 30% at L4-5 and 35% at L5-S1. Following a one-level fusion with BAK and pedicle screws at L4-5, the in vivo ROM distribution from L-3 to S-1 at the 2-year time point was 45% at L3-4, 9% at L4-5 and 46% at L5-S1.

CONCLUSIONS

The baseline as well as the 2-year in vivo data confirmed previously published in vitro data. One-level arthroplasty was shown herein to replicate the normal distribution of motion of the intact spine.

Effect of age on clinical and radiographic outcomes and adverse events following 1-level lumbar arthroplasty after a minimum 2-year follow-up

Object

Lumbar arthroplasty is approved in the US for the treatment of degenerative disc disease at 1 level in skeletally mature patients. However, a bias toward older patients (> 45 years of age) who are otherwise indicated for the procedure may exist. In this study, the clinical outcomes of patients from the Charité Investigational Device Exemption (IDE) study were analyzed on the basis of patient age.

Methods

There were 276 patients enrolled in the IDE study of the Charité Artificial Disc who underwent 1-level arthroplasty at either L4–5 or L5–S1, including 71 nonrandomized and 205 randomized individuals. Patient data were analyzed based on age (18–45 years [217 patients, Group 1] compared with 46–60 years [59 patients, Group 2]). Statistical analyses were performed based on 2-year postoperative improvements in Oswestry Disability Index (ODI), 36-item Short Form Health Survey (SF-36), and visual analog scale (VAS) scores (clinical outcome), as well as range of motion (radiographic outcome), and adverse events.

Results

There was no significant difference between the groups with respect to level implanted, operative time, blood loss, changes in ODI and VAS scores or any of the 8 component scores of the SF-36, compared with baseline, at all time points throughout the 24-month follow-up period (p > 0.10). Patient satisfaction was equivalent at 24 months, with 87% satisfaction in Group 1 and 85% satisfaction in Group 2 (no statistical difference). In addition, no significant differences were identified with respect to adverse events including approach related, neurological, technique related, or reoperation.

Conclusions

Although patients > 45 years of age may have comorbidities or contraindications for arthroplasty for a number of reasons, particularly osteopenia, this analysis demonstrates that patients who are indicated for 1-level arthroplasty experience similar clinical outcome, satisfaction, or adverse events compared with their younger counterparts.

Patient selection for lumbar arthroplasty and arthrodesis: the effect of revision surgery in a controlled, multicenter, randomized study

Object

Patient selection is perhaps the most important factor in successful lumbar surgery. In this study, the authors analyzed the clinical outcomes of patients enrolled in the CHARITÉ investigational device exemption (IDE) trial who underwent revision surgery after primary total disc replacement with CHARITÉ or an anterior lumbar interbody fusion (ALIF) with placement of a BAK cage and iliac crest autograft. This revision surgery was either a supplemental posterior lumbar fixation or a 360° fusion. Statistical analysis was conducted to compare clinical success in patients who underwent revision surgery with those who did not.

Methods

The patients enrolled in the CHARITÉ IDE study were divided into 6 groups according to treatment and repeated operation status, and their Oswestry Disability Index (ODI) and visual analog scale (VAS) scores at the 2-year follow-up and at baseline were compared. The patients had received the following treatments by group: A) ALIF without reoperation; B) ALIF with conversion to 360° fusion; C) arthroplasty (randomized) without repeated operation; D) arthroplasty with supplemental posterior lumbar fixation; E) arthroplasty (nonrandomized) without repeated operation; and F) arthroplasty (nonrandomized) with supplemental posterior lumbar fixation. Outcome scores in the groups of patients who required revision surgeries (Groups B, D, and F; 23 patients) were compared with the groups that did not require revision surgery (Groups A, C, and E; 299 patients).

Results

Patients who required revision surgery had a significantly lower level of clinical improvement than those who did not. The mean change in ODI score was −53.0% in Groups A, C, and E, but just −12.7% in Groups B, D, and F (p < 0.0001). The mean change in VAS score was −59.1% in Groups A, C, and E, compared to −23.4% in Groups B, D, and F (p < 0.0001). No significant differences were identified in analyzing absolute change in scores and the percentage change. A comparison of outcomes in patients who had undergone arthroplasty without reoperation (Groups C and E) with all patients who had undergone revision surgery (Groups B, D, and F) demonstrated similar results (p < 0.0001).

Conclusions

The 7.1% of patients who underwent a secondary stabilization procedure had poor clinical improvement. This finding may indicate that if the alternative treatment had been the initial treatment, these patients would not have benefited, and further implies a 7.1% rate of imprecision in preoperative evaluation.

Effects of a cervical disc prosthesis on maintaining sagittal alignment of the functional spinal unit and overall sagittal balance of the cervical spine

The object of this study is to review the early clinical results and radiographic outcomes following insertion of the Bryan Cervical Disc Prosthesis (Medtronic Sofamor Danek, Memphis, TN), together with its effect on maintaining sagittal alignment of the functional spinal unit (FSU) and overall sagittal balance of the cervical spine for the treatment of single-level or two-level symptomatic disc disease. Forty-seven patients with symptomatic single or two-level cervical disc disease who received the Bryan Cervical Artificial Disc were reviewed prospectively. A total of 55 Bryan disc were placed in 47 patients. A single-level procedure was performed in 39 patients and a two-level procedure in the other eight. Radiographic and clinical assessments were made preoperatively and at 1.5, 3, 6, 9, 12, and 18 and up to 33 months postoperatively. Mean follow-up duration was 24 months, ranging from 13 to 33 months. Periods were categorized as early follow up (1.5-3 months) and late follow up (6-33 months). The visual analogue scale (VAS), neck disability index(NDI), Odom's criteria were used to assess pain and clinical outcomes. Static and dynamic radiographs were measured by hand and computer to determine the range of motion (ROM), the angle of the functional segmental unit (FSU), and the overall cervical alignment (C2-7 Cobb angle). With all of these data, we evaluated the change of the preoperative lordosis (or kyphosis) of the FSU and Overall sagittal balance of the cervical spine during the follow-up period. There was a statistically significant improvement in the VAS score from 7.0 +/- 2.6 to 2.0 +/- 1.5 (paired-t test, P = 0.000), and in the NDI from 21.5 +/- 5.5 to 4.5 +/- 3.9 (paired-t test P = 0.000). All of the patients were satisfied with the surgical results by Odom's criteria. The postoperative ROM of the implanted level was preserved without significant difference from preoperative ROM of the operated level. Only 36% of patients with a preoperative lordotic sagittal orientation of the FSU were able to maintain lordosis following surgery. However, the overall sagittal alignment of the cervical spine was preserved in 86% of cases at the final follow up. Interestingly, preoperatively kyphotic FSU resulted in lordotic FSU in 13% of patients during the late follow-up, and preoperatively kyphotic overall cervical alignment resulted in lordosis in 33% of the patients postoperatively. Clinical results are encouraging, with significant improvement seen in the Bryan Cervical Artificial disc. The Bryan disc preserves motion of the FSU. Although the preoperative lordosis (or kyphosis) of the FSU could not always be maintained during the follow-up period, the overall sagittal balance of the cervical spine was usually preserved.

Superiority of multilevel cervical arthroplasty outcomes versus single-level outcomes: 229 consecutive PCM prostheses

STUDY DESIGN/SETTING

Class 2 level of evidence: This is a prospective, consecutive series of 229 prosthetic implantations that were concurrently enrolled between single-level versus multilevel cervical arthroplasty comprising an FDA Pilot Study.

OBJECTIVE

This study investigated multilevel cervical disc replacement in relation to single-level cervical arthroplasty to find if the same reduction in clinical success would occur with this alternative treatment.

SUMMARY OF BACKGROUND DATA

Usually, the clinical outcomes of instrumented cervical fusions deteriorate as the number of vertebral levels of involvement increases.

METHODS

A total of 229 patients presented with cervical herniated nucleus pulposus, cervical spondylosis, and/or adjacent segment disease with cervical radiculopathy or myelopathy. Following anterior cervical neurologic decompression seventy-one patients required porous coated motion (PCM) cervical arthroplasties from C3-C4 to C7-T1 (Group S, single level). Sixty-nine patients underwent 158 multilevel PCM cervical arthroplasties (Group M, multilevel) during the same time interval, for the same indications, performed by the same surgeons under the same clinical protocol: double level, 53 cases; three levels, 12 cases: and 4 levels, 4 cases.

RESULTS

The self-assessment outcomes instruments showed significantly more improvement for the multilevel cases. The mean improvement in the NDI for the single cases was 37.6% versus the multilevel cases mean improvement in NDI was 52.6% (P = 0.021). The difference between the two was statistically significant. The mean improvement in the VAS showed the same association: single-level mean improvement 58.4% versus the multilevel cases mean VAS improvement was 65.9%. The Odom's were also more improved for the multilevel versus the single-level group: 93.9% versus 90.5% in the excellent, good, and fair categories. The reoperation rates and serious adverse events were similar between the single-level (S = 3) to the multilevel arthroplasty (M = 2) groups. Kaplan-Meier implant survivorship analysis at 3 years for the cohort of 229 prostheses was 94.5% (confidence interval, 1.00-0.820).

CONCLUSIONS

This prospective study of cervical arthroplasty is the first report to date showing significantly improved clinical outcomes for multilevel cervical arthroplasty compared with single-level cervical disc replacement using an FDA validated outcome instrument.

A Comparative Biomechanical Analysis of Spinal Instability and Instrumentation of the Cervicothoracic Junction

OBJECTIVE

Stabilization of the cervicothoracic junction is challenging but commonly required in patients with traumatic, neoplastic, congenital, and postlaminectomy conditions. Although extensive research has been performed on stabilization of the cervical spine, there remains a paucity of published data on instrumentation at the cervicothoracic junction. Using 2-column, 3-column, and corpectomy instability models, a biomechanical analysis was performed on the effects of increasing the number of posterior segmental fixation points and/or anterior column reconstruction at the cervicothoracic junction.

METHODS

Multidirectional flexibility testing was performed utilizing a 6-degree-of-freedom spine simulator and 7 fresh-frozen human cadaveric spines (occiput-T6). After intact spine analysis, each specimen was destabilized and reconstructed as follows: (1) C7/T1 2-column injury with posterior instrumentation; (2) C7/T1 3-column injury with posterior instrumentation; (3) C7/T1 3-column injury with anterior interbody cage/plate and posterior instrumentation; and (4) C7/T1 3-column injury plus C7 corpectomy with anterior cage/plate and posterior instrumentation. All reconstruction groups were tested with posterior instrumentation (screws connected by dual-diameter rods) from C5-T1, C5-T2, and C5-T3.

RESULTS

For 2-column injuries, there were no statistically significant differences in flexibility (P>0.05), although there was a trend toward reduced flexibility with increasing levels of thoracic fixation. For 3-column injuries, posterior fixation alone resulted in excessive flexibility in flexion/extension even with instrumentation to T3 (P<0.05). With the addition of anterior column instrumentation, there were no observed differences in flexion/extension and lateral bending. For axial rotation, instrumentation to T1 alone demonstrated increased motion relative to the intact spine (P<0.05). The 3-column injury with corpectomy model demonstrated similar flexibility properties to the 3-column injury model.

CONCLUSIONS

With 3-column instability posterior segmental fixation alone from C5-T3 was inadequate, and the addition of anterior instrumentation restored flexibility to the intact condition. There was a strong trend toward reduced flexibility with increasing levels of thoracic fixation in all instability models.

Complications of Lumbar Artificial Disc Replacement Compared to Fusion: Results From the Prospective, Randomized, Multicenter US Food and Drug Administration Investigational Device Exemption Study of the Charité Artificial Disc

Background

Previous reports of lumbar total disc replacement (TDR) have described significant complications. The US Food and Drug Administration (FDA) investigational device exemption (IDE) study of the Charité artificial disc represents the first level I data comparison of TDR to fusion.

Methods

In the prospective, randomized, multicenter IDE study, patients were randomized in a 2:1 ratio, with 205 patients in the Charité group and 99 patients in the control group (anterior lumbar interbody fusion [ALIF] with BAK cages). Inclusion criteria included confirmed single-level degenerative disc disease at L4-5 or L5-S1 and failure of nonoperative treatment for at least 6 months. Complications were reported throughout the study.

Results

The rate of approach-related complications was 9.8% in the investigational group and 10.1% in the control group. The rate of major neurological complications was similar between the 2 groups (investigational = 4.4%, control = 4.0%). There was a higher rate of superficial wound infection in the investigational group but no deep wound infections in either group. Pseudarthrosis occurred in 9.1% of control group patients. The rate of subsidence in the investigational group was 3.4%. The reoperation rate was 5.4% in the investigational group and 9.1% in the control group.

Conclusions

The incidence of perioperative and postoperative complications for lumbar TDR was similar to that of ALIF. Vigilance is necessary with respect to patient indications, training, and correct surgical technique to maintain TDR complications at the levels experienced in the IDE study.

The first 18 months following food and drug administration approval of lumbar total disc replacement in the United States: reported adverse events outside an investigational device exemption study environment

Background

Introduction of a new surgical technology may result in higher rates of adverse events compared with rates reported in the study performed to gain regulatory approval. The purpose of our study was to describe the incidence of reported adverse events during the first 18 months following US Food and Drug Administration (FDA) approval of the first lumbar arthroplasty device available in the United States and to discern data trends.

Methods

Reports of adverse events submitted to the FDA in patients receiving the Charité artificial disc were reviewed and pooled by similarity. We analyzed 135 medical device reports filed with the FDA regarding the Charité artificial disc between October 26, 2004, and April 26, 2006. Sixteen reports were excluded for lack of information regarding cause or because described events were vague or unrelated to the procedure.

Results

Rate of adverse events reported to the FDA as a percentage of devices of which the device manufacturer was aware had been dispensed at 6, 12, and 18 months following approval was 0.58%, 2.34%, and 2.13%, respectively. The adverse event reported most frequently through 18 months was anterior migration with reoperation (0.65%); other reported adverse events were, in decreasing order, sizing and malposition errors resulting in reoperation (0.36%), posterior element fracture resulting in reoperation (0.30%), major vascular injury requiring a blood transfusion (0.23%), and subsidence requiring reoperation (0.20%). Three non–device-related patient deaths were reported following FDA approval. The reported rate of sizing/malposition errors leading to reoperation of 0.36% was the same rate as that seen in the investigational device exemption (IDE) study of the Charité artificial disc. All other reported rates were lower than rates of the same events reported in the study.

Conclusions

Medical device reporting is an important yet highly anecdotal and incomplete event-tracking process. However, it is the principal means available in the United States for obtaining information on the clinical performance of a device after its approval for sale and does provide some data, albeit imperfect, in this regard. The cumulative medical device reports through the 18 months following FDA approval, measured against the number of devices dispensed, suggests a rate of adverse events that either tracks or is somewhat less than that reported in the IDE study. This suggests that a repeat of the “cage rage,” a “lumbar arthroplasty rage,” has not yet occurred.

Prevalence of heterotopic ossification following total disc replacement. A prospective, randomized study of two hundred and seventy-six patients

BACKGROUND

Despite reports of good clinical outcomes in patients treated with lumbar and cervical disc replacements, varying degrees of heterotopic bone have been observed around these devices. The purposes of the present study were to determine the prevalence of heterotopic ossification following lumbar disc replacement and to investigate whether heterotopic ossification results in loss of motion or negatively affects clinical outcome.

METHODS

All preoperative and postoperative radiographs from a completed prospective, randomized, United States Food and Drug Administration-regulated trial comparing replacement with the CHARITE Artificial Disc with anterior interbody arthrodesis were analyzed. In each of 276 patients treated with disc replacement, heterotopic ossification was categorized with use of a validated 5-point radiographic classification system both preoperatively and at all protocol-specified follow-up intervals to two years. The range of motion on flexion and extension radiographs made preoperatively was compared with that on radiographs made two years postoperatively, and the motion was correlated with the presence or absence of heterotopic ossification. Similarly, validated clinical outcome measures were correlated with the presence or absence of heterotopic ossification at two years.

RESULTS

The prevalence of heterotopic ossification in the 276 consecutive patients treated with lumbar disc replacement with the CHARITE Artificial Disc was 4.3%. There were four cases of Class-I heterotopic ossification and eight cases of Class-II heterotopic ossification. In five of the twelve patients, heterotopic bone was visible as early as six weeks postoperatively, and eleven of the twelve patients had evidence of heterotopic ossification by three months postoperatively. The postoperative range of motion exceeded the preoperative range in all of the patients with heterotopic ossification. With the numbers available, no difference in either the range of motion or the clinical outcome at twenty-four months postoperatively was found between the patients who had and those who did not have heterotopic ossification.

CONCLUSIONS

Heterotopic ossification is infrequent in patients treated with the CHARITE Artificial Disc, and it does not impact the range of motion or clinical outcome.

Evaluation of surgical volume and the early experience with lumbar total disc replacement as part of the investigational device exemption study of the Charité Artificial Disc

STUDY DESIGN

A prospective, randomized, multicenter, Food and Drug Administration regulated Investigational Device Exemption (IDE) clinical trial.

OBJECTIVES

To discern whether there is a correlation between surgical volume and clinical outcomes, as well as the complication rate and perioperative data points, for lumbar total disc replacement. To examine the early experience for lumbar total disc replacement as part of an IDE study.

SUMMARY OF BACKGROUND DATA

To our knowledge, an analysis of the effect of surgical volume has not been performed for any spine surgical procedure. Prior reports of the early experience with lumbar total disc replacement consist of retrospective reviews with nonspecific indications.

METHODS

An analysis was performed of the Food and Drug Administration IDE Study of the Charité Artificial Disc (DePuy Spine, Inc., Raynham, MA). Patients enrolled in the control group were omitted from the analysis. Up to 5 nonrandomized cases (representing the early experience) were performed at each site before beginning the randomized arm of the study. There were 3 comparisons performed: nonrandomized cases (71) versus randomized cases (205); randomized cases performed by high-enrolling surgeons versus low-enrolling surgeons; and randomized cases at high-volume institutions versus low-volume institutions.

RESULTS

The high-enrolling groups had a significantly lower mean hospital stay and operating time compared to the low-enrolling groups (P < 0.05). High-enrolling surgeons and institutions showed significantly shorter operating times, length of hospital stay, and complication rates. High-enrolling surgeons had significantly fewer device failures and cases of neurologic deterioration. Mean operating time and hospital stay were significantly lower in the randomized group (P < 0.05) compared to the nonrandomized group. Blood loss and approach-related complications were similar between the 2 groups. Device failure requiring removal was 4.2% in the nonrandomized group and 1.5% in the randomized group.

CONCLUSIONS

Surgeons and institutions with a high volume of lumbar total disc replacement cases have a reduction in key perioperative and postoperative parameters that provide a clinical and/or economic benefit. Surgeons may expect longer hospital stays, higher blood loss, and a higher rate of certain complications in their early experience with total disc replacement procedures, but there was no effect on clinical outcomes.

Biomechanical analysis of rotational motions after disc arthroplasty: implications for patients with adult deformities

STUDY DESIGN

An anatomic and biomechanical bench-top basic scientific comparative analysis to determine the appropriateness of total disc replacement (TDR) in a lumbar spine with scoliotic tendencies.

OBJECTIVES

Only limited data are currently available studying the application of disc replacement adjacent to scoliosis fusions. Theoretically, motion preservation should help delay the continuum of lumbar degeneration adjacent to scoliosis fusions and rotationally unstable lumbar segments.

SUMMARY OF BACKGROUND DATA

As a tertiary referral center for failed TDR, we noticed an alarming number of lumbar spinal rotational iatrogenic instability patterns but none occurring in the cervical spine. It is appropriate to analyze the bench-top rotational stability of disc replacement to predict whether this new technology is feasible for a larger prospective clinical study in the treatment of degenerative scoliosis.

METHODS

Measurements were taken from 60 human specimens from the Hamann-Todd Osteological Collection: 1) to determine the rotational arc of influence (AOI) = the angle formed from the center of axial rotation to the outermost extent of the facet joints; and 2) to determine the relative anatomic size discrepancy between the left and right facets proportionately with the cross-sectional area of the intervertebral disc = facet/endplate ratio (FER). Biomechanical testing was performed using fresh frozen human cadaveric spines with the following conditions to determine the rotational stability: 1) intact; 2) resection of ALL, anulus, disc, and PLL simulating the preparation for a TDR; 3) a more radical anular resection; 4) entire 360 degrees anular resection; and 4) insertion of the respective unconstrained-type disc replacement. Using a 6 degrees of freedom spine simulator, unconstrained pure moments of +/-8.0 Nm (lumbar) and +/-3.0 Nm (cervical) were used for axial rotation with quantification of the operative level range of motion and neutral zone, with data normalized to the intact spine condition.

RESULTS

There were anatomic limitations in the lumbar spine that make it less desirable to apply uncon-strained disc replacements; indeed, the spine was at risk for iatrogenic lumbar scoliosis. The anulus fibrosis, anterior longitudinal ligament, and the posterior longitudinal ligament are critical structures in preventing iatrogenic scoliosis. The lumbar facet joints are more posteriorly located and are smaller relative to the intervertebral disc, compared with this association in the cervical spine. Because the facet capsular ligaments are mechanically less effective with lower tensile strength in the lumbar spine, multiple-level arthroplasty tends to accentuate scoliotic tendencies; this is independent of prosthetic design and surgical technique.

DISCUSSION

Implantation of the lumbar TDR never restored the motion segment back to the rotational stability of the intact segment achieving a range of 120% to 140% rotational range of motion compared with the intact condition. This rotational instability proved to be additive as a two-level lumbar TDR resulted in between 240% and 260% increase in rotational instability compared with the intact condition.

CONCLUSION

The neutral zone of the intact cervical spine was restored even using an unconstrained cervical TDR. The greater inherent rotational constraints of the cervical spine make it more amenable to stable multilevel arthroplasty compared with the lumbar spine.

A Comparison of Retraction Pressure During Anterior Cervical Plate Surgery and Cervical Disc Replacement

BACKGROUND CONTEXT

Dysphagia is a well-recognized complication after anterior cervical discectomy and fusion, observed in as high as 50% of cases by videofluoroscopic evaluation postoperatively. Esophageal injury due to surgical retraction is a complication due to which swallowing difficulties may ensue. There are limited published data evaluating the effect of soft tissue retraction on intraesophageal pressures during anterior cervical instrumentation procedures.

PURPOSE

The purpose of this study was to (a) measure the intraesophageal pressure secondary to retraction during anterior instrumentation, (b) determine whether any pressure differences exist between plating and cervical disc replacement, and (c) determine whether the surgical level or length of the plate influences the magnitude of intraesophageal pressure during retraction.

STUDY DESIGN

An analysis of soft tissue retraction pressure was performed for anterior single-level and 3-level cervical plating and cervical disc replacement procedures.

METHODS

Using a 4-cm transverse incision, a Smith-Robinson anterior approach to the cervical spine was performed on 7 fresh, frozen cadavers. The correct placement of an esophageal pressure-transducing catheter was confirmed by laryngoscopy, manual palpation of the esophagus, and fluoroscopic imaging. Three surgical instrumentation groups were used for comparisons: (a) single-level plate (b) single-level Porous Coated Motion cervical disc replacement, and (c) 3-level plate. Hand-held appendiceal retractors were used to retract the soft tissues during screw insertion into the plate and during application of the disc prosthesis into the interspace. Care was taken to exert just enough force on the retractors to allow the surgeon to move the desired implant into the correct position. In addition the individual performing the retraction was blinded to the procedure being performed-1-level plating, 3-level plating, or disk replacement. Fluoroscopy confirmed that the pressure sensors were directly behind the retractors during data acquisition.

RESULTS

Significantly greater intraesophageal pressures were demonstrated for single-level cervical plating at C5-6 compared to that at C3-4 (P=0.036). Similarly, significantly greater pressures were recorded at C5-6 versus C3-4 for the 3-level plating group (P<0.001). In contrast, there was no statistically significant difference in pressures observed during disk replacement at C5-6 compared to that at C3-4 (P=0.084). Significantly greater pressures were recorded during single-level plating compared to disc replacement at both C3-4 (P=0.016) and C5-6 (P=0.016). Three-level plating demonstrated significantly greater pressures at C5-6 compared to disk replacement (P<0.001) but no statistically significant difference compared to disk replacement at C3-4 (P=0.333). The highest mean pressure, 154.5+/-49.5 mm Hg, was recorded at C5-6 level during insertion of the 3-level plates.

CONCLUSIONS

On the basis of the data presented here, anterior cervical plating results in significantly greater intraesophageal pressures when performed at C5-6 compared to C3-4. This holds regardless of whether the plate spans the distance from C3 to C6 (3-level plate) or the single C5-6 level. In addition, the insertion of the cervical disc replacement seems to require less esophageal retraction and hence reduced intraesophageal pressures when compared to anterior cervical plating.

Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model

STUDY DESIGN

The biomechanical, histopathologic, and histomorphometric characteristics of cervical disc replacement were assessed in a caprine animal model.

OBJECTIVE

To investigate the biomechanical, porous ingrowth, and histopathologic characteristics of the Porous Coated Motion (PCM) Cervical Disc replacement (Cervitech, Inc., Rockaway, NJ).

SUMMARY OF BACKGROUND DATA

As an alternative to anterior cervical interbody arthrodesis, an artificial cervical disc serves to replace the symptomatic degenerated disc, restore the functional biomechanical properties of the motion segment, and preserve neurologic function.

METHODS

There were 12 mature Nubian goats divided into 2 groups based on postoperative survival periods of 6 (n = 6) and 12 months (n = 6). Using an anterior surgical approach, a complete discectomy was performed at the C3-C4, followed by implantation of the PCM device. Functional outcomes of the disc prosthesis were based on computerized tomography (CT), multidirectional flexibility testing, undecalcified histology, histomorphometric, and immunocytochemical analyses.

RESULTS

There was no evidence of prosthesis loosening, or neurologic or vascular complications. CT showed the ability to image and assess the cervical spinal canal for the presence of compressive pathology in the area of the CoCrMo prosthesis. Multidirectional flexibility testing under axial rotation and lateral bending indicated no differences in the full range of intervertebral motion between the disc prosthesis and nonoperative controls (P > 0.05). Based on immunohistochemical and histologic analysis, there was no evidence of particulate debris, cytokines, or cellular apoptosis within the local or systemic tissues. Moreover, review of the spinal cord at the operative levels indicated no evidence of cord lesions, inflammatory reaction, wear particles, or significant pathologic changes in any treatment. Histomorphometric analysis at the metal-bone interface indicated the mean trabecular ingrowth of 40.5% +/- 24.4% and 58.65% +/- 28.04% for the 6 and 12-month treatments, respectively.

CONCLUSION

To our knowledge, this serves as the first in vivo time-course study investigating the use of the PCM device for cervical arthroplasty. All 12 animals undergoing cervical disc replacement had no evidence of implant loosening, subluxation, or inflammatory reactions. PCM cervical arthroplasty permits unobstructed visualization of the spinal canal based on CT imaging. Segmental intervertebral motion was preserved under axial rotation and lateral bending loading conditions, while at the same time permitting porous osseointegration at the prosthesis-bone interface. Based on histopathologic review of all local and systemic tissues, there was no evidence of particulate wear debris, cytokines, cellular apoptosis, or significant pathologic changes in any treatment.

Revisability of the CHARITE artificial disc replacement: analysis of 688 patients enrolled in the U.S. IDE study of the CHARITE Artificial Disc

STUDY DESIGN

A prospective, randomized, multicenter, FDA-regulated Investigational Device Exemption clinical trial.

OBJECTIVES

To analyze the incidence of, and reasons for, reoperation in all patients (treatment and control) enrolled in the IDE study.

SUMMARY OF BACKGROUND DATA

This is the first report of the incidence and nature of reoperations following lumbar TDR as part of a controlled, prospective, multicenter trial.

METHODS

A total of 688 patients meeting the inclusion and exclusion criteria were enrolled in one of three arms of the study at 14 centers across the United States. This cohort includes 71 nonrandomized cases, 205 randomized cases, and 313 continued access cases, all receiving the CHARITE Artificial Disc, as well as 99 randomized cases in the control group (ALIF with threaded fusion cages and autograft). A detailed analysis was performed of clinical chart notes, operative notes, and adverse event reports for all patients requiring reoperation following their index surgery.

RESULTS

Of the 589 patients with TDR, 52 (8.8%) required reoperation. Of the 99 patients with lumbar fusion, 10 (10.1%) required reoperation, and an additional 2 required surgery for adjacent level disease (P = 0.7401). There were 24 TDR patients who underwent a repeated anterior retroperitoneal approach, with 22 (91.7%) having had a successful removal of the prosthesis. Seven of the 24 TDR prostheses requiring removal were revised to another CHARITE Artificial Disc. The mean time to reoperation in all patients was 9.7 months. A total of 29 patients (4.9%) in the TDR group required posterior instrumentation and fusion as did 10 (10.1%) in the control group (P = 0.0562). At 2 years or more follow-up, 93.9%(553/589 = 93.9%) of patients receiving TDR with the CHARITE Artificial Disc had a successfully functioning prosthesis with a mean of over 7 degrees of flexion-extension mobility.

CONCLUSIONS

Lumbar TDR with the CHARITE Artificial Disc did not preclude any further procedures at the index level during primary insertion, with nearly one third being revisable to a new motion-preserving prosthesis and just over two thirds being successfully converted to ALIF and/ or posterior pedicle screw arthrodesis, the original alternative procedure.