Stabilizing potential of anterior cervical plates in multilevel corpectomies

Clinicoradiological Outcome of 3 or More Levels of Anterior Cervical Corpectomy and Reconstruction

BACKGROUND

Studies reporting multilevel anterior cervical corpectomy (>2 levels) and reconstruction in patients with long-segment anterior cervical compression are few and surgical outcomes are variable with increased surgical morbidity and a high incidence of graft-related complications. The aim of this study is to evaluate the effectiveness and safety of cervical corpectomy and anterior reconstruction of 3 or more levels in patients with long-segment anterior cervical compression.

METHODS

We retrospectively reviewed patients who had undergone 3 or more levels of anterior cervical corpectomy and reconstruction from 2014 to 2018. Clinical and radiological parameters such as Nurick grading, modified Japanese Orthopedic Association (mJOA) score, cervical segmental angle, cervical sagittal angle, graft subsidence, and fusion rate were evaluated preoperatively and at a 2-year follow-up. Patients were divided into 2 groups according to their anterior reconstruction, either with fibular strut autogenous graft or titanium mesh cage and rigid anterior cervical plating for subgroup analysis. Patients whose bone stock was found to be poor had undergone posterior instrumentation as a staged procedure.

RESULTS

There were 48 patients (mean age: 58.17 years) in the cohort: 42 had undergone 3-level and 6 had undergone 4-level cervical corpectomy with an ossified posterior longitudinal ligament and multilevel cervical spondylotic myelopathy being the main surgical indications. C5 to C7 corpectomy was most common. Of the cohort, 83.4% had standalone anterior reconstruction and only 8 patients (16.6%) had supplementation with posterior instrumentation. Our subgroup analysis showed statistically significant change in Nurick grading, mJOA score, cervical segmental angle, and sagittal angle in both groups at a 2-year follow-up (P < .05). Overall fusion rate was 89.5%. Decreased incidence of graft subsidence, statistically significant less graft subsidence (P = .002) and a higher fusion rate (P = .001) were noted in titanium mesh cage group at 2-year follow-up.

CONCLUSIONS

Multilevel anterior cervical corpectomy and reconstruction is a safe and efficacious procedure. A titanium mesh cage filled with autogenous bone graft and a rigid anterior cervical plate gives best results. Posterior instrumentation should be considered along with a multilevel cervical corpectomy construct in patients with poor bone stock.

LEVEL OF EVIDENCE

4.

The Effect of Rod Pattern, Outrigger, and Multiple Screw-Rod Constructs for Surgical Stabilization of the 3-Column Destabilized Cervical Spine - A Biomechanical Analysis and Introduction of a Novel Technique

Objective

Anterior-only reconstructions for cervical multilevel corpectomies are prone to fail under continuous mechanical loading. This study sought to define the mechanical characteristics of different constructs in reducing a range of motion (ROM) of the 3-column destabilized cervical spine, including posterior cobalt-chromium (CoCr)-rods, outrigger-rods (OGR), and a novel triple rod construct using lamina screws (6S3R). The clinical implications of biomechanical findings are discussed in depth from the perspective of the challenges surgeons face cervical deformity correction.

Methods

Three-column deficient cervical spinal models were produced based on reconstructed computed tomography scans. The corpectomy defect between C3 and C7 end-level vertebrae was restored with anterior titanium (Ti) mesh-cage. The ROM was evaluated in a customized 6-degree of freedom spine tester. Tests were performed with different rod materials (Ti vs. CoCr), varying diameter rods (3.5 mm vs. 4.0 mm), with and without anterior plating, and using different construct patterns: bilateral rod fixation (standard-group), OGR-group, and 6S3R-Group. Construct stability was expressed in changes and differences of ROM (°).

Results

The largest reduction of ROM was noticed in the 6S3R-group compared to the standard- and the OGR-group. All differences observed were emphasized with an increasing number of corpectomy levels and if anterior plating was not added. For all simulated 1-, 2-, and 3-level corpectomy constructs, the OGR-group revealed decreased ROM for all motion directions compared to the standard-group. An increase of construct stiffness was also recorded for increased rod diameter (4.0 mm) and stiffer rod material (CoCr), though these effects lacked behind the more advanced construct pattern.

Conclusion

A novel reconstructive technique, the 6S3R-construct, was shown to outperform all other constructs and might resemble a new standard of reference for advanced posterior fixation.

Biomechanical Comparison of Integrated Fixation Cage Versus Anterior Cervical Plate and Cage in Anterior Cervical Corpectomy and Fusion (ACCF): A Finite Element Analysis

Background

Anterior cervical plate and cage fixation system (ACPC) used in anterior cervical corpectomy and fusion (ACCF) is reported to incur excess complications. This study aimed to introduce integrated fixation cage (IFC) into ACCF to eliminate the anterior cervical plate (ACP)-related complications.

Material/Methods

One validated intact and 3 ACCF-simulated C3–C7 cervical spine models were developed. In ACCF models, C5 was corpectomied and fixed by IFC or ACPC. For each model, 1.0 Nm moments of flexion, extension, lateral bending, and torsion were imposed on the C3 vertebra. The range of motion (ROM) of each segment and the stress distribution on screw-vertebra interface, bone graft, and cage-endplate were recorded and analyzed.

Results

ROMs of C3–C7 were not different in any motion condition between IFC and ACPC models. The maximal von Mises stress on screw-vertebra interface of the IFC model was lower than that of the ACPC models in flexion, extension, and lateral bending, but higher in rotation. The maximal von Mises stress on bone graft of the IFC model was higher compared with the ACPC models, except in flexion. The IFC model showed a higher maximal von Mises stress on cage-endplate interface in all motion planes.

Conclusions

Based on finite element analysis, IFC provided identical C3–C7 construct stability as ACPC. Compared with ACPC, IFC showed better biomechanical performance on screw-vertebra interface and bone graft, but worse biomechanical performance on cage-endplate interface.

Incidence and Outcomes of Acute Implant Extrusion Following Anterior Cervical Spine Surgery

STUDY DESIGN

Multi-institutional retrospective case series of 8887 patients who underwent anterior cervical spine surgery.

OBJECTIVE

Anterior decompression from discectomy or corpectomy is not without risk. Surgical morbidity ranges from 9% to 20% and is likely underreported. Little is known of the incidence and effects of rare complications on functional outcomes following anterior spinal surgery. In this retrospective review, we examined implant extrusions (IEs) following anterior cervical fusion.

METHODS

A retrospective multicenter case series study involving 21 high-volume surgical centers from the AOSpine North America Clinical Research Network. Medical records for 17 625 patients who received cervical spine surgery (levels from C2 to C7) between January 1, 2005, and December 31, 2011, were reviewed to identify occurrence of 21 predefined treatment complications.

RESULTS

Following anterior cervical fusion, the incidence of IE ranged from 0.0% to 0.8% across 21 institutions with 11 cases reported. All surgeries involved multiple levels, and 7/11 (64%) involved either multilevel corpectomies or hybrid constructs with at least one adjacent discectomy to a corpectomy. In 7/11 (64%) patients, constructs ended with reconstruction or stabilization at C7. Nine patients required surgery for repair and stabilization following IE. Average length of hospital stay after IE was 5.2 days. Only 2 (18%) had residual deficits after reoperation.

CONCLUSIONS

IE is a very rare complication after anterior cervical spine surgery often requiring revision. Constructs requiring multilevel reconstruction, especially at the cervicothoracic junction, have a higher risk for failure, and surgeons should proceed with caution in using an anterior-only approach in these demanding cases. Surgeons can expect most patients to regain function after reoperation.

Biomechanics of Cervical "Skip" Corpectomy Versus Standard Multilevel Corpectomy

STUDY DESIGN

In vitro biomechanical study of flexibility with finite-element simulation to estimate screw stresses.

OBJECTIVE

To compare cervical spinal stability after a standard plated 3-level corpectomy with stability after a plated 3-level "skip" corpectomy where the middle vertebra is left intact (ie, two 1-level corpectomies), and to quantify pullout forces acting on the screws during various loading modes.

SUMMARY OF BACKGROUND DATA

Clinically, 3-level cervical plated corpectomy has a high rate of failure, partially because only 4 contact points affix the plate to the upper and lower intact vertebrae. Leaving the intermediate vertebral body intact for additional fixation points may overcome this problem while still allowing dural sac decompression.

METHODS

Quasistatic nonconstraining torque (maximum 1 N m) induced flexion, extension, lateral bending, and axial rotation while angular motion was recorded stereophotogrammetrically. Specimens were tested intact and after corpectomy with standard plated and strut-grafted 3-level corpectomy (7 specimens) or "skip" corpectomy (7 specimens). Screw stresses were quantified using a validated finite-element model of C3-C7 mimicking experimentally tested groups. Skip corpectomy with C5 screws omitted was also simulated.

RESULTS

Plated skip corpectomy tended to be more stable than plated standard corpectomy, but the difference was not significant. Compared with standard plated corpectomy, plated skip corpectomy reduced peak screw pullout force during axial rotation (mode of loading of highest peak force) by 15% (4-screw attachment) and 19% (6-screw attachment).

CONCLUSIONS

Skip corpectomy is a good alternative to standard 3-level corpectomy to improve stability, especially during lateral bending. Under pure moment loading, the screws of a cervical multilevel plate experience the highest pullout forces during axial rotation. Thus, limiting this movement in patients undergoing plated multilevel corpectomy may be reasonable, especially until solid fusion is achieved.

The Reliability of the Ultrasonic Bone Scalpel in Cervical Spondylotic Myelopathy: A Comparative Study of 46 Patients

BACKGROUND

In patients with cervical spondylotic myelopathy (CSM), laminectomy is usually performed with a Kerrison rongeur or a high speed drill (HSD).The HSD, which is most often selected for laminectomy, may cause complications such as duratomy, thermal and mechanical neural injuries. With an ultrasonic bone scalpel (UBS), a less traumatic laminectomy can be performed in a shorter time. The aim of this study was to compare the results of laminectomies using HSD and UBS.

METHODS

Evaluation was made in 46 patients who were operated on for CSM. Cervical laminectomy was performed on 23 patients using the UBS (group I) and to 23 using the HSD (group II). A comparison was made of the 2 groups in respect of demographic characteristics, laminectomy levels, mean laminectomy duration, bleeding rates, and surgical complications.

RESULTS

In group I, the mean laminectomy time was 2.2 ± 0.4 min/level, mean blood loss was 180 mL, hospitalization was 3.0 ± 0.0 days, and C5 radiculopathy was seen in 1 patient. In group II, the mean laminectomy time was 7.4 ± 2.6 min/level, mean blood loss was 380 mL, hospitalization was 3.7 ± 1.3 days, C5 radiculopathy was seen in 1 patient and dura injuries in 3 patients. The recovery rate was determined as 47.6% in group I and 48.8% in group II.

CONCLUSIONS

For patients with CSM, laminectomy using the UBS provides a safe, rapid, and effective decompression with a lesser blood loss. The low rate of complications lessens the postoperative morbidity rates and shortens hospital stay.

Biomechanical testing of circumferential instrumentation after cervical multilevel corpectomy

STUDY DESIGN

Biomechanical investigation.

PURPOSE

This study describes ex vivo evaluation of the range of motion (ROM) to characterize the stability and need for additional dorsal fixation after cervical single-level, two-level or multilevel corpectomy (CE) to elucidate biomechanical differences between anterior-only and supplemental dorsal instrumentation.

METHODS

Twelve human cervical cadaveric spines were loaded in a spine tester with pure moments of 1.5 Nm in lateral bending (LB), flexion/extension (FE), and axial rotation (AR), followed by two cyclic loading periods for three-level corpectomies. After each cyclic loading session, flexibility tests were performed for anterior-only instrumentation (group_1, six specimens) and circumferential instrumentation (group_2, six specimens).

RESULTS

The flexibility tests for all circumferential instrumentations showed a significant decrease in ROM in comparison with the intact state and anterior-only instrumentations. In comparison with the intact state, supplemental dorsal instrumentation after three-level CE reduced the ROM to 12% (±10%), 9% (±12%), and 22% (±18%) in LB, FE, and AR, respectively. The anterior-only construct outperformed the intact state only in FE, with a significant ROM reduction to 57% (±35 %), 60% (±27%), and 62% (±35%) for one-, two- and three-level CE, respectively.

CONCLUSIONS

The supplemental dorsal instrumentation provided significantly more stability than the anterior-only instrumentation regardless of the number of levels resected and the direction of motion. After cyclic loading, the absolute differences in stability between the two instrumentations remained significant while both instrumentations showed a comparable increase of ROM after cyclic loading. The large difference in the absolute ROM of anterior-only compared to circumferential instrumentations supports a dorsal support in case of three-level approaches.

Comparison of the safety and efficacy of anterior 'skip' corpectomy versus posterior decompression in the treatment of cervical spondylotic myelopathy

Background

The aim of this study was to compare the therapeutic effects of anterior ‘skip’ corpectomy with posterior decompression for treating four-level cervical spondylotic myelopathy.

Methods

Operation time and blood loss during the operation for the anterior and posterior approach groups were recorded. Patients were examined with cervical lateral radiography before and after the operation to measure Cobb's angle and postoperatively to monitor bony fusion. Surgery-, instrumentation-, and graft-related complications were assessed and recorded.

Results

The surgical aspects of both anterior ‘skip’ corpectomy and posterior decompression went smoothly, with mean durations of 2.5 and 2.1 h, respectively, and mean blood loss volumes of 250 and 380 mL, respectively. In the anterior approach group, the complications included axial pain in five cases and transient hoarseness in two. Radiography revealed titanium mesh subsidence in two cases and plate or screw dislodgement in one case. In the posterior approach group, C5 nerve root palsy was present in 2 patients, axial pain in 15, and cerebrospinal fluid leakage in 3. The mean Japanese Orthopaedic Association scores showed that the recovery rate was significantly higher in the anterior approach group than in the posterior approach group (p < 0.05).

Conclusions

‘Skip’ corpectomy has comparable safety and better efficacy than posterior decompression in the treatment of four-level cervical spondylotic myelopathy.

Anterior cervical interbody constructs: effect of a repetitive compressive force on the endplate

Graft subsidence following anterior cervical reconstruction can result in the loss of sagittal balance and recurring foraminal stenosis. This study examined the implant-endplate interface using a cyclic fatigue loading protocol in an attempt to model the subsidence seen in vivo. The superior endplate from 30 cervical vertebrae (C3 to T1) were harvested and biomechanically tested in axial compression with one of three implants: Fibular allograft; titanium mesh cage packed with cancellous chips; and trabecular metal. Each construct was cyclically loaded from 50 to 250 N for 10,000 cycles. Nondestructive cyclic loading of the cervical endplate-implant construct resulted in a stiffer construct independent of the type of the interbody implant tested. The trabecular metal construct demonstrated significantly more axial stability and significantly less subsidence in comparison to the titanium mesh construct. Although the allograft construct resulted in more subsidence than the trabecular metal construct, the difference was not significant and no difference was found when comparing axial stability. For all constructs, the majority of the subsidence during the cyclic testing occurred during the first 500 cycles and was followed by a more gradual settling in the remaining 9,500 cycles.

A comparative biomechanical study of traditional and in-line plating systems following immediate stabilization of single and bi-level cervical segments

BACKGROUND

Anterior cervical plate fixation has gained widespread acceptance for treatment of cervical spine pathologies by stabilizing the segment and enhancing fusion rates. While it is generally accepted that multiple fusion levels benefit from plating, few studies have compared plate designs. Wider plates can increase surgical complications and cost and are, therefore, not indicated unless biomechanical benefits exist. In this study, a cervical cadaver model is subjected to physiological loads and stabilized with in-line one-screw, and traditional two-screw per vertebral body plates.

METHODS

Three groups of eight fresh frozen human cadaver cervical spines (C2-C7) were tested by applying pure moments of 1.5 Nm. Motion was obtained at C5-C6, and C4-C5/C5-C6 for single-level and bi-level experiments, respectively, in flexion-extension, lateral bending and axial rotation. Specimens were tested, 1) intact, 2) injured (anterior discectomy), 3) with interbody fusion spacer, 4) in-line one-screw plate+spacer, and 5) two-screw plate+spacer, using four available plate brands.

FINDINGS

Single-level plating with interbody spacer restricted range-of-motion with respect to the spacer-alone construct in flexion-extension, regardless of one-screw or two-screw plate design, or brand. Similar behavior was seen in axial rotation, but not in lateral bending, where significance reductions in motion were achieved only with respect to the intact spine, not the interbody spacer group. In bi-level experiments all plate types restricted range-of-motion below spacer-alone levels in all loading modes.

INTERPRETATION

Anterior plating should be selected based on surgical requirements, as a wide (two-screw) over a narrow (one-screw) plating profile does not appear to provide a tangible biomechanical benefit.

Avaliação do tratamento da discopatia degenerativa cervical pela artrodese via anterior utilizando placas associadas a cages ou cages em peek isoladamente

OBJETIVOS: Avaliar comparativamente o tratamento da discopatia degenerativa cervical por discectomia e artrodese cervical via anterior utilizando placas associadas a cages ou cages em PEEK isoladamente. MÉTODOS: Foi realizado um estudo retrospectivo comparativo entre dois grupos de pacientes operados pela técnica de discectomia e artrodese cervical via anterior. Foram selecionados aleatoriamente 70 pacientes, 35 operados com o método de fixação com placas associadas a cages - denominado Grupo I - e 35 com o cage em PEEK isoladamente - Grupo II. Realizou-se anamnese, exame físico, escores de dor (escala visual e analógica da dor) e função (critérios de Odom's, SF-36, Indice de incapacidade do pescoço) o pré e pós-operatório e exames de imagem. RESULTADOS: Houve predominância de pacientes do sexo feminino em ambos os grupos, com média de idade de 55 anos no Grupo I e 47 no Grupo II. Ambos os grupos apresentaram distribuição semelhante quanto ao número de níveis operados, assim como nas complicações encontradas e escores de dor, cervicalgia e SF36 no pré e pós-operatório. Houve 97.1% de fusão com 94.3% de bons resultados no Grupo I e 100% de fusão, com 97 % de bons resultados no Grupo II. CONCLUSÕES: O estudo comparativo da utilização de placas com cages e cages em PEEK isoladamente apresentou resultados semelhantes e satisfatórios para os grupos estudados, não se constatando superioridade ou inferioridade de um método com relação ao outro.

The stabilizing potential of anterior, posterior and combined techniques for the reconstruction of a 2-level cervical corpectomy model: biomechanical study and first results of ATPS prototyping

Clinical studies reported frequent failure with anterior instrumented multilevel cervical corpectomies. Hence, posterior augmentation was recommended but necessitates a second approach. Thus, an author group evaluated the feasibility, pull-out characteristics, and accuracy of anterior transpedicular screw (ATPS) fixation. Although first success with clinical application of ATPS has already been reported, no data exist on biomechanical characteristics of an ATPS-plate system enabling transpedicular end-level fixation in advanced instabilities. Therefore, we evaluated biomechanical qualities of an ATPS prototype C4-C7 for reduction of range of motion (ROM) and primary stability in a non-destructive setup among five constructs: anterior plate, posterior all-lateral mass screw construct, posterior construct with lateral mass screws C5 + C6 and end-level fixation using pedicle screws unilaterally or bilaterally, and a 360° construct. 12 human spines C3-T1 were divided into two groups. Four constructs were tested in group 1 and three in group 2; the ATPS prototypes were tested in both groups. Specimens were subjected to flexibility test in a spine motion tester at intact state and after 2-level corpectomy C5-C6 with subsequent reconstruction using a distractable cage and one of the osteosynthesis mentioned above. ROM in flexion-extension, axial rotation, and lateral bending was reported as normalized values. All instrumentations but the anterior plate showed significant reduction of ROM for all directions compared to the intact state. The 360° construct outperformed all others in terms of reducing ROM. While there were no significant differences between the 360° and posterior constructs in flexion-extension and lateral bending, the 360° constructs were significantly more stable in axial rotation. Concerning primary stability of ATPS prototypes, there were no significant differences compared to posterior-only constructs in flexion-extension and axial rotation. The 360° construct showed significant differences to the ATPS prototypes in flexion-extension, while no significant differences existed in axial rotation. But in lateral bending, the ATPS prototype and the anterior plate performed significantly worse than the posterior constructs. ATPS was shown to confer increased primary stability compared to the anterior plate in flexion-extension and axial rotation with the latter yielding significance. We showed that primary stability after 2-level corpectomy reconstruction using ATPS prototypes compared favorably to posterior systems and superior to anterior plates. From the biomechanical point, the 360° instrumentation was shown the most efficient for reconstruction of 2-level corpectomies. Further studies will elucidate whether fatigue testing will enhance the benefit of transpedicular anchorage with posterior constructs and ATPS.

Total subaxial reconstruction

OBJECT

Certain cervical spinal conditions require decompression and reconstruction of the entire subaxial cervical spine. There are limited data concerning the clinical details and outcomes of patients treated in this manner. The object of this study was to describe the specific technique employed to perform a total subaxial reconstruction and review the postoperative outcomes following surgery.

METHODS

The author performed a review of data prospectively collected in 27 consecutive patients undergoing complete anterior decompression and reconstruction of the anterior cervical spine and followed by posterior instrumented arthrodesis with or without decompression.

RESULTS

There were 16 men and 11 women whose mean age was 59 years (range 35-86 years). The minimum follow-up was 12 months and the mean follow-up period for all patients was 26 months. One patient underwent C2-7 surgery, and in all others the procedure crossed the cervicothoracic junction. Following surgery patients remained intubated for an average of 3.3 days (range 1-22 days). The mean hospital length of stay was 11 days (range 3-45 days). One patient died 6 weeks following an uneventful surgery. Pneumonia developed in 5 patients, 1 patient experienced a minor pulmonary embolism, and 2 patients had posterior wound infections. No patient was neurologically worse following surgery. A single patient presented with a C-8 radiculopathy 6 weeks after surgery. At final follow-up no patient complained of dysphagia when specifically questioned about this potential problem. In all patients solid fusions developed at each treated levels. Preoperatively the mean sagittal Cobb angle was 15.4° (kyphosis) and the postoperative mean angle was -10.9° (lordosis) representing a total average correction of over 25° (p < 0.0001). The mean preoperative Neck Disability Index was 27.6; this score decreased to 15.5 (p = 0.0008) postoperatively. The mean pre- and postoperative visual analog scale neck pain scores were 6.0 and 2.1, respectively (p = 0.0004), and mean visual analog scale arm pain scores decreased by 3.7 following surgery (p = 0.001). Based on Odom criteria, the author found that 8 patients had an excellent outcome and 14 patients a good outcome. There were 4 patients in whom the outcome was judged to be fair and the single death was recorded as a poor outcome. The mean preoperative Nurick score was 2.68. Postoperatively the group improved to an average score of 1.5; the difference between the 2 was statistically significant (p = 0.002).

CONCLUSIONS

Segmental anterior decompression and reconstruction of the entire subaxial cervical spine, combined with an instrumented posterolateral fusion, can be performed with acceptable morbidity and is of significant benefit in selected patients.

“Skip” corpectomy in the treatment of multilevel cervical spondylotic myelopathy and ossified posterior longitudinal ligament

Object

The authors reviewed the results of “skip” corpectomy in 29 patients with multilevel cervical spondylotic myelopathy (CSM) and ossified posterior longitudinal ligament (OPLL).

Methods

The skip corpectomy technique, which is characterized by C-4 and C-6 corpectomy, C-5 osteophytectomy, and C-5 vertebral body preservation, was used for decompression in patients with multilevel CSM and OPLL. All patients underwent spinal fixation using C4–5 and C5–6 grafts, and anterior cervical plates were fixated at C-3, C-5, and C-7.

Results

The mean preoperative Japanese Orthopaedic Association score increased from 13.44 ± 2.81 to 16.16 ± 2.19 after surgery (p < 0.05). The cervical lordosis improved from 1.16 ± 11.74° to 14.36 ± 7.85° after surgery (p < 0.05). The complications included temporary hoarseness in 3 cases, dysphagia in 1 case, C-5 nerve palsy in 1 case, and C-7 screw pullout in 1 case. The mean follow-up was 23.2 months. The final plain radiographs showed improved cervical lordosis and fusion in all cases.

Conclusions

The authors conclude that the preservation of the C-5 vertebral body provided an additional screw purchase and strengthened the construct. The results of the current study demonstrated effectiveness and safety of the skip corpectomy in patients with multilevel CSM and OPLL.

In vitro study of accuracy of cervical pedicle screw insertion using an electronic conductivity device (ATPS part III)

Reconstruction of the highly unstable, anteriorly decompressed cervical spine poses biomechanical challenges to current stabilization strategies, including circumferential instrumented fusion, to prevent failure. To avoid secondary posterior surgery, particularly in the elderly population, while increasing primary construct rigidity of anterior-only reconstructions, the authors introduced the concept of anterior transpedicular screw (ATPS) fixation and plating. We demonstrated its morphological feasibility, its superior biomechanical pull-out characteristics compared with vertebral body screws and the accuracy of inserting ATPS using a manual fluoroscopically assisted technique. Although accuracy was high, showing non-critical breaches in the axial and sagittal plane in 78 and 96%, further research was indicated refining technique and increasing accuracy. In light of first clinical case series, the authors analyzed the impact of using an electronic conductivity device (ECD, PediGuard) on the accuracy of ATPS insertion. As there exist only experiences in thoracolumbar surgery the versatility of the ECD was also assessed for posterior cervical pedicle screw fixation (pCPS). 30 ATPS and 30 pCPS were inserted alternately into the C3-T1 vertebra of five fresh-frozen specimen. Fluoroscopic assistance was only used for the entry point selection, pedicle tract preparation was done using the ECD. Preoperative CT scans were assessed for sclerosis at the pedicle entrance or core, and vertebrae with dense pedicles were excluded. Pre- and postoperative reconstructed CT scans were analyzed for pedicle screw positions according to a previously established grading system. Statistical analysis revealed an astonishingly high accuracy for the ATPS group with no critical screw position (0%) in axial or sagittal plane. In the pCPS group, 88.9% of screws inserted showed non-critical screw position, while 11.1% showed critical pedicle perforations. The usage of an ECD for posterior and anterior pedicle screw tract preparation with the exclusion of dense cortical pedicles was shown to be a successful and clinically sound concept with high-accuracy rates for ATPS and pCPS. In concert with fluoroscopic guidance and pedicle axis views, application of an ECD and exclusion of dense cortical pedicles might increase comfort and safety with the clinical use of pCPS. In addition, we presented a reasonable laboratory setting for the clinical introduction of an ATPS-plate system.

Iatrogenic spinal deformity

Patients with postoperative spinal deformities are being identified with increasing frequency as the number of instrumented spinal operations increases. Thus, it is important for the neurosurgeon to understand ways to minimize postoperative deformity and to understand its operative and nonoperative management. A variety of intra- and postoperative risk factors have been associated with postoperative deformity, including patient age, operative positioning, preoperative medical condition, and the use of prior radiation therapy. The evaluation of all patients who have been suspected of iatrogenic deformity should include a detailed physical examination, plain x-rays, and computed tomographic or magnetic resonance imaging, depending on the condition. Conservative therapy includes physical therapy and pain control, which may be effective in some patients. However, patients with flat-back syndrome typically require reoperation. A wide variety of reoperative procedures may be performed, depending on the area of the pathological deformity, extent of disease, and patient condition.

Cervical anterior transpedicular screw fixation. Part I: Study on morphological feasibility, indications, and technical prerequisites

Multilevel cervical spine procedures can challenge the stability of current anterior cervical screw-and-plate systems, particularly in cases of severe three-column subaxial cervical spine injuries and multilevel plated reconstructions in osteoporotic bone. Supplemental posterior instrumentation is therefore recommended to increase primary construct rigidity and diminish early failure rates. The increasing number of successfully performed posterior cervical pedicle screw fixations have enabled more stable fixations, however most cervical pathologies are located anteriorly and preferably addressed by an anterior approach. To combine the advantages of the anterior approach with the superior biomechanical characteristics of cervical pedicle screw fixation, the authors developed a new concept of a cervical anterior transpedicular screw-and-plate system. An in vivo anatomical study was performed to explore the feasibility of anterior transpedicular screw fixation (ATPS) in the cervical spine. The morphological study was conducted based on 29 cervical spine CT scans from healthy patients and measurements were performed on the pedicle sizes, angulations, vertebral body depth, height and width at C2 to T1. Significant morphologic parameters for the new technique are discussed. These parameters include the sagittal and transverse intersection points of the pedicle axis with the anterior vertebral body wall, as well as the distances between sagittal intersection points from C2 to T1. On the basis of these results, standard spine models were reconstructed and used for the conceptual development of a preclinical release prototype of an anterior transpedicular screw-and-plate system. The morphological feasibility of the new technique is demonstrated, and its indications, biomechanical considerations, as well as surgical prerequisites are thoroughly discussed. In the future, the technique of cervical anterior transpedicular screw fixation might diminish the number of failures in the reconstruction of multilevel and three-column cervical spine instabilities, and avoid the need for supplemental posterior instrumentation.

Cervical anterior transpedicular screw fixation (ATPS)--Part II. Accuracy of manual insertion and pull-out strength of ATPS

Reconstruction after multilevel decompression of the cervical spine, especially in the weakened osteoporotic, neoplastic or infectious spine often requires circumferential stabilization and fusion. To avoid the additional posterior surgery in these cases while increasing rigidity of anterior-only screw-plate constructs, the authors introduce the concept of anterior transpedicular screw (ATPS) fixation. We demonstrated its morphological feasibility as well as its indications in a previous study in Part I of our project. Consequently, the objectives of the current study were to assess the ex vivo accuracy of placing ATPS into the cervical vertebra as well as the biomechanical performance of ATPS in comparison to traditional vertebral body screws (VBS) in terms of pull-out strength (POS). Twenty-three ATPS were inserted alternately to two screws into the pedicles and vertebral bodies, respectively, of six cadaveric specimens from C3-T1. For insertion of ATPS, a manual fluoroscopically assisted technique was used. Pre- and post insertional CT-scans were used to assess accuracy of ATPS insertion in the axial and sagittal planes. A newly designed grading system and accuracy score were used to delineate accuracy of ATPS insertion. Following insertion of screws, 23 ATPS and 22 VBS were subjected to pull-out testing (POT). The bone mineral density (BMD) of each specimen was assessed prior to POT. Statistical analysis showed that the incidence of correctly placed screws and non-critical pedicles breaches in axial plane was 78.3%, and 95.7% in sagittal plane. Hence, according to our definition of "critical" pedicle breach that exposes neurovascular structures at risk, 21.7% (n = 5) of all ATPS inserted showed a critical pedicle breach in axial plane. Notably, no critical pedicle perforation occurred at the C6 to T1 levels. Pull-out testing of ATPS and VBS revealed that pull-out resistance of ATPS was 2.5-fold that of VBS. Mean POS of 23 ATPS with a mean BMD of 0.566 g/cm(2) and a mean osseus screw purchase of 27.2 mm was 467.8 N. In comparison, POS of 22 VBS screws with a mean BMD of 0.533 g/cm(2) and a mean osseus screw purchase of 16.0 mm was 181.6 N. The difference in ultimate pull-out strength between the ATPS and VBS group was significant (p < 0.000001). Also, accuracy of ATPS placement in axial plane was shown to be significantly correlated with POS. In contrast, there was no correlation between screw-length, BMD, or level of insertion and the POS of ATPS or VBS. The study demonstrated that the use of ATPS might be a new technique worthy of further investigation. The use of ATPS shows the potential to increase construct rigidity in terms of screw-plate pull-out resistance. It might diminish construct failures during anterior-only reconstructions of the highly unstable decompressed cervical spine.

4- and 5-level anterior fusions of the cervical spine: review of literature and clinical results

In the future, there will be an increased number of cervical revision surgeries, including 4- and more-levels. But, there is a paucity of literature concerning the geometrical and clinical outcome in these challenging reconstructions. To contribute to current knowledge, we want to share our experience with 4- and 5-level anterior cervical fusions in 26 cases in sight of a critical review of literature. At index procedure, almost 50% of our patients had previous cervical surgeries performed. Besides failed prior surgeries, indications included degenerative multilevel instability and spondylotic myelopathy with cervical kyphosis. An average of 4.1 levels was instrumented and fused using constrained (26.9%) and non-constrained (73.1%) screw-plate systems. At all, four patients had 3-level corpectomies, and three had additional posterior stabilization and fusion. Mean age of patients at index procedure was 54 years with a mean follow-up intervall of 30.9 months. Preoperative lordosis C2-7 was 6.5 degrees in average, which measured a mean of 15.6 degrees at last follow-up. Postoperative lordosis at fusion block was 14.4 degrees in average, and 13.6 degrees at last follow-up. In 34.6% of patients some kind of postoperative change in construct geometry was observed, but without any catastrophic construct failure. There were two delayed unions, but finally union rate was 100% without any need for the Halo device. Eleven patients (42.3%) showed an excellent outcome, twelve good (46.2%), one fair (3.8%), and two poor (7.7%). The study demonstrated that anterior-only instrumentations following segmental decompressions or use of the hybrid technique with discontinuous corpectomies can avoid the need for posterior supplemental surgery in 4- and 5-level surgeries. However, also the review of literature shows that decreased construct rigidity following more than 2-level corpectomies can demand 360 degrees instrumentation and fusion. Concerning construct rigidity and radiolographic course, constrained plates did better than non-constrained ones. The discussion of our results are accompanied by a detailed review of literature, shedding light on the biomechanical challenges in multilevel cervical procedures and suggests conclusions.

Dynamic anterior cervical plates

Graft- and plate-related complications are not uncommon problems associated with anterior cervical fusion surgery. Although the reasons for such complications are protean, one potential set of etiologies centers on plate design. Dynamic cervical plates, which allow for better load sharing while providing overall resistance to motion, address perceived biomechanical deficiencies of rigid cervical plates. However, despite the theoretic advantages of dynamic plating over rigid plate fixation, there are concerns that settling associated with dynamic plates could lead to segmental kyphosis or foraminal narrowing and that excessive motion may lead to inferior fusion rates. The surgeon using these implants should be familiar with differences among the various types of dynamic plates and should perform the plating procedure appropriately to accommodate the expected settling.

Anterior approaches to fusion of the cervical spine: a metaanalysis of fusion rates

Object

Anterior cervical discectomy (ACD), ACD with interbody fusion (ACDF), ACDF with placement of an anterior plate system (ACDFP), corpectomy, and corpectomy with plate placement are used to fuse the cervical spine. The authors conducted a metaanalysis of studies published after 1990 in which fusion rates achieved with each procedure were reported for patients with degenerative disease at one, two, and three disc levels.

Methods

Twenty-one papers each included data on at least 25 patients. In each of the 21 studies the average clinical follow up was more than 12 months, and the results were evaluated according to radiographic evidence of fusion and delineated by the number of levels fused. Chi-square and Fisher exact tests were used for comparisons. The mean age of the patients was 46.7 years, 46.6% were female, and the mean follow-up period was 39.6 months. The studies included 2682 patients and the overall fusion rate was 89.5%. For single disc–level disease, fusion rates were 84.9% for ACD, 92.1% for ACDF, and 97.1% for ACDFP (p = 0.0002). For two disc–level disease, fusion rates were 79.9% for ACDF, 94.6% for ACDFP, 95.9% for corpectomy, and 92.9% for corpectomy with plate placement (p = 0.0001). For three disc–level disease, fusion rates were 65.0% for ACDF, 82.5% for ACDFP, 89.8% for corpectomy, and 96.2% for corpectomy with plate placement (p = 0.0001). The use of anterior plates significantly improved fusion for one-level (p < 0.0001), two-level (p < 0.0001), and three-level (p < 0.05) ACDF. There was no significant difference in fusion rates between two-level ACDF and corpectomy with plate placement.

Conclusions

The anticipated fusion rate is one of several factors that may guide surgical decision making. Anterior cervical decompression and fusion results in high fusion rates. The results of the authors' study show that regardless of the number of levels fused, the use of an anterior cervical plate system significantly increases the fusion rate. For two-disc–level disease, there was no significant difference between ACD with a plate system or corpectomy with a plate system. For three-disc–level disease, however, the evidence suggests that corpectomy with plate placement is associated with higher fusion rates than discectomy with plate placement.

USE OF VENTRAL PLATES FOR CERVICAL ARTHRODESIS

OBJECTIVE

Ventral cervical plates are used to increase the immediate postoperative rigidity of the spine after decompressive and reconstructive procedures. The evidence supporting this practice is reviewed.

METHODS

A computerized literature search of the database of the National Library of Medicine was conducted using PubMed. All relevant articles were reviewed and a critique was performed to explore the utility of ventral cervical plating.

RESULTS

Several randomized controlled trials of ventral cervical discectomy versus ventral cervical discectomy and fusion were identified. Three randomized controlled trials that included a differentiation between anterior cervical decompression and fusion, with and without plating, were identified. Many retrospective series, technical reports, and topical reviews were also identified.

CONCLUSION

There is little support in the literature for the medical usefulness of ventral cervical plates after single-level cervical fusion. There may, however, be a cost-benefit advantage to the use of such devices. In multilevel procedures and in the setting of traumatic instability, there seems to be an advantage to the use of cervical plates.

Successful outcome of six-level cervicothoracic corpectomy and circumferential reconstruction: case report and review of literature on multilevel cervicothoracic corpectomy

The authors report the successful outcome of a six-level corpectomy across the cervico-thoracic spine with circumferential reconstruction in a patient with extensive osteomyelitis of the cervical and upper thoracic spine. To the authors’ knowledge, this is the first report of a corpectomy extending across six levels of the cervico-thoracic spine. Clinical relevance: the authors recommend anterior cage and plate-assisted reconstruction and additional posterior instrumentation using modern spinal surgical techniques and implants.

Early failures following cervical corpectomy reconstruction with titanium mesh cages and anterior plating

STUDY DESIGN

Retrospective case series.

OBJECTIVE

To evaluate the use of titanium mesh cages in the reconstruction of the cervical spine following corpectomy.

SUMMARY OF BACKGROUND DATA

Previous studies have shown high successful fusion rates and low failure rates with this technique. Similar reconstruction techniques using anterior strut bone grafting and anterior plating have shown higher failure rates following multilevel corpectomies.

METHODS

A retrospective review was performed of 23 consecutive patients who underwent anterior cervical corpectomy reconstructed with a titanium mesh cage, local autograft, and fixed anterior plating. Medical records and radiographs were reviewed. Average follow-up was 28 months.

RESULTS

Seven patients (30%) had reconstruction failures. There was 1 failure (6%) in the 1-level corpectomy group and 6 (75%) in the multilevel corpectomy group. All failures occurred before 12 weeks after surgery. The remaining patients had successful fusion (70%).

CONCLUSION

There is a high early failure rate (75%) with the use of a titanium mesh cage and fixed anterior plating for reconstruction of multilevel corpectomies. Posterior fusion and instrumentation should be considered when using this technique for multilevel reconstructions.

Results of surgical treatment for degenerative cervical myelopathy: anterior cervical corpectomy and stabilization

STUDY DESIGN

This retrospective study involves 26 patients with degenerative cervical myelopathy who were surgically treated by anterior corpectomy, titanium mesh cage (TMC) filled with autogenous bone, and anterior plate +/- posterolateral plate and fusion.

OBJECTIVES

This study was conducted to determine the indications, efficacy, and complication rate associated with performing corpectomy to achieve anterior decompression of neural elements or for removing anterior lesions.

SUMMARY OF BACKGROUND DATA

This retrospective study involves patients with degenerative cervical myelopathy who were surgically treated by > or =2-level anterior corpectomy, TMC filled with autogenous bone, and anterior plate +/- posterolateral plate and fusion. The purpose was to evaluate and compare the results in terms of neurologic recovery and function and effectivity of TMC as a structural support.

METHODS

Twenty-six patients with degenerative cervical myelopathy who had surgical treatment and average 30 months (range, 24-52 months) follow up were included. The mean age was 64.9 years (range, 55-74 years) and average period between myelopathic symptoms and surgery was 2.8 years (range, 6 months-5 years). Preoperative evaluation of every patient consisted of anterior-posterior, lateral, bilateral oblique, flexion, and extension radiographs, computed tomography reconstructions and magnetic resonance imaging of the cervical spine, Doppler ultrasound of the carotid arteries, vertebral artery magnetic resonance angiography, neurologic examination, and electromyography. Degree of pre- and postoperative myelopathy was determined according to the scoring systems developed by Nurick and Japanese Orthopedic Association (JOA). Twelve patients had a mild balance problem and difficulty while walking but were able to perform their daily activities. Fourteen patients had spastic quadriparesis ambulating on either crutches or with wheelchairs. Of these, 11 experienced bladder disturbance as well. Surgical treatment in 18 patients consisted of anterior decompressive corpectomy, structural TMC, and anterior plate stabilization in 14 patients who had 2-level corpectomy. Posterior plate stabilization without laminectomy was added to this procedure in another 4 patients who had 3- or more level corpectomy. The remaining 8 patients had first laminectomy and posterolateral plate, then anterior corpectomy, TMC, and anterior plate on the same stage. Corpectomy levels were between C3 and T1, and anterior corpectomy, structural TMC, and anterior plating was the procedure that all patients had in common.

RESULTS

Mean sagittal Cobb angle (C2-C7) was 9 degrees (range, 0-23 degrees) before surgery, 17.1 degrees (range, 11-22 degrees) on the third postoperative month, and 16.9 degrees (range, 10-22 degrees) at last follow-up. The difference in sagittal alignment on the third month and last follow up was not statistically significant (P > 0.05). Average preoperative Nurick score was 3.5 (range, 2-5) and JOA score was 7 (range, 1-14). Major and statistically significant neurologic recovery was within the first 3 months, and average Nurick and JOA scores at 3 months were 2 (range, 0-3) and 11 (range, 8-17) (P < 0.001), respectively. All patients had improved neurologic status at final follow up. As confirmed by plain radiographs and computed tomography reconstructions, solid fusion was achieved across the TMC with no settling or migration, and we had no implant-related complication or failure. As major complications, 1 (3.8%) early deep posterior infection developed but responded to early debridement and antibiotics. Also, 3 patients (11.5%) had transient C5 nerve root injury. At final follow up, all patients were able to ambulate without support and maintain their daily activities.

CONCLUSIONS

Anterior decompression provides good neurologic recovery in patients with degenerative cervical myelopathy. TMC provides good structural support, and solid fusion can be achieved with TMC and anterior plate (for < or =2-level corpectomy) and/or posterior plate (> or =3-level corpectomy). There is increased risk of C5 nerve root injury when first laminectomy and posterolateral plate stabilization are performed.

Treatment of multilevel cervical fusion with cages

BACKGROUND

Multilevel cervical discectomy usually requires plate and screw fixation for maintaining the spinal curvature, and increasing the graft fusion rate. However, the use of plate and screw fixation may cause a few complications, such as screw breakage, screw pullout, esophagus perforation, and cord or nerve root injury. In this study, we try to use cages to replace plate function in multilevel cervical fusion.

METHODS

From January 1997 to June 2001, there were 180 consecutive cases of multilevel cervical degenerative disease. We randomized them into three groups: Group A (60 patients) underwent anterior discectomy and polyetheretherketone (PEEK) fusion, Group B (50 patients) underwent anterior discectomy, autogenous iliac crest graft (AICG) fusion and plate fixation, and Group C (70 patients) underwent anterior discectomy and AICG only. X-ray of cervical spine was taken every 3 months until fusion was complete. Spinal curvature was measured by lateral view of X-ray. The functional and working status were evaluated by Prolo scale. Blood loss and operation time were recorded, respectively.

RESULTS

The total complication rates were 3.3%, 16%, and 54.3% in Groups A, B, and C respectively. The graft complications were evaluated by radiographic findings (graft collapse, nonunion, or dislodged graft). However, only 37.1% of patients (13/35) with graft complications had clinical symptoms (severe neck pain, radicular pain, or neurologic deficits). The fusion rate was better, and the time to fusion was sooner in Groups A and B than Group C, p < 0.001 (chi(2) test). PEEK cage is statistically better than plating group in total complications, p < 0.05. Graft collapse and nonunion were the major graft complications in Group C (AICG without plating). Screw pullout, and screw breakage were the main causes of plating complication. Blood loss was minimum in Group A, p < 0.05. Spinal lordosis increased by a mean of 4.61 +/- 2.93 mm and 1.68 +/- 5.02 mm in Groups A and B, respectively, but spinal kyphosis increased by a mean of -2.09 +/- 4.77 mm in Group C. Group A had a statistically better Prolo scale than Group C, p < 0.0001.

CONCLUSIONS

Both PEEK cage without plating and AICG with plating are good methods for interbody fusion in multilevel cervical degenerative diseases. They increase spinal lordosis and graft fusion rate, and cause fewer surgical complications. However, PEEK cage is preferred in our study for multilevel fusion, because it has the fewest complication rates and the least amount of blood loss.

[Vertebral body replacement in spine surgery]

Autografts and allogeneous bone grafts as well as cages are used for the reconstruction of the anterior column after corpectomy. Recently, expandable cages for vertebral body replacement have been developed. Based on our own experience, the purpose of this study was to summarize the available biomechanical and clinical data of expandable corpectomy cages and to compare it with established fixation techniques. If used correctly, expandable cages offer several surgical advantages in comparison to non-expandable cages. However there were no significant differences between the biomechanical properties of expandable and non-expandable cages. Additionally, design variations of expandable corpectomy cages did not show any significant impact on the biomechanical stability. Currently available mid-term clinical and radiological data on the treatment of fractures, metastasis and infection of the cervical, thoracic and lumbar spine demonstrated no significant difference between expandable and non-expandable cages. However, the increased stress-shielding effect of expandable cages compared to non-expandable cages might result in a deterioration of the long-term clinical outcome.

A new computer-aided technique for analysis of lateral cervical radiographs in postoperative patients with degenerative disease

STUDY DESIGN

Radiographs were measured by four independent observers and remeasured by three of the observers.

OBJECTIVES

To assess the reliability of a new computer-aided measurement technique.

SUMMARY OF BACKGROUND DATA

Many studies have reported sagittal-plane distance and angle measurements in the cervical spine. Common measurement methods involve manual line drawing on lateral radiographs with manual or computer-aided distance and angle computation. In patients with anterior cervical fusion, changes in bony geometry could present difficulties for many existing methods.

METHODS

Digitized lateral cervical radiographs are imported into a graphics software package. Outlines of the vertebral bodies and spinous processes are traced on the best-quality film and transformed to match the bony geometry on each remaining radiograph from the same patient. Intervertebral distance and angulation are calculated from centers of mass of the outlined elements. Segmental measurements were collected for 27 lateral radiographs from nine patients with anterior cervical discectomy and fusion. Intraclass and interclass correlation coefficients were calculated and used to compute standard errors of measurement.

RESULTS

High intraclass and interclass correlations (ICCs) and low measurement errors were calculated for both distance and angle measurements. Intraexaminer mean ICCs were 0.92 for interbody distance and 0.93 for segmental angle, with standard errors of measurement (SEMs) of 3.26% interbody distance (approximately 0.65 mm) and 1.20 degrees sagittal-plane rotation. Mean interexaminer ICCs were 0.91 for interbody distance and 0.86 for segmental angle, with SEMs of 3.58% interbody distance (approximately 0.72 mm) and 1.77 degrees sagittal-plane rotation.

CONCLUSIONS

The measurement method is reliable for both interbody distance and segmental angles within and among examiners. Whereas many existing measurement methods require normal radiographs to locate specific anatomic points, given intact spinous processes, the present method functions even with various radiographic abnormalities and in the presence of surgical decompression, degenerative disease, and cervical hardware. Because it does not rely on specific anatomic points, the present method is robust with respect to changes in the bony anatomy over time.

The biomechanical effects of cervical multilevel oblique corpectomy

STUDY DESIGN

A repeated-measures flexibility test was performed in vitro using human cadaveric spines.

OBJECTIVES

To compare changes in cervical biomechanics associated with multilevel oblique corpectomy and standard grafted corpectomy with or without plating.

SUMMARY OF BACKGROUND DATA

Standard multilevel plated and unplated corpectomies are susceptible to instability in vitro. The authors are unaware of any previous research on the biomechanics of multilevel oblique corpectomy. METHODS.: Six human cadaveric cervical spine specimens (C3-T1) were tested: 1) normal; 2) after 2-level multilevel oblique corpectomy; 3) after expanding multilevel oblique corpectomy to represent standard grafted and plated corpectomy; and 4) after removing the anterior plate. Pure moments were applied to induce flexion, extension, lateral bending, and axial rotation while recording motion stereophotogrammetrically.

RESULTS

Compared to normal, the range of motion after multilevel oblique corpectomy increased 15% during flexion, 18% during extension, 11% during lateral bending, and 18% during axial rotation. These increases were about one-third of the increases observed after standard corpectomy without plating. Multilevel oblique corpectomy caused few alterations in locations of axes of rotation and coupling patterns, whereas standard corpectomy with or without plating significantly altered these parameters in several instances.

CONCLUSIONS

Multilevel oblique corpectomy (without graft) induced significantly less instability and altered kinematics less than standard unplated corpectomy with graft. Multilevel oblique corpectomy allowed significantly more motion than standard plated corpectomy with graft. However, the goal of standard corpectomy is fusion. Our results indicate that plating significantly limits spinal mobility after 2-level corpectomy, improving the environment for fusion.

Geometric results of anterior cervical plate stabilization in degenerative disease

STUDY DESIGN

Consecutive case retrospective computerized analysis of lateral radiographs.

OBJECTIVES

To investigate the time evolution of the geometry of anterior cervical discectomy and fusion using a new computer-aided measurement technique.

SUMMARY OF BACKGROUND DATA

Prior clinical studies have reported benefits and complications of anterior cervical plate stabilization, but have not comprehensively described changes in sagittal geometry following arthrodesis and plating. High fusion success rates have been reported for single-level discectomy and fusion, whereas multilevel procedures have demonstrated lower clinical success rates with increased hardware failure and pseudarthrosis rates.

METHODS

Sagittal alignment data were collected from lateral radiographs of 51 patients with 52 operations for single- or multiple-level anterior cervical discectomy and fusion, both with and without anterior cervical plate stabilization. Intervertebral angulation and separation at the operated segments before surgery, immediately after surgery, and at long-term follow-up were compared using a computer-aided technique.

RESULTS

Single-level anterior cervical discectomy and fusion with plate stabilization lost 0.9 degrees of the operatively obtained lordosis after surgery compared to 7.5 degrees for single-level fusions with bone alone (P = 0.0001). In multilevel anterior cervical discectomy and fusion, the bottommost level of the anterior cervical discectomy and fusion construct was much more prone to collapse than the remaining levels (mean bottom-level loss of 4.0 degrees vs. mean 0.2 degrees increase at the remaining levels, P < 0.0001).

CONCLUSIONS

Anterior cervical plate stabilization helps maintain operatively obtained segmental distraction and lordosis following anterior cervical discectomy and fusion. In multilevel procedures, this study found that postoperative collapse is largely localized to the bottommost level, where hardware failure and pseudarthrosis have been observed most often by others.

Distractive flexion injuries of the subaxial cervical spine treated with anterior plate alone

The clinical and radiographic effect of anterior plate fixation alone was evaluated in 36 consecutive patients with distractive flexion (DF) injuries in the lower cervical spine. Mean follow-up time was 15 months. The aim of the present study was to determine whether anterior plate fixation alone provides sufficient stability when treating DF injuries in the cervical spine. Solid union was seen in 6 of 6 patients with stage 1 injury and in 15 of 17 patients with stage 2 injury. In the patients with stage 3 injury, 7 of 13 of the anterior fixations failed. These failures occurred mainly among the patients with severe neurologic injuries. We believe these findings substantiate the use of anterior plate alone for DF injuries at stage 1 and 2 but disqualify anterior plate fixation alone for DF injuries at stage 3, with neurologic injury present.

Management of strut graft failure in anterior cervical spine surgery

Although successfully used, long strut grafts are vulnerable to dislodgment, displacement, fracture, and nonunion, which can require revision surgery; thus, meticulous preparation of the vertebral endplate along with exact sizing and harvesting of the bone graft with plating are essential for successful outcomes. Biomechanical data and previous clinical studies support the addition of posterior fusion and fixation following multilevel (more than two-level) corpectomy. The additional posterior instrumentation moves the instantaneous axis of rotation posteriorly, thus approximating its normal location in the posterior vertebral body (VB). Biomechanically, this protects the graft from excessive loads while in extension and explains the clinical success of circumferential instrumentation for long-segment corpectomy reconstructions. If strut fracture occurs with minimal displacement and the graft position is still satisfactory, application of a halo vest and judicious observation are recommended. Significant displacement, kyphosis, or loss of contact of the graft and VB require revision surgery. In patients requiring revision surgery for nonunion, placement of fibular autograft or allograft with use of bone morphogenetic protein is likely to be beneficial. If questions remain regarding bone quality or construct stability, the supplemental use of posterior stabilization is recommended. Various surgical approaches have been advocated for treatment of symptomatic anterior cervical pseudarthroses or nonunion. It remains controversial as to whether the anterior or posterior approach is best. Adequate understanding of the graft and implant biomechanics are essential for a successful outcome.

Biomechanical analysis of multilevel cervical corpectomy and plate constructs

OBJECT

The authors compared the biomechanical stability of two multilevel cervical constructs involving the placement of equal size anterior cervical plates (ACPs) after decompressive surgery: the first is placed after three-level corpectomy with strut graft and the second after two-level corpectomy and aggressive discectomy with strut graft. In addition, both constructs were evaluated with and without the application of a screw attaching the ACP to the strut graft to determine whether the additional screw enhanced stability in any mode of loading.

METHODS

Nondestructive repeated-measures in vitro flexibility tests were performed in human cadaveric cervical spines. Nonconstraining pure moments of up to 1.5 Nm were applied while recording three-dimensional angular motion stereophotogrammetrically at each level from C4-5 to C7-T1. Nine specimens underwent the three-level corpectomy/strut graft procedure and eight specimens the two-level corpectomy/discectomy strut graft procedure. Failures during testing eliminated two of the former specimens and three of the latter specimens from analysis. The construct applied after the two-level procedure allowed a significantly smaller normalized neutral zone during flexion-extension than the three-level construct (p = 0.04). Normalized elastic zone and range of motion were consistently smaller in the two- than in the three-level construct, but the differences were not significant. Addition of a screw to the strut graft significantly reduced motion in the three-level procedure-treated specimens during flexion and lateral bending but had no effect on two-level corpectomy-treated specimens.

CONCLUSIONS

The construct associated with the two-level corpectomy/discectomy provided better immediate postoperative stability than that associated with the three-level corpectomy. The addition of a screw to the strut graft conferred stability on the three-level construct but not the two-level construct.

Biomechanical comparison of expandable cages for vertebral body replacement in the cervical spine

OBJECT

Recently, expandable cages for vertebral body replacement in the cervical spine have been developed. The purpose of this study was to compare the biomechanical properties of expandable cages with those of a tricortical iliac crest graft and a nonexpandable cage.

METHODS

Forty human cervical spines (C3-5) were tested in flexion, extension, axial rotation, and lateral bending. First all motion segments were evaluated intact. After corpectomy of C4 the spines were divided into five groups of eight and the following stabilization techniques were used: 1) autologous iliac crest bone graft; 2) mesh titanium cage; 3) anterior distraction device; 4) Synex-C titanium; and 5) Synex-C PEEK. Additionally, anterior plating and anterior plating plus posterior screw/rod fixation were applied. Stiffness, range of motion, and neutral and elastic zones were determined. In comparison with the intact motion segment all implants significantly increased stiffness in flexion and bending, but decreased stiffness in extension. There were no biomechanical differences between the nonexpandable and expandable cages. Furthermore, there were no biomechanical differences between the tricortical iliac crest graft and the cages, except for Synex-C in rotation. Additional anterior plating significantly increased biomechanical stiffness in all test modes; particularly in rotation mode, combined anterior-posterior stabilization increased stiffness by up to 102% compared with anterior plating alone.

CONCLUSIONS

In comparison to a tricortical iliac crest bone graft and a nonexpandable cage, expandable cages have no biomechanical advantages. Due to the low extension and rotational stiffness, none of the implants can be recommended as a stand-alone device. Additional anterior plating increased biomechanical stability adequately. Therefore, additional posterior stabilization should only be considered in cases of severe rotational instability of the cervical spine.

Cervical plates: comparison of physical characteristics and in vitro pushout strength

BACKGROUND CONTEXT

There are many cervical plates available to the spine surgeon today. A single plate design may not be appropriate for every clinical situation. It is important for the surgeon to understand the differences of these plating systems. Plate systems are known to fail by screw pullout from the bone, screw and plate breakage and a less frequent but clinically observed screw pushout from the plate. Pushout testing of the screws from the plate have not previously been subjected to study.

PURPOSE

This compares the features of cervical plating systems and the strength of the locking mechanisms to allow the surgeon to make a knowledgeable choice of plating system.

STUDY DESIGN

This is a review of descriptive geometric characteristics of cervical plate systems and a biomechanical evaluation of locking mechanism screw pushout strength.

METHODS

Physical characteristics of each plate were determined. Features of plates and screws were cataloged. Each of the test plate systems had a different locking mechanism. Biomechanical testing of the locking mechanism-screw-plate constructs was performed to determine the pushout strength of the fixation screw from the plate-locking mechanism.

RESULTS

Physical characteristics of the plating systems, including lengths, widths, shortest screw lengths and distance from edge of plate to nearest screw, were determined. Biomechanical testing showed significant differences in pushout strength, in part explained by the type of locking mechanism.

CONCLUSIONS

Biomechanical screw pushout data demonstrate that a significant range of pushout strengths exist across the available cervical plate systems today. Knowing the physical characteristics of the cervical plating systems available may allow the selection of a plate best suited for a given clinical situation.

Bone grafting, implants, and plating options for anterior cervical fusions

The basic principles of cervical spine surgery continue to include adequate decompression, provision of a structurally competent, biologically functional bone graft, and creation of a stable construct to allow for solid fusion. In recent years, the options to achieve these goals have expanded significantly. Bone banking and bone graft substitutes yield increasingly viable alternatives to autogenous bone graft. New prosthetic implants and cages are currently under investigation. The science of bone growth factors seems to be promising and is expected to revolutionize the approach to spinal arthrodesis. Various plating systems are available to provide internal stability to cervical spine constructs. It is important to understand the biomechanics of plating systems so that the optimal system may be used in a given situation. Long constructs place significant loads on SGs and anterior plates. It is important to consider the use of additional fixation, such as posterior segmental fixation in long constructs, which may be prone to failure using only anterior plate fixation. Anterior cervical plating for single-level ACDF remains controversial, whereas plating has been shown to improve the results of multilevel ACDF. Plating may provide a useful salvage option for a cervical nonunion, especially if deformity or neurologic compression dictates an anterior approach. Hardware failures may occur with anterior cervical plating, but most remain asymptomatic and do not require operative intervention.

The cortical shell architecture of human cervical vertebral bodies

STUDY DESIGN

An anatomic study of cervical vertebral bodies.

OBJECTIVES

To provide quantitative information on the cortical shell architecture of the middle and lower cervical vertebral bodies.

SUMMARY OF BACKGROUND DATA

Some external dimensions have been measured, but little quantitative data exists for the cortical shell architecture of the vertebral bodies of the cervical spine.

METHODS

Twenty-one human cervical vertebral bodies (C3-C7) were sectioned along parasagittal planes into five 1.7-mm thin slices for each vertebra. Radiographs of each slice were digitized, and external and internal dimensions were measured. Averages and standard deviations were computed. Single factor analysis of variance was used to determine significant (P < 0.05) differences between the vertebral levels.

RESULTS

The superior endplate was thickest in the posterior region (range 0.74-0.89 mm) and thinnest in the anterior region (range 0.44-0.56 mm). The inferior endplate was thickest in the anterior region (range 0.61-0.81 mm) and thinnest in the posterior region (range 0.49-0.62 mm). In the central region, the superior endplate (range 0.42-0.58 mm) was thinner than the inferior endplate (range 0.53-0.64 mm). Variation with vertebral level was dependent on the dimension studied.

CONCLUSIONS

Comprehensive quantitative anatomic data of the middle and lower cervical vertebral bodies have been obtained. This may be useful in improving the understanding of the three-column and other vertebral-fracture theories, the fidelity of the finite element models of cervical spine, and the designs of surgical instrumentation.

Development of a system for in vitro neck muscle force replication in whole cervical spine experiments

STUDY DESIGN

An in vitro biomechanical study.

OBJECTIVES

To develop and evaluate a new in vitro whole cervical spine model that provides to the specimen, in vivo-like mechanical characteristics.

SUMMARY OF BACKGROUND DATA

In vitro studies of kinematics, kinetics, and trauma using isolated spine specimens (head-T1 vertebra) have usually applied upward force to the head, resulting in tensile spine forces, contrary to the physiological compressive forces present in vivo. Further, the in vitro load-displacement curves have never been compared with the corresponding in vivo data.

METHODS

A novel muscle force replication (MFR) system is presented. It consists of a set of compressive forces applied to the various vertebrae and occiput of a whole cervical spine specimen. Two protocols, with and without MFR, were evaluated using standardized flexibility testing. Ranges of motion (ROM) and load-displacement curves were documented, and contrasted with similar in vivo data.

RESULTS

Results for the MFR were found to be similar to the in vivo measurements, with respect to the intersegmental and whole neck motions as well as the load-displacement curves, thus validating the MFR approach.

CONCLUSIONS

The new model advances the in vitro testing, which uses whole cervical spine specimens.

Biomechanical evaluation of occipitocervical fixation devices

Human cadaveric occipitocervical specimens were implanted with three types of instrumentation. The devices were tested biomechanically under three modes of loading to determine the stiffness of spinal constructs and the failure mechanisms of the constructs under extreme flexion. The devices tested were the AXIS Fixation System (with custom plate), the Y-Plate, and the Luque rectangle. No significant differences in stiffness among the devices were found under compression and flexion. The stiffnesses of the plate systems were statistically higher than the Luque rectangle in extension and torsion. In extreme flexion, the plate systems failed by fracture of the C2 pedicles. Modern plate systems, for occipitocervical fixation, provide more stiffness and stability than traditional wiring techniques. This study provides surgeons with information on the relative merits of modern plate and screw systems compared with traditional rod and wire constructs.

Increased fusion rates with cervical plating for three-level anterior cervical discectomy and fusion

STUDY DESIGN

A retrospective review of all patients surgically treated by a single surgeon with a three-level anterior cervical discectomy and fusion with and without anterior plate fixation.

OBJECTIVES

To compare the clinical and radiographic success of anterior three-level discectomy and fusion performed with and without anterior cervical plate fixation.

SUMMARY OF BACKGROUND DATA

Previous studies of multilevel cervical discectomies and fusions have shown fusion rates to decrease as the number of surgical levels increases. Anterior cervical plate stabilization can provide more stability and may increase fusion rates for multilevel fusions.

METHODS

Over a 7-year period, 59 patients were treated surgically with a three-level anterior cervical discectomy and fusion by the senior author. Forty patients had cervical plates, whereas 19 had fusions with no plates. These patients were observed for an average of 3.2 years. Clinical and radiographic follow-up data were obtained.

RESULTS

Of the 59 patients, 14 had a pseudarthrosis (7 in each group). The pseudarthrosis rates were 18% (7 of 40) for patients with plating and 37% (7 of 19) for patients with no plating. Although the nonunion rate for unplated fusions was double that of plated fusions, this difference was not statistically significant. There was no statistically significant correlation between pseudarthrosis and gender, age, level of surgery, history of tobacco use, or previous anterior surgery. The fusion rates were improved with the use of a cervical plate. Inferior clinical results were demonstrated in patients with a pseudarthrosis, regardless of the use of a cervical plate.

CONCLUSIONS

The addition of plate fixation for three-level anterior cervical discectomy and fusion is a safe procedure and does not result in higher complication rates. In this study, the pseudarthrosis rate was lower for patients with a cervical plate. However, this difference was not statistically significant. Patients treated with cervical plating had overall better results when compared with those of patients treated without cervical plates. Although the use of cervical plates decreased the pseudarthrosis rate, a three-level procedure is still associated with a high nonunion rate, and other strategies to increase fusion rates should be explored.

The role of bone graft force in stabilizing the multilevel anterior cervical spine plate system

STUDY DESIGN

The role of bone graft force in stabilizing an instrumented cervical spine was evaluated for one-level and three-level corpectomy models using in vitro experiments.

OBJECTIVES

To investigate the role of bone graft force in enhancing stability of anterior cervical plate, and to study effects of fatigue loading.

SUMMARY OF BACKGROUND DATA

The anterior cervical plate system is used widely in stabilizing the cervical spine after spinal corpectomy and grafting. Many factors such as applied screw torque, screw pullout force, plate strength, plate geometry, and type of bone graft have been studied. However, the role of bone graft in stabilizing the anterior plate system has not been explored.

METHODS

Two models (one-level and three-level) incorporating corpectomy, strut graft, and anterior plate were constructed from eight human spine specimens (C2-T1). The flexibility of an intact specimen and two constructs with graft forces of 0 N and 100 N was determined. A flexibility test, simulating physiologic loads, consisted of pure moments of flexion, extension, lateral bending, and axial torques up to 1 Nm. For each moment, range of motion and neutral zone were determined. The stability potential index was defined as the decrease in motion caused by instrumentation, as compared with intact motion. A larger stability potential index indicates a more stable spinal construct. Repeated measures analysis of variance was used to determine the significant changes.

RESULTS

In both models, bone graft force increased during extension, decreased during flexion, and showed minor changes during axial torsion and lateral bending. Higher bone graft force increased stability potential index-neutral zone and stability potential index-range of motion in the three-level model in all directions, but only in flexion-extension in the one-level model. Fatigue loading decreased bone graft force to a greater extent in the three-level model.

CONCLUSIONS

In the corpectomy-graft-anterior-plate model, graft force decreased in flexion and increased in extension. Higher graft force increased and fatigue decreased stability of the spinal construct in the three-level model.