Biomechanical comparison of anterolateral plate, lateral plate, and pedicle screws-rods for enhancing anterolateral lumbar interbody cage stabilization

Spinolaminar locking plates improve fixation strength compared to pedicle screws: a biomechanical analysis

INTRODUCTION

Pedicle screw loosening is a significant complication of posterior spinal fixation, particularly among osteoporotic patients and in deformity constructs. In orthopedic trauma surgery, locking plates and screws have revolutionized the fixation of osteoporotic fractures. We have combined the traumatology principle of fixed-angle locking plate fixation with the spine principles of segmental instrumentation.

METHODS

A novel spinolaminar locking plate was designed based on morphometric studies of human thoracolumbar vertebrae. The plates were fixed to cadaveric human lumbar spines and connected to form 1-level L1-L2 or L4-L5 constructs and compared to similar pedicle screw constructs. Pure moment testing was performed to assess range of motion before and after 30,000 cycles of cyclic fatigue. Post-fatigue fixture pullout strength was assessed by applying a continuous axial tensile force oriented to the principal axis of the pedicle until pullout was observed.

RESULTS

Spinolaminar plate fixation resulted in superior pullout strength compared to pedicle screws (1,065 ± 400N vs. 714 ± 284N, p = 0.028). Spinolaminar plates performed equivalently to pedicle screws in range of motion reduction during flexion/extension and axial rotation. Pedicle screws outperformed the spinolaminar plates in lateral bending. Finally, no spinolaminar constructs failed during cyclic fatigue testing, whereas one pedicle screw construct did.

CONCLUSIONS

The spinolaminar locking plate maintained adequate fixation post-fatigue, particularly in flexion/extension and axial rotation compared to pedicle screws. Moreover, spinolaminar plates were superior to pedicle screw fixation with respect to cyclic fatiguing and pullout strength. The spinolaminar plates offer a viable option for posterior lumbar instrumentation in the adult spine.

Evaluation of the Stability of a Novel Lateral Plate Internal Fixation: An In Vitro Biomechanical Study

BACKGROUND

This study aims to evaluate the biomechanical stability of a novel lateral plate (NLP) that can be used in oblique lateral lumbar fusion (OLIF).

METHODS

In vitro biomechanical tests were performed on 6 fresh calf lumbar vertebrae specimens. The surgical segment was set at L3-L4. Each specimen was tested in the following order: intact state (INT); OLIF cage only/stand-alone (SA); cage supplemented with lateral screw-rod (LSR); cage supplemented with novel lateral plate (NLP); and cage supplemented with unilateral or bilateral pedicle screw-rod (UPS or BPS). A pure moment of ±7.5 Nm was applied to the specimen to produce 6 different motion directions, including flexion and extension, lateral bending, and axial rotation, and the range of motion (ROM) of L3-L4 in each direction was recorded.

RESULTS

In addition to flexion-extension, NLP reduced the ROM of SA (P < 0.05). In flexion-extension, the ROM of NLP was similar to those of SA and LSR (P > 0.05); compared to pedicle screw-rod (PSD), the ROM of NLP was higher (P < 0.05). In lateral bending, the ROM of NLP was close to that of LSR and PSD (P > 0.05). In axial rotation, the ROM of NLP was higher than that of PSD (P < 0.05), and close to that of LSR (P > 0.05).

CONCLUSIONS

NLP can enhance surgical segment stability in all directions of motion, similar to LSR, but weaker than UPS and BPS in flexion-extension and rotation.

Kinematics Following 3-Screw Integrated Interbody Spacers in the Lumbar Spine

BACKGROUND

"Stand-alone" fusion implants attempt to alleviate the need for supplemental posterior instrumentation.

OBJECTIVE

A biomechanical study was conducted to assess the stability of an integrated 3- screw interbody cage with, and without, supplemental posterior fixation.

METHODS

Nondestructive biomechanical testing was performed on 19 healthy cadaver spine segments. Specimens were tested in 6 degrees of motion and a maximum pure bending moment of 10 Nm was applied. Specimens were evaluated in the following sequence: Intact, cage, cage ± facet bolts, and cage ± pedicle screws. Nonconstrained motion was measured at both the index and adjacent levels.

RESULTS

The index levels were L2-L3 and L5-S1. The cage alone provided a significant decrease in motion at the L2-L3 level but not at L5-S1. At L2-L3, cage + pedicle screws decreased motion more effectively than cage + facet bolts, however, both the supplemented constructs outperformed intact (P < .05). At L5-S1, both posterior fixation systems appeared to have smaller degree of displacement compared to intact; however, no significant differences were observed at L5-S1 among the various constructs. Furthermore, the adjacent segments for each level (L1-L2 and L4-L5) had no significantly increased motion, compared to intact, for all 6 degrees of motion tested.

CONCLUSION

The stand-alone cage was more effective at L2-L3, than at L5-S1, in limiting motion. At L5-S1, supplemental fixation may need to be considered. No abnormal motion was identified at the adjacent, normal segments, for the stand-alone, or the circumferential constructs at either level tested.

Circumferential minimally invasive approach for low-grade isthmic spondylolisthesis: A clinical and radiological study of 43 patients

INTRODUCTION

Circumferential fusion for lumbar low-grade isthmic spondylolisthesis (LGIS) provides the best spinal stability and highest fusion rates. The aim of this study is to investigate results of minimal invasive management of LGIS and correlations between Intervertebral Foramen Surface (IFS) and other parameters.

METHODS

We retrospectively reviewed cases of 43 patients who underwent a minimally invasive circumferential fusion (Anterior lumbar interbody fusion followed by percutaneous posterior pedicle screw fixation) for LGIS between January 2010 and December 2014 in our institution. Inclusion criteria were one-level (L4-L5 or L5-S1) LGIS with low back and/or radicular pain. Pre- and postoperative radiographic evaluations were performed at 6, 12 and 24months. Measurements (Percentage of anterior displacement, degree of slip angle, height of the intervertebral space and the IFS) were obtained using Surgimap^®.

RESULTS

Nineteen patients (44.2%) were males. Mean age was 43 years old (19-72years). The mean follow-up of the series was 18.3months (3-72months). Mean preoperative Visual Analogy Scale (VAS) for low back pain decreased from 70mm to 20mm and from 80mm to 10mm as to radicular pain. Anterior displacement was reduced from 18% to 7% (p<0.01), degree of slippage were increased from 9.8° to 15.2° (p<0.01), intervertebral height was restored from 4.4mm to 8.5mm (p<0.01) and increase of the IFS was calculated 48.8%.

CONCLUSION

One stage circumferential fixation for adults' LGIS without decompression, allows restoration of intervertebral height permitting good reduction of the slippage, an increasing of the IFS and liberation of nerve roots.

Biomechanical analysis of the thoracolumbar spine under physiological loadings: Experimental motion data corridors for validation of finite element models

Biomechanical studies that involve normal, injured or stabilized human spines are sometimes difficult to perform on large samples due to limited access to cadaveric human spines and biological variability. Finite element models alleviate these limitations due to the possibility of reusing the same model, whereas cadaveric spines can be damaged during testing, or have their mechanicals behaviour modified by fatigue, permanent deformation or structural failure. Finite element models need to be validated with experimental data to make sure that they represent the complex mechanical and physiological behaviour of normal, injured and stabilized spinal segments. The purpose of this study is to characterize the mechanical response of thoracolumbar spine segments with an analytical approach drawn from experimental measurements. A total of 24 normal and fresh cadaveric thoracolumbar spine segments (T11-L3), aged between 53 and 91 years, were tested in pure flexion/extension, lateral bending and axial torsion using a specific experimental setup. Measurements of global and intervertebral angle variations were performed using three-dimensional mark tracking methods. Load/angle curves for each loading were fitted by a logarithmic approach with two coefficients. The coefficients for the functions describing the response of the spinal segments are given and constitute predictive models from experimental data. This work provides data corridors of human thoracolumbar spine motion segments subjected to pure bending in the three physiological planes. These data could be very useful to validate finite element models of the human spine.

Complete Spondylectomy Using Orthogonal Spinal Fixation and Combined Anterior and Posterior Approaches for Thoracolumbar Spinal Reconstruction: Technical Nuances and Clinical Results

STUDY DESIGN

Retrospective chart review.

OBJECTIVE

To determine the long-term efficacy of 2-stage total en bloc spondylectomy (TES).

SUMMARY OF BACKGROUND DATA

TES is a well-described technique to achieve tumor-free margins, but it is a highly destabilizing procedure that necessitates spinal reconstruction. A 2-stage anterior/posterior approach for tumor resection and instrumentation has been shown to be biomechanically superior to the single-stage approach in achieving rigid fixation, but few clinical studies with long-term outcomes exist.

METHODS

A retrospective review was performed on patients undergoing a 2-stage TES for a spinal tumor between 1999 and 2011. Results were compared with those from a literature review of case series, with a minimum of 2-year follow-up, reporting on a single-stage posterior-only approach for TES.

RESULTS

Seven patients were identified (average follow-up 52.7 mo). Tumor location ranged from T1 to L3 with the following pathologies: metastasis (n=3), hemangioma (n=1), leiomyosarcoma (n=1), giant cell tumor (n=1), and chordoma (n=1). There were no significant surgical complications. All 7 patients had intact spinal fixation. There were no failures of the orthogonal fixation (pedicle screws or anterior fixation). The average modified Rankin Scale scores improved from 2.7 preoperatively to 0.7 at last follow-up. None of the patients in our series suffered local disease recurrence at last follow-up or suffered neurological deterioration. These results were comparable with those noted in the literature review of posterior-only approach, where 12% of patients experienced instrument failure.

CONCLUSIONS

TES is a highly destabilizing procedure requiring reconstruction resistant to large multiplanar translational and torsional loads. A 2-stage approach utilizing orthogonal vertebral body screws perpendicular to pedicle screws is a safe and effective surgical treatment strategy. Orthogonal spinal fixation may lower the incidence of instrumentation failure associated with complete spondylectomy and appears to be comparable with a single-stage procedure. However, larger prospective series are necessary to assess the efficacy of this approach versus traditional means.

Optimization of segmental lumbar spine instability treatment using minimally invasive spinal fusion technique

Open transforaminal lumbar interbody fusion (TLIF) using transpedicular rods and interbody cage, which is used to treat segmental instability, is associated with a significant paravertebral muscle and ligament injury. A new rigid fusion method was introduced to improve the outcome of patients' treatment.

OBJECTIVE

To conduct a comparative analysis of the effectiveness of minimally invasive fusion technique and TLIF to improve the treatment results in patients with symptomatic lumbar spine degeneration concomitant with moderate segmental instability of the lumbar spine.

MATERIAL AND METHODS

The study involved 90 patients, which were divided into 2 groups. Transforaminal interbody fusion with Pezo-T PEEK cage was performed after spinal canal reconstruction in both groups. In the first group (n=45), interbody fusion was augmented by 4-point transpedicular rod fixation (Konmet, Russia); in the second group (n=45), by Coflex-F rigid interspinous spacer. Patients were followed up and the results were assessed within 24 months after surgery.

RESULTS

The intergroup comparison of pain level using the visual analogue scale, the need for analgesics, and quality of life according to Oswestry Disability Index score during the early postoperative period demonstrated significantly better outcomes in the second group of patients due to lesser operative trauma of the paravertebral soft tissue. Interbody fusion was observed within 20-36 months in 95% of group I patients and 94% of group II patients (p>0.05). Postoperative complications were observed in 17.8% of patients in group I and in 2.2% of cases in group II (p<0.001).

CONCLUSION

Stabilization by rigid interspinous spacer and transforaminal interbody cage provides better clinical outcomes and fewer postoperative complications as compared to the conventional TLIF technique with similar X-ray rate of bone block formation in patients with moderate segmental instability of the lumbar spine, thus optimizing the treatment of such patients.

Biomechanical evaluation of a biomimetic spinal construct

Background

Laboratory spinal biomechanical tests using human cadaveric or animal spines have limitations in terms of disease transmission, high sample variability, decay and fatigue during extended testing protocols. Therefore, a synthetic biomimetic spine model may be an acceptable substitute. The goal of current study is to evaluate the properties of a synthetic biomimetic spine model; also to assess the mechanical performance of lateral plating following lateral interbody fusion.

Methods

Three L3/4 synthetic spinal motion segments were examined using a validated pure moment testing system. Moments (±7.5 Nm) were applied in flexion-extension (FE), lateral bending (LB) and axial rotation (AR) at 1Hz for total 10000 cycles in MTS Bionix. An additional test was performed 12 hours after 10000 cycles. A ±10 Nm cycle was also performed to allow provide comparison to the literature. For implantation evaluation, each model was tested in the 4 following conditions: 1) intact, 2) lateral cage alone, 3) lateral cage and plate 4) anterior cage and plate. Results were analysed using ANOVA with post-hoc Tukey’s HSD test.

Results

Range of motion (ROM) exhibited logarithmic growth with cycle number (increases of 16%, 37.5% and 24.3% in AR, FE and LB respectively). No signification difference (p > 0.1) was detected between 4 cycles, 10000 cycles and 12 hour rest stages. All measured parameters were comparable to that of reported cadaveric values. The ROM for a lateral cage and plate construct was not significantly different to the anterior lumbar interbody construct for FE (p = 1.00), LB (p = 0.995) and AR (p = 0.837).

Conclusions

Based on anatomical and biomechanical similarities, the synthetic spine tested here provides a reasonable model to represent the human lumbar spine. Repeated testing did not dramatically alter biomechanics which may allow non-destructive testing between many different procedures and devices without the worry of carry over effects. Small intra-specimen variability and lack of biohazard makes this an attractive alternative for in vitro spine biomechanical testing. It also proved an acceptable surrogate for biomechanical testing, confirming that a lateral lumbar interbody cage and plate construct reduces ROM to a similar degree as anterior lumbar interbody cage and plate constructs.

Allogeneic mesenchymal progenitor cells for posterolateral lumbar spine fusion in sheep

BACKGROUND CONTEXT

Osteoconductive porous ceramic bone graft materials supplemented with mesenchymal precursor cells (MPC) derived from autologous bone marrow aspirates have been shown to stimulate successful interbody and posterolateral spine fusion in preclinical models. Recent advances in immunomagnetic cell sorting have enabled purification and isolation of pluripotent stem cells from marrow aspirates and have expanded stem cell technology to allogeneic cell sources. Allogeneic MPC technology combined with appropriate synthetic biomaterial carriers could provide both the osteogenic and osteoconductive components needed for successful posterolateral spine fusion without the need for autologous bone harvest or expensive recombinant protein technology.

PURPOSE

To determine the safety and efficacy of a hydroxyapatite:tricalcium phosphate graft material supplemented with allogeneic mesenchymal precursor cells in posterolateral lumbar spine fusion using an ovine model.

STUDY DESIGN

Skeletally mature ewes underwent single-level instrumented posterolateral lumbar spine fusion using either autograft (AG), hydroxyapatite:tricalcium phosphate carrier (CP), or CP supplemented with allogeneic mesenchymal progenitor cells (MPCs). Three doses of MPCs were evaluated: 25 × 10⁶ cells (low dose, LD), 75 × 10⁶ cells (mid dose, MD), and 225 × 10⁶ cell (high dose, HD). Animals survived for either 4 or 9 months.

METHODS

Plain radiographs were acquired and scored for bridging bone at regular intervals during healing to monitor fusion development. Hematology, coagulation, and serum chemistry were monitored at regular intervals throughout the study to monitor animal health. After necropsy, computed tomography, high-resolution radiography, biomechanical testing, organ pathology, bone histopathology, and bone histomorphometry were conducted to monitor the safety and ascertain the efficacy of MPC treatment.

RESULTS

MPC treatment in this spine fusion model resulted in no observed adverse systemic or local tissue responses. Radiographically, fusion scores for MPC-treated animals were uniformly higher compared with those treated with carrier alone (CP) after 3 months and continued the same trend throughout 9 month of healing. Quantitative computed tomography confirmed better connectivity of the fusion for MPC treatment groups compared with CP. Biomechanical analyses were not able to differentiate between treatment groups. Histomorphometry results confirmed radiographic and quantitative computed tomography results; cell-supplemented treatment groups and autograft had equivalent amounts of bone within the fusion mass and less bony fusion tissue was found within the fusion mass in specimens from the CP treatment group. No conclusive effects of cell dose of fusion efficacy were noted.

CONCLUSIONS

Adult allogeneic mesenchymal precursor cells delivered via a hydroxyapatite:tricalcium phosphate carrier were both safe and efficacious in this ovine spine fusion model. Results from this preclinical study support that allogeneic mesenchymal precursor cells produced fusion efficacy similar to that achieved using iliac crest autograft, thereby providing a safe and viable option to achieve successful posterolateral spine fusion.

Relation between radiological assessment and biomechanical stability of lumbar interbody fusion in a large animal model

PURPOSE

To relate the progress of vertebral segmental stability after interbody fusion surgery with radiological assessment of spinal fusion.

METHODS

Twenty goats received double-level interbody fusion and were followed for a period of 3, 6 and 12 months. After killing, interbody fusion was assessed radiographically by two independent observers. Subsequently, the lumbar spines were subjected to four-point bending and rotational deformation, assessed with an optoelectronic 3D movement registration system. In addition, four caprine lumbar spines were analysed in both the native situation and after the insertion of a cage device, as to mimic the direct post-surgical situation. The range of motion (ROM) in flexion/extension, lateral bending and axial rotation was analysed ex vivo using a multi-segment testing system.

RESULTS

Significant reduction in ROM in the operated segments was already achieved with moderate bone ingrowth in flexion/extension (71 % reduction in ROM) and with only limited bone ingrowth in lateral bending (71 % reduction in ROM) compared to the post-surgical situation. The presence of a sentinel sign always resulted in a stable vertebral segment in both flexion/extension and lateral bending. For axial rotation, the ROM was already limited in both native and cage inserted situations, resulting in non-significant differences for all radiographic scores.

DISCUSSION

In vivo vertebral segment stability, defined as a significant reduction in ROM, is achieved in an early stage of spinal fusion, well before a radiological bony fusion between the vertebrae can be observed. Therefore, plain radiography underestimates vertebral segment stability.

Biomechanical comparison of anterior lumbar interbody fusion: stand-alone interbody cage versus interbody cage with pedicle screw fixation -- a finite element analysis

Background

Anterior lumbar interbody fusion (ALIF) followed by pedicle screw fixation (PSF) is used to restore the height of the intervertebral disc and provide stability. Recently, stand-alone interbody cage with anterior fixation has been introduced, which eliminates the need for posterior surgery. We compared the biomechanics of the stand-alone interbody cage to that of the interbody cage with additional PSF in ALIF.

Methods

A three-dimensional, non-linear finite element model (FEM) of the L2-5 segment was modified to simulate ALIF in L3-4. The models were tested under the following conditions: (1) intact spine, (2) destabilized spine, (3) with the interbody cage alone (type 1), (4) with the stand-alone cage with anterior fixation (SynFix-LR®; type 2), and (5) with type 1 in addition to PSF (type 3). Range of motion (ROM) and the stiffness of the operated level, ROM of the adjacent segments, load sharing distribution, facet load, and vertebral body stress were quantified with external loading.

Results

The implanted models had decreased ROM and increased stiffness compared to those of the destabilized spine. The type 2 had differences in ROM limitation of 8%, 10%, 4%, and 6% in flexion, extension, axial rotation, and lateral bending, respectively, compared to those of type 3. Type 2 had decreased ROM of the upper and lower adjacent segments by 3-11% and 3-6%, respectively, compared to those of type 3. The greatest reduction in facet load at the operated level was observed in type 3 (71%), followed by type 2 (31%) and type 1 (23%). An increase in facet load at the adjacent level was highest in type 3, followed by type 2 and type 1. The distribution of load sharing in type 2 (anterior:posterior, 95:5) was similar to that of the intact spine (89:11), while type 3 migrated posterior (75:25) to the normal. Type 2 reduced about 15% of the stress on the lower vertebral endplate compared to that in type 1. The stress of type 2 increased two-fold compared to the stress of type 3, especially in extension.

Conclusions

The stand-alone interbody cage can provide sufficient stability, reduce stress in adjacent levels, and share the loading distribution in a manner similar to an intact spine.

Iterative design and testing of a modular anterior plate for lumbar spine fixation applications

In this study, a modular anterior lumbar plate is designed and tested in an iterative fashion. The study starts with a basic design that is built by combining same-sized modules; an approach that allow inventory costs to be decreased. The basic design is iteratively improved guided by the results of biomechanical tests performed on each new design. At the end of three iterations of improvements, the design is complete and the plate is of sufficient quality for it to be used in anterior surgical operations. Using these plates creates the advantage of being able to increase the size and slot count during surgical operations, even when some of the modules are already fixed to vertebrae. The designed modular plate is shown to be as safe for use as a rigid plate in terms of its static and fatigue biomechanical performances.

Evaluation of a robot-assisted testing system for multisegmental spine specimens

Mono- and multi-segmental testing methods are required to identify segmental motion patterns and evaluate the biomechanical behaviour of the spine. This study aimed to evaluate a new testing system for multisegmental specimens using a robot combined with an optical motion analysis system. After validation of the robotic system for accuracy, two groups of calf specimens (six monosegmental vs. six multisegmental) were mounted and the functional unit L3-4 was observed. Using rigid body markers, range of motion (ROM), elastic zone (EZ) and neutral zone (NZ), as well as stiffness properties of each functional spine unit (FSU) was acquired by an optical motion capture system. Finite helical axes (FHA) were calculated to analyse segmental movements. Both groups were tested in flexion and extension. A pure torque of 7.5 Nm was applied. Statistical analyses were performed using the Mann-Whitney U-test. Repeatability of robot positioning was -0.001±0.018 mm and -0.025±0.023° for translations and rotations, respectively. The accuracy of the optical system for the proposed set-up was 0.001±0.034 mm for translations and 0.075±0.12° for rotations. No significant differences in mean values and standard deviations of ROM for L3-4 compared to literature data were found. A robot-based facility for testing multisegmental spine units combined with a motion analysis system was proposed and the reliability and reproducibility of all system components were evaluated and validated. The proposed set-up delivered ROM results for mono- and multi-segmental testing that agreed with those reported in the literature. Representing the FHA via piercing points determined from ROM was the first attempt showing a relationship between ROM and FHA, which could facilitate the interpretation of spine motion patterns in the future.

Interbody device endplate engagement effects on motion segment biomechanics

BACKGROUND CONTEXT

Stand-alone nonbiologic interbody fusion devices for the lumbar spine have been used for interbody fusion since the early 1990s. However, most devices lack the stability found in clinically successful circumferential fusion constructs. Stability results from cage geometry and device/vertebral endplate interface integrity. To date, there has not been a published comparative biomechanical study specifically evaluating the effects of endplate engagement of interbody devices.

PURPOSE

Lumbar motion segments implanted with three different interbody devices were tested biomechanically to compare the effects of endplate engagement on motion segment rigidity. The degree of additional effect of supplemental posterior and anterior fixation was also investigated.

STUDY DESIGN/SETTING

A cadaveric study of interbody fusion devices with varying degrees of endplate interdigitation.

OUTCOME MEASURES

Implanted motion segment range of motion (ROM), neutral zone (NZ), stiffness, and disc height.

METHODS

Eighteen human L23 and L45 motion segments were distributed into three interbody groups (n=6 each) receiving a polymeric (polyetheretherketone) interbody spacer with small ridges; a modular interbody device with endplate spikes (InFix, Abbott Spine, Austin, TX, USA); or dual tapered threaded interbody cages (LT [Lordotic tapered] cage; Medtronic, Memphis, TN, USA). Specimens were tested intact using a 7.5-Nm flexion-extension, lateral bending, and axial torsion flexibility protocol. Testing was repeated after implantation of the interbody device, anterior plate fixation, and posterior interpedicular fixation. Radiographic measurements determined changes in disc height and intervertebral lordosis. ROM and NZ were calculated and compared using analysis of variance.

RESULTS

The interbody cages with endplate spikes or threads provided a statistically greater increase in disc height versus the polymer spacer (p=.01). Relative to intact, all stand-alone devices significantly reduced ROM in lateral bending by a mean 37% to 61% (p< or =.001). The cages with endplate spikes or threads reduced ROM by approximately 50% and NZ by approximately 60% in flexion-extension (p< or =.02). Only the cage with endplate spikes provided a statistically significant reduction in axial torsion ROM compared with the intact state (50% decrease, p<.001). Posterior fixation provided a significant reduction in ROM in all directions versus the interbody device alone (p<.001). Anterior plating decreased ROM over interbody device alone in flexion-extension and torsion but did not have additional effect on lateral bending ROM.

CONCLUSION

The cages with endplate spikes or threads provide substantial motion segment rigidity compared with intact in bending modes. Only the cages with endplate spikes were more rigid than intact in torsion. All devices experienced increased rigidity with anterior plating and even greater rigidity with posterior fixation. It appears that the endplate engagement with spikes may be beneficial in limiting torsion, which is generally difficult with other "stand-alone" devices tested in the current and prior reports.

Biomechanical pullout strength and stability of the cervical artificial pedicle screw

STUDY DESIGN

This study used cadaveric specimens to compare the biomechanical performance of artificial pedicle screws (APS) versus lateral mass screws (LMS).

OBJECTIVE

The goal of this study was to biomechanically characterize APS range-of-motion and pullout strength in surgical instances that preclude LMS insertion.

SUMMARY OF BACKGROUND DATA

Posterior approaches used in instances of ventral spine tumors often necessitate complete facetectomy, thereby removing fixation points for LMS and requiring longer constructs with fewer segmental fixations to span the resected levels. Recently, APS were developed to overcome this obstacle. Although APS have been used successfully in clinical cases, they have yet to be biomechanically validated.

METHODS

Seven fresh-frozen cervical spine segments (C2-C7) were harvested from human donors (F = 1, M = 6; 65 +/- 5 years old, range: 50-72 years old). Nondestructive range-of-motion tests were conducted on each specimen in its intact and surgically destabilized states, and after each of 3 different APS and LMS surgical stabilizations. After nondestructive bending tests, a final pullout test of APS and LMS at the C4 level was performed for each specimen.

RESULTS

The pullout strength of the APS was twice as strong on average as that for the LMS (503.4 +/- 338.3 vs.- 254.3 +/- 142.3 N); this difference approached but was not statistically significant (P = 0.07). There was no significant difference in specimen stiffness between the APS- and LMS-instrumented configurations in all ranges of primary and off-axis motions (P > 0.05). However, all fixation methods increased specimen stiffness in comparison with the intact conditions (1.7-36.5 times increase; P < 0.05).

CONCLUSION

Our results demonstrate that APS provide comparable stability to LMS and can therefore be considered a viable alternative in surgical scenarios requiring the complete removal of lateral masses. Moreover, APS may provide some enhanced strength in the face of destructive pullout forces.

Biomechanical comparison of two different concepts for stand alone anterior lumbar interbody fusion

Segmental instability in degenerative disc disease is often treated with anterior lumbar interbody fusion (ALIF). Current techniques require an additional posterior approach to achieve sufficient stability. The test device is an implant which consists of a PEEK-body and an integrated anterior titanium plate hosting four diverging locking screws. The test device avoids posterior fixation by enhancing stability via the locking screws. The test device was compared to an already established stand alone interbody implant in a human cadaveric three-dimensional stiffness test. In the biomechanical test, the L4/5 motion segment of 16 human cadaveric lumbar spines were isolated and divided into two test groups. Tests were performed in flexion, extension, right and left lateral bending, right and left axial rotation. Each specimen was tested in native state first, then a discectomy was performed and either of the test implants was applied. Finite element analysis (FE) was also performed to investigate load and stress distribution within the implant in several loading conditions. The FE models simulated two load cases. These were flexion and extension with a moment of 5 Nm. The biomechanical testing revealed a greater stiffness in lateral bending for the SynFix-LR compared to the established implant. Both implants showed a significantly higher stiffness in all loading directions compared to the native segment. In flexion loading, the PEEK component takes on most of the load, whereas the majority of the extension load is put on the screws and the screw-plate junction. Clinical investigation of the test device seems reasonable based on the good results reported here.

Biomechanical comparison of three fixation techniques for unstable thoracolumbar burst fractures. Laboratory investigation

OBJECT

Increased structural stability is considered sufficient justification for higher-risk surgical procedures, such as circumferential fixation after severe spinal destabilization. However, there is little biomechanical evidence to support such claims, particularly after traumatic lumbar burst fracture. The authors sought out to compare the biomechanical performance of the following 3 fixation strategies for spinal reconstruction after decompression for an unstable thoracolumbar burst fracture: 1) short-segment anterolateral fixation; 2) circumferential fixation; and 3) extended anterolateral fixation.

METHODS

Thoracolumbar spines (T10-L4) from 7 donors (mean age at death 64+/-6 years; 1 female and 6 males) were tested in pure moment loading in flexion-extension, lateral bending, and axial rotation. Thoracolumbar burst fractures were surgically induced at L-1, and testing was repeated sequentially for each of the following fixation techniques: short-segment anterolateral, circumferential, and extended anterolateral. Primary and coupled 3D motions were measured across the instrumented site (T12-L2) and compared across treatment groups.

RESULTS

Circumferential and extended anterolateral fixations were statistically equivalent for primary and off-axis range-of-motions in all loading directions, and short-segment anterolateral fixation offered significantly less rigidity than the other 2 methods.

CONCLUSIONS

The results of this study strongly suggest that extended anterolateral fixation is biomechanically comparable to circumferential fusion in the treatment of unstable thoracolumbar burst fractures with posterior column and posterior ligamentous injury. In cases in which an anterior procedure may be favored for load sharing or canal decompression, extension of the anterior instrumentation and fusion one level above and below the unstable segment can result in near equivalent stability to a 2-stage circumferential procedure.

[Anterior retroperitoneal L5-S1 arthrodesis and fixation with cage and plate. Prospective study in 40 cases with a follow-up of one to 2.2 year (mean 1.5 year)]

PURPOSE OF THE STUDY

The aim of this prospective clinical study was to assess outcome after anterior retroperitoneal interbody arthrodesis for L5-S1 discopathy. We used a cage filled with an autologous corticocancellous graft and plate fixation.

MATERIALS AND METHODS

Forty patients with low back pain unresponsive to medical treatment for more than six months were included in this series. We noted the clinical signs and radiculalgia. Patients were assessed preoperatively, postoperatively, at one year and at last follow-up using the Oswestry score and a visual analog scale (VAS). The plain X-rays disclosed 30 cases of discopathy (16 primary and 14 postdiscectomy) and 10 cases of spondylolisthesis by L5 isthmic lysis (three Meyerding grade 0 and seven grade 1). MRI revealed signs of disc degeneration in all cases with a black disc or modification of the endplate signals (Modic type 1 in 23 and type 2 in 13). The anterior retroperitoneal approach was used in all cases for complete discectomy, arthrodesis with cage insertion and an autologous corticocancellous graft harvested from the iliac bone and fixation using a triangular plate (Pyramid, Medtronic, Memphis TN). Clinical and radiographic follow-up data were available at six weeks and three, six and 12 months in addition to last follow-up. A follow-up using MRI was performed in the event of complications.

RESULTS

The population was composed of 25 women and 15 men, mean age 44 years and eight months (range 29-693 years). Thirteen patients presented radiculalgia. There were no vascular or gastrointestinal complications with the anterior approach. Bone healing was achieved at one year in 38 patients (95%). The mean Oswestry score improved from 52 to 16% and the mean VAS from 7.8 to 1.83 at 18 months follow-up. All occupationally-active patients but one resumed their former activity at mean 4.7 months. There were no sexual complications in this series. Low back pain persisted in three patients and five developed transient facet joint symptoms. Two patients developed secondary radiculalgia. We noted two cases of nonunion, in one grade 1 spondylolisthesis and in one grade 1 spondlylolisthesis (with revision for complementary posterior arthrodesis and a good outcome at latest follow-up).

DISCUSSION AND CONCLUSION

Anterior L5-S1 interbody fusion with a cage filled with a corticocancellous autograft fixed with a plate provides good clinical and radiographic results for the stabilization of painful discopathy. The results have been good for isolated L5-S1 discopathy and for grade 0 spondylolisthesis. The limit for this technique would appear to be grade 1 spondylolisthesis with a degraded disc. Performed via an anterior retroperitoneal approach, this method is a good alternative to the classical posterior approach, enabling very low morbidity.

In vitro biomechanical comparison of an anterior and anterolateral lumbar plate with posterior fixation following single-level anterior lumbar interbody fusion

Object

Anterior lumbar interbody fusion (ALIF) is often supplemented with instrumentation to increase stability in the spine. If anterior plate fixation provided the same stability as posterior pedicle screw fixation (PSF), then a second approach and its associated morbidity could be avoided.

Methods

Seven human cadaveric L4–5 spinal segments were tested under three conditions: ALIF with an anterior plate, ALIF with an anterolateral plate, and ALIF supplemented by PSF. Range of motion (ROM) was calculated for flexion/extension, lateral bending, and axial torsion and compared among the three configurations.

Results

There were no significant differences in ROM during flexion/extension, lateral bending, or axial torsion among any of the three instrumentation configurations.

Conclusions

The addition of an anterior plate or posterior PS/rod instrumentation following ALIF provides substantially equivalent biomechanical stability. Additionally, the position of the plate system, either anterior or anterolateral, does not significantly affect the stability gained.

Stand-alone anterior lumbar discectomy and fusion with plate: initial experience

BACKGROUND

The stability of the lumbar spine after ALIF with lateral plate fixation and/or posterior fixation has previously been investigated; however, stand-alone ALDF with plate has not. Previous clinical studies have demonstrated poor fusion rates with stand-alone anterior interbody fusion in the absence of posterior instrumentation. We review our initial experience with stand-alone ALDF with segmental plate fixation for degenerative disc disease of the lumbar spine and compare these results with our experience with traditional ALIF and supplemental posterior instrumentation.

METHODS

Forty-nine patients treated at the University of California, San Francisco between 2002 and 2005 were included in this analysis. The study was retrospective in nature. All patients presented with discogram-positive back pain and had failed conservative treatment. Twenty-four patients underwent ALDF with plate, and 25 underwent ALIF with posterior instrumentation. Patients underwent flexion/extension imaging at 6 weeks, 3 months, 6 months, and 1 year postoperatively. All patients completed ODI and VAS questionnaires at 3 months, 6 months, and 1 year postoperatively.

RESULTS

Average follow-up was 11.6 and 21.7 months in the ALDF with plate and ALIF with instrumentation groups, respectively. All patients demonstrated radiographic evidence of fusion at last follow-up. None developed instability at the fusion level, and none developed hardware failure (plate back-out, screw lucency, etc). Average subsidence at 6 months postoperatively was 2.2 and 2.5 mm, respectively. The VAS and ODI scores are presented in Tables 3 and 4.

CONCLUSIONS

Preliminary results of stand-alone ALDF with plate suggest it may be safe and effective for the surgical treatment of patients with degenerative disc disease of the lumbar spine. Long-term follow-up is clearly needed. Subsidence is diminished with ALDF and plating compared with ALIF with posterior instrumentation. It is unclear at this time which subset of patients may ultimately require posterior hardware supplementation, but those with circumferential stenosis or severe facet disease are not ideal candidates for ALDF with plate. For some patients in whom lumbar arthroplasty is not indicated, or as a salvage procedure, ALDF with plate may be a satisfactory alternative and may eliminate the need for a supplemental posterior procedure.

Augmentation of an anterior lumbar interbody fusion with an anterior plate or pedicle screw fixation: a comparative biomechanical in vitro study

OBJECT

Posterior pedicle screw (PS) instrumentation is often used to augment anterior lumbar interbody fusion (ALIF) but at the cost of an increase in the morbidity rate due to the second approach and screw placement. If anterior plates were found to be biomechanically equivalent to PS fixation (PSF) after ALIF, then this second approach could be avoided without decreasing vertebral stability.

METHODS

Eight cadaveric L5-S1 spinal segments were tested under four conditions: intact, following anterior discectomy and interbody spacer placement, after placement of an anterior plate, and following PSF. The elastic zone and stiffness were calculated for axial compression, flexion/extension, lateral bending, and torsion. Neither anterior plate stabilization nor PSF showed significant intergroup differences in stiffness or the elastic zone. Both exhibited greater stiffness in flexion than the intact specimens (p < 0.001). Pedicle screw fixation was associated with a decreased elastic zone in lateral bending compared with the intact specimen (p < 0.04).

CONCLUSIONS

Anterior plate fixation is biomechanically similar to PSF following ALIF. Surgeons may wish to use anterior plates in place of PSs to avoid the need for a posterior procedure. This may lead to a decrease in operative morbidity and improved overall outcomes.

Video-assisted ALIF with cage and anterior plate fixation for L5-S1 spondylolisthesis

INTRODUCTION

Spondylolysis and spondylolisthesis grade 0, 1, and 2 are mainly asymptomatic but with aging process and different factors some back pain can occur and lead to chronic low back pain. The conservative treatment with physiotherapy and steroid injection is the gold standard but in some cases is not efficient enough and a surgical treatment is proposed.

OBJECTIVES

The goal of this study is to propose a new technique to treat grade 0, 1, and 2 spondylolisthesis with an anterior video-assisted fusion and stabilization.

METHODS

Twenty patients with chronic low back pain since more than 2 years and resistant to conservative therapy were included in this protocol. Clinical signs and radicular pain were noted. They were evaluated preoperatively and postoperatively until the last follow up using Oswestry score and visual analog score (VAS) for leg and back pain. X-rays showed grade 0 (8 cases), 1 (10 cases), and 2 (2 cases) spondylolisthesis according to Meyerding classification with disc collapse (bulging disc). MRI showed in all cases a disc degeneration with at least black disc and/or endplates changes with Modic I or II. All patients were operated using an anterior video-assisted retroperitoneal approach, with discectomy and fusion using an anterior impacted cage filled with autologous cancellous bone from the iliac crest and an anterior fixation with a triangular plate (Pyramid, Medtronic, Memphis). The follow up at 3, 6, 12, and 24 months was done with clinical and radiologic evaluation. In case of problem a computed tomography scan was performed.

RESULTS

There were 11 women and 9 men, with and average age of 39 years old and a BMI of 25.6. All spondylolistheses occurred at level L5. The average slippage was 19%. All L5S1 discs were black, 8 had a Modic I changes in the endplates and 2 had Modic II. The shape of L5 vertebra was abnormal (trapezoidal) in 7 cases. All anterior approaches were performed without vascular, urologic, or digestive complication. Blood loss was inferior to 100 mL. All patients had a soft brace for 8 weeks postoperatively. There was no retrograde ejaculation for the 9 men and no sexual dysfunction reported by the women. One patient had no pain relief and was reoperated for posterior pedicular screw fixation. It was obvious that there was a pseudarthrosis even after the posterior fixation and an anterior transperitoneal revision was performed with the removal of the interbody device and iliac crest bone graft packing alone. A propioni bacterium acnes germ was found responsible for the anterior nonunion. This revision surgery with antibiotics treatment was successful. One of the patients with grade 2 had an additional posterior screw fixation with a minimally invasive pedicle screw system (Sextant, Medtronic, Memphis). Nineteen patients had a good fusion at 2 years follow-up (95%), mean Oswestry score improved from 74% preoperative to 21% postoperative at the last follow-up. Visual analog score (VAS) for back pain improved from 6.5 to 2.7 and VAS for leg pain improved from 6.2 to 3.4. Satisfaction rate was 90%. All active patients except two, were back to work at an average of 5.5 months (6 wk to 1 y). The 2 patients still not working were the nonunion and a work compensation.

CONCLUSIONS

The results of this technique compare favorably with posterior stabilization and fusion (posterior lumbar interbody fusion and postero-lateral fusion) reported in the literature. Unlike posterior lumbar interbody fusion, however, it seems that the complication rate due to the approach is much lower, the fusion rate is similar. Grade 2 SPL is the limitation of the technique. The main advantage of the technique is to avoid posterior muscle damage and a quick recovery with no blood loss. Preservation of adjacent level disease can be assessed only after long-term follow-up.

Two column lesions in the thoracolumbar junction: anterior, posterior or combined approach? A comparative biomechanical in vitro investigation

There are various surgical techniques for the treatment of spinal fractures in the thoracolumbar region. Several implants have been developed for anterior or posterior instrumentation. Optimal treatment of unstable thoracolumbar osseous and ligamentous injuries remains controversial. To compare the stabilizing effects of an antero-lateral, thoracoscopically implantable plate system (macsTL, Aesculap, Germany) with the stability provided by a fixateur interne (SOCON, Aesculap, Germany), this in vitro investigation examined six human bisegmental (T12-L2) spinal units. Specimens were tested intact, and with simulation of osseous lesions in the anterior and ligamentous lesions in the posterior column (combined A/B-fracture). While loaded in the main anatomical planes such as flexion/extension, left and right lateral bending and left and right axial rotation with a bending moment of 7.5 Nm in a special testing jigs, motion analysis was performed. Quantitative interpretation of the stabilizing effect was achieved using a contactless three-dimensional motion analysis system. Each specimen was tested in four different scenarios: the first step measured movements of intact spinal segments. For the second step, specimens underwent simulation of combined A/B-fracture provided with bisegmental (T12/L2) antero-lateral fixation and bone strut graft from the iliac crest. For the third step, segments were additionally stabilized by the fixateur interne. The last measurement (fourth step) was performed after removing the anterior instrumentation. Range of motion (ROM) values were compared and statistically evaluated. Compared to the intact specimens the anterior instrumentation of the combined lesion, simulated A/B-fracture, leads to a stabilizing effect in flexion/extension and lateral bending. In contrast to these findings the torsional instability increased for the upper segment and bisegmentally. A maximum rigidity, beyond intact values, was registered for each anatomical plane with the combined instrumentation: antero-lateral and fixateur interne. After removing the anterior screw plate system maximum movements, in all segments for flexion/extension and lateral bending, bisegmentally and for the upper segment in axial rotation, were less than ROM values measured with the anterior system only. With respect to these findings a combined ventro-dorsal stabilization procedure should be considered for ligamentous disruptions of the posterior column in combination with A-fractures in the thoracolumbar junction.

Perioperative complications in transforaminal lumbar interbody fusion versus anterior-posterior reconstruction for lumbar disc degeneration and instability

OBJECTIVES

Multiple different approaches are used to treat lumbar degenerative disc disease and spinal instability. Both anterior-posterior (AP) reconstructive surgery and transforaminal lumbar interbody fusion (TLIF) provide a circumferential fusion and are considered reasonable surgical options. The purpose of this study was to quantitatively assess clinical parameters such as surgical blood loss, duration of the procedure, length of hospitalization, and complications for TLIF and AP reconstructive surgery for lumbar fusion.

METHODS

A retrospective analysis was completed on 167 consecutive cases performed between January 2002 and March 2004. TLIF surgical procedure was performed on 124 patients, including 73 minimally invasive and 51 open cases. AP surgery was performed on 43 patients. Patients were treated for painful degenerative disc disease, facet arthropathy, degenerative instability, and spinal stenosis.

RESULTS

The mean operative time for AP reconstruction was 455 minutes, for minimally invasive TLIF 255 minutes, and open TLIF 222 minutes. The mean blood loss for AP fusion surgery was 550 mL, for minimally invasive TLIF 231 mL, and open TLIF 424 mL. The mean hospitalization time for AP reconstruction was 7.2 days, for minimally invasive TLIF 3.1 days, and open TLIF 4.1 days. The total rate of complications was 76.7% for AP reconstruction, including 62.8% major and 13.9% minor complications. The minimally invasive TLIF patients group had the total 30.1% rate of complications, 21.9% of which were minor and 8.2% major complications. There were no major complications in the open TLIF patients group, with 35.3% minor complications.

CONCLUSIONS

AP lumbar interbody fusion surgery is associated with a more than two times higher complication rate, significantly increased blood loss, and longer operative and hospitalization times than both percutaneous and open TLIF for lumbar disc degeneration and instability.

Primary spinal segment stability with a stand-alone cage: in vitro evaluation of a successful goat model

BACKGROUND

Interbody cages have been developed to restore disk height and to increase stability of the spinal segment, and thereby enhance fusion. However, they often prove inadequate as a stand-alone device. It is unknown how much primary stability is required to facilitate fusion. In various goat studies, we have obtained spinal fusion routinely with a stand-alone cage device. However, data covering the mechanical conditions under which these fusions have been obtained are lacking. In this study, we addressed the issue of primary stability.

METHODS

We used an established goat model for spinal fusion in vitro. 48 native lumbar spine segments were mechanically tested in flexion/extension, axial torsion (left/right), anterior/posterior shear, and left/right lateral bending. Then all segments were provided with a titanium cage using the exact surgical procedure of our earlier in vivo studies, and the mechanical tests were repeated. Under shear force and axial torsion, a significant loss of stiffness was seen in the operated segments as compared to nonoperated controls. No increase in stiffness was found in any of the loading directions.

INTERPRETATION

Cage implantation in a lumbar spinal segment does not increase immediate postoperative stability as compared to the native segment in this goat model. This is attributable to both the annular damage during cage implantation and the subsequent loss of segment height. Yet previous in vivo studies using this goat model have generally shown fusion. This implies that high primary segment stability is not required for fusion or, alternatively, that the tested range of motion of the spinal segment in vitro does not occur at these magnitudes in vivo.

Biomechanical assessment of anterior lumbar interbody fusion with an anterior lumbosacral fixation screw-plate: comparison to stand-alone anterior lumbar interbody fusion and anterior lumbar interbody fusion with pedicle screws in an unstable human cadaver model

STUDY DESIGN

Human lumbosacral cadaveric specimens were tested in an in vitro biomechanical flexibility experiment using physiologic loads in 5 sequential conditions.

OBJECTIVE

To determine the biomechanical differences between anterior lumbar interbody fusion (ALIF) using cylindrical threaded cages alone or supplemented with an anterior screw-plate or posterior pedicle screws-rods.

SUMMARY OF BACKGROUND DATA

Clinically and biomechanically, stand-alone ALIF performs modestly in immobilizing the unstable spine. Pedicle screws improve fixation stiffness significantly, but supplementary anterior instrumentation has not been studied.

METHODS

There were 7 specimens tested: (1) intact, (2) after discectomy and facetectomy to induce moderate rotational and translational hypermobility, (3) with 2 parallel ALIF cages, (4) with cages plus a triangular anterior screw-plate, and (5) with cages plus pedicle screws-rods. Pure moments without preload induced flexion, extension, lateral bending, and axial rotation; linear shear forces induced anteroposterior translation. Angular and linear motions were measured stereophotogrammetrically, and range of motion (ROM) and stiffness were quantified.

RESULTS

Compared to the destabilized spine, interbody cages alone reduced ROM by 77% during flexion, 53% during extension, 60% during lateral bending, 69% during axial rotation, and 71% during anteroposterior shear (P < 0.001, analysis of variance/Fisher least significant difference). Addition of an anterior plate or pedicle screws-rods, respectively, further reduced ROM by 8% or 13% during flexion (P = 0.21), 21% or 28% during extension (P = 0.15), 5% or 25% during lateral bending (P = 0.04), 11% or 18% during axial rotation (P = 0.13), and 18% or 18% during anteroposterior shear (P = 0.17). Compared to stand-alone ALIF, both the anterior screw-plate and pedicle screw-rod fixation reduced vertebral ROM to less than 1.2 degrees of rotation and less than 0.1 mm of translation.

CONCLUSIONS

The anterior screw-plate and pedicle screws-rods both substantially reduced ROM and increased stiffness compared to stand-alone interbody cages. There was no significant difference in the amount by which the supplementary fixation devices limited flexion, extension, axial rotation, or anteroposterior shear; pedicle screws-rods better restricted lateral bending.

Biomechanical evaluation of a novel lumbosacral axial fixation device

BACKGROUND

Interbody arthrodesis is employed in the lumbar spine to eliminate painful motion and achieve stability through bony fusion. Bone grafts, metal cages, composite spacers, and growth factors are available and can be placed through traditional open techniques or minimally invasively. Whether placed anteriorly, posteriorly, or laterally, insertion of these implants necessitates compromise of the anulus--an inherently destabilizing procedure. A new axial percutaneous approach to the lumbosacral spine has been described. Using this technique, vertical access to the lumbosacral spine is achieved percutaneously via the presacral space. An implant that can be placed across a motion segment without compromise to the anulus avoids surgical destabilization and may be advantageous for interbody arthrodesis. The purpose of this study was to evaluate the in vitro biomechanical performance of the axial fixation rod, an anulus sparing, centrally placed interbody fusion implant for motion segment stabilization.

METHOD OF APPROACH

Twenty-four bovine lumbar motion segments were mechanically tested using an unconstrainedflexibility protocol in sagittal and lateral bending, and torsion. Motion segments were also tested in axial compression. Each specimen was tested in an intact state, then drilled (simulating a transaxial approach to the lumbosacral spine), then with one of two axial fixation rods placed in the spine for stabilization. The range of motion, bending stiffness, and axial compressive stiffness were determined for each test condition. Results were compared to those previously reported for femoral ring allografts, bone dowels, BAK and BAK Proximity cages, Ray TFC, Brantigan ALIF and TLIF implants, the InFix Device, Danek TIBFD, single and double Harms cages, and Kaneda, Isola, and University plating systems.

RESULTS

While axial drilling of specimens had little effect on stiffness and range of motion, specimens implanted with the axial fixation rod exhibited significant increases in stiffness and decreases in range of motion relative to intact state. When compared to existing anterior, posterior, and interbody instrumentation, lateral and sagittal bending stiffness of the axial fixation rod exceeded that of all other interbody devices, while stiffness in extension and axial compression were comparable to plate and rod constructs. Torsional stiffness was comparable to other interbody constructs and slightly lower than plate and rod constructs.

CONCLUSIONS

For stabilization of the L5-S1 motion segment, axial placement of implants offers potential benefits relative to traditional exposures. The preliminary biomechanical data from this study indicate that the axial fixation rod compares favorably to other devices and may be suitable to reduce pathologic motion at L5-S1, thus promoting bony fusion.