Purpose: At present there is no reported, valid and reproducible model of degenerative spondylolisthesis for biomechanical testing of spinal implants. The purpose of this study was to create a single functional spinal unit (FSU) model that could demonstrate anterolisthesis consistent with low grade degenerative spondylolisthesis under physiologic shear loads.

Method: Eight fresh-frozen human cadaveric, lumbar FSU’s were potted and secured in a custom jig for pure shear testing. The cranial segment was loaded from – 50N (posterior) to 250N (anterior) over three cycles for each of five test conditions with a 300N preload. Test conditions addressed known restraints to shear translation and were performed in the same order for all specimens, and included: intact, facet capsulectomy and bilateral two mm facet gap, bilateral four mm facet gap, nucleotomy, and annular release. Three-dimensional motion was recorded using an optoelectronic camera system.

Results: Mean anterior translation at 250N for the five test conditions was 0.7 mm (95% confidence interval 0.4 to 0.9), 1.2 mm (0.9 to 1.6), 1.5 mm (1.1 to 2.0), 1.9 mm (1.4 to 2.4) and 3.1 mm (2.2 to 4.0). The mean maximum anterior translation was significantly different for each test condition with two exceptions. The four mm facet gap did not result in a significantly different maximum anterior translation compared to the two mm facet gap or the nucleotomy. There were no differences in off-axis motion (lateral or superior-inferior translation, flexion-extension, axial rotation, lateral bending) between the five test conditions.

Conclusion: Anterior translation consistent with low grade degenerative spondylolisthesis was repeatedly demonstrated under physiologic shear loads using this model. All sequential destabilizations preserved anatomy critical for the application of pedicle screw constructs, interbody devices and interspinous spacers. As such, this model is appropriate for biomechanical testing of implants currently used in the treatment of low grade degenerative spondylolisthesis.

Purpose: A long spinal fusion across the thoracolumbar region is sometimes applied in scoliosis. Adjacent level degeneration below these constructs has been documented. Treatment with an artificial disc replacement below the fusion has been proposed to prevent degeneration there. There is currently little data detailing the expected biomechanics of this situation. The objective of this study was to evaluate range of motion (ROM) and helical axis of motion (HAM) changes due to one- and two-level Maverick total disc replacement adjacent to a long spinal fusion.

Method: A multidirectional flexibility testing protocol with compressive follower preload was used to test seven human cadaveric spine specimens (T8-S1). A continuous pure moment ±5.0 Nm was applied in flexion-extension (FE), lateral bending (LB) and axial rotation (AR), with a compressive follower preload of 400 N. The motion of each vertebra was monitored with an optoelectronic camera system. The test was completed for the intact condition and after each surgical technique:

T8-L4 fusion and facet capsulotomy at L4–L5 and L5-S1;

Maverick total disc replacement and fusion with the CD Horizon system was performed. Repeated measures ANOVA was used to analyze changes in ROM and HAM of the L4–L5 and L5-S1 segments.

Results: Following L4-L5 Maverick replacement, L4-L5 ROMs tended to decrease slightly (on average from 6.2°±2.8° to 5.1°±3.8° in FE, 1.1°±1.1° to 0.9°±0.5° in LB and 1.3°±0.9° to 1.0°±0.6° in AR). With two-level Maverick implantation, L5-S1 ROMs tended to increase slightly in FE (from 6.6°±2.6° to 7.1°±3.9°), and to decrease slightly in LB (from 1.5°±0.9° to 1.0°±0.3°) and AR (from 1.5°±1.5° to 1.1°±0.6°), compared to the fused condition. As a trend, HAM location shifted posteriorly in FE and AR, and inferiorly in LB following Maverick replacement. However, neither ROM nor HAM at these two segments showed any significant change due to the implantation of one-or two-level Maverick total disc replacement in any of the three directions.

Conclusion: The present results suggested that lower lumbar segments with Maverick disc replacement exhibited intact-like kinematics in both extent and quality of motion.

Purpose: Recent studies have shown differences in short term spinal cord pathology between spinal column injury mechanisms, such as contusion and fracture-dislocation. Such differences may exist at longer time points, and thus survival studies are needed in the dislocation models. A more in-depth characterization of the dislocation model is needed for development of a mild-moderate cervical spine dislocation model in a rat that is suitable for survival studies. Specifically, our objective in this study was to determine the dislocation displacement that produces initial spinal column failure in a Sprague-Dawley rat model and to validate a consistent injury at the desired dislocation in-vitro and in-vivo.

Method: For the dislocation model, the dorsal ligaments and facets at C4–C5 were removed to mimic the type of posterior element fracture and ligament injury commonly seen in a bilateral fracture-dislocation. C3 and C4 were clamped together and held stationary while the clamp holding C5 and C6 was connected to an electromagnetic actuator and displaced dorsally to produce the injury while force and displacement were recorded. Twenty-eight isolated cervical spine specimens of Sprague-Dawley rats were used to determine dislocation displacement at initial spinal column failure. The C4–C5 segment sustained dislocation (> 3mm) injury at 0.05mm/s (n=11), 100mm/s (n=4) and 1000mm/s (n=13). Initial spinal column failure was defined at with maximum force during the dislocation. A dislocation displacement of 1.4mm was applied to 7 isolated specimens and 4 anesthetized rats at 430mm/s. The spinal column failure was inspected up to 3 days after injury, as well as hemorrhage of spinal cord in-situ.

Results: The dislocation displacement at in-vitro spinal column failure was 0.95mm±0.32 and not significantly different among specimens at the three dislocation speeds. Under a dislocation displacement of 1.4mm, rupture of the C4–C5 disc occurred in all in-vitro (0.67mm±0.38) and in-vivo (0.65mm±0.17) cases. SCI hemorrhage at epicenter was observed in 3 of 4 cases.

Conclusion: The initial spinal column failure in an innovative SCI model occurs at displacement between 0.65mm and 0.95mm. Dislocation displacement of 1.4mm results in spinal column failure consistently and SCI hemorrhage, and may be suitable for survival studies.

The Dynesys is a flexible posterior stabilization system that is designed to preserve intersegmental kinematics and reduce loading at the facet joints. The purpose of this study was to determine if the length of the Dynesys spacer has an effect on range of motion (ROM) at the implanted level. Spacer length was found to significantly affect ROM in all three loading directions with and without a follower preload. The longer spacer increased ROM and the shorter spacer decreased ROM, largely due to differences in segmental compression between the two.

The Dynesys, a flexible posterior stabilization system that provides an alternative to fusion, is designed to preserve intersegmental kinematics and alleviate loading at the facet joints. Recent biomechanical evidence suggests that motion with Dynesys is less than the intact spine (Schmoelz, 2003). The purpose of this investigation was to determine if the length of the Dynesys spacer contributes to differences in range of motion (ROM) at the implanted level.

Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of ±7.5Nm in three directions of loading with and without a follower preload of 600N. Test conditions included: intact, injury at L3-L4, Dynesys at L3-L4 (standard spacer), long spacer (+2mm), and short spacer (−2mm). Intervertebral rotations were measured using an optoelectronic camera. Pressure sensors placed inside the joint capsules measured facet loads. Statistical significance was determined using repeated measures ANOVA.

Spacer length had a significant effect on ROM in all three loading directions with and without a follower preload. Initial contact loads within the facet joints were 150% and 64% that of the standard spacer for the short and long spacer, respectively.

The magnitude of distraction of the segment affects ROM. Shorter spacers increased segmental compression of the intervertebral disc and facet joints and therefore reduced ROM. With a follower preload, the segment is further compressed and ROM is further reduced.

The results contribute to an understanding of the design of such implants and could help guide future research.

Funding: Synos Foundation, Switzerland, National Science and Engineering Research Council of Canada (NSERC)

Please contact author for table or diagram.

Purpose: The objectives of this study were to determine the effect of posterior instrumentation extension and/or cement augmentation on immediate stabilization of the instrumented level and biomechanical changes adjacent to the spinal instrumentation.

Methods: This study was designed for repeated measures comparison, using 12 T9-L3 human cadaveric segments, to test the effects of posterior rod extension and cement augmentation following T11 corpectomy. The spine was stabilized with a vertebral body replacement device and with posterior instrumentation from T10 to T12. The T12 pedicle tracts were over-drilled to simulate loosened screws in an osteoporotic spine. The T10 screws were not over-drilled but cemented so as to keep the superior segments constant. Flexibility tests were first carried out on the intact specimen, followed by 3 randomized surgical conditions without cement and lastly the 3 conditions after cement augmentation. The 3 conditions were: 1) no posterior extension rods to L1, 2) flexible extension rods, and 3) rigid extension rods. A combined testing/analysis protocol that used both the traditional flexibility method and a hybrid technique [Panjabi 2005] was adopted. Flexibility tests with +/−5 Nm pure moments in flexion-extension, axial rotation and lateral bending were carried out and vertebral bodies’ motion in 3-D were collected. Two-way repeated measures ANOVA analyses were carried out on ROM between cement augmentation (factor 1) and the posterior rod extension (factor 2) on each flexibility test direction. An alpha of 0.05 was chosen. Newman-Keuls post-hoc analyses were carried out to compare between surgical techniques.

Results: Using the flexibility protocol, a reduction in ROMs at the destabilized level was observed with cement augmentation of screws or extension with rigid or flexible posterior rods to adjacent distal level. With the hybrid protocol, ROMs at adjacent level (T12-L1) were reduced with rod extension, but not with cement.

Conclusions: The results of this study suggest that cement augmentation would enhance stabilization, but create possible adjacent level effects due to increased motion and strain, while additional flexible extension rods would reduce biomechanical changes at the level of extension. Funding: 2 Funding Parties: CIHR