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.

Evidence suggests that femoral neck fractures initiate in the superolateral cortex, where it is significantly thinner in older than younger individuals (Mayhew, et al. Lancet 2005). Thus, we sought to determine the relative time-course of crack initiation and propagation during a simulated hip fracture.

Four unembalmed frozen, human cadaveric specimens (mean age = 78 yrs) were loaded to failure in sideways fall configuration at a rate of 100 mm/sec using a materials testing system. Images of the fracture were captured with two high-speed video cameras at a resolution of 384x384 pixels, and sample rate of 9,111 Hz (frames/second).

Test A: The load-displacement (L-D) curve had three distinct peaks: at the first peak (4390 N), the head and neck rotated slightly. At the second peak (4607 N), a visible local compressive fracture appeared in the superior cortex of the proximal neck. At the third peak (3582 N), a neck-spanning tensile failure occurred in the inferior neck. Test B: At the first and second peak loads (1714 N and 3040 N) fluid was released from the posterior then superior and inferior surfaces. The third peak load (3361 N) corresponded to a local compressive failure in the lateral superior neck, followed by a neck-spanning tensile failure medially. Test C: The L-D curve was linear until ultimate load (3038 N). A compressive crack first appeared on the anterior-superior surface of the neck cortex, then fractured in the inferior neck. Test D: The L-D curve was linear until ultimate load. A small local crack appeared in the superior cortex of the proximal neck at ultimate load (3841 N).

We found that during ex vivo simulations of hip fracture, the femur failed initially in the superior cortex of the neck, and then failed in the inferior cortex. This is the first study to demonstrate, with high speed video data, the location of crack initiation and its propagation. These preliminary data support the hypothesis of Mayhew et al. (Mayhew, et al. Lancet 2005) in terms of fracture development and could relate to clinically relevant fracture types.

We studied various aspects of graft impaction and penetration of cement in an experimental model. Cancellous bone was removed proximally and local diaphyseal lytic defects were simulated in six human cadaver femora. After impaction grafting the specimens were sectioned and prepared for histomorphometric analysis.

The porosity of the graft was lowest in Gruen zone 4 (52%) and highest in Gruen zone 1 (76%). At the levels of Gruen zones 6 and 2 the entire cross-section was almost filled with cement. Cement sometimes reached the endosteal surface in other Gruen zones. The mean peak impaction forces exerted with the impactors were negatively correlated with the porosity of the graft.

We performed a biomechanical study on human cadaver spines to determine the effect of three different interbody cage designs, with and without posterior instrumentation, on the three-dimensional flexibility of the spine. Six lumbar functional spinal units for each cage type were subjected to multidirectional flexibility testing in four different configurations: intact, with interbody cages from a posterior approach, with additional posterior instrumentation, and with cross-bracing. The tests involved the application of flexion and extension, bilateral axial rotation and bilateral lateral bending pure moments. The relative movements between the vertebrae were recorded by an optoelectronic camera system.

We found no significant difference in the stabilising potential of the three cage designs. The cages used alone significantly decreased the intervertebral movement in flexion and lateral bending, but no stabilisation was achieved in either extension or axial rotation. For all types of cage, the greatest stabilisation in flexion and extension and lateral bending was achieved by the addition of posterior transpedicular instrumentation. The addition of cross-bracing to the posterior instrumentation had a stabilising effect on axial rotation. The bone density of the adjacent vertebral bodies was a significant factor for stabilisation in flexion and extension and in lateral bending.