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The effect of cage positioning on lumbrosacral vertebral endplate failure in compression Labrom, Robert David


Anterior column cage or graft subsidence remains a biomechanically and clinically serious problem that affects the performance and patient outcome of any spinal arthrodesis surgery. To assess the best position to place an interbody fusion cage, a posterior interbody fusion construct was simulated. Specifically, the hypothesis to be tested was that two smaller, posterolaterally positioned interbody cages would provide superior construct stiffness and strength in compression. Nine human cadaver spine specimens from L3-S1 were dissected and continuously posteriorly instrumented with pedicle screws and rods. This continuously instrumented construct was then potted in dental cement and plaster of Paris in such a way as to enable sequential individual axial compression testing of each functional spinal unit (FSU) from L3/4 to L5/S1. All specimens were x-rayed, and scanned with DEXA for bone mineral density pre-testing. Stiffness properties of the FSU's with intact disc and without disc were tested. Three patterns of titanium mesh cages were then used to test stiffness and gross failure under compression: one large central, two small central, or two small posterolaterally positioned cages. After digitizing points on the cage and vertebral bodies pre-test, anoptoelectronic camera system was used to track motion of the cage and vertebrae. The compressive stiffness of the construct at all spinal levels was significantly higher with the intact disc compared to without the disc, and with any of the three cage patterns, and these differences were significant. Mean failure loads for the three cage positions ranged between 2000 N and 2500 N and were not significantly different, though tended to be higher for the 2 posterolateral cage position. Mean bone mineral density values for both superior and inferior vertebrae of the FSU tested, were significantly correlated with failure load values, yet did not appear predictive of cage subsidence direction. Motion analysis of the cage- either single or double combinations, revealed no trend for either superior or inferior subsidence into the endplates. Mode of endplate failure appears to involve a mass shear displacement of the underlying trabecular bone, with condensation of the trabecular architecture in the immediate sub-endplate region. Results of this study have supported the biomechanical validity of PLIF and TLIF type surgeries, with the preferred placement of two smaller posterolaterally positioned mesh cages (Harms et al, 1997).

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