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To maintain an intervertebral disc space during fusion, a structural allograft, typically utilizing dense cortical bone, must sustain functional loads. Difficulty with surgical placement of structural allografts has resulted in reports of intraoperative graft fracture but no data have been generated to quantify insertional loads. In this study, simulated intraoperative and immediate postoperative strengths of allograft femoral rings were quantified. Three types of processed allograft were used: Tutoplast® processed bone, frozen-thawed and freeze-dried. To replicate postoperative in vivo functional loading, axial compressive strengths were evaluated by crushing femoral rings between flat platens under displacement control. To replicate surgical placement into an intervertebral space, samples were oriented so that the ring would be loaded transversely and an “insertion” displacement was imposed at 1m/sec. In axial compression, all three allograft types sustained loads far greater than estimated in vivo spinal loads. In insertion loading, all allograft types yielded substantially smaller strengths. Because intraoperative insertional loads are unknown and subjective to individual surgeon technique, the adequacy or inadequacy of any group is unclear. Testing guidlelines should be developed upon both anticipated in vivo loading and intraoperative demands.
orthopaedic medical devices — bone, allograft, compressive strength, strength testing
Director, Spine Biomechanics Research, Minneapolis, MN
Vice President, Scientific Affairs, Minneapolis, MN