• Paul B Suh, Christian Puttlitz, Chad Lewis, B Sonny Bal, and Kirk McGilvray.
• From the Orthopaedic Specialists of North Carolina, Raleigh, NC (Dr. Suh); the Orthopaedic Bioengineering Research Laboratory, Department of Mechanical Engineering, and School of Biomedical Engineering, Colorado State University, Fort Collins, CO (Dr. Puttlitz and Dr. McGilvray); Amedica, Salt Lake City, UT (Dr. Lewis and Dr. Bal); and the Department of Orthopaedic Surgery, University of Missouri, Columbia, MO (Dr. Bal).
• J Am Acad Orthop Surg. 2017 Feb 1; 25 (2): 160-168.

BackgroundInterbody cages used in spinal fusion surgery can subside into the adjacent vertebral bodies after implantation, leading to loss of spinal height, malalignment, and possible radicular symptoms. Several factors may contribute to cage subsidence.MethodsThis in vitro investigation examined the possible contribution of substrate density, cage contact area (ie, cage footprint), cage filling, cage end plate surface texture, and cage material composition on the magnitude of subsidence. Commercially available cervical interbody cages of two sizes (16 × 12 mm and 17 × 14 mm) were implanted between foam blocks of two different densities and were cyclically loaded. Cages were made of titanium alloy (Ti4Al6V), silicon nitride ceramic (Si3N4), or polyether ether ketone (n = 8 cages of each material type). Additional testing was performed on Si3N4 cages of the smaller size with nontextured surfaces and with filled cores.ResultsSubsidence measurements showed that lower foam density had the greatest influence on subsidence, followed by smaller cage footprint. Cage material had no effect on subsidence. In the additional testing of small-footprint Si3N4 cages, the cages in which the core was filled with a load-bearing porous material had less subsidence in lower-density foam than the cages with an empty core had, whereas cage end plate surface texture had no effect on subsidence.ConclusionRanking of the relative impact of these factors indicated that substrate density had the greatest contribution to the measured subsidence (approximately 1.7 times and approximately 67 times greater than the contributions of cage footprint area and material, respectively). The contribution of cage footprint area to subsidence was found to be 40 times greater than the contribution of cage material to subsidence.

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