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The Effect of Anterior-Posterior Shear on the Wear of CHARITÉ Total Disc Replacement

Study Design. An in vitro study of the wear rates of the CHARITÉ lumbar total disc replacement (TDR). Objective. To investigate the effect of anterior-posterior shear on the in vitro wear rates of the CHARITÉ lumbar TDR. Summary of Background Data. Current standards prescribe only 4-degrees of freedom (DOF) inputs for evaluating the in vitro wear of TDRs, despite the functional spinal unit incorporating 6 DOF. Anterior-posterior shear has been highlighted as a significant load, particularly in the lumbar spine. A previous study investigated the effect of an anterior-posterior shear on the ProDisc-L, finding that wear rates were not significantly different from 4-DOF wear tests. Methods. Six CHARITÉ lumbar discs were mounted in a 5 active DOF spine wear simulator and tested under 4-DOF (ISO18192) conditions. Six further CHARITÉ lumbar discs were tested under 5-DOF conditions, consisting of 4-DOF conditions plus an anterior-posterior shear displacement of +2/–1.5 mm. The displacement was decreased and then increased by a factor of 2 to investigate the effect of the magnitude of displacement. Micro-computed tomographic scans of the discs were taken before and after wear testing, and the height loss of the discs was calculated. These were compared with the same measurements taken from explanted CHARITÉ discs, micro-computed tomography scanned at another institution. Results. Wear rates for 4 DOF (12.2 ± 1.0 mg/MC) were not significantly different from 4-DOF tests on the ProDisc-L. Wear rates were significantly increased (P < 0.01) for “standard” 5-DOF conditions (22.3 ± 2.0 mg/MC), decreased 5 DOF (24.3 ± 4.9 mg/MC), and increased 5 DOF (29.1 ± 7.6 mg/MC). The height loss of the explants and in vitro tested discs were not significantly different (P > 0.05). Conclusion. The addition of anterior-posterior shear to wear testing inputs of the CHARITÉ lumbar TDR increases the wear rate significantly, which is in direct contrast to the previous 5-DOF testing on the ProDisc. This study highlights the importance of clinically relevant testing regimens, and that test inputs may be different for dissimilar design philosophies.