Spinal kinematics and facet load transmission after total disc replacement

Study Design: In vitro human cadaveric biomechanical study. Objective: The objectives were to determine the effect of total disc replacement (TDR) on kinematics, especially range of motion (ROM), helical axis of motion (HAM), and facet joint contact force. Summary Of Background Data: Ball-and-socket type artificial discs are designed to mimic normal motion, but the biomechanical effect on kinematics has not been thoroughly clarified. Methods: Fourteen human cadaveric L4-L5 units were tested before and after TDR. In 7 specimens, facet contact forces were directly measured with thin-film piezoresistive load transducers inserted in the facet joints. In the other 7 specimens, the facet joint capsules were kept intact. Moments (±7.5 Nm) were applied in flexion/extension, lateral bending, and axial rotation motion, with and without an axial compressive preload of 400 N. Three-dimensional motion was recorded, and each angular ROM and HAM were calculated. Results: Without axial compressive preload, the TDR did not produce significant differences in ROMs in all cases. However, under compressive preload, the TDR produced significantly larger ROMs for flexion (4.0° and 8.7°) and lateral bending (2.4° and 5.6°) (intact state and TDR, respectively). The TDR did not alter the HAM significantly except the location in lateral bending without compressive preload and the orientation in flexion/extension against horizontal plane. The location of HAM was slightly shifted caudally by the compressive preload in intact and TDR states. Despite the increased ROMs, the facet contact forces were not significantly altered by the TDR either with or without compressive preload (26 N and 27 N in extension, 41 N and 41 N in lateral bending, 117 N and 126 N in axial rotation). Conclusion: TDR using a ball-and-socket type artificial disc significantly increased ROM under axial load and maintained the HAM with similar facet contact forces to the intact state.