Aim of the study: Maintaining segmental motion is one of the most reported theoretical advantages of total disc replacement (TDR). Several inlay sizes are available for reconstruction of the physiological disc height. The influence of the implant height on the range of motion (ROM) was investigated in a biomechanical study.
Methods: A total of 10 human lumbar cadaver spines were subjected to biomechanical testing. Flexion/extension and side-bending moments were applied from 2.5-7.5 Nm on a spine load simulator allowing for all 6 degrees of freedom. Motion under different loads was monitored by the Zebris system in 3 dimensions. Initially intact specimens were tested in 3 load cycles. Then a total disc prothesis was implanted with an 8.5 mm inlay and the cycles were repeated. Finally in 5 cases a 1-mm larger inlay was inserted while in the remaining 5 cases the inlay was exchanged with a 2-mm larger implant. Neutral zone (NZ) and ROM were recorded under the different loads.
Results: The average motion for the various loads showed no significant difference when the intact motion segment was compared to the specimen containing the 8.5-mm inlay. After the larger inlay had been mounted the average reduction of the ROM in flexion/extension was 25% under the load of 7.5 Nm, 26% under a torque of 5.0 Nm and 30% when 2.5 Nm were applied. The NZ was reduced by 37%. For side-bending the ROM was reduced by 21% under a load of 7.5 Nm, by 26% under 5.0 Nm and by 35% under a torque of 2.5 Nm. The NZ was decreased by 27%. The reduction of the ROM was significant (p=0.0057).
Conclusion: Segmental lumbar motion is maintained after TDR. The size of the inlay can significantly change the ROM in lumbar spine segments treated by TDR.