Comparison of Artificial Vertebral Body Analogs to Evaluate Initial Stability of Cervical Disc Replacements

Recent studies have raised concerns regarding migration of cervical disc replacements as a significant clinical complication associated with failure. To date, no laboratory models have addressed migration. Bone analog models have been established for fixation studies of large joint replacements. Therefore, this study aimed to develop models to assess micromotions of cervical disc replacements. Five cervical disc replacement designs were biomechanically tested in flexion/extension, lateral bending, and axial rotation. These were selected to represent different clinical outcomes, including some with significant in vivo migration. Each device was tested in a (1) previously validated 3D-printed biomimetic model and (2) commercially available rigid polyurethane foam blocks. Sagittal and coronal plane micromotions were continuously measured throughout testing. Cyclic displacements were compared as a function of device design and bone analog model type. One ball-and-socket cervical device, the PCM, exhibited significantly greater micromotion in the polyurethane foam model than in the 3D-printed biomimetic model during flexion-extension and lateral bending, specifically 25.8 ± 11.4 µM versus 15.0 ± 9.5 µM (p = 0.04) and 122 ± 64 µM versus 14.5 ± 6.4 µM (p = 0.06), respectively. The large amount of micromotion with the PCM device design was consistent with clinical reports of migration leading to failure. In contrast, motions measured in the 3D-printed biomimetic model did not establish the same differences. In summary, the polyurethane foam model indicated differences between devices better in comparison to the 3D-printed biomimetic model. However, the 3D-printed model has greater potential for further material refinements to more precisely predict clinical performance with better simulation of bone mechanical properties.