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Disc degeneration is a natural process and is widely prevalent. The severity of disc degeneration and the type of treatment varies from person to person. Fusion is a commonly chosen treatment option. However, clinical and biomechanical studies have shown that intervertebral discs adjacent to a fusion experience increased motion and higher stress which may lead to adjacent-segment disease. Cervical disc arthroplasty achieves similar decompression of the neural elements, but preserves the motion at the operated level and may potentially decrease the occurrence of adjacent segment degeneration.
Computationally, a validated intact 3D finite element model of the cervical spine (C2-T1) was modified to simulate single (C5-C6) and bi-level (C5-C7) degeneration. The single level degenerative model was modified to simulate single level fusion and arthroplasty with the Bryan and Prestige LP artificial discs. The bi-level degenerative model was modified to simulate a bi-level fusion, bi-level arthroplasty with Bryan and Prestige LP discs and a disc replacement adjacent to fusion.
An in-vitro biomechanical study was also conducted to address the effects of arthroplasty and fusion on the kinematics of the cervical spine. A total of 11 specimens (C2-T1) were divided into two groups (Bryan and Prestige LP). The specimens were tested in the following order; intact, single level TDR at C5-C6, bi-level TDR C5-C6-C7, fusion at C5-C6 and TDR at C6-C7 (Hybrid construct) and finally a bi-level fusion. The intact state was tested up to a moment of 2Nm. After surgical intervention, the specimens were loaded until the primary motion (C2-T1) matched the motion of intact state (hybrid control).
In all cases; computational and experimental, an arthroplasty preserved motion at the implanted level and maintained normal motion at the nonoperative levels. A fusion, on the other hand, resulted in a significant decrease in motion at the fused level and an increase in motion at the un-fused levels. In the hybrid construct, the TDR adjacent to fusion preserved motion at that level, thus reducing the demand on the other levels.
The computational models were used to analyze disc stresses at the adjacent levels and facet forces at the index and adjacent levels. The disc stresses followed the same trends as motion. Facet forces though, increased considerably at the index level following a TDR. There was a decrease in facet forces however at the adjacent levels. The adjacent level facet forces increased considerably with a fusion. The hybrid construct had adjacent level facet forces between the bi-level TDR and bi-level fusion models.
To conclude, this study highlighted that cervical disc replacement with both the Bryan and Prestige LP discs not only preserved the motion at the operated level, but also maintained the normal motion at the adjacent levels. Under hybrid loading, the motion pattern of the spine with a TDR was closer to the intact motion pattern, as compared to the degenerative or fusion models. Also, in the presence of a pre-existing fusion, this study shows that an adjacent segment disc replacement is preferable to a second fusion.