[Impact of artificial disc height on the morphology of cervical facet joint]

Objective: To investigate the impact of implant height on the morphology of cervical facet joints in artificial cervical disc replacement (ACDR). Methods: A finite element model of the cervical spine from C(2) to C(7) was constructed. The surgical processes of ACDR were simulated by insertion of 5 mm, 6 mm or 7 mm Prestige-LP artificial cervical discs. The morphological changes of cervical facet joints were recorded in terms of facet overlap distance, facet overlap area and facet angle. Results: The normal height Prestige-LP cervical disc could restore the natural status of the cervical spine. Compared with the intact condition, the model with ≥2 mm height implant would reduce the overall range of motion (ROM) by 19.2% in flexion, 19.4% in extension, 18.4% in lateral bending and 19.0% in axial rotation. Besides, the ROM at the surgical level reduced by 15.8% in flexion, 15.4% in extension and lateral bending and 19.0% in rotation. The upper and lower adjacent levels (C(4/5) and C(5/6)) showed more obvious changes in ROM, which reduced by 26.8% and 26.5% in flexion, 26.6% in extension, 26.5% and 26.4% in lateral bending, and 26.4% and 26.3% in rotation, respectively. The reduction of facet overlap distance, facet angle and facet overlap area with the ≥2 mm height artifact was 9.6%, 30.7%, and 17.9% respectively at the upper adjacent level, and 22.3%, 35.3% and 28.2% respectively at the surgical level, compared with the intact model.The morphology of facet joints at the lower adjacent level were slightly changed. Conclusion: The implant height selection in ACDR should be in accordance with the physiological intervertebral disc height. Artifact with ≥2 mm in height would obviously change the morphology of facet joints, thus should not be used in clinical practice.