A Review of Cervical Disc Arthroplasty Biomechanics

Background: Adjacent segment degeneration (ASD) occurs after anterior cervical discectomy and fusion (ACDF) at a rate of 2.9% per level per year. Cervical disc arthroplasty was developed to theoretically reduce the transfer of forces onto adjacent levels while allowing motion at adjacent sites. We present a review of the biomechanics of cervical disc arthroplasty. Methods: A systematic review of the literature was performed by three junior authors regarding the biomechanics and/or the alteration of the biomechanics following a cervical disc arthroplasty. PubMed and Medline databases were used. Keywords for search results utilized included; cervical disc arthroplasty, adjacent segment disease, cervical disc biomechanics, cervical arthroplasty biomechanics. Level IV evidence or higher was utilized for the purposes of this review. Both articles of historical interest on the subject and articles of recent publication were included in the analysis. Articles included in the analysis were published from 1965 to present. 70% of the 50 articles were chosen for inclusion in the analysis. The reviewers consisted of one experienced fellowship trained orthopedic spine surgeon, one junior fellowship trained orthopedic spine surgeon, one current spine surgery fellow, and one current orthopedic surgery resident. Exclusions were left at the discretion of the senior author. Results: Biomechanics studies suggest that cervical disc arthroplasty more accurately mimics normal range of motion (ROM) and kinematics at the native vertebral functional unit. Some studies suggest that this will delay adjacent segment disease but this has not yet been proven in long term followup. In terms of patient outcomes, early results of cervical disc arthroplasty are at minimum non inferior to ACDF. Emerging evidence suggest higher patient reported outcome scores with CDA. Conclusions: The literature reviewed in this review suggests that cervical disc arthroplasty more accurately mimics native kinematics and ROM at the functional spine unit. CDA is at least non inferior to anterior cervical discectomy and fusion and may in fact be superior to ACDF in recent patient centered outcome studies. Long term studies are ongoing and will help elucidate whether CDA reduces adjacent segment disease and results in lower rates of revision surgeries.