In Cervical Arthroplasty Only Prostheses with Flexible Biomechanical Properties Should be Used for Achieving a Near-Physiological Motion Pattern

Abstract Background: In cervical arthroplasty, qualitative motion analysis generally investigates the position of the Center of Rotation (COR) before and after surgery. But is the prae-op COR suitable as reference? We believe that only a comparison against healthy individuals can answer whether a physiological motion pattern has been achieved. The aim of our study was to examine how the COR for flexion / extension after insertion of 3 biomechanically completely different types of disc prostheses compares to healthy volunteers, and whether and how prosthesis design contributes to a more natural or maybe even worse motion pattern.Methods: In 15 healthy volunteers MRI in flexion and in extension was taken, and the coordinates for the CORs (COR-HV) from C3 to C7 were determined. Then prae- and post-op flexion-/extension x-rays from 30 patients with a one-level disc prosthesis underwent analysis for determination of COR from C3 to C7; 10 patients who received a Bryan-, a Prestige STLP- or a Discover prosthesis were chosen, respectively. Change of post-op COR position was investigated in relation to the COR-HV.Results: The pre-operative COR is not congruent with the COR found in healthy subjects and therefore cannot be used as reference for investigation whether a disc prosthesis resembles natural motion. However, the comparison with healthy individuals shows that Prosthesis-insertion can change the coordinates of the COR to any direction in all levels from C3/4 to C6/7 regardless of the operated segment. Prostheses with flexible biomechanical properties can contribute to shift the COR towards normal, but devices with unphysiological biomechanical design, like fixed ball-socket designs, for instance, can make the motion pattern even worse.Conclusions: Even if the small cohorts in our study do not allow strong conclusions, it seems that in cervical arthroplasty, the biomechanical concept of the prosthesis has a significant impact whether a near-physiological motion pattern can be achieved or not. As it is rumor but not scientifically proofed that prosthesis-design has no influence on clinical outcome, surgeons should only choose devices with flexible biomechanical properties for disc replacement.