In cervical arthroplasty, only prosthesis 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 pre-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 pre- 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 toward 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 a rumor but not scientifically proven that prosthesis design has no influence on clinical outcome, surgeons should only choose devices with flexible biomechanical properties for disc replacement.

Cervical disc prostheses need a variable center of rotation for flexion / extension below disc level, plus a separate COR for lateral bending above disc level to more closely replicate in-vivo motion: MRI-based biomechanical in-vivo study

“…Some have one fixed COR below the motion segment (ProDisc C, DePuy-Synthes; PCM, Cervitech; Discover, DePuy-Synthes), others a COR above the motion segment level with capability for additional sagittal translation (Prestige STLP, Medtronic), again others a variable COR (Bryan, Medtronic) or no articulating surfaces at all but an elastic nucleus (M6, Spinal Kinetics). An analysis of the motion pattern after insertion of a Bryan-, a Prestige STLP or a Discover prosthesis in patients after cervical discectomy could show that a more flexible biomechanical design can contribute to a better physiological motion [1].…”

Mid-long-term follow-up of operated level kinematics after single-level artificial cervical disc replacement with Bryan disc

“…There are some limitations in this study. First, the location of COR at the level with cervical disc degeneration will change, and the preoperative COR may not represent the physiological COR [45]. Second, our analysis was limited to data collected at two time points: preoperation and the last follow-up.…”

Long-term follow-up MRI shows no hastening of adjacent segment degeneration following cervical disc arthroplasty

“…However, biomechanical characteristics and their influence on clinical outcome have not been assessed thoroughly so far. Muhlbauer et al recommended that only prostheses with flexible biomechanical properties should be used in clinical practice 29 . The MRI assessment showed that progressive degeneration most often occurred at the cranial adjacent segment.…”