Likelihood of Achieving Physiological Mobility After Cervical Disc Arthroplasty—Assessing Physiological Motion Restoration vs Physiological Motion Preservation

Background A previous study investigated the likelihood of cervical artificial discs achieving post-cervical disc arthroplasty (CDA) range of motion (ROM) in the physiological range, defined as 5° to 16°. Here, we delve deeper into the capability of a disc prosthesis to restore physiological motion when the patient is preoperatively hypomobile (ROM < 5°) or hypermobile (ROM > 16°), and capability to preserve physiological mobility when the preoperative ROM falls in the 5° to 16° range. Methods This is a secondary analysis of prospectively collected radiographic data from 131 patients who participated in a 1-level clinical trial of the M6-C (Orthofix) artificial disc. We chose this prosthesis for demonstration purposes due to the availability of pre- and postoperative flexion-extension ROM data on each individual patient. This allowed calculation of the number of implanted levels with preoperative and postoperative flexion-extension ROM in the following motion ranges: hypomobile (ROM < 5°), physiological mobility (ROM: 5°–16°), and hypermobile (ROM > 16°), resulting in a preoperative vs postoperative 3 × 3 mobility matrix. Results Preoperatively, 35 of the 131 implanted levels were hypomobile (ROM: 2.2° ± 1.5°). Of these, physiological ROM was restored to 26 implanted segments by 2 years after CDA (ROM: 7.6° ± 2.0°). A total of 89 of 131 levels had physiological mobility preoperatively (ROM: 9.1° ± 3.3°). Of these, 73 segments preserved their physiological range 2 years after CDA (ROM: 10.2° ± 3.1°). Seven of 131 segments were hypermobile preoperatively (ROM: 18.5° ± 0.9°). Of these, physiological motion was restored to 4 segments by 2 years after CDA (10.9° ± 1.4°). Since the group of patients with preoperative hypermobility was small (n = 7), we combined preoperatively hypo- and hypermobile groups in calculating the restoration rate, resulting in a 2 × 2 mobility matrix. The calculated physiological motion restoration rate was 71% (30/[35 + 7] = 0.71), and preservation rate was 82% (73/89 = 0.82). The combined result was that the prosthesis had a 79% likelihood of yielding physiological mobility at 2 years post-CDA follow-up ([73 + 30]/131). Conclusions The ability to restore and preserve physiological motion are 2 distinct mechanisms that combine to yield physiological motion after CDA. The cervical disc prosthesis analyzed in this study restored physiological motion to 71% of segments with preoperatively nonphysiological (hypomobile plus hypermobile) ROM, while preserving physiological mobility in 82% of segments with preoperative ROM in the physiological range. Clinical Relevance Knowing the likelihood of achieving an ideal postoperative motion outcome for a range of preoperative mobilities can help in guiding patient expectations about CDA and give clinicians additional information to use during implant selection. Level of Evidence 2.