Influence of 1-Level Versus 2-Level Cervical Disc Replacements and Disc Prosthesis Design on Motion Outcomes 2-Year Postdisc Arthroplasty in 835 Patients From 4 US IDE Clinical Trials

Study Design/Setting: Analysis of 24-month postoperative flexion-extension range of motion (ROM) data from 1-level and 2-level clinical trials of cervical disc arthroplasty (CDA). Objective: To investigate the influence of (a) 1-level versus 2-level implantations, and (b) prosthesis design, on the proportions of reconstructed segments that achieved physiological mobility (5–16 degrees). Summary of Background Data: The functional goals of CDA are to restore enough ROM to reduce the risk of accelerated adjacent segment degeneration but also limit excessive motion to obtain a biomechanically stable segment. This motion range is termed “physiological” and is defined as 5–16 degrees based on scientific evidence. Methods: We analyzed 24-month post-CDA flexion-extension (FE) ROM data from investigational device exemption studies of artificial disc prostheses approved for both 1-level and 2-level use (Mobi-C, Prestige LP, and Simplify). Only the Prestige LP and Mobi-C trials had data presenting how many implanted levels yielded postoperative ROM of a given degree, and only these 2 discs are included in this analysis. Eight hundred thirty-five patients participated in the 1-level and 2-level clinical trials. We calculated the proportion of implanted levels with postoperative FE-ROM in the following motion ranges: hypomobile (0–4 degrees), physiological (5–16 degrees), and hypermobile (≥17 degrees). Results: On average, two-thirds of implanted levels in these clinical trials yielded FE-ROM of 9.5±3.1 degrees (physiological mobility). Twenty-five percent of the levels moved 2.4±1.2 degrees (hypomobility), and 9% had ROM of 19.8±2.9 degrees (hypermobility). Prestige LP disc had significantly fewer patients exhibiting hypermobility at 24 months in both single-level and 2-level cases than the Mobi-C disc ( P <0.01). In 2-level Mobi-C constructs, the superior level was where most of that hypermobility occurred. Conclusions: Two-level arthroplasty did not significantly influence the likelihood of achieving post-CDA motion in the physiological range when compared with 1-level CDA ( P =0.58). Prosthesis design did influence segmental mobility in both 1-level and 2-level constructs. Surgeons may choose to use a specific implant depending on whether a more- or less-constrained design would be best in a specific clinical situation.