16. Likelihood of achieving physiological range of motion after cervical disc arthroplasty: analysis of IDE clinical trial data

BACKGROUND CONTEXT The functional goals of cervical disc arthroplasty (CDA) are to restore enough range of motion (ROM) to reduce the risk of accelerated adjacent segment degeneration, and limit excessive motion to maintain a biomechanically stable index segment—a motion-range termed the “physiological motion-range.” Mobility and stability are two essential functional requirements which allow a spinal segment to function in harmony with its neighboring segments. Yet, clinical studies do not report what proportion of reconstructed segments yield ROM in the physiological motion-range following CDA surgery. PURPOSE To calculate what proportion of reconstructed segments yield flexion-extension ROM (FE-ROM) in the physiological motion-tange (defined as 5°-16°) by analyzing the 24-month postoperative FE-ROM data reported by clinical trials of various cervical disc prostheses. STUDY DESIGN/SETTING Retrospective analysis of 24-month postoperative FE-ROM data from clinical trials. PATIENT SAMPLE Data from 995 patients from 1-level disc replacement clinical trials of 6 cervical disc prostheses. OUTCOME MEASURES Postoperative index-level FE-ROM. METHODS We analyzed 24-month post-CDA FE-ROM data from 995 patients reported in the 1-level IDE clinical trials of 6 cervical disc prostheses that were available in the public domain (ProDisc-C, Prestige, Bryan, Mobi-C, Secure-C, and M6-C). The reported data allowed calculation of the proportion of implanted levels with postoperative FE-ROM in the 0-4°, 5°-16°, and 17° motion-ranges. Preoperative motion data were not available. The lower bound of the physiological range, defined as 5°, was based on the clinical observations of significantly reduced incidence of progressive radiographic adjacent level degeneration in patients with 5° or greater ROM after CDA.The upper bound of 16° was based on laboratory data from 133 C5-C6 and C6-C7 segments from 102 cervical spines with mild-to-moderate degeneration evaluated in our laboratory, which showed a mean FE-ROM of 12.1°±4.0°. RESULTS Data from 6 clinical trials showed that 655/995 implanted levels yielded post-CDA ROM in the 5°-16° range. The proportions ranged 62%-79% across the 6 disc prostheses, with an average of 65.8±6.3%. 234/995 implanted levels yielded ROM below the 5° lower-bound. The proportions ranged 17%-34% over the 6 disc prostheses with an average of 23.5±7.3%. Of the 995 implanted levels, 106 yielded ROM of 17° with an average proportion of 10.7±7.6% and a wide range 3%-21% over the 6 disc prostheses. The type of cervical disc prosthesis significantly influenced the likelihood of achieving FE-ROM in the 5°-16° range, while avoiding hypomobility (ROM<5°) or hypermobility (ROM17°) (P