Prosthesis design and likelihood of achieving physiological range of motion after cervical disc arthroplasty: analysis of range of motion data from 1,173 patients from 7 IDE clinical trials.

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 but limit excessive motion to maintain a biomechanically stable index segment. This motion-range is termed the "Physiological mobility range." Clinical studies report postoperative ROM averaged over all study subjects but they do not report what proportion of reconstructed segments yield ROM in the Physiological mobility range following CDA surgery.

Purpose: To calculate the proportion of reconstructed segments that yield flexion-extension ROM (FE-ROM) in the Physiological mobility range (defined as 5°-16°) by analyzing the 24-month postoperative data reported by clinical trials of various cervical disc prostheses.

Study Design/setting: Analysis of 24-month postoperative FE-ROM data from clinical trials.

Patient Sample: Data from 1,173 patients from single-level disc replacement clinical trials of 7 cervical disc prostheses.

Outcome Measures: 24-month postoperative index-level FE-ROM.

Methods: The FE-ROM histograms reported in Food and Drug Administration-Investigational Device Exemption (FDA-IDE) submissions and available for this analysis were used to calculate the frequencies of implanted levels with postoperative FE-ROM in the following motion-ranges: Hypomobile (0°-4°), Physiological (5°-16°), and Hypermobile (≥17°). The ROM histograms also allowed calculation of the average ROM of implanted segments in each of the 3 motion-ranges.

Results: Only 762 of 1,173 patients (implanted levels) yielded 24-month postCDA FE-ROM in the physiological mobility range (5°-16°). The proportions ranged from 60% to 79% across the 7 disc-prostheses, with an average of 65.0%±6.2%. Three-hundred and two (302) of 1,173 implanted levels yielded ROM in the 0°-4° range. The proportions ranged from 15% to 38% with an average of 25.7%±8.9%. One-hundred and nine (109) of 1,173 implanted levels yielded ROM of ≥17° with a range of 2%-21% and an average proportion of 9.3%±7.9%. The prosthesis with built-in stiffness due to its nucleus-annulus design yielded the highest proportion (103/131, 79%) of implanted segments in the physiological mobility range, compared to the cohort average of 65% (p<.01). Sixty-five of the 350 (18.6%) discs implanted with the 2 mobile-core designs in this cohort yielded ROM≥17° as compared to the cohort average of 9.3% (109/1,173) (p<.05). At 2-year postCDA, the "hypomobile" segments moved on average 2.4±1.2°, those in the "physiological-mobility" group moved 9.4±3.2°, and the hypermobile segments moved 19.6±2.6°.

Conclusions: Prosthesis design significantly influenced the likelihood of achieving FE-ROM in the physiological mobility range, while avoiding hypomobility or hypermobility (p<.01). Postoperative ROM averaged over all study subjects provides incomplete information about the prosthesis performance - it does not tell us how many implanted segments achieve physiological mobility and how many end up with hypomobility or hypermobility. We conclude that the proportion of index levels achieving postCDA motions in the physiological mobility range (5°-16°) is a more useful outcome measure for future clinical trials.