Effect of intervertebral disc height on postoperative motion and clinical outcomes after Prodisc-C cervical disc replacement.

Background Context: Cervical total disc replacement (TDR) is an emerging technology. However, the factors that influence postoperative range of motion (ROM) and patient satisfaction are not fully understood.

Purpose: To evaluate the influence of pre- and postoperative disc height on postoperative motion and clinical outcomes.

Study Design/setting: Retrospective review of patients enrolled in prospective randomized Food and Drug Administration (FDA) trial.

Patient Sample: One hundred sixty-six patients with single-level ProDisc-C arthroplasty performed were evaluated.

Outcome Measures: ROM and clinical outcomes based on Neck Disability Index (NDI) and Visual Analog Scale (VAS) were assessed.

Methods: Preoperative and postoperative disc height and ROM were measured from lateral and flexion-extension radiographs. Student t test and Spearman's rho tests were performed to determine any correlation or "threshold" effect between the disc height and ROM or clinical outcome.

Results: Patients with less than 4mm of preoperative disc height had a mean 1.8 degrees increase in flexion-extension ROM after TDR, whereas patients with greater than 4mm of preoperative disc height had no change (mean, 0 degrees ) in flexion-extension ROM (p=.04). Patients with greater than 5mm of postoperative disc height have significantly higher postoperative flexion-extension ROM (mean, 10.1 degrees ) than those with less than 5mm disc height (mean, 8.3 degrees , p=.014). However, patients with greater than 7mm of postoperative disc height have significantly lower postoperative lateral bending ROM (mean, 4.1 degrees ) than those with less than 7mm disc height (mean, 5.7 degrees , p=.04). It appears that the optimal postoperative disc height is between 5 and 7mm for increased ROM on flexion extension and lateral bending. There was a mean improvement of 30.5 points for NDI, 4.3 points for VAS neck pain score, and 3.9 points for VAS arm pain score (all p<.001). No correlation could be found between clinical outcomes and disc height. Similarly, no threshold effect could be found between any specific disc height and NDI or VAS.

Conclusion: Patients with greater disc collapse of less than 4mm preoperative disc height benefit more in ROM after TDR. The optimal postoperative disc height range to maximize ROM is between 5 and 7mm. This optimal range did not translate into better clinical outcome at 2-year follow-up.