Purpose: Cervical arthroplasty is an emerging technology with the potential of motion preservation and reduced adjacent level disease. However, the factors that influence postoperative range of motion (ROM) and patient satisfaction is not fully understood. The aim of this study was to evaluate the influence of disk height on the postoperative motion as well as clinical outcomes.

Method: 167 patients from a multi-center prospective randomized FDA trial with single level ProDisc-C arthroplasty performed were evaluated radiographically utilizing Medical Metrics (QMATM, Medical Metrics, Inc.). Preoperative and postoperative disk height and ROM were measured from standing lateral and flexion-extension radiographs. Of these 167 patients, 19 patients from a single center had clinical outcomes based on ODI and VAS scores evaluated pre and postoperation with a mean follow-up of 22 months. Two-tailed student’s T-test and Spearman’s Rho tests were performed in order to find out if there was any correlation or “threshold” effect between the disk height and ROM.

Results: Patients with less than 4 mm of preoperative disc height had a 1.8° increase in their flexion-extension ROM after TDA as compared to no change in ROM in patients with more than 4 mm of preoperative disc height (p=0.04). Patients with more than 5mm of postoperative disc height have significantly higher postoperative flexion-extension ROM (10.1°) than those with less than 5mm disc height (8.3°, p=0.014). However, patients with more than 7mm of postoperative disc height have significantly lower postoperative lateral bending ROM (4.1°) than those with less than 7mm disc height (5.7°, p=0.04). It appears that the optimal postoperative disc height is between 5 to 7 mm for increased ROM on flexion-extension and lateral bending. No correlation could be found between clinical outcomes and disc height. Similarly, no threshold effect could be found between any specific disc height and ODI or VAS.

Conclusion: Patients with greater disc collapse benefit more in ROM from a TDR. The optimal range to maximize ROM for postoperative disc height is between 5 to 7mm. This optimal range did not translate into better clinical outcome at 2 year follow-up. A longer follow-up will yield if less ROM ultimately leads to worse clinical outcomes.