The Effect of a Mismatched Center of Rotation on the Clinical Outcomes and Flexion-Extension Range of Motion

Abstract

Retrospective clinical and radiographic assessment of 21 levels of 18 consecutive patients treated using total lumbar disk replacement (TDR) for degenerative disk disease. To report clinical and radiographic outcomes after TDR using the Mobidisc prosthesis. In addition, to determine whether there is a correlation between clinical and radiologic outcomes and prosthesis positioning. TDR for lumbar degenerative disk disease is reported to provide good clinical and radiographic outcomes. However, TDR can alter the kinematics of the facet joint during flexion and extension. If prosthesis positioning is poor, the facet joint loading is increased upto 2.5-fold. No study has examined whether differences between the prosthesis center of rotation (COR) and the individual's COR have an effect on the clinical or radiographic outcomes after TDR. A retrospective study of 21 levels from 18 consecutive degenerative disk disease patients who underwent lumbar TDR. The Mobidisc prosthesis was used in all cases. Clinical parameters measured were lower back and leg pain [both assessed using the Oswestry Disability Index (ODI)]. These parameters were measured preoperatively and at the last follow-up. Radiographic assessment involved examining standard lateral flexion/extension views taken at the preoperative, postoperative 6-month, and the last follow-up assessments to determine disk space height (DSH) and range of motion (ROM). Patient satisfaction (subjective outcome) was determined by telephone questioning. For analysis, TDR cases were categorized into 3 groups on the basis of the size of the "COR index," which represented the difference between an individual's inherent COR and the inherent prosthesis COR. Group 1, COR index <5 mm, consisted of 13 levels; group 2, COR index >5 mm, and <10 mm, consisted of 5 levels; and group 3, COR index >10 mm, consisted of 3 levels. Overall, 77.8% of patients stated that they were highly satisfied with their surgical outcome. Low back pain visual analogue scale scores decreased from 7.61±2.17 (mean±SD) preoperatively to 2.33±2.679 at the last follow-up (P<0.001). The function increased postoperatively (ODI: 25.89±7.77 preoperative vs. 5.89±7.21 at last follow-up; P<0.001). The difference between preoperative and the last follow-up ODI was greater in group 1 than in groups 2 and 3 (P=0.034). Radiographic findings showed that TDR resulted in improved disk space height and segmental ROM (P<0.05). Analysis of the 3 groups showed that ROM preservation decreased as the COR index increased. The present study found that lumbar TDR using the Mobidisc prosthesis resulted in good clinical and radiologic outcomes and good patient satisfaction. Furthermore, we found that patient satisfaction, function, and ROM preservation correlated with correct COR positioning of the prosthesis.