163. Unconstrained cervical disc replacements: is segmental hypermobility a concern?

BACKGROUND CONTEXT Unconstrained cervical disc replacements prostheses (UCDR) have become more commonly used among surgeons performing cervical disc arthroplasty (CDA). CDR have favorable outcomes in treating cervical radiculopathy and restoring normative motion in the spine. More recently, UCDR have become popular due to their ease of insertion, however the unconstrained design's effect on segmental range of motion (ROM) has not been evaluated. PURPOSE To evaluate the unconstrained design's effect on segmental ROM. STUDY DESIGN/SETTING Single-center retrospective cohort study. PATIENT SAMPLE A total of 148 patients (58 single-level, 90 two-level). OUTCOME MEASURES Patient demographics, implant characteristics, baseline (BL) ROM, and follow-up ROM. Methods Patients diagnosed with cervical radiculopathy who underwent a primary 1- or 2- level UCDA from 2015-2019 with pre- and postoperative flexion-extension radiographs. Segmental sagittal measurements were taken to determine baseline (BL) and follow-up segmental ROM from the flex-extension views. Hypermobility was classified as ≥11.0° in ROM from BL to follow-up &/or >3mm translation. Mean BL and follow-up ROM were compared at each level using t-test analyses (α=0.05). Relationships between patient and implant demographics and hypermobility were determined using multivariate logistic regressions. Results A total of 148 (58 single-level, 90 two-level) patients met the criteria. Age, gender and BMI were similar between normal (Age: 42.7±8.7, gender [% female]: 41%, BMI: 28.1±5.5) and hypermobile groups (Age: 44.4±9.4, p=0.272; gender [% female]: 41%, p=0.98; BMI: 29.6±12.6, p=0.309); however, hypermobile patients (0.9±1.0) had a significantly higher CCI than the normal groups (0.4±0.8, p=0.003). Single-level patients: 23 (40%) developed hypermobility at follow-up. Follow-up ROM (12.2±6.8°, p=0.003) was significantly higher compared to the average BL ROM (9.2±4.9°). C5-C6 had significantly higher follow-up ROM (C5-C6: 13.7±6.9°, p=0.003) than BL ROM (C5-C6: 9.2±4.9°). Two-level patients: Although there was no significant difference between follow-up ROM (6.12±6.95°, p=0.749) and BL ROM (7.87±10.54°), 21(23.3%) cephalad and 20 (22.2%) caudal implants developed hypermobility postoperatively. Multivariate regression analysis of implant factors, patient demographics and disc level demonstrated no risk factors for hypermobility except a slightly shorter implant depth (13.6±0.9 mm vs 14.3±1.2 mm, p=0.004; OR:0.565, p=0.005). Conclusions UCDR on average increased segmental ROM with more than forty-percent of these implants being hypermobile at follow-up. Furthermore, shorter implant depth (AP) was indicated to be a risk factor for hypermobility. Patients and surgeons should be aware of the prevalence of hypermobility of UCDR and consider the long-term outcomes of the procedure. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.