Impact of Heterotopic Ossification on Range of Motion Following Cervical Disc Replacement

STUDY DESIGN: Retrospective cohort study. OBJECTIVE: To evaluate the impact of heterotopic ossification (HO) severity on segmental and global cervical range of motion (ROM) following cervical disc replacement (CDR). SUMMARY OF BACKGROUND DATA: Heterotopic ossification is the most commonly reported complication following CDR, with rates ranging from 40% to over 70% at mid-term to long-term follow-up. While high-grade HO is known to restrict motion, the dose-response relationship between HO severity and ROM across multiple spinal levels remains incompletely characterized. MATERIALS AND METHODS: Patients who underwent single-level CDR with a minimum one-year follow-up and flexion-extension radiographs were identified from a single-institution database. HO was graded per McAfee (Grade 0-4) and patients were stratified into Grade 0-2 (nonsevere HO), Grade 3 (severe HO), and Grade 4 (severe HO with bridging ankylosis). ROM was measured at the operative disc space and C2-7 levels at final follow-up. Pairwise Mann-Whitney comparisons were Bonferroni-corrected across six planned tests (three disc space, three C2-7). Reported pairwise P -values reflect adjustment. Kruskal-Wallis omnibus tests were not adjusted. RESULTS: A total of 146 patients met the inclusion criteria (mean age 45.6±9.0 yr, 63.7% male, mean follow-up 2.8±1.9 yr). Grade 0-2 HO was present in 88 (60.3%), Grade 3 in 47 (32.2%), and Grade 4 in 11 (7.5%). Disc space ROM differed significantly across groups (Kruskal-Wallis P <0.001): Grade 0-2 mean 9.1°±5.4°, Grade 3 mean 6.2°±4.2° ( P =0.009), Grade 4 mean 1.3°±2.0° ( P <0.001). C2-7 ROM did not differ significantly across HO groups (Kruskal-Wallis P =0.090), with all pairwise comparisons nonsignificant. CONCLUSION: Higher-grade HO following CDR is associated with a stepwise reduction in disc space ROM. Grade 3 HO significantly reduces segmental motion compared with Grade 0-2, and Grade 4 HO leads to near-complete segmental motion loss. Regardless of HO severity, C2-7 ROM is preserved, potentially suggesting transfer of motion to adjacent segments. LEVEL OF EVIDENCE: Level IV.