Endplate Undercoverage Predicts Location and Severity of Heterotopic Ossification After Cervical Disc Arthroplasty.

STUDY DESIGN: Retrospective radiographic analysis of patients undergoing cervical disc replacement (CDR). OBJECTIVE: To assess the impact of anteroposterior endplate coverage on the development, location, and severity of heterotopic ossification (HO) following CDR, and to establish clinically actionable coverage thresholds. SUMMARY OF BACKGROUND DATA: Heterotopic ossification (HO) is a common complication following CDR that may limit the range of motion. While endplate coverage has been implicated as a risk factor, the direction-specific relationship between undercoverage location and HO location, as well as dose-response thresholds, remains poorly defined. METHODS: Patients undergoing CDR (2015-2024) with a minimum 1-year radiographic follow-up were retrospectively assessed (221 levels). Implant-to-endplate depth ratio (ID: ED) and exposed endplate ratio (EER) were measured on the first postoperative radiographs. HO was graded on final radiographs using the McAfee classification (0-4). Dose-response, direction-matched correlations, and ROC analyses were performed. RESULTS: Heterotopic ossification (HO) was present in 85.8% of levels, with 42.7% developing severe HO (grade 3-4). Posterior HO was more common than anterior (84.3% vs. 41.3%, P<0.001). ID: ED correlated inversely with HO grade at both endplates (P<0.001). Dose-response analysis demonstrated that severe HO rates increased from 22.2% (Q1) to 57.9% (Q4; RR=2.61, P<0.0001). Direction-matched analysis confirmed anterior undercoverage predicted anterior HO (r=0.422, P<0.0001) and posterior undercoverage predicted posterior HO (r=0.300, P<0.0001). ROC analysis identified an ID: ED ≤0.850 as the optimal threshold (AUC=0.668, P<0.001). CONCLUSIONS: Heterotopic ossification (HO) following CDR is strongly associated with inadequate endplate coverage. A direction-specific relationship exists whereby undercoverage location predicts HO location. Maintaining ID>85% and EER <15% may help limit severe HO risk. LEVEL OF EVIDENCE: Level III-prognostic study.