The fate of heterotopic ossification associated with cervical artificial disc replacement.

STUDY DESIGN: This is a retrospective study designed to observe the progressive appearance of heterotopic ossification (HO) after cervical artificial disc replacement (ADR). OBJECTIVE: To investigate the development and progressive appearance of HO at cervical ADR sites. SUMMARY OF BACKGROUND DATA: HO is defined as the formation of bone outside the skeletal system. The reported HO occurrence rate in cervical ADR is high and varied. However, the long-term outcome of HO has not been investigated by longitudinal observation. METHODS: Sixty-seven consecutive patients undergoing cervical ADR with complete radiological data were selected for this study. Cervical lateral radiographs and computed tomographic scans were obtained during the first and second studies and compared to identify any transitions in HO. The change in the HO occurrence rate, distribution, and grade of HO were investigated at 2 follow-up time points (18.6 and 36.9 mo postsurgery). To compare the amount of progression between participants' HO rates, the relative increase rate was calculated. RESULTS: HO showed a progressive pattern. The grade of HO was redistributed toward a higher grade in the second study. The number of patients with HO had increased from 31 (46.3%) to 43 (64.2%) of 67 patients at the final follow-up with statistically significant difference (P = 0.037). Twenty-six patients (39.4%) had progressed toward a higher HO grade, but 41 patients (60.6%) showed no change in HO grade. The overall RI rate and the weighted RI rate were 39.4% and 48.7%, respectively. CONCLUSION: This study found that HO has a progressive appearance and determined that HO is not a static but rather a dynamic phenomenon with progressive pattern. Future elucidation of the environmental factors affecting HO, including genetic or proteomic analyses, is recommended. LEVEL OF EVIDENCE: 3.ope