Do Postoperative Biomechanical Changes Induce Heterotopic Ossification After Cervical Arthroplasty? A 5-Year Follow-up Study

Study Design: Prospective clinical study. Objective: To evaluate the factors that would predispose a patient to heterotopic ossification (HO) formation after cervical arthroplasty. Summary of Background Data: HO after arthroplasty is one of the complications of cervical total disk replacement (TDR). However, the predisposing factors and pathophysiology of HO have not been precisely described. Materials and Methods: We prospectively enrolled and followed up 23 patients, who received single-level arthroplasty with ProDisc-C, for 5 years after the operation. The patients who developed grade 3 or 4 HO were classified into the "high-grade HO group," whereas the patients with grade 0, 1, or 2 HO were classified into the " low-grade HO group." We compared the postoperative changes in the range of motion (ROM) and height of the functional segmental unit (FSU) of the implantation segments between the 2 groups. Results: The mean differences in height and ROM of the FSU were 2.59 +/- 1.42 mm and 6.7 +/- 3.2 degrees in the high-grade HO group, and 0.87 +/- 0.72 mm and 3.1 +/- 2.8 degrees in the low-grade HO group. The mean differences in height and ROM of the FSU were significantly higher in the high-grade HO group than in the low-grade HO group (P < 0.05). After cervical arthroplasty, the height of the FSU and ROM of the implantation segments were significantly increased in the high-grade HO group compared with the low-grade HO group. Conclusions: Overcorrection of the height of the FSU and increase in the ROM of the implantation segment may influence the formation of HOs after cervical arthroplasty.