Bone Formation in Cervical Total Disk Replacement (CTDR) up to the 6-Year Follow-up

Background: Bone formation or so-called heterotopic ossification (HO) has been recognized as a complication of cervical total disk replacement (CTDR), potentially decreasing the range of motion (ROM) or even fusing the operated level. The purposes were to evaluate HO occurrence after CTDR with the PCM device and to determine the corresponding clinical significance. Methods: A prospective functional study and a retrospective radiographic review were performed. Two hundred seventy-two levels with CTDR using PCM device were enrolled. The follow-up average was 4.5 years (±1.37; range, 1 to 6). Radiographic images and clinical outcomes were collected. Results: Of the 272 levels studied, 21 (7.7%) revealed some degree of HO. Of these 21 levels, 10 (47.6%) levels were rated to be grade I, 7 (33.3%) were rated as grade II, 3 (14.28%) as grade III, and 1 (4.76%) as grade IV. Whereas 16.9% of the single-level cases developed HO, only 4.4% of the multilevel cases had some bone formation. Surprisingly, the HO patient group did not demonstrate a worsening of the clinical outcome. In 90.5% of patients with HO, preoperative radiographs showed an incipient osteophyte. The mean ROM in flexion-extension images was 11.2 degrees postoperatively, decreasing to 8.3 degrees at 12 months postoperatively and to 6.0 degrees at 72 months. Conclusions: The incidence of HO has a low rate in our series and does not seem to adversely affect clinical outcomes up to this point. Nevertheless, it is an important event that influenced the mean ROM after 72 months and may be further studied for assessment of any late consequences.