Background: The phenomenon of heterotopic ossification (HO) is a well-known complication of joint replacement surgery. However, the clinical associations and effects of HO in cervical arthroplasty are unclear. This study investigates the incidence of HO in cervical disc replacement, identifies potential associated risk factors, and assesses the relationship of HO with clinical outcomes.

Method: The patient population (n=90; 39 men, 51 women; mean age 45 years; range 26–79 yr; standard deviation, 9.8 yr) was acquired from the original multi-center, prospective, observational Bryan Disc Study by the European Consortium. The presence of HO was defined in accordance with the McAfee classification based on cervical lateral x-rays at 12 months post surgery and interpreted by an independent neurosurgeon and a radiologist. Secondary outcome measurements included Odom’s criteria and the Medical Outcomes Study Short-Form 36-Item Health Survey.

Results: 16 out of the total 90 patients (17.8%) experienced HO. 6 patients (6.7%) experienced Grade 3 and 4 HO. 10 patients’ (11%) artificial discs demonstrated movement of less than 2 degrees on flexion and extension cervical x-ray at 12 months post-operatively. 4 of these patients had HO of Grade 3 or 4. Male sex ([chi]² = 4.1; P = 0.0407) and older patients (P = 0.023; odds ratio = 1.10; 95% confidence interval = 1.01–1.19) were associated with development of HO.

Discussion: HO is known to restrict movement of large joint and lumbar disc replacements. Our study demonstrates male sex and older age as potential risk factors in development of HO after cervical disc arthroplasty. There is a strong association between HO and subsequent loss of movement of implanted cervical artificial discs. Failure to preserve motion may expedite the onset of symptomatic adjacent segment level disease making this an important complication requiring further investigation. Grade 3 or 4 HO limited movement in 4.4% of patients.