Risk factors of cage subsidence after removal of localized heterotopic ossification by anterior cervical discectomy and fusion: A retrospective multivariable analysis

The purpose of the study was to identify risk factors of cage subsidence and evaluate surgical outcome by at least 12 months postoperative follow-up. We retrospectively investigated 113 consecutive patients who underwent anterior surgery to relieve spine cord compression resulted from localized heterotopic ossification, from July, 2011 to February, 2016. We divided the patients into 2 groups: cage subsidence <2 mm group and ≥2 mm group. According to magnetic resonance imaging (MRI), the severity of increased signal intensity (ISI) was classified into grade 0, 1, and 2. Clinical outcome was assessed by the Japanese Orthopedic Association (JOA) scoring system. Logistic regression analysis and receiver-operating characteristic (ROC) curve were utilized for predicting risk factors of cage subsidence, and the recovery rate was evaluated by Kruskal–Wallis test or Mann–Whitney U test. Logistic regression with cage subsidence as the dependent variable showed independent risks associated with a cervical sagittal malalignment (odds ratio [OR] 11.23, 95% confidence interval [CI] 3.595–35.064, P < .001), thoracic 1 (T1) slope angle (OR 1.59, 95% CI 1.259–1.945, P < .001), and excisional thickness (OR 2.38, 95% CI 1.163–4.888.0, P = .018). The cut-off values of T1 slope and excisional thickness were 19.65 angle and 3.7 mm, respectively. Patients with high occupying ratio (P = .001) and high ISI grade (P = .012) are more likely to occur lower recovery rate. Patients with high T1 slope angle or preoperative kyphotic deformity should avoid excessive removal of endplate and vertebral body so as to reduce the occurrence of cage subsidence. Poor outcome was closely related to cervical sagittal malalignment and higher ISI grade.