title: Heterotopic ossification after cervical total disc replacement: determination by CT and effects on clinical outcomes.
abstract: OBJECTIVE
  Heterotopic ossification (HO) after cervical total disc replacement (TDR) has been reported to impede artificial disc motion. In all previously reported cases of HO, assessment was based on plain radiographs. The authors hypothesized that CT scan is a more sensitive and accurate detector. The aims of this study were to assess the actual incidence of HO and its effect on outcome in a cohort of patients undergoing cervical TDR with the Bryan disc and to compare HO detection by means of plain radiographs and CT.
  
  
  METHODS
  The authors retrospectively assessed data from medical records, radiological studies, and clinical evaluations of patients who underwent 1- or 2-level cervical TDR with the Bryan disc and were followed up for more than 12 months. The presence and grading of HO according to the McAfee classification were assessed by CT scan, and these findings were compared with findings on plain radiographs. Thirty-six patients (mean age 46.61 ± 7.24 years; range 29-60 years; 21 men and 15 women) who underwent Bryan TDR at 52 levels were included in the study. The mean duration of CT follow-up was 19.03 ± 4.64 months; the mean duration of clinical follow-up was 26.78 ± 7.20 months.
  
  
  RESULTS
  On the basis of CT, HO was identified in 18 (50%) of 36 patients and 25 (48.1%) of 52 levels treated. Grade 1 HO was present in 9 of the levels treated (17.3%), Grade 2 in 13 levels (25.0%), Grade 3 in 2 levels (3.8%), and Grade 4 in 1 level (1.9%). Nineteen (76%) of the 25 affected levels were in patients who had undergone 2-level TDR. There was no significant association with patient sex or disc pathology. There was a tendency for HO development among older patients, but this finding was not statistically significant (mean age 48.8 ± 6.8 in patients with HO vs 44.4 ± 7.2 in those without HO, p = 0.065). Although HO was found in 25 levels, 96.2% of the treated levels (50 of 52) had segmental range of motion on dynamic (flexion and extension) radiographs. The concordance between HO grading by CT and radiography was high, with an intraclass correlation coefficient of 0.822 (lower limit of 95% CI: 0.710, p < 0.001). Patients who had HO had the same clinical success rate as those who did not (94.4% vs 94.4%, p = 1.00). The visual analog scale scores for neck and arm pain were significantly improved in both the HO and the non-HO group.
  
  
  CONCLUSIONS
  The rate of HO detected by CT scan in this cohort of patients undergoing cervical TDR with a Bryan disc was 48.1% per level treated and 50% per patient with minimal limitation of segmental motion (96.2% of levels remained mobile), but plain radiograph is an acceptable detection tool. Two-level surgery has a higher risk of HO, although development of HO does not affect clinical outcome.
authors.affiliation: Department of Neurosurgery, Neurological Institute, Taipei Veterans General Hospital, Beitou, Taipei, Taiwan.
authors.forename: Tsung-Hsi
authors.initials: TH
authors.lastname: Tu
authors.forename: Jau-Ching
authors.initials: JC
authors.lastname: Wu
authors.forename: Wen-Cheng
authors.initials: WC
authors.lastname: Huang
authors.forename: Wan-Yuo
authors.initials: WY
authors.lastname: Guo
authors.forename: Ching-Lan
authors.initials: CL
authors.lastname: Wu
authors.forename: Yang-Hsin
authors.initials: YH
authors.lastname: Shih
authors.forename: Henrich
authors.initials: H
authors.lastname: Cheng
mesh_terms: D000328:Adult; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D017586:Diskectomy / Q000009:adverse effects*; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000601:surgery*; D007405:Intervertebral Disc Displacement / Q000000981:diagnostic imaging / Q000601:surgery; D008297:Male; D008875:Middle Aged; D009999:Ossification, Heterotopic / Q000000981:diagnostic imaging* / Q000209:etiology / Q000601:surgery; D019919:Prosthesis Implantation / Q000009:adverse effects*; D011859:Radiography; D016059:Range of Motion, Articular; D012189:Retrospective Studies; D016896:Treatment Outcome