title: Prevalence of heterotopic ossification following total disc replacement. A prospective, randomized study of two hundred and seventy-six patients.
abstract: BACKGROUND
  Despite reports of good clinical outcomes in patients treated with lumbar and cervical disc replacements, varying degrees of heterotopic bone have been observed around these devices. The purposes of the present study were to determine the prevalence of heterotopic ossification following lumbar disc replacement and to investigate whether heterotopic ossification results in loss of motion or negatively affects clinical outcome.
  
  
  METHODS
  All preoperative and postoperative radiographs from a completed prospective, randomized, United States Food and Drug Administration-regulated trial comparing replacement with the CHARITE Artificial Disc with anterior interbody arthrodesis were analyzed. In each of 276 patients treated with disc replacement, heterotopic ossification was categorized with use of a validated 5-point radiographic classification system both preoperatively and at all protocol-specified follow-up intervals to two years. The range of motion on flexion and extension radiographs made preoperatively was compared with that on radiographs made two years postoperatively, and the motion was correlated with the presence or absence of heterotopic ossification. Similarly, validated clinical outcome measures were correlated with the presence or absence of heterotopic ossification at two years.
  
  
  RESULTS
  The prevalence of heterotopic ossification in the 276 consecutive patients treated with lumbar disc replacement with the CHARITE Artificial Disc was 4.3%. There were four cases of Class-I heterotopic ossification and eight cases of Class-II heterotopic ossification. In five of the twelve patients, heterotopic bone was visible as early as six weeks postoperatively, and eleven of the twelve patients had evidence of heterotopic ossification by three months postoperatively. The postoperative range of motion exceeded the preoperative range in all of the patients with heterotopic ossification. With the numbers available, no difference in either the range of motion or the clinical outcome at twenty-four months postoperatively was found between the patients who had and those who did not have heterotopic ossification.
  
  
  CONCLUSIONS
  Heterotopic ossification is infrequent in patients treated with the CHARITE Artificial Disc, and it does not impact the range of motion or clinical outcome.
authors.affiliation: Scoliosis and Spine Center of Maryland, O'Dea Medical Arts Building, 7506 Osler Drive, Suite 104, Baltimore, MD 21204, USA.
authors.forename: P Justin
authors.initials: PJ
authors.lastname: Tortolani
authors.forename: Bryan W
authors.initials: BW
authors.lastname: Cunningham
authors.forename: Mmech
authors.initials: M
authors.lastname: Eng
authors.forename: Paul C
authors.initials: PC
authors.lastname: McAfee
authors.forename: Gwen A
authors.initials: GA
authors.lastname: Holsapple
authors.forename: Karen A
authors.initials: KA
authors.lastname: Adams
mesh_terms: D004185:Disability Evaluation; D006801:Humans; D007403:Intervertebral Disc / Q000473:pathology*; D007405:Intervertebral Disc Displacement / Q000000981:diagnostic imaging / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000473:pathology*; D009999:Ossification, Heterotopic / Q000000981:diagnostic imaging / Q000453:epidemiology* / Q000503:physiopathology; D011183:Postoperative Complications / Q000000981:diagnostic imaging / Q000453:epidemiology* / Q000503:physiopathology; D011446:Prospective Studies; D011859:Radiography; D016059:Range of Motion, Articular; D016896:Treatment Outcome