title: The risk of adjacent-level ossification development after surgery in the cervical spine: are there factors that affect the risk? A systematic review.
abstract: METHODS
  Systematic review.
  
  
  OBJECTIVE
  To answer the following clinical questions: (1) What is the risk of adjacent-level ossification development (ALOD) in patients receiving noninstrumented cervical fusion, instrumented cervical fusion with a plate, or cervical total disc arthroplasty?; (2) What are the risk factors for ALOD?; (3) What is the time course for the development of ALOD?; and (4) Does ALOD affect outcomes and rates of reoperation?
  
  
  BACKGROUND
  Anterior cervical plating, total disc arthroplasty, and noninstrumented fusion have all been used in the treatment of cervical disc disease. There are numerous reports that identify ALOD, a form of heterotopic ossification, as a major risk factor after performing these procedures. Few studies have compared these 3 procedures to evaluate the risk, timing, and outcomes related to postoperation ALOD.
  
  
  METHODS
  A systematic search was conducted in PubMed and the Cochrane Library for articles published between January 1, 1990, and December 31, 2011. We included all articles that described the risk of or risk factors for ALOD after surgical treatment of the cervical spine. Studies with patients older than 18 years or those treated for tumor or trauma were excluded from the study. In addition, those with posterior fusions, case reports, and case series with less than 10 patients were excluded.
  
  
  RESULTS
  A total of 5 studies met the inclusion criteria for our systematic review. The risk of ALOD with anterior cervical discectomy and fusion ranged from 41% to 64%, whereas the risk of ALOD after total disc replacement ranged from 6% to 24%. When ALOD did occur, there was a 2-fold higher risk of development at the cranial adjacent segment. The most important risk factor for the development of ALOD was the use of instrumentation and the plate-to-disc distance, although the surgical procedure type (corpectomy vs. discectomy and fusion) neared but did not reach statistical significance. Insufficient evidence was available to delineate the time course for its development and how ALOD affected outcomes.
  
  
  CONCLUSIONS
  The current body of literature suggests that ALOD will develop with the use of instrumentation and especially so if anterior instrumentation is placed within 5 mm of the adjacent cranial disc segment. In addition, total disc replacement showed lower rates for the development of ALOD compared with anterior cervical discectomy and fusion at both short- and long-term follow-up.
  
  
  METHODS
  We recommend that the surgeon make every effort to keep the plate as far away from the adjacent disc as possible. Strength of Statement: Strong.
authors.affiliation: Hospital for Special Surgery, New York, NY, USA.
authors.forename: Han Jo
authors.initials: HJ
authors.lastname: Kim
authors.forename: Michael P
authors.initials: MP
authors.lastname: Kelly
authors.forename: Claire G
authors.initials: CG
authors.lastname: Ely
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Dettori
authors.forename: K Daniel
authors.initials: KD
authors.lastname: Riew
mesh_terms: D002574:Cervical Vertebrae / Q000473:pathology* / Q000601:surgery*; D006801:Humans; D012307:Risk Factors; D013122:Spinal Diseases / Q000473:pathology* / Q000601:surgery; D013123:Spinal Fusion / Q000009:adverse effects* / Q000379:methods; D059145:Total Disc Replacement / Q000009:adverse effects* / Q000379:methods; D016896:Treatment Outcome