title: A fourth atlantoaxial joint: an initial description.
abstract: BACKGROUND
  The presence of a supernumerary posterior atlantoaxial facet joint has not been reported in the literature.
  
  
  OBJECTIVE
  To describe a patient who presented with a unilateral anomalous posterior facet joint and discuss its possible embryological origin.
  
  
  METHODS
  Case report.
  
  
  METHODS
  A 35-year-old woman presented with severe neck pain, headaches, and tingling radiating down her left arm. Radiological studies showed an anomalous right C1-C2 facet joint causing dorsal compression of the spinal cord and cord compression secondary to a C5-C6 herniated disc. The patient underwent resection of the abnormal facet via a posterior approach followed by anterior cervical discectomy and fusion at C5-C6.
  
  
  RESULTS
  The patient reported marked improvement in her neck pain and partial improvement in her left upper extremity numbness and tingling. Postoperative radiographs showed no abnormal motion at C1-C2.
  
  
  CONCLUSIONS
  The authors hypothesize that, in this patient, the C1 and C2 sclerotomes may have fused abnormally posteriorly, followed by the initiation of events at this site that eventually led to the formation of a morphologically normal synovial joint in an abnormal location.
authors.affiliation: Department of Neurosurgery, Boston Institute of Neurosurgery, Tufts Medical Center, 800 Washington St, Boston, MA 02111, USA. rriesenburger@yahoo.com
authors.forename: Ron I
authors.initials: RI
authors.lastname: Riesenburger
authors.forename: Paul
authors.initials: P
authors.lastname: Klimo
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Yao
mesh_terms: D000328:Adult; D001268:Atlanto-Axial Joint / Q000002:abnormalities* / Q000601:surgery*; D019299:Decompression, Surgical; D017586:Diskectomy; D005260:Female; D006261:Headache / Q000209:etiology / Q000601:surgery; D006801:Humans; D007405:Intervertebral Disc Displacement / Q000601:surgery; D019547:Neck Pain / Q000209:etiology / Q000601:surgery; D013117:Spinal Cord Compression / Q000209:etiology / Q000601:surgery; D013123:Spinal Fusion