title: Upper cervical spine instability in pseudoachondroplasia.
abstract: BACKGROUND
  Pseudoachondroplasia (PSACH) is a rare autosomal dominant skeletal dysplasia associated with os odontoideum and atlantoaxial instability. This study aims to define the characteristics of upper cervical spine instability in patients with PSACH and analyze the relation between the incidence of upper cervical instability and os odontoideum.
  
  
  METHODS
  Fifteen patients (10 women and 5 men) with PSACH of Korean ethnicity with mean age of 23.7 years (range, 3-44 years) at presentation to our hospital with varied complaints, including short stature, limb deformity, neck pain, and neurological symptoms, were evaluated clinicoradiologically for upper cervical spine instability. The patients were separated into group 1 (n = 9) with os odontoideum and group 2 (n = 6) without os odontoideum. Comparisons were made using parameters such as instability index, rotational instability, atlantodens interval and space available for cord, and analysis done to correlate cervical instability with age and Japanese Orthopedic Association (JOA) score.
  
  
  RESULTS
  Significant differences were found statistically when the 2 groups were compared on the basis of the space available for the cord (SAC), JOA scoring, and rotational instability. Linear relationship was found between instability and age and JOA score. Incidence of os odontoideum was 60% in our study group.
  
  
  CONCLUSIONS
  Os odontoideum led to an increase in the incidence of upper cervical spine instability. Instability increased with the age. The presence of os odontoideum and atlantoaxial instability did not warrant for surgery because no signs of cervical myelopathy developed or progressed in our patients during the follow-up period, but these patients should undergo regular clinical and radiological evaluation.
  
  
  METHODS
  Level IV prognostic study.
authors: Department of Orthopedic Surgery, Korea University Guro Hospital, Rare Disease Institute, Seoul, Korea.
authors: Gautam M
authors: GM
authors: Shetty
authors: Hae Ryong
authors: HR
authors: Song
authors: Ranjith
authors: R
authors: Unnikrishnan
authors: Seung Woo
authors: SW
authors: Suh
authors: Seok-Hyun
authors: SH
authors: Lee
authors: Chang Yong
authors: CY
authors: Hur
mesh_terms: D000293:Adolescent; D000328:Adult; D001268:Atlanto-Axial Joint / Q000503:physiopathology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007593:Joint Instability / Q000453:epidemiology / Q000503:physiopathology*; D007723:Korea / Q000453:epidemiology; D008297:Male; D009809:Odontoid Process / Q000503:physiopathology*; D010009:Osteochondrodysplasias / Q000503:physiopathology*; D011379:Prognosis; D018709:Statistics, Nonparametric