title: A Twin Study of Perthes Disease.
abstract: BACKGROUND
  Legg-Calvé-Perthes disease (LCPD) is an idiopathic avascular necrosis of the femoral head. Its etiology is poorly understood, although previous studies have implicated low birth weight and possible genetic determinants. The aim of this study was to identify potential birth weight and genetic associations with LCPD.
  
  
  METHODS
  We extracted all twin pairs from the Danish Twin Registry (DTR) in which at least 1 individual had LCPD. The DTR captures every twin pair born alive in Denmark, and those with LCPD were identified by using health record linkage. Probanwise concordance was calculated to describe the likelihood that any given individual had LCPD if their co-twin was also diagnosed.
  
  
  RESULTS
  There were 81 twin pairs: 10 monozygotic, 51 dizygotic, and 20 unclassified (unknown zygosity [UZ]). There was no association between birth weight and being the affected co-twin. Four pairs (2 dizygotic and 2 UZ) were concordant for LCPD, which is greater than would be expected assuming no familial aggregation. There were no concordant monozygotic twin pairs. The overall probandwise concordance was 0.09 (95% confidence interval [CI]: 0.01-0.18): 0.00 for the monozygotic, 0.08 (95% CI: 0.00-0.18) for the dizygotic, and 0.18 (95% CI: 0.00-0.40) for the UZ twin pairs.
  
  
  CONCLUSIONS
  This study found evidence of familial clustering in LCPD but did not show a genetic component. The absolute risk that a co-twin of an affected individual will develop LCPD is low, even in the case of monozygotic twin pairs.
authors.affiliation: Center for Surgery and Public Health, Harvard Medical School, Boston, Massachusetts; Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Oxford, United Kingdom; Warwick Medical School, University of Warwick, Coventry, United Kingdom;
authors.forename: David
authors.initials: D
authors.lastname: Metcalfe
authors.affiliation: Department of Orthopaedic Surgery, Middlemore Hospital, Auckland, New Zealand;
authors.forename: Stephanie
authors.initials: S
authors.lastname: Van Dijck
authors.affiliation: Warwick Medical School, University of Warwick, Coventry, United Kingdom;
authors.forename: Nicolas
authors.initials: N
authors.lastname: Parsons
authors.affiliation: The Danish Twin Registry, University of Southern Denmark, Odense, Denmark; and.
authors.forename: Kaare
authors.initials: K
authors.lastname: Christensen
authors.affiliation: Institute of Translational Medicine, University of Liverpool, Liverpool, United Kingdom danperry@doctors.org.uk.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Perry
mesh_terms: D002648:Child; D002675:Child, Preschool; D003718:Denmark / Q000453:epidemiology; D004200:Diseases in Twins / Q000235:genetics; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D015994:Incidence; D007873:Legg-Calve-Perthes Disease / Q000453:epidemiology / Q000235:genetics*; D008297:Male; D012189:Retrospective Studies; D014429:Twins, Dizygotic / Q000235:genetics*; D014430:Twins, Monozygotic / Q000235:genetics*