title: Seasonal trends in the incidence of hip osteoarthritis in Japanese patients.
abstract: OBJECTIVE
  There is an association between winter birth and developmental dysplasia of the hip, formerly termed congenital dislocation of the hip. The purpose of this study was to clarify the relationship between the month and season of birth and hip osteoarthritis in Japanese patients who had undergone total hip arthroplasty or transpositional osteotomy of the acetabulum.
  
  
  METHODS
  One thousand eight hundred and 52 female patients that presented for the surgical treatment of hip osteoarthritis between 1999 and 2005 were included in this study. Female out-patients who were seen at the initial visit during same period were used as controls to correct the bias of the month of birth.
  
  
  RESULTS
  There was a significant deviation of the observed number of births from the expected values calculated on a monthly basis (p<0.001). A comparison by seasons (periods of 3 months) revealed more births than expected in winter (p<0.001). The risk of hip osteoarthritis was significantly associated with the month of birth. Namely, it was highest for births in January (p<0.001).
  
  
  CONCLUSIONS
  These data clearly revealed that the prevalence of hip osteoarthritis was significantly higher in those patients born in winter. The seasonal trend in hip osteoarthritis might be due to the winter prevalence of congenital dislocation of the hip in childhood.
authors.affiliation: Department of Orthopedic Surgery, Faculty of Medicine, Saga University, 5-1-1 Nabeshima, Saga, Japan.
authors.forename: Satomi
authors.initials: S
authors.lastname: Nagamine
authors.forename: Motoki
authors.initials: M
authors.lastname: Sonohata
authors.forename: Masaru
authors.initials: M
authors.lastname: Kitajima
authors.forename: Synsuke
authors.initials: S
authors.lastname: Kawano
authors.forename: Kenji
authors.initials: K
authors.lastname: Ogawa
authors.forename: Masaaki
authors.initials: M
authors.lastname: Mawatari
authors.forename: Takao
authors.initials: T
authors.lastname: Hotokebuchi