title: Developmental dysplasia of the hip in twins: the importance of mechanical factors in the etiology of DDH.
abstract: BACKGROUND
  Developmental dysplasia of the hip (DDH) is a disease of multifactorial etiology. Genetic and intrauterine mechanical factors influence the development of the hip joint and the incidence of DDH. Breech position is a risk factor for DDH. The aim of this study is to analyze whether in twins, the intrauterine mechanical factors, such as a breech position, limitation of fetal mobility, limitation of hip abduction, and increase the incidence of DDH may, therefore, be considered as risk factors.
  
  
  METHODS
  A pair of twins with either breech or cephalic presentation, and no family history of DDH or any other sicknesses were included in this study. During a 6-year period between 2002 and 2007, data were collected on 105 consecutively born twins (210 children with 420 hips) of whom 48 were a cephalic breech, 35 were a cephalic-cephalic, and 22 were a breech-breech presentation. The presentation was defined by regular prenatal ultrasounds and the obstetric records at birth. The average gestational age was 36.7 weeks and average weight at birth was 2290 g. Control group 1 consisted of 274 single gestation children, with a cephalic presentation at birth. The average gestational age was 39.1 weeks and average weight at birth was 3200 g. Control group 2 consisted of 48 single gestation children, with a breech presentation at birth. The average gestational age was 36.8 weeks and average weight at birth was 2870 g. Ultrasonography of the hips was done according to the Graf technique. Statistical analysis was done using a χ² test.
  
  
  RESULTS
  The χ² test was used and it showed no significant association between the presence or absence of pathologic hip types in the twins group and in the 2 control groups. A higher incidence of DDH (3.4%) was present in groups 1 and 2.
  
  
  CONCLUSIONS
  This comparison of a large series of twins with matched controls shows that there is no increased incidence of DDH in twins when compared with single birth infants with no family history of DDH. This study supports the idea that breech presentation in twins is not a risk factor, when knees are flexed.
  
  
  METHODS
  Retrospective study. Level 2 study.
authors.affiliation: Department of Orthopaedics, Paediatric Orthopaedics Unit, Vita-Salute University, IRCCS San Raffaele Hospital, Milan, Italy.
authors.forename: Maurizio
authors.initials: M
authors.lastname: De Pellegrin
authors.forename: Desiree
authors.initials: D
authors.lastname: Moharamzadeh
mesh_terms: D001696:Biomechanical Phenomena; D001848:Bone Diseases, Developmental / Q000209:etiology*; D004200:Diseases in Twins / Q000209:etiology*; D005260:Female; D005865:Gestational Age; D006621:Hip Joint; D006801:Humans; D007223:Infant; D007592:Joint Diseases / Q000209:etiology*; D007746:Labor Presentation; D008297:Male; D011247:Pregnancy; D012189:Retrospective Studies; D012307:Risk Factors