title: Gender differences in the ratio between humerus width and length are established prior to puberty.
abstract: CONCLUSIONS
  On a sample of 1,317 children aged 9.9 years we developed a novel method of measuring humeral dimensions from total body dual-energy X-ray absorptiometry (DXA) scans and showed that gender differences in the ratio between humeral width and length are established prior to puberty.
  
  
  BACKGROUND
  It is recognised that long bone cross-sectional area is greater in males compared to females, which is thought to reflect more rapid periosteal bone growth in boys. However, it is currently unclear whether these findings reflect gender differences in bone size or shape. In the present study, we investigated whether gender differences exist in the balance between longitudinal and periosteal long bone growth in children, leading to gender differences in bone shape, based on a novel method for evaluating shape of the humerus. We also examined whether these differences are established prior to puberty.
  
  
  METHODS
  Length, area and width of the humerus were estimated from total body DXA scans in 1,317 children aged 9.9 +/- 0.33 years, who had participated in a nested case-control study of fractures within the Avon Longitudinal Study of Parents and Children (ALSPAC) (a geographically based birth cohort based in South West England). No differences were observed with respect to parameters of humeral geometry according to fracture history, and so both groups were pooled for further analysis. Aspect ratio (AR) of the humerus was calculated as humeral width divided by length. Total body height and weight were measured at the same time as the DXA scan. Puberty was assessed using self-completion questionnaires.
  
  
  RESULTS
  Humeral width and length were positively associated with age and height in boys and girls combined (P<0.001), and with Tanner stage in girls (P<0.002). In contrast, age, height and Tanner stage were not related to humeral AR. We then examined gender differences in humeral shape according to pubertal stage. In prepubertal children (i.e. Tanner stage 1), humeral length was similar in boys and girls, but width (1.92 vs 1.88 cm, P<0.001) and area (47.7 vs 46.9 cm(2), P<0.001) were greater in boys, resulting in a greater AR (7.78 vs 7.53, P<0.001). Similar gender differences were observed in early pubertal children (i.e. Tanner stage 2).
  
  
  CONCLUSIONS
  We conclude that the greater periosteal diameter of boys compared to girls reflects differences in the balance between longitudinal and periosteal bone growth. Interestingly, resulting gender differences in humeral AR are established in prepubertal children.
authors.affiliation: Community Based Medicine, University of Bristol, Bristol, UK. Emma.Clark@bristol.ac.uk
authors.forename: E M
authors.initials: EM
authors.lastname: Clark
authors.forename: A R
authors.initials: AR
authors.lastname: Ness
authors.forename: J H
authors.initials: JH
authors.lastname: Tobias
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods; D000367:Age Factors; D001827:Body Height / Q000502:physiology; D001835:Body Weight / Q000502:physiology; D002648:Child; D015331:Cohort Studies; D005260:Female; D006801:Humans; D006811:Humerus / Q000033:anatomy & histology* / Q000254:growth & development; D008297:Male; D011627:Puberty / Q000502:physiology*; D012727:Sex Characteristics