Fractures and bone mass in urban South African children of different ethnic backgrounds

Abstract

Aims: 1) To determine the incidence or rates of fractures, the common sites of fractures, the causes of fractures and grades of trauma causing fractures in urban South African children of different ethnic groups from birth until 17/18 years of age. 2) To investigate the association between fracture prevalence, bone mass and physical activity in South African children. 3) To assess associations of fracture prevalence and bone mass in adolescents with maternal fracture history and bone mass and sibling fracture history. Design: Using the Birth to Twenty longitudinal cohort of children, we obtained retrospective information on fractures and their sites from birth to 14.9 years of age on 2031 participants. The ethnic breakdown of the children was black (B) 78%, white (W) 9%, mixed ancestry (MA) 10.5% and Indian (I) 1.5%. Using the Bone Health cohort of the Birth to Twenty longitudinal study, we retrospectively obtained information of lifetime fractures until age 14.9 years in 533 subjects. Bone mass (measured by DXA), anthropometric data, physical activity scores and skeletal maturity were obtained at age 10 and 15 years. Comparisons were made between those who did and did not fracture within the same sex and ethnic groups. The third component of the thesis utilized data from 1389 adolescent-biological mother pairs of the Birth to Twenty (Bt20) longitudinal study. Questionnaires were completed on adolescent fractures until 17/18 years of age and on sibling fractures. Biological mothers completed questionnaires on their own fractures prior to the age of 18 years. Anthropometric and bone mass data on adolescent-biological mother pairs were collected. Results: Twenty two percent of children had sustained a fracture one or more times during the first 15 years of life (males 27.5% and females 16.3%; p<0.001). The percentage of children fracturing differed between the ethnic groups (W 41.5%, B 19%, MA 21%, I 30%; p<0.001). Of the children reporting fractures, 20% sustained multiple fractures. The most common site of fracture was the upper limb (57%). In the second component of the thesis, white males who fractured were found to be significantly taller (10 years p < 0.05), more physically active (15 years p < 0.01) and had higher lean body mass (10 years p=0.001; 15 years p<0.05) than those who did not fracture; while white females, who fractured, were fatter (10 and 15 years p< 0.05), than their nonfracturing peers. White males who fractured had greater BA (bone area) and BMC (bone mineral content) at most sites at 10 and 15 years; BA and BMC were no different between fracturing and non-fracturing children in the other ethnic groups. No anthropometric or bone mass differences were found between black children with or without fractures. The third component of the thesis showed that an adolescent’s risk of lifetime fracture decreased with increasing maternal lumbar spine (LS) BMC (24% reduction in fracture risk for every unit increase in maternal LS BMC Z-score) and increased if they were white, male or had a sibling with a history of fracture. Adolescent height, weight, male gender, maternal BA and BMC, and white ethnicity were positive predictors of adolescent bone mass. White adolescents and their mothers had a higher fracture prevalence (adolescents: 42%, mothers: 31%) compared to the black (adolescents: 20%, mothers: 6%) and mixed ancestry (adolescents: 20%, mothers: 16%) groups. Conclusion: More than twice as many South African white children fracture compared to black and mixed ancestry children. This is the first study to show ethnic differences in fracture rates among children; a pattern that is similar to that found in South African postmenopausal women. The factor associated with fractures in white boys appears to be participation in sports activities, while in white girls obesity appears to play a role. We were unable to find any factors that could explain fractures in black children. Unlike the findings of some other studies, fractures in these children were not associated with lower bone mass or reduced skeletal size. Maternal bone mass also appears to play a role in determining fracture incidence in children, as the mother’s bone mass has a significant inverse association with their off-springs’ fracture risk throughout childhood and adolescence. Furthermore, there is a strong familial component in fracture risk among South African adolescents and their siblings, as evidenced by the increased risk of fracture in siblings of index children who have fractured during childhood and adolescence. Differences in fracture rates and bone mass between families and individuals of different ethnic origins may be due to differing lifestyles and/or genetic backgrounds.