The association of BMI and physical activity with acetabular dysplasia in children

Abstract

Purpose: Several studies have shown that the morphology of the hip strongly influences the development of osteoarthritis of the hip. Especially the evidence for cam morphology and acetabular dysplasia is strong. Both morphological variants develop during childhood. In order to potentially prevent development of these morphologies and thus to prevent hip OA in later life, it is crucial to understand how these morphologies develop during childhood. Especially little is known on the natural course of primary dysplasia in the developing hip in children. In this study we focused on the association of hip dysplasia with overweight and physical activity in 9 year old children. Methods: The population for this cross-sectional study was drawn from the ongoing prospective pregnancy cohort: Generation R. 9,778 mothers with a delivery date from March 2002 until January 2006 were enrolled. In a random subgroup of children DXA scanning was performed of the left hip and whole body at age 9. Body mass index, standardized for the Dutch population was divided in four categories based on extended international Obesity Task Force cut-offs: underweight, normal BMI, overweight and obesity. Physical activity was based on self-reported data and defined in 3 areas: time spent on playing outdoors, playing sports and walking to school. The amount of acetabular dysplasia was determined with the center-edge angle (CEA) in DXA images of the (left) hip. Mild and severe dysplasia were defined as CEA<20 and CEA<15 respectively. Associations were tested using logistic regression, adjusting for age, ethnicity, sex, first born, breech birth, gestational weight, gestational age and caesarean section. Results: 1,026 DXA images of the children’s hip were available for analysis. The median age of the children was 9.7 years (range: 9.2 - 12.0 years). Mean prevalence of mild and severe dysplasia was 25.6% and 6.3% respectively (table1). Weight was negatively associated with dysplasia. Obese children showed less dysplasia compared to normal children, while underweight children showed more dysplasia. This was significant only for the unadjusted association between being overweight and mild dysplasia, although the pattern was present for both crude and adjusted associations in both mild and severe dysplasia. Physical activity showed a negative association with dysplasia for playing outside and walking/cycling to school, not for sport activity. Meaning the children who play more outside or walk or cycle to school showed a lower prevalence of dysplasia. This association was significant especially for mild dysplasia after adjustment. Stratification for sex revealed no sex-specificity: all associations were similar for boys and girls. Conclusions: The results of this study suggest that being overweight and mild physical activity might be protective against developing (mild) dysplasia of the hip. The cross-sectional nature of this study prevents making conclusions on causality. For instance, the negative association between dysplasia and mild physical activity could point to a reverse causality as well. Obesity is a known risk factor for many types of OA, though not for hip OA. The current result might provide a possible explanation. The direct (positive) effect of obesity on hip OA could be attenuated by the indirect (negative) effect that is mediated by dysplasia, which is one of the strongest risk-factors for hip OA. These results warrant further investigation of developing hip dysplasia using the 14-year time point of this prospective cohort. In conclusion, obesity and mild physical activity were negatively associated with acetabular dysplasia of the hip in 9 year old children.Tabled 1Associations between dysplasia and weight/physical activityMild dysplasia (CEA<20) n=263 out of 1026Severe dysplasia (CEA<15) n=65 out of 1026crude ORadjusted ORcrude ORadjusted ORWeight (normal=ref)Underweight1.14 (0.65 to 0.20)0.99 (0.51 to 1.92)1.51 (0.62 to 3.68)1.75 (0.64 to 4.82)Overweight0.72 (0.48 to 1.08)0.82 (0.52 to 1.28)0.84 (0.40 to 1.74)1.07 (0.50 to 2.31)Obese0.39 (0.17 to 0.94)0.54 (0.22 to 1.33)0.65 (0.15 to 2.76)0.91 (0.21 to 3.96)Physical activity (hours per week)Sport activity1.00 (0.91 to 1.10)1.00 (0.89 to 1.11)1.05 (0.88 to 1.251.02 (0.84 to 1.25)Playing outside0.98 (0.95 to 1.00)0.97 (0.94 to 1.00)0.95 (0.88 to 1.02)0.92 (0.83 to 1.02)Walking/cycling to school0.87 (0.76 to 0.99)0.86 (0.74 to 0.99)0.82 (0.63 to 1.08)0.76 (0.54 to 1.06) Open table in a new tab