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DETERMINANTS OF OSTEOPENIA IN ADOLESCENT IDIOPATHIC SCOLIOSIS – ABNORMAL GROWTH, ESCALATED BONE TURNOVER, POOR NUTRITION OR INADEQUATE PHYSICAL ACTIVITY?

¹ Department of Orthopaedics & Traumatology , ³ Centre for Epidemiology and Biostatistics, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong. ² Centre of Nutrition and Food safety, School of Biomedical and Molecular Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, U.K.

Background: Adolescent Idiopathic Scoliosis is a 3-dimensional deformity of the spine affecting peri-pubertal adolescents (10-17-y) mostly. Although generalised osteopenia is well documented in AIS, the patho-physiology of AIS related osteopenia is uncertain.

Aim: We studied the association between pubertal-growth, BMD, bone-turnover, calcium intake (CA) and physical-activity (PA) in AIS and compared to those of healthy girls.

Methods: 894 girls (594 AIS & 300 healthy controls) aged 11–16-y entered the study. Anthropometric parameters, areal-BMD of the proximal-femur and volumetric-BMD of the distal-tibia were determined by Dual-x-ray-Absorptiometry and peripheral QCT respectively. Bone-turnover-markers: bone-alkaline-phosphatase (bALP) and deoxypyridonine (Dpd) were assayed. CA and weight-bearing PA were assessed by FFQ method.

Results: Weight of AIS at <12-y and 13-y was significantly lower than controls (P<0.05). From 13-y, corrected right and arm-span of AIS were significantly longer than the controls (P<0.02). aBMD and vBMD were 6.7% and 8.4% respectively lower than the controls across the ages (P<0.05). The disparity in BMD compared with controls increased with age. CA was not different between the AIS and controls (361 mg/d, IQR:230–532mg/d vs. 319 mg/d, IQR:220–494mg/d; P=0.063). Weight-bearing PA of AIS was significantly lower than those of controls (P<0.02).

CA of AIS and controls reached <40% of the Chinese calcium DRI (1000 mg/d). Both CA and weight-bearing PA were correlated with BMD in AIS (P<0.04 & P=0.002 respectively). Both CA and PA were independent predictors on the variations of aBMDs (P<0.03) and vBMDs (P<0.04) in AIS after controlling for confounders in multivariate analysis. Regarding bone turn-over rate, bALP in AIS was 38.6% higher than the controls from 13-y onwards (P<0.005) while Dpd of AIS was 30.4% lower than controls at age 15-y (P=0.003). Furthermore, bALP in AIS was negatively correlated with age-adjusted BMD (r=–0.34, P<0.001) while the correlation was weaker in the controls (P=0.14, P<0.002).

Conclusion: The correlation of calcium intake and physical activity with BMD occurred predominantly in AIS only and that these two factors were also independent determinants on BMD of AIS implying that calcium intake and physical activity were significant modulators on BMD in AIS. Significantly faster physical-growth, higher bone formation rate were associated with lower BMD. Osteopenia in AIS could be interplayed by abnormally faster pubertal-growth and bone-turnover. In fact, Calcium intake of AIS was too low to meet the calcium demand for bone-mineralization. A controlled calcium supplementation and programmed physical activity intervention trial is merited to confirm the effect of Calcium intake and physical activity on bone acquisition in AIS at peripubertal period.

Correspondence should be addressed to Jeremy C T Fairbank at The Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX7 7LD, UK