A New Prognostic Factor in Predicting Curve Progression of Adolescent Idiopathic Scoliosis

Generalized osteopenia has been well reported in adolescent idiopathic scoliosis (AIS) patients. Studies have shown greatest probability of curve progression in AIS during the period of peri-pubertal rapid growth. However, the association between osteopenia and curve progressive has not been report. The aim of this study was to investigate whether osteopenia would serve as one of the important prognostic factors in predicting curve progression in AIS patients.

326 untreated AIS girls, aged 11 to 16, with initial Cobb angle of over 10°, were recruited. Dual energy X-ray absorptiometry (DXA) were used to measure the spinal bone mineral density (BMD) and bone mineral content (BMC) at the time of diagnosis. Age-adjusted spinal BMD and BMC that below -1SD was defined as osteopenia and low BMC respectively. Cobb angle was measured from a PA standing radiograph in every clinical visit. Any increment of at least 6° was classified as evidence of curve progression. All patients were followed up longitudinally for 2.4 years. Potential risk factors including: age-adjusted spinal BMD and BMC, age at diagnosis, menstruation status, family history, Risser grade, curve pattern and initial curve magnitude. Univariate Chi-square test and logistic regression were used for statistical analysis. Receiver operating characteristic (ROC) curve were plotted for testing the accuracy of the model.

The univariate analysis showed that age-adjusted spinal BMD (c2; p=0.062) and BMC (c2; p=0.031), age at diagnosis, menstruation status, Risser grade and initial curve magnitude (all, c2; p<0 .01) were significantly associated with curve progression, while family history and pattern (both, c2; p>0.3) did not. The regression model showed that low spinal BMC, younger age at diagnosis, menarche not yet started, lower Risser grade and greater initial Cobb angle were the risk factors identified in predicting curve progression of AIS girls. The odd ratio for each risk factors were: 2.04 for low bone mass, 1.86 for younger age group (11-13 years of age), 2.80 for patient who has not started their menarche, 4.77 for lower Risser grade (grade: 0-1), 4.52 and 4.84 for initial Cobb angle between 30°-39° and greater than 40° respectively. The area under ROC curve of the model was 0.79.

This is the first study to show that low BMC is one of the important prognostic factors in predicting curve progression in AIS girls. BMD value of a scoliotic spine might be underestimated by increasing of bone area due to the vertebral rotation. Present study showed that BMC, a more reliable bone quantity parameter, might have a higher predictive value than BMD. The sensitivity of the model including BMC parameter showed a fairly good result. In addition to the well-known prognostic factors, such as age, menses and initial Cobb angle, bone density measurement at the time of diagnosis might serve as an additional objective measurement in predicting curve progression in AIS girls. The clinical significance of this finding is that spinal BMC may in the future help us in the planning and possible interventional treatment of the osteopenia which may affect the natural progression of the scoliosis deformity.