Variability in the association between angle of trunk rotation and Cobb angle based on demographic factors

AIS is a 3D deformity characterized by vertebral body rotation, often quantified by the angle of trunk rotation (ATR). However, there is not necessarily a linear relationship between ATR and Cobb angle. We aimed to examine demographic factors that affect this association.

Initially, univariate and multiple linear regression compared ATR and Cobb angle in 4329 AIS patients. Given marked differences between Black patients and other racial groups in the thoracic region, we repeated our analysis after dichotomizing patients. Males and females were also compared in the thoracic and lumbar regions. For BMI and age, we repeated these analyses, including BMI and age as interaction terms in each regression.

A sample of 4329 AIS patients had mean age 12.7 ± 2.3 years (1-21 years) for patients with available data. Mean age at the time of Cobb angle and ATR measurements was 15.0 ± 2.2 years (9.6-26.5 years) for 4326 patients with available data for date of birth and visit date. Among 4262 patients with available thoracic scoliometer measurements of 35 degrees or less, the mean thoracic Cobb angle was 54.0 ± 15.0 degrees (0-127 degrees), and the mean thoracic rib hump angle was 13.8 ± 5.8 degrees (0-33 degrees). Among 4054 patients with available lumbar scoliometer measurements of 35 degrees or less, the mean lumbar Cobb angle was 40.9 ± 13.8 degrees (4-110 degrees) and the mean lumbar rib hump angle was 8.3 ± 5.7 degrees (0-30 degrees). The mean BMI was 21.5 ± 4.4 (10.6-39.8) for 3890 patients with available data. After dichotomizing patients, black patients' slope was 0.3 and 0.2 less than other groups when using univariate (p = 0.01) and multiple linear regression (p = 0.04), respectively. Females demonstrated a significantly greater slope than males in the thoracic region and significantly lesser slope in the lumbar region when using both univariate and multiple linear regression. With each point increase in BMI, thoracic slope decreased by 0.03 for both univariate (p = 0.001) and multiple (p = 0.002) linear regression.

The relationship between ATR and coronal curvature varies significantly by race, sex, and BMI. In particular, Black patients demonstrate lower thoracic curves for the same ATR measurements. Females demonstrate greater curves at lower ATR measures in the thoracic region, and males demonstrate greater curves in the lumbar regions. As BMI increases, the slope between thoracic ATR and Cobb angle decreases.