Upper arm length model suggests transient bilateral asymmetry is associated with right thoracic adolescent idiopathic scoliosis (RT-AIS) with implications for pathogenesis and estimation of linear skeletal overgrowth.

INTRODUCTION

In girls with adolescent idiopathic scoliosis (AIS) the finding of abnormal extra-spinal bilateral skeletal length asymmetries in upper limbs, periapical ribs, and ilia begs the question whether these bilateral asymmetries are connected in some way with pathogenesis.

MATERIAL AND METHODS

We investigated upper arm length (UAL) asymmetries in two groups of right-handed girls aged 11-18 years with right thoracic adolescent idiopathic scoliosis (RT-AIS, n=95) from preoperative and screening referrals (mean Cobb angle 46°) and healthy controls (n=240). Right and left UAL were measured with a Harpenden anthropometer of the Holtain equipment, Asymmetry was calculated as UAL difference, right minus left, in mm. Repeatability of the measurements was assessed as technical error of the measurement and coefficient of reliability.

RESULTS

In girls with RT-AIS, UAL asymmetry was greater than in healthy girls, regressed negatively with age and correlated significantly with Cobb angle and apical vertebral rotation. In healthy girls, UAL asymmetry was unrelated to age. Plotted against years after estimated menarcheal age, UAL asymmetry decreased significantly for girls with RT-AIS but not for healthy girls.

DISCUSSION AND CONCLUSIONS

The apparent transience of the abnormal UAL asymmetry suggests it is not secondary to spinal deformity but pathogenetically associated with it. We suggest two hypotheses to account for these changes: (1) a transient asymmetry process with growth velocity; and (2) in the light of subsequent research, early skeletal overgrowth with catch-down growth affecting right but not left upper arm. The relation of the upper arm length asymmetry to the increased length of periapical left ribs reported for RT-AIS is unknown. Right upper arm length may provide a more simple model than arm span, for estimating linear skeletal overgrowth of girls with RT-AIS.