Abstract
Introduction
In patients with adolescent idiopathic scoliosis (AIS), anomalous extra-spinal left-right skeletal length asymmetries in upper limbs, periapical ribs, and ilia beg the question as to whether these bilateral asymmetries are connected in some way with pathogenesis. The upper arm and iliac length asymmetries correlate significantly with adjacent spinal curve severity respectively in thoracic and lower (thoracolumbar and lumbar) spine. In lower limbs, skeletal length asymmetries and proximo-distal disproportion are unrelated to spinal curve severity. Overall, these observations raise questions about mechanisms that determine skeletal bilateral symmetry of vertebrates in health and disorder, and whether such mechanisms are involved in the cause of this disease. 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=98) from preoperative and screening referrals (mean Cobb angle 45°) and healthy controls (n=240).
Methods
Right and left UAL were measured with a Harpenden anthropometer of the Holtain equipment, by one of four observers (RGB, AAC, RKP, FJP). UAL asymmetry was calculated as UAL difference, right minus left, in mm. Repeatability of the measurements was assessed by technical error of the measurement (TEM) and coefficient of reliability (R).
Results
In girls with RT-AIS, UAL asymmetry was greater than it was in healthy girls (mean 5·9 mm vs 2·5 mm, ANOVA p<0·001, correcting for age), regressed negatively with age (p<0·001, r= –0·374), and correlated significantly with Cobb angle (r=0·342, p=0·001) and apical vertebral rotation (Perdriolle, r=0·291, p=0·004). In healthy girls, UAL asymmetry was unrelated to age. Plotted against years after estimated menarcheal age, right UAL overgrowth reduced significantly for girls with RT-AIS (r= –0·312, p=0·006, n=76) but not for healthy girls (r=0·000, p=0·985, n=121), which was a significant finding (p=0·052, ANOVA).
Conclusions
The abnormal overgrowth of right upper arm length may be secondary, or pathogenetically associated with the RT-AIS trunk deformity. The negative regression of UAL asymmetry may result from (1) older girls having less residual growth and/or (2) a transient, or resolving, asymmetry process common to arm and trunk. We hypothesise that the pathogenetic process of RT-AIS may include two components: a transient bilateral asymmetry process and growth velocity, both of which affect trunk and arm growth. In the spine, these rarely lead to scoliosis resolution because biomechanical, postural, melatonin-signalling, and other factors sustain and aggravate the curve. Four pathomechanisms may induce the asymmetry process in trunk and arms involving (1) neuromuscular function, (2) motor cortex, (3) sympathetic nervous system, and (4) intrinsic time-tallies in growth plates, some of which suggest therapeutic possibilities.
