ADOLESCENT IDIOPATHIC SCOLIOSIS: METRIC ANALYSIS OF THE DEFORMITY

Abstract

Introduction: A parameter in surface topography was developed to measure left-right differences in back surface of different scoliosis patterns, and to relate these to biological asymmetry and the evolution of deformity. Because of the close association between scoliosis and growth, the hypothesis that scoliosis is growth, that it affects not just the spine but the whole body and that it falls into well-described biological patterns of asymmetry, was explored.

Methods: The new measure compares the positions of three points (mid way between the first thoracic vertebra and axilla, and one and two thirds from axilla to posterior superior iliac spines) on either side of the mid-line, reflecting right onto left and expressing the displacement along Cartesian axes in millimetres. The purpose is to measure size and growth differences at diagnosis and during follow-up. Statistical analysis was of prospectively collected topographic, radiographic and clinical data. There were three groups, all female: 1. mild asymmetry (N=84, no radiograph); 2. thoracic (N=65, mean Cobb angle 61.4°±19.5) and 3. thoracolumbar or lumbar (N=40, mean Cobb angle was 51.8°±23.0). Comparisons were made between each group and theoretically perfect symmetry (test value zero). Correlations with Cobb angle change over time were analysed.

Results: Groups one and three showed directional asymmetry in the coronal plane only, and were not statistically different from each other. Group two showed directional asymmetry at all levels, the side of the scoliosis convexity being larger in all three dimensions (left-right, antero-posterior and cranio-caudal). Changes in Cobb angle correlated with statistical significance with change in the vertical height of the convex side.

Conclusions: This topographic measure was developed specifically to quantify the asymmetry of the back surface, to assign it to a biological pattern and to observe how it might change during growth and scoliosis evolution. All levels of asymmetry, the minor as well as the true scoliosis, showed directional asymmetry (normal distribution of left-right differences about a mean that is not zero, genetically determined) which suggests an origin of scoliosis lying in the biology of growth and the evolution of morphology, rather than in a particular disease process. This asymmetry does not cause scoliosis: it is the result of asymmetric growth processes, it is scoliosis. The relevance of this view is that it obviates the need for an identifiable disease process, as scoliosis is a non-specific developmental response to physiological stress. It is the destabilising of the genetic control “programme” that operates in the growing organism to produce an adult phenotype which is an accurate expression of its genotype. This interpretation can explain observations of natural history that currently cause problems viz. the association with growth and development, lateralisation, increased incidence with other medical conditions, and female predominance, the recurrence of deformity after surgical correction and perhaps even the difficulty in reaching a final conclusion on the efficacy of brace treatment.