Facet asymmetry in normal vertebral growth: Characterization and etiologic theory of scoliosis

STUDY DESIGN. The shape and orientation of the thoracic and lumbar zygapophyseal facets at the T1-L5 level in children were measured and analyzed. OBJECTIVE. To detect the pattern of zygapophyseal facet asymmetry in the thoracic and lumbar spines in children. SUMMARY OF BACKGROUND DATA. Whereas many studies have defined the pattern of zygapophyseal facet asymmetry in adults, there is insufficient data in children. METHODS. A 3-dimensional digitizer was used to measure zygapophyseal facet size, topography (length, width, concavity, convexity, and lateral interfacet height), and orientation (transverse and sagittal facet angles) at the T1-L5 level. Thirty-two complete, nonpathologic skeletons of children (age range from 4 to 17 years), housed at the Hamman-Todd Human Osteological Collection (Cleveland Museum of Natural History, Cleveland, OH) were assessed. Statistical analysis included paired t tests and analysis of variance. RESULTS. In general, zygapophyseal facet asymmetry in children exists only in the superior facets of the thoracic spine and is independent of age: The right superior facet is significantly shorter than the left in all thoracic vertebrae T1-T12 (up to -2.91 mm at T1), and significantly wider than the left in thoracic vertebrae T1-T9 (T8 excluded) (P < 0.003). The right superior transverse and sagittal facet angles are significantly greater than the left in thoracic vertebrae T1-T11, indicating a lesser inclination (in the sagittal plane) and more frontally positioned facet (in the transverse plane) (P < 0.003). Facet asymmetry was not evident in the superior or inferior facets of the lumbar vertebrae. CONCLUSION. Facet asymmetry in thoracic vertebrae appears in early childhood. The pattern of this asymmetry differs from that reported for adults and may be considered as a possible contributing etiological factor in the development of different types of idiopathic scoliosis.