STUDY DESIGN: Frontal plane geometry of postoperative curves was analyzed using a geometric model to investigate the relationship between coronal decompensation and postoperative apical shifts from the center sacral line for various thoracic and lumbar Cobb angles. OBJECTIVE: To determine if a balanced spinal configuration is possible when the postoperative lumbar curve is larger than the thoracic curve, and to determine the limits on the postoperative magnitude of the lumbar curve relative to the thoracic curve beyond which a spinal configuration with acceptable balance cannot be achieved. SUMMARY OF BACKGROUND DATA: Previous studies have suggested that overcorrection of the primary thoracic curve may be the principal cause of coronal decompensation after selective thoracic correction and fusion in King Type II curves. Also, other causative factors, such as inappropriate selection of fusion levels and hook patterns, have been implicated as possible reasons for decompensation after Cotrel-Dubousset instrumentation for idiopathic scoliosis. METHODS: Postoperative thoracic curves of 20 degrees, 25 degrees, and 30 degrees were simulated on a model spine. For each thoracic Cobb angle, three left lumbar curves were simulated with the lumbar curve larger than thoracic by 5 degrees, 10 degrees, and 15 degrees. For each combination of thoracic and lumbar Cobb angles, spinal configurations corresponding to different lateral shifts of the thoracic and lumbar apical vertebrae from the center sacral line were obtained. RESULTS: For a given combination of postoperative thoracic and lumbar Cobb angles, there is an optimal range of postoperative lateral distance between the thoracic and lumbar apices (relative apical distance) that will maintain acceptable balance (decompensation < or = 10 mm). Smaller values of the relative apical distance will decompensate the spine. For a constant postoperative thoracic Cobb angle, the postoperative distance between the thoracic and lumbar apices needed to maintain a balanced spine increases with increasing postoperative lumbar Cobb angle. Similarly, for a constant difference between the postoperative thoracic and lumbar Cobb angles, the postoperative distance between the thoracic and lumbar apices needed to maintain a balance spine increases with increasing postoperative thoracic Cobb angle. For postoperative thoracic curves of 20 degrees-30 degrees, acceptable balance can be achieved when the magnitude of the postoperative lumbar curve is up to twice the thoracic curve as long as adequate postoperative relative apical distance can be maintained. CONCLUSIONS: Decompensation does not appear to be caused by the relative magnitudes of the postoperative thoracic and lumbar curves, but is a result of inadequate relative distance between the thoracic and lumbar apical vertebrae in the postoperative geometry.