No relationship exists between the correction of the thoracic and the lumbar curves after selective thoracic fusion for adolescent idiopathic scoliosis King type II

In this study we focus on idiopathic scoliosis with a primary thoracic curve and a secondary lumbar curve. We were interested in how the lumbar curve corrects following selective thoracic fusion and whether one can explain or predict the behaviour of the lumbar curve. In the literature it is said that, if the lower level of fusion is properly selected, the lumbar curve spontaneously corrects to balance the thoracic curve after selective thoracic fusion. Most authors have assumed a mechanism whereby improvement of the lumbar curve occurs through counterbalancing the surgical correction of the thoracic curve. The correction of the lumbar curve is said to echo the correction obtained for the thoracic curve. Because of these hypotheses, we postulate there should be a correlation between the correction of the lumbar and the thoracic curves of a scoliosis. To validate this hypothesis, we performed a retrospective study on 27 patients with King type II adolescent idiopathic scoliosis treated by selective thoracic fusion. The mean preoperative Cobb angles were 54degrees for the thoracic curve and 34degrees for the lumbar curve. Postoperatively they were 31degrees and 22degrees respectively. Using Pearson correlation analysis, we found no significant correlation between the relative corrections of the individual thoracic and lumbar curves. Moreover, there was a decrease in the correlation between the thoracic and lumbar curve after operation (preoperative R=0.787, postoperative R=0.364). These results show that the correction of the lumbar curve is not a reflection of the thoracic correction. The exact mechanism by which the lumbar spine corrects remains to be elucidated.