Effect of rib cage deformity in primary thoracic idiopathic scoliosis on pulmonary function, airway morphology and lung volumes

Although scoliosis is defined as a Cobb angle of 10° or more as measured in the coronal plane, the combined three-dimensional distortions to both the spine and rib cage in scoliosis are best conceptualised as a thoracic deformity. There is much interest in the relationship between scoliosis and pulmonary function due to the increased morbidity of respiratory failure and mortality of those with untreated progressive scoliosis. Establishing the mechanisms of pulmonary function impairment in scoliosis is important in identifying patients with compromised lung function or those who will benefit from surgical intervention. In this work, the relationship between thoracic deformity in patients with right-sided, Lenke type 1 or 2, adolescent idiopathic scoliosis (AIS) and pulmonary function impairment is investigated. From radiographic and computer tomographic imaging, measurements of the thoracic deformity were found to be superior predictors of pulmonary function in AIS than conventional Cobb angles. A statistical shape model was constructed from biplanar radiographs to identify modes of variation in the thoracic configuration. Thoracic features such as the extent of the rib hump, narrowed convex hemithoracic width and spinal intrusion were found to be factors contributing to lung function impairment. Morphological analysis of the tracheobronchial tree demonstrated the presence of right-sided airway narrowing. In particular, patients with hypokyphosis demonstrated significant narrowing of the bronchus intermedius and its bifurcation as a result of extrinsic compression by the vertebral column. Right-sided airway obstruction was found to correspond to the presence of atelectasis and air-trapping in the right middle and lower lobe. Post-operative analysis demonstrated that restoration of natural kyphosis in patients with hypokyphosis and scoliosis resulted in improved lung function post-operatively. Although reduction in lung function in patients with scoliosis is multifactorial, variance in the sagittal thoracic profile plays a more important role in impairing lung function than is generally appreciated