Certain connective tissue disorders occur in patients with funnel chest, such as Marfan syndrome (5-8%), Poland syndrome (5.4%), Ehlers-Danlos syndrome (3%) and Sprengle deformity (0.6%), suggesting an intrinsic factor in the development of funnel chest. 2% of funnel chest cases are associated with congenital heart disease and 15-20% with scoliosis. Funnel chest can also be part of some rare syndromes, often with mental retardation or neurological disorders, such as syndromes and spinal muscular atrophy. Thoracic deformities may not be apparent at birth, but become apparent during the rapidly growing adolescent years. Because the thorax is completely ossified after 18 years of age, this defect no longer develops. The sternal depression is caused by abnormal growth of the sternal body and rib cartilage. The exact cause is unknown, although it is suggested that its onset may be related to connective tissue defects, with the majority of affected children having onset at birth or in early infancy. It may be caused by incongruent rib and thoracic growth, such that the rib growth exceeds the rate of cardiopulmonary expansion (which can push the sternum outward) and the sternum may be depressed inward. Once this deformity occurs, it can persist or worsen.