or chondrodystrophy, a congenital disease that begins prenatally and is manifested by impaired growth of the extremities in length. The infant is born with short arms and legs, which later lag considerably in growth or do not grow at all while the trunk, neck, and head grow normally. Achondroplasia is often associated with other developmental anomalies and mental retardation. The basis of the disease is derangement of the process of ossification at the boundary between the epiphysis and diaphysis. The causes of achondroplasia are unknown.