Absence of tibia is a rare birth defect that is characterized by deficiency of the tibia (the shinbone) with other bones of the lower leg relatively intact. The condition may affect one or both legs. Some cases are isolated birth defects, while others are associated with a variety of skeletal and other malformations. It can also be a part of a recognized syndrome such as Werner's syndrome, tibial hemimelia-polysyndactyly-triphalangeal thumb syndrome, and CHARGE syndrome. The underlying cause is generally unknown. Although most isolated cases occur sporadically in people with no family history of the condition, absence of the tibia can rarely affect more than one family member. Treatment varies based on the severity of the condition, but generally involves surgery (i.e. amputation or reconstructive surgery with a prosthesis adapted to growth).