The cause of<unk>  is uncertain. Two hypotheses have been advanced. The theory of sclerotome subtraction posits injury to neural crest cells that are the precursors to sensory nerves at the level of L4 and L5. Histologic studies of a fetus with unilateral<unk>  have prompted an alternative hypothesis that<unk>  is caused by a defect in maturation of chondrocytes (cartilage cells) at the growth plate. In either hypothesis, the agent causing the injury is usually not known. Thalidomide is known to cause<unk>  when the mother is exposed to it in the fifth or sixth week of pregnancy, and it is speculated that exposure to other toxins during pregnancy may also be a cause. Other etiologies that have been suggested, but not proven, include anoxia, ischemia, radiation, infection, hormones, and mechanical force.<unk>  occurs sporadically, and does not appear to be hereditary.
Proximal femoral focal deficiency
Bipartite patella
Luxating patella
Tarsal coalition
Radial dysplasia
Ankle fusion
Attenuated patella alta
Plantar calcaneal bursitis