Hypoplasia or aplasia of the tibia with polydactyly (THYP) is an extremely rare disorder, characterized by under-development of both the shank bones (tibial hypoplasia) and the presence of extra fingers and/or toes (polydactyly). Patients may also show some additional features, including thickened and/or duplicated fibula, hand syndactyly, and triphalangeal thumb. Only very few cases of this congenital malformation syndrome are known worldwide, and very little information is available.
Mutations in the SHH (sonic hedgehog signaling molecule) regulatory element of the LMBR1 gene is known cause THYP. The few families in which the disease has been described, it appears to follow an autosomal dominant mode of inheritance.