Triphalangeal thumb (TPT) is a congenital malformation where the thumb has three phalanges instead of two. The extra phalangeal bone can vary in size from that of a small pebble to a size comparable to the phalanges in non-thumb digits. The true incidence of the condition is unknown, but is estimated at 1:25,000 live births. In about two-thirds of the patients with triphalangeal thumbs, there is a hereditary component. Besides the three phalanges, there can also be other malformations. It was first described by Columbi in 1559. Signs and symptoms The triphalangeal thumb has a different appearance than normal thumbs. The appearance can differ widely; the thumb can be a longer thumb, it can be deviated in the radio-ulnar plane (clinodactyly), or thumb strength can be diminished. In the case of a five-fingered hand it has a finger-like appearance, with the position in the plane of the four fingers, thenar muscle deficiency, and additional length. There is often a combination with radial polydactyly. Complications Generally, triphalangeal thumbs are non-opposable. In contrast to most people with opposable thumbs, a person suffering from TPT cannot easily place his or her thumb opposite the other four digits of the same hand. The opposable thumb's ability to effortlessly utilize fingers in a "pinch" formation is critical in precision gripping. For the thumb to adequately grip, certain thumb criteria must be met (e.g. suitable position and length, stable joints and good thenar muscle strength). Because triphalangeal thumbs cannot easily oppose and do not possess many of the optimal qualities found in most opposable thumbs, they tend to cause the hand to be less effective in use and, therefore, prove to be more problematic in daily life. Cause Malformations of the upper extremities can occur in the third to seventh embryonic week. In some cases the TPT is hereditary. In these cases, there is a mutation on chromosome 7q36. If the TPT is hereditary, it is mostly inherited as an autosomal dominant trait, non-opposable and bilateral. The sporadic cases are mostly opposable and unilateral. Syndromes Triphalangeal thumb can occur in syndromes but it can also be isolated. The triphalangeal thumb can appear in combination with other malformations or syndromes. Syndromes include: Holt–Oram syndrome Aase syndrome Diamond–Blackfan anemia Townes–Brocks syndrome Malformations include: Radial polydactyly Syndactyly Claw-like hand or foot Diagnosis Classifications There are multiple classifications for the triphalangeal thumb. The reason for these different classifications is the