Brachydactyly type D, also known as short thumb or stub thumb and inaccurately referred to as clubbed thumb, is a condition clinically recognised by a thumb being relatively short and round with an accompanying wider nail bed. The distal phalanx of affected thumbs is approximately two-thirds the length of full-length thumbs. It is the most common type of brachydactyly, or shortness of digits, affecting approximately 2–3% of the population, and is associated with the HOXD13 gene, located on chromosome 2q31.1

Physiology
Brachydactyly type D is a skeletal condition which exhibits a partial fusion or premature closing of the epiphysis with the distal phalanx of the thumb, according to Goodman et alia (1965). J.K. Breithenbecher (1923) found that distal phalanges of stub thumbs were one-half the length of full-length thumbs, while R.M. Stecher (1957) claimed that it is approximately two-thirds. The condition may either be unilateral (affecting one thumb) or bilateral (affecting both). Genetics
A genetic trait, brachydactyly type D exhibits autosomal dominance and is commonly developed or inherited independently of other hereditary traits. The condition is associated with the HOXD13 gene, which is central in digital formation and growth.Various other studies supported an autosomal dominant pattern with reduced penetrance. Hereditary trait
A 1965 scientific study in Israel found that 3.05% of Palestinians in Israel had one or two stub thumbs, compared with 1.57% among Ashkenazi as well as non-Ashkenazi Jews.
The lower extremities often show atrophy and seldom show hypertrophy resulting in limb circumference discrepancy.When located on the trunk, the lesions of CMTC tend to show mosaic distribution in streaks with a sharp midline demarcation seen across the abdomen. The lesions are primarily localized, but can be segmental or generalized, often unilateral in appearance. Diffuse involvement of the skin is usually not observed. Although its course is variable, the majority of lesions in mild cases fade by adolescence. Ulceration and secondary infection are complications in severe cases and can be fatal if present in the neonatal period. Causes
Fewer than 100 cases of CMTC have been published worldwide. Petrozzi reported the first case of CMTC in the United States in 1970. CMTC is believed to be more common than suspected, as studies have shown that milder forms of the disease are not being recognized as CMTC.The pathophysiology is still unclear, with most cases occurring sporadically, although rare cases were reported in families. Studies indicated the primary involvement of capillaries, venules and veins, and possibly also that of arterioles and lymphatics. Hypotheses that have been proposed include: environmental/external factors; peripheral neural dysfunction; failure of the development of mesodermic vessels in an early embryonic stage; autosomal dominant inheritance with incomplete penetrance and, finally, the theory of Happle. Diagnosis
Differential diagnosis
During the first few weeks after birth, when the lesions are not very reticulated, CMTC may look very similar to vascular lesions such as port-wine stains. However, during follow-up, CMTC lesions become characteristic in their appearance.