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Thumb hypoplasia is a spectrum of congenital abnormalities of the thumb varying from small defects to complete absence of the thumb.〔Riley, S.A. & Burgess, R.C. (2009). Thumb Hypoplasia. Journal of Hand Surgery, vol 34A, 1564–1573〕 It can be isolated, when only the thumb is affected, and in 60% of the cases〔Ashbaugh, H. & Gellman, H. (2009). Congenital Thumb Deformities and Associated Syndromes. Journal of Craniofacial Surgery, vol 20, number 4, 1039–1044〕 it is associated with radial dysplasia〔 (or radial club, radius dysplasia, longitudinal radial deficiency). Radial dysplasia is the condition in which the forearm bone and the soft tissues on the thumb side are underdeveloped or absent.〔Manske, P.R. & Goldfarb, C.A. (2009). Congenital Failure of Formation of the Upper Limb. Hand Clinics, 25, 157 – 170〕 In an embryo the upper extremities develop from week four of the gestation.〔 During the fifth to eighth week the thumb will further develop.〔Hovius, S., Foucher, G. & Raimondi, P.L. (2002). The Pediatric Upper Limb. London, United Kingdom: Informa Healthcare〕 In this period something goes wrong with the growth of the thumb but the exact cause of thumb hypoplasia is unknown.〔 One out of every 100,000 live births shows thumb hypoplasia.〔 In more than 50% of the cases both hands are affected, otherwise mainly the right hand is affected.〔〔 About 86% of the children with hypoplastic thumb have associated abnormalities.〔〔 Embryological hand development occurs simultaneously with growth and development of the cardiovascular, neurologic and hematopoietic systems.〔 Thumb hypoplasia has been described in 30 syndromes wherein those abnormalities have been seen. A syndrome is a combination of three or more abnormalities. Examples of syndromes with an hypoplastic thumb are Holt-Oram syndrome, VACTERL association〔 and thrombocytopenia absent radius (TAR syndrome).〔 ==Classification== In general there are five types of thumb hypoplasia, originally described by Muller in 1937 and improved by Blauth, Buck-Gramcko and Manske.〔 - Type I: the thumb is small, normal components are present but undersized.〔 Two muscles of the thumb, the abductor pollicis brevis and opponens pollicis, are not fully developed ,.〔〔 This type requires no surgical treatment in most cases.〔〔Light, T.R. & Gaffey, J.L. (2010). Reconstruction of the Hypoplastic Thumb. Journal of Hand Surgery, vol 35A, 474 – 479〕 - Type II is characterized by a tight web space between the thumb and index finger which restricts movement,〔 poor thenar muscles and an unstable middle joint of the thumb metacarpophalangeal joint.〔 This unstable thumb is best treated with reconstruction of the mentioned structures.〔 - Type III thumbs are subclassified into two subtypes by Manske. Both involve a less developed first metacarpal and a nearly absent thenar musculature.〔 Type III-A has a fairly stable carpometacarpal joint and type III-B does not.〔〔〔 The function of the thumb is poor.〔 Children with type III are the most difficult patients to treat because there is not one specific treatment for the hypoplastic thumb. The limit between pollicization and reconstruction varies. Some surgeons have said that type IIIA is amenable to reconstruction and not type IIIB. Others say type IIIA is not suitable for reconstruction too.〔 Based on the diagnosis the doctor has to decide what is needed to be done to obtain a more functional thumb, i.e. reconstruction or pollicization. In this group careful attention should be paid to anomalous tendons coming from the forearm (extrinsic muscles, like an aberrant long thumb flexor – flexor pollicis longus).〔〔〔 - Type IV is called a pouce flottant, floating thumb.〔〔〔〔 This thumb has a neurovascular bundle which connects it to the skin of the hand.〔〔〔 There’s no evidence of thenar muscles and rarely functioning tendons.〔〔 It has a few rudimentary bones.〔〔 Children with type IV are difficult to reconstruct.〔〔 This type is nearly always treated with an index finger pollicization to improve hand function.〔〔 - Type V is no thumb at all〔〔 and requires pollicization.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thumb hypoplasia」の詳細全文を読む スポンサード リンク
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