Radial dysplasia is the condition in which the forearm bone and the soft tissues on the thumb side are underdeveloped or absent.
[2] In general there are five types of thumb hypoplasia, originally described by Muller in 1937 and improved by Blauth, Buck-Gramcko and Manske.
[1][5] - Type II is characterized by a tight web space between the thumb and index finger which restricts movement,[5] poor thenar muscles and an unstable middle joint of the thumb metacarpophalangeal joint.
[2] Children with type III are the most difficult patients to treat because there is not one specific treatment for the hypoplastic thumb.
[4] 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.
[3][4][5] - Type IV is called a pouce flottant, floating thumb.
[1][4] This type is nearly always treated with an index finger pollicization to improve hand function.
[4] Severe thumb hypoplasia is best treated by pollicization of the index finger.
[3][5] It has been recommended that pollicization is performed before 12 months, but a long-term study of pollicizations performed between the age of 9 months and 16 years showed no differences in function related to age at operation.
[3][4] The less developed the index finger, wrist and fore-arm is, the less strength the reconstructed thumb will have after surgery.