Thumb hypoplasia is a part of the spectrum of radial longitundial deficiency (RLD). Classically, there are three parts to the small thumb: looseness or laxity at the MCP joint, a tight first webspace, and poor muscles around the thumb. I have previously written about the small thumb and these key points at Small thumb .
Decisions on how to address the small thumb follow an algorithm or treatment plan. Most congenital hand surgeons agree with the following basic principles.
1) If the thumb is small, has poor muscle to power movement, and has instability, then the thumb is reconstructed to allow the best long term function.
2) If the thumb is very small, without good muscle and with poor bone development, then pollicization, or making a thumb from the index finger, may be the best option. This may be a difficult choice for the family but can offer a really great functioning thumb/ hand and also a hand that looks very good. I have blogged about pollicization on several occasions.
3) The most difficult situation is the “tweener” thumb which seem be too big to remove and replace with a pollicized index finger but still too small or too unstable to expect great function.
An untreated hypoplastic thumb can provide acceptable function in the young child but with tasks that require finer skills and dexterity (i.e., as the child reaches school age and beyond), the thumb may not be able to do its job. Without a good thumb, hand function will be poor. Additionally, with time, the thumb may assume a progressive worsening posture as in the case below. In this patient, the thumb (and really the hand in general) was simply not used for many activities. The fingers were useful, the thumb not at all.
|Thumb deformity with hypoplastic thumb|
|Thumb deformity with hypoplastic thumb. Note that there are some thumb muscles but smaller than typical.|
|Marked thumb deformity with hypoplastic thumb|
|The thumb cannot serve its normal purpose given the position and deformity.|
|Note the zig-zag deformity of the thumb. Both the CMC joint and MCP joints are markedly out of position.|
Reconstruction of such a thumb is a real challenge. The thumb most importantly needs a stable MCP joint and CMC joint. The distal joint of the thumb has already been fused and so, at most, fusion of either the MCP OR the CMC joint can be considered but not both. But the thumb also needs a tendon transfer for muscle power. The other option, given how good the fingers are, is to consider removal of the thumb (which clearly has a large number of serious problems) and pollicization of the index finger.
Charles A. Goldfarb
My Washington University Bio