The treatment of thumb duplication depends upon the level of the anomaly. Pre-operatively, the child must have a paediatric review to assess for other anomalies. Splintage is a useful adjunct for a tight first webspace. The family must be aware that the reconstructed digit will often be smaller than expected but function is usually excellent. Plain radiography will identify the level of the duplication prior to surgery but the level of joints and their stability are confirmed under anaesthesia prior to surgery.
Typically, surgery is carried out when general anaesthesia is more facile at around 6 to 9 months of age. If there will be a predicted requirement for bone modification eg osteotomies or the removal of associated anomalies such as a delta phalanx, there is an argument for delaying surgery until the hand is larger at one year of age.
The overall principles of thumb duplication surgery are reduction of the anomalous part and reconstruction. Often, the radial-sided digit is excised and the ulnar digit preserved. However, the decision about which digit to keep may be more difficult if the the radial digit is more completely formed or stable from a functional perspective. A C-shaped (delta) phalanx may produce abnormal angulation with time and is best treated with an osteotomy. A highly unstable interphalangeal joint may make fusion the only option. Each case must be carefully assessed.
Longterm problems after surgery include stiffness, joint instability and angular deformity with growth. Secondary surgery is not uncommon in adolescence.
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