Congenital Clasped Thumb

A thumb held flexed and adducted into the palm with an inability to actively extend it suggests congenital clasped thumb, which is a progressive flexion-adduction deformity caused by a deficient or absent extensor mechanism - typically a hypoplastic or absent extensor pollicis brevis, sometimes with extensor pollicis longus involvement - often with first-web narrowing and metacarpophalangeal-joint laxity. First I would distinguish it from the normal infantile cortical thumb, which is a thumb-in-palm posture in the first few months that resolves, and from a congenital trigger thumb. Then I would assess and classify it: I would test active and passive thumb extension to determine whether it is flexible, meaning passively correctable, or complex and rigid; examine the first web space and thenar muscles; test MCPJ stability with a stress test; and look for associated anomalies and syndromes such as arthrogryposis. The classification drives treatment. For a flexible clasped thumb I would use early splinting first - serial extension splinting started in infancy and worn for a prolonged period, which is frequently successful. For a complex, rigid thumb, or one that fails splinting, I would plan surgery tailored to the deficient structures: a tendon transfer to restore active extension, release of the first-web contracture, lengthening of a tight flexor pollicis longus, and stabilisation of an unstable MCPJ as required. So my approach is to classify the thumb, splint the flexible type early, and reserve tailored surgery for the complex or splint-resistant thumb.