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Anomalous composition of musculature of the first dorsal fibro-osseous compartment of the wrist.

The anomalous orientation of musculature of the first dorsal fibro-osseous compartment of the wrist is clinically relevant to De Quervian's stenosing tenosynovitis and reconstructive surgeries. Split insertion of the abductor pollicis longus (APL) is commonly found in chimpanzees, gorillas and gibbons. A comparable identical pattern of anomalous slips in humans is of anthropological and phylogenetic importance and could be a result of atavism. This case report describes an unusual fused muscle belly of the APL and extensor pollicis brevis (EPB), which split into three slips--medial, intermediate and lateral. Further, the medial slip was seen to divide into two tendons, inserting on the base of the first metacarpal along with the intermediate slip. The lateral slip divided into three tendons, inserting into the base of the proximal phalanx, base of the first metacarpal and abductor pollicis brevis muscle. The fusion and unusual insertion pattern of the APL and EPB merits documentation for reconstructive procedures such as tendon transfer and interposition arthroplasty.