Attention has been drawn to the variations found in the anatomy of the tendons of the abductor pollicis longus and extensor pollicis brevis muscles as they lie in the first extensor compartment of the wrist. Such variations involve reduplication of the abductor pollicis longus tendon and more rarely the extensor pollicis brevis tendon. An accessory tendon may occasionally lie in a separate osseo-fibrous canal. It is not unusual to find the abductor pollicis longus and extensor pollicis brevis tendons lying in separate compartments, because an accessory fibrous septum is subdividing the first extensor compartment into two parts. Furthermore, the extensor pollicis brevis tendon may also lie in its own separate fibrous canal in the depths and the more distal part of this common compartment.

Such variation could lead to two possible misinterpretations during the course of surgical decompression for de Quervain's disease and these misinterpretations could reasonably be linked with the failure rate for this particular operation.

1. The finding of the abductor pollicis longus and its accessory tendon in a single compartment (12 per cent of wrists in the series) after a limited surgical incision could be mistaken for the abductor pollicis longus and extensor pollicis brevis tendons. Such a mistake would lead to failure to decompress the extensor pollicis brevis.

2. A failure to identify the deeper-lying separate canal for the extensor pollicis brevis in the distal part of the compartment would again be responsible for failure to decompress the extensor pollicis brevis (14 per cent of wrists in this series).