Variant course of extensor pollicis brevis tendon in the third extensor compartment

According to the anatomical literature, the extensor pollicis brevis (EPB) tendon passes through the first compartment and enters the base of the proximal phalanx of the thumb. There have been a few reports on the different types of supernumerary EPB tendons; however, an unusual course of the EPB tendon is extremely rare. During routine cadaveric dissection in the Department of Gross Anatomy, we detected an variant EPB muscle in a 96-year-old fresh female cadaver. The EPB muscle originated from the posterior surface of the radius and interosseous membrane. However, the EPB tendon passed through the third compartment instead of the first compartment. It ran parallel to the extensor pollicis longus (EPL) tendon and entered the base of the thumb proximal phalanx. The EPL tendon was attached to the base of the first distal phalanx, as normally observed. Both EPB and EPL muscles were innervated by the posterior interosseous nerve. We report a case of a variant course of the EPB tendon appearing in the third extensor compartment of the wrist with the EPL tendon. The knowledge of this anatomic variation will be helpful for accurate diagnosis and surgical planning.