The authors describe two unique clinical cases of closed extensor digiti minimi (EDM) tendon injuries after hyperflexion of the wrist with full finger flexion and one case of chronic tenosynovitis around the EDM tendon. All three cases were thought to be related to the bifurcation of the EDM tendon and synovial septum. Subsequently, variations in EDM tendons were investigated in 49 cadaveric hands with a focus on patterns of tendon bifurcation and their relationships with the surrounding synovial sheath. The EDM tendon was found to be bifurcated in 74% (n = 36) of hands and all of these hands contained a synovial septum. In 9 (25%) hands, the EDM tendon bifurcated proximal to the retinaculum, in 15 (42%), it bifurcated distal to the retinaculum, and in the other 12 hands (33%), the tendon bifurcated at the retinacular level. In 6 of the 15 hands with an infraretinacular bifurcation, the tendon was found to impinge on the synovial septum during passive flexion of the wrist with full finger flexion, and the mean distance between the synovial septum and the bifurcation point in these specimens was 0.6 cm (range, 0.4-0.7 cm), which was differed significantly from hands not showing impingement (P = 0.01). This study shows that distal bifurcation of the EDM tendon may lead to tendon impingement on the septum and suggests that this is a potential etiology of chronic tenosynovitis of the fifth compartment and of acute closed tendon injuries.
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