Prevalence, evaluation, and clinical implications of a reticular tunnel formed by uncharacteristic distal fibers of the abductor pollicis longus

Introduction: Variations in muscles of the pollex are common, and some can elicit significant clinical implications. The abductor pollicis longus (APL) frequently exhibits non-typical morphologies; however, descriptions of uncharacteristic distal fibers from APL (APLudf) that form a reticular tunnel around the tendons of extensor carpi radialis longus (ECRL) and brevis (ECRB) are scarce in current literature. The objective of this study is to investigate the prevalence of APLudf with concurrent report of in situ and cross-sectional gross imaging. Methods: The distal aspect of 110 APL muscles was examined for APLudf and reticular tunnel formation. Reticular tunnel length was measured over the long axis of the underlying ECRL tendon, and APLudf pennation angle was recorded. Characteristic APL and APLudf examples were photographed in situ, and one example of APLudf was cross-sectioned to show the bony attachment of the atypical fibers in relation to contiguous anatomy. A unique finding of an APLudf accessory tendon inserting onto an accessory abductor pollicis brevis muscle was removed, splayed, and photographed. Results: APLudf were present in 65 (59.1%) of upper limbs. The APLudf originated from a free origin over the ECRL and ECRB tendons and a fixed origin from the lateral margin of the distal radius and inserted onto the main APL tendon or an accessory tendon, creating a distinct reticular tunnel around the ECRL and ECRL with a mean length of 20.25 ± 5.55 mm (range = 11.09 mm–36.09 mm) and 12.99° ± 1.44° pennation. All APL muscles displayed one main tendon and at least one accessory tendon which originated solely from APLudf on 43 (39.1%) of the specimens. Conclusions: The present study suggests the prevalence of APLudf is 59.1% and often occurs bilaterally. The restriction of intersecting movements within the APLudf reticular tunnel could cause ECRL and ECRB tendon inflammation (i.e., intersection syndrome), directly affect respective functions (e.g., radial extension of the wrist, abduction and extension of the thumb), and complicate surgical management in the region. This report may inform educators and healthcare providers when deliberating diagnosis, treatment, and healing of associated forearm, wrist, and pollex conditions