Exhibition of the rare deep abductor-flexor of the fifth digit with concurrent hypothenar aberrations

Intrinsic coordination of the fifth digit of the hand is largely controlled by the abductor digiti minimi (ADM), flexor digiti minimi brevis (FDMB), and opponens digiti minimi (ODM) muscles (i.e., the hypothenar muscles). Hypothenar muscle variations have been reported; however, biomechanical analyses are seldom performed and descriptions of rare variants such as the deep abductor-flexor of the fifth digit (DAF5) are especially scarce. This study aims to investigate an especially unique case involving the rare DAF5 among additional hypothenar muscle aberrations with biomechanical analysis and discussion of clinical implications. An especially unique case of multiple unilateral (left) hypothenar muscle aberrations was discovered during routine human cadaver dissection. The aberrations were cleaned of extraneous fascia and photographed in situ . Mass and fascicle length were measured and mean postmortem fixed sarcomere states were determined via light microscopy to calculate a normalized maximal isometric force ( F max ) for each muscle. The concurrent aberrations included two accessory ADM muscles, three accessory FDMB muscles, an accessory ODM muscle, and a bicipital DAF5. The curved long head of DAF5 ( F max = 4.77 N) originated from the transverse carpal ligament (across midline), while the straight short head ( F max = 1.23 N) originated from the pisiform. The two heads united to form a prominent 3 cm tendon which inserted on the medial aspect of the base of the fifth proximal phalanx. Notably, the long head separated the ulnar artery (superficial) from the ulnar nerve (deep). The presence of consecutive hypothenar anomalies could influence fifth digit coordination for proper hand function, and the concomitant DAF5 could implicate the underlying median and ulnar nerves. Insights from this report may inform occupational and physical therapists, orthopedic surgeons, and medical anatomy educators when treating or deliberating related cases. • Gross imaging of the rare deep abductor-flexor of the fifth digit. • Biomechanical analysis of consecutive intrinsic hand muscle variations. • Comparative maximal isometric forces of individual hypothenar muscles. • Clinical implications of hypothenar muscle variations.