An Especially Unique Case of Accessory Flexor Muscles to All Five Digits with Clinical Implications for Occupational Therapy
Authors: Snow Ethan , Tyce Alexis 
Published: May 2022

INTRODUCTIONAnatomical variations in antebrachial musculature are relatively common. Most are asymptomatic, but some develop serious sequelae. Extra tendon structures in the carpal tunnel can cause carpal tunnel syndrome (CTS), accessory muscle attachments can cause symptomatic supernumerary muscle belly (SSMB) syndrome, and compression of the anterior interosseus nerve from overlaying supernumerary muscles can cause anterior interosseus nerve syndrome (AINS). Many studies have reported accessory muscles to the flexor pollicis longus (aFPL) or flexor digitorum profundus (aFDP), but few have examined co‐occurrences of these muscles, and none have documented a case where they collectively send isolated flexion forces to all five digits. The objective of this study is to analyze an especially unique case of accessory flexor muscles to all five digits with respect to CTS, SSMB syndrome, and AINS clinical implications for Occupational Therapy (OT).METHODSOne aFPL and four aFDPs were found co‐occurring during routine cadaver dissection at the University of Nebraska Medical Center. All neurovascular and muscular components within the flexor compartment of the antebrachium were carefully cleaned of extraneous fascia. Contiguous muscles were reflected to view the muscle bellies, tendons, proximal attachments (origins), and distal attachments (insertions) of the aFPL and aFDPs. The aFPL and aFDPs were measured and photographed in situ, and maximal isometric force (Fmax) was calculated for each muscle. The normal FPL and FDP were also analyzed to gauge accessory muscle impact. A review of relevant literature was conducted to discuss clinical implications of these muscles for OT practice.RESULTSA bicipital accessory muscle to FDP, with radial (aFDPr) and ulnar (aFDPu) heads, was discovered adjacent to an accessory muscle to FPL (aFPL). The aFDPr/u and aFPL originated from the coronoid process of the ulna and were innervated by the posterior‐lying anterior interosseus nerve. The aFDPr flexed digits 3 and 4, and aFDPu flexed digits 4 and 5. The aFPL was especially unique: part of its tendon merged with the proximal third of FPL (to flex digit 1) while the remainder continued distal, drawing two additional distinct muscle bellies from FPL (aFDPFPL) and FDP (aFDPFDP) and distributing these adjoining forces to digits 2, 3, and 4 via an aponeurosis in the carpal tunnel. The calculated Fmax for aFPL, aFDPr, aFDPu, aFDPFPL, and aFDPFDP were 3.80 N, 1.22 N, 0.98 N, 2.47 N, and 0.62 N respectively and are used to discuss potentials for eliciting a quadriga effect associated with SSMB syndrome. CTS is discussed as a result of carpal tunnel crowding, and AINS is discussed in regard to neuropathy of FDP (radial half), FPL, and pronator quadratus. Each of these conditions is conferred with their predominant implications for OT intervention and treatment.CONCLUSIONS & SIGNIFICANCEThe present study reports a previously undocumented and especially unique case of accessory flexor muscles to all five digits. Its use as a reference may help OT providers better determine when supernumerary muscles are present in clients being treated for prehension, distal pinch, and digit independence limitations caused by CTS, SSMB syndrome, or AINS and adapt treatment modalities accordingly.