The Extensor Pollicis et Indicis Accessorius Muscle: Cadaveric Case Analysis and Pertinence to Occupational Therapy

Abstract

INTRODUCTION & OBJECTIVE Independent movement of the pollex (thumb) and index finger is critical for normal hand function. Accordingly, movements of these digits are controlled by separate muscles; for example, extension of the pollex and index finger is performed by the extensor pollicis longus (EPL) and extensor indicis proprius (EIP) muscles, respectfully. Accessory muscles such as the extensor pollicis et indicis accessorius (EPIA) can develop and inhibit the independent movements of the pollex and index finger by connecting the otherwise autonomous digits. EPIA's attachment to both the pollex and index finger merits special consideration for human occupation and occupational therapy (OT) treatment. The objective of this study was to analyze bilateral EPIA muscles in a human cadaver in regard to occupational therapy. MATERIALS & METHODS The tendons of EPIA were discovered during routine cadaver dissection at the University of Nebraska Medical Center. T1-weighted magnetic resonance imaging (MRI) was used to visualize the entire EPIA's location and relation to surrounding structures. The EPIA was photographed (iPad Pro, Apple Inc.), and deep dissection was performed to identify its origin, insertions, and innervation. The muscle bellies of EPL, EIP, and EPIA were eviscerated, weighed (Ohaus Scale, Model V22PWE1501T), and photographed. Pennation angles and muscle fiber lengths for each of these muscles were measured (ImageJ, 1.52q) to calculate physiological cross-sectional areas (PCSAs) and maximum isometric force (MIF) generating capacities. All calculations were derived from mean values between the bilateral muscles. MIFs for EPL, EIP, and EPIA were compared to substantiate the effect of EPIA on digit movement. RESULTS MRI and gross imaging showed an independent muscle belly of EPIA originating on the posterior ulna and interosseus membrane (distal third) and positioned between EPL and EIP. Its common tendon passed through the fourth extensor compartment of the wrist, bifurcated over the trapezoid, and inserted on the first distal phalanx and extensor expansion of the index finger. The EPIA's 11.77 cm unipennate muscle belly was innervated by the posterior interosseus nerve. The PCSA of the EPL, EIP, and EPIA was 0.76 cm2, 0.68 cm2, and 0.41 cm2, generating MIFs of 17.00 N, 15.21 N, and 9.19 N, respectively. In this regard, MIFEPIA would have increased the collective synergistic MIFs of EPL and EIP by 28.53%. EPIA's split MIF would exert 27.02% and 30.21% more force for independent digit extension by EPL and EIP, respectively. CONCLUSIONS & SIGNIFICANCE While the EPIA would assist simultaneous extension of the first and second digits, it may considerably impact hand function by inhibiting flexion of one digit when extending the other. This outcome could impact range of motion and strength progress during OT treatment following an EPL or EIP injury and require adjusted treatment. Similarly, the traditional OT treatment protocol for EIP or EPL tendonitis may be inadequate with the presence of an unsuspected EPIA or EPIA tendonitis. This study presents radiologic imaging, gross photography, and biomechanics of EPIA, all of which can be used to inform OT and other related healthcare fields about EPIA and the complications it can cause.