The extensor pollicis et indicis accessorius muscle: Case analysis and clinical implications

Ethan L. Snow, Jay J. Byrd, Alexis M. Tyce, Travis L. McCumber

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Introduction: Independent movement of the pollex (thumb) and index finger is critical for proper hand function. Accordingly, extension of the pollex and index finger is predominantly performed by the separate extensor pollicis longus (EPL) and extensor indicis proprius (EIP) muscles, respectfully. Non-typical muscles such as the extensor pollicis et indicis accessorius (EPIA) can inhibit pollex and index finger independence by connecting the otherwise autonomous digits. The objective of this study was to present a case of bilateral EPIA muscles with gross imaging, MRI, histological inspection, biomechanical analysis, and clinical implications. Materials and methods: Left and right EPIA muscles and tendons were discovered during routine cadaver dissection. Three-tesla (3T) T1-weighted magnetic resonance imaging (MRI) visualized the left EPIA in relation to contiguous anatomy. The dissection field of the left EPIA was cleaned and photographed, and the innervation and sites of proximal and distal attachment for EIP, EPIA, and EPL muscles were noted bilaterally. The bilateral EIP, EPIA, and EPL were detached, splayed, photographed, and weighed. Pennation angle, muscle length, and fascicle length for each muscle were measured, and left and right fascicle lengths were normalized via histological inspection of EPIA mean sarcomere lengths. Physiological cross-sectional area (PCSA), maximum isometric force (Fmax), and difference index (δ) were calculated to compare EIP, EPIA, and EPL biomechanics and structural similarity. Mean results of left and right structures were discussed when no significant difference existed between sides. Results: Gross analysis and MRI showed an independent muscle belly of EPIA originating on the distal third of the ulna and interosseus membrane (posterior aspects), positioned between EIP and EPL. Its common tendon coursed through the fourth extensor compartment of the wrist, bifurcated over the trapezoid, and inserted on the first distal phalanx (via EPL and extensor expansion of the index finger. The posterior interosseous nerve (PIN) coursed through the EPIA to innervate EIP. Mean sarcomere lengths of left and right EPIA muscles were 2.38 ± 0.12 μm and 2.92 ± 0.07 μm, respectively. The PCSA of the EIP, EPIA, and EPL was 0.68 cm2, 0.41 cm2, and 0.76 cm2 and constituted a Fmax of 15.22 N, 9.19 N, and 17.00 N, respectively. Left and right EPIA muscles were structurally similar (δ < 0.30), and EPIA was structurally similar to EIP bilaterally. Conclusions: While EPIA generates synergistic movement of the pollex and index finger during simultaneous extension, it may limit extension and/or flexion during complex, antagonistic movements of the pollex and index finger. According to the present study, EPIA may increase the force of extension from EIP or EPL by 60.38% or 54.06% respectively, or 30.19% and 27.03% if distributed equally. EPIA may entrap PIN and the resulting compression could cause potential paralysis of EIP, exacerbating clinical implications and complicating diagnosis and symptom treatment. The gross analysis, MRI, histological inspection, biomechanical analysis, and clinical implications in this report may serve as valuable resources for orthopedic surgeons, occupational and physical therapy providers, and medical educators when presented with a similar clinical or educational case.

Original languageEnglish (US)
Article number100227
JournalTranslational Research in Anatomy
StatePublished - Nov 2022


  • Accessory muscle
  • Biomechanical analysis
  • Extensor indicis
  • Extensor pollicis
  • Magnetic resonance imaging
  • Sarcomere histology

ASJC Scopus subject areas

  • Anatomy


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