Anomalous Extensor Indicis With an Intermediate Tendon: Surgical and Functional Considerations

The extensor indicis proprius (EIP) is regarded as one of the most consistent muscles of the dorsal forearm, originating from the distal third of the posterior ulnar surface and passing through the fourth extensor compartment to insert into the dorsal digital extensor expansion (DDEE) of the index finger. Despite its relative constancy, the EIP exhibits a notable range of morphological variations, including supernumerary tendons, anomalous insertions, and accessory muscle bellies. A double-bellied configuration of the EIP, particularly with an intermediate tendon bearing a capsular attachment, represents an exceptionally rare anatomical variant with significant biomechanical and clinical implications.  During routine cadaveric dissection of a formaldehyde-fixed female cadaver (approximately 55 years of age), an unusual double-bellied EIP with an intermediate tendon was identified. The proximal belly followed a typical course, transitioning into a tendon that traversed beneath the extensor retinaculum. Distal to the retinaculum, this intermediate tendon gave rise to a second, distinct fleshy belly extending approximately 3.5 cm over the dorsum of the hand before resuming a tendinous form and inserting normally into the DDEE of the index finger. The intermediate tendinous segment also gained an attachment to the dorsal capsule of the wrist joint near the third metacarpal. The extensor digitorum communis (EDC) slip to the index finger demonstrated a normal insertion pattern, and no supernumerary EIP tendons were identified. This variant is interpreted in the context of embryological, evolutionary, and functional frameworks. Incomplete regression of the distal premuscle mass during development likely underlies the persistence of a secondary distal belly, while the capsular attachment of the intermediate tendon may reflect retained embryonic fascial connections analogous to capsular muscles in other joints, such as the popliteus. Functionally, the additional contractile element may augment force distribution and fine motor control of index finger extension, though compartmental crowding within the fourth extensor compartment raises the risk of tenosynovitis, extensor lag, and localized dorsal hand pain. From a surgical standpoint, this variation is directly relevant to tendon transfer procedures, reconstructive hand surgery, and decompression of the fourth dorsal compartment, where failure to recognize such anatomy may lead to inadvertent tendon injury or suboptimal outcomes.  This case expands the documented morphological spectrum of EIP variants and elaborates the importance of recognizing double-bellied configurations with intermediate capsular attachments. Surgeons and anatomists should remain vigilant for such anomalies during preoperative planning and cadaveric dissection, particularly in the context of tendon harvest, transfer, or dorsal wrist decompression procedures.