Novel Concomitant Variations of the Tibialis Anterior and Extensor Hallucis Longus: Clinical Implications of Accessory Tendons

The tibialis anterior (TA) and extensor hallucis longus (EHL) muscles are critical dorsiflexors of the foot and essential for gait stability. During a routine cadaveric dissection, a novel combination of anatomical variations involving the TA and EHL was discovered bilaterally in a 90-year-old male donor. Two accessory tendinous slips emerged from the TA primary tendon, one inserting into the proximal phalanx of the hallux and the other onto the shaft of the first metatarsal. Concurrently, an accessory EHL tendon arising from the EHL muscle belly merged with the tendon of the extensor hallucis brevis (EHB) to form a conjoined tendon inserting on the proximal phalanx of the hallux. A literature review was then used to categorize these findings according to established classification systems and reported prevalences. Knowledge of this concomitant variation is useful in preoperative planning and treatment of pathologies such as tendon ruptures and hallux valgus, the development of which has been linked to variations in the EHL and TA similar to those in the case presented here.