VARIANTS OF THE EXTENSOR HALLUCIS LONGUS MUSCLE AND THEIR IMPORTANCE IN TENDON TRANSFER SURGERIES

The extensor hallucis longus (EHL) muscle inserts into the dorsum of the base of the distal phalanx of the big toe, yet variants may possess one or more accessory tendinous slips that insert into other neighboring bones or muscles/tendons. The most common insertion variant includes one or two additional slip(s) inserting into base of the proximal phalanx of the big toe, either distal to, medial to, or directly into the extensor hallucis brevis tendon. This variant is called the extensor primi internodii muscle of Wood, and it is usually present as a tendinous slip and rarely found as a separate muscle. Other reported insertions of these EHL tendons include the dorsal aspect of the distal segment of the first metatarsal, the capsule of the first metatarsophalangeal joint (a variant known as the extensor hallucis capsularis), or the tendon of extensor digitorum longus to terminate into the extensor expansion of the second or third toe. These variants are clinically important due to the use of the EHL tendons in tendon transfer surgeries to correct deformities such as hallux varus, equinovarus foot, clawed hallux associated with a cavus foot, and dynamic hyperextension of the hallux. Presence of EHL variants can complicate surgeries of the foot and ankle; therefore, knowledge of EHL variants is crucial for radiologists, clinicians, and surgeons in order to prevent adverse surgical events.