Fifth-Toe Deformities

Anatomically, the forefoot is considered the portion of the foot that extends from the tarsal-metatarsal joint to the tips of the toes, and pathology of the toes is typically subdivided into the pathology of the hallux, or great toe, and pathology of the lesser toes. The fifth toe is the forefoot's most distal and lateral structure and comprises the proximal, middle, and distal phalanges. The proximal phalanx articulates with the metatarsal at the metatarsophalangeal joint, and in turn, the proximal phalanx articulates with the middle phalanx at the proximal interphalangeal joint, and the middle phalanx articulates with the distal phalanx at the distal interphalangeal joint. Layer 1, or the most superficial layer, is comprised of the following structures: The abductor hallucis muscle serves to abduct the great toe, whereas the flexor digitorum brevis muscle which inserts on the base of the middle phalanx of toes 2 to 5 and flexes the proximal interphalangeal joints and the abductor digiti minimi muscle which serves to abduct the fifth toe. Layer 2 is immediately deep to layer 1 and comprises the flexor digitorum longus tendons, which insert on the base of the distal phalanx of toes 2 to 5 and serve to flex the proximal and distal interphalangeal joints. The flexor hallucis longus tendon inserts on the base of the great toe's distal phalanx and flexes the great toe's interphalangeal joint. The quadratus plantae muscle, which inserts on the tendon of the flexor digitorum longus, assists with the flexion of toes 2 to 5. Finally, the lumbrical muscles, which originate from the tendon of the flexor digitorum longus and insert on the extensor digitorum longus, serve to flex the metatarsophalangeal joints and extend the interphalangeal joints of toes 2 to 5 Layer 3 comprises the flexor hallucis brevis muscle, which inserts on the base of the proximal phalanx of the hallux and flexes the great toe. Within the 2 heads of the flexor hallucis brevis lie the sesamoid bones of the great toe. The adductor hallucis muscle, comprised of an oblique and transverse head, adducts the great toe. Finally, the flexor digiti minimi brevis muscle, which inserts on the base of the fifth toe proximal phalanx, flexes the fifth toe at the metatarsophalangeal joint. Layer 4 is the deepest and comprises the dorsal interosseous muscles, which abduct the toes at the metatarsophalangeal joints. In contrast, the plantar interosseous muscles serve to adduct the toes at the metatarsophalangeal joints. The peroneus longus tendon travels from lateral to medial within the foot and inserts on the medial cuneiform, providing eversion and flexion to the ankle joint. Finally, the tibialis posterior tendon inserts on the navicular and acts as a foot supinator and inverter, crucial to maintaining the foot arch. The dorsum of the foot contains the muscle bellies of the extensor digitorum brevis and extensor hallucis brevis. The extensor digitorum longus originates in the anterior compartment of the lower leg and inserts on the dorsum of the middle and distal phalanges of toes 2 to 5, serving to extend the toes and assist in ankle dorsiflexion while the extensor hallucis longus inserts on the dorsal base of the distal phalanx of the great toe and thus extend the interphalangeal joint of the hallux. Understanding the anatomy of the foot is critical to understanding its various deformities. Fifth-toe deformities are often congenital and include an overlapping fifth toe, a congenital curly toe, and a bunionette deformity.