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Third Plantar Interosseous of the Foot

musculus interosseus plantaris tertius pedis

The third plantar interosseous is the most lateral of the plantar interossei, adducting the fifth toe medially from the fifth metatarsal origin. It works in balance with the abductor digiti minimi, which abducts the little toe. Together they control little toe position during the stance phase of gait.

Nerve: Lateral plantar nerve (deep branch) Blood Supply: Fourth plantar metatarsal artery Region: Foot & Ankle

Anatomical Data

Origin, Insertion & Supply

Origin	Medial side and base of the fifth metatarsal (unipennate)
Insertion	Medial base of the proximal phalanx of the fifth toe and extensor hood
Nerve Supply	Lateral plantar nerve (deep branch)
Blood Supply	Fourth plantar metatarsal artery

Biomechanics

Function & Actions

Actions	Adducts the fifth toe toward the second toe axis; Assists MTP flexion of the little toe

Clinical Relevance

Clinical Notes

The third plantar interosseous is rarely assessed in isolation but is relevant in little toe deformities and in patients with peripheral neuropathy affecting the deep lateral plantar nerve. Bunionette deformity (tailor's bunion) at the fifth metatarsal head alters the mechanical advantage of this muscle. Fifth toe hammer toe and underlapping fifth toe involve imbalance between the third plantar interosseous and abductor digiti minimi.

Palpation

Not directly palpable due to plantar soft tissue depth and small muscle size.

Pathology

Common Injuries & Conditions

Little Toe Adduction Deformity

Fifth toe underlapping from contracted third plantar interosseous, producing chronic fifth toe adduction under the fourth toe, managed with flexor to extensor tendon transfer or proximal phalangectomy.