Dorsal Interossei

The dorsal interossei of the foot are four intrinsic muscles located between the metatarsal bones. They form part of the deepest, or fourth, layer of the plantar musculature and are distinguished from the plantar interossei by their bipennate structure and their principal action of abducting the toes. The second toe serves as the reference axis for abduction and adduction in the foot.^[1][2]

Each dorsal interosseous muscle arises from the adjacent sides of two metatarsals. Their tendons pass distally to the proximal phalanges and extensor expansions of the second, third, and fourth toes. The first dorsal interosseous inserts on the medial side of the second toe, while the remaining three insert on the lateral sides of the second, third, and fourth toes respectively. This arrangement allows the second toe to be abducted in either direction.

In addition to spreading the toes, the dorsal interossei contribute to control of the metatarsophalangeal joints and participate in coordinated stabilization of the forefoot during weight bearing and locomotion. They are innervated by the lateral plantar nerve, predominantly through its deep branch.

Location and Organization

The four dorsal interossei occupy the four intermetatarsal spaces. Although their name reflects their visibility from the dorsal aspect of the foot, they are deep intrinsic muscles extending through much of the depth between adjacent metatarsals.

They are classified with the plantar interossei in the fourth muscular layer of the sole. The deep plantar arterial arch and deep branch of the lateral plantar nerve are closely related to this layer.

The dorsal interossei are numbered from medial to lateral according to the intermetatarsal space in which they lie:

• First dorsal interosseous: Between the first and second metatarsals.
• Second dorsal interosseous: Between the second and third metatarsals.
• Third dorsal interosseous: Between the third and fourth metatarsals.
• Fourth dorsal interosseous: Between the fourth and fifth metatarsals.

Structure

Each dorsal interosseous muscle is bipennate. Its fibers arise from the facing surfaces of two neighboring metatarsals and converge onto a central tendon.

This arrangement differs from the plantar interossei, which are generally unipennate and arise from single metatarsals. The bipennate architecture allows each dorsal interosseous to occupy a substantial portion of its intermetatarsal space.

The first dorsal interosseous is usually the largest of the group. It occupies the first intermetatarsal space and is related to the passage of the dorsalis pedis artery into the sole through the proximal part of this space.

Origins and Insertions

The attachments of the four muscles are determined by the axis of the second toe. All four arise from adjacent surfaces of the metatarsals, but their distal attachments differ.

Muscle	Origin	Insertion	Abduction Produced
First dorsal interosseous	Adjacent surfaces of first and second metatarsals	Medial side of base of proximal phalanx and extensor expansion of second toe	Moves second toe medially
Second dorsal interosseous	Adjacent surfaces of second and third metatarsals	Lateral side of base of proximal phalanx and extensor expansion of second toe	Moves second toe laterally
Third dorsal interosseous	Adjacent surfaces of third and fourth metatarsals	Lateral side of base of proximal phalanx and extensor expansion of third toe	Moves third toe laterally
Fourth dorsal interosseous	Adjacent surfaces of fourth and fifth metatarsals	Lateral side of base of proximal phalanx and extensor expansion of fourth toe	Moves fourth toe laterally

The fifth toe does not receive a dorsal interosseous insertion. Its abduction is produced principally by abductor digiti minimi. Similarly, abduction of the great toe is produced by abductor hallucis.

First Dorsal Interosseous

The first dorsal interosseous lies in the first intermetatarsal space and is generally the largest of the four. It arises from the adjacent surfaces of the first and second metatarsals and inserts on the medial side of the base of the proximal phalanx of the second toe and its extensor expansion.

Its position medial to the second toe allows it to abduct that toe medially, away from the longitudinal axis through the second digit.

The first intermetatarsal space is also an important vascular region. The deep plantar artery, the terminal continuation of dorsalis pedis, passes between the two heads of the first dorsal interosseous to reach the sole, where it contributes to the plantar arterial arch.^[1][3]

Second Dorsal Interosseous

The second dorsal interosseous occupies the second intermetatarsal space and arises from the facing surfaces of the second and third metatarsals.

Its tendon inserts on the lateral side of the base of the proximal phalanx of the second toe and contributes to the extensor expansion. It therefore abducts the second toe laterally.

The first and second dorsal interossei act on opposite sides of the second toe. This is why the second toe can be abducted either medially or laterally, while the third and fourth toes are abducted only laterally relative to the second-toe axis.

Third and Fourth Dorsal Interossei

The third dorsal interosseous arises between the third and fourth metatarsals and inserts on the lateral side of the proximal phalanx and extensor expansion of the third toe.

The fourth dorsal interosseous arises between the fourth and fifth metatarsals and inserts on the lateral side of the proximal phalanx and extensor expansion of the fourth toe.

Both muscles draw their respective digits away from the second toe during abduction.

Innervation

The dorsal interossei are supplied by the lateral plantar nerve, a terminal branch of the tibial nerve. Most of their motor supply is provided by the deep branch of the lateral plantar nerve.^[1][2]

The deep branch curves medially across the deep sole in association with the plantar arterial arch and supplies the interossei as well as adductor hallucis and other deep intrinsic structures. The spinal root contribution is generally described as predominantly S2 and S3.

The pattern is functionally comparable to the deep branch of the ulnar nerve supplying most interosseous muscles in the hand, although the detailed organization of the hand and foot should not be treated as identical.

Blood Supply

The dorsal interossei receive arterial blood from branches of the plantar arterial arch and the dorsal and plantar metatarsal arterial systems.

The plantar arch lies deep in the sole near the bases of the metatarsals and gives rise to plantar metatarsal arteries. Perforating branches communicate through the intermetatarsal spaces with the dorsal metatarsal arteries.

This vascular arrangement creates extensive communications between the dorsal and plantar arterial networks around the interosseous muscles.

Actions

Abduction of the Toes

The principal action of the dorsal interossei is abduction of toes 2 to 4. In the foot, movements of the toes are defined relative to a longitudinal axis passing through the second digit.

This relationship is commonly remembered using the anatomical mnemonic DAB, meaning dorsal interossei abduct. The corresponding plantar interossei adduct toes 3 to 5 toward the second toe.

The first and second dorsal interossei act on opposite sides of the second digit. The first moves it medially and the second moves it laterally. The third and fourth muscles move the third and fourth toes laterally.

Metatarsophalangeal Flexion

The dorsal interossei also assist flexion at the metatarsophalangeal joints. Their tendons pass plantar to the transverse axes of these joints before contributing to the extensor apparatus.

This flexion action is coordinated with the lumbricals and other intrinsic muscles of the toes.

Interphalangeal Extension

Through their attachments to the extensor expansions, the dorsal interossei can contribute to extension of the interphalangeal joints. Their effect is produced through the dorsal digital apparatus rather than through direct insertion onto the distal phalanges.

The combined ability to assist metatarsophalangeal flexion and interphalangeal extension is important for coordinated positioning of the digits.

Relationship to the Plantar Interossei

The dorsal and plantar interossei occupy the deepest muscular plane of the forefoot but differ in structure, attachment, and primary action.

Feature	Dorsal Interossei	Plantar Interossei
Number	Four	Three
Structure	Bipennate	Generally unipennate
Origin	Adjacent sides of two metatarsals	Single metatarsal
Digits	Second to fourth	Third to fifth
Primary action	Abduction	Adduction
Reference axis	Second toe	Second toe
Innervation	Lateral plantar nerve	Lateral plantar nerve

The conventional mnemonic DAB and PAD summarizes these opposing actions: dorsal interossei abduct, while plantar interossei adduct.

Relationship to the Extensor Expansions

The distal tendons of the dorsal interossei attach not only to the proximal phalanges but also to the dorsal extensor expansions of their respective toes.

The extensor expansions receive contributions from the long extensor tendons and intrinsic muscles. Through this arrangement, force generated by the interossei can influence more than one joint within a digit.

This relationship explains how a muscle positioned primarily between the metatarsals can participate in both metatarsophalangeal flexion and interphalangeal extension.

Relationship to the Plantar Arterial Arch

The plantar arterial arch lies in the deep sole in close relationship to the bases of the metatarsals and the interosseous muscles. It is formed mainly by the lateral plantar artery and is usually completed medially by the deep plantar branch of dorsalis pedis.

Branches of the arch pass in relation to the intermetatarsal spaces. The plantar metatarsal arteries run distally and communicate with the dorsal arterial network through perforating vessels.

The deep branch of the lateral plantar nerve accompanies the arterial arch and provides motor innervation to the interossei.

Relationship to Dorsalis Pedis

The relationship of the first dorsal interosseous to the dorsalis pedis artery is particularly important. Near the proximal first intermetatarsal space, dorsalis pedis gives rise to its deep plantar branch.

This vessel passes between the heads of the first dorsal interosseous and enters the sole. It then joins the lateral plantar artery to contribute to the plantar arterial arch.

The first dorsal interosseous is therefore an important landmark in understanding the connection between the dorsal and plantar arterial systems of the foot.

Role in Forefoot Stability

The dorsal interossei occupy the spaces between the metatarsals and can contribute dynamically to forefoot stability. Their attachments allow them to control the positions of the proximal phalanges relative to the metatarsal heads while also influencing the relationships between adjacent rays during muscular activity.

Their stabilizing contribution occurs together with the plantar interossei, lumbricals, flexor tendons, extensor apparatus, plantar ligaments, and other intrinsic muscles.

Although the interossei can influence transverse forefoot mechanics, the structural integrity of the transverse arch should not be attributed to these muscles alone. Bony geometry, tarsometatarsal relationships, ligaments, and extrinsic tendons are also important.

Role During Gait

During stance and propulsion, the toes and metatarsophalangeal joints must remain sufficiently controlled to provide a stable interface between the forefoot and the ground. The dorsal interossei participate in this control by stabilizing the digits and contributing to coordinated metatarsophalangeal flexion.

As the heel rises during late stance, the metatarsophalangeal joints extend and load shifts toward the forefoot. The intrinsic muscles act with the plantar soft tissues and extrinsic tendons to regulate the positions of the toes during this phase.

The dorsal interossei should therefore be considered dynamic stabilizers of the forefoot rather than major independent generators of propulsion.

Anatomical Variations

The size and detailed attachment patterns of the dorsal interossei can vary. Individual muscles may have accessory slips or differences in the extent of their attachment to the proximal phalanx and extensor expansion.

Variation can also occur in the relationship of the first dorsal interosseous to the deep plantar artery. Such differences are usually of greatest importance during anatomical dissection, imaging, and surgical procedures involving the intermetatarsal spaces.

Clinical Significance

Lateral Plantar Nerve Injury

Because the dorsal interossei are supplied by the lateral plantar nerve, lesions affecting this nerve or its deep branch can produce weakness of the interosseous muscles. The functional deficit depends on the level and extent of the nerve lesion and commonly involves other intrinsic muscles supplied by the same pathway.

Weakness may reduce active abduction of the affected toes and impair fine muscular control of the metatarsophalangeal joints.

Intermetatarsal Surgical Anatomy

The dorsal interossei fill the intermetatarsal spaces and are encountered in procedures involving the metatarsals and deep forefoot. Their relationships to metatarsal vessels, plantar neurovascular structures, and the deep plantar artery are particularly important.

The first intermetatarsal space requires special attention because the deep plantar branch of dorsalis pedis passes between the heads of the first dorsal interosseous.

Compartment Anatomy

The interosseous muscles are enclosed within deep fascial spaces associated with the metatarsals. This organization is relevant to the compartmental anatomy of the foot, particularly when swelling or increased pressure affects the deep forefoot.

Precise descriptions of foot compartments vary according to anatomical and surgical classification, so the interossei are best understood as muscles occupying distinct deep intermetatarsal regions rather than assuming a single simplified compartment model.

Imaging

The dorsal interossei are readily demonstrated on MRI and can also be assessed with ultrasound in accessible regions. On cross-sectional imaging, they occupy the spaces between adjacent metatarsals and can be distinguished from the plantar interossei by their larger bipennate configuration and position.

Imaging may demonstrate atrophy or altered muscle signal associated with denervation, as well as direct injury or abnormalities involving neighboring metatarsal structures.

The four dorsal interossei form a deep muscular system linking adjacent metatarsals with the proximal phalanges and extensor expansions. Their bipennate architecture and arrangement around the second-toe axis allow them to abduct toes 2 to 4 while assisting coordinated control of the metatarsophalangeal and interphalangeal joints. Their close relationships with the deep plantar neurovascular structures also make them important landmarks of the deepest layer of the forefoot.