Metatarsophalangeal (MTP) joints

Articular surfaces

The metatarsophalangeal joints are articulations between the rounded heads of metatarsal bones and the shallow concavities found on the bases of proximal phalanges. The articular surface of the metatarsal bone heads covers mainly the distal and plantar surfaces, and are vertically convex. The prominent plantar articular surface allows for plantar flexion at these joints. The head of the first metatarsal bone is also transversely convex, allowing for a wider degree of abduction/adduction relative to the other toes.

The plantar articular surface of the first metatarsal head is unique as it bears  two well defined facets which are separated by an anteroposteriorly oriented ridge; a larger medial facet covers approximately two thirds of the palmar articular surface, while a smaller lateral one occupies the remaining one-third. These articulate with the sesamoid bones found within the plantar ligament/plate of the big toe.

Similarly, a sesamoid bone may variably be found at the metatarsophalangeal joints of the second and fifth toes.

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Joint capsule and ligaments

Each metatarsophalangeal joint is enclosed by a loose joint capsule that attaches close to the articular margins and is lined by synovial membrane. This fibrous capsule is supported by collateral ligaments on each side and a plantar ligament on its plantar aspect. The capsule is inseparable from these ligaments. The thin dorsal surface of the capsule is reinforced by fibers from the extensor tendons.

There are three types of ligaments associated with the metatarsophalangeal joint: collateral, plantar and deep transverse metatarsal ligaments.

Innervation

The plantar digital nerves are the main nerves that supply the metatarsophalangeal joints. The medial plantar nerve supplies the plantar aspects of the first, second, third and medial half of the fourth metatarsophalangeal joint. The lateral half of the fourth joint and the fifth joint are innervated by digital branches of the lateral plantar nerve.

On the dorsal aspect, the metatarsophalangeal joints are mainly supplied by digital branches of the superficial fibular (peroneal) nerve including the medial aspect of the first metatarsophalangeal joint. Only the lateral side of the first metatarsophalangeal joint and the medial side of the second metatarsophalangeal joint receive innervation from the digital branches of the deep fibular nerve.

Blood supply

On the dorsal surface, the metatarsophalangeal joints are supplied by the dorsal metatarsal arteries which are branches of the dorsalis pedis artery.

The arterial blood supply to the plantar aspects of the metatarsophalangeal joints are by the plantar metatarsal arteries which branch off the deep plantar arch.

The deep plantar arch is formed by the lateral plantar artery (from posterior tibial artery) and the deep plantar artery (from dorsalis pedis artery).

Movements

The metatarsophalangeal joints permit flexion, extension and limited abduction, adduction and circumduction. Each joint has 2° of freedom.

Flexion (plantarflexion) of the metatarsophalangeal joints causes the toes to be pulled together and to bend towards the plantar aspect of the foot. This movement is performed by flexor tendons that pass to the digits. During active flexion, the first metatarsophalangeal joint allows approximately 45° range of motion whereas that of the lateral four metatarsophalangeal joints permit about 40° of flexion.

Extension (dorsiflexion) of the metatarsophalangeal joints is performed by extensor tendons that pass to the digits. During active extension of the metatarsophalangeal joints, the toes are elevated towards the dorsal surface of the foot and are to a small degree spread out and pointed slightly laterally. The range of extension in the first metatarsophalangeal joint is about 70° whereas that of the lateral four metatarsophalangeal joints permit about 40°. The close packed position of the metatarsophalangeal joints in full extension, while the resting, or open packed position is at 10° of extension. The capsular pattern (or loss of passive range of motion during inflammation) of these joints is variable, however it is generally considered to be extension more limited than flexion, in the case of the metatarsophalangeal joint of the big toe.

Abduction and adduction at the metatarsophalangeal joints are minimal and occur relative to the second digit which serves as the midline.

During passive movement of the metatarsophalangeal joints some accessory movements can be elicited. These include minimal sliding (up and down) and rotation of the bases of the proximal phalanges over the metatarsal heads.

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Muscles acting on the metatarsophalangeal joint

All the movements of the metatarsophalangeal joints are performed by the muscles of the leg and foot.

• Flexion of the first digit (great toe) at the MTP joint is primarily by the flexor hallucis longus, the flexor hallucis brevis and the oblique head of the adductor hallucis muscle. Flexion of the lateral four digits is mainly by the flexor digitorum brevis, lumbricals, and both dorsal and plantar interossei. These are assisted by flexor digitorum longus and the quadratus plantae, with the flexor digiti minimi brevis assisting in the fifth digit.
• Extension of the MTP joint of the first digit is carried out by the extensor hallucis longus muscle. Extension of digits 2 to 5 are produced by the extensor digitorum longus and the extensor digitorum brevis muscles.
• Adduction of the MTP of the first digit is by the adductor hallucis muscle, whereas that of the digits 3 to 5 is produced by the corresponding plantar interossei.
• Abduction of the first digit is carried out by the abductor hallucis muscle, while that of the fifth digit is produced by the abductor digiti minimi muscle. Abduction of the MTP of digits 2 to 4 is by the dorsal interossei muscles.