The metatarsus of the foot consists of five long bones, which are called the metatarsals. Like the metacarpals of the hand, the metatarsals are comprised of a proximal base, a shaft and a distal head. Along with the tarsals, the metatarsals help form the arches of the foot, which are essential in both weight bearing and walking.
The metatarsals connect the ankle with the toes. They are named I to V medially to laterally, from the dorsal surface of the foot. The metatarsal bones are convex on their dorsal surfaces but concave on their plantar surfaces. The proximal base articulates with one or more of the distal tarsal bones; namely the cuboid and the cuneiform bones. These articulations are known as the tarsometatarsal joints.
The distal heads articulate with their corresponding proximal phalanx to form the metatarsophalangeal joints. The head of metatarsal I also articulates with two sesamoid bones on the plantar surface of the foot.
In addition, the bases of the metatarsals articulate with each other to form intermetatarsal joints. The smooth areas on the metatarsals that articulate with other bones are known as articular facets.
The first metatarsal is the thickest and shortest of the metatarsals. At the base there are normally no facets but sometimes there is a facet laterally, where it articulates with the second metatarsal. Proximally, it articulates with the medial cuneiform at the base. Here, there is a tuberosity medially. The shaft is strong and a prismoid shape. Distally, there are two grooved facets where the two sesamoid bones articulate on the dorsal surface. It also articulates distally with the 1st proximal phalanx.
The muscle attachments of the first metatarsal are the following:
- Base – tibialis anterior and fibularis longus
- Tuberosity – fibularis longus
- Lateral part – 1st dorsal interosseus
The second metatarsal is the longest of the metatarsals and has four articular facets at its base. These articulate with the medial, intermediate and lateral cuneiforms as well as the third metatarsal. Occasionally there is a medial facet at the base, which articulates with the first metatarsal. Distally, it articulates with the 2nd proximal phalanx.
The muscle attachments of the second metatarsal are the following:
- Medial shaft – 1st dorsal interosseus
- Lateral shaft – 2nd dorsal interosseus
The third metatarsal has a triangular base, which articulates proximally with the lateral cuneiform. Medially it has two facets where it articulates with the second metatarsal and laterally it articulates with the fourth metatarsal by a single facet. The head articulates with the 3rd proximal phalanx.
The muscle attachments of the third metatarsal are the following:
- Medial shaft – 2nd dorsal interosseus and 1st plantar interosseus
- Lateral shaft – 3rd dorsal interosseus
The fourth metatarsal is smaller than the third and has 3 articular facets at its base. Proximally, it has a quadrilateral facet, which articulates with the cuboid. There is an oval facet medially which articulates with the third metatarsal and there is another single facet on the lateral surface for articulation of the fifth metatarsal. Distally, the head articulates with the 4th proximal phalanx.
The muscle attachments of the fourth metatarsal are the following:
- Medial shaft – 3rd dorsal interosseus and 2nd plantar interosseus
- Lateral shaft – 4th dorsal interosseus
The fifth metatarsal has a tuberosity lateral to the base, which can be both seen and felt on the lateral border of the foot. The base articulates with the cuboid proximally by a triangular surface and medially with the fourth metatarsal. Its head also articulates with the 5th proximal phalanx.
The muscle attachments of the fifth metatarsal are the following:
- Dorsal base – fibularis tertius
- Tuberosity – fibularis brevis
- Plantar base – flexor digiti minimi brevis
- Medial shaft – 4th dorsal interosseus and 3rd plantar interosseus
Along with the calcaneus, the metatarsals are involved in supporting the weight of the body. The metatarsus has 5 main points of contact with the ground:
- The 1st metatarsal head and two sesamoid bones
- The 2nd metatarsal head
- The 3rd metatarsal head
- The 4th metatarsal head
- The 5th metatarsal head
The majority of the load is supported by the thicker 1st metatarsal, with the 2nd-5th metatarsals providing balance and comfort.
Fractures of the metatarsals are uncommon but occur when a heavy object falls or rolls over the foot. These fractures can also occur in ballet dancers when they lose balance whilst on the tips of their toes. This results in the metatarsals supporting the whole body weight, which may cause a fracture of one or more of the metatarsals.
Gout is an inflammatory arthritis characterised by high levels of uric acid in the blood and crystal deposits in the joints and surrounding tissues. The metatarsophalangeal joint of the great toe is often the first joint affected by gout. Gout can cause tenderness and oedema in this joint. When this joint is affected, it is referred to as podagra. Osteoarthritis can also cause severe pain in this joint.
Hallux valgus is a foot deformity characterised by medial deviation of the 1st metatarsal and deviation of the great toe (hallux) laterally. It is often caused by degenerative joint disease or by pressure from footwear. More commonly occurring in females, hallux valgus causes the 1st metatarsal to shift medially and the sesamoid bones to shift laterally. This results in the sesamoid bones lying between the heads of the 1st and 2nd metatarsals. When the surrounding tissues swell, a subcutaneous bursa can form. When this bursa is inflamed this can cause great pain. A painful hallux valgus deformity is referred to as a bunion.