Overview
The metatarsal bones (ossa metatarsalia) are homologous to the metacarpals of the thoracic limb, but longer and somewhat more compressed from side to side. The dog stands on its digits (digitigrade stance), so the metatarsals form an oblique column between the tarsus and the phalanges and do not touch the ground. The four main metatarsals (II to V) are held together by interosseous ligaments and by the interosseous muscles, which arise from their plantar surfaces.
Number and arrangement
- Metatarsal I: almost always rudimentary, reduced to a small proximal bone, or absent. When a dewclaw is present on the pelvic limb it usually articulates with it; in some breeds the dewclaw is double and often has no bony connection to the tarsus.
- Metatarsals II to V: weight-bearing. The central ones, III and IV, are the longest and strongest; the peripheral ones, II and V, are shorter and diverge slightly, giving the paw its oval outline.
Parts of each bone
- Base (basis): irregular proximal end that articulates with the distal row of the tarsus (II with tarsal bones I and II, III with tarsal III, and IV and V with tarsal IV) and with the neighbouring bases.
- Shaft (corpus): roughly triangular in section, with a smooth dorsal surface and a concave plantar surface.
- Head (caput): distal end with a convex articular surface and a sagittal ridge on its plantar face that fits between the two proximal sesamoid bones of the corresponding digit. It articulates with the proximal phalanx at the metatarsophalangeal joint.
Relations
The tendons of the digital extensors and the dorsal vessels and nerves of the paw run over the dorsal surface; the tendons of the superficial and deep digital flexors run over the plantar surface, protected by the metatarsal pad. The dorsal pedal artery passes between the second and third metatarsals to reach the plantar arch.
Clinical relevance
- Metatarsal fractures: common after road traffic accidents and crush injuries. If only one peripheral bone is involved a bandage or splint is enough; when two or more, or one of the main bones (III or IV), are fractured, internal fixation is recommended.
- Stress fractures: described in racing greyhounds from repeated loading on the track.
- Radiographic landmark: on the dorsoplantar view the metatarsals should line up with the tarsal bones; deviation indicates tarsometatarsal luxation.
- Bandaging: the thin soft-tissue cover on the dorsal surface favours pressure sores under poorly padded splints.