Overview
The tarsus (ossa tarsi) is the group of short bones of the hock, between the tibia proximally and the metatarsus distally. The horse has six tarsal bones in three rows. They form four joints: the tarsocrural, between tibia and talus, where practically all flexion and extension happens; and the proximal intertarsal, distal intertarsal and tarsometatarsal joints, which barely move and absorb load. Unlike ruminants, the equine talus has only one trochlea, the proximal one.
Bones
- Talus (talus): medial in the proximal row. Its trochlea has two tall ridges set in a spiral, oblique to the limb axis, that fit the cochlea of the tibia; as a result the foot moves slightly laterally as the hock flexes.
- Calcaneus (calcaneus): lateral in the proximal row. Its tuberosity (tuber calcanei) forms the point of the hock and receives the common calcaneal tendon; the superficial digital flexor tendon forms a cap over it. The sustentaculum tali guides the deep digital flexor tendon medially.
- Central tarsal bone (os tarsi centrale): middle row, distal to the talus.
- First and second tarsal bones: fused into a single small bone on the plantaromedial side.
- Third tarsal bone: in the distal row, between the central tarsal bone and Mt III.
- Fourth tarsal bone: lateral, spanning the height of the two distal rows.
Clinical relevance
- Bone spavin: osteoarthritis of the distal intertarsal and tarsometatarsal joints, one of the most common causes of pelvic limb lameness in sport horses. It is diagnosed by intra-articular anaesthesia and radiography.
- Osteochondrosis of the tarsus: at the distal intermediate ridge of the tibia and the lateral trochlear ridge of the talus; it causes distension of the tarsocrural joint (bog spavin).
- Subcutaneous calcaneal bursitis (capped hock): soft swelling over the point of the hock, almost always from knocks against walls or the trailer.
- Plantar desmitis (curb): thickening of the plantar aspect of the tarsus, distal to the point of the hock, from injury to the long plantar ligament or the neighbouring tissues.