Overview
The tarsus of the dog consists of seven bones arranged in three rows between the tibia and fibula proximally and the metatarsal bones distally. Together they form the hock joint (articulatio tarsi), a composite joint in which almost all flexion and extension takes place in the proximal row, while the middle and distal rows are nearly rigid and act as shock absorbers.
Proximal row
- Talus (talus): the medial bone of the proximal row. Its trochlea (trochlea tali), with two sagittal ridges, fits into the tibial cochlea and defines the hinge axis of the tarsocrural joint. Its head articulates distally with the central tarsal bone.
- Calcaneus (calcaneus): the lateral bone and the largest of the tarsus. It extends proximally as the calcaneal tuber (tuber calcanei), the lever of the hock where the common calcanean tendon (Achilles tendon) inserts. On its medial face it carries the sustentaculum tali (sustentaculum tali), over whose plantar surface the tendon of the lateral digital flexor glides. Distally it articulates with the fourth tarsal bone.
Middle row
- Central tarsal bone (os tarsi centrale): interposed between the talus and tarsal bones I, II and III. It exists only in the medial half of the tarsus.
Distal row
- Tarsal bones I, II, III and IV (ossa tarsalia I–IV): I is small and sometimes rudimentary; II and III lie between the central bone and metatarsals II and III; IV is the largest, occupies both the middle and distal rows on the lateral side, and articulates proximally with the calcaneus and distally with metatarsals IV and V.
Joints
- Tarsocrural (tibia and fibula with talus and calcaneus): the main hinge, stabilised by the medial and lateral collateral ligaments, each with a long and a short part.
- Proximal and distal intertarsal and tarsometatarsal joints: low-motion joints reinforced by strong plantar ligaments.
Clinical relevance
- Osteochondrosis of the talus: mainly affects the medial ridge of the trochlea in young Rottweilers and Labradors.
- Central tarsal bone fractures: the typical injury of the racing greyhound, almost always in the right limb because of the loading in the bends.
- Calcaneal fractures and rupture of the common calcanean tendon: the animal rests the hock on the ground (plantigrade stance).
- Tarsal luxations and shearing injuries: loss of a malleolus or a collateral ligament destabilises the tarsocrural joint; irreparable injuries are managed by pantarsal or partial tarsal arthrodesis.