The hind paw of the dog is a complex structure made up of many small bones that form multiple joints. The top joint (ankle) is flexible, whereas the other joints are held together by strong ligaments that prevent them from over-extending (hyperextension).
The tarsal bones are the seven skeletal elements of the ankle that articulate with each other and with the tibia and fibula to form the ankle joint. These bones, from proximal to distal, are the talus; the calcaneus, or heel bone; and the six tarsal metatarsal bones. The tarsal bones are joined by a fibrous component of the joint capsule that extends from the distal tibia over the articulation points between the tarsal bones and tibia.
In the wolf, the central tarsal bone is a triangular in shape with a concave medial face and two facets at its distal end that articulate with the large and small metatarsal bones. It extends downward into a nodular projection. The first tarsal bone is wedge shaped, and the second tarsal is a quadrilateral. The third tarsal is irregular cube-like and has one facet on its proximal surface that articulates with the central tarsal and two on its distal surface that articulate with the first metatarsal.
Fractures of the central tarsal bone occur in wolves and a variety of other ruminants. The lateral and medial articular surfaces of the central tarsal are separated by a shallow longitudinal groove, similar to that seen in other ruminants.
In a clinical setting, the diagnosis of a central tarsal fracture is often based on radiographic appearance. A radiograph of the paw with 10 basic views is recommended, but this is not feasible in practice as it requires the patient to be repeatedly repositioned and may cause increased discomfort.
Although isolated fractures of the central tarsal bone have been reported, they are seldom observed in other non-racing dogs. In the few reports that have been described, most involved dogs with concomitant fractures of other tarsal bones and were associated with major complications.
Most tarsometatarsal instability is associated with traumatic hyperextension of the plantar supporting ligaments and collateral ligaments in combination with damage to the articular cartilage. This is common in middle-aged to older, overweight dogs such as collies or Shetland sheepdogs and may be accompanied by laxity of the plantar supporting ligaments. Progressive, degenerative tarsometatarsal hyperextension is also known to occur in these breeds and has been associated with degeneration of the talus, calcaneus, or both. This condition is difficult to treat and usually requires arthroplasty or arthrodesis with the use of plate fixation. This procedure should be reserved for those patients in whom the prognosis is favorable for a successful outcome.