THE HOCK AND HINDPAW

(See also pp. 90-91 and 93.)

Inspection of the distal part of the limb reveals the distinctive conformation of the hock; when it is taken apart, there is little external difference between the fore­paws and hindpaws beyond the absence of any analogue of the carpal pad.

Although the hock skeleton is complete—there is no suppression or fusion of the standard elements—most bones cannot be individually identified on palpation. The most distinctive feature is the long, rather slender calcaneus, which provides the leverage for the effective extension of the hock; the arrangement carries an intrinsic risk, and the bone is occasionally fractured by the force exerted by the powerful muscles attaching to its slightly swollen tip. The calcaneus extends a medial process, the sustentaculum tali, over the plantar aspect of the talus, where it may be felt despite being covered by the deep flexor tendon (Figure 17-7/3'). The more distal tarsal bones do not have identifying surface fea­tures, but their locations and extents may be deduced after reference to a skeleton or to radiographs. The other prominent surface features of the region are the projections of the tibial and fibular malleoli at the lower limit of the leg and the equally prominent swellings at the proximal ends of the second and fifth metatarsal bones. A long collateral ligament may be traced from the malleolar to the metatarsal thickening on each side of the limb. The extensor tendons can be followed over the dorsal surface of the hock; the retinacula that hold them in place over the distal tibia and again at the proximal end of the metatarsus can also be appreciated in many dogs.

Only the tarsocrural joint is large enough to be punc­tured in the live animal. This is done on the lateral side just distal to the malleolus; the needle is directed distally toward the lateral surface of the palpable lateral troch­lear ridge of the talus.

Similar impressions of the bones and soft structures are obtained as on a palpation of the hind as of the forepaw.

Figure 17-7 Dorsoplantar and lateral radiographic views of the canine (A and B) and feline (C and D) hocks and hindpaws. 1,Tibia; 2, fibula; 3, calcaneus; 3', sustentaculum tali; 4, talus; 5, fourth tarsal bone; 6, central tarsal bone; 7, 7, second and fifth metatarsal bones.

Although a complete radiographic examination of the hock calls for exposures in dorsoplantar, mediolat- eral, and oblique projections, the most useful general picture is obtained from the dorsoplantar view because it permits identification of all the bones, of which some are more easily identified than others as there is consid­erable superimposition (Figure 17-7, A). Both the talus and the calcaneus are well outlined despite the overlap of the sustentaculum tali. The two bones in the subja­cent tier, the fourth and central tarsals (Figure 17-7 Z 5,6), are also generally well outlined, although the mediodis- tal part of the fourth is superimposed on the third. The second tarsal is clearly shown with the smaller first tarsal superimposed on it. The distal extremities of the tibia and fibula appear closely related in this projection; the gap between them is unexpectedly wide in slightly oblique projections obtained of the cat’s hock and is a feature that is occasionally misinterpreted as evidence of luxation.

The lateral projection (Figure 17-7, B) depicts the calcaneus and talus clearly, although they overlap toward the center of the field. The more distal bones are less easily identified in this view, apart from the fourth, which is betrayed by a protuberance on its plantar aspect (Figure 17-8Z√'). Because the central bone is occasionally dislocated, it is important to note the normal alignment of the dorsal borders of the bones of successive tiers. Two previously unrecorded sesamoid bones have recently been described in the Greyhound at the plantar aspect of the hock about the level of the tarsometatarsal joint. They appear to have the usual significance of such sesamoids; that is, they can potentially be misinterpreted as chips fractured from the major bones.

There are no distinctive features of the radiological anatomy of the metatarsal bones and phalanges. The short digital muscles are comparable with those in the front limb.