The Hock Joint

This is a composite joint made up of a number of articulations (Articulationes tarsi). These are: (1) The tibio-tarsal articulation; (2) the intertarsal articulations; (3) the tarso-metatarsal articulation.

The tibio-tarsal articulation (Articulatio talo-cruralis) is a typical ginglymus formed by the trochlea of the tibial tarsal bone (astragalus or talus) and the corresponding surface of the distal end of the tibia. The ridges and grooves of these surfaces are directed obliquely forward and outward at an angle of about 12° to 15°, with a sagittal plane. The trochlear surface is about twice as extensive as that on the tibia, and its ridges have a spiral curvature. The other articulations are arthrodia, which have joint surfaces and ligaments of such a nature as to allow only a minimal amount of gliding motion.

As in the case of the carpal joints, it is convenient to describe first the common capsule and ligaments, which are the more important practically, and then to consider very briefly the special ligaments.

The fibrous part of the joint capsule is attached around the margin of the tibial articular surface above and the metatarsal surfaces below; it is also attached in part to the free surface of the bones which it covers, and blends with the lateral ligaments. Its anterior part (anterior ligament) is rather thin; in distention of the capsule, as in “bog-spavin,” its antero-internal part, which is not bound down by the tendons passing over the joint, forms a fluctuating swelling over the inner ridge of the trochlea. The posterior part (posterior and tarso-metatarsal ligaments) is very thick, and is intimately attached to the tarsal bones. Its superficial face is in part cartilaginous, and forms a smooth surface for the perforans tendon. Superiorly, it pouches upward behind the distal end of the tibia for a distance of about two inches (ca. 5 cm.). Interiorly, it is continued downward, forming the subtarsal or check ligament, which unites with the perforans tendon about the middle of the metatarsus.

There are four synovial sacs: 1. The tibio-tarsal sac lubricates the proximal joint, and is much the largest and most important. It is chiefly involved in the swelling produced by excess of fluid in the joint cavity, when the capsule bulges antero-internally and postero-superiorly. 2. The first intertarsal sac lines the joints formed by the tibial and fibular tarsal bones above, and the central and fourth tarsals below; it communicates in front with the proximal synovial capsule. 3. The second intertarsal sac lubricates the joints formed between the central tarsal and the bones below and on either side. 4. The tarso-metatarsal sac lubricates the joints formed between the tarsal and metatarsal bones, those between the proximal ends of the metatarsal bones, and those formed by the third tarsal with the bones on either side.

Common Ligaments.—The external lateral ligament (Ligamentum collaterale fibulare longum et breve) consists of two distinct bands which cross each other. The long (superficial) ligament arises on the posterior part of the external malleolus, is directed almost straight downward, and is attached to the fibular and fourth tarsal bones and the large and external small metatarsal bones. It forms a canal for the lateral extensor tendon. The short (deep) ligament arises on the anterior part of the external malleolus, is directed chiefly backward, and ends on the rough excavation on the external surface of the tibial tarsal and the adjacent surface of the fibular tarsal bone.

The internal lateral ligament (Ligamentum collaterale tibiale longum et breve) also consists of two parts which cross each other. The long ligament arises on the posterior part of the internal malleolus, becomes wider below, and is attached on the lower tuberosity of the tibial tarsal, the large and outer small metatarsal bones, and the inner surface of the lower tarsal bones which it covers. The short ligament lies largely under cover of the long one. It extends from the anterior part of the internal malleolus, runs backward and somewhat downward, and divides into two branches; one of these ends on the upper tuberosity on the inner surface of the tibial tarsal bone, the other on the sustentaculum tali.

The plantar or calcaneo-metatarsal ligament (Ligamentum tarsi plantare) is a very strong flat band which covers the outer part of the posterior surface of the tarsus. It is attached to the posterior surface of the fibular and fourth tarsal bones and the proximal end of the external metatarsal bone.

The dorsal or oblique ligament (Ligamentum tarsi dorsale) is a triangular sheet which is attached above to the lower tuberosity on the inner face of the tibial tarsal bone, and spreads out below on the central and third tarsal bones, and the proximal ends of the large and inner small metatarsal bones, to all of which it is attached.

Special Ligaments.—A considerable number of short bands which connect adjacent bones of the tarsus and metatarsus are described by various authors; some of these are quite distinct; others are difficult to isolate. Most of them are not of sufficient importance to justify detailed description.

(1) The tibial and fibular tarsal bones are united by four bands (astragalo-calcaneal ligaments). The internal ligament extends from the sustentaculum tali to the adjacent part of the tibia tarsal, blending with the short lateral ligament. The external ligament extends from the anterior process of the fibular tarsal to the adjacent part of the external ridge of the trochlea. The superior ligament extends from the posterior margin of the trochlea to the fibular tarsal. The interosseous ligament is deeply placed in the sinus tarsi between the two bones, and is attached in the rough areas of the opposed surfaces.

(2) The smaller bones are attached to each other as follows: The central and third tarsal are united by an interosseous and an oblique anterior ligament (scaphoido-cunean ligaments). The central and fourth tarsal are united by an interosseous and a transverse external ligament (cuboido-scaphoid ligament). The third and fourth tarsals are similarly connected (cuboido-cunean ligaments). The third tarsal is joined by an interosseous (intercunean) ligament to the (fused) first and second tarsals; the latter are connected with the fourth tarsal by a transverse posterior ligament.

(3) The smaller bones are connected with the upper row as follows: The central is attached to the tibial tarsal by posterior and interosseous (astragalo-seaphoid) ligaments, and to the fibular tarsal by a short oblique (calcaneo-scaphoid) band. The fourth is attached to the fibular tarsal by interosseous and posterior (calcaneo-cuboid) ligaments. The (fused) first and second tarsals are connected with the fibular tarsal by a posterior (calcaneo-cunean) ligament.

(4) The lower tarsal bones are connected with the metatarsus by tarso-metatarsal ligaments, which are not distinct from the common ligaments, except in the case of the interosseous ligament between the third tarsal and metatarsal bones.

Movements.—These are flexion and extension, which take place at the tibio-tarsal joint. The movements between the tarsal bones, and between the latter and the metatarsus, are so limited as to be negligible so far as the action of the joint as a whole is concerned. In the standing position the articular angle (in front) is about 150° to 160°. Complete extension is prevented by tension of the lateral ligaments. Flexion is checked only by contact of the metatarsus with the leg, provided the stifle joint is also flexed. Owing to the fact that the axis of motion is slightly oblique, the lower part of the limb deviates somewhat outward during flexion.

The movements of the hock joint must correspond with those of the stifle on account of the tendinous bands in front and behind (peroneus tertius and flexor perforatus), which extend from the lower part of the femur to the tarsus and metatarsus.

The remaining joints differ in no material respect from those of the thoracic limb.