THE JOINTS OF THE HINDLIMB

The hip joint (Figure 2-62) is a spheroidal joint formed between the lunate surface of the acetabulum and the head of the femur. The acetabular surface is enlarged by an articular labrum (Figure 2-62Z2') continuous with the transverse acetabular ligament (Figure 2-62Z2") that bridges the notch interrupting the medial wall of the socket.

Although a spheroidal joint, the hip does not enjoy the full range of movement expected of this class of joint. In the large animals movement is largely restricted to flexion and extension; the capacity for rotation, abduction, and, especially, adduction is limited. In con­formity with the dominance of sagittal movement, the articular area tends to extend onto the neck in rumi­nants. The restriction on movement owes much to the intraarticular ligament(s) but something to the massive medial muscles of the thigh. The joint has a more ver­satile employment in the dog.

The stifle joint (Figure 2-63), corresponding to the human knee, comprises femorotibial, femoropatellar, and proximal tibiofibular joints; in the dog it also includes the joints between the femur and paired sesa­moids in the origins of the gastrocnemius and that between the tibia and the sesamoid in the popliteus tendon.

The femorotibial joint is unusual in having two fibro­cartilaginous menisci (Figure 2-63Z10,17) interposed between the femoral and tibial condyles. The menisci, which compensate for the incongruence of the articular surfaces, are each semilunar in plan and wedge-shaped in section and have concave proximal and flattened distal surfaces. Each is secured by ligaments that extend between its cranial and caudal extremities and the central nonarticular area of the proximal extremity of the tibia; the lateral meniscus is also attached caudally to the intercondylar fossa of the femur.

Four ligaments join the femur to the bones of the leg. A medial collateral ligament passes between the femoral epicondyle and the proximal part of the tibia, toward the caudal part of the joint. The corresponding lateral ligament has a similar disposition but attaches to the fibular head. The cruciate ligaments are centrally placed. The cranial (lateral) cruciate ligament (Figure 2-63Z16) arises from the lateral condyle of the femur within the intercondylar fossa and runs craniodistally

Figure 2-61 Skeleton of right pes of the dog, lateral (A) and dorsal (B) views. Dorsal (C) view of left equine tarsus. Roman numerals identify the metatarsal bones. 1, Tibia; 2, fibula; 2’, lateral malleolus; 3, calcaneus; 3’, sustentaculum tali; 3", calcanean tuber (point of hock); 4, talus; 5, central tarsal; 6, fourth tarsal; 7, first, second, and third tarsal bones in distal row; 7, third tarsal in the horse; 8, proximal sesamoid bones; 9, dorsal sesamoid bones; 10,11,12, proximal, middle, and distal phalanges; 12’, claw.

to attach on the tibia. The caudal (medial) cruciate liga­ment (Figure 2—63/15) runs at right angles to the cranial one and attaches far back on the tibia near the popliteal notch.

The femoropatellar joint is formed between the femoral trochlea and the patella and is extended by its parapatellar cartilages, of which the medial one is espe­cially well developed in the large animals. Relatively weak collateral femoropatellar ligaments (Figure 2-63/12) run between the cartilages and the femur. Dis­tally the patella is joined to the tibial tuberosity by a single patellar ligament, except in the horse and ox, in which three ligamentous thickenings are present— medial, intermediate, and lateral—connected by a fibrous sheet (see Figure 24-4). The middle (or single) patellar ligament represents the insertion tendon of the

Figure 2-62 Schematic transverse section through the left hip joint of a dog. The femur has been drawn in relief. 1, Gluteus medius; 2, acetabulum, connected to the femoral head by the ligament of the head of the femur; 2’, fibrous rim (labrum) of acetabulum; 2'', transverse acetabular ligament; 3, femur; 4, biceps; 5, rectum; 6, vagina; 7, urethra; 8, obtura­tor foramen; 9, pelvic floor.

quadriceps femoris; the others, when present, represent the continuation of other muscles inserting about the joint.

The synovial membrane attaches around the periph­eries of the articular surfaces and the menisci. It covers the cruciate ligaments and here forms a partition, com­plete only in the horse, between the medial and lateral femorotibial joints. The femoropatellar portion of the cavity extends proximally between the femur and the quadriceps. In the horse it generally communicates only with the medial femorotibial compartment, but in other species it has free communication with both. Divertic­ula of the capsule embrace the lesser joints with the fibula and the sesamoid bones and extend along the tendons of origin of the long digital extensor and pop­liteus muscles.

Despite its complexity, the stifle functions as a hinge joint whose free movement is restricted to flexion and extension.

The tarsal joint of quadrupeds is usually known as the hock. It possesses four levels of articulation, but in most species almost all movement occurs at the cruro- tarsal level. This is a hinge joint but not a typical one as the obliquity of the interlocking ridges and grooves of the tibia and talus imposes a lateral deviation of the foot when it is carried forward on flexion. In ruminants and carnivores, limited flexion is also possible at the curved surfaces of the talocentral joint.

The ligaments are numerous. Those most important are the medial and lateral collateral ligaments, which extend, with intermediate attachments, from the tibia (and fibula) to the proximal extremity of the metatarsus. Each comprises a long superficial part of full extent and a shorter deeper part restricted to the proximal level of articulation. Another long ligament is found caudally, extending from the plantar surface of the calcaneus over the fourth tarsal bone to the metatarsus. The remaining smaller ligaments firmly hold the tarsal bones together.

There are several compartments to the joint. That between the tibia and talus is most capacious and may possess a number of local pouches, as the less supported parts of joint capsules are known. The other synovial sacs are much tighter and often communicate. The details are most important in the horse (p. 631).

The remaining joints of the hindlimb are considered in the regional chapters, insofar as they require to be differentiated from the corresponding forelimb joints.