The distal pelvic limb is made up of the crus (lower thigh), tarsus, metatarsus and phalanges. Anatomical information was obtained from Miller et al. (1964) and Evans and DeLahunta (1971).
1.6.1 Bones
The tibia and fibula are the bones of the crus, also known as the lower thigh or shank. The tibia is much larger than the fibular and lies in the medial part of the crus. It articulates proximally with the femur, distally with the tarsus and on its lateral side, both proximally and distally with the fibula. The fibula is a long, thin, laterally compressed bone located in the lateral part of the crus. It articulates proximally with the caudolateral part of the lateral condyle of the tibia and distally with the talus. It main purpose is muscle attachment as it supports little weight. The tarsus refers to the section of the limb that lies between the crus and the metatarsus (Figure 1.8). There are seven bones arranged into three irregular rows. The proximal row is made up of a long, laterally located calcaneus and a shorter, medially located talus. Both the tibia and fibula articulate with the talus. There are four bones in the distal row numbered from medial to lateral. Three small bones, the first, second and third tarsal bones are located alongside each other and are separated from the proximal row by the central tarsal bone. The large fourth tarsal bone completes the distal row laterally and is as long as the combined lengths of the third and central tarsal bones against which it lies.
The metatarsus (Figure 1.8) contains five metatarsal bones that resemble the metacarpal bones. However, unlike the first metacarpal bone, the first metatarsal bone is not always present. The phalanges resemble those of the thoracic forepaw.
Figure 1.8: Dorsal view of the bones of the distal pelvic limb. Adapted from Goody (1997).
1.7.2 Muscles and tendons
The superficial and deep digital flexor tendons and their corresponding muscles are the structures of interest (Figure 1.9). The SDF is a spindle shaped muscle arising from the lateral supracondylar tuberosity of the femur along with the lateral head of the gastrocnemius. Proximal to the calcaneal process the SDF tendon twists across the medial surface of the gastrocnemius. Further distally, the tendon widens, caps the tuber calcanei and attaches on each side of the tuber calcanei. Metatarsal bones 2‐5 Talus Tarsal bone 4 Fibula Digits 2‐5
(three phalanges each) Tarsal bone 3 Calcaneus Tarsal bone 1 Tibia Metatarsal bone 1 Central tarsal bone Tarsal bone 2
Figure 1.9: Plantar view of a canine (right) distal pelvic limb showing the muscles and tendons of interest. The medial and lateral deep digital flexor muscles are out‐of‐view behind the gastrocnemius muscles. Adapted from Evans and DeLahunta (1971). SDF tendon l.DDF tendon m.DDF tendon Medial gastrocnemius muscle SDF muscle Lateral gastrocnemius muscle
At the level of the distal row of tarsal bones, the tendon divides twice forming four tendons of nearly equal size. These extend distally over metatarsals two, three, four and five. At the metatarsophalangeal joints, each tendon forms a cylinder for the passage of the tendons of the DDF. The tendons insert at the bases of the second phalanges of digits two, three, four and five. The pelvic limb SDF acts to flex the digits and stifle, and to extend and fix the tarsus.
The gastrocnemius muscle is divided into a lateral and medial head. The lateral head arises by a large tendon on the lateral plantar tuberosity of the femur and the medial head from the medial plantar tuberosity. The two heads of the gastrocnemius almost totally enclose the SDF muscle; they all fuse together distally forming a flat muscle. After crossing the SDF tendon laterally, the tendon of the gastrocnemius inserts on the tuber calcanei.
The DDF consists of a large lateral head and a weaker medial head. The smaller medial head of the DDF lies between the lateral head and the popliteus muscle. It arises from the head of the fibula and the proximal end of the tibia and it runs distomedially. The medial DDF tendon lies on the caudomedial side of the tibia. At the level of the distal row of tarsal bones it unites with the tendon of the lateral head. The lateral head of the DDF arises from the caudolateral border of the proximal two‐thirds of the tibia, most of the proximal half of the fibular and the adjacent interosseous membrane. The lateral DDF tendon begins as a wide expanse on the plantar side of the muscle which condenses distally. Medial to the tuber calcanei it is surrounded by the tarsal synovial sheath and bound in the groove over the sustentaculum tali of the calcaneus by the flexor reticulum. At the level of the distal row of tarsal bones the lateral DDF tendon fuses with the medial DDF tendon. At the middle of the metatarsus, the joined tendon then divides into four branches; these four branches behave in the same way as those of the DDF in the thoracic limb. The DDF acts to flex the digits and stifle and to extend the tarsus.