Page 313 - Canine Lameness

18.2 ­ormal Anatomy 285

limits its ability to flex when the stifle is extended (Chapter 5). The mean tarsal standing angle for dogs
is around 137°, but there is considerable variation due to breed, with an average angle of 116° for
German Shepherds and 140–147° for other large-breed dogs (Sabanci and Ocal 2018). Tarsal extension
is maintained by the CCT.
When the angle between the cranial aspect of the tibia and the dorsal aspect of the pes decreases,
the tarsocrural joint (and thus the tarsus as a whole) is in flexion. There is no consensus on termi-
nology for dorsoplantar movement of the joint levels distal to the tarsocrural joint, since normally
the tarsal bones remain in a straight line and move very little relative to each other. In this chapter,
movement of the tarsus (regardless of joint level) and pes towards the cranial aspect of the tibia
will be called flexion (or hyperflexion if the angulation at that level is excessive). When the pes
angles away from the cranial aspect of the tibia, the tarsus will be said to be extended (or hyperex-
tended if degree of angulation is abnormal). TARSAL REGION
The tarsus is composed of seven bones that are stacked to form joints at four levels on the medial
side and two on the lateral side (Figure 18.2). Two small, variably present sesamoids have been
described in Greyhounds (Wood and McCarthy 1984). These sesamoids are located on the lateral
(termed lateral plantar tarsometatarsal sesamoid bone) and medial (termed intra-articular tarso-
metatarsal sesamoid bone) side of the tarsometatarsal joint. These sesamoids are rarely described
in the current literature and generally not visible on radiographs; however, with the increased use
of advanced imaging, they are more frequently observed in other breeds as well (Deruddere et al.
2014). The tarsocrural joint, which is responsible for almost all of the motion in the tarsus, is
formed by the junction of the trochlea of the talus with the tibia and the fibula. The malleoli of
these bones overhang the talus to provide mediolateral stability. The talar trochlea has two nearly
circular ridges that correspond with two recesses in the tibial articular surface that are divided by
an intermediate ridge. The calcaneus does not participate in the tarsocrural joint but instead has a
long proximal projection, the tuber calcanei, that serves as an attachment point for the powerful
CCT. The sustentaculum tali of the calcaneus is a shelf on the plantar aspect of the tarsus that
hosts the lateral digital flexor muscle and is closely bound to the talus. Both bones end distally at
the proximal intertarsal joint, which is a collective joint formed by the junction of the talus with
the central tarsal bone (the talocalcaneal central joint) on the medial side and the junction of the
calcaneus and the fourth tarsal bone (the calcaneoquartal joint) on the lateral side (Figure 18.2).
The proximal intertarsal joint allows for a small amount of flexion/extension as well as motion in
the mediolateral plane.
The medially located central tarsal bone has a small articulation with the calcaneus, but it is
most broadly mated with the head of the talus proximally, the fourth tarsal bone laterally, and the
first, second, and third tarsal bones distally. Its junction with tarsal bones I–III is called the centro-
distal or the distal intertarsal joint. Unlike the proximal intertarsal joint, the distal intertarsal joint
is only present on the medial side, since the long fourth tarsal bone spans this level on the lateral
side. All of the bones of the tarsus are bound to their neighbors on either side with short, strong
ligaments in what are termed vertical intertarsal joints. Finally, the numbered tarsal bones articu-
late with corresponding metatarsal bones at the collective tarsometatarsal joint.
Each tarsal bone forms from a single ossification center except for the calcaneus, which also has
an apophysis, the tuber calcanei. Ossification centers of the talus and the body of the calcaneus
are present at birth, with the central and numbered tarsal bones (along with the ossification
center of the tuber calcanei) appearing between 3 and 6 weeks of age on radiographs (Thrall and
Robertson 2016). There are several growth plates around the tarsus that can complicate radio-
graphic interpretation in young animals. The distal tibia has a physis that closes between 5 and
15 months of age, while its attached medial malleolus has its own growth plate that closes by