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.
[Illustration: