Now we’ll move on, to look at the two joints of inversion and eversion. There’s one directly beneath the main part of the talus, called the subtalar joint; and there’s one below and in front of the head of the talus that has an unwieldy name, the talo-calcaneo-navicular joint. We’ll call it the T.C.N. joint for short.
To understand these joints we need to get acquainted with the remaining tarsal bones. We already know the talus, the calcaneus, and the navicular. In front of the navicular are the three cuneiform bones, first, second, and third. Lastly, the bone in front of the calcaneus is the cuboid bone.
Now let's look at the calcaneus by itself. The posterior part of the calcanueus forms the heel. The massive calcaneal tendon, also called the Achilles tendon, is attached here. Here on the medial side there’s a projecting shelf which the medial part of the talus sits on, called the sustentaculum tali.
On the front of the calcaneus there’s an articular surface for the cuboid bone. On the upper aspect of the calcaneus there are two articular surfaces for the talus, a small one in front, a larger one behind.
The larger of these two surfaces, together with the corresponding surface on the underside of the talus, forms the subtalar joint. The head of the talus fits into a socket, which we’ll see by taking the talus away. The socket is formed by this surface of the calcaneus, this surface of the navicular bone, and by a strong ligament here which we’ll see in a minute. These surfaces, together with the head of the talus, form the talo-calcaneo-navicular joint.
Here’s what these joint surfaces look like in the living body: the surface for the subtalar joint, and the two surfaces for the T.C.N. joint. This structure in between, which forms part of the TCN joint, is the upper surface of the strong calcaneo-navicular ligament, also misleadingly called the spring ligament, which helps to hold up the head of the talus. It goes from here on the calcaneus to here on the navicular.
The movement that happens at the subtalar and T.C.N joints is a rocking motion, that takes place around an obliquely placed axis.
This rod shows the position of the axis: it’s oblique to the long axis of the foot both in this plane, and in this plane. Here’s eversion...here’s inversion. Again, eversion...and inversion
Several strong ligaments hold the malleoli, the talus, the calcaneus and the navicular bone together. On the medial side, which we’ll see first, there’s one extensive ligament to look at, the deltoid ligament. We’ve seen part of it already,
Now here’s the whole of the deltoid ligament. This is the part we saw before, going from from the medial malleolus to the talus. In addition, parts of the deltoid ligament fan out below onto the sustentaculum tali of the calcaneus, and in front onto the navicular bone, so that the deltoid ligament holds all four of these bones together.
On the lateral side there are two important ligaments, the calcaneo-fibular ligament which goes from the lateral malleolus to the side of the calcaneus, and this strong ligament, the interosseous talo-calcaneal ligament, which goes from here on the calcaneus, to here on the talus.
To see that ligament better, we’ll remove the talus. The interosseous talo-calcaneal ligament lies between the subtalar joint and the T.C.N. joint.
Now that we’ve seen the ankle joint and the joints of inversion and eversion, we’ll look very briefly at the remaining joints of the tarsus. Between the navicular and its neighbors, the cuneiform bones and the cuboid bone, there’s hardly any movement. But there is a small amount of rotation between the cuboid and the calcaneus, which lets the front part of the foot invert and evert a little, independently of the calcaneus.