Among the many joints in the human body, one of the most integral is the talocrural region, which most people know as the ankle joint. This connection between the foot and the lower leg carries out many useful functions for the leg and the body as a whole. Though most people refer to the area as a singular joint, the ankle bones actually form two or three joints, depending on how experts classify them.
Many bones can be viewed as “ankle bones,” but the only true ankle bone is the talus. Technically, the talus belongs to the tarsus group of foot bones, but it is unique in several ways. Unlike most other bones, the talus does not attach to any muscles. As such, its position is entirely reliant on the surrounding bones. Additionally, in comparison to other bones, it is covered in one of the highest percentages of articular cartilage.
The other bones that could classify as ankle bones belong to the lower leg and the foot.
What many people refer to as the main ankle joint has several names. Some experts call it the talocrural joint, while others prefer the ankle joint proper or just the ankle joint. Three separate bones meet to form the structure. The ends of the tibia and fibula connect and create a mortise or hole into which the body of the talus fits snugly. Interestingly, the tibia and the fibula form their own joint here, which some people consider a distinct third ankle joint. This synovial hinge joint has limited motion capabilities.
Synovial hinge joints are only capable of extending or flexing a body part. Where the ankle is considered, this joint achieves dorsiflexion and plantarflexion. Dorsiflexion refers to flexing the top side of the foot, meaning that the action brings the toes closer to the shin, such as walking on tiptoes. Plantarflexion is the opposite motion, meaning the heel moves closer to the back of the leg, as when walking on the heels.
Though the upper ankle joint is the primary joint, the lower ankle joint is also important. Some experts refer to it as the talocalcaneal joint because it occurs at the meeting point of the talus and the calcaneus, though most people use the term “subtalar joint.” Some medical texts include the navicular as part of this joint, though the true anatomical subtalar joint is only the talus and the calcaneus. This joint is a plane synovial joint; it is capable of only a few motions, like a synovial hinge, but on a different axis.
In addition to extending and flexing, the ankle is also capable of moving side-to-side: pronation and supination. Eversion moves the foot outward, placing the weight on the big-toe side. Inversion points the foot inward, placing the weight on the outside of the foot or the little-toe side.
Beyond being the main ankle bone, the talus is important for several reasons. Its main functional role is transmitting force from the tibia to the calcaneus. One of the reasons the talus can do this is its high percentage of articular cartilage. Synovial joints typically have protective layers of this type of cartilage to allow for smoother movements. Because the talus has so much, it smoothly and easily transitions from motion to motion at each of its joints.
Among its many important functions, the ankle joints support and shift a significant amount of weight and absorb shock. The various movements allow humans to perform many actions, such as jumping, running, and walking. Medical experts and researchers do not fully understand the extent of the role the ankle plays in these movements. The common consensus is that the ankle provides a significant amount of force from push-off, but it is unclear if this contributes primarily to leg swinging or full-body motion. Some studies suggest the ankle contributes to both relatively equally.
There are two important sets of ligaments in the ankle joint proper. The medial or deltoid ligament attaches to a projection on the tibia and consists of four separate ligaments. Each of these individual pieces attaches to the navicular, calcaneus, and talus bones. The medial ligament prevents over-eversion. The lateral ligament attaches to a similar projection on the fibula and has three distinct ligaments. Two connect to the talus, and the remaining ligament attaches to the calcaneus.
The subtalar joint sits within a protective joint capsule. A synovial membrane lines its interior and a strong fibrous layer lines its exterior. In addition to this, the joint and the capsule receive support from three key ligaments that each attach to the talus and the calcaneus at different points. Another ligament, the interosseous talocalcaneal ligament, is responsible for holding the talus and the calcaneus together. This ligament is substantially stronger than the others and provides the most stability for the joint.
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