The Ankle Joint

The ankle joint (or talocrural joint) is a synovial joint located in the lower limb. It is formed by the bones of the leg and the foot – the tibia, fibula and talus. Functionally, it is a hinge type joint, permitting dorsiflexion and plantarflexion of the foot. In this article, we shall look at the applied anatomy of the ankle joint; the articulating surfaces, ligaments, movements, and any clinical correlations.


Articulating Surfaces

The ankle joint is formed by three bones; the tibia and fibula of the leg, and the talus of the foot:

  • The tibia and fibula are bound together by strong tibiofibular ligaments, producing a bracket shaped socket, which is covered in hyaline cartilage. This socket is known as a mortise.
  • The body of the talus fits snugly into the mortise formed by the bones of the leg. The articulating part of the talus is wedge shaped. It is wider anteriorly, and thinner posteriorly. During dorsiflexion, the anterior part of the bone is held in the mortise, and the joint is more stable (vice versa for plantarflexion).
Fig 1.0 - X-ray of a normal ankle joint. Note the bracket shaped socket formed by the tibia and fibula.

Fig 1.0 – X-ray of a normal ankle joint. Note the bracket shaped socket formed by the tibia and fibula.

Fig 1.1 - Superior view of the tarsal bones of the foot. Note the wedge shape of the talus.

Fig 1.1 – Superior view of the tarsal bones of the foot. Note the wedge shape of the talus.


Ligaments

Fig 1.2 - Lateral ligament of the ankle joint.

Fig 1.2 – Lateral ligament of the ankle joint.

There are two sets of ligaments, which originate from each malleolus. The medial ligament (or deltoid ligament) is attached to the medial malleolus. It consists of four separate ligaments, which fan out from the malleolus, attaching to the talus, calcaneus and navicular bones. The primary action of the medial ligament is to resist over-eversion of the foot.

The lateral ligament originates from the lateral malleolus. It resists over-inversion of the foot. It is comprised of three distinct and separate ligaments:

  1. Anterior talofibular: Spans between the lateral malleolus and lateral aspect of the talus.
  2. Posterior talofibular: Spans between the lateral malleolus and the posterior aspect of the talus.
  3. Calcaneofibular: Spans between the lateral malleolus and the calcaneus.

Clinical Relevance: The Ankle ‘Ring’

The ankle joint and associated ligaments can be visualised as a ring in the coronal plane:

  • The upper part of the ring is formed by the articular surfaces of the tibia and fibula.
  • The lower part of the ring is formed by the subtalar joint (between the talus and the calcaneus).
  • The sides of the ring are formed by the medial and lateral ligaments.

A ring, when broken, usually breaks in two places (The best way of illustrating with is with a polo mint – it is very difficult to break one side without breaking the other). When dealing with an injury to the ankle joint, a clinician must bear this in mind. For example, a fracture of the ankle joint may occur in association with ligament damage – which would not show up on x-ray.

Movements and Muscles Involved

The ankle joint is a hinge type joint, with movement only possible in one plane. Thus, plantarflexion and dorsiflexion are the only movements that occur at the ankle joint. Eversion and inversion are produced at the other joints of the foot, such as the subtalar joint.

Clinical Relevance: Common Conditions Affecting the Joint

Ankle Sprain

An ankle sprain refers to partial or complete tears in the ligaments of the ankle joint. It usually occurs in a plantarflexed weight-bearing foot, which is excessively inverted. The lateral ligament is more likely to be damaged for two main reasons:

  • The lateral ligament is weaker than the medial ligament

    Fig 1.4 - Trimalleolar fracture at the ankle. The arrows denote fractures of the medial malleolus and proximal tibia.

    Fig 1.3 – Trimalleolar fracture at the ankle. The arrows denote fractures of the medial malleolus and distal tibia.

  • The lateral ligament resists inversion

The anterior talofibular ligament is the lateral ligament most at risk of irreversible damage.

Pott’s Fracture-Dislocation

A Pott’s fracture is a term used to describe a bimalleolar (medial and lateral malleoli) or trimalleolar (medial and lateral malleoli, and distal tibia) fracture.

This type of injury is produced by forced eversion of the foot. It occurs in a series of stages:

  1. Forced eversion pulls on the medial ligaments, producing an avulsion fracture of the medial malleolus.
  2. The talus moves laterally, breaking off the lateral malleolus.
  3. The tibia is then forced anteriorly, shearing off the distal and posterior part against the talus.

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