The Human Ankle

The ankle is often classified as a mortise joint and is a very special joint due to its position in the body. The ankle hinge performs forwards and backwards movement during gait and confers stability so that we can balance during motion and on rough surfaces. The lower leg is connected to the foot by the ankle and this permits the body weight to be transmitted across the joint to the propulsion unit which is the foot. Most of the upper part of the joint consists of the tibia with a small contribution made by the fibula on the lateral side.

The upper ankle mortise closely encloses the talus or ankle bone which itself balances on top of a series of tarsal and mid foot bones of the foot arch. The talar dome on the upper surface makes the joint with the under surface of the tibia but there are two other joints which the talus participates in. The talo-navicular joint is to the front and through which weight is transferred forwards. The complex talo-calcaneal joint is below the talus and takes body weight straight down.

Weight passing vertically down through the tibia is transmitted through the talus in an anterior direction through the navicular and towards the metatarsal bones and backwards to the calcaneum. The metatarsals are slender bones lying almost parallel to each other as they fan out towards the toes, conveying important degrees of stability and mobility to the foot. The foot has arches and in the front of the foot the weight is primarily taken on the metatarsal heads of the first and fifth rays. If the arch collapses the other metatarsal heads of the second to the fourth may also weight bear.

The ankle upward and downward movements are known as dorsiflexion (up) and plantarflexion (down) and the inwards and outwards movements of the foot do not occur at the ankle. The inwards movement is known as inversion, the outwards as eversion, and both of these movements occur at the talo-navicular, forefoot and talo-calcaneal joints. Together these complex joints allow the body weight to be held stable over the feet as the body moves and to allow the feet to cope with irregular surfaces. The foots design allows it to satisfy these competing demands.

The human foot is very well designed to manage the severe demands which are put upon it. The vertical forces which are developed in weight bearing are very significant and the foot has to cope with these and transmit them onwards. The central arch of the tarsal bones takes a good degree of weight, with the interconnecting ligaments between the joints absorbing much of the forces developed inside the foot. The foot muscles are also important in maintaining the structural integrity of the foot under the pressure of body weight and the momentum of large movements.

The front of the shin is the origin for the long and bulging muscle called the tibialis anterior, whose tendon clearly runs down across the ankle towards the midline. The tendon inserts to three bones making up the apex of the foot arch and when the muscle contracts it pulls the arch up and maintains its structure to some degree. The posterior tibialis muscle has its origin at the rear of the tibia and its tendon make its way around the inner ankle to finally insert close to the tibialis anterior tendon. This pulls the bone towards the back and contributes again to arch maintenance.

These two muscles together pull the arch up from above and clench it from the side, stopping the weight of the body from flattening the arch. This maintains the springiness of the foot which is so important in walking and running. They are helped by the peroneus longus muscle which comes from the rear calf again and round the outside of the ankle and acts as a sling as it travels under the foot from the outside towards the big toe. This way the foot can be kept stable by strong muscles able to pull from every direction to maintain the posture of the foot under the pressure of the body weight.

Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Kent. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.