by Jonathan Blood Smyth

Joint classification puts the ankle joint into the category of a mortise joint and it is specialised for its function. A front and rear movement action is performed by the ankle hinge for walking and it stabilises the lower leg on rough ground and when moving. The ankle forms the connection between the lower leg and the foot to facilitation the transfer of the weight of the body to the mobile and stable foot. The upper joint structures are mostly made up of the tibial surface with the fibula making up a small part of the lateral side of the joint.

The mortise of the upper part of the ankle encloses the ankle bone or talus, which is set on top of a group of mid foot or tarsal bones which make up the foot arch. The upper dome of the talus articulates with the tibia surface and the talus makes two other important joints. One is with the navicular bone in front of it so it can transfer weight forward onto the forefoot. The other is the joint below the talus, the talo-calcaneal joint, a very complex and important foot joint.

The tibia takes the weight of the body and carries it down to be borne by the talus which passes it on vertically and rearwards to the calcaneum and forwards through the navicular to the forefoot metatarsals. Spreading out forwards with an almost parallel radiation , the metatarsals are slender bones which give significant mobility and stability to the foot. The foot arches are important structures and mean that the forefoot weight bearing areas are mainly under the first and fifth metatarsal heads, with the others bearing weight if the arch gives way to some degree.

The ankle has predominantly the downwards and upwards movement of a hinge, respectively known as plantarflexion and dorsiflexion. The other movements, which do not occur at the ankle but rather at the forefoot, talonavicular and talocalcaneal joints, are the inward and outward foot turning known as inversion and eversion. When on rough ground these complex joints allow the foot to accommodate to unstable surfaces and also to perform as a reliable support for the bodyweight. Both demands are conflicting and the foot solves these.

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.

On the outside of the shin lies the prominence of the tibialis anterior muscle, with its tendon obvious and prominent as it runs across the front of the ankle towards the inside to insert into the foot. Three bones which make up the top of the arch of the foot are the insertion points for the tibialis anterior tendon and it helps lift the arch as the muscle contracts. From the posterior calf the tibialis posterior muscle’s tendon runs around the inner ankle bone and finally inserts near the tendon of the tibialis anterior, drawing the bone posteriorly and increasing the arch.

The arch is pulled up to some degree by these two muscles working in concert to pull it up and stabilise it from the side against the weight of the body. The spring of the foot, vital in running and walking, is maintained by this. Another important muscle is the peroneus longus which runs down the leg and its tendon runs under the outside of the foot to insert over towards the first toe. This arrangement stabilises the foot from any direction as there are muscles which pull from each direction to maintain the foot posture against the forces generated by movement and body mass.

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

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