All the synovial joints in the body have accessory movements and the ankle is no exception. Accessory movements are little sliding or gliding motions which occur inside the joint normally during activity but which cannot be performed on their own. Joints need the accessory movements to function normally and a joint will lose some of its ability if these tiny movements are restricted or lost. An example is the inward slide of the talus in response to the vertical forces of the body as the toes and the forefoot are placed flat on the ground during stepping.
The foot twists during this manoeuvre, storing up tension energy which allows the recoil to occur in an elastic manner as the foot begins to lift off, a recoil which is supported by the longitudinal ligament stretch as the arch is flattened. From this we feel a little push of energy each step, very appropriately often referred to as the spring in the step. Rough ground presents challenges to the foot in terms of differing angles of surface and differing levels and the foot adapts to this with the subtalar joint, the joint between the calcaneum and the talus.
Whilst the talus is solidly maintained within the ankle mortise the heel can move outwards and inwards underneath it by lateral movements of the subtalar joint, there being much less outward movement than inward. Our foot posture can vary depending on our walking patterns and this can interfere with normal foot function and precipitate painful conditions in the feet. For example lateral (outward) rotation of the feet makes us hit the ground initially with the outer border of the feet and forces the foot to roll significantly inwards to reach the ground each step.
As the foot rolls in this amount the arch tends to flatten, stretching the sling of supporting muscle tendons from the tibialis posterior and tibialis anterior muscles of the calf. This allows the talus to slip from its top position in the foot arch to some degree. As the forces involved in this medial rolling of the arch are very great this gradually over time makes the foot become abnormal when it performs like a static platform rather than a springy, dynamic part.
As the ankle is much more likely to be forcibly wrenched in a medial direction it is the lateral foot ligaments which suffer the sprain. With a significant sprain of the ankle joint this always involves some joint or ligamentous sprain of the subtalar joint which can lead to local joint instability and the increased likelihood of repeated sprains. With repeated damage the local tissue swelling gradually organises and thickens on its way to maturing into scar and if the joint is kept still such as in a cast this can lead to a loss of both joint mobility and stability.
The lateral ligaments of the foot are the ones typically sprained due to the tendency of the ankle to go over as the foot moves forcibly inwards. The subtalar joint is always involved and damaged when there is a significant sprain of the ankle joint and this can cause instability of this area and lead to repetitive ankle sprains. Multiple sprains make the local tissues swell and this with time thickens up, turning into scar and giving problems if the ankle has been held still for some weeks in a cast, reducing both the stability and mobility of the area.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Newcastle. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
