The Ankle " Part Two "
The arch of the foot is also partly maintained by a strong ligament under the foot called the spring ligament, connecting the navicular with the calcaneus. It holds the arch together by resisting the tendency for the bones to separate under vertical stresses, becoming tighter the more weight is placed upon it. Due to its great strength the spring ligament is particularly useful when we are doing very vigorous movements such as running or jumping which might otherwise be too much for the muscle to resist the splaying of the arch. The muscles work more continuously at a lower level but between them and the spring ligament the talus is maintained at the top of the arch of the foot.
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.
This tensions the foot by twisting it slightly and along with the stretching of the longitudinal ligaments by the arch being flattened this gives the foot some elastic recoil when the weight begins to be removed from the foot. This gives a little kick of propulsion as we step forwards, perhaps being what we refer to as a spring in the step. As the foot encounters the unstable surface of rough ground it must accommodate to the differing levels and the joint between the talus and the calcaneum, the subtalar joint, is important in this adjustment.
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.
This rolling inwards of the foot tends to flatten the arch, leading to a stretching of the supporting sling made up of the tibialis anterior and tibialis posterior muscle tendons, allowing the talus to slip down from its apex position at the top of the foot arches. As the process continues the forces involves in rolling the arch inwards are very strong, gradually leading to an abnormal foot which acts much more like a platform and much less like a spring loaded tool.
The foot has greater ranges of movement in an inwards direction than outwards, the outwards seemingly blocked by the more anteriorly placed fibula if you try and turn your foot that way. This makes it more likely, if subjected to lateral instability, that the foot will turn rapidly inwards and cause an ankle strain. Although flat feet may not be painful in many cases, they can affect a whole series of joints above them from the ankle and knee to the hips and back. The sideways arch of the forefoot, a minor arch compared to the longitudinal arch of the foot, can also flatten and allow excessive weight to be borne by the second metatarsal head.
The inwards turning movement is much greater in range of motion than in the outwardly direction which the lower end of the fibula restricts. So it is much more likely that the foot will move rapidly inwards in response to a sudden inwards stress, resulting in a sprain to the ankle ligaments. Flat foot may not be a painful condition but it alters the alignment of the entire lower limb, with knock on effects as far up as the lumbar spine. The side to side arch in the forefoot is much less obvious than the primary foot arch, but when it loses its strength this places the second metatarsal head under greater stress.