There are two major classifications of flat foot, congenital flat foot which is often asymptomatic and cannot be classed as a pathology, and acquired flat foot which occurs in adulthood secondary to some pathological condition. The causes of adult flat foot are many and include fractures or dislocations of the foot, abnormalities of the foot, neurological problems and arthritic conditions. The most frequent cause of this acquired form is a dysfunction of the tibialis posterior tendon, the tendon of one of the calf and foot muscles. Trauma to the area, inflammation or degenerative changes can all affect this tendon.
In scientific studies changes to the tibialis posterior tendon have been shown to be more frequent in people who are diabetic, overweight, have had operations or trauma to the midfoot and have a history of taking steroids. A higher incidence is also shown in patients who have an arthritic condition in the group of spondyloarthropathies, having a history in the family of psoriasis or joint related inflammatory conditions. A mechanical cause may also be common as older people without any explanatory pathology can also suffer from this tendon dysfunction. Rheumatoid arthritis sufferers may show this in 10% of cases.
Just underneath the inside bones of the ankle and for a short distance forward there is an area of reduced blood supply which affects the tendon which runs through this area, perhaps helping to explain why degenerative changes might be more important in this area. This tendon forms part of the support for the medial arch of the foot which has both active and passive components. The passive or static supports for the stability of the arch are the plantar fascia, the short and the long plantar ligaments and the spring ligament, also called to calcaneonavicular ligament. The spring ligament supports the ankle bone or talus and prevents it from sliding downwards or inwards.
The tendon of the posterior tibialis muscle is the most powerful support for the medial arch of the foot. Muscle contraction through the tendon raises the inside of the medial arch of the foot and turns the foot inwards if it is not planted. Loss of this muscle function from a rupture or damage to the tendon deprives the foot arch of its most powerful supporting influence which allows the muscles which turn out the foot to act without restraint. The foot can then undergo three main postural alterations: flattening of the medial foot arch; turning out of the forefoot and turning out of the hindfoot area.
The forefoot and the hindfoot combine to be a rigid and stable platform in gait and the tendon changes lead to a reduction or loss of this with a less efficient gait pattern. Because the tibialis posterior muscle''s strong influence on the foot is diminished or removed by the tendon problems this allows the major calf muscles to act more at the ankle rather than further forward. Pressure on the talus or ankle bone leads it to move down and inwards which puts the spring ligament on a stretch and allows a collapse of the inside arch as the joints assume new positions.
On presentation with acquired flat foot symptoms patients typically report that the inner side of the ankle and foot suffers pain and swelling whilst weight bearing. They may notice a gradual reduction in the arch and observe that they are weight bearing on the inner half of the foot. Push off in walking becomes less easy as strength reduces and a limp may develop, with the soles of the shoes showing evidence of a change in the gait pattern. Physiotherapy assessment of a person with flat foot typically starts with a comparison of both feet and their arches in standing.
On observing a foot from behind it is usual to see the two lateral toes on the outside and if more are visible this indicates forefoot abduction. The angle between the line of the heel and the line of the lower leg will be measured by the physiotherapist assessing the foot, indicating the valgus angulation of the heel. Asking a patient with a normal foot to go up on tiptoe will show an internal movement of the heel as the calf muscle contracts.