In this common condition the longitudinal arch of the foot is reduced so that, on standing, its medial border is close to, or in contact with, the ground.  

Cause

In many cases it probably has a congenital basis, but it may be caused by selective muscle weakness or paralysis.

Pathology

All infants have flat feet for a year or two after they begin to stand. When the deformity persists into adult life it becomes a permanent structural defect, the tarsal bones being so shaped that when articulated they tend to form a straight line rather than an arch.

Clinical features　

In Children, flat feet are usually symptomless, but the parents commonly complain that the uppers of the shoes persistently bulge inwards and that the heels wear down quickly at the inner sides.

In adults, too, flat feet are often free from symptoms, but they are more liable than are normal feet to suffer foot strain, and when pain is complained of it is usually from that cause.

In later life pain may also arise from osteoarthritis of tarsal joints consequent upon their mal-alignment.

Treatment　

In Children under 3 years old treatment is not required.

In children over 3 the accepted method of treatment is to tilt the shoe slightly to the lateral side by inserting a wedge, base medially,  between the layers of the heel. This may help to  overcome the valgus twist and to reduce the  bulging- over of the uppers at the medial side, but  it  must be accepted that in most cases it is little more  than a placebo. In order children it is better to insert a  valgus insole into the shoe, and this may be  supplemented by a course of supervised exercises to  strengthen the intrinsic muscles of the foot.

In cases of severe valgus deformity – which is usually the consequence of selective muscle imbalance, as after poliomyelitis – operation to restore the correct relationship between talus and calcaneus, and to fuse the two bones together, may be considered.

In adults treatment is not needed unless symptoms are present, when the advisability of fitting an arch support should be considered. Supports are seldom of benefit when the foot is completely flat, but they often afford relief when the longitudinal arch is diminished but not lost.