What Is It?
A fallen arch or flatfoot is known medically as pes planus. The foot loses the gently curving arch on the inner side of the sole, just in front of the heel. If this arch is flattened only when standing and returns when the foot is lifted off the ground, the condition is called flexible pes planus or flexible flatfoot. If the arch disappears in both foot positions — standing and elevated — the condition is called rigid pes planus or rigid flatfoot.
Flexible flatfeet are considered normal in young children because babies are not born with a normal arch. The arch may not form fully until sometime between ages 7 and 10. Even in adulthood, 15% to 25% of people have flexible flatfeet. Most of these people never develop symptoms. In many adults who have had flexible flatfeet since childhood, the missing arch is an inherited condition related to a general looseness of ligaments. These people usually have extremely flexible, very mobile joints throughout the body, not only in the feet. Flatfeet also can develop during adulthood. Causes include joint disease, such as rheumatoid arthritis, and disorders of nerve function (neuropathy).