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).
Unlike a flexible flatfoot, a rigid flatfoot is often the result of a significant problem affecting the structure or alignment of the bones that make up the foot's arch. Some common causes of rigid flatfeet include:
Congenital vertical talus — In this condition, there is no arch because the foot bones are not aligned properly. In some cases, there is a reverse curve (rocker-bottom foot, in which the shape is like the bottom rails of a rocking chair) in place of the normal arch. Congenital vertical talus is a rare condition present at birth. It often is associated with a genetic disorder, such as Down syndrome, or other congenital disorders. The cause is unknown in up to half of cases.
Tarsal coalition (peroneal spastic flatfoot) — In this inherited condition, two or more of the foot bones are fused together, interfering with the flexibility of the foot and eliminating the normal arch. A rare condition, it often affects several generations of the same family.
Lateral subtalar dislocation — Sometimes called an acquired flatfoot, it occurs in someone who originally had a normal foot arch. In a lateral subtalar dislocation, there is a dislocation of the talus bone, located within the arch of the foot. The dislocated talus bone slips out of place, drops downward and sideways and collapses the arch. It usually occurs suddenly because of a high-impact injury related to a fall from a height, a motor vehicle accident or participation in sports, and it may be associated with fractures or other injuries.