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Flat feet (pes planus) refer to a change in foot shape in which the foot does not have a normal arch when standing.
Flat feet are a common condition. The condition is normal in infants and toddlers.
Flat feet occur because the tissues holding the joints in the foot together (called tendons) are loose.
The tissues tighten and form an arch as children grow older. This will take place by the time the child is 2 or 3 years old. Most people have normal arches by the time they are adults. However, the arch may never form in some people.
Aging, injuries, or illness may harm the tendons and cause flat feet to develop in a person who has already formed arches. This type of flat foot may occur only on one side.
Rarely, painful flat feet in children may be caused by a condition in which two or more of the bones in the foot grow or fuse together. This condition is called tarsal coalition.