Flat arches or a collapsed foot
Some people are simply "built" with flat feet (shallow arches). Most have no problems, so much so that this flat feet have to be considered a normal variety of the human foot (flexible idiopathic flafoot). A shallow arch is often inherited. In fact, flat footedness is normal for the toddler so that even when the foot is destined to have a normal arch it often does not develop until 3, 4 years of age or older. Even if the arch stays flat there are usually no symptoms. Some children do have achey shins or feet when walking long distances and shoes can wear out rapidly creating a rather expensive problem. Arch support insoles can then be helpful although they will not give the child a better arch later in life.
Sometimes there is a true abnormality in the structure of the foot (not necessarily serious) causing a shallow arch and symptoms. For example, there might be an abnormal join between some of the foot bones (tarsal coalition) characteristically causing a stiff, flat foot which becomes painful along the outside of the hindfoot and shin in late childhood. Another condition present since birth but not causing symptoms until later is an accessory navicular bone. This extra bone sits at the side of the foot where a strong tendon (tibialis posterior tendon) attaches and causes pain, often with a bony bump above the rather flat arch.
In adults, this tendon can become over-strained, inflamed, stretched and even torn (posterior tibial tendon dysfunction). The arch becomes even flatter and the foot begins twisting outwards in the process. The same process of arch collapse can also happen if the midfoot joints become arthritic due either to wear and tear (osteoarthritis) or inflammatory disease (such as rheumatoid arthritis). Rheumatoid arthritis can also attack the tendons directly. An arch collapsing because of arthritis in the joints can be pictured as a stone bridge arch whose mortar between the stones starts crumbling. As this happens the stones approach one another and the arch sinks.
Many of these causes of flat foot benefit from biomechanical treatment such as exercises and the use of an arch support. However the prognosis and correct treatment should a conservative approach fail, depend upon on the underlying cause.