Flat foot What and why?
This most commonly occurs as a result of arthritic changes in the subtalar joint plus or minus the midfoot joints. There are many pathologies which may ultimately result in this situation. The most common are progression following a Tibialis posterior dysfunction, following Calcaneal or other Midfoot fractures, as a manifestation of inflammatory joint disease(such as Rheumatoid Arthritis), or as a result of a tarsal coalition. 
Osteoarthritic subtalar(1), Talo-navicular(2) and calcaneo-cuboid (3) joints.
A Calcaneo-navicular bony tarsal coalition(arrow).
Are there any catches? - If the ankle is also involved in the arthritic process this may also require intervention
- A rapid and stiff collapse may be a Charcot joint ,for which the mainstay of management is initially orthotic with a Total Contact Cast.
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