There are many reasons for flat feet but if flat feet was something that you only noticed as you got older or you had sudden pain and swelling on the inside of the arch, a tendon injury may be involved. Treatment for Acquired Adult Flat Feet (AAFF) depends on the stage of the pain and deformity. Early diagnosis and treatment may include a walking boot or cast to immobilize the tendon in acute flare ups with eventual orthotics for continued support. In cases of a torn posterior tibial tendon, a custom brace may be appropriate. If all conservative measures fail, surgery should be discussed with your foot and ankle specialist. The longer you wait, the more limited choices you have. At the end stage of flat feet is joint degeneration (arthritis), which may require a joint fusion. Early treatment has the best prognosis.