One of the most common causes for a flat foot in the adult is a rupture of the posterior tibial tendon.
The posterior tibial tendon (along with other supportive ligaments) is responsible for maintaining the arch of the foot. This tendon passes behind the ankle and winds around one of the bones inside the ankle called the medial malleolus.
Those prone to developing a rupture of the posterior tibial tendon include women who are overweight, and those who have a flat foot that has been present since early adulthood or childhood.
In these cases, the posterior tibial tendon may be prone to stretch out and tear. As the tendon tears, its supportive function is lost.
Once the foot begins to flatten, other structures including supportive ligaments on the inside of foot begin to stretch and tear and the foot becomes very flat. Frequently, the patient will note pain which begins on the inside of the foot, just behind the ankle. This is due to inflammation of the tendon, which is also associated with partial rupture or tearing of the tendon.
One of two things will happen to the foot at this stage: Either the foot remains mobile and flexible, or it will start to become stiff. If the condition deteriorates further, the flattening of the foot is associated with stiffening of the joints of the back of the foot. This will limit the inward and outward movement of the foot (inversion and eversion).
The stiffer the foot, the more difficult it becomes to treat the condition. For this reason, it’s important to initiate treatment as early as possible, once the diagnosis of a rupture of the posterior tibial tendon is determined.
The bones of the foot occasionally develop abnormally in a child - and an extra bone, called an ‘accessory navicular,’ is present towards the inside of the foot, in front of the ankle.
This bone shows up in approximately 10-percent of the general population, but isn’t large enough to cause symptoms in the majority of individuals.
The extra bone lump present in childhood can be quite uncomfortable because it rubs on shoes. Invariably, feet associated with an accessory navicular are flat. If a child is active and involved in various athletic activities, it typically aggravates the inflammation of the tendon that attaches to the accessory navicular.
This tendon is called the posterior tibial tendon - and is responsible for maintaining the strength of the arch of the foot. The flat-footedness associated with the accessory navicular is what usually prompts parents to bring children for treatment.