Posterior tibial tendon insufficiency is also known as adult acquired flatfoot. Although it is a disease of the posterior tibial tendon, it is associated with a variety of foot and ankle deformities. The pathologic changes are not limited to the posterior tibial tendon, but also to changes in the arch of the foot. Failure of the arch ligaments to support the arch, followed by abnormalities in the function of the tendons, both contribute to the deformity of the foot. The deformity in the later changes causes painful weakness in the foot.