One of the most common causes of acquired flatfoot deformity in adults is d
ysfunction of the posterior tibial tendon. The main function of the posteri
or tibial tendon is to invert the midfoot and lock the transverse tarsal jo
ints (talonavicular and calcaneocuboid joints). When the tendon fails to fu
nction properly, a progressive flatfoot deformity develops. Because the dis
ease process is a continuum, a staging system has been devised to offer gui
delines for nonoperative and operative treatment of this problem. The ratio
nale for nonoperative treatment of this disorder is to support the longitud
inal arch and to decrease the valgus angulation of the calcaneus for flexib
le flatfoot deformity, and to immobilize and support the hindfoot and midfo
ot for rigid flatfoot deformities. The success of nonoperative treatment fi
rst requires the assessment of the flexibility of the flatfoot deformity. F
or a flexible deformity, the custom orthosis should be fitted with the foot
and ankle in a corrected position as close to the neutral position as poss
ible. Whereas, for a rigid deformity, it is imperative for the custom ortho
sis to be fitted with the affected foot and ankle in an in situ position.