Pes Planus
A loss of normal medial longitudinal arch leads to pes planus, which can be flexible or rigid.  It may arise as a consequence of hyper-pronation or from increased eversion of the subtalar joint.  The calcaneus will then lie in valgus and external rotation relative to the talus,  Associated midfoot sag may be due to dorsal subluxation of the navicular on the talus.

Many toddlers have flattening of the long arch, forefoot pronation, & heel valgus on weight bearing, severity of which is variable.  Ligamentous laxity is apparent, & degree of abnormality in bone-ligament complex probably is determined genetically.  Usually within the first decade, these children spontaneously develop a strong normal arch.

In the adult population, pes planus may occur in 20 % of adults, most of which are flexible.  Flat feet with a concomitant heel cord contracture may limit function.  They are also associated with a forefoot valgum

If this is a flexible or functional condition it is very effectively treated with custom orthotics.  The goal in this case is to address the overpronation and arch collapsing problem.  This is based on the proper casting of the foot in subtalar neutral position.  This is done to obtain the foots normal or natural position before it has a chance to fail biomechanically by weight bearing.  The orthotic will then maintain this idea posture.  In the case of a rigid foot the procedure will be the same.  Here our goal is not to "create" an arch but to maintain the existing arch and prevent an increase in the collapsing of this arch as well as to prevent additional pronation.
A cavus deformity of the foot (elevated longitudinal arch) is due to fixed plantar flexion of the forefoot.  In pes cavus, the upward axis is increased and therefore  the subtalar joint allows less inversion and eversion (more internal and external rotation).  Because the longitudinal axis is closer to the mid-line, less than normal dorsiflexion and plantarflexion occurs at this joint.  The deformity is progressive, and rigidity increases over time.

The orthotic treatment of this foot is based on the correction of the supination at the subtalar joint.  We can ease the foot in a slight pronated position the relieve stresses on the associated soft tissues.