A flare is used to widen the base of support under the foot usually under the heel. A flange provides rigidity to the heel portion of the uppers allowing for a greater force to be placed on it without the heel counter buckling.
Typically, the intervention will center around whether the person has a fixed or mobile hindfoot deformity. In the presence of excessive inversion at initial contact, a lateral flare will improve heel strike by increasing the lever arm and applying a valgus tourque moment at the lateral heel as the foot loads. A mobile hindfoot will correct and overall limb alignment will improve during weight bearing. With a fixed deformity, a flare will shift the valgus stress to the next more proximal joint, usually the knee. Unless there is a correctable degree of valgum present at the knee, a flare alone is not recommended for the correction of excessive inversion in a fixed hind foot deformity.
A flange works differently. Again, if we use the excessive inversion example, Stiffening the heel counter places greater force coupling around the superior aspect of the lateral calcaneous. This modification is not used to correct, but to accommodate the deformity, decelerate ballistic frontal plane motion, and minimize the effects of an inverted hind-foot. Ultimately this results in a better fit, less stress on the knees and hips, and improved shoe wear.
Combining the two provides maximum control over hind foot frontal plane mechanics.
Chris Gracey MPT, Cped