38 C.F.R. § 4.57

It is essential to make an initial distinction between bilateral flatfoot as a congenital or as an acquired condition. The congenital condition, with depression of the arch, but no evidence of abnormal callosities, areas of pressure, strain or demonstrable tenderness, is a congenital abnormality which is not compensable or pensionable. In the acquired condition, it is to be remembered that depression of the longitudinal arch, or the degree of depression, is not the essential feature. The attention should be given to anatomical changes, as compared to normal, in the relationship of the foot and leg, particularly to the inward rotation of the superior portion of the os calcis, medial deviation of the insertion of the Achilles tendon, the medial tilting of the upper border of the astragalus. This is an unfavorable mechanical relationship of the parts. A plumb line dropped from the middle of the patella falls inside of the normal point. The forepart of the foot is abducted, and the foot everted. The plantar surface of the foot is painful and shows demonstrable tenderness, and manipulation of the foot produces spasm of the Achilles tendon, peroneal spasm due to adhesion about the peroneal sheaths, and other evidence of pain and limited motion. The symptoms should be apparent without regard to exercise. In severe cases there is gaping of bones on the inner border of the foot, and rigid valgus position with loss of the power of inversion and adduction. Exercise with undeveloped or unbalanced musculature, producing chronic irritation, can be an aggravating factor. In the absence of trauma or other definite evidence of aggravation, service connection is not in order for pes cavus which is a typically congenital or juvenile disease.