Tibialis posterior insufficiency occurring in a patient without peronei: a mechanical etiology.

A patient presented with a painful flatfoot deformity, which developed after the onset of a drop-foot secondary to a herniated lumbar disk. On examination, the only functioning muscles were her gastrocnemius-soleus complex and her intrinsic toe flexors. Her affected foot had taken the classic deformity seen with tibialis posterior dysfunction--a valgus heel, midfoot collapse and an abducted forefoot. Peroneus brevis was not functioning and therefore could not be implicated as part of the etiology of this patient's acquired flatfoot deformity. The mechanism in which the ground reaction force produces the foot deformity in a tibialis posterior insufficient foot will be presented.