Hyperpronation: more than flat feet.
The stability of the subtalar joint complex is essential for proper foot function. A deviation in either the anterior or posterior joint facet leads to excessive motion in both portions of the complex. Subluxation of the talus on the calcaneus leads to abnormal excessive forces acting on the articular cartilage of the joints as well as pathologic strain to the supporting soft tissues structures. Overall, a normal subtalar joint complex occurs when the articular facets are in constant alignment. Any deviation from the joint facet contact indicates an abnormal alignment leading to slippage of the talus to excessively rotate.
Hyperpronation is considered present when there is an increased amount of static or dynamic subtalar joint pronation or when the period of subtalar joint pronation is prolonged during the gait so as to interfere with resupination of the foot (Aquino, A., Function of the Windlass Mechanism in Excessively Pronated Feet, JAPMA Vol 91, No 5 245, May 2001). The excessive pronation leads to an unlocking of the mid-tarsal joint and immediate places significant strain to the supporting structures with every step taken. It is essential to have a rigid lever effect of the foot for the propulsive phase of the gait cycle. An over-pronating foot therefore leads to musculoskeletal tissue damage and depending on the weakest link a symptom will develop.
The goal of a physician is to rid the patient not only of the presenting symptom, but more importantly to eliminate the source of symptom. In a pathologic deformity, such as most musculoskeletal deformities, physical intervention is necessary. An infectious disease process may be eliminated by antibacterial medicines whereas an osseous mal-alignment requires physical correction. Instability of the talus on the calcaneus results in the obliteration of the sinus tarsi. This non-articular, non-weightbearing space must be maintained in an “open” attitude to restore the normal mechanics to the foot and ankle. The goal is to reorient the posterior facet of the talus to its corresponding calcaneal surface which restores the proper dimensions of the sinus tarsi. Interosseous impingement is prevented, abnormal strain eliminated and the chronic overuse injury to the knees, hips, pelvis, and back is no longer present.
