10/01/2020
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Effects of Medial and Lateral Deviation of the Subtalar Joint on Subtalar Joint Moments Produced by Ground Reaction Force
In a foot which functions normally during gait, the subtalar joint (STJ) axis passes from the posterior-lateral calcaneus to about the first metatarsal head (see middle photo below). Ground reaction force (GRF) acting medial to the STJ axis will cause a STJ supination moment (i.e. a tendency to supinate the STJ) and GRF acting lateral to the STJ axis will cause a STJ pronation moment (i.e. a tendency to pronate the STJ).
The location of the STJ axis in the normally functioning foot is no accident. There is a very good biomechanical reason why the normally functioning foot has its STJ axis passing from the posterior-lateral calcaneus to the first metatarsal head, splitting the foot into two halves.
With this STJ axis location in the normally functioning foot, the plantar foot is divided nearly equally into pronation and supination halves. In other words, in feet which function most normally, one half of the plantar foot is on the pronation side of the STJ axis, and the other half of the plantar foot is on the supination side of the STJ axis. The two halves of the plantar foot are nearly equally balanced in supination and pronation surface areas, thereby allowing more normal pronation and supination moments and motions during gait.
However, in feet with either a medially deviated STJ axis or a laterally deviated STJ axis, the STJ axis does not divide the plantar foot into nearly equal halves as it does within the normally functioning foot. This unequal balancing of pronation and supination moments creates abnormal moments and motions during gait.
In the medially deviated STJ axis foot, there is much less plantar surface area of the foot on the supination side of the STJ axis and a much greater plantar surface area on the pronation side of the STJ axis. Then, when GRF acts on the plantar foot during weightbearing activities, this creates an overwhelming STJ pronation moment that will tend to make the STJ maximally pronated during gait and will also make the STJ very difficult to supinate even with anti-pronation foot orthoses.
However, In the foot with a laterally deviated STJ axis, there is much greater plantar surface area of the foot on the supination side of the STJ axis and much less plantar surface area on the pronation side of the STJ axis. Thus, when GRF acts on the plantar aspect of the foot with a laterally deviated STJ axis during weightbearing activities, this creates an overwhelming STJ supination moment that will tend to cause the peroneal muscles become tonically active during standing and have much greater contractile activity during gait in order to prevent inversion ankle sprains with each step. In addition, the increased STJ supination moment seen in the laterally deviated STJ foot makes the STJ very difficult to pronate with foot orthoses.
Understanding the biomechanics of STJ axis location allows the podiatrist and foot-health clinician to develop a much better appreciation of the kinetics of the STJ so that better treatments for patients with STJ axis deviation can be designed and implemented.
