Thursday, November 17, 2005

Does genu valgum cause the foot to pronate?

This one has troubled me for a while.

When teaching the pediatrics part of the course here, especially the genu varum and genu valgum, it always comes up about the effect they have on foot function. Genu vaurm affects the foot as it has to pronate to get the foot flat on the ground. According to so many podiatric texts, genu valgum also causes the foot to pronate as the center of body weight is medial to the subtalar joint -- this has always troubled me ... how can opposite alignments at the knee cause the same affect on foot function Is this just another one of those podiatric myths

As part of the lecture, I mention this troubling aspect --- sometimes, I get the students to do this exercise:
Stand up with feet wide apart (simulated genu valgum, due to wide base of gait) -what does your subtalar joint feel as though its doing? -- the answer is always "supinating" ---- so how does a genu valgum pronate a foot, like so many podiatric texts claim??

Now thanks to Bart Van Gheluwe we finally got some real data:

Effects of Simulated Genu Valgum and Genu Varum on Ground Reaction Forces and Subtalar Joint Function During GaitBart Van Gheluwe, Kevin A. Kirby and Friso Hagman
Journal of the American Podiatric Medical AssociationVolume 95 Number 6 531-541 2005
"The mechanical effects of genu valgum and varum deformities on the subtalar joint were investigated. First, a theoretical model of the forces within the foot and lower extremity during relaxed bipedal stance was developed predicting the rotational effect on the subtalar joint due to genu valgum and varum deformities. Second, a kinetic gait study was performed involving 15 subjects who walked with simulated genu valgum and genu varum over a force plate and a plantar pressure mat to determine the changes in the ground reaction force vector within the frontal plane and the changes in the center-of-pressure location on the plantar foot. These results predicted that a genu varum deformity would tend to cause a subtalar pronation moment to increase or a supination moment to decrease during the contact and propulsion phases of walking. With genu valgum, it was determined that during the contact phase a subtalar pronation moment would increase, whereas in the early propulsive phase, a subtalar supination moment would increase or a pronation moment would decrease. However, the current inability to track the spatial position of the subtalar joint axis makes it difficult to determine the absolute direction and magnitudes of the subtalar joint moments." (J Am Podiatr Med Assoc 95(6): 531–541, 2005)

What say you?

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