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Does genu valgum cause the foot to pronate?

Discussion in 'Pediatrics' started by Craig Payne, Nov 16, 2005.

  1. Craig Payne

    Craig Payne Moderator

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    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 ponate 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 :confused: Is this just another one of those podiatric myths :confused:

    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 Gait
    Bart Van Gheluwe, Kevin A. Kirby and Friso Hagman
    Journal of the American Podiatric Medical Association
    Volume 95 Number 6 531-541 2005

    What say you?
     
  2. summer

    summer Active Member

    This one has always troubled me as well. It is similar to the "chicken or the egg" scenario. Truthfully, I don't think it can be answered, however I do know that knee evaluation and alignment to the extremity as a whole is often forgotten. Many times I have seen patient who had subtalar implants inserted with rather significant genu valgum, all necessitating removal. In my opinion, when diagnosing and treating either pediatric or adult acquired flatfoot, the knee needs to be addressed initially.
     
  3. I have made similar observations, Craig, and have taught podiatry students for the past 20 years that I thought that genu valgum was a supination influence on the STJ, contrary to what I was taught at CCPM. This led me to write a Precision Intricast newsletter on this in August 1998 titled "Genu Valgum and Pronated Feet" in which I detailed the mechanics of genu valgum and supination moments acting across the STJ axis (Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 121-122).

    Bart asked me a few years ago for an idea on research projects and the paper we just had published in JAPMA represents over a year's worth of collaboration with Bart and Friso in trying to answer this question in a dynamic fashion on normal subjects. The next step would be to study patients with real genu valgum and genu varum to see how the GRF vectors were applied on their plantar feet.

    Glad to see the paper finally in print!
     
  4. David Smith

    David Smith Well-Known Member

    Dear All

    As everyone else, I have seen more often then not that a person with a genu valgum type gait has a pronated SJT in gait, some times tho they are supinated in resting bipedal stance. I have been confused by this and am not entirely sure about posting in some cases. However, after recent studies looking at how inclined/oblique axes change net moments about a joint, it may be that if one analyses moments about a joint in terms of a global reference frame ie the lab and force plate and assumes no obliquity in the joint axes (which may be the most intuitive way to imagine these motions)then this may give a very different picture than if the forces and moments are rotated into the true anatomical axis and using a local axis reference frame ie proximal limb in relation to distal limb.
    I think the fact that the CoM is medial to the STJ may be a red herring, as as far as I know and with reference to above, the position and magnitude of the force applied distally, ie at the foot, and the force position in relation to the axis is more indicative of a certain motion or moment.
    I have come to the conclusion that moments in the Y axis of the limb eg tibia/shank (using a local reference frame ie Y = vertical if in normal resting stance) and the effect of an oblique axis on all forces will account for pronation in a genu valgum stance.
    In my own case if I stand (RCSP) in a genu valgum stance with my shoes on my STJ most definintely does supinate and the high pressure is central or nearer to the lateral border, without shoes however there is a definite tendency to pronate and high pressure is on the medial border. This would indicate that despite the knee position being similar and the CoM in the same relative position the change of distal grf forces on the foot changes the moments and motion of the STJ.
    Cheers Dave Smith
     
  5. The first question that one must ask is whether the pronated foot has anything to do with the genu valgum deformity in the patient. Since "pronated feet" are probably present in over half to two thirds the human population, then an association of genu valgum with a pronated foot may not actually mean very much since most of the population has a pronated foot.

    The higher the subtalar joint (STJ) axis is located from the ground, then the greater will be the moment arm for the medial-lateral shearing vector component of ground reaction force (GRF) to cause a STJ supination moment in stance phase. This is why adding shoes of increased sole thickness will increase the magnitude of STJ supination moment while both standing in a genu valgum type of stance or walking in this gait style. We found that the latter half of stance phase would be the part of stance phase where the STJ supination effect should be the greatest due to the inclination angle of the STJ axis (Van Gheluwe B, Kirby KA, Hagman F: Effects of simulated genu valgum and genu varum on ground reaction forces and subtalar joint function during gait. JAPMA, 95:531-541, 2005).

    As one stands with increasing base of support during relaxed bipedal stance (i.e. feet more wide apart), then one will feel an increased STJ supination effect as the feet are spread wider apart. This is because, as one spreads the feet wider apart, the GRF must cause an increasingly larger magnitude of medial-lateral shearing component on the plantar foot to counterbalance the increasingly larger magnitude of internal hip abduction moment that arises from the vertical component of GRF acting at a longer distance lateral to the anterior-posterior hip joint axis.

    The mechanical effect of the increased internal hip abduction moment with increased base of support is best personally experienced and demonstrated by standing in sock feet or nylons on a wood or smooth-surfaced floor with the feet increasingly wider apart. As the feet become more wide apart, the hip abduction moment increases and the feet will experience increased lateral acceleration on the floor due to the decreased coefficient of friction between the sock and smooth floor. However, with barefoot standing, the coefficient of friction is increased which will then prevent the feet from sliding laterally but will also increase the magnitude of medial-lateral shearing component from GRF which, in turn, increases the STJ supination moment.

    The above described exercise should be a standard experiment/demonstration for all undergraduate courses in podiatric biomechanics.
     
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