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Originally Posted by Scorpio622
I have not applied wedges clinically for OA but have thought about the concept for some time. I have a much more simplistic view of how these work- which is purely in the frontal plane. I feel that the lateral wedges tilt the tibia in the frontal plane when the STJ can't accommodate to the wedge. I don't think much internal leg rotation is happening.
In view of all the studies, it seems that lateral wedges have more of an affect on the knee then medial. I assume this is because the STJ has the ability to invert much more than evert, esp in the max pronated patient, which can't evert at all. That being said, most can accommodate better to the varus wedge rather than the valgus at the level of the STJ due more available ROM.
With respects to the max pronated patient (which most knee OA are), they would repond to a lateral wedge much like someone with a triple arthrodesis. Both are standing in heel valgus without the ability to pronate further. As such, the GRF of the lateral wedge must be resolved proximally with little triplanar motion since the STJ has little/no movement. The tibia is thrust medially in the frontal plane and genu valgum is the result.
This is how I see it, I could be wrong....
Nick
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From a biomechanical standpoint, it is more accurate to say that the valgus insoles used when treating medial knee OA cause a lateral shift in the center of pressure which will, in turn, cause a decrease in external knee adduction moment. The problem with saying that valgus wedges "tilt the tibia in the frontal plane" is that this does not accurately portray what is happening mechanically. No measureable tibial motion need to occur in order for these valgus wedges to produce the reduction in pressure in the medial compartment of the knee that produces the symptomatic results seen in patients with medial knee OA. One of my previous Thought Experiments explains this mechanical idea more graphically.
http://www.podiatry-arena.com/podiat...ght+experiment
In addition, I don't believe that the common idea that feet that are maximally pronated are the only ones that will respond successively to valgus wedging for medial knee OA is mechanically accurate either. I could easily make the argument that since a foot that is not maximally pronated has a greater potential to pronate at the STJ (and, therefore, to abduct more laterally to the longitudinal axis of the tibia), then these patients that are not maximally pronated also have the greatest potential to have a more lateral repositioning of their CoP relative to the longitudinal axis of the tibia due to the valgus wedged insole, and, as a result, have the greatest potential to have decreased external knee adduction moment from these valgus wedges.
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Sincerely,
Kevin
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Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
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