Today we have a question from a podiatrist regarding the forefoot correction in the our ProLab P3 prefabricated orthoses.
To set the stage for this question, you should be aware that we designed this prefab to be as ckose to a custom functional orthosis as a prefab could be. To do so, we incorporated a 2mm medial heel skive and 3 degrees of forefoot valgus correction in the forefoot (as if it was a custom orthoses balanced for a person with 3 degrees of forefoot valgus). So, with no further adieu, here is the question:
Why do you have a 3 degree VALGUS forefoot post when it appears that most people need a 3 to 4 degree varus post? I understand that in open kinetic chain you lock the midtarsal joint by dorsiflexing the 5th ray, but in closed kinetc chain the ray gets dorsiflexed by ground reactive forces, hence it is supposed to lock the midtarsal joint. But when the forefoot is in varus the extra motion to the neutral position causes excess pronation. We learned years ago in school this is stopped by posting the forefoot to its abnormal varus or valgus component. Have I missed a change in biomechanics somewhere along the line?
Actually, only a very small percentage of the population has a structural varus forefoot to rearfoot relationship, although a large percentage has a supinatus (soft tissue varus) position of the foot.
During gait, anyone who has an everted heel or an everted forefoot will develop increased ground reactive force under the medial forefoot. This results in a dorsiflexory force on the first ray. Over time, the first ray adapts to this position (likely due to shortening of the dorsal ligmentous structures) and remains in a dorsiflexed position. This foot appears to have be a varus, but if you apply a few ounces of plantarflexion force on the first ray, around the first met/cuneiform area, you will bring the foot back to it’s natural valgus position.
A number of studies, including Hicks’ series of classic articles on the “Weightbearing Mechanisms of the Foot” from the 1950’s have shown that the windlass mechanism of the first MPJ will work more effectively if the first ray is allowed to plantarflex during gait. The opposite is also true – “pushing up” under the first metatarsal head will dorsiflex the first ray and lead to jamming of the first mpj during toe-off. This in turn leads to functional hallux limitus and increased tension on the plantar fascia..
Thus, since most feet have a structural valgus deformity (although it may be a positional varus), a prefab with a varus forefoot would only maintain the positional deformity and cause a jamming of the first MPJ during gait. By putting a valgus forefoot to rearfoot relationship in our prefab, we are helping the first ray plantarflex and decompressing the first mpj and reducing tension on the plantar fascia.
ProLab takes a scientific approach with our orthoses by integrating evidence-based medicine into orthotic therapy. Our team of Medical Consultants regularly evaluates the medical literature pertaining to orthotic therapy and biomechanics. ProLab clients are encouraged to contact a medical consultant whenever they have questions about an orthotic prescription.
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