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Feb 2

Written by: Larry Huppin, DPM
2/2/2012 7:00 AM

 I want to talk today about the "Blake Inverted Paradox". Blake inversion refers to balancing the positive cast more than 10 degrees inverted. Additional modifications are performed to make the arch height tolerable. This technique is available only in plaster.  The paradox is the fact that a 9 degree inverted orthotic has a higher arch than a 10 degree inverted orthotic.  

I consulted with a ProLab client yesterday who wanted to order an orthosis with an arch that would conform extremely close to the arch of the foot. He ordered a standard cast fill, but then inverted the device 10 degrees.

He was surprised when he received the orthotic back and found that it did not conform particularly well to the arch of the foot. He was under the impression that as you invert the positive casts the arch of the orthosis increases. This is true up to a point. That is what I want to discuss today.

When you prescribe an orthosis to be inverted, what is actually happening is that we are inverting the positive casts. Essentially, we are building the positive casts up under the first metatarsal. For example, let’s say that you ask us to invert the orthosis by 5 degrees. When we are making the anterior platform of the positive cast, we will make it thicker under the first metatarsal. This results in the heel of the positive cast sitting inverted to the supporting surface. In addition, as you invert the positive cast the arch increases in height. When an orthosis is made off of this cast, that orthosis will have a higher arch. For every degree that you increase the inversion, the arch height will increase up to 9 degrees inverted.

When the cast is inverted more than 9 degrees, it is considered a Blake inverted orthosis. During production of a Blake inverted orthosis, once the positive cast is inverted, we then fill in the medial arch with plaster to bring the arch height back down to what it was initially on the negative cast. This is necessary because some Blake orthoses are inverted 15, 20, even 35 degrees or more. Without the modifications, no one could tolerate the arch height on such an orthosis.

So, this is why an orthosis made with 9 degrees of inversion has a much higher arch than one made with 10 degrees of inversion.

If you prescribe an orthosis with 9 degrees of inversion, we are not going to add any extra plaster to the medial arch. The orthotic made off of this cast will have a very high arch. However, if you prescribed 10 degrees of inversion, then we are going to add plaster back into the medial arch to bring that arch back to what the foot was originally. This will have a substantially lower arch than the 9-degree inverted device.

This is somewhat of a confusing issue and I hope to do a video explanation of this soon.

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.

For an easy way to stay up-to-date on evidence-based orthotic therapy, subscribe to our free E-Journal. Your will receive a monthly email synopsis of the research that impacts your practice.   


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