My name is Robert (name changed) and I work in Australia. At our practice we use your inversion devices, however we have started to increase the amount of inversion correction we use, and as such seem to be getting an issue with the medial fat pad at the heel where the devices are coming out with less correction than the required platform degree.
Also must state we use weight bearing box impression rather than POP.
My question is when modifying at the plaster stage, do you have a method of measuring or checking the ratio of platform to heel position. Obviously if the heel is not modified correctly the ratio is not accurate or will vary.
Many thanks in advance, we look forward to seeing if you have an answer.
Robert, I am going to attempt an answer in the next 2 days, however, you can look at my blog post from Saturday on the inverted technique. This was from a Podiatry student from Spain. Rich
|The medial heel area is off the ground with this 25 degree positioning. This will create the inverted cant that predictably gets 5 degrees of pronation correction.|
|This is another left foot flatter on the bottom of the heel, but also set at 25 degrees inverted.|
|If you compare this 2nd left foot to the first photo, you can tell the subtle increase in pronation support just be the flatter heel.|
|A goniometer is always used to check the inversion cant.|
You mention the medial fat pad and how that definitely, and negatively, influences this. However, what neutralizes that are 2 important factors. The first is that the box technique tends to create a flatter heel. The flatter the heel, the more varus cant you actually get when you do invert it. The rounder the heel, as you invert, sometimes you just rotate the rounded heel and do not get much inversion at the heel. You will always get more talo-navicular and naviculo-first cuneiform support with more inversion. It is with more rounded heels that I need more Kirby skives to help get the correction I need.
Second of all, the amount of forefoot varus or forefoot valgus makes a big difference. And, I believe you get more forefoot varus angulation in a box cast versus a suspension cast, but I have not seen this written. Take 2 feet, one with 15 degrees of forefoot varus, and the other with 15 degrees of forefoot valgus. If you ask the lab to make 25 degrees of inverted correction in both, which one will have more support. Which positive cast, after it has been poured, has to be corrected further. The 15 degree forefoot varus foot cast will sit 15 degrees everted at rest and you have to correct 40 degrees to get the heel 25 degrees inverted. The 15 degree forefoot valgus foot cast will sit 15 degrees inverted and will have to be corrected only 10 degrees to get the desired 25 degree inversion. This subtlety can also make a big difference. You definitely get more correction of the medial column with a 25 degree inverted correction in a forefoot varus foot than a forefoot valgus foot.
I bring all this up now just to get you thinking. Hope we can have some good exchanges. Rich