If a foot is abnormally pronating, there is only one way to stop it. It has to be stopped by applying a force on the medial side of the subtalar joint axis. The force can’t be applied on the axis as it will do nothing (and usually feels uncomfortable) and the force can not be applied lateral to the axis as that will just add to the force pronating the foot.
While this makes sense, the problem arise as that position of the axis varies so much in individuals and each type of foot orthotic varies as to where it applies the force. Fortunately we have clinical tests that can tell us where the axis is, so we can work out where to apply the force from a foot orthoses.

When the axis is more medially located, the problem comes that there is very little plantar surface medial to the joint axis to apply the force. To apply the force various orthotic design parameters have been proposed. There is the Kirby medial skive, the DC Wedge (high medial wedge; minimal arch full; no lateral heel seat and a cuboid notch), the Blake Inverted foot orthotic. These are all just medial wedging design parameters.

Recently the MOSI foot orthotic has been proposed by Paul Harradine, Lawrence Bevan, Chris Webb and Simon Collins. MOSI stands for medial oblique shell inclination. The aim of this design parameter is to incline upwards the orthotic shell on the entire medial side of the subtalar joint axis. A very low arch profile is also used, as in those with a more medial subtalar joint axis, the arch is on the lateral side of the foot orthotic, so any sort of arch support is a pronatory influence. It is an interesting innovative design that makes good theoretical sense. More on the MOSI here.

Craig Payne