How is Motion Placed into an Orthotic Device and Evaluated?
The motion of pronation must occur at heel contact, and the motion of big toe joint dorsiflexion must occur at push-off.
Not trying to be too simplistic, but there are only 2 motions of the foot while on the ground, that are crucial for me. The majority of foot function during weight bearing as we progress from heel contact to push-off concerns stability or stabilization. I want to observe a rear foot that pronates at heel strike. The orthotic extrinsic rear foot post plantar surface of the heel on the lateral 1/2 can be ground 1/8 inch or so to encourage that subtalar joint pronation. You should not grind medially which will de-stabilize the crucial medial column. And, I also want to observe a healthy push-off from a side view to make sure that nothing I have added to the distal edge of the plastic blocks that motion.
This image is the bottom of a rearfoot extrinsic post with the medial (M) and lateral (L) sides marked
I have colored about 1/8 inch of the plantar lateral post that can safely be ground down to produce pronatory motion.
One of the most important aspects of a stable orthotic device is that the medial corner or the heel post should be on the ground when the distal medial corner of the said orthosis is also on the ground.
You check this by firmly loading the top of the arch and aligning the front of the orthotic device even from the first metatarsal to the fifth metatarsal. The heel at this point should be in full contact with the ground medially. The medial column is now stable making the ankle, knee, hip, and back stable as well. The heel posts are initially made flat and parallel to the distal edge of the plastic. Then motion is produced by typically grinding about 1/8 inch of the lateral 1/2 of the plantar post surface. If you think about our human weight transference, we land on the lateral side of the heel (normally) and then we gradually spread our weight medially and then of the big toe (hallux). If we are to impact that motion, and remain stable, we need to adjust the plantar lateral post at the point of contact.
When I design (prescribe) CFOs, I am commonly going distal to the metatarsal heads with my lifts and extensions. Here you run the risk of blocking the normal forward propulsive motion. Even metatarsal pads can block this motion. If you add in a patient with a dorsally jammed first metatarsal, a very weak foot and especially at the achilles tendon, the gait can become apropulsive with orthotic devices, from being propulsive without orthotic devices. Simple rules can help like using the least material that is necessary in the front 1/2 of the custom orthosis, cutting through the material in cross striations from medial to lateral (not at the edges however), making sure the plastic itself is not too far up the foot, and adding a heel lift (both sides) if possible when the patient complains of not being able to push through their foot. These are the questions I always ask: Do you feel stable? Do you feel anything off? Do you feel like you can roll through your feet with ease? And, I even ask them to go onto the ball of their feet, and ask if they are clearing the plastic.
