Orthotic > Dynamic AFO
The search for a functioning AFO (within a larger KAFO) has proven to be the most difficult component. The requirements of my unique anatomy and desire for a high level of function create a complex case. Mechanically speaking, the anatomy of my leg is such that the geometry of my leg looks very different in extension then it does in flexion. My leg in extension is characterized by knee in valgus, and an ankle in varus, caused by an externally rotated foot. My leg in flexion is characterized by even more knee valgus and even more ankle varus caused by a collapsing knee and external hip rotations. Before you ask, none of these limb malfunctions are "correctable" beyond a certain point, they are deformations. The descriptions I am giving are after having them "corrected" as much as they can be. This is not an exercise in, "let's get everything into neutral on all 3 planes and call it a day", it is very much one of, "we have a limb with non-standard, sub-functional geometry, how can we build a exo-structure that fills in the structural gaps". Now that we have that out of the way, buckle up, here are the design requirements for my AFO:
- Counter Knee Valgus Support: I require a robust 3-point counter knee valgus system. The AFO needs to contribute two of these forces points, the distal lateral tibia and medial knee. This is accomplished with a medial/anterior knee wall and lateral paddle. We originally had the lateral paddle coming the posterior strut, but ran into durability issues. Our current testing model has the paddle coming off of the footplate, however in next version we are now looking to separate foot plate / strut completely from the valgus control component.
- Ankle Flexibility vs. Dynamic Return: In the past we have incorporated some fancy dynamic energy return into the AFO design and had some success, but ultimately learned that my varus ankle doesn't tolerate impact well while in plantarflexion . So unless we find an energy return design w/o a plantarflexed foot plate, I am giving up this idea. With dreams of energy return dashed we have turned our attention to a design that allows increased ankle plantar and dorsiflexion. Having a fixed ankle greatly decreases the functional effectiveness of a limb because it decreases the amount of foot surface area that can be in contact with the ground through out various movements. Currently we are looking for a design that follows the 2 anatomical flex points of the foot and accomplish this without breaking.
- Durability & Light Weight: The AFO needs to not fail. No cracking of the foot plate or snapping of the strut. This must be achieved with at a minimum weight. Of all the components of the brace it is most weight sensitive portion is the AFO. More weight = more swing weight. My current working model weights a ton.
- Foot Control: In full knee extension my tibia is externally rotated 40 degrees (uncorrectable). As such to hold my foot in a comfortable "neutral" position with respect to the talus (ankle) joint, it also needs to be externally rotated 40 degrees. Having one's foot externally rotated during activity is a functional disaster. Fortunately (I guess) over the years my ankle has created a varus deformation to allow my foot to remain forwardish for increased function. Now for the complicating factor, due to the geometry of my knee flexion, as I bend my knee the angle of external rotation of my talus joint increases dramatically, going from the 40 degrees in extension to about 80 degrees of rotation at 90 degrees of knee flexion. So my ankle that was already in varus in knee extension now goes into extra super duper extreme varus during knee flexion in order to maintain a forward foot position. The problem areas in the foot that result are pain in the medial calcaneus (heel) and on the bottom of the foot around the 4th and 5th metatarsal head. In the past we have focused on making relief points directly in the carbon foot walls and lateral foot wedges. Recently we have been experimenting with a DAFO design to utilize better suited material. In our next design we are going to push this idea further, relying more on the DAFO for foot control and less on the carbon walls.
- Accommodation of Hip Rotation: Coming soon