ORLANDO, Fla. — Current prosthetic feet designed for children do not compensate for the sudden locomotor activities they perform. Optimizing a foot for walking will produce substantial deficits in running while optimizing the foot for running will cause instability during walking. However, kinetic data presented here may help in the design of a durable prosthetic foot for children.
“Designing prosthetic feet for children is an especially difficult challenge, partly because of the wide range of activities they do,” Michael Orendurff, PhD, senior scientist and director of the Biomechanics Laboratory at Orthocare Innovations Foundation, said. “For an adult amputee who runs, they have a running limb and go off for 30 or 40 minutes and that is their exercise protocol. But for children most of their development is play based, and they just suddenly start running. You want to enable that level of play with a prosthetic foot but it’s an especially challenging problem. The goal of this project is to collect kinetic data on children participating in their usual activities and use that information to help design and refine these mechanical, electrical, microprocessor engineering models in a foot.”
Prosthetic feet for children
Orendurff and colleagues from the University of Strathclyde, Glasgow, Scotland, and the department of engineering, Seattle Pacific University, placed Europa load cells at various levels between the prosthetic socket and foot of six transtibial amputees aged 7 years to 15 years. Researchers used Bluetooth to stream forces and moment data to a laptop and videotaped activities of the children, which included walking and running on level ground and walking up and down 6° and 20° slopes on grass, loose gravel, asphalt and dirt. Each child completed between 18 and 54 trials over a 1-hour period.
Researchers found children avoided putting a high load on their prosthetic feet and adopted strategies to change loading so they landed on their sound limb more than their prosthetic limb. They also found the loading rate was almost twice as high for children as the ISO standard for accelerated wear tests on adult feet. Overall, study results showed the peak moment was 100 Nm and max loading rate was 500 Nm per second.
“If you’re going to put the [prosthetic] foot into a test machine and try to make sure it’s safe before you let a human walk on it, it would be nice to test somewhere near the appropriate loading rate,” Orendurff said. “It doesn’t make a big difference in the ultimate strength, but it does make a difference in how you assess the hysteresis of the foot or how much energy it absorbs. If you are going to think about damping the spring a little bit, you need to load at a physiologic rate that actually happens out in the community with these children.”
Currently, the researchers are entering patient testing at three locations. According to Orendurff, the next stage is to try the prosthetic foot on a treadmill with a harness and test up and down slopes and the various controls. However, several challenges remain.
Orendurff said researchers are trying to incorporate a controls algorithm that anticipates what the child is doing and adapts the foot to the movements they are performing.
“This is very much like how humans work,” Orendurff said. “As you walk around you don’t think about every single step. Everything is a spinal reflex happening as you elicit your gait, but if you suddenly step off a curb you didn’t know was there, the human system has only one response to that and it’s a startle response. So we have something like that in the controls algorithm.”
Although the peak loads for children are much lower than adults, their loading rates are much faster, making it difficult for the control algorithm to compensate for the different activities children participate in.
“For an adult, the transitions between various gait states are relatively smooth. We don’t suddenly break into a run because one of our schoolmates came and tagged us, but this happens a ton for children,” Orendurff said. “They just suddenly break into a very intense activity and we had to very quickly adapt to what was happening.”
The researchers still need to figure out how to move the foot into plantar flexion for running, which is difficult because children start running so suddenly.
“We have been slowly letting the foot migrate into plantar flexion as their running continues, but usually we keep them at neutral for most of their running because they’re only doing three or four or five steps in a row and then stopping. So there isn’t a long period of adaptation like there is in an adult,” Orendurff said. — by Casey Tingle
Disclosure: Orendurff works for Orthocare Innovations, the developer and manufacturer of the Europa system.