Variable-Stiffness Prosthesis Improves Walking, Prevents Osteoarthritis—study
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Most users of stiff passive prosthetic legs develop health issues like hip and knee osteoarthritis. Research shows that people with lower limb loss are 17 times more likely to develop knee osteoarthritis compared to the general population, and the rate of developing knee pain is twice as high compared to those without amputations.
Passive prostheses and osteoarthritis
Osteoarthritis is more common in individuals with below-knee limb loss, due to the reduced power they get from their ankles when pushing off. This lack of push-off can result in more weight being placed on the opposite limb when walking. For those with a prosthetic leg, the sound side leg has to work harder, which may contribute to the development of knee or hip osteoarthritis over time.
Fortunately, osteoarthritis doesn’t have to be a given for people who have experienced limb loss. Research found that powered prosthetics can lower the stress on the knee and reduce the forces that lead to osteoarthritis. Additionally, enhancing push-off power in powered prosthetics can help decrease the energy needed to walk.
As of this writing, the Empower by Ottobock is the only powered ankle-foot prosthesis available for consumers. Although it offers various advantages, only 5% of individuals with below-knee limb loss are using this device, primarily because insurance doesn’t cover it. Additionally, compared to conventional passive prosthetics, Empower is generally heavier, bulkier, and pricier.
To address these challenges, researchers are exploring the potential of variable-stiffness ankle-foot prostheses.
How joint stiffness works in biological legs
Joint stiffness adjustment is an essential function of the biological ankle. It behaves as a spring that changes its stiffness. For example, the ankle becomes stiffer when the body needs more support (like pushing off during a sprint) or more flexible when simply walking.
Considering the design of the natural foot, researchers realize that it’s crucial for a prosthetic foot to have adjustable joint stiffness. So, they studied the variable-stiffness ankle-foot prosthesis, which is designed to change stiffness to mimic the way a natural ankle functions.
Variable-stiffness prosthesis: the study
In the study, researchers explored the advantages of a variable-stiffness ankle-foot prosthesis with seven participants with below-knee limb loss. They compared this adaptable prosthesis, which could operate at six different stiffness levels, to the Ottobock Taleo, a standard passive prosthesis with fixed stiffness.
The researchers adjusted the stiffness of the variable-stiffness ankle-foot to levels similar to those of a biological ankle. They found that it allowed the study participants to experience increased energy storage (which assisted with the next step or movement), as well as increased peak and push-off power.
The study also found that the variable-stiffness ankle-foot reduced impact forces on the other leg, compared to the fixed-stiffness Taleo prosthesis. This means that the prosthesis could lessen the strain on the other leg and reduce the risk of knee osteoarthritis.
The bottom line
The variable-stiffness ankle-foot prosthesis offers a promising solution to the challenges individuals with below-knee limb loss face.
As the field continues to evolve, variable-stiffness ankle-foot prostheses could provide a more effective, accessible, and cost-efficient alternative to traditional and powered prosthetic options. This would ultimately improve the quality of life for amputees and promote healthier mobility.
