System analysis enhances artificial limb design

Tom Shelley reports on a mechanical coupling that dramatically improves the lives of leg amputees

Medical technology company, Blatchford, has developed an artificial ankle joint that uses an adjustable hydraulic damping device to restore much of the lower limbs natural movements.

The design approach was based on the analysis of the combined leg-ankle-foot system, rather than the requirement of each individual component part which then need to be integrated together. The philosophy has brought great benefit to both the company and the individuals that have use it.

It demonstrates the benefit that a design based on complete mechanical system analysis can bring. In this case, the resulting 'Endolite' has moved to the front of a market already occupied by several hundred competitive products. It has rapidly gained popularity in the US since its release last October with many veterans of the Iraq and Afghan wars regaining much in the way of mobility.

As soon as we arrived at its headquarters in Basingstoke, technical director Professor Saeed Zahedi introduced us to Nigel Kingston. Kingston was standing on a slight slope at the entrance of the building and commented: "You quickly realise how many uneven surfaces there are when you become an amputee."

He was unable to work or do much at all for twelve and a half years. It was the result of severe arthritis, a disastrous operation, and subsequently having his leg amputated. He was then fitted with an artificial leg and ankle combination that was less than optimal.
However, having been fitted with the new ankle, he says: "It's that getting your foot back feeling, I have had a leg given back to me, the foot has transformed my life. It's now a normal walk for me."

He then described how he had been able to walk 27 miles in one go with it. We could see that his standing gait was natural, and he could easily sit down and stand up, all of which stem from the ability of the ankle joint to bend in either direction, which is described technically as plantarflexion and dorsiflexion.

Support is provided by having the joint connected to a short hydraulic cylinder, with an adjustable amount of flow restriction for each direction of movement. Professor Zahedi showed us some of the other solutions that different designers have come up with to try to solve the problem of restoring amputee's abilities to lead normal lives.

"We found that amputees can align their feet better than clinicians because they can feel it," says Zahedi. "A human muscle is a spring and dashpot. As long as you can control the damping, you control natural optimisation.

"The ankle joint provides a limited range of motion to allow the amputee to self align - the range being a total of nine degrees."

The hydraulic element in the Endolite combined with carbon foot springs mimics the visco-elastic nature of human muscle. It essentially used biomimetics to see how the leg, ankle and foot work together to control movement and adapt to varying terrain.

On sloping ground the foot adapts to the environment, so the body doesn't have to. The result is perfect loading symmetry between the limbs. When the wearer sits the toe relaxes downwards giving a totally natural appearance.

The amount of damping set and the procedure was explained by prosthetist-orthotist, Laura Ritchie. She said: "We start with the restrictors in both directions set to '7', out of a possible range of 0 to 9. We get users to stand up and then we can see if they are leaning back a bit. We then get them to walk back and see if they get foot slap. After this the settings are adjusted and then we get them to go up on a ramp and adjust them again."

The goal is that the loading on the sound leg and prosthetic leg should be equal and Ritchie says before the invention of the new ankle people were putting 80% of their weight on to the sound limb. "A lot of amputees suffer from lower back pain, which is accentuated by an uneven posture.

"When you interface a machine with a human, you tend to lose the ability to compensate, so we design devices that restore the natural ability to do that."

Alternative designs that we were shown include a single carbon fibre spring element, mechanisms with pneumatic damping, and use of large pieces of elastomer. There was a plethora of different geometries.

Because of the wide range of designs, Professor Zahedi is part of an ISO – CEN working group set up to define standards for artificial ankles, and other prosthetic products. "Define minimum standards of what products should be able to do," he says. "This should include no more discomfiture of stumps and users should feel safe coming downhill."

The next stage in development for Blatchford is to build in adaptive intelligence, as has already been done with the intelligent knee joint. As well as advancing the engineering of human prosthetics and showing the advantages of incorporating increased sophistication in mechanical couplings, it also shows a how a long established British company successfully applies targeted innovation to improving its business.

Zahedi says: "The company lost market share in 2004, but we then took a strategic decision to put innovation first. As a result, five years on we have regained that lost market share.

"But the innovation and 'Eureka moment' came from understanding unmet needs. We asked a amputee sprinter who could do 100m in 11.2s what he had trouble with and he told us that it was going upstairs. We added this and such tasks that the fully able bodied take for granted such as getting off a chair."

Pointers

* Ankle joint includes a very short hydraulic piston and adjustable restrictions to damp limited movement in both directions
* It works in conjunction with a carbon fibre sprung foot to constitute a classic spring and two dashpots configuration
* In its present form, is facilitates the user to adapt to using it, but in future, electronics are likely to be added to make it adaptive within itself, as has already been done for other company prosthetic products

Author
Tom Shelley

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