Science Interviews

Interview

Sun, 9th Jan 2011

Naked Engineering - Artificial Limbs

Saeed Zahedi, Nigel Kingston, Blatchford Prosthetics

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Meera -   For this week’s Naked Engineering, Dave and I are looking into prosthetics or artificial limbs.  You've got us walking up and down in front of a building here.  Why are we doing this?

Echelon_FootDave -   Well we’re looking at artificial limbs, particularly an artificial leg.  If you want to make an artificial leg, you've got to understand how the natural one works, and what it’s actually doing.  So if you stop now and walk very, very slowly, first of all, your leg just swings forward, almost like a pendulum.  The knee slightly bends as it does it which lifts your foot off the ground, so it doesn’t drag across the floor.  Then your heel hits the ground and then you roll over the top of that heel and you move forward, and then eventually, your toe leaves the ground, you push off with the toe, and then your leg swings forward again.  So, if you're going to build artificial legs, then you've got an awful lot of things to think about and you don't have the advantage of muscles or any way of controlling them.  So you have got to somehow copy these effects with, effectively, a much more simple mechanical system.

Meera -   To find out how artificial limb designers mimic our bodies in this way, we’ve come along to Blatchford Prosthetics here in Hampshire to meet their technical director, Saeed Zahedi.

Saeed -   Historically, the prosthetic leg has been making the sockets from wood or leather, metal, aluminium sheet, and then having components like a knee-joint and the foot and ankle joint, and a pylon connecting the two together which is like a peg leg, like a Long John Silver leg.  If you think about the peg leg, you can just stand on it.  You can hop on it, swing your leg outside and step from A to B in a very, very difficult way, very energetic way.  When you apply a knee-joint, you can swing the leg a bit more easily and there’s a bit less energy involved but that's about it.

Meera -   You're really then using other parts of your body and compensating for the fact that there’s this limited movement in the prosthetic.

Saeed -   Exactly and I think that's where the major breakthrough has been for us, to understand better this nature of compensation.

Meera -   In order to understand a bit more about this compensation and just how the body can react to using prosthetics, we’ve also got Nigel Kingston here, a fellow Blatchford employee, who at the age of 30, due to an arthritic knee joint, had to lose his left leg.  So Nigel, I guess when you were first fitted with a prosthetic, how did your body compensate?  What did you have to think about when simply just walking down the street?

Nigel -   I think probably the biggest thing is slopes for an amputee.  There are very few flat surfaces and it involves the whole of the body having to compensate.  Something typical is if I'm standing on a slope with a standard fixed ankle.  I’d be spending most of my time weight bearing on my sound side which is the right side.  You can feel a lot of pressure in your lower back.  My right thigh muscle is twitching because after awhile, it’s just fighting and saying, “I want to give up now.  I've had enough.”

Dave -   So the fundamental problem is that if you've got a fixed ankle at 90 degrees, if you try to line that up with a slope of 10 degrees, you'd either got to be sort of hanging off in mid-air, leaning forwards or you just got to almost ignore that leg entirely.

prosthetic_legNigel -   You're entirely balancing on the sound side really and the prosthetic side is just there as something of a minor support.

Meera -   So Saeed, as Nigel mentioned, the real problem to-date has been uneven surfaces and to do with this has been the fact that prosthetics have had fixed feet.  So your new design here at Blatchford, the Echelon Foot, a flexible ankle prosthetic, overcomes that problem.

Saeed -   The way it was conceived was basically trying to find out what are the limitations that the amputee like Nigel will be facing.  And to do that, we started looking at the way the ankle and foot function in relation to the ground.  So we started looking more closely at the function of the ankle and we looked at their way the muscles work and the muscles on the normal human body, effectively, they can stretch but in a controlled manner.  They can generate forces at the rate which is in the control of the person.

Meera -   We have one of your Echelon Foot designs here in front of us and this one is about a foot long and it’s made of carbon fibre.

Saeed -   It has about three elements, the toe spring which is made up of carbon fibre composites and that toe spring acts like a cantilever spring which enables the body mass energies to be absorbed and then returned at the right moment in the gate cycle to effectively deliver some sort of a push-off force in the absence of the active calf muscles that is lost in the amputation.

Dave -   So, as the foot rolls forward, you're storing energy in that spring which is then released as you push off.

Saeed -   Correct.  You're almost bending that spring to its limit and then at the right moment, when you take the weight off from the prosthetic side, then that's released.  The other element is the heel spring which again, is effectively designed to be able to absorb the energy which is created at the point of the collision.  If you can visualise the gate cycle as a rhythmical sequence of collision and push-off, you want to be able to absorb the energy at the collision and use some of the energy during the push-off phase to be able to assist the amputee.  This sequence of or the rhythm of collision and push-off needs to be smoothed out.  Hence, we’ve got the hydraulic element in the middle of all this.

Meera -   This is attached really at where the leg would be attached to a foot.

Saeed -   That’s almost roughly where the ankle joint is.  That element effectively acts as a damper.  Now if you have got a damping element in there, the amputee naturally under natural launch and control, they can effectively bring their body into the right orientation.  Hence, the combination of the hydraulic damping and the carbon fibre composite spring, very much acts like your muscles and that’s why we called it the biomimetic design.

Meera -   And now Nigel then.  So, when you were first fitted with a prosthetic, it did have a fixed foot, but you're now wearing the Echelon Foot design.  What differences have you felt then?

Nigel -   Shortly after trying it, I realised I had all my foot to the floor and then I was actually feeling more secure, more stable.  I was able to walk much longer distances.  It was not necessarily about the time of the distance, it was about the lack of fatigue that I was feeling.

Meera -   This type of prosthetic, how would you summarise the real benefits of them?

Nigel -   I think we’re looking at major advance in the comfort, the functionality, and  the general usability of the new generation of feet.  We actually want to, as an amputee to be able to walk more easily.  The new generation are enabling amputees now to do what they want, when they want, and I think it’s the way forward for us.

Chris -   Nigel Kingston and Saeed Zahedi from Blatchford.  They were showing Dave and Meera how to engineer artificial limbs and specifically ones with flexible ankles, so they can more closely mimic the process of human locomotion, resulting in more comfort for amputees. 

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