The basics of getting an artificial knee

jacobandjennyOur names are Jacob and Jennifer, and we are the designers of the VGK. First off, we would like to give you an introduction to artificial limbs for transfemoral amputees. Losing a limb can be a very serious loss, and leave you feeling very alone and not knowing where to start. This website is naturally set up to inform you about the VGK (Very Good Knee), but firstly we’d like to explain a bit more about the process of getting a new limb, and what to reasonably expect from it.

Getting fitted with a leg: the socket

After (trans femoral-) amputation, an artificial limb may be made for you. You may have been told what you could do with it in a positive way, or what you cannot do with it in a negative way. Let’s leave that asidefor the moment, and see what it is all about in a technical way. An artificial limb would typically be fitted to you with a ‘socket’, which is a snugly fitting cup to go around the leg stump (residual limb, or as some call it ‘foot’).  (Neither of these words fit the experience very well, and after much deliberations, the word ‘stump’ seems to prevail in practice). Below the socket all the mechanics of the leg are fitted, which we will explore in the next stage of our tour, but right now the socket fitting comes first. The  socket is really the most important part, and  a well fitting socket is still not a fixed recipe. The problem is that the amount and consistency of the flesh and bone change over time, and there is a skill factor from the prosthetist how to work with and around the anatomy, and unfortunately even a bit of luck, because even ‘duplicate’ sockets do not necessarily feel the same. However, most trans femoral amputees can be fitted comfortably, or a diagnosis can be made why such comfort cannot be. If satisfaction is still not found, a ‘second opinion’ can at times help.  (moved  to below)The shape of the socket is made from either:

1) A plaster cast, (made from wrapped plaster bandage that sets in the form and shape the prosthetist gives it by firm hand pressure. Naturally hands get near delicate places, but so be it, job needs doing).

2) Measurements, (a tape measure is used to select/modify a suitable pre-existing template).

3) A computer image based method (where the size and shape of the stump is captured by camera or tracer, and the data are converted into a digital representation of the socket).

Any method better than the other? Depends. If the result is good, then all is well. If the result is mediocre, then reflect. If the result is poor, try may be another method, or do the same method better next time. From the model from above methods, the socket is made one of two dominant methods:

a) a fibrous material is soaked full with resin that goes hard to make a strong structure, or

b) a plastic is heated up and stretched over the positive model to create a socket.

c) less usual is the manufacture of a flexible socket, due to high input of expense and expertise. See to see a flexible socket as used.

What must a prosthesis do? Knee and Foot.

Hierarchy of needs of the Transfemoral amputee

The artificial limb will have to take care of what the lost leg used to do: keep you from the floor (prevent falling and collapsing), help restore your shape and posture, allow you to walk in a good looking fashion, allow you walk down stairs and slopes, and so many more things in life. A good knee joint makes sure that under normal circumstances and under normal abnormal circumstances the knee joint does not collapse under your weight. So what is an ‘abnormal’-normal circumstance? Think about walking in strong wind, think about walking in long grass or sand, think about walking downslope and stairs. Even a late night out when driving is not an option. There are many more situations: opening a door, stepping down from more than ordinary step height, cycling and getting of the bicycle, rowing a boat, fishing in long boots in the river, shooting arrows, crawling or kneeling under a table, and the lists go on. The relative importance of the various features of a prosthetic knee joint is indicated in the ‘hierarchy of needs of the transfemoral amputee’ as show in the figure to the left. The concept shows that a stable stance phase (when you have your weight on the knee joint) is the basis for all other activity.  The figure also shows how higher needs can get fulfilled after the basic needs are met.  The details of this figure are discussed elsewhere. This is where the VGK (Very Good Knee) comes into the picture. There is a lot of information elsewhere on this site, so let’s move on to the foot. The artificial foot is also of great importance, be it more for comfort than for safety.  There are a few basic foot designs:

  1. The SACH foot (Solid Ankle Cushion Heel),  typically a budget foot with very little ‘life’ in it. The soft heel may cause a sense of lack of stability in standing.
  2. The uniaxial and poly axial foot, typically provided with one or more hinges, that can bend under load as rubber springs are compressed. These give a little  more freedom of motion.
  3. The ‘energy storing foot’, typically made of carbon or glass fibre. These feet are the most dynamic, and generally the most comfortable, but are significantly higher up in price.  A well designed heel allows a well cushioned heel strike, and an energy storing foot design will recoil just in time to help give some ‘push-off’ on the end of the step.

Fortunately the VGK is compatible with virtually any foot, but needless to say, an energy storing foot does add to walking comfort and confidence. Then there are other elements that can be built into the limb, like a ‘torque absorber’ (which allows the leg to twist a little during walking, which may reduce any chafing within the socket), a vertical shock absorber (to enhance the qualities of the prosthetic foot), and rotator, which allows cross legged sitting. We hope you are still comfortable, and that the information makes sense. Would you find you have questions, please do not hesitate to write to us and ask your question. In the meantime, let us look at the role of the knee in the prosthesis.

What must a knee do for me?

This is a difficult question to answer, as it depends a little on the ‘You’ who is with me on this tour. ‘You’ may be in good health other than having lost the limb through a cause like an accident or a tumor. ‘You’ may also be in the latter stages of life when general health is no longer optimal. To help answer your question we introduce a few people to you (fictitious but representative), who will tell something about themselves and their needs. Hopefully one of these may give you an answer you are looking for. Alternatively you stay with me a little bit longer. Let’s imagine the aspirational-‘You’ wherein general health is not a restricting factor, then life’s demands dictate what a knee must do, then the following minimum functions can be considered.

  1. Give stability on placing weight on the prosthesis.
  2. Assist you in recovery from a stumble.
  3. Allow you to walk leg over leg downstairs and down slope.
  4. Walk properly fast and properly slow, in good time with you.
  5. Keep its good properties when getting a little hot in use.

If we bear in mind that there are more than 300 different artificial knee joints on the market, than it will not be a surprise that many will not meet even basic needs.  Naturally this site is not the place to discuss each of these, but we can explain a bit more about each of these needs, and see how the general design principles behind these 300+ knee joints allow or disallow the basic needs to be met. In case ‘health’  is a factor,  then very energetic movements may not be on the menu. However, the need for stability and stumble recovery may actually increase. The VGK is deemed to be suitable for category  K2, K3, and K4 type users, and these ‘grades’ are medical terms to indicate the range from housebound walkers to active living to very active users in the outdoors and light sports.

Is it difficult to walk on a prosthesis?

To begin with, it may indeed be difficult. To make the prosthesis move with you is something that needs to be learned, and depending on the type of knee joint there will be different difficulties. Simple mechanical knee joints will often require you to place the weight on the leg correctly to make it secure, and these will continue to require your effort to keep the knee stable. The more sophisticated knee joints, like the VGK, will need you to learn to make it free to swing through (a movement that later on will be totally second nature), but is otherwise ready to keep you stable when you place the weight onto the knee.

To answer this question in another way, have a look at this video of a person telling their story of taking a longer time to get used to the leg (due to other problems that people sometimes encounter). This is not the best or worst case scenario, but just something to look at the process from a certain perspective.

Where do I go from here?

You will possibly have already seen a prosthetist/CPO, or will soon see one. Please ask her/him all about your prosthesis, and about the knee joint they may select for you. The VGK is now used by hundreds of people all over the world, and is likely to be a good alternative for you. Please ask your CPO as much as you can to get yourself well informed, and keep the ‘Hierarchy of needs of the Transfemoral amputee’ in mind as a check list, and ask how the proposed knee joint will cope with any of the six key items.