by John W. Michael, MED, CPO, FAAOP, FISPO

For people who have lost their biological knees, the function and reliability of the mechanical knees in their prostheses is critically important. However, no presently available man-made device can replace the subconscious voluntary control of knee motion we have over our natural body parts. So, the prosthetist and amputee are faced with choosing the best from the available alternatives, understanding that even with the most advanced available technology, walking with a prosthetic knee will still require concentration and practice as well as increased reliance on the non-amputated leg. This primer is intended to help the interested amputee and family participate actively in this important discussion with their prosthetist.

The walking cycle can be divided into two parts: the period when the leg is on the ground supporting the body is called the “stance phase” while the time it is off the ground is termed “swing phase.” One convenient way to evaluate a person's prosthetic requirements is to look more closely at their stance and swing phase capabilities. Once the individual's biomechanical needs in both these areas have been clarified, the prosthetist can then suggest artificial knees that provide the necessary function.

Please note that the cost of a prosthetic device is usually directly related to its complexity, but not to its suitability. For some people, one of the more basic prosthetic knees will be just fine while others will require a very complex device to fully meet their goals. As a general rule, the more limited a person's walking goals the simpler and less expensive the prosthesis.

One of the obviously important requirements during stance phase is that the prosthesis remain sufficiently stable so the person does not fall while walking. Such stance stability may come from the knee design or from the amputee's muscle power or a combination of both factors. When the amputee is in good health and has a relatively long residual limb for leverage, they may be able to use their hip muscles on the prosthetic side to control knee stability perfectly when walking indoors. If they can also compensate instantly for irregular surfaces, and readily negotiate ramps, curbs, stairs, and similar barriers, then they may consider the most basic of prosthetic knees: the single axis type.

This is basically a “door hinge” in the prosthesis at the level of the anatomical knee that bends freely. The major advantage of the single axis knee is the simplicity of its design, which makes it the lowest cost, most durable, and lightest option available. This knee is used most frequently for children's prostheses because it is so rugged and because kids will outgrow their artificial limbs every year or so. It is also the knee of choice for adults who live in remote areas, find obtaining prosthetic follow-up difficult, and therefore value mechanical reliability above all else. This is one of the most commonly used knees in the developing parts of the globe, but it is used selectively in countries where other options are readily available.

There are two major limitations to the single axis mechanical knee. As noted previously, it is only safe if the amputee can use their muscle power to make it so. Although this presents little challenge to small children brimming with energy, it is seldom possible for older amputees to control such a simple knee so perfectly. In addition, all knees with mechanical swing phase control can only be adjusted to walk optimally at one walking speed. Since most adults walk faster at some times than others, this swing phase limitation is a significant one.

The swing phase limitation can be overcome by the addition of what are called “fluid control units” to a basic single axis knee frame. Such fluid controlled knees, either pneumatic or hydraulic, have a rotary or linear piston linked to the knee that automatically increases or decreases the swing phase resistance as the amputee speeds up and slows down. This has been demonstrated to result in a more normal gait pattern and to allow the person to participate in activities that would be impossible with a single speed knee. When they are properly aligned and adjusted, fluid controlled knees “keep up with” the amputee's pace across a variety of speeds from fairly slow to moderately fast. Providing they can always keep the knee stable in stance phase with their muscles, active people may prefer the basic “fluid swing control only” prosthetic knees. Figure two (in the magazine) shows an example of a single axis frame with a hydraulic fluid control unit weighing only a few ounces more than the basic single axis knee. The majority of amputees, however, are not able to control their prosthetic knee perfectly under every circumstance. For example, walking indoors on a level hardwood floor is much easier than negotiating deep pile carpets, cobblestones, or grassy slopes. Hiking in the park or carrying grandchildren are more difficult yet. To make such everyday tasks easier and safer, many amputees prefer a knee that has stance phase stability built in.

Three levels of stance control are available. Very feeble individuals, or those with poor hip control from a stroke or other medical complications, sometimes feel safest with a manual lock knee. This knee is normally locked completely straight for walking. With this device, the amputee provides no voluntary stance control since the prosthesis does it all for them.

Although it may sound desirable to have the prosthesis maximally stable at all times, there is ample evidence that walking with a locked knee is not ideal. In fact, a stiff knee may be dangerous should the amputee stumble since it cannot be bent to control the direction of the fall. In addition, a stiff knee forces the person to walk with a limp similar to the character “Chester” from the old “Gunsmoke” television series. Finally, in order to sit down with the knee bent, the person must pull a release lever or cable to unlock the knee and this is often awkward.

For all the above reasons, most clinicians consider the manual lock knee to be the choice of last resort. On the other hand, if this is the only prosthetic knee you can handle it may be your best option. But, the majority of amputees can learn to use one of the more stable but free-swinging knees, after a little practice in the safety of parallel bars.

In the developed areas of the world, the average age of the new amputee is over 60 and many new amputees in their 70s and 80s and even 90s are candidates for prosthetic rehabilitation. Particularly when the leg was lost due to poor circulation, the doctors may have been forced to amputate at a very high level. The combination of a relatively short residual limb, when combined with weakness from poor blood circulation, may create the situation where the amputee can only control the prosthesis for a few steps before becoming too tired to continue safely. Commonly, such persons may need to use a walker for balance and therefore take very short steps. The stance control knee was developed for just such a situation.

Sometimes colloquially called “safety knees,” these devices typically contain a weight-activated friction brake that can stop knee motion. When properly aligned and adjusted, stance control knees swing freely when there is little or no weight applied to the prosthesis. Although it is not possible to have a normal gait with such devices, they do permit knee movement in swing phase, making them superior to the manual lock alternative for those who can master their use.

Stance control knees are best used to supplement limited amputee knee control by providing extra security in the event of a misstep. When the amputee is rested and alert and keeps the knee straight during weight bearing, added stance stability is not needed. But, if they make a mistake and try to step on a knee which is partially bent, then as they shift weight onto the artificial leg the brake stops it from collapsing permitting them to complete the step safely. This feature makes the stance control knee a common choice for the initial prosthesis.

The greatest limitation to all stance control knees is that they disrupt the preswing phase of gait because they cannot be flexed under much weight bearing load. The braking feature that is so useful in early stance phase becomes a liability later in the gait cycle, forcing the amputee to walk slowly and to take small steps. If the person's medical condition prevents them from ever walking faster or more naturally, this is not a problem. But, like the manual lock type, stance control knees should be used selectively and should be replaced with more functional alternatives as soon as the amputee's walking ability increases sufficiently.

The final basic prosthetic knee configuration is the most mechanically complex. A polycentric knee can be visually identified by its multiple axes of rotation. Sometimes called “four bar” knees, these prosthetic devices can be designed to be very stable in early stance and yet easy to flex during preswing.

The biomechanics of why this is possible are beyond the scope of this article, but the result is a knee that can supplement the amputee's voluntary control without disrupting swing phase movements. For this reason, polycentric knees may be suitable for many amputees with the potential to be independent household or community ambulators. Certain versions may offer sufficient stability for those who cannot walk securely with other knees or for people with bilateral lower limb loss. And, a special group of polycentric knees are available specifically designed to minimize the distance from the top to the front of the prosthetic knee for people with very long residual limbs.

The biomechanical versatility of polycentric knees is probably the major reason for their increasingly popularity worldwide. The primary drawback of the polycentric design is the resulting additional weight and moving parts that may require servicing over time. But, in many cases, the benefits of the enhanced biomechanical function outweigh such concerns. The standard polycentric knee has only mechanical swing phase control and therefore is a single speed design. Although this may be quite sufficient for some people, amputees who want to walk at various speeds can consider one of the many polycentric knees that incorporate fluid swing phase controls. Prosthetic knees incorporating the features of two or more basic designs are sometimes referred to as “hybrid” designs.

All presently available prosthetic knees can be described using one or more of the basic designs discussed. Is the basic structure a single axis or polycentric one? Does it also include friction brake stance control or a manual lock feature? Is the swing phase control the basic single-speed mechanical type, or is there a hydraulic or pneumatic swing control unit included? The diagram at the top of the next column (in the magazine) is an example of a “prescription criteria decision tree,” which may help determine from a biomechanical control standpoint what basic features and designs would be well suited for an individual's needs. Readers are encouraged to discuss these concepts with their prosthetist and other members of the rehabilitation team.

In closing, the recent availability of computerized or “microprocessor-controlled” prosthetic knees must be mentioned. There are several such new knees available, and they all use the power of the computer to enhance the clinical function of basic mechanical knee designs. For example, some use a computer-regulated valve to adjust the swing phase resistance of a pneumatic cylinder. In principle, this is similar to having your prosthetist inside your prosthetic knee, constantly adjusting and readjusting it to offer the smoothest possible swing phase movement. When you slow down, the “computerized prosthetist” opens the valve and makes it easier to swing the leg. As you speed up, this “computer wizard” gradually closes the valve and the knee moves faster and faster.

Another design uses the computer to control hydraulic resistance for increased stance phase stability in addition to enhanced swing phase control. In the more advanced systems, such adjustments are based on readings taken from multiple on-board sensors which measure how the prosthesis is walking, and adjust it as frequently as fifty times per second!

Today, the great majority of amputees will continue to use the proven mechanical prosthetic knees that are much more widely available and significantly less costly or complex than the microprocessor-controlled designs. But, as was the case in the design of gasoline engines for automobiles, if the early results from microprocessor control continue to be positive and researchers continually improve the responsiveness, reliability, and ease of use of these systems, it may well be that computerized devices become the future standard.

In the final analysis, a lower limb prosthesis is a mobility device. Much like when choosing an automobile, the question to be answered is not “What is the most advanced design?” or “What is the most expensive version?” but rather “What is the lowest cost alternative that will fully meet my present and anticipated needs?” The concepts presented in this primer should help the amputee consumer enter into this dialog in an informed and more confident manner, as an effective collaborator on the prosthetic rehabilitation team.

Last updated: 09/18/2008
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