Category Archives: Prosthetic knees

Comparing prosthetic knees for above knee prosthetic legs.

Prosthetic knee for short stump

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Very Good Prosthetic Knee for Short StumpAs an amputee having a short stump, a major requirement for my prosthetic leg is low weight of the prosthetic knee joint.

My new prosthetic knee, the Very Good Knee – Short Transfemoral (VGK-S), is specifically designed for above knee amputees having a short stump (as well as hip disarticulation amputees). Previously, I had to choose between acceptable weight OR advanced functionality. The Very Good Knee – Short Transfemoral gives me both, and that has been another major improvement for my quality of life.

The major innovation of this prosthetic knee is the high centre of gravity, having the hydraulic above the knee axis, unlike all other prosthetic knee joints on the market. Why? Because the manufacturers have a One Size Fits All strategy. For the majority of above knee amputees, there is no room ABOVE the knee axis because of the length of the stump, and for those above knee amputees having a longer stump weight is not so much a problem. But the shorter the stump is, the more the weight of the prosthesis -and in particular of the prosthetic knee- does become a problem.

Image you hold a hammer at the distant end of the handle. Now imagine you hold that same hammer at the hammerhead. It will feel MUCH lighter. This is the concept of the Very Good Knee – Short Transfemoral: By placing the hydraulic unit close to the body, there is a high centre of gravity resulting in a low moment of inertia – that is what an above knee amputee feels when wearing a prosthetic leg.

The Very Good Knee – Short Transfemoral has the same functions as the traditional Very Good Knee – Go!, as described in my comparison of prosthetic knee joints. Very important for me: This prosthetic knee is waterproof. See the video.

Compare Prosthetic Knees

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This article is a comparison of a selection of mechanical and MPK microprocessor prosthetic knees for above knee prosthetic legs. I have been testing prosthetic knee joints such as Orthomobility‘s innovative VGK „Very Good Knee“, which I compare to Otto Bock‘s 3R80 mechanical knee and Ottobock‘s microprocesser C-Leg knee, as well as Otto Bock‘s electronic Genium knee.

All of these are first-class prosthetic knee joints for active above-the-knee amputees, have both swing and stance control. In other words, all of these prosthetic knees enable different walking speeds, going down hills by putting weight on the knee while it is bending, going down stairs step-over-step. Also, each of these knees supports a natural walking style with the knee bending slightly and bouncing back after heel strike.

And yet there are differences, which, according to my opinion, become apparent when it comes to activities beyond walking and taking stairs:

  • Kneeling on the floor (should not make scratches, neither on the floor nor to the prosthesis!)
  • sitting down (knee should ideally swing freely)
  • going down really long hills or very many stairs (knee hydraulics becoming warm!)
  • having contact with water (most electronic prosthetic knees don‘t like that!)
  • bearing additional weight (knee function might be adjusted for a fixed body weight only)
  • wearing different shoes (having different weight, which has an impact on the swing phase!)
  • sitting in a narrow place (the more knee angle, the better!)
  • feeling the weight (a heavy prosthesis can be a real problem with a short stump!)
  • Design. For amputees who do not want a cosmetic covering, design matters!

So this is how I compare Orthomobility‘s „Very Good Knee“ VGK with Otto Bock‘s mechanical 3R80 and electronic C-Leg 3 and Genium resp. Genium X3. I don’t include polycentric constructions such as Össur’s TOTAL KNEE or Endolite KX06 (with a Mauch hydraulic unit) because with my activity level I regard the geometric locking mechanism more a hindrance than a feature.

I use colours to say whether an aspect is

very good
quite good
could be a problem
bad disadvantage
  VGK 3R80 C-Leg Genium
Manufacturer Orthomobility Otto Bock Otto Bock Otto Bock
Type Mechanical, with innovative autoadaptive fluidics control Mechanical Electronic Electronic, with more sensors than C-Leg, being able to adjust to more different situations
Swing phase control
  VGK 3R80 C-Leg Genium
Different walking speeds yes, slow to very fast yes, but not such a wide speed range. I haven’t found a setting supporting both very slow and very fast. yes, slow to fast yes, slow to very fast.
Effect of different shoe weight none. fluidic control adjusts automatically to different shoe weight bad. Heavier shoe means slower swing phase. Would require adjusting a screw setting to compensate bad. Havier shoe means slower swing phase. Would require different programming to adjust. none. Electronics compensates by adjusting the hydraulic unit automatically.
Natural walking style with knee bending and bouncing after heel strike yes, requires a bit of practice and a proper adjustment of the foot yes, requires a bit of practice and a proper adjustment of the foot yes, initial knee bending motion (before bouncing back) is damped smoothly yes, automatically having a few degrees of knee flexion on heel strike (Otto Bock calls it „pre-flex“ function)
Safety, preventing falling during sudden interruption of swing mode (stumbling) yes, stumble recovery function in swing mode no. Requires quick body reaction to prevent falling in stumbling situation. yes yes
Stance phase control
  VGK 3R80 C-Leg Genium
Safety, reliability of switching into stance mode, preventing falling in difficult situations on heel strike (e.g. stone under heel) yes depends. Requires very precise setting of the weight activation switch yes yes
Effect of additional weight, e.g. when bearing something none. Fludic control is able to compensate bad. Stance control is weight activated. Bearing additional weight could lead to stumbling none. Electronics compensates. none. Electronics compensates.
Effect of long descends or or long stairs down none. Fluidic control compensates. bad. The warmer the hydraulic unit gets, the more it looses its stance control function. OK. Electronics compensates by adjusting the hydraulic valves. However, due to the nature of the hydraulics, it might become too warm and then switch into a „safe mode“ with no knee bending function. OK, same as with C-Leg
Swing characteristics when not walking Stance control active. knee is relatively stiff. This is the price for having a “safe” knee, which does not bend unintentionally. OK. Swings almost freely, slightly damped by spring Bad. Stance control active, knee is relatively stiff Best. Knee swings freely.
Cycling Mode yes, to be activated with a small handle. With configurable knee resistance and security mode: In cycling mode, a quick movement of the knee will give sufficient stability to stand on a flexed knee. yes. Normal mode is fine for cycling. yes, to be activated with a special 3-times tipping on forefoot. Second mode characteristics can be configured. yes, to be activated with a remote control, or as with C-Leg. Multiple „second modes“ with configurable characteristics.
Aspects for every day usage
  VGK 3R80 C-Leg Genium
Maximum knee-angle perfect! Only restricted by the socket! With my short stump and socket, there is sufficient space so the knee can be bound to almost 175 degrees, which is the physiological kneeling angle of a sound leg. good! knee can be bound to 160 degrees bad. Only about 120 degrees, prothesis could feel like an obstacle, depending on the situation OK, much better than C-leg.
Kneeling down Well supported by a plastic knee cap that protects the floor and prevents slipping away Moderate. Weight is supported by plastic cover. However, the ottobock 3R80 logo gets scratches. With previous 3R80 model, foot goes up. OK, but only if knee protector is mounted. Very bad. Would slip away on hard floor and/or make scratches in knee frame or floor.
Weight heavy, 1400 g OK, 1200 g heavy, 1400 g Very heavy, 1600 g. Could be too much for a short stump
Water-proof yes! Can be used under shower, and even when doing water-sports. However, should not be used in salt water. Don’t know. Probably a few rain drops don’t matter.Update: The new version (2014) is waterproof! No! Must be kept away from any water! Moreover, Charge socket is located such that even a few rain drops could cause damage. Update 2014: The Genium X3 is waterproof (however: even heavier and more expensive than the standard Genium)
Handling nothing special nothing special Requires almost daily battery recharge, battery connector is a bit difficult to attach. Requires battery recharge every few days, however, battery connector is easy to attach
Restriction on the choice of prosthetic foot No restrictions. Can probably be used with any prosthetic foot of your choice No restrictions. Bad. Only allowed with a few Otto Bock feet, none of which are my personal preference Bad. Same as with C-Leg.
  VGK 3R80 C-Leg Genium

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