The “upside-down” knee

Heinrich Grimsehl
By Heinrich Grimsehl
4 Min Read


A breakthrough in knee prosthesis technology has proven to be very good news for above-the-knee amputees

Transfemoral amputees with especially short residual limbs are finally offered a prosthetic solution with incredible results. Just as the name suggests, the Very Good Knee, or VGK-S (S for short stump), is an exciting new prosthetic knee that is proving to be a fierce competitor in the world of prosthetic knees.

An age-old known fact in the field of prosthetics is that the longer the residual limb, the larger the lever arm that makes walking with prostheses easier. Having more muscles, and especially joints, intact in the residual limb can significantly enhance the energy output required to ambulate (move around) with the prosthesis.

The VGK-S replaces the function of the quadriceps muscles, which assist with movement and propulsion.

The quadriceps muscles in the thigh consist of four large muscles. These “quads” are powerful and assist in movement and propulsion. One of their most important functions is to stabilise the normal and prosthetic knee joint from flexing, essentially keeping the wearer safe from falling. This is, however, increasingly more difficult with a short stump!

In everyday life, quad muscles help you get up from a chair, walk, climb stairs and squat. You can imagine how difficult it must be to walk when you have lost almost all of your quadriceps muscles and are left with only a short little femur to act as a lever arm.

Thankfully, the VGK-S is specifically designed to replace the function of the quadriceps muscles. Some people even refer to it as the upside-down knee joint. It uses the force patterns flowing from the hip to control the stance and swing modes. In other words, your own weight stabilises the knee joint, even if you stumble.

This minimises the effort involved in pushing the joint into flexion against your body weight, as is necessary with most other knee joints. The fluidic processor evaluates consistently and continuously the progression of the swing phase. This allows for responsive control, even for an odd single-swing phase such as kicking a ball.

Stance phase is also controlled, securing underweight bearing and reacting to activities such as stairs and slope descent. It is easy to see why this knee should not be underestimated! It caters for a group of amputees who otherwise never might have used a prosthesis. It has the added benefit of being surprisingly affordable, compared to other knees.

It has always been my personal feeling that a normal knee is stabilised by the thigh and not by the calf. In most prosthetic knees the stabilisation mechanism is situated in the calf area, so the design of the VGK-S makes a lot of sense to me. Short residual limbs could officially be a problem of the past as the VGK-S poses a very good solution!


Heinrich Grimsehl is a prosthetist in private practice and a member of the South African Orthotic and Prosthetic Association (SAOPA). email:

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Heinrich Grimsehl
By Heinrich Grimsehl Prosthetist
Heinrich Grimsehl is a prosthetist in private practice and a member of the South African Orthotic and Prosthetic Association (SAOPA).
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