VGK Very Good Knee Short TransferalVery Good Knee – Short Transfemoral  (VGK-S)

VGK-S, with its centre of gravity (998 gr. / 2.2 lb) proximal to the knee centre, reflects a new way of thinking with respect to amputee healthcare, with special reference to amputees with short transfemoral stumps. The development of VGK-S is rooted in years of clinical practice and nurtured by amputee experience. The VGK-S represents a bespoke innovation, for short transfemoral stump that allows, due to shortness, space for a functional prosthetic component. Whereas most transfemoral amputees have sufficient stump length to control a standard prosthesis, it is estimated that 30% have femoral bone length that is simply too short to handle the mass distribution of a standard prosthesis comfortably.

These individuals would be explained that their stump is too short, as if it were their fault.

Instead of focussing on the shortfall, Orthomobility take the opportunity to make the best of the situation, which is the courage to be real, and have conceptualised a knee joint situated between end of stump and the knee centre where the knee bends. Some call it an upside down knee joint, which is surely not true, but it reflects the revolutionary approach and the lack of anything like it. In Fact, the VGK-S replaces the powerful Quadriceps that were lost from amputation.

The VGK-S uses a polykinetic processor  for:

-automatic walking speed recogntion
-self balancing resistance to slope and stair descent
-stumble recovery reflex detection.

This is a further development of the known and established technology in the VGK-Go!  To date, most knee joints (whether electronic or not) use the force patterns entering the foot to determine the stance or swing of the joint. The VGK-S uses the force patterns flowing from the hip to control stance or swing mode. This avoids the effort involved to push the joint into flexion against residual body weight as necessary in the default stance knee joints. The control of this novel, hip activated stance, allows highly versatile user control, even when the user is tired, and reacts fast to spinal reflex hip extension during stumble recovery.

The polykinetic processor  evaluates continuously the progression of the swing phase and responds to changes in cadence with immediacy. This allows for a responsive control, even for one odd single swing phase such as in kicking a ball.

Stance phase is also polykinetically controlled, securing a highly controlled rate of knee flexion under weight bearing (yielding), such as in stairs and slope descent.

Last but not least, and for those who like to know the figures, the mass, that is the weight required to make this function possible, is not only minimal (998 gr, 2.2lb) but the proximal location of the centre of gravity 6 cm proximal to the knee centre provides a very low moment of inertia of 0.11 kgm2 (3.8 lbft2). Just for contrast, a 850 gram polycentric joint, (no yielding, no stumble recovery support), with centre of gravity 6 cm distal to the knee centre would have a moment of inertia of 0.17 kgm2 (6.2 lbft2). This is a 35% reduction in dynamic forces on the stump on account of the mass of the knee joint. The savings relative to a distal joint mass of say 1200 gram would be 58%.

In fact the VGK-S has been designed to be compatible with light weight feet and shoes, the significance of that will be understood by the experienced user!

The benefit gained from the low moment of inertia is reduced pressures in the stump-socket interface with respect to shear strain in the skin (skin comfort), and reduced soft tissue compression and decompression during walking. This reduction in soft tissue deformation supports increased proprioception since the position of the prosthesis is better related to the position of the hip joint. This in turn supports confidence in walking and a wider range of walking speeds.


Would the VGK-S fit the stump in terms of build height? Please  refer to the cartoon that indicates the space requirement.

And the foot? A lightweight foot is very recommendable to take full advantage of the VGK-S, and one foot is reviewed in the facebook blog of Rush Foot

Rush foot