Prosthetic knee joints can be classified by the way that the flexion and extension (bending and straightening) of the knee joint is regulated.
There are two types of fluid control systems: pneumatic (air) and hydraulic (fluid). In each case, pistons move through the control medium as the knee bends and extends. As the pistons move, control valves provide varying degrees of resistance depending on the angle of the knee. These systems allow a different stiffness during different phases of gait. The result allows the user to walk more comfortably at different speeds.
Hydraulic (fluid) controlled knees have fluid filled inner chambers which house a sliding piston. The piston seals against the side of the chamber much like a bicycle pump. As the piston moves, liquid (usually silicone oil) is transferred from one chamber to another. The flow of fluid is regulated by control valves which can be adjusted to moderate the bending and straightening resistance of the knee. The movement of fluid allows the user to walk more comfortably at different speeds.
- May be used over one axis of rotation (monocentric knees) or multiple axes (polycentric knees).
- Accurately mimics anatomical knee function.
- Provides better swing control and stability than constant friction or pneumatic systems.
- Generally lighter and less expensive that microprocessor knees.
- Allows more variation walking speeds than pneumatic units due to the fine control afforded by the valves.
- Piston seals are prone to wear.
- Can produce heat when actively worked for long periods.
- Heavier and more expensive than mechanical friction knees.
- Require more accuracy when being adjusted.
- More expensive than pneumatic or mechanical friction systems.