1. Stability requirements

That is, the knee joint should be stable under stress and should not bend to cause amputees to fall.

2. Extension requirements

When moving forward, knee joint should be able to compensate for the function of the quadriceps muscle, drive the leg to swing forward, and cannot make the leg lag behind the thigh.

3. Swing control requirements

It should be able to make crus acceleration in swing metaphase and be able to make crus deceleration in swing end. Do not let the leg impact too much when the leg straighten which causing the sound of knee joint collision. In addition, there are small size, light weight, strength, long life and other requirements.

Commonly used AK prosthesis knee joint mechanism

In order to meet the above requirements, various knee joint mechanisms have been designed. The simplest knee joint mechanism is a free swinging uniaxial joint. A more complex knee joint has adjustable friction swing control knee joint; There is a four-bar linkage joint that simulates instantaneous change of rotation axis of real knee joint. A load-bearing self-locking joint; More complex are hydraulic or pneumatic controlled joints; The most advanced is the microcomputer-controlled knee joint mechanism.

1. Stability control mechanism for prosthesis knee joint during support period

(1) Hand-controlled locking knee joint: it is the simplest mechanism to control the standing stability of knee joint. Once locked, the knee remains in a straight position and cannot be flexed, ensuring absolute stability of the knee joint. When sitting down, the knee lock must be opened by hand.

(2) Force line blockade: it is traditionally the most commonly used method to ensure the stability of knee joint support, that is, when the prosthetic is fitted with alignment, the axis of the knee joint is located behind the weight line of the prosthetic, and the knee joint is kept stable in the front and middle sections of the support period by the action of gravity. The requirements for force line deviation adjustment vary according to the degree of movement and control ability of amputees.

(3) Load-bearing self-locking mechanism: there are generally two friction surfaces that can move relative to each other and do not contact in normal state; When the prosthesis supports gravity, which compacts the surface of friction, friction prevents further relative motion. A well-designed load-bearing self-locking mechanism can keep the knee joint stable even at a certain buckling angle. If the weight of the prosthesis is insufficient, stability is not guaranteed.

(4) Hydraulic or pneumatic transmission support period stability mechanism

2. Prosthetic knee swing control mechanism

The traditional control method of knee swing is to use sliding friction damping, that is, to make the surface of knee relative motion have a certain friction. At the beginning of the swing, the thigh swings forward, the calf lags behind, and the knee joint has a trend of flexion movement. Friction counteracts this movement trend and helps drive the calf forward swing. And in swing end, ham stops before swing, crus continues to swing as a result of inertia, knee joint shows extensional motion trend, friction is counter to this trend, make crus swing decelerate. The frictional damping does not change with the velocity, so there is an optimal damping state for a certain walking speed. It is necessary to adjust the damping if you want to move quickly or slowly. At present, the method of air pressure and hydraulic damping is used to control the knee swing. The hydraulic cylinder provides the damping required to control the swing of the knee joint. Different from the mechanical friction damping, the characteristic of hydraulic damping is proportional to the speed of movement, so it can better adapt to the change of walking speed and greatly improve the performance of swing control.

3. Knee extension device

The main function of the extension device is to help the leg swing forward, partially compensate the function of the quadriceps muscle, and reduce the loss of walking energy. Extension device has internal and external forms. External extension devices are usually simple elastic bands that fit in front of the knee joint. The internal extension device is usually a spring. The starting and ending position of the internal extension spring is specially designed, so that the spring will no longer help the extension and flexion when the knee bend exceeds a certain Angle, so as to ensure that the knee joint remains stable when sitting down.