The AK prosthesis is composed of prosthetic foot, ankle joint mechanism, below knee leg, knee joint mechanism, above knee leg, prosthetic socket and suspension device. This chapter mainly describes some simple working principles of prosthetic foot and ankle joint components, knee joint mechanism, prosthetic socket and suspension device.

Prosthetic foot and ankle components

Prosthetic foot and ankle joint, also known as ankle-foot mechanism of lower limb prosthesis, are the common basic components of various lower leg prosthesis and thigh prosthesis. There are many kinds with different characteristics and applicability. The most commonly used prostheses are the SACH foot, which fixes the ankle, and the uniaxial multi-ankle foot. Recently, there are two main directions for the development of prosthetic foot structure. One is multi-axis ankle foot, which is represented by Greissinger made in Germany.

The second is the "energy storage" prosthetic foot, which opens up a new field for the application of prosthesis.

A singlefoot is a kind of moving ankle foot. Its main mechanical component is a rotating shaft perpendicular to the sagittal plane. The leg and foot of the prosthesis can rotate relative to each other around this rotation axis to achieve false toe flexion and dorsiflexion. There is an elastic buffer block made of hard rubber on the front and rear of the rotation axis to adapt to the toe flexion and back flexion force suffered by the prosthetic ankle joint. The main part of the foot is made of wood. Its bottom and front toe part is made of rubber or polyurethane material, so called wood glue foot or polyurethane foot. The low stiffness of the toe flexion buffer behind the uniaxial ankle allows most of the impact of heel landing to be absorbed, thus helping to improve knee stability. By adjusting the elasticity of the front and rear buffer blocks, the prosthetic foot can be adapted to the needs of different amputees. The bending deformation of toes under stress makes walking more natural and comfortable. The disadvantage of single-axis ankle foot is that the single-axis foot can only have toe flexion and dorsiflexion movement, which can not realize valgus and horizontal rotation, so it can not compensate for the force of other aspects when walking on uneven road surface. Compared with fixed ankle prosthesis, uniaxial ankle prosthesis is heavier and its appearance is not as good as fixed ankle prosthesis.

2. The multi-axis foot usually uses an elastic block that can move in any direction as the connection between the lower leg of the prosthetic foot, which is the basic structure of the most popular universal foot -- Greissinger foot. This kind of prosthetic foot can reduce the force of other parts of the prosthesis on the lateral and horizontal planes. It is suitable for amputees to walk on the level road. Its disadvantages are complex structure, high maintenance demand, more expensive, weight is also large.

3. Solid ankle cushion heel (SACH foot for short).

Unlike the uniaxial foot, the SACH foot does not have a movable ankle, and the lower leg and foot of the prosthesis are bolted together. The foot is made of rubber or polyurethane and has a wedge shaped cushion at the heel. During walking, the cushioning function is similar to that of the toe flexion cushioning block of the uniaxial ankle foot, which is achieved by the overall deformation of the forefoot of the colloidal foot. The whole of the foot is flexible, and the SACH foot allows for a certain amount of roll-in, roll-out and horizontal rotation. Because of its simple structure, there is little need for maintenance, and its weight is very light, thus reducing the energy consumption during movement; The SACH foot can be made to look like a real foot, with no gap between foot and calf like an ankle foot. The disadvantage of SACH foot is that it can not easily adjust the toe flexion and dorsiflexion angle of the foot like the uniaxial foot. If the heel is hard, the stability of the SACH foot is not as good as that of the universal foot when walking on uneven road. As rubber and other materials age, the SACH foot gradually loses elasticity, and the plantar joint at the front of the foot breaks due to repeated flexion.

4. "Energy storage" foot "energy storage" foot is a variant of SACH foot, which belongs to the fixed ankle prosthetic foot, with many styles.

Its main feature is a foot core made of a special elastic material, called a "keel", with rubber or polyurethane outside cast into the shape of the foot. The "energy storage" foot was developed to accommodate the movement needs of amputees. An elastic keel is used to give the foot good resilience, or "energy storage." The prosthetic foot would then be able to assist in movement, partially compensating for the loss of leg muscle function. The world's famous "energy storage" foot has the United States FLEX foot, made of carbon fiber material, light weight, good resilience, suitable for lower limb athletes running, jumping.