A prosthesis for amputation from 10cm below the ischium tubercle to 8cm above the knee joint. Due to the loss of the normal knee joint, the function of the AK amputation lost a lot, but after fitting the appropriate prosthesis, and after systematic use of training, which can walk with a good gait. With high-performance prosthetics, people can not only ride a bicycle, but also run and participate in appropriate sports.
AK prosthesis is composed of prosthetic foot, ankle joint, lower leg, knee joint, prosthetic socket and suspension device. Due to the complex structure of AK prosthesis, different prosthetic socket and knee, ankle and other joint parts can be used, so there are many kinds of prosthesis.
1. According to the overall structure type
It can be divided into traditional leg prosthesis and modern component leg prosthesis, shell leg prosthesis and bone leg prosthesis.
① Traditional AK prosthesis: the shell structure is adopted, the prosthetic socket is conical insertion type, and it needs to be suspended by belt. According to the material of the prosthetic socket, the traditional AK prosthesis mainly includes aluminum AK prosthesis and leather AK prosthesis. These prostheses are cheaper to make but largely obsolete because of their antiquated assembly techniques, inadequate fitting of the prosthetic socket and their bulkiness, and are made only for a small number of patients with special needs.
② Shell componentized leg prosthesis: Componentized leg prosthesis was developed after the 1980s. In the early stage, it was mostly shell prosthesis, and the knee joint was made of block structure. The prosthesis adopts resin composite material vacuumizing to form the prosthetic socket, which is fully contact and bearing of key parts. Knee, ankle, foot and their connectors are made of standard parts, which are easy to assemble, adjust and maintain. The mouth shape of the prosthetic socket is made according to physiological and anatomical requirements, and the bearing capacity is reasonable. The lower end of the prosthetic socket is equipped with an exhaust valve, and the negative pressure between the prosthetic socket and the stump is used to suspend the prosthesis (also known as the suction AK prosthesis), which is convenient to wear and take off without belt and other suspension devices. For the condition of the stump is too bad or the patient is not used to wear the suction type of prosthetic socket, it can also be made into incomplete contact (especially the stump end) of the prosthetic socket, and then add a belt for suspension.
③ Skeletal componentized leg prosthesis: in modern times, the component prosthesis generally adopts skeletal structure, that is, the standard componentized joints, connectors and supports are in the shape of endoskeleton, with the addition of decorative foam and knitted leg cover, which makes the appearance more realistic. As components such as knee joints are becoming more versatile, high-strength and lightweight, the performance of prosthetics is also improving greatly.  
2. According to the type of prosthetic socket
With the progress of modern prosthetic assembly technology, the form of AK prosthetic socket is also developing. At present, there are mainly the following types, which can be selected according to the needs of patients.
(1) Quadrilateral prosthetic socket AK prosthesis: This is an earlier and more conventional suction AK prosthesis, which adopts a full contact quadrilateral prosthetic socket. The ischial tubercle is load-bearing, and the ischial bone bearing is mainly at the upper back edge of the prosthetic socket. The prosthetic socket is equipped with an exhaust valve, and prosthesis is suspended by the negative pressure between the prosthetic socket and the stump.
(2) ISNY AK prosthesis: AK prosthesis with ISNY prosthetic socket. ISNY prosthetic socket, also known as rigid frame soft prosthetic socket, is separated into two parts: residual body storage and body weight support. The internal prosthetic socket is made of elastic transparent polyethylene, and then combined with the external load-bearing frame made of carbon fiber composite material. This kind of prosthetic socket is flexible and also meets the requirements of supporting weight transfer force, so patients are comfortable and light to wear. The ISNY socket was initially developed on the basis of the quadrilateral socket. In recent years, with the popularization of CAT/CAM AK socket, the CAT/CAM ISNY socket has also been adopted. The mouth shape of CAT/CAM ISNY frame structure with combination of prosthetic socket, was collectively known as the IRC prosthetic socket (Ischial Ramal Containment Socket, sciatic tolerance to accept cavity) in international.
⑶CAT/CAM AK prosthesis: an AK prosthesis with CAT/CAM prosthetic socket (or IRC prosthetic socket). CAT/CAM prosthetic socket is literally translated as "contour-adduction of the greater trochanter controlled contraline prosthetic socket", which also known as "ischium inclusive prosthetic socket", "m-L narrow prosthetic socket". From the perspective of mouth shape characteristics, this is a longitudinal oval cavity, which is very different from the conventional quadrilateral AK prosthetic socket, that is, the transverse flat square prosthetic socket in shape and shape taking method. It increases the weight of the soft tissue (gluteus muscle) and femur by adduction and proper compression of the soft tissue of the residual limb and its inclusion in the prosthetic socket.
CAT/CAM prosthetic socket is generally made of ISNY frame structure, that is, IRC prosthetic socket, which has high production process requirements. Characteristics of CAT/CAM prosthetic socket:
① There is no obvious ischium supporting plane, and the prosthetic socket contains and supports the ischium from the medial and posterior side;
② The inner and outer diameter of the prosthetic socket is quite narrow, and the anterior and posterior diameter is quite wide, forming a longitudinal ellipse, and the blood vessels and nerves in the femoral triangle avoid compression;
③ The lateral margin of the prosthetic socket was too high to the rotor, which kept the femur in adduction and increased the lateral stability of the prosthetic socket.
(4) In addition to the ischium containment and the lower part of the lateral greater trochanter, the prosthetic socket is mainly supported by soft tissue and femur, so that the force is distributed on the whole stump surface.
(5) The operative point of the prosthetic socket subjected to the resultant force is closer to the center of the hip joint, making it closer to the natural physiological state. Therefore, the prosthetic socket is more comfortable to wear and easier to control the prosthesis, especially for the elderly and amputees with circulation disorders.