(1) Static force transfer

When standing on both legs, weight is transferred to the bones of both lower limbs through the hip joints on both sides. After amputation, the statics of the border species are broken. To re-balance the body, a prosthesis must be used to support the amputation side. The ischium bearing AK prosthesis uses the ischium to support the force of gravity. This is different from how normal people's weight is transmitted through the hip joint. The ischium tuberosity is the main area of force transmission and lies medial and posterior to the hip joint. The resultant force vector acting on the socket is therefore biased inward and backward. This is of great significance to the static alignment of the socket.

                                   Principle of force transfer in equilibrium   Balance was broken after unilateral AK amputation The force on the prosthesis

(2) Force analysis of prosthesis during walking

(1)Heel strike

If the inertia caused by the movement of the center of gravity during walking is not taken into account, when the prosthesis is bearing, the gravity and the ground reaction force will produce the moment that causes the hip joint to bend. In addition, as the weight line passes behind the knee joint of the prosthesis, the torque that causes the knee to bend is generated, resulting in obvious softening and backsliding. The joint effect of these two torques greatly enhances the possibility of hip joint and knee joint flexion. In order to combat the flexion moment acting on the two joints, the amputee uses the hip extension moment generated by the contraction of the hip extensor muscle to prevent joint flexion and ensure the stability of the knee joint. In addition, clinical experience has shown that the inertia of the center of gravity movement is also helpful in preventing knee flexion. Under the action of the coxa extensor muscle, there will be displacement between the socket and the stump, which will compress the proximal front and distal rear of the stump. In order to prevent pain from the upper edge of the socket against the stump, the upper edge of the socket should be made into a gentle flared shape. Furthermore, in order to make the movement of the stump accurately transmitted to the prosthesis, appropriate compression should be applied to the quadriceps femoris and the medial femoral triangle in front of the socket.

The external force line is located behind the knee axis, resulting in a flexion moment diagram  The effect of the stump on the socket during contraction of the extensor hip muscle

(2) Early stance phrase to middle stance stage

During this period, the weight of the body shifts from the tendon side to the prosthesis side. Easy to cause the prosthesis to lateral offset. The pelvis has a tendency to incline to the lateral tendons. The action of the hip abductors prevents this movement. However, the effect of hip abductor muscle will cause displacement between the socket and the stump, resulting in compression of the medial proximal end and lateral distal end of the stump.

(3) Middle stage to late stage of stance

After mid-stance, the center of gravity begins to shift toward the tendon. Reduces lateral instability. At this point, the center of gravity is gradually moved forward, in front of the knee joint. It acts to hyperextend the knee joint and acts to stabilize it.

Force on the socket by the stump during lateral muscle contraction                   Force on the socket by the stump during hip flexion

④ Early walking phrase to early swing phrase

During this period, the knee joint is stabilized by external forces. In order to move the prosthesis forward, a flexion force is applied to the hip joint. At this point, a force is generated between the stump and the socket. The posterior proximal and anterior distal part of the stump is compressed.

⑤ late swing phrase

The late swing phase is the preparation for the following walking phase. During this period, there is no special muscle activity and gravity shift.

(3) Stability of AK prosthesis

① Lateral stability

During the prosthetic leg standing, gravity exerts a torque that causes the pelvis to dump toward the healthy side. The only thing that works against it is the opposing moment from the contraction of the hip abductor muscle. When the size of the two is equal, the lateral internal stability of the pelvis can be maintained. In addition to its role in stabilizing the pelvis, when the abductor muscles contract, the femur has a tendency to abduct axially at the hip joint, creating pressure on the lateral wall of the socket. In contrast, the lateral wall of the socket produces a reaction force on the stump. This force acts as a support for gravity. However, because the lateral femur is covered by soft tissue, the pressure exerted on the femur by the lateral wall of the receptive cavity is absorbed to some extent. Therefore, the lateral stabilization effect will be reduced. After amputation, the abductor strength is reduced and the lever arm is reduced due to the shorter length of the femur, all of which adversely affect the lateral stability of the prosthesis. Therefore, it is necessary to change the shape of the socket and set the alignment of the socket to obtain the hip abductor strength, that is, to obtain lateral stability. It can be concluded that the size of the reaction force F on the lateral wall of the socket is inversely proportional to the length of the femur. The shorter the femur, the higher the F-score and the more concentrated the pressure.

(2) The stability of the front and back direction of the AK prosthesis mainly manifested as the stability of the knee joint. The most likely time for knee instability of the AK prosthesis is when the prosthesis bearing. At this time, the body weight transfers from the healthy side of the leg to the prosthesis side. The ground reaction force from the prosthesis is positioned backward, which tends to produce a flexion moment that causes the knee to bend. So we use this moment to analyze the stability of the prosthesis. While bearing, the hip extension moment generated by the hip extensor muscle maintains the stability of the prosthesis. According to the balance equation, when the extension of the hip extender muscle takes the prosthesis as the research object, the hip torque MH is: MH=H×L(4-2) and the knee joint as the research object, MK=P× D-H × H (4-3). From equations (A) and (b), the equation is: MH=L/H (P×d-MK) (4-4), which is the Radclitte knee equation. Wherein, MH is the hip extension torque exerted by muscles, MK is the mechanical friction torque of prosthetic knee joint against flexion, L is the height of hip joint, H is the height of knee joint, D is the distance between hip joint and heel posterior end and knee rotation center, and P is the force related to the weight of human body. It can be seen from the knee joint stability equation, the stability of the knee joint can be controlled by the hip extensor muscle. If the strength of the hip extensor is insufficient, other parameters in the equation need to be compensated by changing the alignment of the prosthesis. If the knee is placed back in alignment, that is, the D value is reduced, then the patient can control the stability of the prosthesis with less muscle strength. The patient can also easily control the stability of the prosthesis by rubbing the knee joint, which increases the MK. In other words, the stability of the prosthetic knee is improved.