Fixation of the prosthesis Bone defects are one of the major problems faced in knee revision. Preoperative radiographs show signs of osteolysis, and the true bone defect below the trochanter may be larger than the surgeon anticipates. Thus, it is necessary to characterize the bone defects of patients before revision surgery.AORI bone defect characterization is a widely used method to characterize bone defects.AORI1 bone defects are mild, and most of the bone defects are inclusive bone defects that do not require special prostheses, while AORI2 bone defects are more extensive, and require special preoperative preparations. Bone defects in knee revision cases have their own characteristics, tibial plateau bone defects mostly occur in medial non-inclusive bone defects, and femoral lateral bone defects may be caused by excessive osteotomies, loosening of prostheses, etc. Bone defects of the distal femoral condyles and posterior condyles not only make it more difficult to anchor the revision prosthesis, but also make the reconstruction of the joint line more difficult. While the joint line of the tibial plateau is relatively easy to find and easy to achieve intraoperatively (25 px above the head of the fibula), the determination of the joint line of the lateral femur is more difficult. Femoral and tibial osteotomies also differ in the degree to which they affect the joint line/joint space. The amount of distal femoral osteotomy and posterior femoral condylar osteotomy affected the joint line to different degrees, and tibial prosthesis thickness affected the flexion and extension gaps to a comparable degree. Different surgical procedures also have different effects on the joint space. If the operator first reconstructs the posterior condylar Offset at 90 degrees of patient flexion before balancing the extension gap, the knee joint line is likely to be elevated. If the surgical sequence is changed, a thick spacer (affecting the joint line) may be used to balance the flexion gap. Regardless of the surgical approach, the operator should try to find the anatomic landmarks of the joint line intraoperatively. Ideal anatomic landmarks include the meniscus stump and the medial epicondyle of the femur. The patella height has a wide range of variability, so the position of the patella is not suitable as an anatomic landmark for finding the joint line. The operator may also refer to the contralateral knee as a reference landmark. Bone defects in zone 1 (tibial side, femoral side) can be filled with metal pads. In younger patients, autogenous bone can be considered, but this approach has its drawbacks: the patient needs to be partially weight-bearing for a long period of time after surgery, and there is a high risk of bone resorption. assisted fixation with a knee prosthesis is considered for zone 2 and 3 defects, and cemented or bioprosthetic fixation can be used for zone 3 defects. Although bioprostheses are easy to fixate and remove, a study by FehringTK and others showed that cement fixation is the preferred method of fixation. Of course, there are some complications in the long term, and metaphyseal resorption occurs in patients, and Beckmann et al. pointed out that how to choose the optimal length, width, and surface finish of the cemented stem is worth exploring further. Joint stability One of the important purposes of knee revision is to help patients achieve the state of “knee extension gap = knee flexion gap”. This is important for the patient to achieve full mobility and a stable knee. If intraoperative knee instability occurs, the surgeon needs to assess the cause. In some patients, correcting the position of the prosthesis may resolve the instability. The surgeon should not rely too heavily on the alignment of the prosthesis stem in the medullary cavity, as reference to this alignment alone can lead to internal and external rotation of the plateau. Often the use of a PS prosthesis will help the patient to achieve a balanced, stable knee, but PS prostheses do not provide support for the medial and lateral aspects of the knee, so it is important to prepare at least a semi-restrictive prosthesis for patients with loss of medial and lateral stabilizing structures. It is necessary to prepare a hinged knee joint in patients with severe bone defects, multiple fractures of the femoral condyles, loss of femoral condylar underlying structures, loss of the knee-extension device, and severe neurologic deficits. Before revision surgery, the patient is thoroughly analyzed and ligaments are examined, but intraoperatively, the degree of joint instability may be found to be much higher than the surgeon imagined, so it is necessary to prepare the RK for knee revision. The principle of prosthesis selection is “low restriction prosthesis – high restriction prosthesis”, because a prosthesis with a high degree of restriction will lead to more wear and tear of the polyethylene, which will lead to loosening of the prosthesis, and the surgeon should try to avoid the use of a high restriction prosthesis. However, in order to achieve adequate knee stability, we have to use a highly restrictive prosthesis. It is therefore important that we find a balance between stability and restriction.