The evaluation of knee return bone loss is based on the radiographic presentation and what is seen during revision surgery. The principles of treatment vary according to the staging criteria. Gross classifies bone loss into two categories: inclusion and non-inclusion, where bone loss does not involve the cortical rim and can be treated with bone cement alone or with screws for small defects; large inclusion defects require repair with homogeneous granulated bone graft; and structural bone graft or various custom reinforcements depending on the site and size of the bone loss (shown).Rorabeck considers that for bone loss less than 15 mm in depth For limited bone loss, such as a unilateral tibial plateau or a single femoral condyle, bone grafting or adjustable wedges can be used. For large tibial bone loss, traditional revision systems such as thick polyethylene pads and compression plates can be used for reconstruction. For defects greater than 40 mm, structural bone grafting or custom prosthetic reconstruction is required. For femoral defects larger than 10 mm, bone grafting is required. The Anderson Orthopaedic Research Institute (AORl) has performed typing based on frontal and lateral x-rays of the knee, and its typing criteria and principles of treatment for each type: Type 1 (F1 and T1, not involving the bone of the stem condyle end), tiny bone defects that do not endanger the stability of the revision prosthesis. x-ray shows no subsidence of the prosthesis, no osteolysis under the prosthesis, normal femoral contour and joint line, tibial prosthesis at the head of the fibula and the intact stem condyle end above. Treatment does not require bone cement, reinforcement or structural implant intervention, but if intraoperative removal of the prosthesis causes a small cancellous bone defect, bone cement or bone graft treatment is required. Type 2 (F2 and T2, bone loss involving the dry condylar end), bone loss at the dry condylar end. It often occurs at the single femoral condyle or (and) the tibial plateau and shows radiographic signs of elevation on the joint line, blurred femoral condyle contours, and reduced tibial bone density with the tibial prosthesis at or below the level of the fibular head. Bone cement, reinforcement and bone grafting are required to restore the normal joint line level. A revision prosthesis with a stem and reinforcement is preferred to relieve the stress at the bone-implant interface, which can be supplemented with bone cement, autogenous or homograft bone. In the case of bone loss with granuloma, lax bone or marginal osteosclerotic lesions, revision requires debridement to viable cancellous bone, excision of necrotic or sclerotic bone, preservation of cancellous bone as much as possible for the use of bone cement or bone graft, and reinforcement to ensure 50% contact with the host bone to avoid sagging of the prosthesis. In type 3 (F3 and T3, segmental loss of the stem condyle), the bone loss involves the femoral condyle or (and) part of the tibial plateau and may be accompanied by bilateral avulsion of the collateral or patellar ligaments. x-ray shows the femoral implant moving to the level of the condyle, the tibial prosthesis moving or subsiding, and the condylar line disappearing. It is often due to severe osteolysis, medical loss due to multiple revisions, and hinged prosthetic supracondylar fractures. Requires greater than 1 cm thick reinforcement or co-integrated bone filling, along with the use of a suitable prosthetic fixation implant to replace bone loss or (and) unstable ligaments. The New York Hospital for Special Surgery (HSS) classifies bone loss into the following 5 types: 1. Capsule bone loss, which often occurs when the bone cement is used to bond the prosthesis, the bone cement penetrates into the surrounding bone and is distributed in a capsule, and when the prosthesis is removed the bone cement is removed along with the surrounding bone, forming a capsule bone loss; focal osteolysis or granuloma growing into the bone can cause focal bone loss, which can fuse into a cavity. The bone loss can be filled with bone cement, intraoperative resected bone or autologous bone, and the large area needs to be filled with a mixture of autologous bone and homogeneous bone. 2, platform or condylar bone loss, commonly caused by asymmetric overloading on the basis of poor joint alignment tibial wedge loss or distal femoral collapse. For inclusion loss, autologous or homogeneous granular bone graft can be given, and a stemmed prosthesis can be selected to transfer the pressure load to the medullary cavity across the lost area; for non-inclusion bone loss, a wedge-shaped reinforcement of appropriate size can be selected, and if necessary, homogeneous bone implants can be used. 3, central cavity-shaped bone loss, commonly formed after the removal of the stemmed prosthesis tunnel-like bone loss. The cortical margin is intact. Granulated bone and/or coimplant bone may be used to strengthen the contact between the revision prosthesis and the host bone, with optional reinforcement for combined wedge-shaped loss. 4, perforation and fracture, common in multiple revisions, removal of the prosthesis on the basis of bone weakness causes perforation or fracture of the distal femur or upper tibia. For perforation, a long-stemmed prosthesis should be selected, with the tip of the stem exceeding the perforation by more than 3 cm; for severe fractures, internal fixation is required to maintain the normal shape and position of the bone, and a stemmed prosthesis is selected to transmit the pressure load to the fracture, combined with autogenous or homograft bone to strengthen the perforation and fracture healing. 5. Segmental loss of bone is common due to multiple revisions that cause vascular damage, resulting in ischemic loss of large bone; further bone ischemia caused by TKA followed by supracondylar fracture of femur, resulting in collapse of large bone and poor alignment. The same bone implant, custom-made or adjustable implant intervention is required; if combined with a support ligament defect, a restrictive implant or hinge device is required to restore joint stability. Commonly used treatments and their efficacy Bone defects are extremely common during knee revision, and even unicondylar replacements can sometimes be secondary to asymmetric bone defects. Therefore, in order to avoid unexpected intraoperative difficulties, it is important to prepare adequately before surgery. The attending surgeon must ensure that a variety of materials are readily available, including various shaped spacers, allograft bone, thickened modules, special prostheses, etc. (i) Bone cement: It is often applied in conjunction with bone graft, reinforcement, and prosthesis. The use of bone cement alone is limited to contained bone loss less than 5 mm in height and can be supplemented with screens or screws; for bone loss greater than 5 mm in height or involving more than half of either side of the tibial plateau, the non-contained nature of the bone cement prevents it from being effectively pressurized, making it difficult to fuse the bone cement with the host bone interface, with osteophyte lines at the interface appearing on X-rays early postoperatively. Ritter compared the application of bone cement alone and supplemented with screens or screws and found no significant difference, with a 23% rate of osteoporotic lines. (ii) Reinforcement: There are two types of reinforcement: combination type and custom type integrated with the prosthesis. Customized reinforcement has the advantages of good strength, not easy to loosen, and produces less wear particles, but the accuracy of preoperative evaluation requires strict. Intraoperatively, the bone needs to be removed to the level of the base of bone loss so that the implant is in close contact with both condyles to restore the normal joint line level. The revision prosthesis is limited by the number of options available for the revision. With the development of adjustable combination stiffeners, the use of custom stiffeners has decreased. Adjustable stiffeners are available in a variety of thicknesses, and the right size and shape can be effective in restoring joint line and joint stability up to 16 mm thick, and can be increased with cement bonding to fill severe bone loss. For asymmetric bone loss in the distal femur or proximal tibia, adjustable stiffeners can be used to eliminate excessive bone resection; Denham was the first to use polyethylene wedge stiffeners to successfully fill peri-trochanteric bone loss, avoiding resection of healthy bone near the joint; Brand found that metal stiffeners had better mechanical properties than cemented or combined screw stiffeners. Small bone defects anterior to the distal femur can be filled with bone cement; distal and posterior bone defects, can be reinforced. Select a prosthesis of appropriate size and thickness. Generally, 5 mm posteriorly may be sufficient, but sometimes 10 mm distally may be needed. the thickness required should be judged according to the flexion and extension of the knee joint, and the joint line must be kept constant. (C) Bone graft: The two main clinical applications are autologous and homogeneous bone graft. Autologous bone has osteogenic, osteoblastic, osteoinductive and osteo-mediated properties, so the fusion after bone grafting is fast, not easily absorbed, does not spread disease, and has no immunogenic rejection. The bone source can be intraoperatively resected bone or autologous iliac volume. However, its bone volume is small and taking the iliac bone has certain complications. With the establishment of bone bank, autologous bone graft has gradually become an adjunct. Its application alone is limited to small contained bone loss, and for large bone loss it needs to be used in conjunction with homograft bone. Homograft bone has no bone volume limitation and can be cut into various shapes to achieve fusion with graft bone through crawling replacement of host bone. However, homograft bone is only osteo-mediated and can be resorbed during reanimation at the implant site, which is influenced by immune rejection, mechanical stress response, contact stability between implant bone and host bone, and choice of internal fixation device. During the fusion process between implant bone and host bone, the graft bone is more fragile and prone to fracture or collapse resulting in the destruction of joint integrity. In addition to fracture, postoperative complications such as infection, interface non-union, osteolysis, and joint dislocation can occur. Among them, infection is of the greatest concern, and its incidence can be as high as 10-20%. Risk factors include poor general condition of the patient, presence of foci of infection in distant compartments, local skin abscess, multiple revisions, infection prior to revision, large segmental bone graft, and long and complex surgical time. Homogeneous bone grafting includes small piecewise bone grafting and large structural bone grafting. The bone source is mostly the femoral head resected during THA in patients with degenerative arthritis, which can retain 75% of its strength despite often being from the elderly; for those who need complete segmental bone grafting due to multiple revisions with large bone loss, unconnected supracondylar femoral fractures, and progressive osteolytic excess, bone can be taken from the cadaver, frozen and sterilized for use. Granular bone grafting alone is limited to contained bone loss and when the revision prosthesis has adequate bone support. It has the advantage of faster and more complete fusion and fewer complications than large structural bone grafts. Large structural homograft bone can fill a larger range of bone loss, mainly in cases where the femoral condyle cannot be filled with reinforcement (distal or posterior about 1 cm), where the tibial plateau has a unilateral bone loss greater than 2 cm or where bilateral bone loss cannot be filled with reinforcement and thick polyethylene implants). During the bone grafting process, if the structural bone graft does not fit the host bone completely despite shaping, it can be supplemented with small pieces of homogeneous or autologous bone graft to facilitate close contact and fusion of the structural bone with the host bone. However, this fusion is incomplete and needs to be supplemented with bone cement, reinforcement, lamellar implants, screws, bone plates, wires, and stemmed prostheses to fix the structural bone graft, of which stemmed prostheses are particularly important. Successful bone grafting depends on timely fusion of the host bone and graft bone interface and the ability to withstand body loading without fracture or pain after grafting. A series of reports have shown that complete fusion of the granulated bone can be achieved. Bone density increases, and joint alignment, stability, and pain are significantly improved; large structural bone graft knee stability, function, and pain are significantly relieved. Fourth, the choice of revision prosthesis in the treatment of bone loss In principle, we choose a less restrictive prosthesis with high joint stability in the process of bone loss treatment. For AORI type l bone loss available original or prototype prosthesis; AORI type 2 and 3 with stem prosthesis: for bone loss is serious, with post-stable prosthesis can not be effective, can choose inward and outward flip restrictive prosthesis; for those with ligamentous defects, need to choose custom or hinge type prosthesis. Internal and external rotation-restricted prostheses and rotating hinged prostheses can increase the stress at the interface between the host bone and the graft and promote fusion at the interface. The diameter and length of the stem are closely related to the stability of the fixed graft, and short or medium length stems are available when the bone quality of the stump is good. Internal and external rotation restriction type and hinge type prosthesis need more than medium length shank to maintain good stability. Custom shanks are generally only 1 or 2 diameters and 50-120 mm in length. When large bone grafting or segmental bone grafting is required, custom shanks cannot meet the requirements, and poor anastomosis between custom shanks and the medullary cavity often requires bone cement filling to reduce the axial load that promotes the connection between the grafted bone and the host bone, preventing the connection between the prosthesis and the host bone. Adjustable shanks with stems are available in a variety of lengths and diameters. The appropriate length and diameter reduces the pressure load on the graft-prosthesis interface, fits the medullary cavity without cementing, and pressure fixation appropriately increases the pressure load on the graft-host bone interface to promote interface fusion. Whether the prosthetic stem is fixed with bone cement or pressure fit depends not only on the nature of the stem, but also on the quality of the bone supporting the prosthesis and the degree of prosthetic alignment. When the medullary cavity is branched or the epiphysis is fractured and deformed, fixation with pressure fit can cause poor joint alignment, so bone cement fixation is required, which can form stress protection for the lower condylar end of the prosthesis. In addition, the experience of the surgeon is an important factor in the selection of the revision prosthesis.