1.Infection When analyzing the causes of postoperative joint pain, infection is always placed first. the consequences of infection after TKA are very serious, and when the pain cannot be explained by other causes, the biggest possibility is infection. Acute infection is the most common cause of early postoperative pain, with typical inflammatory manifestations such as fever, local redness, swelling and heat pain, which are easily diagnosed when combined with laboratory tests. Chronic infection whose clinical manifestations are difficult to distinguish from aseptic loosening of the prosthesis, making the diagnosis difficult to confirm. Especially in the case of application of antibiotics, the diagnosis is more difficult. Patients with chronic infection may only have resting pain or joint discomfort. Early joint swelling and oozing, localized redness, prolonged wound drainage, and poor incision healing may increase the likelihood of deep infection. Although a nuclear scan can be helpful in diagnosis, the definitive diagnosis of infection relies primarily on laboratory tests. 2. Joint instability Knee instability includes axial instability and flexion instability. Axial instability is most commonly caused by unilateral collateral ligament or popliteal tendon insufficiency, and although it can be caused by trauma, it is primarily the result of intraoperative soft tissue balance failure and is relatively easy to diagnose. Flexion instability is easily missed because the joint is stable when the knee is extended. Flexion instability is a common cause of persistent knee pain and dysfunction, and the typical presentation of a patient with flexion instability after TKA includes joint pain, recurrent swelling, widespread knee tenderness, and knee instability. Stable and good knee mobility. If a patient has these clinical signs of joint instability after TKA, it suggests an imbalance in the knee flexion/extension gap. A variety of surgical errors can result in excessive flexion gap, with excessive osteotomy of the posterior femoral condyle (small femoral prosthesis selection) and excessive posterior tibial plateau tilt being the most common. These problems are more pronounced when the sagittal plane of the tibial plateau prosthesis is too flat. Flexion instability may occur even if the knee functions well for some time after TKA. For example, ligament degeneration, recurrent ligament involvement in rheumatoid disease, and medically induced ligament injury can cause postoperative posterior cruciate ligament weakness or tears in the posterior cruciate ligament preserved prosthesis, resulting in flexion instability; early post-TKA knee flexion tightness can also lead to progressive posterior cruciate ligament tears, which can result in flexion instability despite improved knee flexion function; wear of the polyethylene liner can also lead to progressive Wear of the polyethylene liner can also lead to progressive chronic instability. The diagnosis of knee instability relies heavily on physical examination. If flexion instability is suspected, the examination should focus on anterior-posterior stability of the knee in the flexed position. This is done by sitting with the patient’s foot on the floor, when the knee is very relaxed and a significant posterior tibial subsidence can be observed, especially in patients with failed posterior cruciate ligament preserving prostheses. The anterior and posterior drawer tests are performed when the patient is seated and the foot is fixed, and when there is excessive laxity in the anterior-posterior direction, this suggests the presence of flexion instability. Rupture of the rouge tendon due to surgical injury or long-term wear of the rouge tendon and the posterior lateral aspect of the femoral condyle can cause extreme instability of the knee joint, and a popping sensation can be felt in the posterior lateral corner of the knee joint during the examination. 3. Poor alignment The internal rotation of the femoral and tibial prosthesis is the main cause of poor patellar trajectory and patellofemoral instability, which can lead to postoperative anterior knee pain and flexion instability. Poor alignment of the prosthesis causes increased wear due to edge loading of the polyethylene liner, and excessive wear of the polyethylene causes asymmetry of the medial and lateral gaps, resulting in increased poor alignment of the lower extremity, which can cause joint pain. Therefore, good rotational alignment and axial alignment of the prosthesis in TKA is important to prevent excessive polyethylene wear and patellofemoral complications and prevent failure of prosthetic loosening. Poor alignment of the lower extremity can be detected on weight-bearing radiographs, but rotational alignment of the prosthesis is usually determined by CT measurements. 4, knee joint adhesions and joint stiffness Joint adhesions and joint stiffness can be both the cause and the result of pain. Insufficient functional exercise due to pain early after TKA is an important cause of knee adhesions. Poor joint motion due to surgical technical errors must be ruled out before treatment can be provided by postoperative strengthening of functional exercise, or by manipulation or surgical release. There are a number of technical errors that cause limited joint motion, such as inadequate release of the posterior cruciate ligament resulting in limited flexion and extension; anterior tibial plateau resulting in posterior impingement of the femoral prosthesis and limited joint flexion; posterior tilt of the femoral prosthesis with protrusion of the anterior wing of the prosthesis, resulting in increased femoral trochanter impingement and patellofemoral wear, limiting knee motion; anterior femoral prosthesis or over-selection of the prosthesis type displacing the knee extension device forward, limiting joint The femoral prosthesis is too far back to damage the anterior femoral cortex, which not only causes supracondylar fracture, but also tightens the joint flexion and restricts the movement; the femoral prosthesis is internally rotated to cause lateral patellar dislocation or subluxation, resulting in pain and limited knee extension; the patellar prosthesis is too large when the choice of flexion is limited. Long-term restriction of joint movement will lead to intra-articular fibroplasia and joint stiffness. The above problems can usually be clearly diagnosed through physical examination and imaging analysis. 5, joint line abnormalities Joint line elevation changes the mechanical properties of the patellofemoral joint, causing pain and patellar subluxation. The relative low position of the patella after joint line elevation causes premature contact and impingement of the patella with the intercondylar fossa during knee extension, resulting in tension and limited flexion of the knee extension device, which is a common cause of anterior knee pain early after TKA. Data from prospective studies of the Knee Society Clinical Score show that the risk of these problems is greatly increased with an upward shift of the joint line of more than 8 mm. The addition of a tibial osteotomy to address the preoperative knee flexion contracture is a serious error in TKA and will result in increased flexion clearance and a low joint line, which not only causes knee flexion instability, but also results in a high relative patellar position, which can cause pain and dysfunction with patellar subluxation during the final phase of knee extension. The normal joint line is located approximately 1 cm proximal to the fibular head, 2.5 cm distal to the medial femoral epicondyle or 1 cm distal to the lateral femoral epicondyle, and X-rays of the contralateral limb can be used to determine the joint line position. 6. knee extension device problems Knee extension device problems are the most common cause of anterior knee pain after TKA and have many causes. Internal rotation of the femoral and tibial prostheses mentioned earlier is the main cause of poor patellar trajectory and patellofemoral instability; imbalance of the knee extension device, abnormal joint line, inward placement or poor alignment of the femoral prosthesis, asymmetric patellar osteotomy and malposition of the patellar prosthesis lead to patellofemoral instability, resulting in poor patellar trajectory, patellar subluxation, dislocation, aseptic luxation and wear of the patella. Another potential complication associated with knee extension devices is soft tissue impingement of the patellar prosthesis in conjunction with the knee extension device. A fibrous nodule sometimes forms on the very deep upper surface of the patella and enters the femoral talus during knee flexion, and the fibrous nodule can extrude on the surface of the femoral prosthesis during knee extension causing pain, popping and instability. This phenomenon is called patellar popping syndrome. The most serious complication associated with the knee extension device is complete rupture of the knee extension device, including quadriceps tendon rupture and patellar tendon rupture. The usual causes of tendon rupture are intraoperative medical injury (especially partial tears of the patellar tendon stop), or the presence of tendinitis or tendinopathy. The characteristic signs of knee extension rupture are sudden weakness in knee extension, weakness in knee extension, and difficulty walking. In addition to the above mentioned knee extension device problems, too large or too small patella thickness, failure to perform patellofemoral plication or denervation, and patella fracture are all causes of anterior knee pain. 7. Prosthesis failure Prosthesis failure includes prosthesis loosening, osteolysis, and prosthesis fracture, which can cause sudden pain or chronic delayed pain and increased pain during joint activities. Surgical errors, improper patient selection, defective prosthesis design, or a combination of the above factors, eventually lead to aseptic loosening of the prosthesis and failure. Loosening of the prosthesis is often accompanied by osteolysis. Diagnosis of prosthetic failure is relatively easy and comparative serial radiographs can reveal any change in prosthetic position and progression of radiolucent areas. ct and nuclear scans are helpful in diagnosing and differentiating osteolysis from aseptic loosening. It should be noted, however, that infectious factors must first be excluded in the diagnosis of osteolysis and aseptic loosening. In addition, a small number of patients may present with pain in the tibial epiphysis after TKA, and the mechanism of this pain is similar to that of thigh pain after compression-matched femoral prosthesis implantation. Some studies have shown that the design type of tibial prosthesis shank is closely related to pain, such as long shank, thick shank, pressure-matching tight, non-slotted shank occurs more tibial epiphysis pain, this type of design makes the stress concentrated at the end of the shank, so that the contact stress between the bone and the distal end of the prosthesis shank is higher and causes pain. 8, extra-articular causes Nerve problems, such as spinal stenosis, herniated discs compressing nerve roots, lumbar radiculopathy, etc. are the most common causes of pain caused by extra-articular lesions. These lesions are common complications in patients undergoing TKA and are easily diagnosed by careful history taking and thorough physical examination.