1.Epidemiology Distributional characteristics Discoidal cartilage refers to abnormalities in the morphology of the meniscus, and the incidence of discoidal cartilage varies greatly between different regions or races, with a very low incidence of less than 1% reported abroad. However, in China, Korea and Japan, the incidence is very high, accounting for about 26% to 50% of the number of meniscus surgeries. The incidence of men is more than women, about 2~7:1. The incidence is mostly in young adults, the incidence of left and right knees is similar, many bilateral simultaneous incidence, mostly seen in the lateral, medial rare. The etiology and pathogenesis of discoid cartilage is not clear. Some scholars believe that discoid cartilage is a remnant of the dysplasia of the fetal cartilage disk of the knee joint, and the meniscus is differentiated from the femur and tibia mesodermal cells. During the fetal period, the inner and outer cartilage plates of the knee joint are connected to form a disk. In the process of fetal development, the central part of the cartilage plate is gradually absorbed, forming a typical meniscus, if for some reason, this physiological absorption process is interrupted, resulting in discoid cartilage. However, in recent years, some scholars at home and abroad on the observation of the fetal meniscus reached the opposite conclusion to the above, that is, in the early stage of embryonic development, the medial and lateral meniscus that is, the shape of the typical meniscus, and did not see the discoid cartilage, but in the cadaveric autopsy and clinical case data, but the discoid cartilage exists, so that the discoid cartilage is proposed to be born after the early childhood period of gradual development of the formation of the discoid cartilage. The real cause of the disease is still to be further studied. [Diagnostic Points] Diagnostic Points Overview Discoidal cartilage is easy to diagnose if the clinical manifestations are typical, and should be distinguished from meniscus injury. Staging Discoidal cartilage can have different shapes, such as round, square, disk, kidney, etc., and is roughly divided into three types (Figure 1): Type I: completely disk-shaped or square, thick and large, with the medial portion present, sometimes as thick as 8mm, and the thickness of the outer edge of the disk and the medial portion is very little difference. The entire femur is separated from the tibial plateau. Type II: Also disc-shaped, with a hypertrophied edge of the meniscus and a thinner medial aspect. The medial free edge has a double concave notch with a projection towards the center of the joint between the two depressions. Type III: The anterior and posterior widths are close to those of the normal meniscus in terms of structure, except that the central portion is significantly wider than that of the normal meniscus. Clinical manifestations Discoid cartilage is wider and thicker than normal meniscus, the surface is not smooth, the edge of the attachment is strong, thus limiting the activities in the joint, under the action of various stresses in the process of activities, it is very easy to be injured, wear and tear, denaturation, or tear, so clinically about 1/3 of the patients do not have a history of trauma. Discoid cartilage is not always symptomatic, and the appearance of symptoms is mostly seen in young adults, but not uncommon in children. The most common knee symptoms and signs are as follows: 1, joint popping: specific signs of discoid cartilage of the knee joint, the rate of occurrence is as high as 95%, which is of decisive significance to the diagnosis, and there can be a clear ringing sound when the knee is flexed and extended in bed, which is more obvious than bending the knee, and can be seen in the joint throbbing and rotation of the calf, such as lateral discoid cartilage, when the knee is extended to the position of about 20 °, it will be abducted and externally rotated, and it will be reversed when the knee is flexed to the position of 120 ° or so. The opposite is true when the knee is flexed at about 120°. Joint popping is not always accompanied by obvious pain, the mechanism of its occurrence may be due to the uneven surface of the discoid cartilage, there is a ridge-shaped bulge, or the discoid cartilage itself tear caused by the fissure or overlap of the knee flexion activities of the femoral condyles on which the femoral condyles slide due to, or due to the loosening of the discoid cartilage, the femoral condyles by the margins of the femoral condyles to slip and jump. When doing joint flicking, due to the wide discoid cartilage being squeezed by the femoral condyles, the discoid cartilage can be palpated or seen to protrude anteriorly when the knee is flexed, and the cartilage is retracted when the knee is extended or protrudes into the N fossa. This sign is unique to the discoid cartilage, which can be used to identify with meniscus injury. Gravity test: the lateral gravity test of the knee joint has significant diagnostic value for discoid cartilage. If the patient lies on the side, with the affected leg under, make the calf hanging outside the bed, do the extension and flexion of the knee activities, there is obvious popping, change the other side of the side, make the inner side of the knee to the bed, and then do the extension and flexion of the knee, there is no popping or the popping becomes small, for the gravity test is positive. 3.Persistent joint interlocking Only 40% of patients have a history of interlocking, interlocking occurs in a constant orientation and can be unlocked on its own. If the disc cartilage is worn out or ruptured longitudinally, the damaged disc cartilage prevents the activity of femoral condyle, resulting in interlocking. Due to the thickness and width of disc cartilage, it is not easy to unlock, which results in the limitation of the knee joint’s extension activity for a long period of time. 4, other clinical manifestations: discoid cartilage patients, the incidence of pain in the knee joint is 100%, the joint space can have pressure pain, especially the edge of the cartilage and the anterior corner of the most obvious, 1/3 of the patients have a feeling of stepping on the ground or the joint instability, there is a history of trauma in the early joint swelling. Patients with a long course of disease often have quadriceps atrophy, about 20% of the patients with limited extension, 20% of the over-extension pain and total flexion pain, 75% of the joint space tenderness, 90% of the Mai’s sign is positive, 60% to 65% of the grinding test and lateral compression test is positive. Imaging examination 1, knee joint X-ray film can be seen on the affected side of the gap widening, tibial plateau and the edge of the femoral condyle osteophytes, fibular tuberosity position is slightly higher than normal. The main manifestation of arthrography is hypertrophic and wide disk-like cartilage shadow, extending to the intercondylar spine. 2, CT examination can show the morphology of discoid cartilage and the damage situation, which can help to confirm the diagnosis of atypical cases. MRI can show the various levels of the joint structure. Other tests Knee arthroscopy can see the discoid cartilage, and sometimes can also find its surface tear. Overview of treatment] After the diagnosis of discoid cartilage is confirmed, the only reliable treatment is early surgery, total or partial resection of the discoid cartilage, in order to relieve the joint movement obstacles, prevent and reduce the occurrence of traumatic arthritis, the operation can be carried out through the incision of the joint or arthroscopy, the surgical procedure and preoperative and postoperative treatment are basically the same as meniscectomy. Since the 1980s, discoidal cartilage reshaping has been popularized, that is, the discoidal cartilage is modified to be similar to the normal meniscus, which not only eliminates the signs and symptoms produced by the discoidal cartilage, but also preserves the function of the meniscus in transmitting loads, so the biomechanical state of the knee joint is close to the normal state, and prevents the late degenerative changes. The long-term efficacy of partial resection of discoid cartilage has been reported differently, which may be related to the characteristics of pathological changes in the discoid cartilage, the choice of surgical indications and the level of technology. For total resection of the discoid cartilage, the quadriceps muscle should be strengthened after surgery to prevent the stability of the knee joint from being affected by the laxity of the lateral collateral ligament.