The synovial folds of the knee are synovial compartments that exist during embryonic life but begin to degenerate in late fetal life and are referred to as residual tissue in the developing knee joint. It is divided into suprapatellar, infrapatellar, and medial patellar folds by location. The synovial crease of the knee often causes knee pain and a series of dysfunctions due to strenuous exercise, trauma, and inflammation, which is called Plica Syndrome (synovial crease of the knee). When the medial crease is abnormal, it can cross the medial ridge of the patellofemoral articular surface and be squeezed between the patella and femur, producing the so-called medial crease syndrome. The clinical manifestations are mainly medial knee pain and abnormal shaking of the patella during knee movement. The disease is mostly seen in young adults. The disease can be treated with good results, and the treatment can be divided into non-surgical and surgical therapies. I. Histological occurrence of synovial crease There are still differences in the academic community regarding the cause of synovial crease. There are various views, such as congenital variation, fibrin adhesions produced by injurious inflammation, and reactive repair of synovial membrane. Among the various views, the theory of congenital variation is generally accepted. This theory suggests that in early embryonic life, the knee is divided into 3 chambers: medial, lateral, and suprapatellar, separated by a septum of loose elastic fibrous tissue. At 3 months of embryonic age, the septum degenerates and the 3 chambers begin to fuse. If septum degeneration is incomplete, synovial folds are formed. Second, the classification of synovial folds customarily divided into four types of synovial folds: suprapatellar folds, infrapatellar folds, medial patellar folds and lateral patellar folds. The most common ones are infrapatellar and suprapatellar folds. The medial patellar crease is less common than the first two, but it is more likely to cause clinical symptoms and is therefore the focus of clinical research. The lateral patellar crease is less common. The main causes of synovial crease syndrome (1) direct trauma, blunt impact on the crease. (2) Indirect injury, excessive exercise, forced repeated flexion and extension of the knee joint, twisting, pulling the crease and repeated extrusion and friction of the patellofemoral joint surface. (3) Chronic inflammation caused by other pathologies within the knee that involve the crease. These factors cause the synovial folds to become inflamed and congested and edematous, and over time they become hyperplastic, thickened, fibrotic, and lose their original elasticity. When the knee joint flexes and extends, the fibrotic folds cannot be deformed and elongated, resulting in mechanical stimulation of the femoral condyles, which can lead to secondary inflammation of the synovial membrane at the edge of the femoral condyles in mild cases and to erosion of the patella and femoral condylar cartilage in severe cases. In the early stage of the disease, the symptoms mostly come from the inflamed folds themselves; in the late stage, the symptoms are mostly caused by fibrosis and damage to the articular cartilage caused by the strain on the folds. (4) Synovial folds are thick, wide, fibrotic, hard and inelastic, and most people believe that such folds are more prone to pathological changes, leading to clinical symptoms. Clinical manifestations of synovial crease syndrome (1) weakness of the affected knee, leg weakness when moving, especially when going up and down stairs or standing suddenly after sitting; (2) pain in the medial knee, aggravated by overexertion when running or extending and flexing the knee too long; (3) joint popping sound when extending and flexing the knee more than 45°; (4) patients have varying degrees of quadriceps atrophy, knee swelling, fluid accumulation (5) positive knee hyperflexion or hyperextension test; (6) positive McSweeney’s sign; (7) positive knee flexion and extension test with compression of the medial femoral condyle. When it is difficult to make a clear diagnosis, imaging and arthroscopic investigation can be used. MRI: The sequences that can better show the crepitus are gradient-echo T2-weighted image, pressure-lipid T2-weighted image and proton density-weighted image. In case of inadequate joint fluid, arthrography can be used to dilate the knee joint by injection of contrast agent to make the crease image more clear. In MRI, the crease appears as a low-signal band or line shadow located in high-signal joint fluid. The suprapatellar crease is most easily visualized in the sagittal position, often as a band-like low-signal structure, and is located in the upper posterior aspect of the patella. The infrapatellar crease is a linear, low-signal structure that lies anterior to and parallel to the ACL in the sagittal position. The T2-weighted image of the medial patellar crease is clearest in axial and sagittal views, and is more effective when arthrography is performed. Arthroscopy: The clinical signs and symptoms of synovial crease syndrome are not easily distinguishable from other knee pathologies and are often confused. Therefore, the final diagnosis of synovial crepitus syndrome often requires arthroscopy. Most synovial folds are discovered during other arthroscopic procedures. Arthroscopically, a banded, thin, translucent or hypertrophic fibrotic synovial crease may be observed, either attached to the suprapatellar capsule, parallel to the ACL, or sandwiched between the patellofemoral joint, and a wide crease may wrap around the femoral condyle or even separate the joint cavity into two chambers. VI. Differential diagnosis The diagnosis of synovial crease syndrome should be made by the exclusion method, gradually excluding other knee pathologies, such as knee extension device injury, patellofemoral arthritis, meniscal injury, etc. However, because its clinical manifestations are more similar to other knee lesions, the differential diagnosis is more difficult and requires a comprehensive history and sign analysis. (1) Differentiation between crepitus syndrome and knee extension device injury: the former has a long course, mostly starting in adolescence, with intermittent onset of symptoms and gradual aggravation; the latter has a history of excessive physical activity or exercise, with onset in all age groups. Comparison of clinical findings: in the former, striae can be palpated on the patellar rim with pressure pain, and the pressure pain is relieved or disappears when the knee extension device is tense, while in the latter, swelling and pressure pain can be palpated on the peripatellar knee extension device, which is still not relieved when it is tense; in the former, the patellar grinding test can be felt with flicking of the striae and pain between the patellofemoral joint, while the latter can be negative; in the former, the patellar jerk test is positive, while the latter is mostly negative. Although both squat test results are positive, the former mostly produces pain between 30° and 60°. (2) Differentiation between crepitus syndrome and patellofemoral arthropathy: the former mostly begins to show symptoms in adolescence and can lead to patellofemoral arthropathy in those with a long history; the latter has a high age of onset. In terms of clinical symptoms, the pain symptoms are very similar, but the former has frequent crisp ringing, while the latter mostly has no ringing, and the latter has a rubbing sound; the former may have mild joint swelling intermittently, while the latter has no joint swelling when not accompanied by other diseases. Clinical examination revealed that although the results of both patellar compression and patellar grinding tests can be positive, the former can be palpated with patellar edge strips of pressure pain, and the patellofemoral joint can be felt with interpatellar strips of flicking during patellar grinding, while the latter is only patellar pressure pain, and the patellofemoral joint feels rough and uneven during patellar grinding. (3) Differentiation between crepitus syndrome and meniscal injury: the former has a history of excessive exercise and may not have a clear history of trauma, while the latter has a history of acute trauma. In terms of clinical symptoms, the former pain is mostly chronic dull pain, and in mild cases only discomfort can be located above the suprapatellar or joint line; the latter pain is obvious in the acute phase of trauma, and can be relieved when it turns old, but it is aggravated after every strangulation and is located in the medial-lateral joint space. The former has a variety of crisp popping sounds, which can be “babbling”, short “thump” and “crackling”, etc., and is located between the patellofemoral joint in front of the knee; the latter is a low, muffled sound. The former has frequent “pseudo” locking, which is only a popping sensation and does not need to be unlocked; the latter has occasional locking, which is not easy to unlock, or even cannot be unlocked by itself. (4) Distinction between crepitus syndrome and infrapatellar fat pad inflammation: this disease is due to pathological hypertrophy of the infrapatellar fat pad and extrusion by the patellofemoral joint resulting in pain caused by congestion, edema and other inflammatory reactions, when the knee joint is straightened the fat pad is extruded by the patellofemoral joint and causes pain, the pressure point is at the infrapatellar fat pad. Conservative treatment: mainly rest, local physiotherapy, closure, oral non-steroidal anti-inflammatory and analgesic drugs, strengthening the functional exercise of the quadriceps muscle, etc., can play a role in reducing the inflammatory reaction, improving the symptoms and slowing down the pathological changes of the synovial membrane, but cannot remove the source of the disease, once the trigger reappears, the patient’s symptoms and signs will reappear and worsen. Surgical treatment: Patients who take conservative treatment is ineffective should promptly choose to undergo surgical treatment and arthroscopic removal of the pathological state of the crepitus. Especially those folds with inflammatory changes, hypertrophy, fibrosis and loss of elasticity, which have obvious clinical symptoms, often achieve ideal results after arthroscopic surgical excision. Timely removal of pathologic folds prevents them from causing secondary damage to other structures in the knee joint, especially the articular cartilage. When removing the crease, its full length should be removed from its attachment so that it is completely removed; if it is only cut from the middle, the crease may heal on its own and become symptomatic again.