Synovial folds are the residual normalized membrane structures of the knee joint with incomplete degeneration of the primitive septum during the embryonic period, and are divided into suprapatellar, infrapatellar, and medial patellar folds by location. Synovial folds in the knee often cause knee pain and a series of dysfunction due to strenuous exercise, trauma, and inflammation, which is called Plica Syndrome. In recent years, due to the prevalence of synovial crease syndrome and the in-depth research on the etiology, typology, diagnosis, treatment and prognosis of synovial crease syndrome at home and abroad, new progress has been made, which is reviewed in this paper.
Synovial crepitus syndrome is one of the common causes of knee dysfunction, and its main clinical manifestations are pain and limitation of movement in the medial knee joint, which is similar to meniscal injury without any specificity. Therefore, it is difficult to diagnose it under general physical examination and has become another problem for orthopedic surgeons and pain physicians due to its high incidence. We hope that this review will help physicians in the diagnosis and treatment of synovial crease syndrome.
1. Current status of anatomical studies of the synovial crease of the knee
The knee joint is separated by a non-chondrogenic bud base between the tibia and femur during the 7th week of embryonic life; generally, the solid embryonic synovial interstitium does not yet have a joint cavity until the 9th week of embryonic life, and continues to develop before producing a fibrous joint capsule. The synovial tissue divides the joint capsule into a suprapatellar space and two anterior tibiofemoral spaces. Around the 12th week of embryonic life, these interstitial spaces separating the joint cavities degenerate further, forming a single joint cavity, for example. During this time, the synovial compartments of the intervening chambers are resorbed, and the important synovial folds are the remnants of these incompletely resorbed compartments.
There is also a wide variation in the reported values of the residual rate of synovial folds in the knee. According to Sakakibara, synovial folds are present in approximately 50% to 60% of knees, and in an anatomical study by Jouanin [4], the three most important folds were present in 11% of knees, whereas 10% of knees had no folds at all. Regarding the suprapatellar crease, Joyce et al. found that the residual rate was 89%, Japanese scholars reported 18%, and some domestic scholars reported 20%-70%; while the residual rate of the medial patellar crease was 18.5%-55% in Europe and the United States, and 39%-45% in China.
2.Classification of synovial folds
Synovial folds are now commonly classified into four types: 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.
2.1, Suprapatellar synovial crease
The suprapatellar synovial crease begins at the medial or lateral interval of the suprapatellar fossa, dividing the suprapatellar capsule from its inferior joint cavity into two chambers, but rarely completely occludes the suprapatellar capsule, leaving a small opening in the middle to communicate with the knee joint. The more common remnant is a half-moon synovial crease left medially or laterally, which is more common medially than laterally, and its superior border can be located anywhere in the suprapatellar capsule, but most of them are flat on the superior patellar border.
2.2, medial patellar crease
The medial patellar crease, also known as the synovial scaffold and pterygoid crease, is the crease most likely to cause clinical symptoms. It begins in the medial wall of the knee joint cavity and travels diagonally downward to embed the synovial membrane in the infrapatellar fat pad. The broad medial patellar crease can cover the medial femoral condyle via the medial patellofemoral joint[6 ]. It can be connected to the suprapatellar crease, but the medial patellar crease alone is more common. Currently, the generally accepted classification of the medial patellar crease is the Sakakibara classification: type A, which is located in the synovial wall of the knee cavity in the form of a band;
Types A and B have the potential to cause symptoms, while types C and D are located between the patellofemoral joint and are hypertrophic, fibrotic, and impinging on the patellofemoral cartilage in the flexed position, making them susceptible to symptoms. The patellofemoral cartilage can be impinged upon in flexion, causing symptoms and even damage to other structures in the knee joint.
2.3, Subpatellar crease
A ligament-like remnant structure, one end of which originates in the intertrochanteric fossa, crosses the anterior part of the joint space, attaches to the distal end of the suprapatellar fat pad, and then gradually widens inferiorly, with the entire crease being flattened and band-shaped, or membranous. The fetal one is like a thin silk, so it is called filiform type. Most of the folds are surrounded by fat is called fat type. There are also the upper end of the band, the lower end surrounded by more fat, called the band type.
2.4, lateral patellar crease
The most rare crease. It is longitudinal, very thin, and located 1 to 2 cm lateral to the patella. It starts at the lateral wall above the N tendon fissure of the knee and ends at the lateral patellar fat pad.
3. Pathological mechanism of crepitus syndrome
3.1 Excessive motion of the knee joint
Pecina et al. reported that excessive knee motion is the main cause of synovial crepitus syndrome in the knee. As the sportsman does a particular sport, specific body position and movement will lead to good sports performance, such as running, tennis, golf and other sports. This forces the knee to flex and rotate in the same position repeatedly for long periods of time or at the same amplitude, so that parts of the synovial membrane in the knee are repeatedly squeezed, impinged and folded to form synovial folds, leading to synovial fold syndrome.
Most often seen in the medial synovial crease, it does not necessarily require direct trauma or trapping between the patella and condyle causing pain. This is due to overuse of the knee joint in specific knee flexion and rotation of the external stalk, resulting in a double limitation of the range of motion of the crease and the range of motion of the knee joint. This, coupled with sports activities, causes an effect of excessive rotation and wear of the tissue in the restricted area. This effect causes the medial synovial crease to pull up on both ends of the nerve endings, especially its distal attachment to the patellar fat pad, resulting in pain.
3.2 Injury
This structural change destroys the original coordination between the crease and the joint space, so it is easy to rub against the articular cartilage during joint extension and flexion, or even become embedded in the joint space and cause pain and discomfort.
(1) Direct trauma, blunt impact on the crease.
(2) Indirect injury. The knee joint is forced to flex and extend and twist repeatedly due to excessive exercise, and the crease is stretched and the patellofemoral joint surface is repeatedly squeezed and rubbed.
3.3 Inflammation, adhesions
Repeated synovial inflammation often affects synovial folds, causing congestion and edema, degenerative adhesions, and eventually forming inelastic fibrous bundle-like tissue, which hardens the fibrous bundle-like structure as tight as a bowstring, easily causing symptoms due to crease inlay and cartilage wear.
3.4 Causes and pathogenesis of the four types of synovial crease syndrome due to their anatomical specificity
(1) The suprapatellar synovial crease has a low incidence and is flattened when the knee is flexed, so that it is not pinched between two bones and causes clinical problems.
Although the longitudinal course is not affected by joint motion, the lateral patellar border is flat and the length of the crease is not sufficient to cause clinical symptoms.
(3) The subpatellar synovial crease has the longest protrusion, but it is located between the patellar ligament and the intercondylar fossa of the femur, and any movement of the knee joint does not cause it to extend into the contact surface of the bone.
The medial patellar synovial crease runs longitudinally between the patella and the femur, and because the medial edge of the patella is mostly beveled, this crease can develop longer. However, under normal circumstances, this fold does not extend to the contact surface of the patellofemoral joint and does not affect the function of the joint when the knee is flexed and extended. If this fold is abnormally enlarged or thickened due to trauma, chronic irritation, inflammation and scarring, it often crosses the medial ridge of the patellofemoral joint surface and is squeezed between the patellofemoral joint, resulting in synovial fold syndrome of the knee.
4, the main clinical manifestations of knee synovial crease syndrome
(1) Weakness of the knee joint on the affected side, leg weakness when moving, especially when going up and down stairs or suddenly standing after sitting for a long time;
(2) Pain in the medial knee joint, aggravated by excessive running or prolonged extension and flexion of the knee joint;
(3) A joint popping sound when the knee is extended and flexed more than 45°;
(4) Patients have varying degrees of quadriceps atrophy, swelling and fluid accumulation in the knee joint, and sliding of the strips on the medial femoral condyle during extension and flexion activities;
(5) Positive hyperflexion or hyperextension test of the knee joint;
(6) Positive McDonald’s sign;
(7) Positive knee flexion and extension test with compression of the medial femoral condyle. The synovial crease syndrome is often palpable on examination, and pressure pain is obvious. They are located at the medial part of the suprapatellar capsule, near the medial edge of the patella, and above the medial and lateral patellar fat pads, respectively. If the pressure pain is not obvious, the pain can be induced by passive flexion of the knee joint after tight pressure on the strips, which is due to passive pulling of the end of the crease. The patellar grinding test can also trigger painful symptoms by squeezing and rubbing the crease between the patellofemoral joints, and the flicking of the crease between the patellofemoral joints can be clearly felt.
4.2, Differential diagnosis.
4. 2. 1 chondromalacia patellae: there is no history of trauma, the pain sensation is below the patella, while the pain site after tearing of the medial patellar synovial crease is on the medial patella and has obvious pressure points, patellar grinding can cause pain without the friction sensation of chondromalacia patellae, but if both exist at the same time, it is difficult to differentiate.
4. 2. 2 Proliferative osteoarthritis of the knee: this disease is mostly painful on the medial side of the knee, positive patellar grinding test, frictional sensation with joint movement, but no history of trauma, gradual onset, more medial and widespread pressure points, negative MC Murray sign, and x-ray examination shows proliferative degeneration of the knee joint. It is difficult to differentiate between a torn medial patellar synovial crease due to trauma on the basis of proliferative osteoarthritis of the knee.
4. 2. 3 Meniscal injury: Most often there is a history of acute trauma. The pain is obvious in the acute phase of the 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. There may be a low, muffled sound, joint locking is not easy to unlock, or even unable to unlock itself, and leg weakness, while the medial synovial crease tear of the patella is a stuck feeling and accompanied by pain, and does not produce a range of motion of the locking, for pseudocollapse. The medial hemimelia pressure point is in the joint space, not the medial aspect of the patella, and the medial MC Murray sign is positive.
4. 2. 4 Infrapatellar fat pad inflammation: This disease is caused by pathological hypertrophy of the infrapatellar fat pad which is compressed by the patellofemoral joint and causes pain due to inflammatory reactions such as congestion and edema, and when the knee is straightened the fat pad is compressed by the patellofemoral joint and causes pain.
4. 2. 5 Patellofemoral arthropathy: The age of onset of this disease is relatively high. In terms of clinical symptoms, the pain symptoms are very similar, but there is no popping sound, and in severe cases, there is a rubbing sound; and there is no joint swelling in the absence of other diseases. Clinical examination reveals that although both patellar compression and patellar grinding tests are positive, synovial crepitus syndrome can be palpated with patellar rim strips, and patellofemoral joint strips can be felt during patellar grinding, while patellofemoral arthropathy is only patellar pressure pain, and patellofemoral joint roughness and unevenness during patellar grinding.
5.Treatment:
5.1 Conservative treatment
After the diagnosis is clear, the treatment of synovial crepitus syndrome should first adopt conservative therapy. This includes rest, AISIDS medications (nonsteroied anti inflammatomy medica tions). After the acute phase, recovery exercises should be performed. The treatment includes ultrasound and acoustic therapy, all of which can reduce the local inflammatory response. Generally speaking, conservative treatment for trauma-induced disease of less than 3 months’ duration is ideal.
5.2 Surgical treatment
Traditional surgery is very traumatic, bleeding and slow to recover, therefore, arthroscopy is now used for surgical treatment. Knee arthroscopy is a minimally invasive and safe means of examination and treatment, and with the continuous updating of arthroscopic instruments and improvement of surgical skills, knee arthroscopy has become a more routine treatment method. It has also been shown that complete arthroscopic resection of the crease is the preferred method of surgical treatment.
The resection can be partial or complete depending on the nature of the fold (shape, elasticity) and the length of the history. If the history is short and the elasticity of the crease is still acceptable, a small part of the crease can be removed transversely with scissors or biopsy forceps in the middle of the crease to eliminate the action of the bowstring tension band and its friction with the femoral condyle, but complete removal is not necessary; if the history is long and the crease is stiff, complete removal is required. The synovial crease is excised during surgery, and the band below the synovial crease is excised, and a good surgical result is received. Some scholars have shown that the medial crease if interferes with the dynamic patellofemoral joint then arthroscopic resection of the crease can effectively relieve the symptoms.
6. Problems and outlook
Synovial folds are normal structures that are produced by the degeneration of normal tissues within the knee joint and do not produce any symptoms in themselves. Only when it encounters trauma, long-term wear and tear, chronic inflammation and other triggering factors, the crease may become inflamed and become congested and edematous, fibrotic, and lose its original elasticity, resulting in a series of clinical symptoms such as anterior knee pain, popping, limp leg, pseudogang, and even limited flexion and extension, and may damage other intra-knee structures such as articular cartilage.
The clinical symptoms and signs of synovial crepitus syndrome are similar to other knee lesions, and there are few physical examination methods, so it is easy to miss or misdiagnose. If conservative treatment is ineffective, arthroscopic surgery is the most effective treatment to remove the crease in its pathological state. This has become a common understanding among current doctors.
However, in the prevention of synovial crepitus syndrome, since synovial crepitus does not show clinical symptoms in the absence of causative factors, even if the presence of synovial crepitus is detected early, no intervention can be given. Therefore, the early detection of synovial crepitus and the prevention of its development after detection will undoubtedly pose new challenges to researchers and physicians in synovial crepitus syndrome in future studies.