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. Synovial folds of the knee are often caused by strenuous exercise, trauma, inflammation and other causes of knee pain and a series of functional disorders, called synovial fold syndrome of the knee.
I. Histological occurrence of synovial folds
There is still academic disagreement about the cause of synovial folds. There are various views such as congenital variation, fibrin adhesions from injurious inflammation, and reactive repair of the 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 sparse 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.
II. Classification of synovial folds
It is customary to classify synovial folds into 4 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.
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, the crease is subjected to strain 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 make the synovial crease inflammatory changes and congestion and edema, and over time it becomes hyperplastic, hypertrophic, fibrotic, and loses its original elasticity. When the knee joint flexes and extends, the fibrotic folds cannot be deformed and elongated, resulting in mechanical irritation of the femoral condyles, which can lead to secondary inflammation of the synovial membrane at the edge of the femoral condyles in mild cases, or 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 crease itself; in the late stage, the symptoms are mostly caused by fibrosis and damage to the articular cartilage caused by the strain on the crease.
(4) Synovial fold variation, thick, wide, fibrotic, hard and inelastic, most people believe that such folds are more prone to pathological changes, resulting in clinical symptoms.
Fourth, the clinical manifestations of knee synovial crease syndrome
(1) Weakness of the affected knee joint and weakness of the leg when moving, especially when going up and down stairs or suddenly standing after a long sitting period;
(2) Pain on the medial side of the knee joint, aggravated by overexertion from running or prolonged extension and flexion of the knee joint;
(3) A joint popping sound when extending and flexing the knee joint more than 45°;
(4) The patient has varying degrees of quadriceps atrophy, knee swelling, effusion, and sliding of the strips over the medial femoral condyle during extension and flexion activities;
(5) Positive hyperextension or hyperflexion test of the knee joint;
(6) Positive McSweeney’s sign;
(7) Positive knee flexion and extension test with compression of the medial femoral condyle.
V. Auxiliary tests
When a clear diagnosis is difficult, imaging and arthroscopic investigation can be used. x-ray films usually have no positive findings, and general CT has no significant effect on the diagnosis of crepitus.
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 band of thin, translucent or hypertrophic fibrotic synovial folds may be observed, either attached to the suprapatellar capsule, parallel to the ACL, or sandwiched between the patellofemoral joint, with wide folds that may wrap around the femoral condyles 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, which gradually excludes 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 comprehensive history and sign analysis.
(1) Differentiation between crepitus syndrome and knee extension device injury: the former has a longer 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 reduced or disappears when the knee extension device is tense, while in the latter, swelling and pressure pain can be palpated on the knee extension device around the patella, and there is no relief when it is tense; in the former, the patellar grinding test can be felt as a flicking of the striae between the patellofemoral joint and pain, while the latter can be negative; in the former, the patellar twitch 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 is mostly symptomatic since 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 popping sounds, while the latter has no popping sounds, 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 reveals that although the results of both patellar compression and patellar grinding tests can be positive, the former can be palpated with patellar rim strips of pressure pain, and the patellofemoral joint can be felt as 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. Clinical symptoms, the former pain is mostly chronic dull pain, light only discomfort location can be in the suprapatellar or above the joint line; the latter in the acute phase of trauma pain is obvious, can be relieved when it turns old, but every time after the strangulation lock aggravated, the location of the medial-lateral joint space. The former has a variety of crisp popping sounds, which can be “babbling”, short “thump” and “crackling”, etc., located in the patellofemoral joint in front of the knee; the latter is a low, muffled sound. The former is frequently “pseudo” strangulation, only the popping sensation, do not need to unlock; the latter occasionally strangulation, not easy to unlock, or even unable to unlock themselves.
(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.
VII. Treatment
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 response, improving the symptoms and slowing down the pathological changes of the synovial membrane, but cannot remove the source of the disease, once the cause 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 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.