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 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.
Two, the type of synovial folds
There are 4 types of synovial folds: suprapatellar folds, infrapatellar folds, medial patellar folds and lateral patellar folds.
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 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, hypertrophic, fibrotic, and lose their 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 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 knee synovial crease syndrome
(1) Weakness of the affected knee joint, leg weakness 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 excessive running strain or prolonged extension and flexion of the knee joint.
(3) A joint popping sound when extending and flexing the knee joint greater 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 knee hyperflexion or hyperextension test.
(6) Positive McSweeney’s sign.
(7) Positive knee flexion and extension test with compression of the medial femoral condyle.
V. Auxiliary tests
When the diagnosis is difficult, imaging and arthroscopy 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, banded, 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 over the femoral condyles or even separate the joint cavity into two chambers.
VI. 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, 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 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.