I. Definition and etiology
Meniscal cysts (m en iscal cysts) are cysts formed by the accumulation of synovial fluid under the meniscal envelope or within the meniscus through the injured meniscus and are a clinical cause of knee pain. The horizontal fissure of the meniscus provides access to the joint and the cyst. The synovial fluid flows through the torn meniscus parenchyma to form a tortuous channel around the periphery of the meniscus, which acts as a one-way valve to prevent synovial fluid from flowing back into the knee joint, resulting in a meniscal cyst.
However, the etiology of meniscal cysts is not fully understood and there are many different theories, the main two being meniscal injury and mucus-like degeneration within the meniscus. The literature reports that most patients with meniscal cysts (56%-80%) have a clear history of trauma, and the majority of patients (94%-100%) have combined meniscal injury, and the cysts can be found to have old mechanized hemorrhagic fluid (red jelly-like fluid) intraoperatively. However, some studies have reported that only a minority of patients with meniscal cysts (37%-41%) have a history of trauma, and the rate of combined meniscal injury is only 50%-88%. Therefore, these authors suggest that the development of meniscal cysts is mainly related to mucinous degeneration within the meniscus. They suggest that this mucinous degeneration results in the formation of tiny vesicles within the meniscus, and as the lesion progresses, horizontal fissures and lateral cystic protrusions gradually form within the meniscus, eventually resulting in meniscal laminar fracture injury and meniscal cysts. In traditional open knee surgery, it is controversial whether meniscectomy is performed at the same time as lateral meniscus cyst removal. In recent years, arthroscopic knee surgery has become a minimally invasive procedure recognized by orthopaedic surgeons and patients alike. Arthroscopy allows complete resection of the meniscal cyst and preservation or repair of the meniscus at the same time.
II. Clinical presentation and diagnosis
The patient may or may not have a history of trauma; the mass is visible or palpable in the joint space, is tough, and has mild pressure pain; the mass protrudes significantly on knee extension and disappears on knee flexion. MRI of the knee is the main basis for the diagnosis before surgery and shows round or round-like cystic changes between the outer edge of the meniscus and the joint capsule or within the meniscus, with clear borders and uniform low signal on T1-weighted images and uniform high signal on T2-weighted images. The cysts may be compartmentalized or separated, and sometimes a beak-like junction with the meniscus is seen. The diagnosis of knee meniscal cyst is based on: knee pain with localized swelling; examination of the joint space with swelling and pressure pain; MRI is the best tool to diagnose (or even confirm) knee meniscal cyst; arthroscopic exploration is the final diagnosis.
Differential diagnosis
Meniscal cysts should be differentiated from the following other cystic lesions that occur around the knee joint.
Tendon sheath cysts: The walls of both tendon sheath cysts and meniscal cysts are composed of flattened spindle-shaped cells, and most of the cysts are filled with jelly-like and high-protein concentrated material. The essential difference between tendon sheath cysts and meniscal cysts is that meniscal cysts are closely associated with meniscal tears, whereas tendon sheath cysts are not associated with meniscal tears. In addition, it is important to distinguish between cysts that occur around the cruciate ligament and tenosynovial cysts of the cruciate ligament, and Lektraku et al. suggested the following two points to help distinguish them:
( 1) Meniscal cysts are usually centrally located next to the cruciate ligament, whereas tendon sheath cysts are variable in location, often located at the tibial or femoral end of the cruciate ligament;
( 2) Meniscal cysts may encircle the cruciate ligament, whereas tenosynovial cysts rarely do so.
(2) Popliteal cyst: A typical popliteal cyst is located in the posterior medial aspect of the popliteal fossa, adjacent to the medial head of the gastrocnemius muscle, formed by a pouch-like herniation of the synovial membrane of the knee or an abnormal expansion of the synovial bursa of the semimembranous muscle of the gastrocnemius.
3. Synovial cysts: Synovial cysts are mostly caused by inflammation, fluid accumulation in the bursa, and thickening of the synovial membrane. Meniscal cysts are not usually associated with thickening of the synovial membrane.
4. Joint effusion: Knee joint effusion is often diffusely distributed with unclear borders, not related to meniscal tears and degenerative changes.
Lateral parameniscal meniscal cysts
Intra-medial meniscal cysts
Synovial cyst type meniscal cyst
Cruciate ligament tendon sheath cyst
IV. Treatment
Meniscal cysts are classified as tendon sheath cysts and are generally divided into three main types: intra-meniscal cysts, which manifest as fluid collection within the meniscus and are rare, along with meniscal tears; parameniscal cysts, which manifest as peri-meniscal cysts; synovial cysts, which manifest as band-like protrusions of the joint capsule and are rare, and meniscal cysts, which have multi-housing characteristics and are easily missed during surgery, determine the choice of treatment. For the first two types of meniscal cysts, the treatment is relatively complicated because of the associated meniscal injury, and meniscal cysts combined with meniscal injury are mostly horizontal tears or complex tears, and the history of the disease is long, so the indications for suture surgery need to be strictly controlled. The main treatment for meniscal cysts is surgery. The treatment of meniscal injury is often neglected by incisional excision alone, which can easily leave symptoms or recurrence, so arthroscopic treatment is preferred. Arthroscopy can explore the meniscus, and in addition to cleaning the cyst, the meniscus injury can be treated together, and the meniscus can be removed, trimmed or sutured. If the cyst is too large, it can be supplemented with incisional surgery. The meniscus should be carefully investigated during surgery, especially the simple laminar fracture that does not reach the articular surface is most likely to be missed, and if only simple cyst excision is performed, symptoms may remain after surgery and recurrence of the cyst may occur. In recent years, the use of ultrasound-guided percutaneous cyst aspiration and sclerotherapy has emerged, but only in patients who are not candidates for surgical treatment for various reasons.
The cyst should be excised as thoroughly as possible using a planing knife to open the cyst cavity sufficiently to avoid recurrence of the cyst, which may be divided or separated. For injuries beyond the above-mentioned range or complex tears and horizontal tears of the meniscus, suture surgery should be abandoned, and suture surgery can be performed on the basis of partial removal of the meniscus (e.g., removal of the unstable piece of the two pieces of the horizontal tear) if conditions permit. The suture procedure is performed on the basis of a partial meniscus excision (e.g., excision of the unstable piece of the horizontal tear). The damaged meniscus is then carefully trimmed with a planer or grinder to neaten the alignment and reveal fresh tissue; a working cannula is placed in the lateral infrapatellar approach with an additional median approach via the patellar ligament for strict anatomic repositioning. A probe is placed through the median approach to fix the meniscal tear, and a rotator cuff suture is used to insert the working cannula through the lateral infrapatellar approach. The tip of the suture first penetrates the inferior surface of the meniscus from the junction of the anterior meniscus edge and the coronary ligament, then crosses the meniscal fissure and penetrates the superior surface of the meniscus from bottom to top on the opposite side. The sutures were cut by tying two more sleeve knots. The surgical procedure for synovial cysts is relatively simple: the cyst is removed microscopically and the defective area between the meniscus and the coronary ligament is sutured.
The treatment of meniscal cysts is still controversial. In China, it is advocated to remove the meniscus together with the meniscus to avoid the duplication, but with the continuous biomechanical research, it is recognized that the meniscus is essential for the function of the joint, and joint degeneration occurs in up to 21% after total removal of the meniscus and up to 57.5% after 15 years. We recommend that patients should be treated arthroscopically, but the meniscus should be preserved and the injury repaired as much as possible. Arthroscopic decompression of the cyst and excision of the meniscal tear site is a reliable method, but recurrence has also been reported.
V. Summary
There is no obvious association between meniscal injury and cyst in patients, whether it is a different type of lesion or a micro-injured pathway of the meniscus or the regenerative ability of the marginal meniscus is unclear. The application of arthroscopic technique for surgical treatment of knee meniscal cysts resulted in significant reduction of knee symptoms, significant improvement in function, and significant improvement in Ly-sholm score after surgery compared with that before surgery. Compared with previous open surgery, the near- and long-term outcomes of arthroscopic knee cyst removal, partial meniscal tear removal or repair were better, and the incidence of osteoarthritis was lower. Therefore, the knee arthroscopy technique for meniscal cysts has significant advantages in preserving meniscal function and reducing the incidence of osteoarthritis in the affected knee, and open surgery should be abandoned.