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Increase of mucoid ground substance in connective tissue containing glycoprotein and mucoprotein is the unique characteristic of mucoid degeneration (MD) [12]. There is excessive accumulation of proteoglycans in interstitial tissue [46]. MD of the meniscus can appear in one of two pathological forms: stromal MD and cystic parameniscal degeneration. Stromal degeneration is an intrameniscal degeneration characterized by degeneration of fibrocartilage with an increase in mucoid ground substance that starts around the cells and extends progressively around the interstitial area. However, cystic parameniscal degeneration is located in the parameniscal area and is characterized by integrated clefts and pseudocysts [12, 13]. MD has also been referred to as cystic degeneration [13, 42].

Fig. 1
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Increased signal intensity in whole meniscus (“empty” meniscus)

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Etiology of meniscal MD is not clear. Endogenous and exogenous trauma [39, 42, 46], endothelial inclusions in the cartilage during its development [33], chronic infection with hemorrhage [18, 46], and intraparanchymal hemorrhage [20] have been proposed as causes. In a recent study, authors could not find bacterial infection as a causal factor [5]. Mechanical stresses have been asserted as an etiologic factor; intrameniscal stromal degeneration is a nonspecific reaction to injury and a physiological condition in all knees [12]. It is possible that chondrocytes synthesize proteoglycans in response to altered mechanical stresses [28]. The meniscus can respond aggressively to alternations in its environment and possibly to changes in load by increased synthesis and deposition of proteoglycans in its extracellular matrices [29]. Aging is another proposed etiologic factor in degeneration of the meniscus; it certainly leads to degeneration of the meniscus, as in any other tissues, but MD seems to be a different entity that leads to premature degeneration [5, 23].

Although some studies report lateral predominance [13, 42], one study suggests higher incidence of medial involvement [5]. Medial involvement is usually confined to the body of the medial meniscus due to its firm capsular attachment, palpable cystic swelling is rare. However, lateral cases may also be in the form of stromal degeneration or present as a cystic swelling.

Fig. 2
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The Arthroscopic appearance of degenerated meniscus

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Grade I and II lesions on magnetic resonance imaging (MRI) show focal or linear increased signal intensity not extending to articular surfaces [10]. Stoller et al. [43] examined the correlation between MRI findings and a pathological model. In most cases, on MRI, there is tearing superimposed on extensive grade I and II lesions because the entire meniscus in a single section showed increased signal and only the borders of the meniscal triangle displayed normal signal intensity and such meniscus is called empty meniscus (Fig. 1) [5].

Fig. 3
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Lateral meniscal “cyst” on coronal image

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We believe that meniscal mucoid degeneration with tear is not uncommon. Accumulation of mucopolysaccharides at the meniscal interstitial area disrupts the collagen network and weakens the meniscus. Such meniscal tears are not appropriate for repair most of the time (Fig. 2). When the menisci affected by MD are torn, the diagnosis may be delayed due to the lack of history of trauma and to relatively less severe symptoms [5].

There are three types of meniscal cysts in the knee: intrameniscal cyst, parameniscal cyst (extraarticular), and intraarticular cyst [3, 22, 24]. MD has been proposed as etiologic factor of meniscal cysts [12, 13]. Inevitably, factors which have been proposed for etiology of MD are responsible for formation of meniscal cysts [18, 20, 33, 39, 42, 46]. Despite proposed theories on the etiology of meniscal cysts, recently the most accepted theory is that meniscal cysts are a focal collection of synovial fluid located within or adjacent to meniscus [26]. Intrameniscal cysts are believed to be formed by an accumulation of joint fluid within a torn or degenerated meniscus. Parameniscal cysts are thought to form when there is fluid extravasation through a meniscal tear into the parameniscal soft tissue [1, 26, 27]. McCarthy et al. [27] suggested that horizontal meniscal tear is common with meniscal cysts. Primarily, on the basis of arthroscopic and surgical findings, numerous authors have reported that meniscal cysts occur more commonly in the lateral compartment of the knee [2, 7, 13, 25, 30, 33, 34, 45, 47]. Tasker et al. [44] reported nearly equal involvement of both compartments of the knee by meniscal cysts on the basis of MRI findings. Others suggest that medial meniscal cysts are more common [8, 9]. Clinically, the most frequent localization for a meniscal cyst is the peripheral portion of the mid-third of the lateral meniscus (Fig. 3) [21, 33].

Medial cysts are most commonly located adjacent to the posterior horn of the meniscus, but may also be primarily located adjacent to the anterior horn, next to the meniscal body, or extending superficially to the medial collateral ligament [1, 9, 11, 26]. Most lateral cysts are located adjacent to the anterior horn or body. Anteriorly, meniscal cysts may be found deep in the iliotibial band, and posteriorly, they may be seen deep in the lateral collateral ligament [1, 11, 26]. Meniscal cysts may also extend into the intercondylar notch of the knee, around the cruciate ligaments [3, 22].

Meniscal cysts may be symptomatic, and patients may present with pain, tenderness, swelling, or a palpable mass (especially on the lateral side of the knee). When the cystic swelling is visible or palpable, there is little difficulty in making the diagnosis [36]. However, ganglion cysts, inflamed bursa around the knee, synovial inflammation, arthritic spurs, loose bodies, meniscal tears, and tumors should be considered in the differential diagnosis [4, 21, 35, 38, 41, 42, 47]. In the lateral compartment of the knee, the mass is usually palpable at the level of joint line, most prominent when the knee is in 20–30° of flexion, being firmly fixed to underlying tissues and not mobile in subcutaneous tissue [37]. Pisani [37] reported that the cystic mass disappears on acute flexion of the knee due to rotation of the tibia. According to our observations, the mass is most prominent at 45° of flexion and disappears not only in full flexion, but also in full extension [36]. It is more beneficial to use a modified maneuver for lateral meniscus cysts: with the knee held at 45° of flexion, the prominence is also inspected during external and internal rotation of the leg [36]. With external rotation of the leg, even doubtful lateral meniscus cysts become apparent; disappearance of the cystic mass with internal rotation further confirms the diagnosis (Fig. 4) [36].

Fig. 4
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(a) A prominent cyst in external rotation (left knee). (b) Disap­pearance of the cyst with internal rotation of the leg (L lateral, M medial, P patella, arrow lateral meniscal cyst)

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MRI is important in making the correct diagnosis and providing accurate information regarding the size and location of meniscal cysts, and can also detect associated meniscal disorders [26].

The oldest meniscal cyst treatment strategies are open surgical excision [15] and excision of the cyst combined with open total meniscectomy [4, 6, 15, 45, 47]. Surgical excision of the cyst alone is associated with a high rate of recurrence [4, 6, 16]. Arthroscopic treatment of meniscal lesion with open cyst excision has favorable results [31, 35, 38]. Arthroscopic treatment of both meniscal lesion (partial or subtotal meniscectomy) and cyst (intraarticular decompression) is an effective treatment option [14, 17, 32, 40, 41]. Because of cyst decompression through the substance of meniscus, adequate decompression can be achieved without extensive meniscectomy [17, 40]. Creating a window through the capsule into the cyst away from the meniscal substance is another possible treatment option which saves the meniscal tissue and its capsular attachment and can be described as an “arthroscopic marsupialization” of the meniscal cyst [19].