Introduction	Choose Top of pageABSTRACTIntroduction <<Materials and MethodsResultsDiscussionReferencesCITING ARTICLES

Numerous reports in the literature state that meniscal cysts occur more commonly (range, 3- to 7-fold) in the lateral compartment than in the medial compartment of the knee [1,2,3,4,5,6,7,8,9,10,11,12,13]. These studies are based primarily on arthroscopic and surgical findings. However, one recent study based on MR imaging findings states that meniscal cysts occur with nearly equal frequency in the medial and lateral compartments [14]. Several theories have been proposed regarding the origin of meniscal cysts, but the most widely accepted theory states that these cysts represent fluid collections in the parameniscal soft tissues and result from the extrusion of joint fluid through a meniscal tear [3, 7, 9, 13,14,15,16,17,18,19,20]. This view is supported by the fact that meniscal cysts are nearly always associated with horizontal cleavage tears of the adjacent meniscus [1, 2, 10,11,12, 14, 17,18,19,20,21,22].

Because medial meniscal tears are more common than lateral meniscal tears [13, 15, 23,24,25,26,27], we hypothesize that meniscal cysts should also occur more commonly in the medial compartment of the knee. To further evaluate this hypothesis, we retrospectively studied a large series of patients who had previously undergone MR examination of the knee. The incidence of medial versus lateral meniscal cysts was determined, and the relative frequency of the location of these cysts was compared with the frequency and location of meniscal tears. We describe our findings and then discuss their clinical significance.

Materials and Methods	Choose Top of pageABSTRACTIntroductionMaterials and Methods <<ResultsDiscussionReferencesCITING ARTICLES

A total of 2572 consecutive knee MR examinations were performed at two institutions (September 1995-July 1998 at institution 1, and January 1997-April 1999 at institution 2). All knee MR examinations at both institutions were interpreted by experienced musculoskeletal radiologists. The reports of the 2572 knee MR examinations were reviewed retrospectively for the presence of meniscal cysts and meniscal tears.

The images from all knee MR examinations with reported cysts about the knee were reviewed retrospectively by two musculoskeletal radiologists, and a consensus interpretation was obtained. In each case, the presence, location, size, and morphology of the meniscal cysts were recorded. “Meniscal cyst” was defined as a focal parameniscal or intrameniscal fluid collection that showed fluid signal intensity on both T1- and T2-weighted images.

The presence, location, and type of each meniscal tear were recorded. Criteria for a meniscal tear included grade 3 signal [28] extending to an articular surface of the meniscus or abnormal morphology of the meniscus, including an absent or displaced meniscal fragment in the absence of a history of prior meniscal surgery. In each case, whether the meniscal tear directly contacted the adjacent meniscal cyst was also noted. Symptoms of each patient were obtained from the radiographic request form and from a questionnaire completed by the patient at the time of the MR examination, and the results were recorded.

The protocol for MR imaging of the knee varied between the two institutions, but all images were obtained on a Signa 1.5-T scanner (General Electric Medical Systems, Milwaukee, WI). The patients were placed in a supine position and scanned using a 6.5-inch (16.5 cm) extremity coil for the knee (Quadrature coil; General Electric Medical Systems). At institution 1, coronal T1-weighted spin-echo images (TR/TE, 400/14) were acquired. The field of view was 16 × 16 cm, and the slice thickness was 4 mm with a 1-mm interslice gap. The matrix was 256 × 192 with 3 excitations. This sequence was followed by sagittal dual-echo T2-weighted images using frequency-selective fat saturation (TR/first-echo TE, second-echo TE, 2200/30, 90). The field of view was 16 × 16 cm and the slice thickness was 4 mm, with a 1 mm interslice gap. The matrix was 256 × 192 with 1 excitation. Finally, coronal and axial fast spin-echo T2-weighted images were obtained with frequency-selective fat saturation (TR range/effective TE range, 3200-4000/60-64; echo train, 12). The field of view was 16 × 16 cm and the slice thickness was 4 mm, with a 1-mm interslice gap. The matrix was 256 × 192 with 4 excitations.

At institution 2, sagittal and coronal T1-weighted images were obtained (TR range/TE range, 400-600/10-16). The field of view was 16 × 16 cm and the slice thickness ranged from 3 to 4 mm, with a 1-mm interslice gap. The matrix was 256 × 192 with 2 excitations. This sequence was followed by axial, sagittal, and coronal fast spin-echo T2-weighted images with frequency-selective fat saturation (TR range/effective TE range, 3200-5200/60-80; echo train, 8). The field of view was 16 × 16 cm and the slice thickness ranged from 3 to 4 mm, with a 1-mm interslice gap. The matrix size was 256 × 192 with 3 excitations. Because of the retrospective nature of this study, the imaging parameters of a few MR examinations varied slightly from the standard imaging protocols.

Data concerning meniscal cyst and meniscal tear location were analyzed using a chi-square statistical analysis.

Discussion	Choose Top of pageABSTRACTIntroductionMaterials and MethodsResultsDiscussion <<ReferencesCITING ARTICLES

A meniscal cyst is considered to be an uncommon lesion of the meniscus, with cited incidences in the range of 1-2% [1, 15] to 7-8% [2, 17] as diagnosed arthroscopically or surgically. Meniscal cysts reportedly occur at the lateral joint margin more commonly than at the medial joint margin [1,2,3,4,5,6,7,8,9,10,11,12,13], with the estimated ratio of lateral to medial ranging from 3:1 [1,2,3,4,5] to as high as 7:1 [11]. Meniscal cysts are nearly always associated with an adjacent meniscal tear, and they most commonly occur in men between 20 and 40 years old [2, 5, 10,11,12, 14, 15, 17,18,19,20,21,22,23]. Medial meniscal cysts are most commonly located adjacent to the posterior horn [1,2,3, 14], and lateral meniscal cysts are most commonly located adjacent to the anterior horn or body [1, 2, 14]. Most patients studied for meniscal cysts in the orthopedic literature have presented with a palpable mass [9, 11, 12, 15, 29], especially on the lateral side of the knee.

Various theories concerning the etiology of meniscal cysts have been proposed. Some have held that synovial cells congenitally or traumatically displace into the meniscus and produce mucin that over time results in the formation of a cyst [13, 15]. Others have suggested that cystic degeneration may take place as a result of chronic infection, contusion, or hemorrhage [2, 7, 9], or that mucopolysaccharides produced by mesenchymal cells may progressively accumulate to form a cyst [16]. The most widely held view is that meniscal cysts simply result from the extrusion of synovial fluid through an adjacent meniscal tear [3, 14, 17,18,19].

In our population, medial meniscal cysts were nearly twice as common as lateral meniscal cysts. Because medial meniscal tears were also approximately twice as common as lateral meniscal tears, the proportion of medial to lateral meniscal cysts was essentially equal to the proportion of medial to lateral meniscal tears (7.8% vs 7.7%). The frequent association in this series of a meniscal tear that directly contacts an adjacent meniscal cyst (98%) supports the theory that these cysts arise in the parameniscal soft tissues as a result of the extrusion of fluid through a meniscal tear. The 109 patients in this study who showed meniscal cysts on MR imaging were demographically similar to patients in previous studies [1,2,3,4,5, 7, 9,10,11,12,13,14,15, 19, 21, 23]. The location of the meniscal cysts in the involved compartment (medial cysts most commonly located adjacent to the posterior horn of the meniscus [Figs. 1A,1B,1C and 3], lateral cysts most commonly located adjacent to the anterior horn or body) was also similar to findings in previous studies [2,3,4, 9, 10, 14, 18]. In our study, only a small percentage of patients presented with a palpable mass, which differs from most prior clinical studies [1,2,3,4, 6,7,8,9,10,11,12,13]. We think this difference is because of the increased sensitivity of MR imaging over physical examination in the early detection of small meniscal cysts.

Our findings contradict those of numerous clinical studies that cite lateral meniscal cysts as occurring more commonly than medial meniscal cysts [1,2,3, 5,6,7,8,9,10,11,12,13, 15,16,17, 22, 23). However, our findings support MR imaging studies that have suggested that meniscal cysts occur with greater frequency in the medial compartment of the knee [14, 18]. Several possible explanations may help account for the discrepancies between MR imaging studies and clinical studies regarding the location of meniscal cysts.

First, MR imaging is thought to be more sensitive than physical examination in the detection of small meniscal cysts, as evidenced by the small percentage of patients in this study (15%) with palpable meniscal cysts. Therefore, the number of meniscal cysts detected at physical examination alone [1,2,3, 6,7,8,9,10,11,12,13] appears to fall far short of the true incidence of meniscal cysts seen on MR imaging. The relatively scant amount of fatty soft tissue on the lateral aspect of the knee, compared with the medial aspect, may explain why lateral meniscal cysts present as a palpable mass more commonly than medial meniscal cysts.

Second, other diagnostic methods, such as conventional knee arthrography, also underestimate the true incidence of meniscal cysts. Diagnosis of a meniscal cyst using conventional arthrography requires extravasation of contrast material through the meniscal tear and into the adjacent cyst [1, 4], but in some cases the cyst may not fill with contrast material, again resulting in an underestimation of the true incidence of meniscal cysts.

Finally, meniscal cysts are occasionally overlooked at the time of arthroscopy [10]. The surgical approach to the posterior horn of the medial meniscus is difficult [2, 30] and, as a result, small meniscal cysts located along the posterior aspect of the medial meniscus may be overlooked at surgery. In our series, 73% of the medial meniscal cysts were located adjacent to the posterior horn of the medial meniscus. The difficulty associated with surgically identifying meniscal cysts in this location may account for the large difference in the incidence of medial versus lateral meniscal cysts when comparing surgical and MR imaging results. The lower sensitivity of other diagnostic techniques (surgery, physical examination, and conventional arthrography) relative to MR imaging in the detection of meniscal cysts may account for the discrepancy in incidence between medial and lateral meniscal cysts.

Historically, the treatment of meniscal cysts has varied to include isolated cyst excision [31], cyst excision combined with total meniscectomy [5, 7, 32, 33], and, more recently, arthroscopic surgery combined with either intraarticular cyst decompression or open cystectomy [10, 11, 20, 34,35,36]. Success rates vary, but overall the best results occur when adequate meniscal repair has been combined with surgical treatment of the cyst. The surgical literature now strongly supports the view that treatment of meniscal cysts should include decompression of the cyst as well as appropriate arthroscopic treatment of associated meniscal abnormalities. Treatment of one abnormality without treatment of the other leads to a lower rate of success. One study in particular showed 80% excellent and good results when arthroscopic partial meniscectomy was combined with open cystectomy versus 50% excellent and good results when arthroscopic partial meniscectomy was performed without cystectomy [22].

The findings in our study support a growing body of evidence that suggests that medial meniscal cysts occur with greater frequency than was earlier reported [10, 14, 18]. Before the widespread use of MR imaging, large numbers of meniscal cysts located in the posteromedial compartment of the knee may have gone undiagnosed, and many may have been inadequately treated. On the basis of these findings and other recent reports [10, 14, 18], MR imaging appears to be the diagnostic method of choice for detecting the greatest number of medial meniscal cysts. MR imaging can provide accurate information regarding the size and location of meniscal cysts and can also accurately detect associated meniscal disorders [37, 38], making MR imaging invaluable as a preoperative planning tool.

This study had several limitations, including the retrospective method of evaluating the MR images for both meniscal tears and meniscal cysts. However, because a high percentage of patients with suspected internal derangement of the knee were sent for MR imaging, and because the meniscal cysts were largely incidental findings, our findings are thought to represent a random sampling. Another limitation is that we reviewed reports to find all cases of meniscal cysts rather than reviewing the actual MR images, although all images of patients with meniscal cysts were reviewed. In addition, because of the retrospective nature of this review, no standards were set for the diagnosis of meniscal cyst in advance, and no control was in place for interobserver variations. No surgical proof exists of the overall incidence of meniscal tears or cysts. However, all initial MR interpretations were performed by experienced musculoskeletal radiologists, which should provide an accuracy rate of approximately 90% in the detection of meniscal disorders [37, 38].

In conclusion, meniscal cysts as seen on MR imaging are more commonly located in the medial than in the lateral compartment, but they occur with nearly equal relative frequency when compared with the incidence of medial versus lateral meniscal tears. These findings, when viewed in the context of the literature on meniscal cysts, appear to indicate that MR imaging can detect greater numbers of medial meniscal cysts than physical examination or arthroscopy alone, and MR imaging can play a valuable role in ensuring proper surgical treatment of patients with these cysts.