MRI is widely used for assessing meniscal tears. Many MR studies have documented the high accuracy of knee MRI [1-6]. A meta-analysis based on 22 studies described an overall sensitivity of 88% and specificity of 94% for detecting meniscal lesions [7]. MRI affects the treatment of patients with knee problems [2, 8]. However, there is a baseline prevalence of meniscal tears in asymptomatic knees [9-11]. Depending on the patient's age, a prevalence of up to 36% has been reported [10]. On the basis of experiences in other musculoskeletal fibrocartilage structures, this figure may be substantially higher when the prevalence of asymptomatic lesions in the contralateral asymptomatic side is determined. A congenital bilateral predisposition of lesions has been shown in several paired musculoskeletal structures [12, 13]. A high prevalence of abnormal findings in asymptomatic volunteers hampers the value of any imaging method in clinical decision making.

The purpose of the study was to evaluate the prevalence of MR abnormalities of the knee on the symptomatic and on the contralateral asymptomatic sides in patients with suspected meniscal tears.

Meniscal tears were found in 57 symptomatic knees and in 36 contralateral asymptomatic knees (p < 0.001, McNemar test). All patients without a meniscal tear on the symptomatic side (n = 43) had normal menisci on the asymptomatic contralateral side. Therefore, the prevalence of a meniscal tear on the asymptomatic contralateral side in patients with a symptomatic meniscal tear was 63% (36/57) (Table 1). Oblique or horizontal meniscal tears were found medially in 32 and laterally in 11 symptomatic knees, and medially in 29 and laterally in eight asymptomatic knees (p = 0.68 for the medial side, p = 0.75 for the lateral side, McNemar test) (Figs. 1A, 1B, 1C, 1D, 1E, and 1F). Twenty (63%) of 32 partial-thickness tears on the medial side had a similar tear on the contralateral asymptomatic side (Table 2). For statistical analysis, the remainder of the tears (radial, vertical [Fig. 2], complex full-thickness [Fig. 3], and displaced meniscal tears [Fig. 4]) were combined into one group of patients because of the low numbers in each subgroup. These tears were found medially in 18 and laterally in five symptomatic knees. They were present medially in five and laterally in none of the asymptomatic knees (p = 0.004 for the medial side, p = 0.06 for the lateral side).

Symptomatic partial-thickness tears were associated with a history of trauma in 16 cases and were not associated with trauma in 27 cases. Symptomatic full-thickness tears were evenly distributed between the patients with and those without trauma (n = 7 each). All symptomatic displaced tears (n = 5) were associated with trauma. (Table 3). The mean age of patients with bilateral meniscal tears was 49 years (± 16 years [SD]), of patients with unilateral meniscal tears was 43 years (± 16 years), and of patients without meniscal tears was 37 years (± 3 years). The mean age of patients with unilateral or bilateral meniscal tears was not significantly different (p = 0.21, unpaired Student's t test).

Collateral ligament abnormalities were found in 53 symptomatic and in six asymptomatic knees (p < 0.001) (Table 4). Ligament tears were never found in asymptomatic knees. Pericapsular edema was found in 64 symptomatic and in 12 asymptomatic knees (p < 0.001). Edema-like bone marrow abnormalities were found in 36 symptomatic and in three asymptomatic knees (p < 0.001) (Table 5). Additional bone marrow abnormalities consistent with necrosis and fracture were found exclusively on the symptomatic side (n = 11). Cartilage lesions were found in 32 symptomatic and in 25 asymptomatic contralateral knees (p = 0.23) (Table 6). The prevalence of the most advanced cartilage lesions (grades 3-5) (Fig. 5) differed substantially on the medial side between the symptomatic (n = 16) and asymptomatic (n = 5) sides but not on the lateral side (symptomatic side, n = 5; asymptomatic side, n = 7).

In our series, clinically suspected meniscal lesions were confirmed by MRI in 57% (57/100) of patients. In those 57 patients with a meniscal tear on the symptomatic side, the prevalence of asymptomatic tears on the contralateral side was 63% (36/57). This prevalence is substantially higher than previously reported in asymptomatic volunteers (6-36%) [9-11] and suggests a predisposition for bilateral meniscal changes. This situation is comparable to the diagnostic problems for triangular fibrocartilage lesions in the wrist, for which a high prevalence (69-88%) of communicating triangular fibrocartilage lesions on the asymptomatic contralateral side was found [12, 22]. In the wrist, however, the closer image analysis of triangular fibrocartilage lesions allows a differentiation to a certain degree between symptomatic and asymptomatic abnormalities. The ulnar-sided and often noncommunicating triangular fibrocartilage defects have a higher association with the symptomatic side than the radial-sided and often-communicating defects have [22]. In the knee, the differentiation of various types of meniscal lesions seems similarly important. Partial-thickness tears with communication to only one side of the articular surface were the most common meniscal abnormality in our study. This type of meniscal lesion had the highest prevalence in the contralateral asymptomatic side, whereas full-thickness and displaced meniscal lesions were rarely seen in the asymptomatic side.

In the orthopedic literature, so-called leave-alone lesions of the menisci have been described [23, 24]. DeHaven [24] considered partial-thickness tears to be stable tears that should be left alone. More debate exists about vertical full-thickness tears, which may also be stable, short (< 5 mm) tears. The conclusion of Weiss et al. [23] about such vertical full-thickness was equivocal: “The tear (stable vertical tears) should be left alone unless it is the only abnormality that is found and it is causing symptoms that warrant treatment.” This statement reveals a common dilemma for orthopedic surgeons: when symptoms are present, abnormalities found during arthroscopy or MRI are often considered to be responsible for the patient's complaints. This approach to treatment decisions may become more problematic with the increasing availability of noninvasive imaging techniques. Potentially self-limited or minor complaints are more commonly assessed with imaging. In such situations, many meniscal tears, especially horizontal or oblique partial-thickness tears, need not be associated with symptoms. Surgery may be inadequate and may even cause complications [25].

Studies have shown that surgery of the knee is less frequently performed after MRI than initially planned before MRI [2, 8, 26]. The change to a nonoperative treatment plan was explained by the high prevalence of medial collateral ligament abnormalities found during MRI that may mimic a meniscal lesion clinically. Medial collateral ligament abnormalities are rarely treated surgically. Our study confirms that clinical signs of meniscal tear may be mimicked by collateral ligament abnormalities even if the physical examination is performed by a qualified referring physician. The 88% (88/100) of referring clinicians in our study were specialists in orthopedic surgery, general surgery (with special interest in musculoskeletal disorders), or rheumatology.

Pericapsular soft-tissue abnormality was the most common finding in our symptomatic knees (62/100). Apparently these pericapsular soft-tissue abnormalities may mimic the clinical symptoms of a meniscal tear. Pericapsular edema in the knee has been suggested as an indirect sign for a meniscal tear [16]. However, such an association between posterolateral pericapsular edema and a lateral meniscal tear could not be confirmed in a subsequent study [27]. De Smet et al. [16] believe that this edema is probably caused by hemorrhage resulting from capsular tearing or stretching. Stretching of the pericapsular structures causing signal abnormalities has also been described in association with osteoarthritis. This relationship is most pronounced on the medial side [17].

In our study, edema-like bone marrow abnormalities were identified in 35 symptomatic and in only three asymptomatic knees. These findings are also called “bone bruise” when they are associated with a distinct trauma. Histologic examinations of so-called bone marrow edema zones have rarely been reported in the literature. However, the few reports revealed unspecified histologic findings, including bone marrow necrosis, bone marrow fibrosis, trabecular bone abnormalities, and fragments of cartilage, but only a small portion of the reports revealed bone marrow edema [28-30]. Therefore, the term “edema-like” has been used in this investigation for such abnormalities. Histologically, bone marrow necrosis and trabecular necrosis are consistently found in the presence of such signal abnormalities [30]. Such necrotic areas are not only found in avascular necrosis of bone but may also accompany an insufficiency fracture [31]. On the basis of our results, these edema-like bone marrow abnormalities are associated with symptoms, which is not unanticipated because of experiences in osteoarthritis of the knee in which a strong association between pain and edema-like bone marrow lesions has been shown [32]. A strong association between edema-like changes and pain has been also seen in early-stage osteonecrosis of the femoral head [33].

In conclusion, horizontal or oblique meniscal tears are frequently encountered in both asymptomatic and symptomatic knees and may not always be related to symptoms. On the contrary, radial, vertical, complex, or displaced meniscal tears, as well as abnormalities of the collateral ligaments, pericapsular soft tissues, and bone marrow, are found almost exclusively on the symptomatic side and appear to be clinically more meaningful. These findings should be specifically emphasized in radiology reports.

Address correspondence to M. Zanetti ([email protected]).