MR Imaging of the Meniscus: Review, Current Trends, and Clinical Implications
Section snippets
Anatomy
The menisci are wedge-shaped, semilunar (C-shaped), fibrocartilage structures composed of thick collagen fibers primarily arranged circumferentially, with radial fibers extending from the capsule, between the circumferential fibers. The superior surface of the meniscus is concave and the inferior surface is flat, allowing for maximal congruency between the femur and tibia. With weight bearing, the curved femoral condylar surfaces radially displace the menisci, creating circumferential hoop
Meniscal variants
Many meniscal variants have been reported. Some of the variants described more commonly include the discoid meniscus, meniscal ossicles, and the meniscal flounce.
The discoid lateral meniscus has a reported incidence of 0.4% to 16.6% and is more common in the Japanese and Korean populations (Fig. 3) [9]. Joint line tenderness is noted in 73%, “snapping” in 49%, and locking of the knee in 21% of patients [10]. The three types of discoid lateral meniscus are complete, incomplete, and the Wrisberg
Meniscal extrusion
Meniscal extrusion is measured from the outer meniscal edge to the proximal tibial margin. Extrusion of the medial meniscus more than 3 mm is considered abnormal (Fig. 5). This degree of extrusion can be seen in patients who have advanced meniscal degeneration, and various types of meniscal tears [22]. Although extrusion of the anterior horn or body of the lateral meniscus sometimes is considered a normal variant [23], others consider extrusion of the lateral meniscus more than 1 mm to be
Tears: etiology
The cause of meniscal tears can be divided into two categories: increased force on a normal meniscus, usually resulting in longitudinal or radial tears, and normal forces on a degenerative meniscus, usually producing horizontal tears in the posterior half of the meniscus [26]. Tears are more common in the medial meniscus [1], [6], possibly because the medial meniscus is less mobile, and it bears more force during weight-bearing than the lateral meniscus [22], [27], with 56% of tears involving
Tears: diagnosis
The diagnosis of a meniscal tear requires high spatial resolution and an optimized signal-to-noise ratio [31], achieved with the use of a dedicated extremity coil, a slice thickness of 3 to 4 mm, a field of view of 16 cm or less, and a matrix size of at least 256 × 192 (frequency and phase). Many MR sequences have been used to evaluate for meniscal tears, and although they vary in other parameters, they all share a short echo time (TE) [32]. The advantages of a short TE include a reduction in
Horizontal tears
Horizontal or cleavage tears are parallel to the tibial plateau and divide the meniscus into upper and lower segments (Fig. 7, Fig. 8) [28].
Vertical tears
A vertical longitudinal tear occurs between the circumferential collagen fibers, parallel to the long axis of the meniscus, perpendicular to the tibial plateau, with the tear equidistant from the peripheral edge of the meniscus (Fig. 9, Fig. 10) [28].
A vertical radial tear occurs on a plane perpendicular to the long axis of the meniscus and perpendicular to
Meniscal pitfalls
Seventy percent of false-positive MR imaging findings occur in the posterior horns of the menisci, which are the most difficult areas to evaluate at arthroscopy [29], [52], [71], [72]. The standard arthroscopic technique for evaluating the posterior horn of the medial meniscus is to probe the tibial surface while compressing the femoral surface [52]. Because evaluation of the meniscal gutters is also difficult, the accuracy of arthroscopy for diagnosing meniscal tears is 69% to 98%, depending
Meniscocapsular separation
Meniscocapsular separation occurs when the meniscus detaches from the capsular attachments [82], which is more common medially and usually is associated with other injuries [83]. The medial capsuloligamentous structures can be thought of as three layers, from superficial to deep: layer 1: crural fascia; layer 2: superficial portion of the MCL; and layer 3: capsule and deep portion of the MCL [84]. The medial meniscus is attached to the femur by way of the meniscofemoral ligament, and to the
Findings associated with meniscal tears
The use of indirect signs to increase the accuracy for the detection of lateral meniscal tears has been reported [85]. A torn or absent superior popliteomeniscal fascicle was noted in 31% of patients with, and 4% without, lateral meniscal tears [86]. Presumptive subarticular stress reactions of the knee are characterized by an edema-like pattern in the subarticular marrow, which encompasses a focal, linear, or curvilinear low-signal area. Of these patients, 76% have a meniscal tear in the same
Meniscal cysts
Meniscal cysts are identified on 4% to 6% of knee MR examinations, are located twice as often medially, and may be lobulated or septated in appearance [32], [91], [92]. The cysts can be confined within the meniscus (intermeniscal) or can extend into the adjacent soft tissue (perimeniscal), with the latter more common [93]. The most widely accepted cause of a meniscal cyst is extension of fluid through a meniscal tear [91], with 57% noted in horizontal cleavage and 33% in complex tears with a
MR imaging field strength
Several studies suggest similar accuracy for diagnosing meniscal tears with 0.2 T and 1.5 T MR imaging, although some controversy remains [94], [95]. Undoubtedly, scanning time is longer at lower field strengths, 15 minutes longer at 0.2 T than at 1.5 T in one article [95]. A higher confidence for diagnosing meniscal tears at 1.5 T, compared with 0.2 T, has been reported, with the exception of the posterior horn of the lateral meniscus, likely because of the inherent increased signal-to-noise
Treatment
The four main options for treating meniscal tears are complete meniscectomy, partial meniscectomy, meniscal repair, and conservative treatment without meniscal surgery [28]. The treatment of meniscal lesions depends on many factors, including the type, location, and size of the tear. Initially, meniscal lesions were treated with complete meniscectomy because the importance of the meniscus and its function were not understood well [97], [98]. Unfortunately, complete meniscectomy has been shown
Postoperative imaging
Postoperative imaging of the meniscus is complicated. The standard criteria for a tear has limited diagnostic usefulness when diagnosing a tear at the site of meniscal repair or partial resection, with sensitivity up to 100% but specificity as low as 23% [50], [51], [100], because of either intermeniscal granulation tissue, which can have abnormal T1-weighted or proton density signal extending to the meniscal surface in the repaired or healing meniscus, or the possible “conversion” of grade 1
Future imaging: ultrashort echo time imaging, parallel imaging, and 3 Tesla
Ultrashort TE imaging (TEs of 0.08–0.2 ms) is a technique in which the normal meniscus demonstrates increased signal and tears have decreased signal, and is performed best without fat suppression [118]. In contrast to fat-suppressed T1 and Fast Low Angle Shot (FLASH) sequences with intravenous gadolinium, which cannot differentiate between the vascular and avascular zones of the meniscus [4], contrast administration on ultrashort TE images can make this differentiation [118], [119]. The full
Summary
MR imaging is the preferred imaging modality for evaluating meniscal pathology, with high accuracy reported in most studies. Achieving this high accuracy requires a thorough knowledge of the anatomy of the meniscus and perimeniscal structures, an understanding of normal variants and interpretive pitfalls, an awareness of the common findings associated with meniscal tears, and an understanding of the diagnostic criteria for a meniscal tear. In patients who have partial meniscal resection or
Acknowledgments
I would like to thank D. Laurie Persson and K. Fox for their invaluable assistance in the preparation of the figures and manuscript, respectively.
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2018, Radiologic Clinics of North AmericaCitation Excerpt :However, a large percentage of meniscal injuries remain unamendable to repair, as reflected in the persistent greater than 10:1 ratio between meniscectomy and meniscal repair. As well as location in the periphery of the meniscus, factors associated with improved meniscal healing include being in the lateral meniscus, tear length of less than 25 mm, acute tears, patient age less than 30 years, and accompanying anterior cruciate ligament (ACL) reconstruction.7,8 The favorable effect of ACL reconstruction on meniscal healing is attributed to biologic stimulation.
This article was originally published in Magnetic Resonance Imaging Clinics of North America 15:1, February 2007.