The treatment of meniscal tears has evolved secondary to a better understanding of the essential roles that the menisci play in the normal function of the knee, including load transmission, stress distribution, shock absorption, joint lubrication, resistance to capsular and synovial impingement, and maintenance of joint congruity. Imaging evaluation of the menisci requires an understanding of the normal anatomy, the imaging criteria necessary to accurately diagnose a meniscal tear, meniscal tear patterns, and awareness of common diagnostic pitfalls.

Keywords

Meniscus

Knee

Tear

Magnetic resonance imaging

MRI

ISAKOS (International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine)

Key points

Introduction

Arthroscopic partial meniscectomy is the most common orthopedic surgery performed in the United States.1 Perspectives on the function of the menisci, biomechanical effects after meniscectomy, and treatment algorithms continue to evolve, placing a greater emphasis on meniscal preservation and outcome measures. The potential deleterious effects of surgery have been known for some time: the landmark study in 1948 by Fairbank2 recognized that “meniscectomy is not wholly innocuous…” in the sentinel article recognizing the chronic maladaptive changes after a meniscectomy. More recently, studies1 have shown no difference in long-term improvement between patients undergoing partial meniscectomies and a sham procedure in the treatment of degenerative meniscal tears. In addition, symptomatic patients with meniscal tears and underlying chondrosis showed no difference in functional status when comparing surgery versus physical therapy alone.3

Since its inception into clinical practice in the 1980s, magnetic resonance (MR) has become the preferred imaging method for evaluating the meniscus, with reported accuracies, sensitivities, and specificities ranging between 85% and 95% in detecting meniscal tears.4 Given the evolving treatment strategies, one must not only identify a tear but describe the location, extent, tear pattern, and any associated chondrosis to guide treatment options. The International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine (ISAKOS) Knee Committee formed a Meniscal Documentation Subcommittee in 2006, with the objective of developing a reliable classification system in the evaluation of the meniscus to facilitate outcome assessment. The tear patterns in this classification system include longitudinal-vertical, horizontal, radial, vertical flap, horizontal flap, and complex.5

Therefore, the role of MR imaging has expanded to be not only a simple diagnostic study but a critical decision-making tool providing information that may not only alter the surgical technique but also provide information that would obviate surgery. This review focuses on normal anatomy, technical factors involved when imaging the meniscus, the imaging criteria for diagnosing meniscal tears, the imaging appearance of the various patterns of meniscal tears, secondary signs of meniscal injury, and common diagnostic pitfalls.

Anatomy

The shape and composition of the menisci confer an ability to absorb shock, distribute axial load, assist in joint lubrication, and maintain joint congruity in extremes of flexion and extension.6 The semilunar, triangular, fibrocartilaginous menisci are C-shaped, with a concave surface tapered centrally, conforming to the morphology of the femoral condyle, and a flat base attached to the condylar surface of the tibia via the anterior and posterior root ligaments (Fig. 1). The intimate anatomic relationship and contiguous fibers between the anterior root ligament of the lateral meniscus and anterior cruciate ligament (ACL) insertion site result in a striated or comblike appearance, which can be mistaken for a meniscal tear (Fig. 2).7 Although rarely identified on MR, a similar connection between the ACL and medial meniscus through the meniscocruciate ligament has been noted in several anatomic studies.8 A common variant of the anterior root of the medial meniscus is an insertion along the far anterior margin of the tibia, giving the false impression of extrusion or pathologic subluxation (Fig. 3).9 The typical meniscal tibial attachment sites and their relationship with the cruciate ligaments are shown in Figs. 1 and 2.

The configuration of 3 distinct layers of collagen within the meniscus and formation of collagen bundles oriented along both the long and short axes of the menisci allow for efficient load transmission and shock absorption. The longitudinal fibers are circumferentially oriented, resulting in the ability of the meniscus to distribute axial loads and provide what is commonly referred to as hoop strength. The more loosely organized radial fibers help form a lattice and act to tie the longitudinal bundles together and resist forces that would lead to longitudinal splitting of the meniscus (Fig. 4).10

Each meniscus can be divided into thirds: an anterior horn, body, and posterior horn. The shape of the meniscus can be described best as an elongated semilunar triangle, with a concave hypotenuse and tapered ends. The result is a cross-sectional appearance, resembling either a slab or triangle, based on the orientation of the imaging plane in respect to the axis of the meniscus. The menisci resembles a triangle when imaged perpendicular to the free edge or long axis of the meniscus, such as sagittal images through the horns or coronal images through the body. The menisci takes on a more slab or bow tie configuration if the imaging plane parallels the long axis of the meniscus (eg, sagittal images through the body or coronal images through the horns).

Despite the similarities, the medial and lateral menisci are distinctly different. The larger, more open C-shaped medial meniscus increases in width from anterior to posterior, resulting in a larger posterior horn compared with the anterior horn when viewed in cross section. The more circular lateral meniscus maintains a relatively constant width, resulting...