AUTHOR AND EDITOR INFORMATION

Section 1 of 11  

• Authors and Editors
• Introduction, Anatomy, Physiology, And Histology
• Pathology And Embryology
• Clinical Details
• Preferred Examination
• MRI Techniques, Normal Anatomy, Grading, And Criteria
• Diagnostic Accuracy
• Pitfalls
• Treatment
• Multimedia
• References

INTRODUCTION, ANATOMY, PHYSIOLOGY, AND HISTOLOGY

Section 2 of 11  

• Authors and Editors
• Introduction, Anatomy, Physiology, And Histology
• Pathology And Embryology
• Clinical Details
• Preferred Examination
• MRI Techniques, Normal Anatomy, Grading, And Criteria
• Diagnostic Accuracy
• Pitfalls
• Treatment
• Multimedia
• References

Introduction

The knee menisci were once thought to be functionless remnants of a leg muscle (Sutton, 1897) and expendable components of the knee (Johnson, 1999). Much has been learned in the last 30 years through laboratory investigation, clinical experience, and radiologic imaging. The meniscus is now known to play an important role in the complex biomechanisms of the knee (Fairbanks, 1948; Arnoczky, 1992; Burr, 1982; Spilker, 1992; Spilker, 1992). For instance, it is involved in joint stability (Krause, 1976; Henning, 1985; Shoemaker, 1986; Gershune, 1989), load sharing and transmission (Krause, 1976; Walker, 1975; Shrive, 1978; Ahmed, 1983), shock absorption, and nutrition and lubrication of the articular cartilage (Newman, 1993; Cameron, 1972; Clark, 1983).

Meniscal injuries are a common sports-related problem and the most frequent injury to the knee joint (Renstrom, 1990). Such injuries are especially prevalent among competitive athletes, particularly those who play soccer, football, basketball, and (sometimes) baseball (Johnson, 1974; Koski, 2000). In the last 25 years, the number of people participating in sports has greatly increased, resulting in a higher number of knee injuries.

MRI accurately depicts the anatomy and pathology affecting almost every joint in the body. The reported high accuracy of MRI in the knee has resulted in its being preferred to arthroscopy by most leading orthopedic surgeons (Helms, AJR Am J Roentgenol, 2002).

Repair or resection of meniscal injuries is one of the most common orthopedic operative procedures. Meniscal injuries have a tremendous physical and financial impact on the population. It has been estimated that more than 1.7 million patients undergo meniscal surgery every year (Rodkey, 2000).

A variety of treatment options have been used for meniscal repair and reconstruction. The optimal treatment of meniscal lesions remains controversial. With the evolution of diagnostic MRI, it is no longer sufficient for the orthopedist to simply know a meniscal tear is present. Treatment options depend on knowledge of the exact type, location, and extent of the meniscal tear.

Severe damage, loss or removal of the menisci frequently leads to joint instability and later accelerated degenerative joint disease (DJD) (see Images 62 and 66) (Levy, 1982; Cox, 1977; Newman, 1989), resulting in further disability and joint replacement.

The purpose of this article is to describe the gross and microanatomy of the meniscus, pathology and mechanism of meniscal injuries, clinical presentation, diagnostic tests used to discover meniscal injuries, treatment options, and new frontiers in treatment.

Anatomy

The menisci are C-shaped fibrocartilaginous structures attached to the condylar surface of the tibia (Renstrom, 1990; Cooper, 1990). The limbs of the C face centrally (Thornton, 2000), and the superior meniscal surface is concave, improving contact with the curvilinear-shaped femoral condyle. Conversely, the undersurface of the meniscus is flat, improving contact with the flattened tibial plateau (Rodkey, 2000). The periphery (outer portion) of the meniscus is convex and thicker than the pointed central portion. The thick periphery allows for a firm attachment to the joint capsule (Rodkey, 2000) (see Image 1).

The adult meniscus is relatively avascular, except for the peripheral 10-25% (Arnoczky, 1982; Fairbanks, 1948). The meniscus of the child is better vascularized than that of the adult, but this regresses with advancing age. The normal meniscus is 3-5 mm in height. The meniscus is arbitrarily divided into anterior horn, body, and posterior horn.

Medial meniscus

The medial meniscus (MM) has a more crescentic shape than the more circular lateral meniscus (LM) (Rodkey, 2000). It occupies 50% of the articular contact area of the medial joint compartment (Rath, 2000) and is larger than the LM.

The posterior horn is thicker than that of the LM (Thornton, 2000) (see Image 2). The MM varies in width from about 6 mm in the anterior horn to 12 mm in the posterior horn (Mink, 1990; Rodkey, 2000). The transverse (intermeniscal) ligament connects the anterior horns of the MM and LM (Rodkey, 2000). The posterior fibers of the root of the anterior meniscus merge with the fibers of the transverse ligament (Thornton, 2000; DeHaven, 1994) (see Image 3).

The attachment of the MM to the joint capsule is more rigid than that of the LM, making the MM more susceptible to injury (Thornton, 2000). The posterior horn attaches to the tibia posterior to the intercondylar fossa between the attachment of the posterior cruciate ligament (PCL) and the more anteriorly located posterior horn of the LM (DeHaven, 1994; Cooper, 1990; Arnoczky, 1992; Johnson, 1995; Thornton, 2000), by the meniscotibial ligament (Thornton, 2000; DeHaven, 1994).

The periphery of the MM is attached to the joint capsule throughout its length (Fairbanks, 1948). The tibial portion of the capsular attachment of the MM is called the coronary ligament. At its mid portion, the body of the MM is more firmly attached to the tibia and femur through a thickening of the joint capsule called the deep medial collateral ligament (MCL) (DeHaven, 1994). The body is tapered when compared with the wider anterior and posterior horns. In 11% of patients, the anterior horn is not attached to the tibia. In this situation, it attaches directly to the transverse ligament or the anterior cruciate ligament (ACL) (Ohkoshi, 1997).

Lateral meniscus

The LM is more circular than the MM (Rodkey, 2000) and covers more of the tibial plateau (about 70%) than the MM (Rodkey, 2000; Rath, 2000; Thornton, 2000). Also, the LM is more uniform in width from anterior horn to posterior horn than the MM (Rodkey, 2000).

The LM contains the popliteus recess in its posterolateral margin. The recess allows passage of the popliteus tendon and sheath through the knee joint (Rath, 2000) (see Image 6). The popliteus recess is located between the lateral recess and the joint capsule (see Images 5-7). The LM attaches to the lateral aspect of the medial femoral condyle through the meniscofemoral ligaments: the ligament of Wrisberg, which is posterior to the PCL and the ligament of Humphrey, which is anterior to the PCL (Renstrom, 1990; Koski, 2000). Both ligaments originate from the posterior horn of the LM (see Image 9).

The anterior horn of the LM attaches to the tibia between intercondylar eminence and the insertion of the ACL (see Image 14). The fibers of the anterior horn and ACL insertion may be contiguous (Shankman, 1997) (see Images 15 and 40). The posterior horn is secured to the tibia between the intercondylar eminence and the insertion of the MM (Cooper, 1990; Arnoczky, 1992; Johnson, 1995). The LM measures about 10 mm in width.

At the posterolateral corner of the knee joint, the LM is attached to the joint capsule by the superior and inferior fascicles (Thornton, 2000) (see Images 7-8). The LM has a loose attachment to the joint capsule and is not anchored to the lateral collateral ligament (LCL), as the MM is to the deep portion of the MCL. The attachments of the LM to the femur and popliteus tendon enable it to couple its motion with that of the femur during rotation. Its greater mobility makes it less likely to become injured when contrasted with the more firmly attached MM (Rath, 2000). It may normally move 1 cm in the anteroposterior direction (Renstrom, 1990).

Meniscomeniscal ligament

This recently described, uncommon (1-4%), normal variant runs from the anterior horn of 1 meniscus to the posterior horn of the other meniscus. It is named according to its anterior attachment (Sanders, 1999).

Discoid meniscus (see Image 41)

The discoid meniscus (DM) is a dysplastic meniscus and frequently bilateral. This structure is enlarged and shaped like a half moon. It does not have the usual semilunar shape of the normal meniscus. The DM is more common in the LM by a ratio of 5:1 to 50:1 (Crues, 1993; Dickason, 1982). The incidence of occurrence in the LM is between 1.4 and 15.5% (Aichroth, 1992). That in the MM is about 0.3% (Crues, 1993).

The DM may be described according to morphology as complete and incomplete. Both types are firmly attached to the tibia (Rath, 2000). A complete DM extends to the intercondylar notch of the tibia. The Wrisberg type of DM may have a normal appearance (Crues, 1993; Jaureguito, 1995; Rath, 2000), but lacks any attachment to the posterior tibia and joint capsule. As a result, the meniscus retracts anteriorly from the pull of the meniscofemoral ligaments. This unrestricted motion puts it at risk for hypertrophy and trauma (Fujikawa, 1990; Rath, 2000).

An anterior megahorn DM is an overgrowth of the anterior horn and body with a normal posterior horn. The DM is more susceptible to tears and cyst formation than the normal meniscus. An incomplete DM is the most common type (Stoller, 1997). Criteria for DM is that the horizontal diameter of the body of the meniscus is greater than 13-15 mm on coronal MRIs, as measured from the capsular margin to the free edge (Crues, 1993; Quinn, 1988).

The DM is associated with other musculoskeletal anomalies (Arnoczky, 1988; Smillie, 1978; Jeanopoulos, 1950; Weiner, 1974; Resnick, 1976), including the following: high fibular head, fibular muscular defects, hypoplasia of the lateral femoral condyle with lateral joint-space widening, hypoplasia of the lateral tibial spine, abnormally shaped lateral malleolus of the ankle, and enlarged inferior lateral geniculate artery.

Transverse, or intermeniscal, ligament

The transverse (intermeniscal) ligament connects the anterior horns of the MM and LM (see Images 3-4, 43-44). It originates anterolateral to the central rhomboid attachment of the LM and inserts on the anterior superior aspect of the anterior horn of the MM. The transverse (intermeniscal) ligament is absent in up to 40% of individuals.

Meniscofemoral ligaments

The meniscofemoral ligaments are oriented obliquely from the posterior horn of the LM to the medial femoral condyle (see Image 9). The more posterior ligament of Wrisberg is the larger of the 2. Its thickness may be half the thickness of the PCL (Mink, 1993). This ligament is present in 60% of cadavers.

The presence and size of these ligaments vary greatly (Heller, 1964; Arnoczky, 1988). Their exact function is uncertain (Rodkey, 2000), but they may pull the posterior horn of the LM anterior to increase the coverage of the lateral femoral condyle and the LM (Heller, 1964).

Popliteus tendon

The popliteus tendon originates from the posterolateral surface of the femur, above the lateral femoral epicondyle in a groove (see Images 6 and 10-12). It retracts the posterior horn of the LM in flexion and internal rotation, reducing entrapment of the LM between the femur and tibia (Rath, 2000).

Blood supply

The MM is supplied by the supreme, superior and inferior branches of the medial geniculate artery. The LM is supplied by similar branches of the lateral geniculate artery (Fairbanks, 1948).

The middle geniculate branch of the popliteal artery supplies the menisci through the vascular synovial covering of the anterior and posterior attachments to the joint capsule. These synovial vessels penetrate these attachments and give rise to the meniscocapsular vessels that branch into the menisci (DeHaven, 1994; DeHaven, 1989; Rodkey, 2000; Wirth, 1981).

The microvascular structure of the meniscus consists of a perimeniscal capillary plexus originating in the capsular and synovial tissues of the joint (Renstrom, 1990; Arnoczky, 1982; Cooper, 1990). This plexus extends through 10-25% of the peripheral aspect of the meniscus (Arnoczky, 1982; Rodkey, 2000). This limited blood supply has important prognostic considerations when treating meniscal tears.

On the basis of degree of vascular penetration, the meniscus is divided into a red or vascular zone (outer one third) and a white or avascular zone (inner two thirds). The border between these areas is called the red-white zone or junction.

There is an absence of penetrating vessels in the posterolateral aspect of the LM adjacent to the popliteus tendon (Arnoczky, 1982).

Nervous enervation

Nervous enervation is limited to the meniscal horns. The central one third of the menisci are devoid of enervation (Rodkey, 2000).

Physiology

The meniscus is a biphasic medium: it has a fluid phase and a solid phase (Spilker, 1992; Adams, 1992). The fluid phase is the interstitial water, and the solid phase is the extracellular matrix, which is composed predominately of collagen. The interstitial water flows through the porous solid matrix at different rates under different circumstances to prevent damage and deformity of the meniscus as it fulfills its biomechanical function. The menisci are viscoelastic under load (Mow, 1989; Favena, 1983). That is, the tissue deforms and behaves differently depending on the amount of load placed on it, as well as the rate at which the load is applied (Rodkey, 2000).

The menisci provide added mechanical stability to the normal gliding of the femur on the tibia by deepening the surface of the tibial plateau to increase the congruity between the femoral condyles and the tibial plateau (Mow, 1989). It has been suggested that the posterior horn of the MM is the most important part of the meniscus in providing this function (Levy, 1982; Fu, 1992).

The meniscus protects the articular cartilage by acting as a buffer between the articular surfaces of the femur and tibia. The meniscus transmits axial and torsional forces across the knee joint. It prevents capsular interposition between the tibia and femur. It increases the surface area for motion of the femoral condyles. Vertically oriented collagens, located predominately in the periphery of the meniscus, are converted into hoop stress in the menisci, decreasing the axial load on the articular cartilage. Loss of this function through a meniscal tear or meniscectomy may accelerate the breakdown of articular cartilage with resultant DJD.

The menisci cushion mechanical loading and support 50% of the load about the knee in extension. In 90% flexion, 85% of the load is transmitted through the menisci (Ross, 1998). They act as a shock absorber (Cooper, 1990; Johnson, 1978; Rodeo, 1986). As the knee joint compresses, the circumferential collagen fibers elongate and the meniscus extrudes peripherally, absorbing energy and reducing the shock to the adjacent cartilage and subchondral bone (Newman, 1993; Johnson, 1978). They act as a spacer, separating the tibia and femur at low loads (Newman, 1989; Burke, 1998).

Nerves of the menisci may play an important role in proprioception of the knee. Evidence suggests that knees treated with meniscectomy have less proprioception than normal knees (Ryu, 1998).

The menisci contribute significantly to joint lubrication. Because 74% of the total weight of the meniscus is water, compression of the menisci squeezes water out from them into the joint space to allow smoother gliding of the joint surfaces. This pumping action also helps to distribute synovial fluid throughout the joint and aids in the nutrition of the articular cartilage (Rath, 2000).

Medial meniscus

The MM facilitates the transmission of 40-50% of the load of the medial compartment, whereas the LM may transmit as much as 65-75% of the load on the lateral compartment (Seedhorn, 1976; Shrive, 1974). The MM is subject to higher stresses than the LM. This has prognostic considerations when treating acute MM tears in the presence of ACL tears. In this situation, it is imperative to repair the meniscal tear while repairing the ACL tear. The medical meniscus also acts as a secondary stabilizer in anteroposterior stability, becoming more important in the ACL-deficient knee (Levy, 1982; Rubin, 1999).

Lateral meniscus

The LM may play a role in rotational control, especially in patients with posterolateral instability (Koski, 2000; Rubin, 1999). The LM causes a greater proportion of the load on the lateral side of the knee when compared with the medial side, where the load is shared equally by the MM and the articular surface (Fitzgibbons, 1995).

Histology

The menisci are composed of dense fibrocartilage consisting of networks of collagen fibers and cells (Arnoczky, 1992). The cells of the meniscus are called meniscal fibrochondrocytes because they are a mixture of both fibroblasts and chondrocytes (Webber, 1989). The cells in the superficial portion of the meniscus are similar to the chondrocytes found in the more superficial layer of the articular cartilage of the femur and tibia (Webber, 1989). The cells in the deeper portion of the meniscus are more oval in shape, but also resemble the chondrocytes found in the tibia and femur (Webber, 1989). The function of the cells within the meniscus is to synthesize and maintain the extracellular collagen matrix, which is the scaffold of the meniscus.

The collagen is formed into bundles and the bundles are organized into 2 zones: circumferential and transverse. The circumferential bundles are located primarily in the peripheral one third of the meniscus. The transverse bundles bridge the circumferential bundles and extend to the free (inner) edge of the meniscus, adding structural integrity to the meniscus and resisting compressive forces (Bullough, 1970).

The middle perforating collagen bundle divides the transverse bundles into superior and inferior portions. Some of the transverse fibers act as tie fibers between the circumferential bundles that resist longitudinal splitting (Bullough, 1970; DeHaven, 1994). The middle perforating collagen bundle may demarcate the sheer plane of the meniscus. In internal degeneration, the middle perforating collagen bundle corresponds to the predominately horizontal region of increased signal of grade 2 menisci, seen on proton density (PD)– and T2-weighted images (Stoller, 1997). Elastic fibers bridge the collagen fibers throughout the meniscus (Ahmed, 1983).

Ultrastructural studies demonstrate 3 layers of collagen fibers: superficial, surface and middle. Going from superficial inward, the fibers tend to become larger and more structurally significant (Bullough, 1970).

Several types of collagen have been identified within the meniscus. Type I collagen predominates and makes up 95% of the meniscal collagen (Fairbanks, 1948). It serves as the fibrous network for the dense and durable fibrocartilage of the meniscus. Types II, III, V, and VI collagen have also been identified within the meniscus, but in smaller amounts (Bullough, 1970).

For excellent patient education resources, visit eMedicine's Foot, Ankle, Knee, and Hip Center; Imaging Center; and Arthritis Center. Also, see eMedicine's patient education articles Knee Injury, Knee Pain, Knee Joint Replacement, Magnetic Resonance Imaging (MRI), and Osteoarthritis.

PATHOLOGY AND EMBRYOLOGY

Section 3 of 11  

• Authors and Editors
• Introduction, Anatomy, Physiology, And Histology
• Pathology And Embryology
• Clinical Details
• Preferred Examination
• MRI Techniques, Normal Anatomy, Grading, And Criteria
• Diagnostic Accuracy
• Pitfalls
• Treatment
• Multimedia
• References

Pathology

Two pathways for healing in response to a tear in the periphery of the meniscus are described: extrinsic and intrinsic. Two types of tears are also discussed: traumatic and degenerative.

Extrinsic pathway

Once a meniscal tear occurs, a fibrin clot forms within its margins, creating a scaffold into which angiogenesis develops from the perimeniscal capillary plexus. The fibrin clot contains factors, such as platelet-derived growth factor and fibronectin, that act as chemotactic and mitogenic agents for reparative cells to migrate and develop (Koski, 2000). Undifferentiated mesenchymal cells also migrate into the clot and a fibrovascular scar develops, gradually sealing the lesion. Further inflammatory response and angiogenesis result in healing of the lesion in about 10 weeks in the dog (Arnoczky, 1983).

It may take months or even years for the scar tissue to change into fibrocartilage, resembling that of the meniscus (Rodkey, 2000). Differences between the newly formed fibrocartilage and mature fibrocartilage are recognizable and include increased cellularity and, at times, increased vascularity in the repair tissue (Deutsch, 1990; King, 1936).

Intrinsic pathway

The chondrocytes within the meniscus have an inherent capability to generate a healing response, even in the avascular region. Chondrocytes are assisted by the fibrin clot, which not only acts as a scaffold, but also provides the chemotactic and mitogenic stimuli to promote healing. Meniscal tears can be classified into 2 types: traumatic and degenerative.

Traumatic tears

Traumatic tears are most commonly found in young, athletically active individuals (DeHaven, 1992). Traumatic tears are not necessarily associated with contact injuries (Rodkey, 2000). They are frequently associated with ACL tears and less commonly with PCL tears. Vertical longitudinal tears are the most common; transverse or radial tears are also common (DeHaven, 1992).

Degenerative tears

Degenerative tears tend to occur in patients older than 40 years. No history of a traumatic event is present. These tears have minimal or no healing capacity, and horizontal cleavage tears, flap tears, and complex tears are most common (DeHaven, 1992).

Embryology

The menisci develop from condensations of mesenchymal tissues within the limb bud and assume an adult shape by the end of the eighth week of fetal development (Canham, 1986).

At birth, the menisci are completely vascularized, but this regresses so that by age 10 years an avascular central third of the meniscus is present (Arnoczky, 1983). The transition from vascular to avascular regions progresses so that by adulthood, only the peripheral 10-30% of the meniscus remains vascular. Weight bearing and knee motion may be responsible for this progressive change (DeHaven, 1981).

CLINICAL DETAILS

Section 4 of 11  

• Authors and Editors
• Introduction, Anatomy, Physiology, And Histology
• Pathology And Embryology
• Clinical Details
• Preferred Examination
• MRI Techniques, Normal Anatomy, Grading, And Criteria
• Diagnostic Accuracy
• Pitfalls
• Treatment
• Multimedia
• References

Mechanisms of injury

Injuries to the healthy meniscus are usually produced by compressive forces coupled with rotation of the flexed knee as it starts to move into extension. The final type and location of the tear is determined by the direction and magnitude of the force acting on the knee and the position of the knee when injured (Muellner, 1999; Buckwalter, 1993). Most meniscal tears in sports are noncontact in nature and occur from deceleration, cutting, or landing from a jump (Washington, 1995). Other mechanisms of injury include twisting of the knee or squatting (Connor, 1994).

In knees with chronic ACL tears, recurrent episodes of anterior tibial subluxation on the femur occur, resulting in shearing forces on the menisci (Shelbourne, 1995).

In skiers with acute LM tears and ACL tears, the mechanism of injury to the meniscus is an anterolateral rotatory translation. As the ACL disrupts, excessive anterolateral rotation of the tibia on the femur traps the LM between the posterolateral aspect of the tibial plateau and the central portion of the lateral femoral condyle. The LM is distorted when the tibia reduces, often resulting in a tear (Duncan, 1995).

The orientation of the transverse tie fibers between the circumferential bundles is responsible for the tendency of the meniscus to tear with rotational forces (Newman, 1993). Rotation, especially when accompanied by axial compression, can squeeze the meniscus between the tibia and femur, generating tensile forces in the body of the meniscus high enough to damage either the transverse fibers, resulting in a longitudinal tear, or the circumferential fibers, resulting in a radial tear (Newman, 1993; Aspden, 1985).

Findings on physical examination

General findings include the following:

• Knee giving way (Newman, 1993)

• Clicking (Newman, 1993)

• Locking of the knee in fixed flexion (This may occur immediately after displacement of a meniscal fragment between the tibia and the femur.)

• Pseudo-locking, which may be related to muscle spasm (Cannon, 1994)

• Limping (This is common, and patients may not be able to bear weight.)

Findings related to a DM include the following:

• Snapping knee syndrome: When the knee is flexed or extended, a snapping sound is heard (Silverman, 1989) because the DM may be hypermobile (Carrino, 2002). The snapping is associated with knee joint pain or lateral joint line tenderness. This occurs only in a minority of cases.

• Nonspecific physical findings are common (Rath, 2000; Washington, 1995).

• The Wrisberg type of DM may present before adolescence with pain and with or without an audible palpable click (Aspden, 1985).

Tests to evaluate the menisci

No one test provides predictive results for diagnosing meniscal tears. A combination of several positive results is highly predictive of meniscal tears.

Appley test

With the patient prone and the knee flexed at 90° the foot is rotated internally and externally, first with distraction and then compression. A positive result elicits pain with compression. This test is inaccurate, and its use is discouraged (Richmond, 1996).

McMurray test

The patient is supine with the hip and knee flexed. The foot is alternatively internally and externally rotated, while the posteromedial and posterolateral joint line are palpating. To evaluate the MM, the knee is extended, and the foot is externally rotated. To evaluate the LM, the knee is extended, and the foot is internally rotated. Hearing or feeling a click during these maneuvers is indicative of a meniscal tear.

A true-positive result is rare, but the specificity is nearly 100%. More commonly, the tear elicits pain with a meniscal tear (Reeder, 1989). This test is the most widely used clinical test to detect meniscal injuries, but its accuracy is inconsistent (Zairul-Nizam, 2000).

Range-of-motion tests

Flexion pain or limitations in range of motion often result from posterior meniscal horn tears. Extension pain can be linked to anterior horn tears. Tenderness on palpation of the joint line is probably the most important finding.

Boehler test

This test is performed in the same way as testing for stability of the collateral ligaments. Valgus stress results in pain with LM tears. Varus stress results in pain with MM tears.

Apley grinding test

With the patient prone, the hip is extended and the knee is flexed more than 90°. Downward pressure is placed on the foot and the joint surfaces of the knee are rotated and compressed. Eliciting pain is indicative of a tear.

Payr test

The knee is flexed to 90°. The application of varus stress compresses the posterior horn of the MM. Eliciting pain indicates a meniscal tear.

Accuracy and limitations of tests

Accuracy

Large, blinded prospective studies with diverse patient populations suggest that for experienced clinicians, the sensitivity of physical examination for detecting meniscal tears is between 70-90%. Higher sensitivity can be achieved only at the expense of lower sensitivity (Bessette, 1992; Daniel, 1982; Fowler, 1989; Munk, 1998; Oberlander, 1993; Simonsen, 1984; Spiers, 1993; Thornton, 2000).

The negative predictive value (NPV) is no greater than 67%; therefore, about one third of meniscal tears are missed with clinical screening alone (Fowler, 1989; Spiers, 1993). In situations of multiple knee lesions, the accuracy of clinical examination in diagnosing meniscal tears decreases to 30% (Oberlander, 1993).

In the presence of acute ACL tears, the sensitivity for diagnosing MM tears is 45% and 58% for LM tears (Fowler, 1989; Jorgenson, 1987). The sensitivity of joint line tenderness for diagnosing meniscal tears is 75%. The sensitivity of the Apley grinding test for meniscal tears is about 45%. The sensitivity of the Payr test for diagnosing meniscal tears is about 40%.

Limitations

Unlike MRI, the clinical examination cannot demonstrate the location, shape, or length of a meniscal tear. These factors are important in treatment decisions.

The clinical diagnosis of meniscal tears becomes more difficult and unreliable in the presence of acute ligamentous injuries of the knee (Kimori, 1989; Fowler, 1989; Keene, 1993). The sensitivity for diagnosing MM tears decreases to 45% and the sensitivity for diagnosing LM tears decreases to 58% when the ACL is ruptured (Fowler, 1989; Shelbourne, 1995). Specificity also decreases, most likely due to the presence of tibial and femoral bone bruises that frequently accompany acute ACL tears (Fowler, 1989; Shelbourne, 1995). Pain from these bone injuries can cause joint-line tenderness, a finding that otherwise suggests the presence of a meniscal tear.

Distribution of meniscal tears

In knees with intact ligaments, MM tears are more common (De Smet, 1994; Dandy, 1990), due to higher biomechanical stresses in this location and a firmer attachment to the joint capsule (Miller, 1988). Only 18-20% of meniscal tears occur without associated ligament damage (Johnson, 1999).

Meniscal tears associated with acute ACL tears

The incidence varies between 35% and 78% (Indelicato, 1985; Keene, 1993; Keene, 1987; Satku, 1986), and the majority are bucket handle tears (Rubin, 2000; De Smet, 1994; Dandy, 1990; Tolin, 1993; Paletta, 1992; Barber, 1992; Indelicato, 19815; Thornton, 2000).

These tears usually involve the vascularized outer one third of the posterior horn or meniscocapsular junction. This favorable location, within the richly vascularized portion of the meniscus makes these tears amenable to repair (Indelicato, 1985), or conservative treatment (Fitzgibbons, 1995). In downhill skiers, the LM is more often involved. Tears tend to be longitudinal and are located at the periphery of the posterior horn, or the root of the meniscus at the joint capsule, medial to the popliteus tendon sheath hiatus (De Smet, 1994; Duncan, 1995; Heron, 1992; Paletta, 1992; Thornton, 2000). Tears are either full thickness or partial thickness (Paletta, 1992; Noyes, 1980).

Meniscal tears associated with chronic ACL tears

After the initial ACL tear, the incidence of subsequent meniscal tears increases to between 53% and 97% over a 10-year period (Indelicato, 1985; Keene, 1993; Keene, 1987; Satku, 1986).

The incidence of MM tears increases with time. The incidence of LM tears with chronic ACL injuries was the same as with acute ACL injuries (Indelicato, 1985; Thornton, 2000). The MM is a significant restraint to anterior tibial translation after ACL disruption, suggesting the possibility of increased sheer forces on the meniscus after ACL tear (Paletta, 1992). The relatively immobile MM may also be injured during abnormal knee motion during periods of instability (Duncan, 1995; Keene, 1993).

Meniscal tears with chronic ACL injuries are often degenerative and complex, supporting the concept that chronic instability, seen with chronic ACL tears, leads to or accelerates degenerative meniscal tears and predisposes to early and accelerated DJD (Glashow, 1989).

They are less amenable to surgery (Keene, 1993; Keene, 1993).

Meniscal tears with multiple ligament tears

With both ACL and MCL tears, LM tears are more common (Thornton, 2000).

Isolated MM tears occurring without LM tears are uncommon (Duncan, 1995; Barber, 1992; Shelbourne, 1991). O'Donoghue's triad, the unhappy triad of combined ACL, MCL, and MM injury, is uncommon (Duncan, 1995).

Many MM injuries in this setting are actually ruptures of the medial joint capsule and the deep fibers of the MCL in conjunction with peripheral meniscocapsular separation (MCS) (Barber, 1992; Rubin, 1996).

Other tears

Degenerative tears are more commonly occur in patients older than 40 years, and patients often are not aware of any injury, only the subsequent symptoms (Washington, 1995).

Meniscal tears in children are often associated with ACL tears (Iobst, 2000).

Patients with DM tears may present with mechanical symptoms without a tear because the abnormally large DM may be hypermobile causing painful snapping with flexion and extension (Carrino, 2002).

Differential diagnosis

Differential diagnoses of meniscal tears include the following (Stoller, 1997):

• Bone contusion

• Plica syndromes

• Popliteus tendinitis

• Osteochondritis desiccans

• Chondral damage from trauma

• Loose bodies

• Patellofemoral pain and instability

• Fat pad impingement syndrome

• Inflammatory arthritis

• Fracture

• Meniscotibial ligament sprain

• Discoid meniscus

• Synovial lesions

Differential diagnoses of a DM include the following:

• Any condition that presents as a snapping knee on physical examination (A snapping knee yields a snapping sound heard during flexion and extension of the knee.)

• Patellofemoral joint subluxation or dislocation

• Meniscal cysts

• Congenital subluxation of the tibiofemoral joint

• Subluxation and/or dislocation of the proximal tibial-fibular joint

• Snapping of the tendons about the knee due to the presence of osteophytes or a roughened bone surface

PREFERRED EXAMINATION

Section 5 of 11  

• Authors and Editors
• Introduction, Anatomy, Physiology, And Histology
• Pathology And Embryology
• Clinical Details
• Preferred Examination
• MRI Techniques, Normal Anatomy, Grading, And Criteria
• Diagnostic Accuracy
• Pitfalls
• Treatment
• Multimedia
• References

Magnetic resonance imaging

MRI is the most powerful, accurate, and noninvasive method for diagnosing meniscal tears. It is more accurate than physical examination (Renstrom, 1990) and has influenced clinical practice and patient care by avoiding unnecessary diagnostic arthroscopies (Renstrom, 1990; Bui-Mansfield, 1997; Carmichael, 1997; Mackenzie, 1996; Rangger, 1996; Ruwe, 1992), or by identifying alternative diagnosis that can mimic meniscal tears (Mackenzie, 1996; Rangger, 1996; Maurer, 1997; Rubin, 2000; Reicher, 1986; Trieshmann, 1996).

In many cases, MRI results change the proposed management. One study determined that about one third of all diagnostic arthroscopies are unnecessary if MRI is used (Rangger, 1996). Another study showed the use of MRI prevented 51% of diagnostic arthroscopic studies, resulting in a savings of more than $103,000 (Ruwe, 1992), and avoiding the morbidity associated with arthroscopy (Spiers, 1993).

MRIs show many of the essential characteristics of meniscal tears critical to management, such as their location, shape, length, and depth (Rubin, 2000). In this way, MRI helps to provide an accurate assessment of stability, the likelihood of tear propagation, and a determination of whether it can be repaired. It is advantageous to know ahead of time if a given meniscal tear is repairable; the additional equipment, surgical assistants, and time needed for repair can be anticipated (Cannon, 1994; Matava, 1999). Patients also benefit from an earlier knowledge for need of surgery. The recovery time for meniscal repair is longer than that for partial meniscectomy (PM). Patients may want to time surgery to fit with their other obligations.

When combined with clinical data, such as the patient's age, athletic requirements, and physical findings (eg, possible associated ligamentous injuries), a treatment plan can be developed by assessing the need for and timing of surgery and the type of surgery (meniscal debridement, rasping, repair, partial or total resection, or meniscal transplantation) (Rubin, 2000). MRI can be used to identify other injuries, such as ligament tears, especially ACL tears, which can also influence the need for surgery.

With MRI, physicians can obtain images in several planes, providing multiple perspectives on meniscal and ligamentous injuries. Other advantages include the following: (1) the lack of patient exposure to ionizing radiation; (2) it does not normally involve the intravenous administration of contrast material, which has a small but definite number of adverse effects; (3) it does not require joint manipulation; and (4) it is painless and can be performed in less than 35 minutes (Reicher, 1986); and (5) it prevents the intra-articular injection of iodinated radiographic contrast material, which is needed for arthrography. MRI results alter therapy in about one third of cases (Rangger, 1996; Spiers, 1993).

Arthrography

Two decades ago, arthrography was commonly used to diagnose meniscal tears (Butt, 1969; Pavlov, 1978). It was an accurate, but invasive test requiring the intra-articular injection of iodinated contrast material. Similar to MRIs, arthrograms did not give much information about the underlying tendons, ligaments, bones, and bursa. Arthrography has been replaced by MRI, except when the patient is too large for the MRI unit or when contraindications to MRI (eg, intracranial aneurysm clips, orbital metallic foreign bodies) are present.

Plain radiography

Plain radiography is extremely limited in the assessment of meniscal tears. Radiographs may be obtained to rule out unsuspected lesions, such as osteochondritis desiccans and loose bodies.

In the presence of a DM, radiographs may show widening of the medial or lateral joint compartments, hypoplasia of the lateral femoral condyle related to the increased size of the LM, a high fibular head, cupping of the lateral tibial plateau, or a squared-off lateral femoral condyle (Schonholtz, 1993).

Magnetic resonance arthrography

Magnetic resonance (MR) arthrography is used to evaluate residual or recurrent meniscal tears after meniscal surgery. The detection of residual or recurrent meniscal tears following meniscectomy or meniscal repair is difficult with conventional MR images. MR arthrography is more accurate when over 25% of the meniscus is resected (Steinbach, 2002).

MRI TECHNIQUES, NORMAL ANATOMY, GRADING, AND CRITERIA

Section 6 of 11  

• Authors and Editors
• Introduction, Anatomy, Physiology, And Histology
• Pathology And Embryology
• Clinical Details
• Preferred Examination
• MRI Techniques, Normal Anatomy, Grading, And Criteria
• Diagnostic Accuracy
• Pitfalls
• Treatment
• Multimedia
• References

Imaging systems, protocols, and sequences

Careful attention to detail must be made to achieve high accuracy in diagnosing meniscal tears.

MRI systems

MRI systems of low, medium, and high field strength can all produce accurate, diagnostic images for identifying meniscal abnormalities (Cotten, 2000). In units with lower field strength, the number of signals averaged may need to be increased to obtain an adequate signal-to-noise ratio (Boeve, 1991; Barnett, 1993; Franklin, 1997; Glashow, 1989; Reicher, 1986; Rothschild, 1990; Thornton, 2000). This adjustment, however, increases the imaging time, which increases the risk of patient motion. Even a small amount of motion can degrade the images, jeopardizing the ability to diagnose meniscal tears.

An extremity coil is used to optimize the signal-to-noise ratio. A surface coil may be used for better detail in evaluating subtle lesions or suspicious areas (Antonio, 2004).

Protocols and imaging planes

The knee is usually positioned in extension with slight external rotation to facilitate imaging the ACL.

High spatial resolution is required to show subtle tears. This requires a field of view of 16 cm or less, a section thickness of 5 mm or less (3-4 mm is preferred), and a matrix of at least 192 X 256 steps in the phase- and frequency-encoding directions (Rubin, 2000; Turner, 1991; Thornton, 2000; Reicher, 1985). A skip of 0.5 to 1 mm is used between imaging sections (Carrino, 2002). These parameters can be achieved by using a solenoid surface coil (Reicher, 1985). An extremity coil is used to optimize the signal-to-noise ratio.

If subtle lesions or suspicious areas are identified by using the standard extremity coil, high-resolution images can be obtained by using a surface coil, provided that the area of interest is superficial enough to be encompassed by the surface coil with the small field of view. Scanning parameters in this situation include the following: field of view as small as 10 X 10 cm, matrix 256 X 512 (displayed at 512 X 512), section thickness of 3 mm with an 0.3-mm intersection gap, and 3 signals acquired (Antonio, 2004).

Images must be obtained in both the sagittal and coronal planes. Sagittal images are obtained with the knee externally rotated to permit imaging in the plane of the ACL. Meniscal and ACL injuries frequently coexist. Axial images are also obtained to study the supporting ligaments around the knee. Changing coils during an MRI examination is not part of the standard examination, but is similar to changing transducers during ultrasonography to look at deeper or more superficial structures. In a small or remote radiology practice, the attending radiologist may not be available to supervise the MRI examination. In this situation, the patient can be called back for additional imaging with the surface coil (Antonio, 2004).

Several factors should be considered in optimizing the imaging protocols. Imaging in all 3 planes is useful; however, not every sequence must be performed in every plane. Fluid-sensitive sequences are mandatory for detecting subtle areas of edema. Typically, some T2-weighted sequence is performed, usually in the sagittal and axial planes (Carrino, 2002). Experience with a particular sequence may outweigh any theoretical improvements from a pulse sequence unfamiliar to the imager (Carrino, 2002).

A repetition time (TR) between 2200 and 2800 ms is needed to generate enough sections to image both menisci in the sagittal plane. Short echo times (TEs) are important when PD–weighted imaging is performed. With a TE of less than 26 ms, more than 90% of all meniscal tears can be detected (Crues, 1987). If the TE is increased to greater than 60 ms, less than 30% of tears are detected (Carrino, 2002).

For reviewing MRIs of the knee, the use of meniscal windows has become popular. This method consists of a region of interest, centered on the meniscus, zoomed to 1.5-2X magnification. A window width of 100 to 150 and a window level of approximately 1000 are used (Carrino, 2002). Data indicate no significant differences in detecting meniscal tears by using these narrow compared with conventional window widths (Buckwalter, 1993).

T1- and PD-weighted sequences

T1-weighted images are not as sensitive as PD-weighted images for diagnosing meniscal tears.

Gradient-recalled echo sequences

Gradient-recalled echo (GRE) sequences (Haggar, 1988; Heron, 1992; Reeder, 1989; Solomon, 1989; Spritzer, 1988; Spiers, 1993) are as accurate as conventional spin-echo images for diagnosing meniscal tears (Kojima, 1996; Quinn, 1992; Heron, 1992).

However, GRE imaging is more limited in diagnosing ligament, muscle, tendon, bone marrow, and articular cartilage abnormalities. It is also less specific for meniscal tears as a consequence of spurious signal from artifacts or degeneration without a tear (Carrino, 2002).

Fat-suppressed sequences

Fat suppression can be applied to meniscal-sensitive sequences to rid the image of distracting high signal originating from the fatty marrow in the bones and the fat in the soft tissues (Helms, 2002). With fat suppression, the dynamic range signal of the menisci is increased, making meniscal tears more conspicuous (Helms, 2002) (see Images 22-23).

No evidence indicates that fat suppression increases the accuracy in diagnosing meniscal tears, but this practice is gaining widespread acceptance (Helms, 2002).

Fast spin-echo sequences

Turbo or fast spin-echo (FSE) pulse sequences are not as effective as conventional spin-echo sequences for diagnosing meniscal tears because images with a short effective TE, as seen with FSE imaging, sacrifice high-spatial-frequency information for speed. Images of the menisci may appear blurred (Mulkern, 1990; Rubin, 1994). Rubin and colleagues (Rubin, 1994) postulated that the presence of ghosting artifact (secondary to phase differences between even and odd echoes in the echo train) or the loss of meniscal signal intensity in meniscal tears from an increased magnetization transfer (as seen with FSE sequences) may be responsible for the lower sensitivity of this sequence (Stoller, 1997; Thornton, 2000).

A review of 6 studies (Blackman, 2005), including the author's, showed a distinct discrepancy between the sensitivities of FSE and conventional spin-echo sequences. The sensitivities of fast spin echo techniques for detecting a meniscal tear was approximately 80% whereas the sensitivities for conventional spin-echo sequences averaged approximately 90%. The authors postulated that abnormal meniscal signal may appear to extend to the meniscal surface secondary to blurring and may be incorrectly interpreted as a tear. Alternatively, the increased blurring and decreased resolution associated with FSE imaging can contribute to false negative results. Blurring is most evident with short TE sequences but short TE sequences are most proficient for detecting meniscal abnormalities. Blurring is also most conspicuous with long echo-train lengths, such as those incorporated with FSE imaging protocols. The authors urged abandoning FSE imaging because a loss of greater than 10% in sensitivity is unacceptable.

When FSE sequences are used, an echo train length of 4-6 should be used to reduce blurring. The sensitivity of FSE sequences for diagnosing meniscal tears is about 80% in all reports (Rubin, 1994; Anderson, 1995; Escobedo, 1996; Cheung, 1997; Kowalchuk, 2000; Major, 1998), whereas the sensitivity of conventional spin-echo techniques is at least 90%. If the sensitivity decreases from over 90% to 80% and if all that is gained is 2-3 minutes in imaging time, the use of FSE sequences for imaging the menisci is hardly justified (Helms, 2002).

The use of FSE sequences with high performance gradients is accurate as conventional spin-echo imaging in diagnosing meniscal tears (Kowalchuk, 2000). The following parameters are used: a TR of 1500 ms and an effective TE of 20 m, with K space centered on the second echo at 2X minimal interecho spacing and a length of 4.

Normal anatomy on MRIs

Structures in the sagittal plane

Centrally, the normal meniscus is composed of 2 separate triangular structures: the anterior horn and the posterior horn. The apices (free edges or inner margins) appear as sharp points of the triangle facing each other (Thornton, 2000) (see Image 34). On the lateral side of the knee, the triangular anterior and posterior horns of the LM are equal in size (see Image 16). On the medial side of the knee, the posterior horn of the MM is larger than the anterior horn (see Image 17).

Peripherally (medially for the MM and laterally for the LM), the menisci have a bow-tie configuration (see Images 1, 8, 16, and 20) (Thornton, 2000). The anterior and posterior horns are taller than the thinner and interposed body of the meniscus. Both menisci have anterior and posterior roots, which attach the anterior and posterior horns to the tibial plateau, on either side of the centrally placed tibial spine (see Image 35) (Shankman, 1997). These attachments are referred to as roots.

Popliteus tendon and sheath (see Images 8-10)

The popliteus tendon and its accompanying sheath course through the posterolateral portion of the posterior horn of the LM in an oblique anterosuperior to posteroinferior direction. It is seen on the more lateral images of the LM.

Two fascicles connect the posterior horn of the LM at the popliteus tendon sheath level to the joint capsule. The inferior fascicle is seen on the more lateral images through the tendon. Here, the superior fascicle is absent. More medially, both superior and inferior fascicles are present. The most medical images through the tendon show the superior fascicle and absence of the inferior fascicle. The thickness of the popliteus tendon sheath varies in size from a thin line to a thick band.

Structures in the coronal plane

This is the best plane in which to image the meniscal bodies (Quinn, 1988). Each meniscal body looks like a triangle with the pointed apex in the innermost part of the meniscus (see Image 13). The anterior and posterior horns appear as flat slabs (Thornton, 2000). The root of the posterior horn of the LM is directed obliquely upward from a lateral to medial direction (see Images 2, 5-6, 34, and 54) (De Smet, 1994; Peterfy, 1994; Thornton, 2000). The popliteus recess is located in the outer portion of the lateral joint compartment. It can be identified either by the presence of joint fluid within it or by the popliteus tendon originating from the distal lateral femur, above the joint, and passing through the sheath toinsertonthe back of the proximal tibia (see Images 5-6).

The insertion of the semimembranosus tendon is located posterior along the subarticular surface of the medial aspect of the proximal tibial metaphysis (see Images 37 and 43). This is not to be confused with a displaced meniscal fragment.

Meniscal flounce

A meniscal flounce is an uncommon meniscal variant characterized by a single symmetrical fold along the free edge of the meniscus (Boeree, 1991; Zarins, 1984). It appears as an S-shaped fold along the free edge on sagittal images and is associated with a truncated but normal meniscus on coronal images.

Normal meniscal signal intensity

The normal meniscus shows uniform, low signal intensity on T1- and T2-weighted images obtained with both conventional and FSE sequences (see Image 6). The low signal is related to a lack of mobile protons in the meniscal fibrocartilage. Subsequent dephasing of hydrogen nuclei results in T2 shortening, contributing to the low signal intensity on all pulse sequences.

Fascicles of the posterior horn of the LM are best evaluated on T2-weighted sagittal images. This is due to the bright fluid in the popliteus tendon sheath and joint space contrasting with the low signal intensity of the fascicles (Crues, 1990).

Discoid meniscus

Differentiation between a true DM and a slightly larger but normal meniscus may be difficult (Watanabe, 1969; Mink, 1987).

On sagittal images, the DM has a thickened, bow-tie appearance on 3 consecutive sagittal images (Howe, 1988; Silverman, 1989; Watanabe, 1969). The anterior and posterior horns of the normal meniscus are seen on several images near the intercondylar notch. With a complete DM, no distinct anterior or posterior horn is present (Stark, 1995). The normal meniscus rapidly tapers from the outer periphery to the center. The presence of equal or nearly equal meniscal height on 2 adjacent peripheral 5-mm-thick images indicates a DM (Howe, 1988). The anterior and posterior horns of the LM are normally equal in height. An asymmetric discoid LM may have an abnormally large anterior or posterior horn (Stoller, 1997).

On coronal views, the abnormal meniscal body extends more medially toward the intercondylar notch (see Image 41).

The posteromedial horn of the MM and the anterior horn of the MM near the roots may have a normal speckled appearance (see Images 34, 44, and 46).

Coronal images show the smallest width of the meniscal body, making this plane the most sensitive for showing meniscal enlargement. An asymmetric DM with an enlarged body may have a wide meniscal body on coronal images but normal anterior and posterior horns on sagittal images (Watanabe, 1969), emphasizing the need for coronal images. Incomplete DM may not extend into the intercondylar notch.

In children, grade 2 signal is frequently seen within the posterior meniscal horns. This is thought to represent normal vasculature, seen in the meniscus of a child. This disappears in adulthood (Quinn, 1988).

Regarding the meniscofemoral ligaments, either the anterior or posterior ligament is present on 33% of MRIs (Stoller, 1997). Both ligaments are present on 3% of examinations (Stoller, 1997). One of the 2 ligaments predominates. The ligament of Humphry is best seen on sagittal images. It is occasionally seen on coronal images. The ligament of Wrisberg is best seen on posterior coronal images.

Meniscal degeneration

Local increases in the degree of freedom of trapped water molecules within the substance of the meniscus occurs with age, resulting in increased T2 times. The appearance is that of increased signal intensity within the substance of the meniscus on short-TE images (Crues, 1993).

MRI grading system for meniscal degeneration

An MRI grading system has been developed and correlated with a histologic model (Stoller, 1987). Regions of degenerative show increased signal intensity in a spectrum of patterns or grades based on the distribution (morphology) of signal intensity relative to an articular surface of the meniscus. This basis is exclusive of the peripheral capsular margin of the meniscus, which is considered nonarticular (Stoller, 1997).

Grade 1

Grade 1 is a nonarticular, focal or diffuse region of increased signal intensity within the substance of the meniscus (see Image 16). This finding is correlated with early meniscal degeneration and chondrocyte-deficient or hypocellular region. The terms mucinous, myxoid, and hyaline degeneration are used interchangeably to describe the production and accumulation of an increased amount of mucopolysaccharide ground substance in stressed areas of the fibrocartilage of the meniscus. Such changes are a response to repetitive mechanical loading.

This appearance is found in healthy volunteers and asymptomatic athletes and not clinically significant.

Grade 2

Grade 2 is a horizontal, linear area of increased signal intensity within the substance of the meniscus that extends to but does not involve the (see Image 21). Such regions of abnormal signal are more extensive than in grade 1 degeneration, and no distinct cleavage plane or tear is present. Grade 2 is a continuation of progressive degeneration from grade 1 changes. Patients are usually asymptomatic.

Histologically, there is microscopic collagen fragmentation and clefting within the hypercellular region of the fibrocartilaginous matrix. The middle perforating collagen bundle, which divides the meniscus into superior and inferior halves, is the site of preferential accumulation of mucinous ground substance. It also represents the shear plane of the meniscus and is also the site of origin of horizontal degenerative meniscal tears.

The posterior horn of the MM is the most common location. It also is the most common site for grade 3 meniscal tears. The presence of grade 2 signal-intensity changes is not predictive of future progression to grade 3 meniscal tears. Grade 2 represents a point of potential structured weakening. Grade 3 tears, when they develop, are adjacent to or are in continuity with areas of grade 2 changes.

Grade 2C is a subcategory in which linear signal intensity extends to the articular surface on a single image. When found in symptomatic patients, about 50% have a tear. There are no additional features that can discriminate a torn meniscus from an intact meniscus with grade 2C signal intensity (Dillon, 1991; Lee, 2000). This is not a very common occurrence, appearing in only 3% of patients in one study (McCauley, 2002). Most patients with grade 2C signal are not treated with arthroscopy because they do not have symptoms referable to the site of abnormality, but about 50% of patients with grade 2C signal and knee symptoms have meniscal tears. Grade 2C signal might represent more extensive degeneration than seen with Grade 2 signal and can progress to degenerative tears.

Grade 3

Grade 3 is a region of abnormal signal intensity within the meniscus extending to and communicating with at least 1 articular surface of the meniscus (see Image 21). Multiple foci of grade 3 signal-intensity changes may be present in 1 meniscus.

About 5% of grade 3 tears are actually intrasubstance cleavage tears. These are closed meniscal tears and diagnosed only with surgical probing of the meniscus. They may be missed on routine arthroscopy if surface extension is not identified (Crues, 1993).

False-negative correlations with arthroscopy have been described (Kaplan, 1991; DeSmet, 1993). This commonly occurs in the LM, either peripheral or posterior when an associated ACL tear is present (DeSmet, 1994; Antonio, 2004). The improved spatial resolution and signal-to-noise ratio achieved with a surface coil may improve diagnostic accuracy in this situation and be related to spurious interpretation of areas of fraying as meniscal tears. Such lesions may present with pain related to edema and ingrowth of the synovium within the tear because the meniscus is an enervated structure.

MRI criteria for meniscal tears

Two MRI criteria have been established for diagnosing meniscal tears. If prior surgery has not been performed on the meniscus, the accuracy in diagnosing tears is 90% (Rubin, 2000).

Criterion 1

Criterion 1 is increased internal signal intensity in the meniscus (see Images 10, 22-25, 32-33, 38-39, 41-42) (Rubin, 2000; Crues, 1987). A recent article discussed the concept of the "Two-Slice-Touch Rule" (DeSmet,2006). The authors describe a positive predictive value of 94% for meniscal tears for the medial meniscus and96% for the lateral meniscus when the tear is present on two consecutive images. The postitve predictive value was 55% and 36% for medial and lateral meniscal tears when seen only on one slice.

The abnormal signal intensity must be in contact with one articular surface, either the superior or interior surface or at the tip (free edge) of the meniscus (see Images 10, 22-25, 32-33, 38-39, 41-42) (Rubin, 2000). If the contact with the articular surface appears on 2 or more consecutive images, the accuracy of the diagnosis of meniscal tear increases (see Images 50-51) (Rubin, 2000).

The need for short TEs is important. Most other tissue disorders are characterized by an increase in free water and unbound protons. In meniscal tears, hydrogen nuclei are bound to macromolecules (Bessette, 1992; Crues, 1990). The bound protons have a shorter T2 relaxation than do protons in free water (Rubin, 2000). Meniscal tears also result in the absorption of synovial fluid in the margins of the tear. This may be related to the loss of the normal tight collagen spiral, resulting in an increased mobility of water molecules. Water molecules are trapped, increasing the local spin density. The increased meniscal signal within the tear probably results from an increase in the local spin density and not from an increase in the T2 signal (Rubin, 2000; Crues, 1990; Mink, 1988).

The rate of proton rotation is shortened by the interaction of synovial fluid and large macromolecules, resulting in shortening of T1 and T2 values, increasing the sensitivity of PD-weighted images in revealing meniscal pathology. Such changes cause a local increase in the degree of freedom of trapped water molecules, resulting in increased T2 times, allowing the detection of increased signal with short-TE sequences (Crues, 1993). Increased signal within the meniscus is best seen on short-TE images obtained by using PD-weighted or GRE sequences.

Although most meniscal tears are well seen on PD-weighted images, they are not visualized as well on T2-weighted images, unless there is a wide cleft at the site of the meniscal tear that freely communicates with joint fluid. If such a situation is present, confidence in diagnosing a meniscal tear is high. However, this finding is not common.

Criterion 2

Criterion 2 is an abnormal meniscal shape in the (see Images 22-33, 38-39, 42, 48-51, 60-62). Comprehensive knowledge of the normal MRI anatomy of the menisci is required.

Meniscal tears are more confidently diagnosed when they are seen on both sagittal and coronal images. The presence of a meniscal tear on both these views decreases the rate of false-positive diagnoses (Heron, 1992). However, some tears at the meniscocapsular junction can be seen only on 1 of these views.

In writing the MRI report about a meniscal tear, the radiologist should understand the use of standard nomenclature for meniscal tears and describe the location, plane, shape, completeness, length, and number of tears (Carrino, 2002).

Planes, shapes, lengths, and thicknesses of meniscal tears

Multiple cross-sectional representations of a meniscal tear have to be translated into a 3-dimensional (3D) description for the benefit of the arthroscopist (Rubin, 2000).

Meniscal tears occur in 2 primary planes: vertical (see Images 22-23, 38-39, and 42) and horizontal (see Images 24 and 33).

The 3 basic meniscal tear shapes are longitudinal (see Image 38-39, 42, and 67), radial (see Images 22-23, and 68), and horizontal (see Image 24, 33, 43, 45, 63, and 69). They occur as traumatic lesions in younger patients (Dandy, 1990).

Vertical tears are aligned perpendicular to the coronal plane of the meniscus and can be subdivided into longitudinal (see Images 38-39 and 42) and radial tears (see Images 22-23). They occur as traumatic lesions in younger patients (Baxamusa, 2001).

Longitudinal tears separate the meniscus into inner and outer fragments and occur parallel to the outer margin of the meniscus (are perpendicular to the tibial plateau and propagate parallel to the circumferential axis of the meniscus) (see Image 67). These tears are equidistant from the outer (peripheral) meniscal margin throughout their entire course (Dandy, 1990; Rubin, 1997; Thornton, 2000). Longitudinal tears are more commonly traumatic in etiology and occur in younger and more physically active patients (Dandy, 1990; Noble, 1980). They are also commonly found in patients who have also acutely torn their ACL. Such meniscal tears are located in the posterior horn of the LM, central to the popliteus tendon (Thornton, 2000).

In addition, these tears do not involve the free-edge (the inner part) of the meniscus on any image (Rubin, 2000). They are often located in the middle or outer third of the meniscus (see Image 38) and usually begin in the posterior horn (Dandy, 1990).

Short tears, or those confined to the posterior horn, may be visible only on sagittal images. Longer tears propagate into the body of the meniscus. These are seen on both sagittal and coronal images.

Meniscal tears are either partial thickness (see Image 25, 29, 31, and 38) or full thickness (see Images 24, 38, 39,and 44). A partial thickness longitudinal tear contacts only the superior or inferior articular meniscal surface, but not both (see Images 21-22, 30, 38-39).

A full thickness vertical tear contacts both the superior and inferior articular meniscal surfaces, completely dividing the torn part of the meniscus into an inner and outer portion (see Images 19-20, 29, 38-39). Such tears can lead to the development of bucket-handle tears (Thornton, 2000).

Bucket-handle tears

These tears are displaced vertical longitudinal tears and usually involve the MM (see Images 38-39, 42, and 52). The separated central (inner) fragment, when viewed axially, resembles the handle of a bucket (Carrino, 2002). The remaining larger peripheral portion of the meniscus resembles the bucket. These tears account for about 10% of all meniscal tears (Helms, 2002; Wright, 1995).

Central or anterior movement of the inner portion of a longitudinal tear results in a bucket-handle tear (Rubin, 1997; Weiss, 1991). The central fragment in a displaced bucket-handle tear may be partially displaced into the intercondylar notch, inferior and anterior to the PCL (Weiss, 1991). Partial displacement is seen with shorter tears. The central fragment is often well visualized on coronal images and poorly visualized on sagittal images (Crues, 1993). Central fragments of bucket-handle tears of the posterior horn of the LM are often displaced anteriorly so that the torn and displaced fragment lies on top of the anterior horn of the LM. This occurs because the ACL prevents the meniscal fragment from completely migrating into the intercondylar notch. In this situation, the "height" of the anterior horn of the LM is almost twice its normal height. This is best seen on sagittal views.

An absent bow-tie sign is helpful for diagnosis of bucket handle tears of the meniscal body (see Image 52). The normal body of the meniscus is 9-12 mm in width and should be seen on 2 consecutive sagittal images and, as described in normal MRI anatomy, has the shape of a bow tie. When a bucket handle tear is present, part of the free edge of the meniscus is missing (Major, 1998). The inner portion of the meniscal body will be absent. Confirmation almost always occurs in the form of a displaced meniscal fragment that is visualized elsewhere in the knee joint.

When no displaced fragments are found, an absent bow-tie sign may be related to a normal but small meniscus. In this situation, both the medial and LM are small; bucket handle tears of both the medial and LM, occurring at the same time, are rare. Another cause of an absent bow-tie sign is a normal postoperative meniscus in patients who have undergone PM (Major, 1998).

These injuries may be further classified as single, vertical longitudinal tears, displaced bucket-handle tears, broken bucket-handle tears, and double and triple vertical longitudinal bucket-handle tears. They are 3 times more frequent in the MM when compared with the LM and may be associated with ACL tears (Rosenberg, 1993). Bucket-handle tears are commonly seen in young adults with a history of locking, extension block, or slipping of the joint due to displacement of the central fragment toward the intercondylar notch (Shakespeare, 1983).

Radial tears (transverse tears)

These are vertical tears and propagate perpendicular to the main axis of the meniscus (see Images 10, 22-23, 32, and 68). These injuries are devastating because a full thickness tear destroys meniscal integrity, ie, the ability of the meniscus to distribute hoop stress. Hoop stress is the normal outward force generated in the meniscus in all directions as a result of weight bearing.

The force is distributed in the meniscus by collagen fibers, located around the circumference of the meniscus from apex to periphery. The collagen preserves the normal shape and integrity of the meniscus in weight bearing. Radial tears transect these fibers. The meniscus is normally attached to the tibia at the anterior and posterior ends. During weight bearing, the meniscocapsular attachments pull the meniscus outward. A radial tear occurring between the tibial attachment points causes the free, unattached edges of the meniscus at the point of the tear to temporarily pull outward, expanding the width of the tear, exposing a bare spot on the adjacent tibia and femur, allowing abnormal stresses on the unprotected articular cartilage and bony surfaces, and resulting in articular cartilage destruction and subsequent bone alteration, leading to accelerated degenerative disease.

A complete radial tear extends all the way through the meniscus from the apex to the periphery (see Image 23). When it involves the meniscal body, the meniscus is split into an anterior and posterior fragment (Thornton, 2000) (see Image 29). The middle third of the LM is a common location (Stoller, 1997). This injury begins at the free edge (inner margin) and extends a variable distance toward the periphery.

Small tears may be difficult to recognize on MRIs. Missed radial tears constitute a large proportion of errors made in image interpretation of meniscal pathology. The key feature of recognition is that they involve the free edge of the meniscus (Tuckman, 1994). Thus, the inner point of the meniscal triangle is absent or blunted on 1 or more images (Tyrell, 1988). Radial tears of the meniscal body are best seen on sagittal images (see Image 32). They disrupt the normal bow-tie configuration of the meniscus on 1 or more images (Tuckman, 1994). Radial tears of the anterior and posterior horns are best seen on coronal images (see Images 10, 22-23).

Like longitudinal tears, radial tears are commonly traumatic and occur in younger, more physically active patients (Dandy, 1990; Noble, 1980). Tears in the MM usually occur in the posterior horn and are more common in older patients (see Images 10, 22-23), (Stoller, 1997). Small tears of 3 mm or shorter may be asymptomatic (Smith, 1990).

Tears located near the posterior horn of the LM are associated with ACL tears. They may be associated with more complex meniscal tears, such as vertical longitudinal tears or peripheral horizontal cleavage tears (Stoller, 1997). They are devastating because the circumferential fibers are disrupted. The meniscus is prevented from developing the necessary hoop stress that normally helps dissipate forces across the knee.

It is common to see a radial tear as one component of a complex tear (see Image 22) (Tuckman, 1994).

Oblique (parrot beak) tears or flap tears are a form of radial tears. They begin at the free (inner) edge, like other radial tears, but then curve into a longitudinal orientation, similar to longitudinal meniscal tears, as the tear extends toward the meniscal periphery (see Image 70) (Tuckman, 1994). As the tear is traced on sequential images, it moves closer to the outer portion of the meniscus and then remains equidistant from the outer meniscal margin on subsequent images, as seen on longitudinal tears (Rubin, 2000). Oblique tears are the most common meniscal tears (Stoller, 1997).

A high prevalence of radial tears is present in postoperative patients who have had partial meniscectomy. Magee et al indicated a 32% prevalence when viewing 100 postoperative MRIs. This may be related to altered biomechanics of knee function after partial meniscus removal. Stresses may be redistributed that predispose to radial tears.

Horizontal tears

These tears are also called cleavage or fish-mouth tears. They divide the meniscal tear into a top (superior) portion and a bottom (inferior) portion (see Images 44-45, 49, 63, and 69-70). They usually begin on the undersurface of the meniscus (see Images 43 and 63) (Thornton, 2000).

Although horizontal tears may appear to extend deep into the meniscal substance on MRIs, the tears, as seen at arthroscopy, may extend only a few mm into the meniscus from the point where the abnormal signal contacts the meniscal surface. When it extends to the periphery of the meniscus, the cleft between the fragments can allow the egress of joint fluid to the meniscosynovial border, where it may become trapped, forming a meniscal cyst (see Images 47 and 59) (Burke, 1998). Most are degenerative, occurring in older patients with osteoarthritis.

Miscellaneous tears

Other tears shapes may be thought of as combinations of these 3 basic patterns (see Image 22) (Carmichael, 1997). Multiple tears can present simultaneously in a meniscus, involving different portions of or the same region. A single meniscal tear, containing a combination of longitudinal, radial or horizontal cleavage planes, is called a complex tear (see Images 24 and 33) (Dandy, 1990). A common type of complex tear is composed of horizontal and radial components (see Image 22). Nearly all of these tears are degenerative in origin (Rubin, 2000).

Fraying or fibrillation of the free edge of the meniscus is seen as an area of increased signal intensity at the apex of a normally shaped meniscus. If abnormal morphology (truncation and foreshortening) is present, a tear is likely (Stoller, 1997). Fraying can occur anywhere along the meniscal surface (see Image 41).

Indirect signs of a LM tear (De Smet, 2001) include an abnormal or absent superior popliteal-meniscal fascicle and the presence of pericapsular edema, posterolaterally, is associated with a tear to the posterior horn of the LM. However, caution is needed in this setting. Johnson and De Smet reported that the superior popliteal-meniscal fascicle was not seen in 2 of 66 patients with intact menisci. De Smet and Asinger showed that 2 of 13 patients with posterior pericapsular edema did not have LM tears.

Displaced meniscal tears

A fragment of a torn meniscus, partially attached to the meniscus proper and migrating to any position within the joint; usually the intercondylar notch anterior and parallel to the PCL (see Images 46-47), anteriorly toward the infrapatellar fat pad, on top of the anterior horn (see Images 48-49), backward to lie above, below or behind the posterior meniscal horn (Wright, 1995), or superomedial to the medial femoral epicondyle or inferomedial to the tibial plateau (Lecas, 2000). Displaced meniscal fragments occur in 9-24% of meniscal tears. Any shape of a meniscal tear can result in a displaced fragment.

MRI diagnosis depends on visualizing the torn, deficient meniscus (Helms, 2002) or the displaced inner meniscal component (Ruff, 1998; Wright, 1995; Singson, 1991; Haramati, 1993; Magee, 1998).

Displaced meniscal fragments are often clinically significant lesions requiring surgery because of pain and knee locking.

Types of displaced meniscal tears

Bucket-handle tears are the most common pattern, occurring in 10% of meniscal tears. They result from vertical, longitudinal, or oblique tears. These tears often involve the entire meniscus but isolated tears of the anterior horn, posterior horn, or (more commonly) the posterior horn and body (Wright, 1995; Dandy, 1982). The most reliable sign is visualization of the displaced fragment. Typical locations of the displaced fragment include the intercondylar notch anterior and parallel to the PCL (double PCL sign) (see Images 46-47); and ventrally or horizontally juxtaposed to the anterior horn (see Images 48-49) (Stone, 1979; Weiss, 1991; Wright, 1995). An absent bow-tie sign indicates the meniscal body is absent. Any meniscalsegment, however, can be involved.

A flap tear is a short-segment horizontal meniscal tear with either superior or inferior displacement of the meniscal fragment (see Images 50-51) (Ruff, 1998). This type is less frequent; superior displacement is more common (Dandy, 1982).

Horizontal tears become displaced either by the top or bottom portion flipping over to lie above or below the remainder of the meniscus (Ruff, 1998) or by sliding toward the inner part of the knee. These tears usually involve the MM (Lecas, 2000).

Inferomedial displaced tears from the MM are uncommon. When the displaced fragment extends inferior and medial to the tibial plateau, deep to the MCL, it may go unnoticed by the arthroscopist because the meniscus surface may appear intact (see Image 58).

Inward displacement of the free edge of an oblique tear results in a displaced parrot-beak tear (Dandy, 1982; Rubin, 1997) and can precipitate mechanical symptoms, such as locking, catching and giving way. Displaced fragments can prevent closed reductions of knee dislocations (Baxamusa, 2001). The fragment can completely separate from the rest of the meniscus to become a free fragment.

A meniscal body appearing unusually small should prompt a careful search for a displaced fragment (Helms, 2002).

Signs and terminology used to describe the displaced meniscal fragment

Several signs may be present in the same patient (Chen, 2001).

An absent bow-tie sign may be identified on sagittal images and represents a torn meniscal body segment (see Image 52) (Helms, 2002). When present, it is indicative of a free meniscal fragment in 97% of patients (Chen, 2001).

Bucket-handle tears with centrally displaced fragments into the intercondylar notch may also be present (Wright, 1995; Simonsen, 1984). A double-PCL sign is seen in 39% of patients (Chen, 2001), and the notch-fragment sign is seen in 51% of patients (see Image 63) (Chen, 2001). In this setting, the fragment is adjacent to but not at the same level as the PCL on sagittal images. The fragment is slightly more medial in location. It originates from the MM. A disproportionate posterior horn sign is seen in 21% of patients (Chen, 2001). A larger central portion of the posterior meniscus, when compared with the smaller periphery of the same posterior meniscus, indicates a displaced meniscal fragment. The meniscal tear usually originates from the anterior horn (Chen, 2001; Dandy, 1982).

Bucket-handle tears with an anteriorly displaced meniscal fragment can be observed (see Images 48-49). The flipped-meniscus sign (Kowalchuk, 2000) is seen in 63% of patients (see Images 50-51) (Chen, 2001). The criterion is a tear or nonvisualization of the posterior meniscus with the maximum height of the anterior horn of the involved meniscus greater than 8 mm (see Images 48-49 and 53) (Haramati, 1993).

Pseudohypertrophy of the anterior horn of a meniscus occurs when an anterior meniscal horn appears abnormally large (see Images 48-49 and 53). The meniscal body or posterior horn is unusually small. This indicates a portion of the torn body or posterior horn has flipped anteriorly and lies behind the anterior horn (Wright, 1995; Haramati, 1993). The abnormal meniscal fragment and the adjacent normal meniscus are separated by joint fluid, which has increased signal intensity on T2-weighted images.

Differential diagnosis of displaced meniscal tears

The differential diagnosis includes the following: ligament of Humphry, loose bodies, osteophytes, and fracture fragments.

Stable versus unstable tears

The stability of tears is determined by a number of factors, including the length, location, and completeness of the tear (Newman, 1993; Weiss, 1989). Probing the meniscal tear during arthroscopy is critical for determining stability (DeHaven, 1981; Newman, 1993; Weiss, 1989; Hanks, 1991).

A stable vertical longitudinal tear occurs when the central (inner) fragment of a meniscal tear cannot be displaced more than 3 mm from the intact meniscal periphery (Hanks, 1991). Any meniscal tear with a displaced fragment is unstable.

Longitudinal tears that are relatively long are unstable (Carpenter, 1996); their length is assessed on multiple 3- to 4-mm sections in either plane and they extend through the full thickness of the meniscus or contain fluid on T2-weighted images.

Some meniscal tears do not show a free fragment at the time of MRI. Whenever, however, the inner margin of