Definition

The menisci serve important roles in maintaining proper joint health, stability, and function. The anatomy of the medial and lateral menisci helps explain functional biomechanics. Viewed from above, the medial meniscus appears C -shaped and the lateral meniscus appears O -shaped ( Fig. 72.1 ). Each meniscus is thick and convex at its periphery (the horns), but becomes thin and concave at its center. This contouring serves to provide a larger area for the rounded femoral condyles and the relatively flat tibia. Menisci do not move in isolation. They are connected by ligaments to each other anteriorly and to the anterior cruciate ligament, the patella, the femur, and the tibia.

Superior view of medial and lateral menisci.

The medial meniscus is less mobile than the lateral meniscus. This is due to its firm connections to the knee joint capsule and the medial collateral ligament. This decreased mobility, in conjunction with the fact that the medial meniscus is wider posteriorly, is cited as the usual reason for the higher incidence of tears within the medial meniscus than within the lateral meniscus. The semimembranosus muscle (through attachments from the joint capsule) helps retract the medial meniscus posteriorly, serving to avoid entrapment and injury to the medial meniscus as the knee is flexed. The lateral meniscus is not as adherent to the joint capsule. Unlike the medial meniscus, the lateral meniscus does not attach to its respective collateral ligament. The posterolateral aspect of the lateral meniscus is separated from the capsule by the popliteus tendon. Therefore the lateral meniscus is more mobile than the medial meniscus. The attachment of the popliteus tendon to the posterolateral meniscus ensures dynamic retraction of the lateral meniscus when the knee internally rotates to return out of the screw-home mechanism, as one proceeds into flexion out of a fully extended and locked knee. Therefore both the medial and the lateral menisci, by having attachments to muscle structures, share a common mechanism that helps avoid injury.

The architecture of the vascular supply to the meniscus has important implications for healing. Capillaries penetrate the menisci from the periphery to provide nourishment. After 18 months of age, as weight bearing increases, the blood supply to the central part of the menisci recedes. In fact, research has shown that eventually only the peripheral 10% to 30% of the menisci, or the red zone, receives this capillary network ( Fig. 72.2 ). Therefore the central and internal portion, or white zone, of these fibrocartilaginous structures becomes avascular with age, relying on nutrition received through diffusion from the synovial fluid. Because of this vascular arrangement, the peripheral meniscus is more likely to heal than are the central and posterolateral aspects.

Vascular zones of the meniscus. Tears within the red zone have a higher healing potential.

The primary, but not sole function, of the menisci is to distribute forces across the knee joint and to enhance stability. Multiple studies have shown that the ability of the joint to transmit loads is significantly reduced if the meniscus is partially or wholly removed. There was a seminal article published in 1948 suggesting that the menisci are vital in protecting the articular surfaces. It reported that individuals who had undergone total meniscectomies demonstrated premature osteoarthritis.

Meniscal tears are classified by their complexity, plane of rupture, direction, location, and overall shape. Tears are commonly defined as vertical, horizontal, longitudinal, or oblique in relation to the tibial surface ( Fig. 72.3 ). Most meniscal tears in young patients will be vertical-longitudinal, whereas horizontal cleavage tears are more commonly found in older patients. The bucket-handle tear is the most common type of vertical (or longitudinal) tear ( Fig. 72.4 ). Tears are also described as complete, full-thickness, or partial tears. Complete, full-thickness tears are so named as they extend from the tibial to femoral surfaces. In addition, medial meniscus tears outnumber lateral meniscus tears from 2:1 to 5:1.

Types of meniscal tears.

Bucket-handle type of meniscal tear.

Meniscal injuries may result from an acute injury or from gradual degeneration with aging. Vertical tears (e.g., bucket-handle tears) tend to occur acutely in individuals 20 to 30 years of age and are usually located in the posterior two thirds of the meniscus. Sports commonly associated with meniscal injuries are soccer, football, basketball, baseball, wrestling, skiing, rugby, and lacrosse. Injury commonly occurs when an axial load is transmitted through a flexed or extended knee that is simultaneously rotating. Degenerative tears, in contrast, are usually horizontal and are seen in older individuals with concomitant degenerative joint changes.

On the basis of arthroscopic examination, the majority of acute peripheral meniscal injuries are associated with some degree of occult anterior cruciate ligament laxity. In addition, true anterior cruciate ligament tears are associated with lesions of the posterior horns of the menisci. Lateral meniscal tears appear to occur with more frequency with acute anterior cruciate ligament injuries, whereas medial meniscal tears have a higher incidence with chronic anterior cruciate ligament injuries. With chronic anterior cruciate ligament injuries, the medial meniscus may be more frequently damaged because its posterior horn serves as an important secondary stabilizer of anterior-posterior instability. Finally, meniscal architecture appears to be mostly unchanged between male and female knees with the notable exception of larger average volumes in male menisci. Meniscal degenerative patterns do vary between the genders, however, with males tending to preferentially wear on the medial side and females the lateral. This has been theorized to be more associated with biomechanical differences imposed by the hip girdle and not intrinsic to the knee itself.

Symptoms

The history will help diagnose a meniscal injury 75% of the time. Young patients who experience meniscal tears will recall the mechanism of injury 80% to 90% of the time and may report a “pop” or a “snap” at the time of injury. Deep knee bending activities are often painful, and mechanical locking may be present in 30% of patients. Bucket-handle tears should be suspected in cases of mechanical locking with loss of full extension. If locking is reported approximately 1 day after the injury, this may be due to “pseudo-locking,” which results from hamstring contracture. Knee hemarthrosis may also occur acutely, especially if the vascularized, peripheral portion of the meniscus is involved. In fact, 20% of all acute traumatic knee hemarthroses are caused by isolated meniscal injury. More typically, however, knee swelling occurs approximately 1 day later as the meniscal tear causes mechanical irritation within the intra-articular space, creating a reactive effusion. Typically, this effusion is secondary to a lesion in the central portion of the meniscus.

In contrast, degenerative meniscal tears are not usually associated with a history of trauma. In fact, the mechanism of injury, which may not be reported by the patient, can be simple daily activities, such as rising from a chair and pivoting on a planted foot. Patients with degenerative tears often also report recurrent knee swelling, particularly after activity.