Meniscus and Articular Cartilage Injuries

Anatomy

The gross shape of the meniscus can be divided into two parts: the medial meniscus and the lateral meniscus. The medial meniscus is C-shaped with a triangular cross section, whereas the lateral meniscus is more circular and covers more of the articular cartilage surface area. The medial and lateral menisci are connected by the transverse (intermeniscal) ligament anteriorly. While there are variants of the meniscal bony attachments, the menisci are connected to the tibia peripherally by the coronary ligaments and posteriorly to the femur by the meniscofemoral ligaments. The anterior meniscofemoral ligament is often referred to as the ligament of Humphrey and runs from the posterior horn of the lateral meniscus anterior to the posterior cruciate ligament (PCL) and inserts on the femur. The posterior meniscofemoral ligament is referred to as the ligament of Wrisberg. This ligament attaches to the posterior aspect of the lateral meniscus and crosses the PCL superiorly and medially to insert onto the medial femoral condyle.

The anatomy of the meniscus allows it to perform its two primary roles: modulating force transmission and acting as a secondary knee stabilizer. Owing to the elastic nature of the meniscus, it allows for more shock absorption than articular cartilage alone. The shape of the meniscus allows for increased contact area with the femoral condyles, thus increasing the congruency at the interface of the femur, meniscus, and tibia. Additionally, the shape of meniscus allows it to functionally deepen the tibial surface, which provides stability. The posterior horn of the medial meniscus acts as the main secondary stabilizer to anterior translation. The lateral meniscus is more mobile. Therefore it confers less stability than the medial meniscus, but in the setting of an ACL-deficient knee, the menisci become the primary knee stabilizers.

Composition

The meniscus is composed of fibroelastic cartilage, which contains collagen, proteoglycans, glycoproteins, and cells. Water comprises 65%–75% of the total meniscal contents and collagen constitutes the majority (60%–70%) of the dry weight of the meniscus. The majority of collagen in the meniscus is type I collagen. The meniscus also contains cells called fibrochondrocytes, which help synthesize the fibrocartilaginous matrix. An important point regarding the composition and structure of the meniscus is the orientation of its fibers. The meniscus contains both radially and longitudinally oriented fibers, which allow the meniscus to expand under compression and this is important for its functions of increasing contact area and providing shock absorption.

Blood supply and healing potential

The blood supply to the meniscus is by the middle genicular artery, which supplies the posterior horns; the medial inferior genicular artery, which supplies the peripheral 10%–30% of the medial meniscus; and the lateral inferior genicular artery, which supplies the peripheral 10%–25% of the lateral meniscus. In general, the innate healing potential of the meniscus is limited, largely because of its limited blood supply. Peripheral meniscal tears are considered to have healing potential because of their proximity to the vascular supply. It is believed that tears in the peripheral 25% of the meniscus can heal via fibrocartilage formation but that tears in the central 75% of the meniscus likely have limited to no intrinsic healing ability. Therefore identifying the location of meniscal tears is crucial, as the location of the tear is very important in determining the treatment plan.

Epidemiology

Meniscal tears are very common and are one of the most frequent causes of knee pain in active patients. The mean annual incidence of meniscal tears is approximately 60 per 100,000. ^, The majority of meniscal tears affect the medial meniscus in the posterior horn, and meniscal tears are most common in the third, fourth, and fifth decades of life. Meniscal tears are more common in males than in females in a ratio of 3:1 and are commonly associated with ACL injuries. Isolated medial meniscal tears in females younger than 30 years and with stable knees are uncommon. Injuries that are often concomitant with meniscal tears and should therefore raise the index of suspicion for a meniscal injury include tibial plateau fractures, femoral shaft fractures, and the presence of a hemarthrosis. ^,

Classification/types of meniscal injury

There are several methods for classifying meniscal tears. Important factors to consider when evaluating meniscal tears include chronicity, location, pattern, and size of the tear. Chronicity (acute vs. degenerative) can be determined from a combination of clinical history and imaging and is a very important factor in determining the treatment plan. Location of the meniscal tear is also important for decision-making regarding management. The “red-red zone” of the meniscus is often referred to as the outer third, which is considered to be the region of the meniscus that has adequate blood supply and more intrinsic healing potential. The “red-white zone” refers to the middle third of the meniscus, and the “white-white zone” refers to the inner third of the meniscus. These areas do not have a direct blood supply and therefore their healing potential is limited. The pattern of meniscal tears is also variable and may include vertical/longitudinal, bucket handle, oblique/parrot beak, radial, horizontal, complex, and/or root tears ( Fig. 6.1 ). Finally, the size of the tear may be difficult to quantify, but it is also important in determining treatment. While no strict guidelines exist, understanding the percentage of the meniscus involved in the tear can be crucial to determine whether the patient needs a meniscectomy (partial removal of the torn meniscal segment) or a meniscal repair.

Meniscal cysts

Meniscal cysts are also common and represent 1%–10% of meniscal pathology. While patients may present with an isolated meniscal cyst, meniscal cysts are very commonly associated with meniscal tears. Patients may present with symptoms similar to those found with meniscal tears or meniscal cysts may be encountered incidentally on a magnetic resonance imaging (MRI). Meniscal cysts can be perimeniscal (within the meniscus itself) or parameniscal (extruded fluid outside the meniscus). Typically, cysts form because the meniscal tear functions as a one-way valve, and synovial fluid is extruded and forms a discrete collection. Symptomatic cysts are more common laterally.

Discoid meniscus

A discoid meniscus is a congenital abnormality in which the meniscus does not have its typical anatomic shape. The incidence of discoid meniscus is 3.5%–5% of the general population. It typically involves the lateral meniscus and can be bilateral in 25% of cases. Although not always symptomatic, the presence of a discoid meniscus often presents in adolescence with symptoms including pain, clicking, and mechanical locking. In a retrospective review of adolescent patients undergoing arthroscopy for isolated lateral meniscal pathology, 75% had a discoid meniscus. However, if patients are not symptomatic and the presence of a discoid meniscus is found only incidentally, surgical intervention is not recommended.

Presentation

The clinical presentation of patients with isolated meniscal tears may be variable. If the tear is isolated to the medial or lateral aspect of the meniscus, there may be pain, which localizes to the medial or lateral aspect of the knee. A careful history should be taken to elicit this. Additionally, patients should be asked whether or not they have mechanical symptoms, such as locking, catching, or clicking. They may also report delayed or intermittent knee swelling. Patients may or may not report a history of acute injury, but patients should always be asked if there were any inciting events that they can identify, as well as the mechanism of the injury. Typically, acute meniscal tears occur due to a twisting or hyperflexion injury and may present with acute pain and swelling subsequently. Degenerative tears, on the other hand, may occur in older patients who may report an atraumatic history of chronic pain and joint swelling.

Physical examination

A careful and thorough physical examination should be performed for any patient presenting with knee pain. To begin with, general alignment and gait should be observed and documented. Any swelling or joint effusion should be noted, and range of motion should be tested. It should be noted if there are mechanical blocks to motion at this time. Thorough palpation should be performed, and joint line tenderness is the single most sensitive physical examination finding for a meniscal tear.

There are several provocative tests that can be used to help identify a meniscal injury on physical examination. The McMurray test consists of flexing the knee, placing one hand on the joint line and the other holding the foot. The leg should be rotated as it is brought from flexion to extension. A palpable click with associated pain is considered a positive test result. The Thessaly test consists of having the patient stand with the knee at 20 degrees of flexion and twisting the knee into internal and external rotation. Any clicking or discomfort elicited is considered a positive test result. Lastly, the Apley compression test involves having the patient lie prone with the knee flexed. The examiner then applies an axial load and internally and externally rotates the tibia, again in an attempt to elicit any discomfort or a click in the knee. Although these tests may be helpful, they each have low sensitivity, specificity, and diagnostic accuracy, and therefore, advanced imaging is still recommended if the clinical suspicion for a meniscal tear is high. Finally, additional ligamentous testing and a thorough neurovascular examination should be performed to rule out any concomitant injury. Other diagnoses that may be confused with a meniscal tear include symptomatic plica, fat pad impingement, chondral lesions, and synovitis.