ANATOMY

Menisci are discs of fibrocartilage interposed between the femoral condyles and tibial plateau. The articular surface of the medial condyle of the tibia is oval, with the longer axis being in the anteroposterior plane (Fig. 6-1). The central portion of the articular surface is not covered by meniscus and represents the articular area. The lateral meniscus plateau is almost circular and less concave. The menisci cover the peripheral two thirds of the tibia, and serve to deepen its articular surface and stabilize the joint. The upper borders of the menisci are concave and the undersurfaces are flat. Peripherally, the menisci thicken and are attached to the synovium and capsule. Centrally, the border is thin and exists as a free edge.

FIGURE 6-1 Axial view of the tibial plateau showing both menisci. Note that the cruciate ligament’s insertion has been removed to allow better visualization (grey rectangle). Transverse ligament, dashed arrow; hiatus popliteus, arrow.

The medial meniscus is oval, 3.5cm in length, and wider posteriorly from its peripheral to its central aspects in the knee. Its anterior horn attaches to the tibial plateau just anterior to the attachment of the ACL. Some posterior fibers originating from the anterior horn cross the knee joint anteriorly, attaching to the lateral meniscus and forming the transverse ligament (see Fig. 6-1). Posteriorly, the meniscus is anchored to the intercondylar fossa of the tibia between the posterior cruciate ligament (PCL) and lateral meniscal attachment. Along its periphery, the medial meniscus is attached to the joint capsule and the deep medial collateral ligament (MCL). The medial meniscus has less mobility than the lateral meniscus, and moves almost 10 mm from extension to flexion.

The lateral meniscus is circular, covering a larger portion of the articular surface than its medial counterpart. It is the same width anteriorly and posteriorly. The anterior horn attaches near the intercondylar spine posterior to the attachment of the ACL, with which it partially blends (Fig. 6-2). The posterior horn attaches just posteriorly to the intercondylar spines, anterior to the attachment of the medial meniscus. The lateral meniscus does not attach to the lateral collateral ligament but is attached loosely to synovium along much of its length. The anterior (ligament of Humphrey) and posterior (ligament of Wrisberg) meniscofemoral ligaments run from the posterior horn of the lateral meniscus to anterior and posterior, respectively, to the PCL attachment on the medial femoral condyle. Moreover, the tendon of the popliteus muscle, on its way to insert into the lateral condyle, crosses the joint through a hole on the lateral meniscus called the hiatus popliteus (see Fig. 6-1). This area along the popliteal hiatus is described as the avascular region.

FIGURE 6-2 Oblique view of the tibia showing the insertion of the ACL. Note how the anterior horn of the lateral meniscus fibers blend with the ACL (arrow).

CLASSIFICATION

A meniscal tear may be classified by its location relative to its blood supply and vascular appearance. The peripheral and central surfaces can be clinically graded as white (relatively avascular) or red (vascular) at the time of arthroscopy. This classification is based on anatomic studies that have depicted a peripheral vascular zone.

A red-red tear is defined as a peripheral capsular detachment, and has the best prognosis for healing. Unfortunately, a significant portion of tears occur in the white-white zone, which lies in the central avascular portion of the meniscus. This area of the meniscus has minimal blood supply, and theoretically is unable to heal. The remainder exist as red-white tears. These are meniscal rim tears through the peripheral vascular zone of the meniscus. The central portion of this tear exists in the avascular zone, so theoretically, these lesions should have sufficient vascularity to heal by fibrovascular proliferation.

Conventional wisdom dictates that meniscal repairs be limited to the peripheral vascular area of the meniscus (i.e., red-red and red-white tears). Both experimental and clinical studies have suggested that white-white tears are incapable of healing, even in the presence of a surgical suturing. This has thus provided the rationale for partial meniscectomy. In an effort to extend the zone of repair more peripherally, techniques such as the creation of vascular access channels by trephination, synovial abrasion, and use of a fibrin clot have been developed.

In addition, meniscal tears can be classified depending on their stability (Box 6-1). A tear is considered unstable when it is more than half the length of the meniscus and subluxes under the femoral condyle when probed with a hook (Fig. 6-3A). This concept is especially important to help decide the treatment options—left alone, trephinated, resected, or repaired.

FIGURE 6-3 A, Unstable longitudinal vertical tear. Note how it subluxes under the femoral condyle when probed. B, Stable incomplete longitudinal vertical tear. Stability should be assessed continuously with the probe. C, Stable complete longitudinal vertical tear. Some tears although complete may also be stable.

Stable tears, which occur particularly in the posterior aspect of the meniscus and do not subluxate into the joint may be left alone (see Fig. 6-3B and C). Fitzgibbons and Shelbourne⁸ have found that of 189 patients whose stable meniscal tears of the posterior horn of the medial meniscus were left alone, no patient was symptomatic at 2.6 years following surgery. Also, a stable longitudinal tear of the lateral meniscus, posterior to the popliteus, may be left alone, even in association with a torn ACL.

Tears can be described depending on their morphology and based on their configuration. Under these criteria, tears could be vertical or horizontal, according to whether the line of the lesion goes from superior to inferior (vertical) or from inside to outside (horizontal), commonly called open book or fishmouth tear (Fig. 6-4). Moreover, tears can be described as longitudinal if the pattern is from anterior to posterior (Fig. 6-5; also see Fig. 6-3), or transverse, also called a radial or parrot beak tear (Fig. 6-6). Combinations of these four basic patterns make up the others types of tears; the oblique tear is vertical and radial, and the so-called bucket handle tear is a vertical longitudinal tear that is unstable and subluxes completely under the condyle (Fig. 6-7; also see Fig. 6-3A). Finally, the complex tear is a combination of all of these, usually in a degenerative setting, located in the posterior horn of the medial meniscus (Fig. 6-8).

FIGURE 6-4 Horizontal degenerative tear. These tears may reach the vascular zone (arrow).

FIGURE 6-5 Diagram showing the longitudinal tear.

FIGURE 6-6 Radial tear. This is also called a parrot beak or flap tear.

FIGURE 6-7 Bucket handle tear dislocated into the intercondylar notch.

FIGURE 6-8 Complex tear. These tears are usually located in the posterior horn of the medial meniscus and are associated with degeneration.

Longitudinal vertical tears usually occur in the young patient in association with an anterior ACL tear, and more frequently in the medial meniscus because of its lower mobility. Oblique tears tend to appear in between the medial and posterior thirds of the meniscus. They may cause mechanical symptoms of entrapment and pain caused by the tension on the meniscus–capsular junction.

Horizontal-shaped tears usually begin as intrasubstance degeneration in the middle of the meniscus, and migrate toward the free surface. Often, they also extend to the capsular junction and can cause the formation of a cyst. Because of the increasing size of the cyst over time, the patient may experience pain and tenderness. They usually appear more frequently in the lateral meniscus and are filled with a gel-like substance chemically similar to synovial fluid. These cysts have been reported to be 1% to 10% of meniscal pathology.⁹

Complex tears appear mainly in older patients and are usually associated with cartilage degeneration. They are considered to be part of the process of arthrosis and degenerative arthropathy. Because of their complex pattern, the fragments can be unstable and cause mechanical symptoms. The pain associated with these tears is an added element to the generalized inflammation involved in the osteoarthritic knee. The associated histologic pathology is myxoid degeneration, hyaline acellular degeneration, and dystrophic calcification.¹⁰

Timing is also important in decision making. Tears can be acute, with more likelihood of healing, or chronic, which are usually associated with complex patterns and degeneration. These usually require resection. The time frame to consider a tear to be chronic is generally 8 to 12 weeks after the lesion has occurred.

The cause or mechanism of the tear should be determined. If the tear is traumatic, which usually happens in a young active patient, and is diagnosed acutely, there is an improved chance of success. On the other hand, degenerative complex tears usually occur in older patients and are associated with arthrosis. Whether they are a cause or consequence of the osteoarthritic process is still unknown.

Finally, tears can be classified depending on the side—that is, the medial or lateral meniscus. Metcalf and colleagues¹¹ have observed that 69% of tears affected the medial meniscus, whereas the lateral was affected 24% of the time. In their review, both menisci were torn at the same time in 7% of patients. Moreover, 80% of the tears were vertical or oblique and affected the posterior medial part of the meniscus.

In summary, the most common meniscal tears are as follows:

Chronic, degenerative, horizontal, or complex in the medial meniscus (see Fig. 6-8)

Acute, traumatic, longitudinal, and vertical in the medial meniscus (see Fig. 6-3)

The degenerative horizontal type tear usually occurs insidiously in older patients, and generally requires resection. On the other hand, the traumatic vertical or longitudinal tear is seen more often in the younger patient, and potentially can be repaired.

PATIENT EVALUATION

Physical Examination

The patient is examined for signs of an effusion, loss of quadriceps bulk, and decreased range of motion. Tenderness to palpation along the medial or lateral joint line is among the most sensitive signs of a meniscal tear (Fig. 6-9A). Studies have shown it to be as sensitive as 74%, with a positive predictive value of 50%.¹² The collateral and cruciate ligaments need to be assessed to determine whether additional injury is present. In the setting of an ACL-deficient knee the sensitivity of joint line tenderness has been shown to decrease to approximately 50%.

FIGURE 6-9 A, Palpating for joint line tenderness. This is the most sensitive sign for meniscal tear. B, The McMurray test. As a mnemonic rule, the heel of the foot points toward the injured meniscus. The test is considered positive when the patient feels pain in the appropriate joint line, accompanied by a thud or click.

Special tests for assessing the meniscus, such as the McMurray, Steinmann, and Apley tests, are not conclusive but can aid in the diagnosis. The aim of all these maneuvers is to trap a fragment of meniscus between the tibia and femur, producing pain, a clunk, or both. Because the meniscus itself is fairly aneural, pain is caused by traction at the synovium-meniscal junction.

The McMurray test is preferred because it is easy, fast, and reliable. In addition, complementary knee tests may be done in the same position. This provocative test is performed with the patient supine, the hip flexed to 90 degrees, and the knee in forced maximal flexion. The foot is grasped by the heel, the knee is steadied, and the joint line is palpated with the other hand. As the knee is slowly brought into extension, an external rotation stress will test the medial meniscus and an internal rotation stress will test the lateral meniscus (see Fig. 6-9B). As a mnemonic rule, the heel of the foot points toward the injured meniscus. The test is considered positive when the patient feels pain in the appropriate joint line, accompanied by a thud or click. When the clunk is present, the test has a sensitivity of 98% but, because it is not always possible to evoke the clunk, its specificity is only 15%.¹³

In conclusion, the hallmarks of a meniscal tear are presence of an effusion, joint line tenderness, and a positive McMurray test. When the history and physical examination are considered together, the overall sensitivity to diagnose a meniscal tear, confirmed with arthroscopy, is approximately 95% and specificity is 88%.¹⁴

TREATMENT