The lateral and medial menisci are crescent-shaped fibrocartilaginous structures that collectively cover approximately 70% of the articular surface of the tibial plateau, and primarily function in load transmission and shock absorption through the tibiofemoral joint. They are wedge-shaped with thicker portions at the periphery of the joint, thereby deepening the articular surface area of the tibial plateau, which improves joint congruency and acts as a secondary stabilizing mechanism within the joint. 70% of each meniscus is made up of a network of type I collagen arranged primarily in a circumferential direction. The inner 2/3 of the adult menisci is avascular and referred to as the "white zone," likely receiving nutrition through diffusion with the synovial fluid. The peripheral 1/3 of the adult menisci, or "red zone," is well-vascularized and supplied via branches of the medial and lateral genicular arteries. The peripheral 2/3 of the menisci contain nociceptive free nerve endings, while mechanoreceptors are located in the anterior and posterior horns of the menisci, suggesting a possible proprioceptive function. The outer border of the medial meniscus is firmly attached to the joint capsule and deep fibers of the medial collateral ligament (MCL), making it less mobile than the lateral meniscus, which does not connect with the lateral collateral ligament (LCL) and has looser attachments with the joint capsule. In addition to their attachments to the tibial plateau via their anterior and posterior horns, the anterior margins of the menisci are connected by the transverse intermeniscal ligament, while the posterior horn of the lateral meniscus connects to the femur via meniscofemoral ligaments and to the adjacent popliteus tendon.
Isolated meniscal tears occur due to rotational or shearing forces placed across the tibiofemoral joint, especially in situations in which increased axial load is placed through the menisci. Such scenarios include positions with increased degrees of closed kinetic chain flexion (kneeling, squatting), lifting/carrying heavy weights, and activities requiring rapid acceleration/deceleration, change of direction, and jumping. A traumatic impact to the knee can also result in either isolated meniscal tears or tears occurring concomitantly with bony lesions or damage to the primary stabilizing ligaments of the knee, such as the anterior cruciate ligament (ACL) and MCL. Relatively less force is required to create tears in those with degenerative changes of the menisci, typically seen in adults over the age of 40y/o, often with concomitant osteoarthritis (OA).
Studies in the United States have indicated a 61 per 100,000 incidence rate of meniscal tears in the general population and up to an 8.7 per 1,000 incidence rate in the active duty military population. Given the etiology of tears, it is not surprising that infantry-related duties, occupations that require frequent squatting/kneeling, and sports such as soccer, rugby, football, basketball, baseball, skiing, and wrestling all increase the risk of meniscal tears. Male gender and age over 40y/o are also associated with increased risk of meniscal tears. Medial meniscal tears are more common than lateral meniscal tears, possibly due to the relatively decreased mobility of the medial meniscus secondary to its connection to the MCL. People with ACL-deficient knees are more at risk for development of medial meniscal tears, especially if ACL reconstruction is delayed for longer than one year from the time of initial injury. Meniscal tears in children under the age of 10y/o are uncommon, although the presence of a discoid meniscus, an anatomic variant most often occurring in the lateral meniscus that involves the additional central extension of the meniscus over the tibial articular surface, may predispose someone to tears at a younger age.
Meniscal tears are categorized by both their shape and location when visualized on magnetic resonance imaging (MRI), in which high-intensity intrameniscal signals communicate with at least one articular surface on an otherwise black-appearing meniscal tissue. Horizontal (cleavage) tears run parallel to the tibial plateau through the mid-substance of the meniscus and are more likely to occur in people over 40y/o with underlying degenerative changes, in the absence of a distinct inciting event. Longitudinal (vertical) tears run perpendicular to the tibial plateau and parallel to the long axis of the meniscus, whereas radial tears run perpendicular to both the tibial plateau and long axis of the meniscus, originating from the inner free edge of the meniscus. Complex tears involve some combination of horizontal, longitudinal, or vertical tears, while displaced tears involve either complete detachment of a piece of meniscus or flipping of a piece of meniscus that is still attached to the rest of the meniscal body. Bucket-handle tears are fragments of complete longitudinal tears that migrate centrally over the remaining menisci, whereas parrot-beak tears are radial tears with partially detached fragments. Flap tears are partially detached fragments of horizontal tears. Tears that are contained within the outer 1/3 vascular zone of the menisci are "red-red" tears, whereas those with central margins extending into the inner 2/3 avascular zone are "red-white" tears. Tears contained within the inner 2/3 avascular zone are "white-white" tears. Tears located within the red zone have the highest potential for either spontaneous healing with conservative management or successful outcome following meniscal repair.
Clinical presentation of the patient with a meniscal tear is variable depending on the mechanism of injury and degree of concomitant tibiofemoral insults. The sensation of a "pop," with immediate effusion of the knee during high-impact activity or trauma is associated with an ACL tear with a possible associated medial meniscal tear, whereas effusion that develops more gradually over the course of 24 hours is more indicative of an isolated meniscal tear. Symptoms can also be insidious in nature, in which low-grade effusion and stiffness develop over the course of several days despite the absence of an inciting event. Pain is typically reported over the anteromedial or anterolateral joint line. Depending on the type and extent of the meniscal lesion, there may also be symptoms of locking, clicking, catching, intermittent inability to fully extend the knee, or feeling of the knee giving way.
Physical examination should consist of inspection of the knee for edema, palpation of the joint line, standing and supine range of motion (ROM), muscle strength testing, and special testing. Anteromedial and anterolateral joint line tenderness performed with the patient's knee at the edge of the table at 90 degrees of flexion is 83% sensitive and 83% specific for a meniscal tear. There may be pain and deficits in either flexion or extension ROM depending on the type of tear and extent of effusion. Deficits in open kinetic chain knee flexion/extension strength testing are unlikely, but antalgic gait or increased pain with single or double-leg squatting is possible due to increased compressive forces over the menisci. There are several provocative special tests for the detection of meniscal tears. The Thessaly test, in which the patient stands on one leg, squats down to 20 degrees of flexion, and internally/externally rotates the knee through active adduction/abduction of the hip is 75% sensitive and 87% specific. McMurray's test has a sensitivity of 61% and specificity of 84% and involves passively extending the supine patient's knee from a fully flexed position to 90 degrees flexion while maintaining full external or internal rotation. Apley's compression test, in which the prone patient's knee is passively flexed to 90 degrees and then externally/internally rotated with axial compressive force through the knee has been found to be < 20% sensitive and up to 80-90% specific.
When a meniscal tear is suspected, imaging should begin with radiographs that include AP, lateral, oblique, sunrise, and weight bearing views to assess for concomitant bony pathologies, loose bodies, and osteoarthritis. Using arthroscopy as the gold standard, MRI is the best mode of imaging to diagnose and characterize meniscal tears. MR imaging has been found to be 93% sensitive and 88% specific for medial meniscal tears, and 79% sensitive and 96% specific for lateral meniscal tears.
Initial treatment of the acutely painful, edematous knee in which meniscal tear is suspected should consist of the RICE principles for acute soft tissue injury (Rest, Ice, Compression, and Elevation). Oral analgesics and NSAIDS may be prescribed to reduce pain and swelling. Bracing or knee sleeves may be used for protection and compression, and early pain-free knee and ankle ROM exercise may help to limit the loss of motion and aid in edema control. For simple tears confined to the outer 1/3 of the meniscus and degenerative tears, it is reasonable to perform a 4-6 week course of relative rest and physical therapy to determine if spontaneous healing and return to the desired level of function will occur. Patients with persistent pain, swelling, and mechanical symptoms despite conservative management should be assessed for potential surgical intervention. For tears requiring arthroscopic surgery, meniscal repair is preferred over meniscectomy if possible, since the risk of accelerated osteoarthritis increases with the removal of meniscal tissue due to decreased cushioning and increased force transmission across articular cartilage surfaces. Factors that increase the likelihood of successful meniscal repair include tears that occur in the red zone of the meniscus, tears that are shorter than 2cm, vertical longitudinal tears, and acute tears. Rehabilitation following meniscal repair typically includes restrictions in knee flexion ROM and weight bearing status during the first 6 weeks postoperatively, depending on the location of the tear and type of repair. Meniscal allograft transplantation is considered a salvage procedure for symptomatic patients less than 50y/o with a meniscus-deficient compartment and otherwise stable knee.