The lateral meniscus is one of two semilunar fibrocartilaginous structures in the knee joint that act as shock absorbers, enhance joint stability and aid in the distribution of synovial fluid. Together with the medial meniscus, the lateral meniscus provides a concave surface for the convex femoral condyles to articulate superior to the relatively flat tibial plateaus.
Structure and Function
The anterior and posterior horns of the lateral meniscus both attach to the tibia. The anterior horn of the lateral meniscus inserts anterior to the intercondylar eminence next to the attachment site of the ACL, while the insertion of the posterior horn lies posterior to the lateral tibial spine and anterior to the insertion of the posterior horn of the medial meniscus. The meniscofemoral ligaments attach the posterior horn of the lateral meniscus to the lateral part of the medial femoral condyle. Although the lateral meniscus is attached to the majority of the anterior and posterior capsule of the knee joint, there is an area posterolaterally in the region of the popliteus tendon where the lateral meniscus is not attached to the joint capsule. This arrangement allows the lateral meniscus more mobility than the medial meniscus and is one reason why the lateral meniscus is less susceptible to tearing than its medial counterpart. The lateral meniscus is also larger than the medial meniscus and carries a more significant percentage of the lateral compartment pressure than the medial meniscus carries for the medial compartment.
The cells of the menisci are termed fibrochondrocytes since they appear morphologically to be a mix of fibroblasts and chondrocytes. Cells in the superficial layers of the meniscus appear more fibroblastic in nature, whereas cells deeper in the meniscus are more chondrocytic. The meniscal extracellular matrix (ECM) is composed primarily of water and collagen with a small percentage of proteoglycans, noncollagenous proteins, and glycoproteins. The collagen found in the meniscus is almost all type I collagen, with some variable amounts of types II, III, V, and VI. Collagen fibers located in the deeper layers of the meniscus are oriented circumferentially, parallel to the peripheral border, while the more superficial layers contain more radially oriented fibers. These radially oriented fibers are interspersed in the deeper layers as well to provide structural integrity. The proteoglycans found in the ECM provide hydration to the tissue, which allows the meniscus a high capacity to resist compressive loads. As a result, the highest concentration of these glycosaminoglycans is present in the primary weight-bearing areas, the meniscal horns and inner half of the menisci.
The roles of the menisci in the knee are primarily clinical inferences from the degenerative changes that take place when removed. Meniscectomy drastically increases contact stress and may contribute to accelerated articular cartilage damage and degeneration. The menisci have also demonstrated an important role in shock absorption, and the inability to absorb shock in the knee has implications in osteoarthritis. The meniscus may also help with nutrition and lubrication of the knee joint, although the exact mechanics of this are still relatively unknown. The thinking is that microcanals within the meniscus communicate with the synovial cavity, allowing for fluid transport to deeper, avascular structures for nutrition and joint lubrication.
Both menisci arise from the intermediate layer of mesenchymal tissue and begin to take their characteristic shape around the eighth to tenth week of gestation. Initially, the menisci are highly cellular and vascular, but there is a gradual decrease in cellularity and a concomitant increase in collagen content as the fetus continues to develop.
Blood Supply and Lymphatics
The lateral meniscus receives its blood supply from the lateral and middle geniculate arteries that branch off the popliteal artery, although the tissue itself is relatively avascular. Only approximately the peripheral 10% to 25% of the lateral meniscus is well vascularized, and the rest of the tissue receives nourishment from the synovial fluid via diffusion or mechanical pumping during the motion of the joint.
The recurrent peroneal branch of the common peroneal nerve penetrates the articular capsule of the knee. It follows the vascular supply to the peripheral regions of the lateral meniscus and the anterior and posterior horns. Neural innervation is most concentrated at the meniscal horns where mechanoreceptors convert tension and compression during extremes of flexion and extension into an electrical nerve signal. Ruffini endings, Pacinian corpuscles, and Golgi tendon organs are all mechanoreceptors that researchers have identified in studies of human menisci, and the belief is that these neurosensory structures are critical for joint motion and proprioception.
Throughout the history of arthroscopic surgery, open meniscal repair via an incision posterior to the collateral ligaments is rarely performed due to associated neurovascular injury. Currently, both inside-out and outside-in repair techniques are common choices. These techniques involve arthroscopically passing a suture from either the inside or the outside of the knee and tying it beyond the joint capsule via a small incision. Inside-out and outside-in repairs are useful for anterior and middle-third tears, but the saphenous nerve medially and the common peroneal nerve laterally are vulnerable when making the accessory incisions. More recently, all-inside arthroscopic meniscal repair techniques have been developed that avoid the need for accessory incisions. Several review studies have found no differences in clinical failure rate or subjective outcomes between inside-out and all-inside repair techniques, although complications were associated with both. Inside-out repairs were associated with more nerve symptoms; whereas, all-inside repair complications were more implant-related.
Meniscal injuries are one of the most common reasons for arthroscopy of the knee. Since the lateral meniscus is more mobile than the medial meniscus, it is the less commonly injured of the two menisci. Acute traumatic meniscal injuries typically occur with axial loading in conjunction with knee rotation; whereas, chronic degenerative tears are more common in older adults with a history of arthritis of the knee. Meniscus tears are the most common tear associated with ACL injuries, with the lateral meniscus affected more often than the medial side. Patients with a meniscal tear will usually present with joint line tenderness and effusion. The McMurray test is the most commonly used physical exam test to evaluate for a torn meniscus, and MRI is the imaging modality of choice for diagnosis and monitoring.
Meniscal tears can be treated non-operatively or through surgical repair or meniscectomy. The most appropriate treatment depends on both patient factors, such as age and co-morbidities, as well as characteristics of the tear. Due to degenerative complications that occur with total meniscectomy, surgical repair is the ideal treatment modality for the long-term health and stability of the joint. Recent evidence has shown that older patients that have degenerative tears without mechanical symptoms can be effectively managed non-operatively with a structured physical therapy program. Even if these patients eventually require meniscectomy, the delay in surgical management also delays the degenerative complications associated with meniscectomy, and patients still achieve functional outcomes similar to if they had initially had surgical treatment. In younger patients, the ability to repair the meniscus depends on the location of the tear. Since just the peripheral third of the meniscus is well vascularized, only tears in this area, and sometimes the middle third, are expected to heal with surgical repair. Tears that involve deep avascular zones of the meniscus must receive treatment with partial meniscectomy as the lack of blood supply will prevent appropriate healing if attempting a repair. Recently, efforts to encourage bleeding in the avascular zones to aid with healing have shown promise in cases where a surgeon attempted a surgical repair on meniscal tears involving tissue with little or no blood supply.