The medial meniscus is semicircular, crescent-shaped, and measures approximately 3.5 cm in length.

The medial meniscus covers 50%-60% of the medial tibial plateau. The posterior horn is wider than the anterior horn in the anteroposterior (AP) dimension (20,26).

The attachment site for the anterior horn is variable, in the area of the intercondylar fossa in front of the anterior cruciate ligament (ACL), often to the anterior surface of the tibial plateau.

The posterior fibers of the anterior horn merge with the transverse fibers of the intermeniscal ligament, which connects the anterior horns of the medial and lateral menisci. The intermeniscal ligament is located approximately 8 mm anterior to the ACL (2).

The posterior horn is firmly attached to the posterior intercondylar fossa of the tibia, anterior and medial to the posterior cruciate ligament attachment site (2).

The periphery is attached to the capsule throughout its length, and the tibial portion of this attachment is called the coronary ligament. In addition, at its midpoint, the medial meniscus is firmly attached to the femur and tibia through a condensation of the joint capsule known as the deep medial collateral ligament (26).

The lateral meniscus is almost circular in gross morphology and covers 70%-80% of the lateral tibial plateau (2,26).

The lateral meniscus is nearly uniform in width from front to back.

The bony attachments of the lateral meniscus are much closer to each other than those of the medial meniscus. The anterior horn inserts adjacent to the ACL, and the posterior horn inserts just posterior to the ACL, anterior to the posterior horn of the medial meniscus.

There is a loose peripheral attachment of the lateral meniscus to the joint capsule that allows greater translation of the lateral meniscus, when compared to the medial meniscus (11.2 vs. 5.2 mm) (2).

The area of the lateral meniscus with no coronary ligament attachment, anterior to the popliteus tendon, is called the bare area of the lateral meniscus, or popliteal hiatus.

Motion of the lateral meniscus is guided by the capsular attachments, as well as additional ligamentous attachments. These ligaments include the meniscofemoral ligaments (MFLs) and the anterior inferior and posterior superior popliteomeniscal fascicles from the popliteus muscle.

The posterior horn has variably present attachments to the medial femoral condyle through the MFLs. The MFLs originate from the posterior horn of the lateral meniscus.

The anterior MFL (of Humphrey) passes anterior to the posterior cruciate ligament (PCL) to insert on the femur between the distal margin of the femoral attachment of the PCL and the edge of the condylar articular cartilage.

The posterior MFL (of Wrisberg) passes posterior to the PCL to insert at the proximal margin of the femoral attachment of the PCL.

The overall incidence of at least one MFL is 91%. In the knees demonstrating at least one structure, the incidence of an anterior MFL is 48.2%, and the incidence of posterior MFL is 70.4%. The incidence of both ligaments coexisting in one knee is 31.8% (13).

Discoid variants occur with an estimated incidence of 3.5%-5.0%, most commonly the incomplete type (11).

Discoid meniscus is almost universally located in the lateral compartment.

Three types exist — incomplete, complete, and Wrisberg.

Both the incomplete and complete types have firm posterior tibial attachments and are considered stable.

The Wrisberg variant occurs when the posterior horn bony attachment is absent, and the posterior MFL of Wrisberg is the only stabilizing structure (11).

The menisci are fibrocartilaginous tissue comprised of cells interspersed in a matrix largely composed of collagen bundles, along with noncollagenous proteins including elastin and proteoglycans.

Two cell types are present — a more fusiform, fibroblastic cell, and a more rounded, chondrocytic cell.

Water constitutes 72% of the extracellular matrix, and collagen makes up 75% of the dry weight (20).

Elastin is estimated to be less than 0.6%, and noncollagenous proteins 8%-13%, of the meniscus dry weight in humans (20).

Type I collagen represents 90% of collagen present, and types II, III, V, and VI are present in varying quantities depending on location and age (20).

The principle orientation of collagen fiber bundles is circumferential, with few radially directed “tie” fibers. Tie fibers provide structural rigidity to help resist forces that would split the circumferential fibers with compressive loading (2,20).

Fiber orientation changes with depth from the surface. Surface fibers are arranged as a network of irregularly oriented bundles. The deeper fibers are primarily circumferential (20).

Both medial and lateral menisci demonstrate an extensive microvascular network, arising from the respective superior and inferior geniculate arteries (16,26).

The perimeniscal capillary plexus is oriented circumferentially and branches extensively into smaller vessels to supply the peripheral border of the meniscus through its attachment to the capsule.

The branches terminate after supplying the peripheral 10%-30% of the meniscus, leaving the remainder avascular (16,20).

Free nerve endings and specialized end-receptors are present within the menisci. The most densely innervated regions are the anterior and posterior horns of both menisci (20).

Nerve fibers originate in the perimeniscal tissues and radiate into the peripheral 30% of the meniscus.

Three receptor types have been identified — Ruffini endings, Golgi tendon organs, and Pacinian corpuscles.

It is hypothesized that the nerves play a proprioceptive role in normal joint function. Meniscal-derived signals generated during deformation and loading may be important to joint position sense and for protective neuromuscular reflex control of joint motion and loading (20).

The bulk of meniscus nutrition is supplied by the synovial fluid, most notably to the avascular regions.

Nutrients reach the tissue via passive diffusion and mechanical pumping with intermittent compression during loading (20).

With knee flexion from 0-120 degrees, the menisci move posteriorly. In the midcondylar, parasagittal plane, the medial meniscus moves approximately 5.1 mm, and the lateral meniscus moves 11.2 mm (2).

The medial meniscus lacks the controlled mobility of the lateral meniscus.

Posterior motion of the medial meniscus is guided by the deep medial collateral ligament and semimembranosus, whereas anterior translation is caused by the push of the anterior femoral condyle (33).

The posterior oblique fibers of the deep medial collateral ligament limit motion in rotation, and therefore, the medial meniscus is at increased risk of tear (2,16).

The lateral meniscus is stabilized, and motion guided, by the popliteus tendon, popliteomeniscal ligaments, popliteofibular ligament, MFLs, and lateral capsule.

Meniscal motion allows continued load distribution during changes of position of the joint, during which the radius of curvature of the femoral condyles changes (33).

The medial meniscus provides greater restraint to anterior translation than the lateral meniscus by acting as a buttress (17,18).

ACL-deficient knees demonstrate increased anterior translation when subjected to an anteriorly directed force, and this translation increases significantly with combined meniscectomy at all angles of flexion. This confirms the role of the ACL as a primary restraint to anterior translation and demonstrates that the medial meniscus acts as a secondary stabilizer to resist anterior translation (18).

With sufficient anterior translation (in the ACL-deficient knee), the posterior horn of the medial meniscus is wedged between the tibial plateau and the femoral condyle and is the mechanism suggested for the resistance provided by the meniscus.

In contrast, the soft tissue attachments of the lateral meniscus do not affix the lateral meniscus as firmly to the tibia. Combined lateral meniscectomy and ACL sectioning does not increase anterior translation significantly over ACL sectioning alone. This implies that the greater mobility of the lateral meniscus prevents it from contributing as efficiently as a posterior wedge to resist anterior translation of the tibia on the femur (17).

The menisci serve additional functional roles, including load bearing and shock absorption (26). The menisci transmit large loads across the joint, and their contact areas change with different degrees of knee flexion and rotation.

Up to 50%-70% of compressive load is transmitted through the menisci in extension, and 85% at 90 degrees of flexion (2,11,16,26).

Removal of a portion of the meniscus results in a decreased contact area between the femur and tibia. Medial meniscectomy decreases the contact area by up to 70%.

Resection of as little as 15%-34% of the meniscus results in increased contact pressure by up to 350% (33).