The menisci are 2 fibrocartilaginous crescents anchored via bony and ligamentous attachments to surrounding structures. Their biochemical composition and multilayered structure make them ideal for converting compressive forces to tensile forces in addition to improving joint congruity and providing shock absorption to weight bearing. The medial meniscus maintains more attachments at both the horns and the midbody than the lateral meniscus, making it more susceptible to injury. Understanding of the gross anatomy, vascular anatomy, biochemical composition, and microstructure is key to understanding causes of meniscal pathology as well as treatment options for restoring its primary functions.
Understanding of the gross anatomy, microvascular anatomy, and meniscofemoral attachments of the meniscus is vital to understanding treatment of meniscal injuries.
The meniscus is a dense extracellular matrix comprising water and collagen, with interspersed cells, glycoproteins, and proteoglycans also contributing to its unique viscoelastic properties.
Different meniscus injuries or tear patterns can lead to different clinical presentations.
The menisci of the knees are 2 fibrocartilaginous discs whose unique biochemical composition and structure play a vital role in their ability to improve joint congruity, to handle load transmission, and to act as shock absorbers. Injuries to the menisci are a significant source of musculoskeletal morbidity with arthroscopic treatment of meniscal injuries accounting for 10% to 20% of all orthopedic surgeries. The unique structures that allow for the various functions of the menisci also make treatment and repair challenging with long-term damage, leading to degeneration of the knee joint. Understanding these structures is vital to understanding treatment options for restoring function of the menisci after onset of injury or degeneration. This article aims to describe the underlying structure, composition, and function of the menisci and discuss current understanding of how that structure contributes to its primary functions within the knee joint.
The menisci are 2 fibrocartilage crescents covering both the medial and lateral tibial plateaus that are anchored at the meniscal horns via bony attachments at the anterior and posterior aspects of the tibial plateau ( Fig. 1 and 2 ). As seen in Fig. 1 , the lateral meniscus is C-shaped, covering 75% to 93% of the lateral tibial plateau compared with the medial meniscus that is more semicircular-shaped, covering 51% to 74% of the medial tibial plateau.
The medial meniscus is approximately 40 mm to 45 mm long and 27 mm wide, with an anterior-posterior diameter of approximately 35 mm and a posterior region that is significantly broader than its anterior region. The posterior horn of the medial meniscus is firmly anchored via bony attachments at the posterior intercondylar fossa directly anterior to the insertion of the posterior cruciate ligament on the tibia, as seen in Fig. 2 .
The anterior horn attachment to the tibia is more variable, with Berlet and Fowler describing 4 different types of attachment in their classification system. In their study of 48 cadaveric knees, they found the anterior medial meniscal horn inserted at the flat portion of the intercondylar ridge (type I) in 59% of knees, on the downward slope from the medial articular plateau to the intercondylar region (type II) in 24% of knees, and on the anterior slope of the tibial plateau (type III) in 15% of knees. There was no bony insertion of the anterior horn (type IV) in 3% of knees. The most common insertion site is 7 mm anterior to the anterior cruciate ligament (ACL) and is 61.4 mm 2 , which is the largest insertion site of all 4 meniscal horns, the smallest of which is the posterior horn of the lateral meniscus at 28.5 mm 2 . The anterior medial meniscal horn is additionally connected to the anterior lateral meniscal horn via the transverse intermeniscal ligament, as seen in Fig. 1 .
Additional attachments of the medial meniscus include the coronary ligaments and the deep medial collateral ligaments. The coronary ligaments are portions of the joint capsule that connect the inferior menisci to the tibia. The deep medial collateral ligament is a thickening of the joint capsule, which attaches to the midpoint of the meniscus. Given all these attachments, the medial meniscus is considered relatively immobile.
The lateral meniscus is C-shaped and relatively uniform in width from anterior to posterior (see Fig. 1 ). The anterior horn inserts anterior to the intercondylar eminence and lateral to the ACL attachment at the tibial spine (see Fig. 2 ). The posterior horn attaches just posterior to the lateral tibial spine and anterior to the attachment of the posterior medial meniscus.
If present, either 1 or both of the meniscofemoral ligaments additionally attaches to the lateral meniscus. These ligaments include the ligament of Humphrey and the ligament of Wrisberg (see Fig. 1 ), both of which arise from the lateral aspect of the medial femoral condyle and respectively insert just anterior or posterior to the posterior cruciate ligament. Although only 46% of people have both of these ligaments, a majority of people have at least 1 of them. The lateral meniscus has no additional attachments to the corresponding collateral ligaments and only loose peripheral attachments to the joint capsule, which is interrupted by the popliteal tendon at the popliteal hiatus. This allows for increased mobility of the lateral meniscus compared with the medial meniscus.
The medial and lateral middle geniculate arteries are branches of the popliteal artery responsible for providing blood supply to the meniscus. A premeniscal capillary plexus formed from branches of these arteries provides the majority of the vascular supply to the meniscus, as seen in Fig. 3 . The adult meniscus remains a largely avascular structure, however, with only the peripheral 10% to 30% of the medial meniscus and 10% to 25% of the lateral meniscus receiving direct blood supply. This has important implications for healing and is the basis of the 3 distinct zones of the meniscus labeled in Fig. 3 : the peripheral vascularized red-red zone, the central avascular white-white zone, and the intervening partially vascularized red-white zone. In order to maintain their structure, it is believed the white-white and red-white zones receive more than two-thirds of their nutrition from synovial fluid via diffusion or mechanical pumping.
The knee receives innervation from the branches of the posterior tibial nerve, obturator nerve, femoral nerve, and the common peroneal nerve, all of which penetrate the capsule and follow the same distribution as the vascular supply. Thus, neural elements are most concentrated in the peripheral third of the meniscus. This was demonstrated by Dye and colleagues, who found little or no pain on manual probing of the central meniscal tissue compared with slight to moderate discomfort on manual probing of the peripheral meniscal tissue in a conscious patient without intraarticular analgesia. Additionally, studies have identified 3 different mechanoreceptor subtypes that are theorized to contribute to joint proprioception and afferent sensory input. Ruffini endings are unmyelinated, slowly adapting sensory fibers present that sense changes in joint deformation and pain. Pacinian corpuscles are myelinated, quickly adapting sensory fibers that respond to tension and pressure changes. Finally, Golgi tendon organs are myelinated, quickly adapting sensory fibers present that contribute to neuromuscular inhibition at terminal ranges of motion.
Microstructure and Biochemical Composition
The meniscal fibrocartilage structure is a dense extracellular matrix composed primarily of water (65%–70%), collagen (20%–25%), and proteoglycans (<1%). This extracellular matrix is maintained by cellular components of the meniscus that vary by region. In the white-white zone, fibrochondrochytes predominate. These are anaerobic interspersed cells of the meniscus with few mitochondria, making them well suited to survive in this poorly vascularized region. In the well-vascularized red-red zone, fibroblasts predominate and form the extracellular matrix.
Collagen and microstructure
Type I collagen is predominant in the red-red zone of the meniscus, whereas type II collagen comprises the majority of the extracellular matrix of the white-white zone. These collagen fibrils are arranged in an intricate 3-layer structure ( Fig. 4 ), ideal for converting vertical compressive load into circumferential hoop stresses. The deep layer is the most collagen dense layer and contains more type I collagen and fewer type II collagen fibers. These fibers are oriented circumferentially to resist circumferential hoop stresses. Radially oriented type I collagen fibers comprise the second layer. These fibers weave themselves through the circumferential fibers, tying them together and providing further structural rigidity as well as resistance to longitudinal splitting. The surface layer is the final layer and comprises fibers oriented parallel to the surface at various angles to provide a smooth, gliding surface.
Water, proteoglycans, and glycosoaminoglycans
In normal meniscal tissue, water comprises 65% to 70% of its total weight and predominantly resides in the posterior regions of the meniscus. It is theorized that the hydraulic permeability of the meniscus allows for drag force generation during compressive loads. This drag may decrease the compressive strain transmitted through the meniscus and, therefore, help absorb shock and limit meniscal injury risk. Proteoglycans are located within the interwoven layers of collagen and are formed with a core protein covalently attached to 1 or more glycosaminoglycans. In healthy menisci, these negatively charged, hydrophilic molecules draw water into the meniscal tissue to allow for fluid transmission with compressive loading of the meniscus.
The primary functions of the meniscus include improving joint congruity and stabilization, load transmission, and shock absorption. Some investigators also theorize that the menisci play a role in proprioception and in joint lubrication. The mechanism by which the menisci carry out each of these functions is strongly rooted in their macroscopic and microscopic anatomy.
Joint Congruity and Stabilization
Because there is a mismatch between the significant convexity of the medial femoral condyle and the slight concavity of the medial tibial plateau, the contact area of the medial compartment of the knee is, without a medial meniscus, focused over a small area. The wedge-shaped cross-section of the meniscus and its semicircular shape when viewed from above allow the meniscus to fill in the empty space between the plateau and condyle. The various attachments the medial meniscus has, as described previously, provide a strong stabilizing force that allows for minimal translation. This relative stability helps the medial femoral condyle remain centered over the tibial plateau, preventing translation.
On the lateral side of the knee, there is an even greater mismatch between the femoral condyle and tibial plateau, both being convex. The increased mobility of the lateral meniscus allows a greater degree of anterior and posterior translation of the femoral condyle on the tibial plateau, with the more mobile lateral meniscus maintaining its orientation around the condyle.
The role of the medial and lateral meniscus as secondary stabilizers of the knee has been shown through both biomechanical and clinical research. Absence of a functional medial meniscus in an ACL deficient knee demonstrates up to 58% increased anterior tibial translation compared with ACL tear alone, and worsened pivot shift. After meniscal transplantation or repair tibial translation is returned to premeniscectomy levels.
Meniscal Tears and Load Transmission
Axial load transmitted across the knee joint is transmitted into hoop stresses by the meniscus. The wedge-shaped cross-section of the meniscus is expressed outward by compressive loads between femur and tibia, and this extrusion is resisted by the circumferential collagen fibers of the menisci and their anchor points at the anterior and posterior root. In this way, compressive forces applied to articular cartilage are converted into tensile forces absorbed by the menisci.
Fig. 5 demonstrates the various types of meniscal tears. Horizontal tears of the meniscus (see Fig. 5 and 6 A), also termed cleavage tears, are tears in the axial plain and are the most frequently observed tear pattern. They typically are caused by sheer forces applied to the meniscus, although many patients deny awareness of an acute event. Horizontal cleavage tears have been demonstrated to increase contact pressure in the knee joint by approximately up to 70%. Débriding the inferior leaflet in horizontal tears leads to contact pressures 35% to 45% above baseline; however, repair may return contact pressures to within 15% of baseline.