About 70–75 % of the wet weight of the meniscus consists of water [2, 5–7]. The dry weight consists of 60–70 % collagen, 1 % proteoglycans, and 8–13 % non-collagenous proteins (such as elastin). Collagens are primarily type I (90 %) with smaller amounts of II, III, V, and VI [6, 8]. The collagen fibres are mainly oriented circumferentially, with some radially oriented fibres (Fig. 25.2). This may be related to the mechanical forces that act on the menisci.

Fibrochondrocytes are the predominant cell type in the meniscus. They produce collagen and its extracellular matrix [8].

Along the inner avascular zone, cells are morphologically similar to articular chondrocytes but on the periphery, cells are more similar to fibroblasts. Arnoczky et al. demonstrated that the outer 10–30 % of the medial meniscus and 10–25 % of the lateral meniscus is vascular [9].

The medial and lateral geniculate arteries form a perimeniscal capillary plexus that supplies the outer surface of the menisci. The menisci have intrinsic innervation. Innervation is most abundant on the periphery and the anterior and posterior horns [10]. The meniscus does have an inherent ability to heal itself, but this is limited to the vascularized periphery [11].

Proprioception is believed that obtained from free nerve endings and they are activated on the anterior and posterior horns during flexion and extension of the knee [12, 13].

The unique architecture of the meniscus consists of circumferentially oriented collagen fibers interspersed with radial collagen fibers [2].

The mechanical properties of this tissue are highly anisotropic (different in opposing directions), and strongly dependent on the prevailing fiber direction [14, 33]. This can be seen in the tensile stress–strain response of samples oriented in the circumferential direction as compared to those oriented in the radial direction. Circumferential samples show a pronounced “toe” region common to fiber-reinforced tissues, and a higher linear modulus thereafter. Radial samples are relatively linear in their stress–strain response, with a much lower modulus. The tensile properties of the meniscus range from 48 to 259 MPa in the circumferential direction and 3–70 MPa in the radial direction, depending on anatomic location and species [2, 14, 15, 33]. The compressive properties of the meniscus are low relative to articular cartilage (50–400 kPa, about one-half) [16]. The meniscus, less stiff in compression, is also much less permeable than articular cartilage [2], suggesting that the tissue is optimized to enhance congruency, load distribution, and shock absorption across the joint [33].

Load distribution over an incongruent joint surface is redistributed by the menisci by maintaining maximal congruency. The functionality of the menisci and their role in load transmission across the knee has been discussed by many authors [2, 17].

The main functions of the meniscus are to increase joint congruency, transmit and distribute compressive load between the femur and the tibial plateau, stabilize the joint, and improve articular cartilage nutrition and lubrication [7]. These functions are achieved by the unique load transfer that occurs between the (more hyaline) inner region and the (more fibrous) outer region of the meniscus. When the joint is loaded vertically, axial loads from the femoral condyles are redirected laterally .Lateral extrusion of the meniscus is resisted by the osseous anchorage of the anterior and posterior horns [33], generating hoop stresses within the dense network of circumferentially oriented collagen fibers [18]. Normally, the menisci transmit 50–100 % of the loads in the knee (multiples of body weight) [19], with tensile deformations limited to 2–6 % [20, 21].

The main role of the menisci in the knee joint is load bearing. When bearing load, the knee joint is subjected to compression. The compressive force through the joint is distributed over an articulating contact area resulting in contact stresses (contact pressure). The menisci optimize the way that load is transferred across the knee joint by increasing the congruency of the articulation. The areas of contact increase and, as a result, the contact pressure on the articulating surfaces decreases. As the femoral condyles bear down onto the menisci, the meniscal wedged cross section causes the menisci to extrude radially out of the joint; this causes their circumference to increase. Each meniscus attaches mainly to the tibia by anterior and posterior insertional ligaments. Thus, the bulk tissue resists radial displacement by developing circumferential tension (hoop stresses) (Fig. 25.3); this is due to the stiffness of the meniscal tissue, with the predominantly circumferential orientation of the collagen fibres.

The lateral meniscus is more mobile than the medial meniscus because it is not as tightly attached to the capsule as the medial meniscus. Also, the concave medial tibial plateau does not allow the posterior aspect of the medial meniscus to displace off the joint posteriorly in deep flexion; however, the convex posterior aspect of the lateral tibial plateau allows the lateral meniscus to displace posteriorly (go “downhill”) in deep flexion. These key factors may explain the increased frequency of medial meniscal tears compared to lateral meniscal tears, at a ratio of 2:1 [22].

The variable effect and frequency of the various types of meniscal tear may depend on the resistance of the menisci to compressive loads by increasing their circumference and developing hoop stresses.

The menisci distribute contact forces over the articular surfaces effectively by increasing the contact surface of the joint. The menisci maintain maximal congruency by redistribution of the load, over an incongruent joint surface. The functionality of the menisci and their role in load transmission across the knee has been discussed by many authors [2, 17].

Havitcioglu et al. investigated the biomechanical properties of meniscus under pull-out and compression forces (Fig. 25.4) [23]. In compression–tensile tests, statistically significant difference was observed between force and displacement values [(p = 0.037) and (p = 0.045), respectively] (Tables 25.1 and 25.2).

Knowledge of the detail properties of the meniscus and cell culture biomechanics will help the artificial meniscus basic studies. Biomechanical knowledge of the properties of natural meniscus are light is keep for future studies.

With respect to meniscus biomechanics and its possible degeneration, finite element simulations help to understand the stress distribution in the human knee joint and prevent joint injuries and pathological degeneration of articular joints. Three-dimensional finite element models can also be used to estimate the consequences of surgical treatments such as total or partial meniscectomies. [24] considered the effect of meniscectomies on the human knee joint using the finite element method.

Some study was to develop a three-dimensional finite element model of the human tibio-femoral joint including the femur, tibia, cartilage layers, menisci, and main ligaments to estimate the contact areas and pressure distributions between menisci and articular cartilage and the stress distribution in the articular cartilage to investigate the effect of meniscal tears and meniscectomies on these variables. This could help to explain the cartilage degeneration after a total or partial meniscectomy [25, 26].

A 3D model was developed of the tibio-femoral human knee joint including femur, tibia, cartilage layers, menisci, and joint ligaments and analyzed under axial compression forces. It allows a better understanding of the role of the menisci in the transmission of forces across the knee and to investigate the effect of meniscal tears and meniscectomies on the biomechanics of human knee joint [27].

Normal menisci are semilunar, fibrocartilage structures (Fig. 25.1). Deviations from this can occur due to disease, degeneration, traumatic injury, or abnormal development. Once torn, or damaged, the chondroprotective ability of the tissue is disrupted and the cascade toward osteoarthritis initiated [28]. Osteoarthritis leads to degenerative changes in the menisci with the surrounding cartilage, and this is related to 75 % of all meniscal tears and extrusions [29, 30].

Several authors suggested that damage in cartilage starts at its surface and extends through the thickness. This can be explained by a maximal shear stress criterion [31]. Moreover, loss of meniscal tissue frequently leads to long-term degenerative joint changes, articular cartilage degeneration, and osteoarthritis [32, 33].

Most authors agree that total meniscectomy leads to progressive articular wear after a few years. This is a fact that the global biomechanics of the knee is altered and the articular instability increases; so this state can be resulted a progressive and degenerative arthrosic pathology [17, 34].

With consider to meniscus biomechanics and its possible degeneration, finite element simulations help to understand the stress distribution in the human knee joint, so this will help us to prevent joint injuries and pathological degeneration of articular joints. Three-dimensional finite element models can also be used to predict the results of surgical treatments such as total or partial meniscectomies.

Wilson et al. considered the effect of meniscectomies on the human knee joint using the finite element method. They determined the stress and strain distributions and fluid velocities in the articular cartilage before and after meniscectomy, but only used an axisymmetric model [24]. In another study, Pena et al. investigated the effect of meniscal tears and meniscectomies on the human knee joint. They use a 3D finite element model that included the femur, tibia, cartilage layers, menisci, and ligaments. Three different situations were compared: a healthy tibio-femoral joint, a tibio-femoral joint with tears in one meniscus, and a tibio-femoral joint after meniscectomy. According to Pena et al., the minimal principal stresses corresponding to a compressive load at 0° flexion were obtained for the posterior zone of the medial meniscus and the corresponding region of the articular cartilage. The maximal contact stress in the articular cartilage after meniscectomy under an axial femoral compressive load was about twice that of a healthy joint [27]. This fact could partially explain the cartilage damage and degeneration that have been occurring after meniscectomy.