Introduction
The biomechanical properties of the menisci have been extensively studied. Once thought to be a functionless structure akin to an embryonic remnant, it is now recognised that the menisci are critical for maintaining proper knee kinematics and function and for the prevention of the development and progression of knee osteoarthritis. The ability of the menisci to assist in maintaining a normal knee environment is largely a result of their material and functional biomechanical properties and associated ligamentous attachments. The goal of this chapter is to discuss the biomechanical properties of the menisci as they relate to normal knee function. This includes the material properties of the menisci and their response to tensile, compressive and shear forces and how the menisci play a role in load transmission, joint stability, proprioception and maintenance of a homeostatic environment for the knee. Finally, the effects of different meniscal tear patterns on the biomechanics of the knee joint are discussed.
Meniscus Kinematics
A number of studies have demonstrated differences in mobility between the medial and lateral menisci. When evaluating the knee in flexion, the medial meniscus has an average anteroposterior displacement of 2 mm, whereas the lateral meniscus has an average anteroposterior displacement of 10 mm. Additionally, there are significant differences in meniscal excursion when taking the knee from extension to flexion, with the medial excursion found to be 5.1 mm versus 11.2 mm of lateral excursion ( Fig. 14.1 ). The smaller posteroanterior distance (the ratio of posterior to anterior meniscal translation during flexion) of the lateral meniscus, as demonstrated in Fig. 14.1 , indicates that the lateral meniscus is more likely to move as a single unit compared with the medial meniscus with its more extensive peripheral capsular attachments. The portion of the meniscus that has the least mobility is the posteromedial corner. It is thought that this is secondary to its attachment to the tibial plateau via the posterior oblique ligament and posteromedial meniscotibial ligament. This lack of motion and trapping of the meniscus between the tibial plateau and femoral condyle in deep flexion is thought to make this portion of the meniscus more at risk for injury.
Material Properties of the Meniscus
The structural makeup of the meniscus is critical in defining the material properties demonstrated by this tissue.
Viscoelasticity
The viscoelastic nature of the menisci is largely responsible for the compressive properties of this tissue. The material makeup of the menisci allows it to have both viscous and elastic properties, and thus it exists as a biphasic structure. The solid phase occurs initially after a compressive force is applied, resulting in an elastic response by the tissue. This largely is due to the collagenous proteoglycan scaffolding of the meniscus. Simultaneously the fluid phase begins as fluid is slowly extruded from the meniscal tissue into the synovial space under a compressive force. The rate at which fluid leaves the meniscal tissue is determined by meniscal permeability. Compared with articular cartilage, meniscal tissue is approximately one-eighth as permeable, thus allowing the menisci to maintain their shape during compressive loading. , If meniscal tissue had increased permeability, it would not be able to maintain its shape and would be essentially nonfunctional.
Compressive properties
When the menisci experience a compressive force, such as with weightbearing, the axial load transmitted to the tissue is converted into meniscal hoop stresses, which are experienced in the circumferential collagenous fibres in the deep layer of the menisci ( Fig. 14.2 ). As an axial load is applied, the wedge shape of the menisci causes meniscal tissue to extrude both medially and laterally. This phenomenon results in a radially oriented force that is converted into tensile strain by the circumferentially oriented fibres of the menisci and their anterior and posterior horn attachment sites. , , The conversion of axial load to tensile strain in the form of hoop stresses is one of the main reasons that the menisci play such an important role in load distribution in the knee. , When meniscal injury occurs that disrupts the circumferential fibres, such as with radial tears or root tear injuries, hoop stresses are not maintained, resulting in joint overloading and the development or progression of destructive changes. ,
Tensile properties
Fibrocartilaginous tissues often undergo several phases when being exposed to tensile or stretching forces where load applied to the tissue does not result in uniform tissue deformation or elongation ( Fig. 14.3 ). Initially, when menisci experience tensile forces, there is little change in elongation of the tissue as the previously relaxed collagen fibres become stretched. This is often described as the ‘toe region’. As the collagen fibres lose their crimp and straighten, the second phase is entered where there is a linear relationship between elongation and load applied. Finally, meniscal tissue reaches its ultimate tensile load, at which point fibres began to fail and tearing occurs.
Variation in fibre orientation of the deep layer of the menisci results in different responses of this layer to tensile stresses. The deep, circumferentially oriented fibres have a substantially greater tensile modulus (80 to 125 MPa) compared with the radially oriented tie fibres (1.7 to 3.6 MPa) of the same layer. Additionally, several studies have suggested a difference in tensile strength between the medial and lateral menisci, as well as variations in tensile strength between the anterior, middle and posterior portions of a given meniscus. , ,
Although the estimated tensile modulus of human menisci (150 MPa) is lower than that of the major knee ligaments (300 MPa), , it is greater than that of the acetabular labrum (65 MPa) and glenoid labrum (25 MPa).
Shear properties
Shear stress arises from a force vector that is applied parallel to the cross-sectional area of a tissue. The ability of a tissue to resist changing shape under shear stress is defined as its shear stiffness. Compared with cartilage, meniscal tissue demonstrates relatively low shear stiffness. The ability of meniscal tissue to tolerate shear forces is important, especially when considering the aetiology of horizontal meniscus tears. Specifically, this tear pattern is believed to result from axial loads being converted to shear forces that were transmitted to the menisci.
Functional Properties of the Meniscus
The menisci play a crucial role in maintaining a properly functioning knee joint through their role in load transmission, maintenance of joint stability and homoeostasis and knee joint proprioception.
Load transmission
The role of the menisci as load-bearing structures is well established. The current understanding initially came from the realisation that meniscus removal was commonly associated with rapid progression of degenerative changes at the tibiofemoral articulation. Several studies have now demonstrated an increased risk of developing degenerative changes in the knee after meniscectomy. , , , , Much of the load-bearing function results from the size, shape and previously discussed material properties of the menisci. It is estimated that the medial meniscus occupies approximately 50% to 60% of the articular surface of the medial compartment, whereas the lateral meniscus occupies approximately 70% to 80% of the articular surface of the lateral compartment. , This large surface area increases the congruency of the tibiofemoral articulations. When contact area increases, contact stresses on the articular cartilage decrease, as does the risk of developing degenerative changes in the knee ( Fig. 14.4 ).
About 50% of the compressive load is transmitted to the menisci when the knee is in extension, whereas 85% of the compressive load is transmitted to the menisci when the knee is in 90° of flexion. This difference in compressive load experienced by the menisci is thought to be result of a transfer of contact as the knee experiences femoral rollback during flexion. There are also differences in load transmitted through the meniscus based on the compartment, with approximately 70% of the load being transmitted to the lateral meniscus, compared with 50% of the load transmitted to the medial meniscus. The increased load experienced by the lateral meniscus is thought to be a result of its large surface area creating a highly congruent articulation with the convex surface of the femoral condyle and the flat to convex surface of the lateral tibial plateau.
Total or subtotal meniscectomy effectively reduces congruency and contact area, which in turn increases contact stresses ( Fig. 14.5 ). Meniscectomy is believed to decrease contact area by 40% to 75%, which results in an increase in contact stresses by 200% to 300%. , , , , , Given that increased contact stresses are a risk factor for the development of articular cartilage damage, there has been a focus on meniscus repair to conserve as much meniscus as possible.
Joint stability
The menisci are important secondary stabilisers in the ACL-intact knee. Much of their function as a stabilising structure is a result of the increased congruency they provide at the tibiofemoral articulation. , The intact meniscus provides multidirectional stability when the knee experiences axial load, preventing excess motion in all directions. Nevertheless, the medial and lateral menisci differ in their primary stabilising function. The main role of the medial meniscus is to prevent anterior translation of the tibia. , , This is largely accomplished by the posterior horn of the medial meniscus, which undergoes compression during loading and is essentially ‘trapped’ by the tibiofemoral articulation preventing anterior translation of the tibia. The robust peripheral capsular attachments and the meniscotibial attachment of the medial meniscus enhance its role as a secondary stabiliser, most notably in preventing anteroposterior motion. , The lateral meniscus is significantly more mobile than the medial meniscus, causing many to question its role as a stabiliser in the knee. However, the role of the lateral meniscus as a secondary stabiliser, most notably in limiting anterior tibial translation during pivot shift manoeuvres, has been identified.
In the ACL-deficient knee, the stabilising role of the menisci is heightened and they function as primary stabilisers. , , , Allen et al. reported that medial meniscectomy in the loaded ACL-deficient knee increased anterior tibial translation by 2.2 mm with the knee fully extended and 5.8 mm with the knee at 60 degrees of flexion. This study also demonstrated a decrease in coupled internal tibial rotation under anterior tibial loading, again highlighting the role of the medial meniscus in resisting anterior tibial translation in the ACL-deficient knee. The lateral meniscus plays a significant role in rotatory stability in the ACL-deficient knee. One study compared tibial acceleration during pivot shift testing and noted significantly greater tibial acceleration in ACL-deficient knees with concomitant lateral meniscus tears compared with ACL-deficient knees with an intact lateral meniscus.
Joint lubrication and nutrition
The viscoelastic nature of the menisci likely accounts for the contribution of the menisci in providing joint lubrication and nutrition. The permeability of the menisci allows synovial fluid to extrude from the tissue when under compressive forces, providing joint lubrication. , It has been reported that meniscectomy results in a 20% increase in friction in the knee.
The menisci are also believed to play a role in providing nutrition to the knee joint. Using electron microscopy, a system of canals in the menisci have been identified that is in close proximity to nutrient blood vessels. These canals communicate with the synovial cavity and are thought to promote fluid transport, nutrition and joint lubrication.
Proprioception
Proprioception is the perception of joint motion and position in space. This phenomenon is mediated by mechanoreceptors such as Pacinian corpuscles, Ruffini endings and Golgi tendons. Pacinian corpuscles mediate the sensation of joint motion, whereas Ruffini endings and Golgi tendon organs are believed to mediate the sensation of joint position. These mechanoreceptors have been identified in the anterior and posterior horns of the menisci and are thought to play an important role in providing sensory feedback to the knee.
Shock absorption
Shock absorption in the knee joint is a multifactorial phenomenon. Many studies have demonstrated the role of the menisci as shock absorbers in the knee; , however, research has called into question the validity of these studies. Additional work has identified that the knee joint’s surrounding musculature and the hyaline cartilage of the knee likely play a larger role in shock absorption compared with the menisci. ,
Biomechanical Properties of the Meniscus-Associated Ligaments
The meniscal ligaments in the knee joint serve a variety of functions, including connecting the medial and lateral meniscus to each other, connecting the menisci to surrounding soft tissue structures and connecting the menisci to surrounding bony structures. As such, they play a significant role in determining the biomechanical properties and function of the menisci under load.
Anterior intermeniscal ligament
The anterior intermeniscal ligament (AIL), or transverse geniculate ligament, is an anteriorly based structure that functions to connect the anterior horns of the medial and lateral menisci ( Fig. 14.6 ). The true biomechanical role of this ligament is not fully known. Given the nature of the structure as a ‘tie’ between the menisci, it has been postulated that the AIL serves to transmit hoop stresses between the menisci and reduce contact pressures. Early work demonstrated that after sectioning the ligament there was no increase in mean contact pressure experienced at the tibiofemoral articulation when under load. Other data have shown that damage to the AIL results in substantial changes in knee biomechanics, including increasing contact pressures, reducing contact areas and changing the force centre or application which may be due to the role of the AIL in stabilising the menisci. Anatomical studies have also suggested the significance of the AIL in stabilising the menisci, most notably in preventing anterior translation of the medial meniscus. Thus it is suggested to avoid damage to the AIL when performing arthroscopic procedures, such as ACL reconstructions.
