Meniscus Biomechanics

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.