Biomechanics of Soft Tissues

Introduction

1. Soft tissues include skin, cardiovascular tissues, articular cartilage, muscles, tendons and ligaments. All soft tissues are composite materials. Collagen and elastin fibers are the common components of soft tissues and they have most important properties affecting the overall mechanical properties of the soft tissues in which they exist. Collagen is a protein in shape of crimped fibrils which are joined together into fibers. Fibril can be considered as a spring and every fibre as an assemblage of fibril springs. The function of collagen is to withstand axial tension. As collagen fibers have high aspect ratio (length to diameter ratio), they are not effective to withstand compressive loads. collagen fiber acts like a mechanical spring as it stores the energy supplied to it by stretching the fiber. When the load is removed, the stored energy is used to return to the unstretched state. The individual fabrics of the collagen fibers are submerged in a gel-like ground substance consisting largely of water. Since collagen fibers consists of solid and water substance, it shows viscoelastic mechanical properties.

2. Elastin is another fibrous protein and its properties are similar to the properties of rubber. Elastin fibers consists of elastin and microfibril. Elastin fibers are highly extensible and the extension is reversible even at high strain. In other words elastin fibers have a low elastic modulus. The mechanical properties of soft tissues depend upon the geometric configuration of collagen fibers and there interaction with elastin fibers. Collagen fibers have comparatively higher modulus and show viscoelastic mechanical behaviour.

Tendons And Ligaments

1. Both tendons and ligaments are fiberous connective tissues (Refer to para 23 of chapter 1). Ligaments are supporting tissues. They join bones and provide support to the joints for stability. Tendons are connective tissues and they join muscles to the bones. Another function of tendons is to help in executing joint motion by transmitting mechanical force from muscles to bones. Both tendons and ligaments are passive tissues i.e., they cannot generate force by contraction as done by muscles.

2. Tendons have higher modulus of elasticity (Stiffer) to stand higher stresses with small strain. They also have higher tensile strength. Hence at joints where space is limited, tendons enable the attachments of muscles with the bones. Since tendons can support large loads with small strains, hence tendons enable the muscles to transmit forces to the bones without wasting energy in its stretching.

3. The mechanical behaviour of both tendons and ligament depends upon their composition which vary considerably in each direction of loading. The stress and strain diagram for a typical tendon is as shown in the figure. As collagen fibers of tendon require very little force to straighten and rubber like elastin fibers of tendon also do not require very high force, we get a large strain (up to 0.05) with a small applied force. The curve is flat in this portion. The tendon becomes stiffer after this as the crimp is straightened. Hence stiff and viscoelastic nature of the collagen fibers begin to take higher load with slight strain. Tendons are tested to function in the body up to ultimate strains of about 0.1 and ultimate stresses of about 60 MPa.

As the area under the curve is small, hence a tendon does not absorb much energy of muscles and maximum energy is passed on to the bones.

4. As a tendon has a viscoelastic nature, its properties are dependent upon the rate of loading. When a tendon is stretched rapidly, there is less time for the ground substance to flow, hence a tendon becomes stiffer. However, a tendon can release to original shape in a slow manner on unloading. Tendon takes more energy on stretching during rapid loading and releases less energy on slow unloading. The hysteresis loop of loading and unloading is shown in the figure. Some energy is dissipated in tendon during loading & unloading process.

5. Ligaments are also composite materials containing crimped collagen fibers surrounded by ground substance. Ligaments contain a greater properties of elastics (elastic fibers) which contribute to their higher extensibility but lead to lower strength and stiffness. Ligaments are viscoelastic like tendons and exhibit hysteresis on loading & unloading. Ligaments rupture at a stress of about 20 MPa, yield at about 5 MPa and deform at strain of about 0.25. Some energy in ligament is dissipated in causing the flow of fluid within the ground substance.