Biomechanics of Cartilage

[문형철]

cartilage의 생체역학적 연구

Biomechanics of Cartilage.pdf

The materials classed as cartilage exist in various forms and perform a range of functions

in the body. Depending on its composition, cartilage is classified as articular cartilage (also

known as hyaline), fibrocartilage, or elastic cartilage. Elastic cartilage helps to maintain the

shape of structures such as the ear and the trachea. In joints, cartilage functions as either

a binder or a bearing surface between bones. The annulus fibrosus of the intervertebral

disc is an example of a fibrocartilaginous joint with limited movement (an amphiarthrosis).

In the freely moveable synovial joints (diarthroses) articular cartilage is the bearing surface

that permits smooth motion between adjoining bony segments. Hip, knee, and elbow are

examples of synovial joints. This chapter is concerned with the mechanical behavior and

function of the articular cartilage found in freely movable synovial (diarthroidal) joints.

In a typical synovial joint, the ends of opposing bones are covered with a thin layer of articular

cartilage (Fig. 5.1). On the medial femoral condyle of the knee, for example, the

cartilage averages 0.41 mm in rabbit and 2.21 mm in humans [2]. Normal articular cartilage

is white, and its surface is smooth and glistening. Cartilage is aneural, and in normal

mature animals, it does not have a blood supply. The entire joint is enclosed in a fibrous

tissue capsule, the inner surface of which is lined with the synovial membrane that secretes

a fluid known as synovial fluid. A relatively small amount of fluid is present in a

normal joint: less than 1 mL, which is less than one fifth of a teaspoon. Synovial fluid is

clear to yellowish and is stringy. Overall, synovial fluid resembles egg white, and it is this

resemblance that gives these joints their name, synovia, meaning “with egg.”

Cartilage clearly performs a mechanical function. It provides a bearing surface with low

friction and wear, and because of its compliance, it helps to distribute the loads between

opposing bones in a synovial joint. If cartilage were a stiff material like bone, the contact

stresses at a joint would be much higher, since the area of contact would be much smaller.

These mechanical functions alone would probably not be sufficient to justify an in-depth

study of cartilage biomechanics. However, the apparent link between osteoarthrosis and mechanical factors in a joint adds a strong impetus for studying the mechanical behavior of articular cartilage.

The specific goals of this chapter are to

■ Describe the structure and composition of cartilage in relation to its mechanical behavior

■ Examine the material properties of cartilage, what they mean physically, and how they can be determined

■ Describe modes of mechanical failure of cartilage

■ Describe the current state of understanding of joint lubrication

■ Describe the etiology of osteoarthrosis in terms of mechanical factors

A comment on terminology seems appropriate. Osteoarthritis is the term commonly used to

describe the apparent degeneration of articular cartilage. Radin has argued that this is a misnomer

since osteoarthritis does not directly involve inflammation. He suggests the term osteoarthrosis,

which is defined as “loss of articular cartilage with eburnation of the underlying

bone associated with a proliferative response [68,69].” In this chapter, the term osteoarthrosis

is used in place of osteoarthritis. Before proceeding through this chapter, the reader should

be familiar with the basic concepts and terminology introduced in Chapters 1 and 2.