**1.3 Metabolism of articular cartilage**

A high level of metabolism exists in articular cartilage. Historically, one of the factors that led to the impression that articular cartilage was inert was the early demonstration that,

Traumatic Chondral Lesions of the Knee Diagnosis and Treatment 183

This hydraulic pressure provides a significant component of the load support of the cartilage, which protects and stress shields the solid phase of the matrix from much of the

Direct blunt trauma, indirect impact loading, or torsional loading of a joint can damage articular cartilage and the calcified cartilage-subchondral bone region without disrupting the surrounding soft tissue. Examples of direct blunt trauma to articular cartilage of the knee include a shoe kick, knees colliding in games such as football and rugby, and falling on a hard surface. Examples of indirect impact and torsional loading include a blow to a bone that forms the subchondral part of a joint, and severe twisting of a joint that is loaded

Clinical experience suggests that there are age-related differences in the risk and patterns of articular surface injuries. High energy bone or joint trauma causes intra-articular osteochondral fractures in people of all age, but older people and people with more osteopenic bone tend to have more severely comminuted fractures (Buckwalter et al, 1993; Buckwalter and Lane, 1996). Chondral fractures associated with participation in sports generally occur in skeletally mature people, whilst osteochondral fractures associated with participation in sports typically occur in skeletally immature people or young adults. This difference may result from age-related changes in the mechanical properties of the articular surface, including the uncalcified cartilage, the calcified cartilage zone, and the subchondral bone (Buckwalter et al, 1993; Buckwalter and Lane, 1996). That is, age-related alteration in the articular cartilage matrix decreases the tensile stiffness and strength of the superficial zone, and the calcified cartilage zone. The subchondral bone region minimizes fully following completion of skeletal growth, presumably creating a marked difference in mechanical properties between the uncalcified cartilage and the calcified cartilage-

Articular cartilage can withstand single or multiple moderate and occasionally high impact loads. However, a number of studies have addressed the effects of either a single excessive high-impact force causing injury to the cartilage without a break in the surface, or repetitive below-trauma threshold loads causing an accumulation of damage to the cartilage by repeated application of the load. Both can lead to chondrocyte death, matrix damage, fissuring of surface, injury to underlying bone, and thickening of the tide mark region. At a certain threshold of impact loading, the cartilage may be sheared off the subchondral bone

Excessive impact or torsional joint loading causes three types of articular cartilage injury: chondral damage without visible tissue disruption; disruption of articular cartilage alone (chondral fractures and flaps); and disruption of articular cartilage and subchondral bone (osteochondral fractures) (Buckwalter et al, 1988). Intensity and rate of loading, muscle contractions that affect the transmission of force to the articular surface, age, and genetically determined differences in articular cartilage may influence the type of articular surface

load burden (Pearle et al, 2005).

(Williams and Wilkins, 1998).

(Mankin et al, 2005).

**1.7 Age related chondral lesions** 

subchondral bone region (Williams and Wilkins, 1998).

**1.8 Response of articular cartilage to blunt impact** 

injury in a given individual (Buckwalter et al, 1988).

**1.6 Mechanism of articular cartilage injury** 

although articular cartilage had a well-defined glycolytic system, oxygen use was considerably lower in articular cartilage than in other tissues. This difference subsequently was found to be related to the spares cell (*sic*) population rather than to a lack of metabolic activity per cell. Nevertheless, articular cartilage chondrocytes rely principally on the anaerobic pathway for energy production (Oegema and Thompson, 1989).

Chondrocytes synthesize and assemble the cartilaginous matrix components and direct their distribution within the tissue. These synthetic and assembly processes are complex. They involve synthesis of proteins; synthesis of glycosaminoglycan chains, and their addition to the appropriate cores; and secretion of the completed molecules into the extra-cellular matrix (Guilak et al, 1997).

Chondrocytes are responsible for the synthesis, assembly and sulfation of the proteoglycan molecule. However, in normal tissue, in repair and degradation processes proteoglycans of articular cartilage are continually being broken down and released from the cartilage. This activity is a normal event in the maintenance of the tissue and can occur at an accelerated rate. The rate of catabolism can be affected by soluble mediators and by various types of joint loading (Mankin et al, 2005b).

Collagen is much more stable than the proteoglycan components. However, the collagen network is subject to metabolism, and in osteoarthritic or injured cartilage the collagen turnover increases, but as yet little is known about the mechanism of collagen breakdown (Mankin et al, 2005b).

 The source of nutrients for articular cartilage is somewhat of an enigma. Because the tissue is avascular in adult life most investigators believe that nutrients diffuse through the matrix, either from the surrounding synovial fluid or from underlying bone (Mankin et al, 2005b).

#### **1.4 Biomechanics of articular cartilage**

The articular cartilage of diarthrodial joints is subject to high loads applied statically and repetitively for many decades. Thus, the structural molecules, which include collagens and proteoglycans must be organized into a strong, fatigue-resistant and tough solid matrix capable of sustainig the high stresses and strains developed within the tissue (Soltz and Ateshian, 2000). The solid matrix is porous and permeable, and very soft. Water, 65 to 80 percent of the total weight of normal articular cartilage, resides in the microscopic pores. This water may be caused to flow through the porous-permeable solid matrix by a pressure gradient or by matrix compaction. The biomechanical properties of articular cartilage therefore are understood best when the tissue is viewed as biphasic material composed of a solid phase and a fluid phase (Soltz and Ateshian, 2000).

#### **1.5 Local transmission of load**

Although it is porous, the solid phase of the cartilage has low permeability due largely to a high frictional resistance to fluid flow. This causes a high interstitial fluid pressurization in the fluid phase, which contributes more then 90 percent of the load transmission function of cartilage (Soltz and Ateshian, 2000). The high pressurization of the fluid phase and the Low permeability of the solid phase establish both the stiffness and the visco-elastic properties of cartilage (Felson et al, 2000).

This hydraulic pressure provides a significant component of the load support of the cartilage, which protects and stress shields the solid phase of the matrix from much of the load burden (Pearle et al, 2005).
