**1.1 Anatomy and knee joint function**

The knee joint is classified as a modified hinge joint, with its structure providing a highly mobile and complex joint (fig.1). It consists of two condylar joints between the medial and lateral condyles of the femur and the corresponding plateaus of the tibia, and anteriorly a synovial plane patellofemoral joint. The articular surface of the knee is covered by Hyaline cartilage that is supported by subchondral bone. This Hyaline cartilage is very soft and yields its interstitial water easily when compressed yet it is very stiff in tension along planes parallel to the articular surface (Buckwalter and Mow, 2003). Intact cartilage provides a smooth, lubricated gliding surface with a coefficient of friction better than most man-made bearing materials (Buckwalter and Mow, 2003). In the knee articular cartilage distributes the loads of articulation, thereby minimizing peak stresses acting on the subchondral bone whilst the tensile strength of the tissue maintains its structural integrity under loading. These biomechanical properties make the tissue remarkably durable and wear resistant, enabling it to last many decades, even under high and repetitive stresses (Buckwalter and Mow, 2003).

Fig. 1. Structure of knee joint (www.arthroscopy.com)

Traumatic Chondral Lesions of the Knee Diagnosis and Treatment 181

Cartilage has an organized layered structure that can be functionally and structurally divided into four zones (fig.3): Superficial zone, the middle (transitional) zone, the deep zone and a calcified cartilage zone. The superficial zone is the articulating surface that provides a smooth gliding surface and resists sheer. This zone makes up approximately 10 to 20 percent of articular cartilage thickness. It has the highest collagen contents of the zones. The collagen fibrils in this zone are densely packed and have a highly ordered alignment parallel to the articular surface (Mow, 1989; cited by Pearle et al, 2005). This superficial zone has the lowest compressive modulus and will deform approximately 25 times more than the

The middle zone encompasses 40 to 60 percent of the articular cartilage volume. This zone has a higher compressive modulus than the superficial zone and a less organized arrangement of the collagen fibers. The deep zone makes up 30 percent of the cartilage, and consists of large diameter collagen fibril layers perpendicular to the articular surface. This layer has the highest compressive modulus. The tide mark separates the deep zone from the calcified cartilage, which rests directly on the subchondral bone. The calcified cartilage contains small cells in a chondral matrix speckled with apatitic salts (Mow, 1989; cited by

Normal adult cartilage is composed of 75 percent water and 25 percent solids. The solids consist of collagen and proteoglycan, and a fluid phase composed of water and ions

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,

middle zone (Pearle et al, 2005).

(Eichellberger, 1951; cited by Hayes et al, 2001).

Fig. 3. Articular cartilage zones (Buckwalter and Mow, 2003)

**1.3 Metabolism of articular cartilage** 

Pearle et al, 2005).

Flexion and extension are the primary movements of the knee. When the knee is flexed a small amount of rotation can occur. The patella glides along the distal portion of the femur during movements of flexion to extension.

Semi-lunar shaped fibrocartilage discs, triangular in cross-section, form the menisci that are found between the tibia and femur. They deepen the articular surface, improve joint congruency and assist in shock absorption. The medial meniscus is attached to the joint capsule and the deep fibres of the medial collateral ligament. The lateral meniscus does not have these attachments, resulting in increased mobility (Marieb, 1992).

The intra-articular ligaments consist of the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL provides stability to anterior translation, and prevents the posterior displacement of the femur on the tibia. The PCL prevents anterior displacement of the femur on the tibia.

The main extra-articular ligaments comprise of the medial and lateral collateral ligament and provide side-to-side stability.

Stability of the knee joint is therefore provided by two menisci, intra and extra-capsular ligaments, and the dense muscular structures surrounding the joint. Injuries frequently occur to one or more of these stabilizing structures.

#### **1.2 Structure of articular cartilage**

Articular cartilage consists of various substances, each of which contribute to its overall integrity, durability, deformability and ability to repair itself (Hayes et al, 2001).

From embryological life articular cartilage is derived from mesenchymal cells and develops at the future end of epiphyseal bone (Hayes et al, 2001). Cartilage is composed of a matrix of collagen surrounded by proteins and negatively charged proteoglycans (fig.2). The roll of collagen is to serve as an anchor to the proteoglycan matrix protecting chondrocytes. Articular cartilage consists mainly of types II, IX, and XI collagen. Collagen helps to resist extrinsic forces during loading (Vigarita, 1999; cited by Hayes et al, 2001).

Fig. 2. Spatial relations of collagen, proteoglycans and cells in cartilage (Kocheta and Toms, 2004)

Flexion and extension are the primary movements of the knee. When the knee is flexed a small amount of rotation can occur. The patella glides along the distal portion of the femur

Semi-lunar shaped fibrocartilage discs, triangular in cross-section, form the menisci that are found between the tibia and femur. They deepen the articular surface, improve joint congruency and assist in shock absorption. The medial meniscus is attached to the joint capsule and the deep fibres of the medial collateral ligament. The lateral meniscus does not

The intra-articular ligaments consist of the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL provides stability to anterior translation, and prevents the posterior displacement of the femur on the tibia. The PCL prevents anterior

The main extra-articular ligaments comprise of the medial and lateral collateral ligament

Stability of the knee joint is therefore provided by two menisci, intra and extra-capsular ligaments, and the dense muscular structures surrounding the joint. Injuries frequently

Articular cartilage consists of various substances, each of which contribute to its overall

From embryological life articular cartilage is derived from mesenchymal cells and develops at the future end of epiphyseal bone (Hayes et al, 2001). Cartilage is composed of a matrix of collagen surrounded by proteins and negatively charged proteoglycans (fig.2). The roll of collagen is to serve as an anchor to the proteoglycan matrix protecting chondrocytes. Articular cartilage consists mainly of types II, IX, and XI collagen. Collagen helps to resist

Fig. 2. Spatial relations of collagen, proteoglycans and cells in cartilage (Kocheta and Toms,

integrity, durability, deformability and ability to repair itself (Hayes et al, 2001).

extrinsic forces during loading (Vigarita, 1999; cited by Hayes et al, 2001).

have these attachments, resulting in increased mobility (Marieb, 1992).

during movements of flexion to extension.

displacement of the femur on the tibia.

occur to one or more of these stabilizing structures.

and provide side-to-side stability.

**1.2 Structure of articular cartilage** 

Type II collagen fibre

Chondrocyte

Proteoglycan

2004)

Cartilage has an organized layered structure that can be functionally and structurally divided into four zones (fig.3): Superficial zone, the middle (transitional) zone, the deep zone and a calcified cartilage zone. The superficial zone is the articulating surface that provides a smooth gliding surface and resists sheer. This zone makes up approximately 10 to 20 percent of articular cartilage thickness. It has the highest collagen contents of the zones. The collagen fibrils in this zone are densely packed and have a highly ordered alignment parallel to the articular surface (Mow, 1989; cited by Pearle et al, 2005). This superficial zone has the lowest compressive modulus and will deform approximately 25 times more than the middle zone (Pearle et al, 2005).

The middle zone encompasses 40 to 60 percent of the articular cartilage volume. This zone has a higher compressive modulus than the superficial zone and a less organized arrangement of the collagen fibers. The deep zone makes up 30 percent of the cartilage, and consists of large diameter collagen fibril layers perpendicular to the articular surface. This layer has the highest compressive modulus. The tide mark separates the deep zone from the calcified cartilage, which rests directly on the subchondral bone. The calcified cartilage contains small cells in a chondral matrix speckled with apatitic salts (Mow, 1989; cited by Pearle et al, 2005).

Normal adult cartilage is composed of 75 percent water and 25 percent solids. The solids consist of collagen and proteoglycan, and a fluid phase composed of water and ions (Eichellberger, 1951; cited by Hayes et al, 2001).

Fig. 3. Articular cartilage zones (Buckwalter and Mow, 2003)
