DEFINING OSTEOARTHRITIS

Ostcoarthritis is no a single disease, but an extremely common group of disorders that affect synovial joints and are linked to advancing age. A satisfactory clinical definition of osteorathritis has not been determined due to the heterogeneity of the condition and the fact that it can result from a number of different joint insults. Although definitions of osteorthritis vary, most acknoledge that osteoarthritis is characterized by degenerative changes in articular cartilage. Thus, osteoarthritis has been difined as an inherently noninflammatory disorder of movable joints characterized by deterioration of articular cartilage and by the formation of new bone at the joint surfaces and margins.

Osteoarthritis has also been defined as a combination of interactive degradative and repair process in cartilage, bone, and synovium with secondary components of inflammation. Although originally thought of as a noninflammatory, hypertrophic form of arthritis, osteoarthritis is now known to involve a disruption in the normal balance between cartilage synthesis and degradation, often involving some degree of inflammation.

A third definition, offered by the American College of Rheumatology (ACR), defines osteoarthritis as a heterogeneous group of conditions that lead to joint symptoms and signs which are associated with defective integrity of articular cartilage, in addition to related changes in the underlying bone at the joint margin.

Joint Physiology

Osteoarthritis is primarily a disorder of articular cartilage and subchondral bone, However, all tissues of the synovial joint are involved in the disease process.

Articular Cartilage

Articular  cartilage distributes loads over bone surfaces and provides a low friction surface over which bones can move. The articular cartilage contains no blood vessels, nerves, or lymphatic vessels, and is largely composed of extracellular matrix tissue. Only 1% to 2% of the cartilage volume is occupied by cells, while 98% to 99% of the tissue volume is composed of matrix tissue.

Chondrocytes

 The cells of articular cartilage are called chondrocytes. Although these cells are metabolically very active, they normally do not divide after adolescence. Since articular cartilage contains no blood vessels, chondrocytes must receive their nutricnts by diffusion from the underlying subchondral bone, or from the synovial fluid that bathes the cartilage surface. These cells are responsible for the production and maintenance of the extracellular matrix.

Extracellular Matrix

The critical biomechanical properties of articular cartilage depend on the integrity of its extracellular matrix. The three primary constituents of the extracellular matrix are proteoglycans, type II collagen and water.

Proteoglycans

Proteoglycans make up most of the volume of the extracellular matrix and are responsible for the stiffness of the tissue. As many as 200 large proteoglycan molecules (called aggrecan) assemble around a hyaluronic acid chain to form conglomerates, or aggregates. The molecular structure of these aggregates creates an overall negative charge, which attracts molecules of water. Together, the water and proteoglycan aggregates are restrained by a network of crosslinked collagen fibers.

Aggrecan consists of a protein core in which proteoglyean molecules, such as chondroitin sulfate, are attached. About 100 chondrohin sulfate chains and about 30 keratan sulfate chains are attached to a single core protein. The protein and its attached chains are referred to as an aggreean monomer. About 100 agrecan monomers are linked to a string of byaluronic acid to form a proteoglycan aggregate. Other proteoglycans and proteins have been identified in articular cartilage. Compounds such as fibromodulin, fibronectin, and anchorin II may play important roles in maintaining the integrity and function of the extracellular matrix tissue.

Collagen

 This type of collagen is found in only a few other body tissues. Smaller quantities of types IX, X, XI and VI collagen may also be found. In the extracellular matrix, type II collagen, forms a tight network that restricts the hydration and expansion of the proteoglycan aggrecans. This collagen network provides articular cartilage with tensile strength between and resistance to shearing forces, Type IX collagen forms cross links between the collagen fibers, increasing the strength of the collagen "net". It also forms links between the collagen fibers and aggrecan molecules. Type X and type VI collagen are associated with metabolically active chondrocytes. Their levels may be increased in patients with osteoarthritis.

Water

When weight is applied to a joint and the cartilage is compressed, water molecules are forced out of the extracellular matrix. When the pressure is removed, water moves back into the matrix until the swelling pressure of the proteoglycans is balanced by the resistance of the collagen network. This gives articular cartilage the property of reversible deformation.

Synovial Joint

Arthritis is a general term for a group of disorders that cause inflamed. swollen, and painful joints. Joints are found wherever two or more bones or cartilage and bone meet. There are many different kinds of joints in the body. They can be categorized according to structure or function. Among the structure variations are:

>   Fibrous joints.

>   Cartilaginous joints, and

>    Synovial joints

Joints can also be classified according to their ability to provide movement immovable joints are called synarthroses, slightly movable joints are called amphiarthroses, and freely movable joints are called diarthroses. Diarthroses are the most common joints in the body and they are the primary type of joint found in the extremities. Diarthrodial joints are often referred to as synovial joints because they contain a synovial membrane and synovial fluid. Arthritis occurs primarily in freely movable, synovial joints.

Fibrous joints, such as those that are found between the separate bony plates that make up the skull are fastened tightly together by a thin layer of fibrous connective tissue. In general, these joints allow little, if any movement and are therefore synarthroses.

The bones of cartilaginous joints are connected by cartilage. One example is the joint are connected by cartilage. One example is the joint between the public bones of the pelvic girdle. These joints allow limited movement, and are, therefore, amphiarthrotic.

The synovial, or diarthrotic, joint is of greatest interest in the study of arthritis. These joints are freely movable, and are much more complex than fibrous and cartilaginous joints.

Among the joints most commonly affected by osteoarthritis and rheumatoid arthritis are the joints in the fingers, toes, hips knees, wrists, and ankles. The proximal and distal interphalangeal joints of the fingers are hinge joints, while the metacarpophalangeal joints are modified hinge joints or condyloid joints. Hinge joints permit movements in different planes.

The hip and shoulder joints are ball and socket joints. This type of joint consists of a bone with a globular or slightly egg-shaped head that articulates with the cup-shaped cavity of another bone. This arrangement permits movement in a wide variety of planes, and includes movement around a central point or axis.

The knee joint is the largest and most complex of the synovial joints. It includes the upper leg bone the lower leg bone (the tibia), and the kneecap (patella). Although the knee is sometimes considered a modified hinge joint, the connection between the femur and tibia more closely resembles a condyloid joint, while the atriculation of the femur and patella is a gliding joint. In addition, this joint contains two crescent-shaped, fibro-cartilaginous menisci that project into the space between the femur and tibia. The menisci contribute to the stability of the joint and help distribute the body weight that is applied to it. The knee also contains bursae that act as cushions and facilitate the movement of tendons over bony surfaces.