Cartilage Breakdown and ArthritisCartilage There are many different types of cartilage in the body, but as far as osteoarthritis is concerned, articular cartilage is the type affecting our joints. Healthy articular cartilage, found at bone ends, is as smooth as ice, providing a friction-free surface for painless joint movement and shock absorption.
It is approximately 65-80% water, plus a mixture of chondrocytes (specialized cells), collagen, and proteoglycans.
Collagen, a versatile protein, provides the elasticity and shock absorption properties within cartilage. It's a connective tissue that helps hold muscles, bones, and other bodily components together, and is a ladder-like structure that holds the proteoglycans in place. It's very strong and resilient.
Proteoglycans, large molecules composed of protein and sugar, attach to and weave through the collagen framework. With the consistency of gelatin, proteoglycans absorb and release water like a sponge. They give cartilage its resiliency. Proteoglycans rapidly absorb synovial fluid when pressure is off your joint, then squeeze the fluid out again when pressure is back on the joint. Proteoglycans direct the flow of fluids circulating in the cartilage and the joint. They're like springs. These "springs" are weaved through the "ladder-like" structure of the collagen.
Chondrocytes, cells present throughtout the cartilage matrix, are the work horses that produce new collagen and proteoglycans. Chondrocytes release enzymes to breakdown and remove old proteoglycans and collagen, and then make new ones.
Water, collagen, proteoglycans, and chondrocytes work together to help maintain smooth, pain-free movement in your joints. When you put pressure on a joint, by stepping or gripping, muscles move the bones and the cartilage is compressed, or squeezed. It's the "springs"(proteoglycans) that are compressed. The water is squeezed out from within the "springs" and evenly spread across the joint, providing a cushion. When pressure is released from the joint (when you lift your leg to take a step) the water rushes back into the cartilage, providing nourishment and fluid.
If your cartilage is damaged or loses its ability to attract and retain water, it becomes dry, thin, cracked, and no longer provides a smooth, slick surface.
What Causes Cartilage Breakdown? With primary osteoarthritis, scientists believe that compounds within the cartilage somehow become scrambled. The proteoglycans(the springs) become separated from the collagen(ladder-like structure) and randomly flow free into the joint fluid and out of the cartilage, taking along with it its water retaining abilities from within the cartilage. The cartilage is left dry and thin, leading to breakdown. At the same time, the newly floating proteoglycans draw excess fluid into the joint capsule, which often results in swelling.
Two Theories by Scientists: In healthy cartilage, enzymes break down and build up collagen and proteoglycans at an even rate. With primary osteoarthritis, the chondrocytes overproduce enzymes that break down collagen and proteoglycan molecules. An excess of destructive enzymes leads to weakened collagen and decreased proteoglycans. OR
In the reverse case of the previous theory, chondrocytes produce too many proteoglycans and collagen molecules. The overabundant molecules draw extra fluid into the joint and wash away most of the chondrocytes, leaving the cartilage without its necessary molecules for regeneration.
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