Cartilage

Articular Cartilage, also called Hyaline Cartilage, covers the joint surfaces where the femur, tibia and patella articulate with each other. This glistening white substance has the consistency of firm rubber but has very low friction to allow sliding motion with almost no resistence. With normal joint fluid for lubrication, the surface is more slippery than ice on ice and allows smooth and easy knee joint motion for decades.

Hyaline cartilage is a tissue composed of Type II collagen and other special molecules, including glycosaminoglycans (GAG), which help to attract water into the cartilage, giving it viscoelasticity to dampen shock and distribute forces to the bone underneath. Embedded in this matrix are chondrocytes, living cells that initially produced the matrix, and now maintain it.

Cartilage Defects

Damage to the articular cartilage can occur for several reasons:

- trauma, such as car accidents or falls;

- injuries, such as patellar dislocations or ACL tears;

- joint abnormalities, such as osteochondritis dissecans (OCD);

- previous surgery, such as removal of meniscal tissue;

- repetitive injury over time.

Articular cartilage damage is common, being present in over 60% of knees that undergo arthroscopy. Unfortunately, articular cartilage has a very low capacity for healing, since it does not contain blood vessels. Both superficial and full-thickness cartilage injuries may lead to progressive damage, similar to a pothole increasing in size with time, and can eventually end in osteoarthritis. The larger the initial cartilage injury, the faster the potential progression of arthritis.

This video demonstrates the appearance of a cartilage defect during arthroscopy.

: Cartilage defect of the medial femoral condyle during arthroscopy (left) and during open surgery for ACI (right)

Osteoarthritis

Degenerative joint disease, unlike an isolated chondral defect, involves the entire joint although it may have originally started locally as a focal cartilage defect. Over time, the cartilage defect enlarges and is worn down to the underlying bone. Increased pressure on the bone causes it to remodel and become stiffer and harder. Osteophytes (bone spurs) form and the joint capsule becomes thicker and inflamed. The end result is a permanent dysfunction of the joint.

This degenerative process is complex with a combination of abnormal mechanical forces and destructive enzymes. There still are a number of unknown questions regarding the rate of progression, but what is known is that once the degenerative process is initiated it is difficult to stop. That is the reason why it is so important to prevent or halt this negative cycle before painful symptoms significantly restrict your level of activity and hamper your quality of life. With end stage osteoarthritis, your only course of treatment then becomes a total joint replacement.