Joint cartilage
As in all joints, the surface of the bones in the knee joint is covered with a layer of cartilage. Cartilage is elastic and allows the joint surfaces to glide smoothly over each other. This gives the joints their mobility.
This tissue, up to 5 mm thick, contains neither nerves nor blood vessels and can absorb shocks and pressure. Nutrients from the blood are released via the synovial membrane into the joint fluid. Pores on the cartilage surface finally take up the nutrients. Movement promotes this process: with every step, joint fluid is squeezed out and taken up again when the load is released. In this way the joint cartilage stays healthy, because tissue wear and tissue renewal are in balance.
Cartilage injury
In principle, joint cartilage allows the joint surfaces to glide optimally throughout life. Damage is caused by injuries, overload due to misalignment, excess weight, or loss of the meniscus or of ligament stability. Metabolic diseases such as gout, bacterial infections and immune diseases such as rheumatism also cause cartilage damage.
Cartilage healing
Overall, the body needs a lot of time to repair cartilage damage, because metabolism in cartilage tissue is slow. In addition, the ability of cells to repair is limited in adults, unlike in children. These two aspects make it impossible for adult joint cartilage to repair itself. Higher-grade cartilage injuries must therefore be treated to avoid serious consequences such as osteoarthritis.
How the body reacts to a cartilage injury depends on its extent. A distinction is made between chondral defects (affecting only the upper layers of cartilage) and osteochondral defects (damage to the cartilage and the underlying bone). In adults, an injury larger than 2 mm has little chance of healing, so that osteoarthritis of the affected joint develops over time.
If the bone beneath the cartilage is also affected, a fibrous replacement cartilage can form from the escaping bone marrow blood. However, this replacement cartilage cannot bear as much load as joint cartilage, so osteoarthritis develops here too. In children, replacement cartilage is much better, because they have a high proportion of repairing stem cells in their bone marrow.