What is Articular Cartilage?

Three different types of cartilage are found in the body. Articular or hyaline cartilage (covers joint surfaces), fibrocartilage (knee meniscus, vertebral disk), and elastic cartilage (outer ear). These different cartilages are distinguished by their structure, elasticity, and strength. In some joints, such as the knee, both articular cartilage and fibrocartilage are found functioning side-by-side, as distinctly different structures with different functions.
Articular cartilage is a complex, living tissue that lines the bony surface of joints. It’s function is to provide a low friction surface enabling the joint to withstand weight bearing through the range of motion needed to perform activities of daily living as well as athletic endeavors. Those daily activities include walking, stair climbing, and work-related activities. In other words, articular cartilage is a very thin shock absorber. It is organized into five distinct layers, with each layer having structural and biochemical differences.

How is Articular Cartilage Injured?

Articular cartilage injuries can occur as a result of either traumatic mechanical destruction, or progressive mechanical degeneration (wear and tear.) With mechanical destruction, a direct blow or other trauma can injure the articular cartilage. Depending on the extent of the damage, and the location of the injury, it is sometimes possible for the articular cartilage cells to heal. Articular cartilage has no direct blood supply, thus it has little or no capacity to repair itself. If the injury penetrates the bone beneath the cartilage, the underlying bone provides some blood to the area, improving the change of healing. Occasionally an articular cartilage fragment completely breaks loose from the underlying bone. This chip, called a loose body, may float in the joint interfering with normal joint motion.

Mechanical degeneration (wear and tear) of articular cartilage occurs with the progressive loss of the normal cartilage structure and function. This initial loss begins with cartilage softening then processes to fragmentation. As the loss of the articular cartilage lining continues, the underlying bone has no protection from the normal wear and tear of daily living and begins to breakdown, leading to osteoarthritis. Also known as degenerative joint disease, osteoarthritis is characterized by three processes: 1) a progressive loss of cartilage, 2) the body’s attempted to repair the cartilage, and 3) finally, the destruction of the bone underneath the articular cartilage. The cause of osteoarthritis is poorly understood, but lifelong moderate use of normal joints does not increase the risk. Factors such as high impact twisting injuries, abnormal joint anatomy, joint instability, inadequate muscle strength or endurance, and medical or genetic factors can contribute to osteoarthritis.

What are the Signs of an Articular Cartilage Defect (Injury)?

In many cases, a patient will experience knee swelling and vague pain. At this point continued activity may not be possible. If a loose body is present, words such as “locking” or “catching” might be used to describe the problem. With mechanical degeneration (wear and tear), the patient often experiences stiffness, decreased range of motion, joint pain, and/or swelling.

How is an Articular Cartilage Defect (Injury) Diagnosed?

The physician examines the knee, looking for decreased range of motion, pain along the joint line, swelling, fluid on the knee, abnormal ailment of the bones making-up the joint, and ligament or meniscal injury. Injuries to the articular cartilage are difficult to diagnose, and evaluation with MRI (magnetic resonance imaging) or arthroscopy may be necessary.

When is Surgery Necessary?

When a joint is injured, the body releases enzymes that may further breakdown the already damaged articular cartilage. Injuries to the cartilage that do not extend to the bone will generally not heal on their own. Injuries that penetrate to the bone may heal, but the type of cartilage that is laid down is structurally unorganized and does not function as well as the original articular cartilage. Defects smaller than 2cm have the best prognosis and treatment options. Those options include arthroscopic surgery using techniques to remove damaged cartilage, and increase blood flow from the underlying bone (e.g. drilling, pick procedure.) For larger defects, it may be necessary to transplant cartilage from other areas of the knee (joint).
For patients with osteoarthritis, non-surgical treatment consists of physical therapy, lifestyle modification (e.g. reducing activity), bracing, supportive devices, oral and injection drugs (i.e. non-steroidal anti-inflammatory drugs, cartilage protective drugs), and medical management. Surgical options are very specific to osteoarthritis severity and can provide a reduction in symptoms that are generally only short lived. Tibial or femoral osteotomics (cutting the bone to rebalance joint wear) may reduce symptoms, help to maintain an active lifestyle, and delay the need for total joint replacement. Total joint replacement can provide relief for the symptom of advanced ssteoarthritis, but generally requires a change in a patient’s lifestyle and/or activity level.