Articular cartilage injury of the knee

Articular cartilage is the firm, smooth coating on the ends of the bones where they meet in a joint. It lets the surfaces glide against one another with very little friction and helps spread load across the joint. It has no direct blood supply of its own, which is part of why cartilage has a limited ability to heal on its own.

The meniscus is a separate structure. It is a C-shaped piece of tissue that sits between the bones and works like a cushion and shock absorber, helping distribute load. A meniscus tear and an articular-cartilage injury are different problems, even though both involve "cartilage" in everyday language and can cause overlapping symptoms. Telling them apart matters, because they are evaluated and treated differently.

A focal cartilage defect is a localized area of damage on an otherwise reasonably healthy joint surface — a defined "pothole," rather than widespread wear. Generalized osteoarthritis is diffuse thinning and change across much of the joint. The distinction is important: the cartilage-restoration options discussed elsewhere on this site are aimed at selected focal defects. They are not a treatment for generalized arthritis, and repairing one focal area does not reverse arthritis that is already widespread.

A focal cartilage defect can follow a distinct traumatic event, or it can develop over time — and sometimes it is a combination. Understanding how it arose helps individualize the evaluation.

• A direct injury to the knee, such as a fall or a blow
• A kneecap dislocation or other episode of instability, which can shear off a piece of the surface
• An osteochondral injury, in which cartilage and a bit of the bone beneath it are damaged together
• Osteochondritis dissecans, in which a segment of bone and its overlying cartilage lose blood supply and can separate
• Repetitive loading over time, or damage found alongside another knee problem

When a focal cartilage injury does cause symptoms, they can include pain (often with activity or with load on the affected part of the knee), swelling, a catching or clunking sensation, or difficulty with specific activities. A loose fragment can sometimes cause the knee to catch or lock.

Symptoms and imaging findings do not always line up. A cartilage change can be visible on an MRI without being the source of a person's pain, and symptoms can be present that imaging only partly explains. For that reason, findings are interpreted together with the history and examination — not treated in isolation — and not every cartilage finding needs treatment.

Evaluation usually begins with the history of what happened and a physical examination. Weight-bearing X-rays are used to look at the joint and the space within it, and alignment views may be added when the way the leg is loaded is relevant. An MRI is used selectively — to characterize the cartilage surface and the bone beneath it, and to look for associated problems such as a meniscus or ligament injury — when that information will affect the plan.

Because the right path depends on the individual, several things are considered together rather than any single measurement:

• The location of the defect and how big it is
• The condition of the bone underneath the cartilage
• The alignment of the leg and how load passes through the knee
• The stability of the knee and the function of the meniscus
• Your activities, goals, and prior treatment
• Your age or skeletal maturity, where relevant
• How much arthritis is already present in the joint

Nonsurgical care can include activity modification, rehabilitation focused on strength and movement, medication when appropriate, and selected injections in some situations. These measures aim to reduce symptoms and improve function. They are not a way to regrow or restore the cartilage surface, and they do not need to promise cartilage restoration to be worthwhile — for many people they meaningfully improve how the knee feels and works.

Surgery is not automatic. It may become part of the conversation when symptoms are persistent and limit function, when there is a focal, treatable lesion, and when appropriate nonsurgical care has not resolved the problem. What kind of procedure is even a possibility depends on the individual defect and the knee as a whole — those options, and their limits, are described on the cartilage-restoration page.

If a cartilage injury is treated but an important contributing problem is left unaddressed, the result can be less durable. Depending on the individual knee, a surgeon may consider addressing alignment, patellar tracking or instability, ligament instability, or meniscus deficiency — either at the same operation or in a staged manner. Whether any of that applies is an individualized decision made with your surgeon; it is not part of every cartilage problem.