Avascular necrosis (osteonecrosis)

What it means

Bone is living tissue that constantly renews itself, and like any living tissue it depends on a steady blood supply for oxygen and nutrients. Avascular necrosis, also called osteonecrosis, happens when that blood supply to a section of bone is cut off or severely reduced. Without nourishment, the bone cells in that area die, and the bone gradually loses its strength and structure. The hip (the femoral head, the ball at the top of the thigh bone) is the most commonly affected site, though it can occur in the knee, shoulder, or other joints.

Why it appears on a CT or MRI report

MRI is the most sensitive way to detect avascular necrosis, often showing changes before an X-ray would, sometimes even before symptoms start. Reports typically describe a well-defined area within the bone, often near a joint surface, with a characteristic band-like border separating dead bone from healthy bone. As the condition progresses, the report may note flattening or collapse of the bone surface, or if the joint cartilage above it has been affected. Staging systems are often used to describe how advanced the changes are, from early bone marrow changes to later collapse and joint damage.

What it usually means

Avascular necrosis has several known causes: long-term corticosteroid use, heavy alcohol use, certain blood disorders, prior trauma or fracture near a joint, and decompression sickness are among the most common. In some cases no clear cause is found. Early on, many people have no symptoms at all, or only mild discomfort, and the finding is picked up incidentally. As the affected bone weakens, pain with weight-bearing typically develops, and if the bone surface collapses, it can lead to joint damage and arthritis. The outlook depends heavily on how early it's caught and how much bone is involved — smaller, earlier lesions have a much better chance of being managed without needing joint replacement.

When to follow up

Any new finding of avascular necrosis should be discussed with an orthopedic specialist, ideally one experienced in joint preservation. Early-stage disease may be monitored closely or treated with measures aimed at reducing pressure on the bone and encouraging blood flow, sometimes including a procedure called core decompression. More advanced disease, especially after the bone has collapsed, often requires surgery, which can range from bone grafting to partial or total joint replacement in more severe cases. If you're on long-term steroids or have risk factors and develop new joint pain, mention it to your doctor promptly rather than waiting.

A plain-language way to picture it

Picture a section of a garden that relies on one underground pipe for water. If that pipe gets blocked, the plants in that patch don't die instantly — but without water, they slowly wither, the soil beneath them loses its structure, and eventually that patch of ground can cave in. The rest of the garden, still fed by working pipes, stays healthy. Finding the blockage early and restoring water flow gives the best chance of saving that patch before it collapses.