Avascular necrosis (also called osteonecrosis) is a condition where bone tissue dies because its blood supply gets cut off. Without steady blood flow, the living cells inside bone starve and break down. Over time, the weakened bone can fracture beneath the joint surface and eventually collapse, destroying the joint. An estimated 20,000 to 30,000 new cases are diagnosed in the United States each year, most commonly in the hip.
How Blood Loss Leads to Bone Death
Bone is living tissue that depends on a network of small blood vessels to deliver oxygen and nutrients. When something disrupts that blood supply, whether an injury, a clot, or pressure from fat buildup inside the bone marrow, the bone cells begin to die. The body tries to repair the damage, but if the affected area is large enough, the repair process can’t keep up. The bone weakens from the inside out, and a thin layer just beneath the cartilage surface starts to crack. This is called subchondral collapse, and once it happens, the smooth joint surface crumbles and arthritis sets in quickly.
Major Causes and Risk Factors
Causes fall into two broad categories: traumatic and non-traumatic. A hip fracture or dislocation can physically sever the blood vessels feeding the bone, which is the most straightforward path to avascular necrosis. Non-traumatic causes are more complex and far more common overall. Alcohol abuse and corticosteroid use together account for up to 80% of non-traumatic cases.
Both alcohol and steroids damage bone through similar mechanisms. They increase fat production inside the bone marrow, trigger chronic inflammation, and alter the blood vessels that supply the bone. Excess fat in the marrow physically compresses blood vessels and can also form small fat clots that block them from the inside. Animal studies have shown fatty deposits accumulating in bone marrow after prolonged alcohol exposure, mirroring what happens in the liver.
Steroid Risk by Dose
Not every course of steroids causes avascular necrosis, but higher doses sharply increase the odds. Roughly 1,000 mg of oral prednisone given within a short window places patients at increased risk. Daily doses above 40 mg of prednisone were present in 93% of patients who went on to develop the condition. For every 10 mg per day increase in oral steroids during the first six months of therapy, risk rises by about 4.6%. One analysis calculated that a cumulative dose of 9 grams of prednisone in a single month carried a 22% chance of developing osteonecrosis. Cutting cumulative doses roughly in half, from 12 grams to 6.5 grams in one study, more than halved the incidence.
Other non-traumatic risk factors include sickle cell disease, lupus, radiation therapy, blood clotting disorders, and HIV. In many cases, no clear cause is found.
Which Joints Are Affected
The hip is by far the most common site, specifically the ball-shaped top of the thighbone (femoral head). This area is especially vulnerable because it relies on a limited set of blood vessels that are easily damaged. Beyond the hip, avascular necrosis can develop in the knee, shoulder, and small bones of the wrist. It occasionally affects the ankle or jaw, though this is less common.
In the knee, the inner part of the thighbone near the joint is the typical location. In the shoulder, the upper arm bone is affected. In the wrist, two small bones called the lunate and scaphoid are most vulnerable because of their naturally tenuous blood supply.
Symptoms at Each Stage
One of the frustrating things about avascular necrosis is that it often causes no symptoms in its earliest stages. Physical exams at this point are usually completely normal, which inevitably delays diagnosis. Many people don’t feel anything until the disease has already progressed significantly.
As it worsens, pain typically starts with weight-bearing activity. In the hip, this shows up as groin pain, sometimes radiating into the buttock or thigh. In the shoulder, the pain tends to be pulsating and may spread toward the elbow. In the knee, tenderness develops over the inner side of the joint. Eventually, the majority of patients experience pain even at rest, along with stiffness, decreased range of motion, and changes in the way they walk. Late-stage disease brings severe pain, significant disability, and in the worst cases, complete joint collapse.
How It’s Diagnosed
Standard X-rays miss avascular necrosis in its early stages. One study found that plain X-rays could only detect the condition in about 62% of affected hips, completely missing the other 38% that MRI picked up. Overall sensitivity of X-rays for early-stage disease is only around 41%.
MRI is the gold standard for early detection. It can reveal bone marrow swelling and areas of damaged tissue beneath the joint surface well before any changes appear on X-rays. A characteristic low-signal band on certain MRI sequences is the hallmark finding in stage I disease, when the bone still looks perfectly normal on an X-ray. Because early treatment has a much better chance of saving the joint, getting an MRI when avascular necrosis is suspected, rather than relying on a normal-looking X-ray, is critical.
Stages of Disease
The widely used ARCO classification system divides avascular necrosis into four stages based on imaging findings:
- Stage I: X-rays look normal, but MRI shows a characteristic band of abnormal signal inside the bone. No structural damage has occurred yet.
- Stage II: Both X-rays and MRI show abnormalities, but the bone surface is still intact. The bone is weakening internally.
- Stage III: A fracture develops just beneath the joint surface. In early stage III, the bone surface sinks by 2 mm or less. In late stage III, it sinks more than 2 mm. This is the point of no return for the bone’s original shape.
- Stage IV: Full arthritis has developed. The joint cartilage is destroyed and the opposing bone surface is also damaged.
Treatment for Early-Stage Disease
When avascular necrosis is caught before the bone surface collapses, the goal is to preserve the natural joint. The most common procedure is core decompression, where a surgeon drills into the affected bone to relieve pressure, restore blood flow, and encourage new bone growth. Success rates depend heavily on how early the procedure is done. For stage I disease, core decompression succeeds about 78% to 79% of the time. At stage II, success drops to around 52% to 72% depending on how much of the bone is involved. By stage III, it falls to roughly 10% to 41%.
Other joint-preserving options include bone grafting (transplanting healthy bone into the damaged area) and osteotomy (reshaping the bone to shift weight away from the damaged zone). Vascularized fibular grafting, where a piece of the smaller leg bone along with its blood supply is transplanted into the femoral head, has shown strong long-term results. In one study following 124 hips, 62% showed stable or improved X-ray findings, and the procedure delayed the need for a hip replacement by an average of 8.4 years in patients who eventually needed one. Survivorship of the natural hip was 93% at ten years and 83% at twenty years. Recovery from this procedure involves staying off the leg for about a week, using crutches for ten weeks, and gradually increasing weight-bearing over six months.
When Joint Replacement Becomes Necessary
Once the bone surface has collapsed and arthritis has set in (stages III and IV), preserving the natural joint becomes much less likely to succeed. At this point, total joint replacement is typically the most reliable option for relieving pain and restoring function. For the hip, this means replacing the damaged ball and socket with artificial components. Untreated avascular necrosis worsens progressively. The bone eventually collapses, and pain shifts from being activity-related to constant, including at rest and while lying down.
Because outcomes are so much better with early intervention, the biggest factor in how avascular necrosis plays out is how quickly it gets caught. If you have persistent joint pain and any of the major risk factors, particularly heavy alcohol use or a history of high-dose steroid treatment, an MRI can detect the problem months or even years before an X-ray would show anything wrong.