AVN, or avascular necrosis, is a condition where bone tissue dies because its blood supply gets cut off. Without steady blood flow, bone cells starve of oxygen and begin to break down. Over time, the affected bone weakens, collapses, and damages the surrounding joint. Between 20,000 and 30,000 new cases are diagnosed each year in the United States, and the hip is by far the most commonly affected joint.
How Bone Dies Without Blood Flow
Bone is living tissue that needs a constant supply of oxygen-rich blood. When something interrupts that supply, the cells just beneath the joint surface enter a state of oxygen deprivation. Cell membranes lose their integrity, and the bone cells begin to die. Normally, your body recycles old or damaged bone by breaking it down and replacing it with new tissue. In AVN, that recycling process breaks down completely. Dead bone cells aren’t cleared away or replaced, so the bone becomes dense and brittle instead of flexible and alive.
As more bone dies, the area just below the joint surface weakens until it can no longer support the weight above it. The bone eventually cracks and flattens, a process called subchondral collapse. Once that happens, the smooth cartilage surface of the joint loses its shape, and the joint itself starts to degenerate. This is why AVN, left untreated, frequently leads to severe arthritis and the need for joint replacement.
What Causes AVN
Anything that damages or blocks the small blood vessels feeding bone can trigger AVN. The two most common causes are long-term steroid use and heavy alcohol consumption, but traumatic injuries like hip fractures or dislocations can also sever the blood supply directly.
Steroid Use
High-dose corticosteroids are the single biggest non-traumatic risk factor. These medications directly trigger bone cell death and disrupt the internal communication network that keeps bone healthy. The risk climbs steeply with dose: for every 10 mg/day increase in oral steroids during the first six months of therapy, the risk of AVN rises by about 4.6%. In some patient populations receiving very high doses, the incidence of steroid-induced AVN reaches as high as 40%. A cumulative dose of roughly 1,000 mg of prednisone given over a short window is enough to put patients at increased risk. When hospitals cut cumulative steroid doses from 12 grams to 6.5 grams in transplant protocols, the rate of hip AVN dropped by more than half. Short, low-dose courses of steroids rarely cause the condition.
Alcohol
Chronic heavy drinking promotes fatty changes in the liver and elevated blood fats. Those fat deposits can form tiny clots (fat emboli) that block the microscopic blood vessels inside bone, starving the tissue of oxygen. Alcohol also directly impairs the body’s ability to repair and remodel bone, compounding the damage.
Blood Disorders
Sickle cell disease is a well-known cause. Abnormally shaped red blood cells repeatedly clog small vessels in the bone during pain crises, cutting off oxygen delivery and causing localized tissue death. Even sickle cell trait, the milder carrier form, may pose some risk under low-oxygen conditions, especially combined with other factors like alcohol use.
Other recognized risk factors include radiation therapy, blood clotting disorders, autoimmune conditions (particularly lupus, which often requires steroid treatment), and organ transplantation.
Symptoms and How They Progress
In its earliest stage, AVN often causes no symptoms at all. The bone is dying, but the joint surface hasn’t changed shape yet, so everything still feels normal. This silent period is one of the most frustrating aspects of the disease, because early treatment works far better than late treatment.
The first symptom is usually a deep, aching pain in the groin that may radiate down the front of the thigh. Initially it appears only with activity or weight-bearing, then gradually shows up at rest and during the night. As the bone beneath the joint surface begins to crack and flatten, the pain intensifies and range of motion shrinks. Walking becomes difficult. In the final stage, the joint collapses fully, cartilage wears away, and the pain becomes constant, similar to advanced arthritis.
Stages of AVN
Doctors classify AVN into four stages based on imaging findings, using systems like the Ficat classification. Understanding the stage is critical because it determines which treatments are still viable.
- Stage 1: X-rays look completely normal. The patient may have groin or thigh pain, but the damage is only visible on MRI. This is the best window for treatment.
- Stage 2: X-rays start showing abnormalities, areas of increased density, small cysts, or patchy bone thinning. The bone surface is still round and intact.
- Stage 3: The bone surface begins to crack and flatten. X-rays show a characteristic “crescent sign,” a thin line of separation just beneath the joint surface. The round shape of the bone is visibly distorted.
- Stage 4: The bone has fully collapsed, the joint space narrows, and arthritis sets in. At this point, joint replacement is typically the only effective option.
How AVN Is Diagnosed
If your doctor suspects AVN, imaging is the key to confirming it. Standard X-rays are useful for later stages but miss early disease. In one study of 220 patients, X-rays detected only 26.7% of early-stage cases. MRI is far more reliable, catching 94.7% of early cases with high accuracy. It picks up changes in bone marrow and blood flow long before any structural damage shows on X-ray. For this reason, MRI is considered the gold standard for early detection.
If you have known risk factors, particularly a history of high-dose steroid use, sickle cell disease, or a recent hip fracture, and you develop unexplained groin or hip pain, requesting an MRI rather than relying on a normal X-ray can make a significant difference in catching the disease early enough to save the joint.
Treatment Options by Stage
Treatment depends almost entirely on how far the disease has progressed when it’s caught. The goal in early stages is to preserve the natural joint. In late stages, replacement becomes necessary.
Core Decompression
This is the most common joint-preserving surgery for AVN. A surgeon drills one or more small channels into the affected bone to relieve pressure, improve blood flow, and stimulate new bone growth. Success rates drop dramatically with advancing disease: 84% of Stage 1 patients avoid joint replacement after core decompression, compared to 63% at Stage 2 and only 29% at Stage 3. These numbers highlight why early detection matters so much.
Bone Grafting and Stem Cell Therapy
Core decompression is sometimes combined with bone grafts or stem cell injections to encourage healing. A recent review of 10 clinical trials involving 545 hips found that stem cells combined with mechanical support or bone grafts reduced the risk of needing a hip replacement and significantly improved joint function scores. Stem cells alone, without additional structural support, did not show clear benefits. This approach is still evolving, but the combination strategy shows promise for earlier-stage disease.
Joint Replacement
Once the bone has collapsed and the joint surface is destroyed (Stage 4), total hip replacement is the standard treatment. AVN accounts for roughly 10% of the 250,000 total hip replacements performed in the U.S. each year. Modern hip replacements are highly effective at relieving pain and restoring mobility, but they have a finite lifespan, which is a particular concern for AVN patients, who tend to be younger than typical hip replacement candidates. Many are in their 30s, 40s, or 50s, meaning they may need one or more revision surgeries over their lifetime.
Protecting Your Joints if You’re at Risk
If you take corticosteroids for a chronic condition, the single most important factor is keeping the dose and duration as low as your disease allows. Even modest reductions in cumulative steroid exposure can substantially lower AVN risk. Limiting alcohol intake, managing cholesterol levels, and staying physically active all support healthy bone circulation. If you develop persistent groin, hip, or thigh pain while on steroids or with any known risk factor, pushing for an MRI early rather than waiting for X-ray changes can be the difference between a small procedure and a full joint replacement.