Avascular necrosis happens when blood flow to a bone is cut off, starving bone cells of oxygen and causing them to die. The hip is the most commonly affected joint, and the condition tends to strike younger than most bone diseases, with an average age of diagnosis around 52. While a single traumatic event like a fracture can trigger it, most cases develop gradually from conditions that slowly choke off the blood supply inside bone, including long-term steroid use, heavy alcohol consumption, and blood disorders like sickle cell disease.
How Bone Dies Without Blood Flow
Bone is living tissue that depends on a steady supply of oxygen-rich blood. When that supply drops, bone cells begin dying within two to three hours, though visible signs of cell death don’t appear for one to three days. The earliest changes happen in the bone marrow, where blood-forming cells and fat cells die first, followed by the bone cells themselves.
What makes avascular necrosis particularly damaging is what happens next. The body tries to repair the dead zone by sending in new blood vessels from the edges of the damaged area. This repair process involves simultaneously breaking down dead bone and laying down new bone. In the area just beneath the joint surface, bone breakdown outpaces new bone formation. That imbalance weakens the internal scaffolding of the bone until it fractures from within, causing the joint surface to collapse. So it’s actually the body’s incomplete repair attempt, not the initial bone death, that leads to joint destruction.
Blood flow to bone can be disrupted in three fundamental ways: physical damage to blood vessels (from a fracture or dislocation), blockages inside blood vessels (from blood clots or fat globules), or external compression of blood vessels within the bone (from swollen fat cells or abnormal cell accumulation). All roads lead to the same outcome: oxygen deprivation, cell death, and a flawed healing response.
Hip Fractures and Joint Dislocations
Trauma is one of the most straightforward causes. When a bone breaks, especially near a joint, the fracture can tear or kink the blood vessels feeding the bone. Femoral neck fractures (breaks in the narrow section connecting the ball of the hip to the thighbone) are the classic example. A systematic review and meta-analysis found that about 20.7% of patients with displaced femoral neck fractures developed avascular necrosis afterward, compared to 4.7% of those with undisplaced fractures. The more the bone fragments shift out of position, the greater the vascular damage.
The timeline for post-traumatic cases varies widely. Some patients show signs of avascular necrosis as early as three months after surgery, while others don’t develop it for five or more years. This unpredictability makes long-term follow-up important after any significant hip injury. Hip dislocations carry similar risks because the blood vessels supplying the femoral head can be stretched or torn when the ball is forced out of its socket.
Corticosteroid Use
Corticosteroids (commonly prescribed for conditions like lupus, asthma, inflammatory bowel disease, and organ transplants) are the most frequent non-traumatic cause. The risk is dose-dependent and climbs sharply at higher levels. Taking more than 20 mg per day of prednisone (or its equivalent) raises the odds of developing avascular necrosis roughly ninefold compared to lower doses. Each additional 10 mg per day adds about a 3.6% increase in risk. Doses above 40 mg per day are associated with a fourfold higher incidence.
Cumulative dose matters too. Patients who receive a total lifetime dose exceeding 2 grams of prednisone-equivalent develop avascular necrosis at a rate of about 6.7%. The mechanism involves several overlapping processes: steroids promote fat cell enlargement inside bone marrow, and these swollen fat cells compress the tiny blood vessels running through bone. Steroids also alter fat metabolism in ways that can send fat globules into the bloodstream, where they lodge in small vessels and block flow. On top of that, corticosteroids interfere with new bone formation and promote bone cell death directly.
Alcohol and Fat Metabolism
Heavy, sustained alcohol use is another leading cause. Alcohol disrupts lipid metabolism and promotes fat accumulation in both the liver and bone marrow. In animal studies, chronic alcohol exposure caused fat cells in the bone marrow to swell with triglycerides and eventually die. As these fat cells enlarge, they physically squeeze the small blood vessels running through the bone, cutting off circulation. The resulting loss of blood flow triggers the same cascade of cell death and failed repair seen in other forms of avascular necrosis.
Alcohol also weakens bone’s ability to form new tissue and resist structural failure, compounding the damage from reduced blood flow. In one large surgical series, alcohol abuse accounted for about 18% of avascular necrosis cases requiring hip replacement. Unlike steroid-related cases, where the dose-risk relationship is well defined, there’s no precise threshold for alcohol. But the pattern is clear: the risk rises with years of heavy drinking.
Sickle Cell Disease
Sickle cell disease is the most common cause of avascular necrosis in children and a major cause in young adults. Roughly 30% of people with sickle cell disease develop it at some point, and an estimated 50% of adults with the most severe form (homozygous sickle cell disease) will have femoral head involvement by age 35.
The mechanism is vascular obstruction from the inside. In sickle cell disease, red blood cells become rigid and crescent-shaped, clumping together and sticking to blood vessel walls. These dense clusters of sickled cells, along with activated white blood cells and platelets, block tiny blood vessels and starve downstream tissues of oxygen. In the hip, this leads to repeated episodes of ischemia and reperfusion injury that damage the bone’s articular surfaces over time. Unlike a single traumatic event, sickle cell-related avascular necrosis results from accumulated damage across many vaso-occlusive episodes, making it particularly difficult to prevent.
HIV and Immune-Related Conditions
HIV infection has emerged as a significant risk factor. In a large surgical series of over 1,400 hip replacement patients, 49% of those with avascular necrosis were HIV-positive, making it the single most common associated condition in that population. The reasons are likely multifactorial: the virus itself may damage blood vessel linings, and many people living with HIV take medications (including corticosteroids for related conditions) that independently raise risk. Autoimmune diseases like lupus also carry elevated risk, partly because of the disease itself and partly because of the corticosteroids used to treat it.
Decompression Sickness and Radiation
Divers, tunnel workers, and others exposed to rapid changes in atmospheric pressure can develop avascular necrosis through a distinct mechanism. When someone ascends too quickly from a high-pressure environment, dissolved nitrogen in the blood forms bubbles. These bubbles trigger clotting cascades that create tiny blockages in the blood vessels feeding bone, leading to infarction and cell death. This occupational cause, sometimes called caisson disease, typically affects the shoulders and hips.
Radiation therapy for cancer can also destroy the blood supply to nearby bones. This complication, known as osteoradionecrosis, affects 4 to 8 out of every 100 people who receive radiation and can appear months or years after treatment. It’s most common in the jaw following radiation for head and neck cancers, especially at doses above 60 grays. The radiation damages blood vessels within the bone, leaving it unable to heal from even minor injuries or infections.
How Avascular Necrosis Is Detected
One of the challenges with avascular necrosis is that standard X-rays often look normal in the early stages. By the time X-rays show visible changes, the bone may already be collapsing. MRI is far more sensitive and can detect changes as early as one week after the blood supply is disrupted. Bone scans can pick up abnormalities within 7 to 10 days as well, initially showing a “cold spot” where the dead bone isn’t absorbing the radioactive tracer, then shifting to a “hot spot” as the body’s repair response kicks in over the following weeks.
This gap between onset and detection matters because early-stage avascular necrosis, before the joint surface collapses, has more treatment options and better outcomes. If you have known risk factors (long-term steroid use, sickle cell disease, a prior hip fracture, or heavy alcohol use), persistent joint pain that worsens with weight-bearing warrants imaging sooner rather than later. Both hips are often affected: in one surgical series, 58% of patients with avascular necrosis had bilateral disease.
Who Is Most at Risk
Avascular necrosis skews younger than other degenerative joint conditions. The average age at diagnosis is around 52, compared to the early 70s for typical osteoarthritis. Men and women are both affected, though the ratio varies by underlying cause. People with avascular necrosis also tend to have a lower body mass index than those with osteoarthritis, likely reflecting the different disease mechanisms at play.
The strongest modifiable risk factors are corticosteroid dose and alcohol consumption. For people who need steroids for a serious medical condition, using the lowest effective dose for the shortest possible duration is the most practical way to reduce risk. For alcohol-related cases, the damage accumulates over years, and stopping heavy drinking is the clearest preventive step. Non-modifiable risks like sickle cell disease or a history of hip trauma can’t be eliminated, but awareness of the connection allows for earlier monitoring and intervention before joint collapse occurs.