Osteonecrosis, also known as avascular necrosis (AVN), is a condition characterized by the death of bone tissue due to a lack of blood supply. This interruption in blood flow means the bone cells cannot receive the necessary oxygen and nutrients to survive. While AVN can affect various joints, it most commonly occurs in the hip, specifically within the femoral head, the “ball” of the hip’s ball-and-socket joint. When the blood supply is cut off, the bone’s structural integrity weakens, potentially leading to joint collapse and severe arthritis.
The Mechanics of Osteonecrosis
The femoral head relies on specific blood vessels for nourishment, making it particularly vulnerable when circulation is compromised. When blood flow is disrupted, the bone cells, or osteocytes, begin to die off, a process that can take months or years to fully develop. This bone death creates a structurally unsound area, or lesion, within the head of the femur.
The damaged bone can no longer support the mechanical stresses of walking or weight-bearing, eventually causing tiny stress fractures to form. These fractures lead to the collapse of the femoral head, sometimes visible on imaging as a “crescent sign.” Once the rounded shape of the femoral head flattens, the smooth articular cartilage covering the joint surface deteriorates rapidly, leading to the painful grinding associated with end-stage arthritis.
Recognizing the Symptoms of Hip AVN
In the initial stages of hip AVN, many people experience no symptoms, making early detection difficult without specific imaging. The first sign is typically a mild, intermittent pain often felt deep in the groin area. This pain may also radiate into the thigh, buttock, or sometimes be referred down to the knee, which can complicate accurate diagnosis.
As the underlying bone damage progresses, the pain becomes more constant and severe, occurring even when resting or lying down. Movements like standing or walking become increasingly difficult and painful due to the inability of the damaged bone to bear weight. Patients often develop a limp as they try to avoid putting pressure on the affected joint.
The collapse of the femoral head restricts the range of motion in the hip joint, making everyday activities challenging. Actions such as bending over to put on a shoe or rotating the leg become limited and painful. This severe pain results from the joint surface deteriorating and the hip losing its natural, smooth motion.
Primary Causes and Contributing Risk Factors
Osteonecrosis of the hip is categorized into traumatic and non-traumatic causes, though the exact mechanism of blood supply disruption is not always clear. Traumatic AVN occurs when a physical injury, such as a hip dislocation or a fracture of the femoral neck, directly damages the blood vessels supplying the femoral head. This type of injury immediately interrupts blood flow, leading to bone death in the following months.
Non-traumatic AVN is much more common and is linked to systemic and metabolic risk factors that indirectly block or reduce blood flow. A major contributing factor is the long-term, high-dose use of corticosteroid medications, which are thought to increase fat levels in the blood. These fatty deposits, known as fat emboli, can then block the small arteries supplying the bone tissue, effectively starving it of blood.
Excessive alcohol consumption over a sustained period is another significant non-traumatic cause, similarly linked to the formation of fat emboli. Several underlying medical conditions also increase the risk by affecting blood vessel health or blood composition:
- Sickle cell disease, which causes abnormally shaped red blood cells that can clog small vessels.
- Gaucher disease, a rare disorder where fatty substances accumulate in the bone marrow.
- Systemic lupus erythematosus, a condition that can cause inflammation and damage to blood vessels.
- Decompression sickness, which affects deep-sea divers.
- Radiation therapy for cancer, which can compromise the blood supply by damaging the bone tissue and blood vessels.
In many cases, however, no specific cause can be identified, and the condition is classified as idiopathic AVN.
Diagnosis and Treatment Approaches
Confirming a diagnosis of osteonecrosis requires a combination of a physical examination and advanced imaging tests. During the physical exam, a doctor will check the hip’s range of motion and identify positions that trigger pain, often finding tenderness around the groin. While standard X-rays are typically ordered, they often appear normal in the earliest stages of the disease, making them unreliable for timely detection.
Magnetic Resonance Imaging (MRI) is considered the most sensitive tool for identifying AVN, as it can detect changes in the bone marrow and early signs of tissue death before the bone structure collapses. This capability allows for diagnosis at the earliest stage, which is when treatment options are most effective. Other tests like CT scans or bone scans may also be used to assess the extent of the damage.
Treatment is highly dependent on the stage of the disease and whether the femoral head has already begun to collapse. Non-surgical approaches are usually attempted for early-stage AVN, focusing on pain management and limiting weight-bearing on the joint using crutches or a walker. Medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs) for pain and bisphosphonates to slow bone deterioration, may also be prescribed.
When the condition is caught before collapse, a surgical procedure called core decompression may be performed. This involves drilling small holes into the dead area of the bone to relieve pressure and encourage the formation of new blood vessels, potentially preventing collapse. If the femoral head has already collapsed, the damage is irreversible, and the most common treatment is a total hip replacement to restore joint function and eliminate pain.