Osteonecrosis (ON) of the hip is a medical condition defined by the death of bone tissue within the femoral head, the ball portion of the hip’s ball-and-socket joint. This tissue death occurs because the blood supply to the bone is cut off or severely restricted. The condition is frequently referred to as avascular necrosis (AVN) or aseptic necrosis, terms describing the lack of blood flow and the absence of bacterial infection. Since bone requires a consistent blood supply to remain healthy, this interruption causes the bone to weaken and ultimately collapse. The hip joint is the most common site for osteonecrosis, and it is a significant cause of debilitating arthritis and the need for total hip replacement surgery, particularly in younger adults.
What Causes Osteonecrosis of the Hip?
The underlying issue is the compromise of the fine blood vessels that nourish the femoral head, leading to localized bone cell death.
Traumatic Causes
One major category of cause is direct physical trauma, such as a hip dislocation or a fracture of the femoral neck, which can physically tear or damage the blood vessels supplying the bone. When the blood supply is immediately compromised by injury, it is termed traumatic osteonecrosis and usually affects only one hip.
Non-Traumatic Causes
The more common form is non-traumatic osteonecrosis, which stems from systemic factors that block or damage small vessels over time. Long-term use of high-dose corticosteroids is a well-established risk factor, as these medications can alter fat metabolism, potentially leading to fatty blockages within the arteries. Similarly, excessive alcohol consumption can result in fatty deposits and elevated cortisone levels that impair blood flow to the bone.
Underlying medical conditions also predispose individuals to non-traumatic osteonecrosis by affecting blood composition or vessel health. These include blood disorders like sickle cell disease, which causes abnormally shaped red blood cells that can clog small vessels, and Gaucher disease. Other conditions, such as systemic lupus erythematosus, HIV infection, and decompression sickness (“the bends”), can also disrupt blood flow and clotting, resulting in bone death. In a significant number of cases, however, no clear cause is identified, and the condition is classified as idiopathic osteonecrosis.
How the Condition Develops and Manifests
Osteonecrosis of the hip develops through a predictable series of stages, with symptoms progressing as the disease advances. The earliest stages often present with no symptoms, or only mild and intermittent pain that might be attributed to a muscle strain. The first noticeable symptom is typically a dull ache or throbbing pain, most often localized in the groin area but sometimes radiating to the buttock or down the thigh.
As the region of dead bone expands, the structural integrity of the femoral head weakens, making it increasingly difficult to bear weight. This progression can take several months to over a year, eventually leading to constant pain, even when resting. The most significant event is the subchondral collapse, where the bone just beneath the joint surface fractures and flattens.
Once the femoral head collapses, the protective layer of articular cartilage covering the joint also collapses, leading to significant joint incongruity and secondary arthritis. At this point, the patient experiences limited range of motion, stiffness, and a noticeable limp. Diagnosis relies heavily on imaging, with Magnetic Resonance Imaging (MRI) being particularly effective at detecting the condition in the early stages before any collapse is visible on a standard X-ray.
Treatment Options Based on Disease Stage
The management of osteonecrosis is dependent on the stage of the disease, with the primary goal being to preserve the hip joint and prevent the collapse of the femoral head.
Early Stage (Pre-Collapse) Treatment
For the earliest stages, before structural collapse has occurred, non-surgical approaches may be attempted. These include limiting weight-bearing activities, using nonsteroidal anti-inflammatory drugs (NSAIDs) for pain relief, and sometimes medications like statins to improve blood flow. However, these non-operative treatments are limited in their ability to halt disease progression.
When the condition is diagnosed early, bone-sparing surgical procedures are often the preferred intervention. The most common is core decompression, a procedure that involves drilling channels from the side of the femur into the necrotic area of the femoral head. This action serves two purposes: it reduces the destructive pressure within the bone and creates pathways that encourage new blood vessels to grow into the damaged area, promoting healing. Core decompression is frequently performed with the addition of bone grafting or the use of concentrated bone marrow cells to enhance regeneration.
Late Stage (Post-Collapse) Treatment
If the disease has progressed to subchondral collapse, joint-preserving options are no longer viable, and surgical reconstruction becomes mandatory. The standard definitive treatment for a collapsed femoral head is a total hip replacement, known as total hip arthroplasty. This procedure involves removing the damaged femoral head and replacing it with a prosthetic ball and socket, which effectively eliminates pain and restores normal function and mobility. Osteonecrosis is a common reason for total hip replacement, especially in younger patients, as it accounts for approximately 10% of all total hip arthroplasty procedures in the United States.