Osteonecrosis of the femoral head (ONFH) is a debilitating condition resulting from the death of bone tissue in the hip joint. This occurs when the blood supply to the femoral head, the top portion of the thigh bone, is severely compromised or interrupted. The condition is also known as Avascular Necrosis (AVN) or aseptic necrosis, reflecting bone death without infection. Since the femoral head forms the ball of the hip joint, this lack of nourishment causes the bone structure to weaken and ultimately collapse. This structural failure destroys the joint surface, resulting in pain and disabling arthritis.
How Interruption of Blood Flow Damages the Femoral Head
The bone tissue within the femoral head is a living structure requiring a continuous supply of oxygen and nutrients delivered by small blood vessels. The cells maintaining bone health, called osteocytes, are highly sensitive to blood flow disruption. When the supply is cut off, osteocytes and bone marrow cells begin to die, a process that takes days to weeks. This area of dead bone loses its structural integrity.
Damage concentrates in the subchondral bone, the layer directly beneath the joint cartilage, where maximum stress occurs. As the dead bone weakens, mechanical forces from walking cause tiny fractures to accumulate. Eventually, the weakened bone cannot support the body’s weight, leading to a visible fracture line beneath the cartilage, known as the “crescent sign.” This sign marks the beginning of the end-stage process where the spherical femoral head flattens and collapses, destroying the hip joint’s mechanics.
Factors That Increase Risk
Osteonecrosis of the femoral head is broadly classified into traumatic and non-traumatic causes, with non-traumatic causes accounting for the majority of cases. Traumatic causes involve a direct physical injury that severs the blood vessels supplying the femoral head. This typically occurs following a hip dislocation or a displaced fracture of the femoral neck, where the trauma compromises the fragile retinacular arteries.
Non-traumatic cases are strongly associated with certain medications and lifestyle factors. High-dose, long-term use of corticosteroids, such as prednisone, is a frequent cause, potentially involving increased fat cells blocking small vessels or changes in blood flow. Similarly, chronic, excessive alcohol consumption can lead to fatty deposits that obstruct the microcirculation within the bone.
Several underlying medical conditions also increase the likelihood of developing ONFH. Patients with sickle cell disease experience distorted red blood cells that physically block small blood vessels, causing ischemic events in the bone. Other systemic diseases, including systemic lupus erythematosus (SLE) and Gaucher disease, are known risk factors. Additionally, treatments like chemotherapy and radiation can impair the bone’s regenerative capacity.
Identifying the Condition (Diagnosis and Staging)
Confirming a diagnosis of osteonecrosis of the femoral head relies on patient history, physical examination, and advanced medical imaging. In the earliest stages, patients may experience hip pain, but standard X-rays often appear normal. This occurs because the bone structure has not yet physically collapsed or changed density enough to be visible on the X-ray.
Magnetic resonance imaging (MRI) is considered the gold standard diagnostic tool. MRI is highly sensitive, capable of detecting changes in the bone marrow that indicate a lack of blood flow before structural damage occurs. The characteristic finding is a distinct, low-intensity band separating the dead bone from the healthy tissue. Computed tomography (CT) scans may also be used later to precisely visualize the extent of subchondral collapse and aid in surgical planning.
Once the condition is confirmed, medical professionals use staging systems, such as the Ficat or ARCO classifications, to determine the severity of the disease. These systems track the progression of the disease:
- Stage 0: Clinically silent with normal imaging.
- Stages I and II: Pre-collapse lesions where the bone is dying but the femoral head retains its spherical shape.
- Stage III: Defined by the presence of the subchondral “crescent sign,” indicating a fracture has occurred beneath the cartilage.
- Stage IV: Signifies frank collapse and flattening of the femoral head, often accompanied by secondary arthritis, which dictates a shift in the treatment approach.
Managing and Treating Osteonecrosis of the Femoral Head
Treatment strategies are directly determined by the stage of the disease, with the goal of preserving the hip joint. In the earliest, pre-collapse stages (Ficat I or II), joint preservation is the primary focus. Non-surgical management, such as limited weight-bearing using crutches, can be attempted for small lesions not causing severe pain. Medications like bisphosphonates may also be used to slow bone breakdown, potentially delaying the need for surgery.
Core Decompression (CD)
The most common joint-preserving procedure for early-stage ONFH is Core Decompression (CD). This surgical technique involves drilling channels into the necrotic area of the femoral head to relieve high intraosseous pressure. Reducing this pressure allows new blood vessels to grow, stimulating bone healing and preventing collapse. CD is often augmented with bone graft material, growth factors, or concentrated bone marrow aspirate containing stem cells to enhance the regenerative response.
Advanced Joint-Saving Procedures
For intermediate-stage lesions, or those that have failed core decompression, more complex joint-saving procedures may be considered. An Osteotomy involves surgically cutting and reorienting the femoral head so the necrotic lesion is moved away from the main weight-bearing surface. Another option is a vascularized fibula graft, which transplants a section of the patient’s own fibula bone, along with its attached blood supply, into the defect to provide structural support and a new source of blood flow.
Total Hip Arthroplasty (THA)
Once the femoral head has significantly collapsed (Stage IV or higher), joint preservation techniques become ineffective. The definitive solution is Total Hip Arthroplasty (THA). This procedure involves removing the damaged femoral head and replacing it with an artificial implant. THA is highly successful for pain relief and restoring function. However, it is typically reserved for late-stage disease because the lifespan of artificial joints can be a concern for active individuals.