Avascular Necrosis of the Knee

Avascular necrosis is a type of osteonecrosis specifically describing when the blood supply to the bone is disrupted. As the underlying bone weakens, the cartilage on top can collapse, leading to rapid and severe arthritis.

Degeneration of the joint occurs in several knee conditions, including osteoarthritis and osteochondritis dissecans (OCD), but they play out differently. Osteoarthritis develops slowly over time as cartilage wears down from age, overuse, or mechanical stress. OCD, on the other hand, happens when a small segment of bone and its overlying cartilage lose blood flow and begin to separate from the surrounding tissue – usually in younger, more active patients. Osteonecrosis is more aggressive than both: it involves a larger area of bone losing blood supply, progresses faster, and can cause sudden, significant pain even before visible cartilage damage occurs.

Causes & Symptoms

Osteonecrosis of the knee can have different origins, and researchers are still puzzled as to why it happens. The cause can be:

• Idiopathic: Spontaneous Osteonecrosis of the Knee (SPONK) can occur without clear cause, usually in women over 60.
• Secondary: Linked to factors that impair blood flow to bone, including high-dose or long-term use of corticosteroids, chronic alcohol use, sickle cell disease, lupus, and other chronic systemic conditions.

Suspected contributing factors can also include trauma to the immediate area, tobacco use, and some chemotherapy modalities, including radiation and medications.

Bone is constantly breaking down and rebuilding itself, but osteonecrosis disrupts this process. When blood flow is compromised, bone tissue deteriorates faster than the body can replace it. If the weakened area develops a structural crack, the bone can collapse under normal weight-bearing. When this collapse occurs near the joint surface, the smooth contour of the knee becomes uneven, leading to arthritis, stiffness, and significant functional loss.

Unlike the gradual ache of osteoarthritis, osteonecrosis pain often has a sudden onset. While osteoarthritis may cause stiffness and pain that worsens with prolonged activity, osteonecrosis symptoms tend to be sharper.

Other symptoms of osteonecrosis can include:

• Severe pain to the inner side (medial aspect) of the knee
• Increased pain at night
• Sudden inability to bear weight or a dramatic worsening of weight-bearing pain

Diagnosis & Treatment

X-rays can look completely normal in the early stages of osteonecrosis. As the disease advances, the affected region can show patchy areas that look either lighter or darker than normal bone.

One classic feature in later stages of the disease is a thin, curved rim of denser bone outlining the damaged area: the surface of the bone may begin to flatten, and a thin, crescent-shaped line may appear just under the cartilage. This “crescent sign” indicates that the underlying bone has weakened and started to collapse. Eventually, the bone can fragment and the joint space narrows, signaling advanced degeneration.¹

An MRI is considered the gold standard, as it’s the most sensitive test for diagnosing osteonecrosis. MRI can detect early signs of injury, sometimes within a week of disrupted blood flow by showing bone marrow edema (fluid within the bone that reflects inflammation) and impaired blood supply.¹

One classic MRI feature is the “double-line sign,” a pattern that shows a thin outer rim of dead or damaged bone with a brighter inner rim where the body is trying to repair the injury. This pattern is highly specific to osteonecrosis and helps confirm the diagnosis.

Bone scans and computed tomography (CT scan) may also be employed to further assess progression, and they can be beneficial in planning for corrective surgery.

Treatment depends on the level of deterioration:

• Early Stage: Unloading the knee and reducing pressure on the affected bone with crutches and using an “unloader brace” can shift weight away from the damaged area and may help the bone recover.
• Core Decompression: Drilling small channels into the affected bone can lower internal pressure and stimulate the growth of new blood vessels. (This is most effective before any significant collapse has happened.)
• Late Stage (Collapse): Once the bone collapses, the joint surface is destroyed and can no longer function normally. Surgical reconstruction (i.e., partial or total knee replacement) is the most reliable way to restore stability and relieve pain.

The primary goal of treatment is to prevent further bone loss and permanent damage.

Preventive Strategies and Prognosis

Prevention can be difficult, but limiting risk factors can lower the chances of developing this disease and slow its progression. Avoiding high-dose or long-term corticosteroid use when alternatives are available, moderating alcohol intake, smoking cessation, and managing chronic health conditions can protect bone health. Maintaining good joint mechanics, like strengthening supporting muscles, protecting the knee from repetitive overload, and addressing injuries promptly, reduces stress on vulnerable areas of bone.

Osteonecrosis of the knee is treatable, but timing is crucial. Patients who seek early evaluation and receive a diagnosis in the initial stages tend to have better long-term outcomes. Healing largely depends on how much of the bone is involved and how soon the condition is identified.

1. Stroie, O. M., & Vu, V. H. (2024, May 25). Osteonecrosis Imaging. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK604199/.