Avascular Necrosis (AVN) of the Knee

Why It Happens – Causes (Etiology and Pathophysiology)

The primary cause of AVN is the disruption of blood supply to the bone, leading to bone death (infarction). In the knee, AVN often affects the medial femoral condyle, which is the inner part of the thigh bone. The lack of blood flow prevents the bone from receiving essential nutrients and oxygen, causing the bone tissue to die and potentially collapse. While many cases of knee AVN are idiopathic (no known cause), risk factors include:

• Alcohol abuse: Excessive alcohol consumption increases free fatty acids in the bloodstream, which can block blood vessels supplying the bone.

• Corticosteroid use: Long-term steroid use, often for conditions like rheumatoid arthritis or asthma, increases marrow pressure and may lead to AVN.

• Trauma or injury: Knee injuries from falls, accidents, or surgeries can disrupt blood flow, increasing the risk of AVN.

• Underlying diseases: Conditions such as thalassemia, Gaucher’s disease, and kidney transplant recipients are at higher risk.

How the Body Part Normally Works? (Relevant Anatomy)

The knee joint is the largest weight-bearing joint in the body, consisting of the femur (thigh bone), tibia (shin bone), and patella (kneecap). The ends of these bones are covered by articular cartilage, and just beneath it lies the subchondral bone. AVN affects this subchondral bone, often starting in the medial femoral condyle, and can lead to joint collapse if untreated.

What You Might Feel – Symptoms (Clinical Presentation)

Symptoms of knee AVN can vary depending on the stage of the disease. Early on, symptoms may be mild or asymptomatic, but as the condition progresses, patients may experience:

• Pain: Pain is usually felt in the inner part of the knee and may start suddenly or gradually. It worsens with weight-bearing activities or during rest in advanced stages.

• Swelling: The knee may become swollen, especially after physical activity.

• Decreased Range of Motion: The knee may become stiff, making it difficult to fully extend or bend the joint.

• Instability: In severe cases, patients may feel as though their knee is unstable or “giving way.”

• Limping: Chronic pain and instability may cause individuals to walk with a limp.

How Doctors Find the Problem? (Diagnosis and Imaging)

Diagnosis begins with a thorough physical examination, where the doctor checks for knee swelling, tenderness, and restricted movement. Early AVN may not show clear findings on plain X-rays, so further imaging tests are often required:

• MRI: MRI is the most sensitive tool for diagnosing AVN, revealing changes in bone structure, subchondral fractures, and early signs of bone death.

• X-ray: X-rays are used to monitor the progression of AVN and may show bone collapse and joint space narrowing in later stages.

• Bone Scan: This can be used to detect areas of abnormal bone activity, especially in the early stages.

Classification

AVN of the knee is classified into stages based on the progression of the disease:

1. Stage 1: Early, asymptomatic stage. The bone appears normal on X-ray but may show subtle changes on MRI.

2. Stage 2: The subchondral bone begins to collapse, and patients experience significant pain. MRI confirms bone damage.

3. Stage 3: The joint surface collapses, and osteoarthritis begins to develop. X-rays show significant joint changes.

4. Stage 4: Severe osteoarthritis with significant joint destruction. The knee’s range of motion is severely limited, and total knee replacement may be necessary.

Other Problems That Can Feel Similar (Differential Diagnosis)

Conditions such as meniscal tears, osteoarthritis, and knee ligament injuries can cause symptoms similar to knee AVN, including pain, swelling, and decreased function. A thorough examination and imaging studies are necessary to differentiate between these conditions.

Treatment Options

Non-Surgical Care

• Stage 1: In the early stages, protected weight-bearing with crutches, a cane, or a knee brace is recommended. Medications such as NSAIDs and bisphosphonates may help manage pain and slow the disease’s progression.

• Stage 2: Core decompression, where small holes are drilled in the affected area to reduce marrow pressure and increase blood flow, can be effective. Bone and cartilage transplants or autologous chondrocyte implantation may be considered.

• Stage 3 & 4: In advanced stages, total knee replacement (TKR) surgery is the most effective treatment, providing relief from pain and restoring knee function.

Surgical Care

• Core Decompression: This procedure involves drilling small holes into the affected bone to relieve pressure and promote blood flow.

• Bone Grafting: Grafts may be used to repair the damaged area of the bone.

• Total Knee Replacement: In severe cases, when the joint is extensively damaged, knee replacement surgery may be required. This procedure involves removing the damaged bone and cartilage and replacing them with artificial components.

Recovery and What to Expect After Treatment

Rehabilitation plays a critical role in recovery, particularly after surgery. For conservative treatments, physical therapy focuses on improving range of motion and strengthening the muscles around the knee. After total knee replacement, patients will follow a rehabilitation plan to restore knee function, with most individuals regaining normal activity levels within 3 to 6 months.

Possible Risks or Side Effects (Complications)

• Infection: As with any surgery, infection is a potential risk.

• Blood Clots: Post-surgical patients may be at risk for deep vein thrombosis (DVT).

• Graft Failure: In some cases, grafts used in bone repair may fail to integrate or function as expected.

• Recurrent Pain: After total knee replacement, there may be some residual pain or discomfort as the body adjusts to the prosthetic joint.

Long-Term Outlook (Prognosis)

The prognosis for knee AVN depends on the stage at which it is diagnosed. Early intervention with non-surgical methods may prevent the progression to advanced stages. In severe cases, knee replacement can significantly improve quality of life, though long-term outcomes will depend on factors like the patient’s age, activity level, and the success of rehabilitation.

Out-of-Pocket Costs

Medicare

CPT Code 20902 – Bone Grafting (Harvest and Placement of Bone Graft): $63.66
CPT Code 27125 – Hip Replacement (Partial Hip Arthroplasty): $268.14
CPT Code 27130 – Hip Replacement (Total Hip Arthroplasty): $303.45
CPT Code 38220 – Bone Marrow Injection (Aspiration for Bone Marrow Concentrate Therapy): $36.05

Under Medicare, 80% of the approved amount for these procedures is covered once the annual deductible has been met. The remaining 20% is typically the patient’s responsibility. Supplemental insurance plans—such as Medigap, AARP, or Blue Cross Blue Shield—usually cover this 20%, leaving most patients with little to no out-of-pocket expenses for Medicare-approved bone grafting, hip replacement, and bone marrow injection procedures. These supplemental plans coordinate directly with Medicare to provide full coverage for the procedures.

If you have secondary insurance—such as Employer-Based coverage, TRICARE, or Veterans Health Administration (VHA)—it functions as a secondary payer once Medicare processes the claim. After your deductible is satisfied, these secondary plans may cover any remaining balance, including coinsurance or any uncovered charges. Secondary plans typically have a modest deductible, usually between $100 and $300, depending on the specific policy and network status.

Workers’ Compensation
If your bone grafting, hip replacement, or bone marrow injection is work-related, Workers’ Compensation will fully cover all treatment-related costs, including surgery, hospitalization, and rehabilitation. You will have no out-of-pocket expenses under an accepted Workers’ Compensation claim.

No-Fault Insurance
If your injury requiring these procedures is the result of a motor vehicle accident, No-Fault Insurance will cover all medical and surgical expenses, including bone grafting, hip replacement, and bone marrow injections. The only possible out-of-pocket cost may be a small deductible depending on your individual policy terms.

Example
David, a 60-year-old patient with severe hip arthritis, underwent total hip arthroplasty (CPT 27130) and also received bone marrow concentrate therapy (CPT 38220) for joint regeneration. His estimated Medicare out-of-pocket costs were $303.45 for the hip replacement and $36.05 for the bone marrow injection. Since David had supplemental insurance through Blue Cross Blue Shield, the 20% that Medicare did not cover was fully paid, leaving him with no out-of-pocket expenses for the procedures.

Frequently Asked Questions (FAQ)

Q. What causes knee AVN?
A. Knee AVN is caused by a disruption in the blood supply to the bone, which can result from trauma, long-term steroid use, alcohol consumption, or underlying diseases.

Q. How is knee AVN diagnosed?
A. Knee AVN is diagnosed using a combination of physical examination, X-rays, MRI, and bone scans to assess the degree of bone damage.

Q. What are the treatment options for knee AVN?
A. Treatment options range from non-surgical methods, such as weight-bearing protection and medications, to surgical interventions, including core decompression and total knee replacement.

Q. Can knee AVN be prevented?
A. While not all cases of knee AVN can be prevented, reducing alcohol consumption, managing underlying health conditions, and preventing knee injuries can help lower the risk.

Summary and Takeaway

Avascular necrosis of the knee is a serious condition that can lead to joint collapse if left untreated. Early diagnosis and management are key to preventing the progression of the disease. Non-surgical treatments may help in the early stages, but surgery, including total knee replacement, may be necessary in advanced cases. By following a proper treatment plan and rehabilitation, individuals with knee AVN can regain function and improve their quality of life.

Clinical Insight & Recent Findings

Avascular necrosis (AVN) is a serious condition that affects the femoral head, leading to the potential need for total hip arthroplasty (THA). While THA is a common and effective treatment, it may not be ideal for younger, more active patients due to the risk of needing revision surgeries later in life.

A recent systematic review highlighted the role of orthobiologic therapies, such as bone marrow aspirate concentrate (BMAC), in delaying the need for hip replacement surgery. The study found that BMAC, in particular, significantly improved long-term survival without the need for THA, providing a promising alternative for managing AVN.

These findings suggest that early intervention with orthobiologics could help preserve the natural hip joint and delay more invasive procedures like THA. (“Study on orthobiologic therapies delaying hip replacement surgery in AVN – see PubMed.“)

Who Performs This Treatment? (Specialists and Team Involved)

Orthopedic surgeons specializing in joint preservation, sports medicine, and knee surgery are typically involved in the diagnosis and treatment of knee AVN. Physical therapists play a crucial role in rehabilitation after treatment.

When to See a Specialist?

You should consult a specialist if you experience persistent knee pain, swelling, or limited movement, especially after an injury or long-term steroid use.

When to Go to the Emergency Room?

Seek emergency care if you experience sudden, severe knee pain, a noticeable shift in knee alignment, or signs of infection following surgery, such as increased redness or fever.

What Recovery Really Looks Like?

Recovery from knee AVN treatment, especially surgery, requires a structured rehabilitation program to regain strength and mobility. Full recovery, particularly after knee replacement, may take several months.

What Happens If You Ignore It?

Ignoring knee AVN can lead to severe joint damage, chronic pain, and loss of function. Timely treatment is essential to preserve knee health and prevent the need for extensive surgery.

How to Prevent It?

Prevention involves managing underlying health conditions, avoiding excessive alcohol consumption, and protecting the knees from injury through proper technique and protective gear.

Nutrition and Bone or Joint Health

Adequate nutrition, including calcium and vitamin D, is essential for bone health, especially for individuals at risk for AVN. Maintaining a healthy weight and staying active can also help reduce the risk of joint damage.

Activity and Lifestyle Modifications

Engaging in low-impact activities, such as swimming or cycling, can help maintain joint function without putting excessive strain on the knee.