What is avascular necrosis of the knee?
Avascular necrosis of the knee, or osteonecrosis, is when cells in the bones of the knee die because they are not getting enough blood. When enough bone cells die, the bones begin to weaken and break down. This causes pain, arthritis, and difficulty moving the knee. It is a rare cause of knee pain and disability.
There are three types of avascular necrosis of knee:
- Spontaneous osteonecrosis of the knee (SONK). This is the most common form of necrosis of the knee. It is most common in adult women older than 60 years. It usually affects only one knee. Researchers believe that it is caused by small fractures (breaks) in the weakened bones of the knee. The fractures can cause fluid to build up in the bone marrow, which cuts off the supply of blood to bone cells.
- Secondary osteonecrosis. This form of necrosis of the knee is usually seen in women younger than 45 years, and affects both knees. It is often related to another illness, such as sickle cell anemia or lupus. It is also seen with heavy alcohol use or the use of corticosteroids.
- Post-arthroscopic osteonecrosis. This form of necrosis of knee can affect anyone who has had surgery to repair a torn meniscus (arthroscopy), the cartilage that cushions the knee bones. This is rare.
The most common symptoms of avascular necrosis of the knee are pain and stiffness in the knee.
SONK usually causes pain on the inner side of one knee. The pain may start suddenly, and get worse at night or when putting weight on the knee. It is sometimes similar to the pain a person might get with a torn meniscus.
Secondary osteonecrosis causes pain that comes on gradually instead of suddenly. The pain is usually on the inner side of the knee, but can sometimes be on the outer side. A person with secondary necrosis of the knee may also have pain in other joints, like the hips, because this condition often involves many joints.
Osteonecrosis that happens after surgery usually causes pain on the inside of the knee and appears suddenly, usually about 6 to 8 weeks after meniscus surgery.
Avascular necrosis of the knee happens when the blood supply is cut off to the bone cells in the knee. Anything that causes pressure to build up and crowd the bone marrow can reduce the blood supply to the bone cells. Anything that clogs blood vessels or makes them more narrow can also make it difficult for the blood to get to the bones.
You are more likely to have avascular necrosis of the knee if you:
- Have weakness in, or injury to, the bones of the knee. This causes fractures, which may lead to fluid build up in the bone marrow.
- Take steroid medicines. Steroid medicines like prednisone are used to treat inflammation from diseases like lupus (systemic lupus erythemetosis). Researchers aren’t sure why steroids can cause avascular necrosis of the knee. It may be because steroids make it harder for the body to break down fats, which can clog blood vessels.
- Excessive alcohol use. Drinking a lot of alcohol over a long period of time can also make it harder for the body to break down fats, which can clog blood vessels.
- Use tobacco. Tobacco use can narrow the blood vessels, so it can make things worse if you have other risk factors.
Other things that can cause avascular necrosis are:
- Blood disorders such as sickle cell anemia, HIV infection, Gaucher’s disease, and Caisson disease
- Radiation therapy
- Organ transplant
Diagnosis & Tests, Treatments, & Prevention
Diagnosis & Tests
Your healthcare provider may do any or all of the following to find out what’s causing the pain in your knee:
- Talk with you about your medical history, including what your knee feels like, where the pain is, when it started, and what makes it worse.
- Look at your knee and ask you to move it certain ways.
- Do the following tests:
- X-ray. An x-ray may not be sensitive enough to see bone changes in the early stages of avascular necrosis, so the doctor may do more tests if the x-ray is normal.CT Scan. A CT is more sensitive than an x-ray. It takes many pictures of the bone and maries them together with a computer.
- MRI can show bone changes from avascular necrosis, even in the early stages.
- Bone scan. The doctor may do a bone scan if you have normal x-rays, especially if you don’t have any reasons to have avascular necrosis of the knee.
- Biopsy. This is a surgery to take a sample of the bone tissue. It is not done very often.
- Functional evaluation of bone. This is a surgery to measure the pressure inside a bone. It is not done very often.