Can you tell me a little about osteonecrosis of the knee? I was diagnosed with this condition after developing knee pain and swelling for no apparent reason. What causes it? Why did I get it?

Osteonecrosis of the knee is fairly rare. Osteonecrosis of the hip is a much more common problem. The term osteonecrosis means the death of bone tissue. There are three types of knee osteonecrosis: 1) spontaneous (occurs without a known cause), 2) post-arthroscopy (occurs after an arthroscopic procedure), and 3) secondary to some other condition such as lupus, use of steroids, or alcohol abuse.

The type you have is spontaneous osteonecrosis of the knee -- also referred to as SPONK. It usually occurs in one compartment or section of the knee. Secondary osteonecrosis (caused by disease or medical therapy) affects more than one compartment. In most cases (no matter what the cause), the bottom, round part of the femur (thighbone) called the femoral condyle is the area damaged.

Spontaneous osteonecrosis usually occurs in patients older than 55 years, while secondary osteonecrosis can occur at any age. Women are affected by SPONK three times more often than men. The reason for this is unknown.

Osteonecrosis after arthroscopy is rare. It usually occurs when some form of heat such as laser or other thermal devices were used during the procedure. The patient starts to develop worse pain after arthroscopy than before. Knee swelling is a common feature of this problem.

Not much is really known about this condition -- who gets it, what causes it, or even what is happening at the cellular level. For those patients who get it without an obvious reason, the physician looks for potential risk factors such as a traumatic accident or injury, alcohol use, blood disorders, history of previous knee surgery, and autoimmune disorders.

Sometimes, none of these are positive and we are left scratching our heads. Further research may help identify specific reasons. It's likely that the problem occurs as a result of many factors, not just one cause.