We often think of bones as dry, mechanical things that have little to do with tissues and organs.
But in fact bones are a kind of tissue and organ in their own right. They’re built from living cells, they produce red and white blood cells for our bodies, and they require a blood supply to stay healthy.
If that blood supply gets disrupted or cut off, it can – understandably – cause some serious issues. That’s basically what happens in osteonecrosis, a relatively rare condition that causes painful problems for some knee patients. Here’s how it works, and how we treat it.
“Osteonecrosis?”
The word literally means “bone death”. You may also hear it called a few other things:
Avascular necrosis
Ischemic necrosis of bone
Aseptic necrosis
They all refer to the same thing – the death of bone tissue due to loss of blood supply. The condition can affect any bones in the body, but most commonly develops in the endings of long bones, such as the bottom of the femur (your thigh bone), where it connects into the knee joint.
What does osteonecrosis do to the bone?
Bones need a constant supply of blood to stay nourished. It enables them to replace old bone tissue after injury or through normal growth and change. If the supply disappears, the bone tissue can no longer do this. It begins to die – the bone breaks down and, eventually, can deteriorate and crack. When this happens in a knee joint, it can cause the smooth cartilage surface to collapse.
Why does it happen?
It’s not always easy to say. We do know that certain things make osteonecrosis more likely to occur. These include:
Clotting problems – the blood struggles to clot (“coagulopathy”), raising the risk of serious bleeding
Injury – trauma (such as a bad break or a dislocation) that damages blood vessels near a bone
Fatty deposits – which collect in the blood, block vessels or reduce blood flow
Certain medical conditions – such as sickle cell anaemia or Gaucher disease
A number of other things can also raise the risk of developing osteonecrosis. These include drinking too much alcohol, use of certain steroids, and radiation therapy in cancer treatment. It’s most common in middle-aged people, between 40 and 60 years old. Quite often, though, it’s difficult or impossible to pinpoint the exact cause of osteonecrosis.
What symptoms does it cause?
In the early stages, there are often no symptoms at all. As it develops, the first signs tend to be pain when putting weight on the knee (or both knees – the condition can develop in one joint, or both at the same time). In time, the joint can start to become stiff and arthritic, and you could start to lose mobility. If the joint surface collapses, the pain can become acute – often suddenly. Osteonecrosis tends to cause pain on the inner side of the knee (see What causes inside knee pain?).
How would you confirm it?
First, we’d do a physical examination of your knee. We’ll also want to know about your medical history. You may have had an x-ray already – the problem being that x-rays often don’t flag up early osteonecrosis. This is one reason why, unfortunately, patients often go undiagnosed at first. So we may well ask you to have an MRI scan, which can help us to identify the area of bone death in the joint; this appears as a dark patch under the bone surface, somewhat similar to tooth decay.
What can you do about osteonecrosis?
The main aim with treatment is to stop the bone loss from worsening and prevent collapse before it happens. There are a number of things we can do here. In the early stages, the focus is on support: we may recommend a “cricket pad splint” or a hinged knee brace for 1-2 weeks, with ice treatment, pain relief and physiotherapy to strengthen your quads and hamstring muscles. We’ll also monitor you over the weeks and months to come, to make sure the damage isn’t progressing. We could also consider a local anaesthetic and steroid injection, and certain types of medication.
What happens in more serious cases?
If the damage to your knee joint becomes more severe, a partial knee replacement could come into play. The goal with this approach is to replace the portion of your joint where the cartilage has been depleted with a new, smooth, artificial section of knee joint. This often proves to be a very effective treatment for patients with serious cases of osteonecrosis. It’s worth bearing in mind, though, that this scenario is relatively rare. In many cases, with careful management, there’s a lot we can do to help the condition to “settle”, and prevent further damage from happening within the joint.
Interested in reading on? You might find these articles helpful
Joint 101: a beginner’s guide to the knee
Knee resurfacing or partial replacement – what’s the difference?
