What you should know about joint injections for knee osteoarthritis

Osteoarthritis refers to the gradual wear and tear of your joints. This occurs because, over time, the cartilage that protects your joints wears away. Cartilage is a smooth tissue that allows the bony surfaces of your joints to move over each other easily. When your cartilage wears away, it doesn’t grow back, and consequently, your bones rub directly against each other. The increased friction causes inflammation, pain and stiffness. Osteoarthritis can affect any joint in your body but often occurs in the hips and knees. 

Treating knee osteoarthritis

Knee osteoarthritis is first treated by changing your activities to reduce the impact on your knees and taking over-the-counter anti-inflammatory medication (eg ibuprofen) to reduce inflammation. 

However, it is also important to build up the strength of the muscles around your knees through physiotherapy. This is because the pain caused by knee osteoarthritis can reduce your activity levels, which can trigger a vicious circle where your muscles weaken, putting more strain on your joints, therefore, leading to more pain. Physiotherapy can help break this cycle and strengthen your knee joints. 

If these treatments aren’t effective, your doctor may recommend steroid injections into your knees to significantly reduce inflammation and, consequently, your pain levels. This will allow you to perform more physiotherapy exercises to build up the strength of your knees and ease your knee osteoarthritis symptoms.  

How do joint injections work?

Your knee joint is encased in a joint capsule. This capsule holds lubricating fluid that helps your knee joint move smoothly. However, in knee osteoarthritis, even if only one part of your knee is affected, the inflammation can spread throughout your knee joint as the cells that cause inflammation travel through the lubricating fluid.

Steroid injections into the knee joint are given directly into the joint capsule. This allows the steroids to act directly on the cells that are causing inflammation and prevent them from releasing the chemicals that trigger inflammation of your knee joint. This reduces pain and allows you to perform more exercises to strengthen your knee joint.

However, it is important to note that in the first few days after your injection, your knee pain may flare up before the steroids start to work. Your steroid joint injection will, therefore, be given alongside a local anaesthetic to reduce the effects of any flare-up on your pain. If your joint injection has worked, you should notice the benefits after about a week.

How long do joint injections last?

How long the effects of a single steroid injection last varies from person to person, ranging from a few weeks to up to five years. If your steroid injection offers pain relief for years, you can delay the need for knee replacement surgery. If your steroid injection offers pain relief for a few weeks, this helps confirm that your pain is due to knee osteoarthritis rather than another condition, such as a trapped nerve. Steroid injections into the knee are, therefore, used for both treatment and diagnosis.

How many joint injections can you have?

The number of steroid joint injections you can have over your lifetime varies depending on the effectiveness of your injections and how long their benefits last. Steroid injections tend to work slightly less with every subsequent injection.

You should not have too many injections over a short period of time (ie no more than three injections over 12 months) as this may start to damage your knee.

Risks of joint injections

Joint injections are very low risk. However, as with any injection, there is a very small risk of infection. There is also a chance that your joint injection will not work.

If you have a steroid injection into your knee joint, it is recommended that you do not then have knee replacement surgery for another four to six months. This is because steroids can temporarily suppress the immunity within your knee and surgery comes with a risk of infection, which your knee would then find difficult to fight off.

When are joint injections not suitable?

If you have a prosthetic implant in your knee, such as a partial or total knee replacement, you should not have a steroid injection into the joint. This is because prosthetic implants do not have a blood supply and steroids can temporarily suppress the immunity in your knee — together, this puts you at greater risk of developing an infection from the injection.

You should also not have a joint injection if there are signs of infection around your knee eg a skin infection, boil or abscess.