Essentially, there are two different things that can be injected into a knee:-
2) hyaluronic acid
‘Steroid’ and ‘cortison’ mean the same thing. When steroid is injected into a knee joint it is corticosteroid that is used (this is not an anabolic steroid). Steroid is a very strong anti-inflammatory that acts locally in the joint, like taking a massive dose of nurofen, but with it acting all in one spot, and the dose that reaches the rest of the body is negligible. The likelihood of it causing any side-effects or complications is tiny.
The effects of steroid in a joint come on slowly, sometimes taking 48 hours or so. Therefore, we normally mix in some local anaesthetic with the steroid – the LA starts to work very quickly and gives rapid pain relief in the joint. However, there can be a window where sometimes the knee temporarily feels more uncomfortable before the effect of the steroid kicks in.
Steroid is very effective indeed for inflammatory joint problems, such as gout or rheumatoid arthritis. It specifically treats inflammation and therefore it tends to be less effective at relieving pain from mechanical damage in a knee.
Steroid works in about 75% of people, giving reasonably good pain relief but for variable amounts of time – the injection can sometimes give pain relief for up to 6 to 9 months.
Importantly, it should always be remembered that is there is any kind of damage inside a knee, then giving a steroid injection might help improve the pain, but it is only masking the symptoms and not actually addressing or curing whatever the underlying problem might be. The risk here is that if there is any damage in the knee joint, then having a steroid injection can actually be dangerous, as by masking the pain it can encourage people to actually do more on their knee, which means that whatever damage there is could well actually get worse.
For conditions such as osteoarthritis of the knee, where someone is going to end up needing a knee replacement but where they are either not medically fit for surgery, where someone is simply averse to the whole idea of having anything surgical done or where for whatever reason they just need to delay the timing of any surgery – then an intra-articular steroid injection can give many people pretty good symptomatic relief and buy them extra time, delaying the time when they might then need surgery anyway.
Hyaluronic acid injections
Hyaluronic acid (HA) can also be injected into knee joints, but HA is a bit more contentious. HA is a very long-chain molecule, which makes it very oily and viscous. When it first became available, there were two main companies selling it; one sold it as a ‘medical product’, saying that it acted as a mechanical lubricant, effectively ‘oiling’ the knee joint. The other company registered it as a drug, saying that it had a pharmaceutical effect in the knee, like a painkiller. Either one of these companies was right and the other wrong, or maybe both were a bit right, or maybe they are both wrong?!
There is research that shows that HA injections into the knee can reduce the pain of knee arthritis. However, like steroid, it does not work on everyone, and it helps in approximately 75% of patients. For those who it does help, it tends to give just temporary pain relief – for about 6 to 9 months (pretty much the same as a steroid injection). The pain relief that it gives is approximately equivalent to that from taking regular anti-inflammatory tablets.
When it first came out, HA used to be injected as a series of repeated small injections, with 2ml of HA being injected in the knee at weekly intervals, with anything up to 5 or 6 injections in total! Thankfully, HA now tends to be given in larger does (e.g. 6ml) in just 1 single injection. This has shown to be equally as effective as multiple smaller injections, but it is much less hassle (and pain) for the patient, and far cheaper.
Unlike steroid, there are some potential side-effects that some patients may suffer with HA injections. The compound is made from purified HA taken from cockerels’ combs – therefore it shouldn’t be used in people who have a bird/egg allergy. About 1% of patients experience a transient synovitis in the knee after HA injection, which causes pain and swelling in the joint – this can sometimes take a few weeks to gradually settle.
One further consideration that should be borne in mind is that many insurance companies don’t actually cover the cost of HA injections, whereas most do cover steroid injections. It can cost £200 - £300 for a vial of HA, plus there is the cost of the doctor’s charge for injecting the HA.
Therefore, given the lack of certainty about the results, given the potential risks and given the costs, HA generally tends to be offered mainly to people who have already tried a steroid injection.