Firstly we need to define which injection are we talking about.
Although not used much these days, corticosteroid injections (often combined with a local anaesthetic) have been used to decrease pain and inflammation. Because they have no lasting benefits they are used more for diagnostic reasons. If the injection helps, even briefly, it confirms the pain is coming from within the joint itself. This may help guide future treatment decisions.
Hyaluronic Acid injections are one of the latest treatments in the area.
They aim to increase the viscosity of the joint fluid as this is decreased in the osteoarthritic knee.
There are 2 products commercially available, Synvisc and Hyalgan.
Each is administered via multiple injections into the joint space over a period of weeks / months.
There is an increasing body of research into the efficacy of these injections but again the results are mixed.
Some studies have shown no long term benefits while others show promising results. This is another treatment that seems to help some more than others. Adverse reactions have been infrequent and minor so it appears a safe option.
There is a significant cost involved and once again this treatment will not make the knee 'unworn'.
As the use of these injections increases more information will be gained on who it helps and how effective it is.
Until then it should be another question to ask your surgeon. What does he / she think about these injections and do they think you will benefit?
For more information read this article by Dr Dennis Wen