According to a recent paper, we are a bit clearer on the positives and negatives of an injection.
First, it is becoming clear that cortisone injections produce short term pain relief but little long term benefit. Also, there are some tendons (tennis elbow) where an injection of cortisone can cause harm. While the evidence for harm is not definite for other tendons, the balance is certainly favouring a negative effect.
Second, platelet-rich plasma (PRP) injections, which shows promising results in arthritis, seem to be less effective in tendonitis. There are a few studies that show a positive effect of platelet-rich plasma (PRP) injections for tennis elbow and plantar fasciitis. However, there is no evidence for benefit in Achilles tendonitis or patellar tendonitis. I outline the evidence for PRP in this recent blog.
A specific type of injection called high-volume injection for Achilles and Patellar tendonitis showed some promise in early studies. However, recent work suggests that the effect is probably short term and limited.
Other injections such needle tenotomy have some evidence for use in tennis elbow. But there is little evidence for other tendons. Recently, a new procedure pioneered by me called tendon scraping is used for some cases of Achilles and Patellar tendonitis with good results.
Finally, injections of tendons are better under direct vision using ultrasound to improve accuracy.