There may be several factors at play here. First, let’s take a look at the tendon problem itself. Scientists have indeed shown that chronic, painful tendon problems are not associated with inflammation. When tissue samples are studied under the microscope, there are NO inflammatory cells.
There probably was an inflammatory response when the injury first occurred. The initial acute phase is often one of inflammation and repair. But with chronic overuse and excessive mechanical load, the tenocytes (tendon cells) start to degenerate or break down. A pain response is initiated by the body to let you know there is a problem and to do something about it.
The steroid injection may be effective strictly from the placebo effect that is present with any treatment. And that refers to the fact that the patient is expecting the treatment to work, so it does. The brain and body respond with a new, improved healing response that is more successful than the previous strategy.
Secondly, the injection includes both the steroidal antiinflammatory but also an anesthetic or numbing agent. By stopping the pain signals being sent to the brain, the patient gets some pain relief.
But in the final analysis, we simply don’t know exactly why steroid injections work for a noninflammatory condition. And since they aren’t effective for everyone, there must be some individual factors that influence the treatment that haven’t been uncovered yet.