Joint injection is usually done with a numbing agent combined with a steroid drug. Painful symptoms from inflammation can be helped with this technique. But in the case of carpal boss, steroid injections have not been shown to help provide symptomatic relief. This may be because the pain comes from joint instability, not an acute inflammatory process.
Most experts agree that painful symptoms from carpal boss require surgery. Conservative care is always tried first in order to avoid complications from surgery. But with a protective cyst forming over the bony bump, you may not get pain relief without surgery.