Most insurance companies pay for this separately, Medicare does not
While it's hard to paint with a broad brush and make a statement that all payers will agree and follow, in order to bill for pain injections after a procedure you have to look at your payer and their specific policy. Medicare does not allow any procedure within the global unless the patient goes back to the OR. Not all insurance companies see it that way, for which I am grateful for.
After surgery there is a point where the patient should be getting better, or is getting better, and then they have increased pain for some reason. If the injection is given after the patient's pain has improved, and then become worse, we do bill for injections as a complication and they do get paid. My previous statement did not refer to normal pain management, but a complication after a procedure. Having increased pain after a surgery would qualify as a complication and could be an indication of infection.
If your clinic is not billing joint injections when the patient experiences increased pain which could be a sign of infection or other complication your clinic is losing money that is deserved, but as always the golden rule applies: The insurance company has the gold so they make the rules.
Don't automatically write them off. If the injection is given for pain management, don't bill it. But if the patient improves, and then has increasing pain later, it's a complication that many insurance companies do pay for.