A presumption is a belief or hypothesis about something. An assumption is taking a piece of information (as in knowledge about a topic) as a given fact. When you put them together, a presumptive assumption is troublesome because you can assume in error.
Pain is an expected component of injuries, illnesses, and surgical procedures. Let's face it, breaking your leg hurts. In some instances, however, the patient's pain is unexpected or is worse than predicted. Sometimes, the pain can last well beyond the time it should have resolved.