You might think that once drugs, devices and medical procedures are shown to be effective, they quickly become available. You might also think that those shown not to work as well as alternatives are immediately discarded.
Reasonable assumptions both, but you'd be wrong.
Instead, innovations in health care diffuse unevenly across geographic regions — not unlike the spread of a contagious disease. And even when studies show a new technology is overused, retrenchment is very slow and seemingly haphazard.