No study has ever documented that stents prevent future heart attack. But, in day-to-day practice, stents are frequently implanted for just this reason.
A little clarification. Stents do prevent heart attack--if the heart attack is already underway, either as an "acute myocardial infarction" or "unstable angina."
In other words, a plaque in a coronary artery can rupture just like a little volcano. Rather than spewing lava, the underlying plaque contents--fibrous tissue, inflammatory cells, cholesterol crystals, fatty material, debris--are exposed to flowing blood and trigger spasm of the artery and blood clot formation. A ruptured plaque is typically found in people who go to the emergency room with severe chest pain or have difficulty breathing.
A heart catheterization is performed, a severe (e.g., 90-100%--completely closed) is found. A stent in this situation is of clear-cut benefit.
What is not clearly beneficial is someone with no symptoms, symptoms only with physical activity that has been present for at least several months, or someone with a high heart scan score and no symptoms. In these circumstances, stent implantation does not reduce risk for future heart attack.
Take a look at this angiogram of a right coronary artery. You can seen plaque all along the artery (represented by areas that appear pinched off. There are at least 4 visible.)
Putting one 15 millimeter stent in the artery will only affect the area of artery stented. (Stents vary in length, but typically are 12-18 millimeters in length.) The right coronary artery is about 10 times or more this length. There are also two other arteries of similar length. A stent at one location will do nothing to affect the potential for rupture in any of the other plaque-laden areas.
Say a stent is implanted in the "worst" blockage in this right coronary artery, the plaque located at around 9 o'clock. What about all the other plaques? They can still rupture.
Why not put in many stents, say, 4 or 5, and stent all the visible plaques?
Two reasons: 1) Plaque you can't even see on an angiogram can still rupture, and 2) it is very costly (easily $30,000 at the very least), 3) incurs greater procedural risk, and 4) messes up the artery for future procedures, since a steel-lined artery that develops more disease in future will be more difficult to re-implant stents, bypass, or perform other procedural manipulations.
The point: Putting in stents does not reduce potential for plaque rupture in the entire artery.
What can prevent plaque rupture? That's the whole point of following an effective prevention program: prevent plaque rupture.
(Of course, this discussion cannot encompass the wide variety of potential situations that may cause your doctor to individualize your approach. Nonetheless, when advised to have an elective heart procedure, a healthy dose of skepticism and is clearly a good practice.)
Top image courtesy National Heart, Lung, and Blood Institute.