Stents are tiny tubes of metal meshwork, which can hold open narrowed arteries. Although coronary stenting is a great idea, and has helped countless people with coronary artery disease (CAD), it has its limitations. First of all, stents are made of metal, and when blood comes in contact with metal it has a tendency to clot. This undesirable process is known as in-stent thrombosis or IST. The clot can enlarge, block the coronary artery, and cause a heart attack-the exact opposite of the result intended by the cardiologist! Medications like clopidogrel (Plavix) are very effective at preventing such clots from forming, and their consistent use is absolutely essential for stents to work successfully. Your doctor was concerned about the planned knee surgery because your clopidogrel would almost certainly be stopped at least a week before the procedure, putting you at potential risk.