Many blockages that are found by angiography can be safely opened by angioplasty. A wire is passed through the blockage, and a small balloon is advanced over the wire so that it can be inflated inside the artery, opening the blockage. However, there is an unacceptably high rate of the artery closing down again if this is the only thing done, so intracoronary stents were developed. These are small expandable metal tubes that hold the artery open. They may be bare metal or they may be coated with a medication that prevents the growth of scar tissue within the stent. Either way, they must be combined with medications to prevent clotting of the stent.