I'm going to have a total hip replacement done next week. I've been told I'll be put on a blood thinner to prevent blood clots. Why are clots so common after hip surgery?

Blood clots or thrombi occur more often after hip and knee surgery than any other orthopedic procedure. They affect between two and 10 percent of adults age 65 and older. In fact, advancing age is a risk factor for blood clots to form.

There are two main reasons blood clots form after total hip or total knee replacement. The first is the body's response to what it considers an "invasion" around the major blood vessels to the leg. The system sends out messages to the blood to "get ready" for bleeding. The best way to combat bleeding is to form blood clots to plug any holes in the blood vessels.

Secondly, during the operation, the major vein to the leg (femoral vein) gets kinked. This stops blood flow to the area below the kink. The result can be injury to the vein. Injury once again sets up a series of steps the body takes to form a blood clot.