Research isn't yet clear on this issue. A recent study compared two such compression devices. One gave a low-pressure pulse, first to the calf and then to the thigh. The other gave a high-pressure pulse to the calf and foot. Patients with major trauma to the pelvis wore one of these devices after surgery until they were discharged from the hospital.

The authors thought the high-pressure calf-foot device would be more effective in preventing blood clots than the low-pressure thigh-calf device. Yet there were no major differences between the two, possibly because there weren't enough patients in the study to show such differences. Patients who had the low-pressure device were a little more likely to have larger clots, but this trend was slight.

What is clear is that either type of device is far better than nothing at all. Without preventive care, the rate of blood clots may be as high as 61 percent with pelvic trauma. In contrast, the rate of blood clots in this study was 14 percent overall.