Venous thrombosis is frequent in patients undergoing hip arthroplasty. Prophylaxis with warfarin and low-molecular-weight heparin (LMWH) effectively reduces the risk of thrombosis. Oral anticoagulation is complicated by the need for routine laboratory monitoring. The start of LMWH therapy is delayed until 12 to 24 hours postoperatively to limit the risk of bleeding. Because the risk of thrombosis starts perioperatively, perhaps an earlier start of LMWH therapy might decrease the incidence of thrombosis. Hull and associates reviewed randomized trials that assessed the efficacy and safety of once-daily LMWH started at different times after elective hip arthroplasty surgery compared with oral anticoagulation in the prevention of deep venous thrombosis.

In the four studies that met the authors' criteria, a significant risk reduction was seen in two trials in which LMWH was initiated at one half the usual dosage close to surgery (from less than two hours before to four to six hours after). A similar risk reduction was not seen in patients receiving LMWH at more conventional times (from 12 to 24 hours before surgery to 18 to 24 hours after surgery). The peak efficacy for LMWH therapy based on odds ratios for deep venous thrombosis in each trial and the starting time of treatment ranged from two hours preoperatively to six to eight hours postoperatively. Major bleeding was more common in a study in which patients received LMWH in close proximity to surgery. Minor bleeding was minimal in all studies.

The authors conclude that waiting 12 to 24 hours postoperatively before starting LMWH therapy in patients undergoing elective hip arthroplasty is not as effective and no safer than more proximal preoperative treatment. Use of one half the usual high-risk dosage in the preoperative period was more effective than using standard dosages 12 hours preoperatively or 12 to 18 hours postoperatively. Administering LMWH four to six hours after surgery offers greater efficacy and a decreased rate of major bleeding than close-proximity perioperative therapy (administered less than two hours before surgery). The specific type of LMWH and the dosage are unlikely to affect these results.