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Aspirin Versus Low-Molecular-Weight Heparin After Total Hip Arthroplasty

TO THE EDITOR: In a multicenter randomized trial involving 786 patients having elective total hip replacement in Canada during a 3-year period, Anderson and colleagues (1) observed how, after 10 days of dalteparin therapy, 28 days of oral aspirin prophylaxis was noninferior to 28 days of dalteparin therapy for preventing venous thromboembolism (VTE). These results are very interesting to clinicians involved in elective hip surgery, particularly because some guidelines consider aspirin to be effective despite the single large randomized trial (2) quoted in the guidelines from the American College of Chest Physicians (3) that showed no effect of aspirin in this setting. We believe that the data from Anderson and colleagues’ trial cannot be considered as definitive as the conclusions indicated. The randomization in this relatively small trial did not result in a satisfactory distribution of important risk factors. Despite the authors’ assurance of the similarity of baseline characteristics of the 2 groups, we can see several important differences. The 3 strongest risk factors for VTE—history of deep venous thrombosis or pulmonary embolism (8 vs. 5 cases; 2.0% vs. 1.3%), active cancer in the past 5 years (13 vs. 8 cases; 3.3% vs. 2.1%), and surgery in the past 6 months (16 vs. 10 cases; 4.0% vs. 2.6%)— occurred more frequently in the dalteparin group. Furthermore, the most important risk factor for bleeding (previous major bleeding) also occurred more frequently in the dalteparin group (6 vs. 2 cases; 1.5% vs. 0.5%). With these being the principal risk factors for deep venous thrombosis and bleeding, we believe that the results should be interpreted with caution. It is also important to emphasize that the sample size is significantly smaller than those of trials in a similar setting (4, 5). As a result, the number of thromboembolic events in the 2 groups is small and chance-related findings cannot be excluded. We agree with Anderson and colleagues that the generalizability of the findings is probably questionable given the recent availability of novel oral anticoagulants for VTE prophylaxis after hip replacement (5). It would now be of major interest to outline a welldesigned randomized, controlled trial comparing aspirin with a novel oral anticoagulant. In conclusion, we believe that this article adds useful information on the role of aspirin in the orthopedic setting; however, the results need to be considered in the context of the unbalanced baseline risks and confirmed by new larger trials. Until this happens, the weak guidelines from the American College of Chest Physicians (3), based on evidence that shows little or no effect of aspirin in total hip replacement recipients, should not be given more attention or credibility.