Ann Arbor, MI—Prescribing heparin or newer blood-thinners after knee replacement might be unnecessary, according to a new study that finds that OTC aspirin works as well for most patients.

The report in JAMA Surgery could have broad effect because more than 700,000 total knee arthroplasties are performed each year in the United States.

University of Michigan–led researchers determined that few patients developed blood clots after surgery, and that those using only aspirin appeared to have no higher rates of venous thromboembolism (VTE) than those on prescription blood-thinners.

“Aspirin alone may provide similar protection compared to anticoagulation treatments,” explained coauthor Brian R. Hallstrom, MD, an orthopedic surgeon and associate chair for quality and safety at the University of Michigan Department of Orthopedic Surgery.

Whether to use aspirin alone to prevent VTE following total knee arthroplasty (TKA) has been an ongoing debate. This noninferiority study looked at a retrospective cohort of TKA cases submitted to the Michigan Arthroplasty Registry Collaborative Quality Initiative at 29 member hospitals, ranging from small community hospitals to large academic and nonacademic medical centers in Michigan. Included were more than 40,000 patients, mostly female, with a mean age of 65.8 years, who underwent primary TKA between April 1, 2013, and October 31, 2015. Clinical events were monitored for 90 days after surgery, and data were analyzed between September and October 2016.

Participants were stratified based on the method of pharmacologic prophylaxis:
• Neither aspirin nor anticoagulants for 668 patients (1.6%)
• Aspirin only for 12,831 patients (30.9%)
• Anticoagulant only (e.g., low-molecular-weight heparin, warfarin, and Xa inhibitors) for 22,620 patients (54.5%)
• Both aspirin/anticoagulants for 5,418 patients (13.0%)

Most patients were also using intermittent pneumatic compression stockings, according to the report.

Results indicate that a VTE event occurred in 573 of 41,537 patients (1.38%)—of those, 4.79% received no pharmacologic prophylaxis, 1.16% were treated with aspirin alone, 1.42% were prescribed anticoagulation alone, and 1.31% took both aspirin and anticoagulation medications.

Researchers add that aspirin only was noninferior for the composite VTE outcome compared with those receiving other chemoprophylaxis (adjusted odds ratio, 0.85; 95% CI, 0.68-1.07, P for inferiority = .007).

In terms of bleeding, that side effect occurred in 1.10% of patients overall and showed little difference among the group. The study team notes that aspirin alone was also noninferior for bleeding complications (adjusted odds ratio, 0.80; 95% CI, 0.63-1.00, P for inferiority <.001).

“In this study of patients undergoing TKA, aspirin was not inferior to other anticoagulants in the postoperative rate of VTE or death,” the authors conclude. “Aspirin alone may provide similar protection from postoperative VTE compared with other anticoagulation treatments.”

With the Michigan Arthroplasty Registry Collaborative Quality Initiative, a statewide effort to give patients the best possible recovery and outcomes after hip and knee replacements, aspirin prescribing has risen to 70% among Michigan surgeons, points out Hallstrom, who is codirector of the initiative and a health services researcher at University of Michigan’s Institute for Healthcare Policy and Innovation.

“This study is truly a real-world experience of what happened in Michigan when the majority of surgeons switched to aspirin,” Hallstrom says. “The incidence of blood clots, pulmonary embolus and death did not increase despite this dramatic change in practice.”

Some clinical guidelines lag behind, however. The American College of Chest Physicians advice favors heparin to reduce the risk of blood clots, while the American Academy of Orthopaedic Surgeons guidelines state that no one drug is better than another for preventing clots, according to the report.

Yet, the cost differences are significant, the Michigan researchers emphasize. They write that the reported cost for a 30-day supply of rivaroxaban is about $379 to $450; heparin is estimated at $450 to $890. Warfarin costs a few dollars for a 30-day supply but most be monitored, which adds costly physician visits, Hallstrom says. Aspirin costs approximately $2 a month, according to the article.

Study authors caution that, before making the decision on what to use, surgeons should take into account a patient’s history of clots, obesity, and ability to mobilize after surgery.

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