Mechanical and suboptimal pharmacologic prophylaxis and delayed mobilization but not morbid obesity are associated with venous thromboembolism after total knee arthroplasty: A case‐control study

BACKGROUND: The FDA-approved dose of low-molecular-weight heparin (LMWH) may not provide adequate thromboprophylaxis in morbidly obese patients after total knee arthroplasty (TKA). Suboptimal dosing, delayed initiation, and overreliance on mechanical methods may also limit the effectiveness of thromboprophylaxis. OBJECTIVE: We explored the associations between the type of thromboprophylaxis, obesity, time of mobilization, and undergoing bilateral TKA on development of symptomatic venous thromboembolism (VTE) after TKA. DESIGN/SETTING/PATIENTS: This was a case-control study of patients undergoing TKA in 15 teaching hospitals between October 2008 and March 2010. Cases were screened using the Agency for Healthcare Research and Quality's Patient Safety Indicator 12 and had objectively documented acute VTE within 9 days of surgery; controls were randomly selected from the same hospital. Multivariable logistic regression was used to analyze risk factors for postoperative VTE, adjusted for age and gender. RESULTS: Among 130 cases with and 463 controls without acute VTE, body mass index (BMI) ranged from 17 to 61 (median = 34). Thromboprophylaxis was LMWH in 284 (48%), warfarin in 189 (32%), both in 55 (10%), and mechanical prophylaxis alone in 120 (20%). Overall, 77% ambulated on day 1 or 2 after surgery. Factors significantly associated with VTE were bilateral simultaneous TKA (odds ratio [OR] = 4.2; 95% confidence interval [CI]: 1.9-9.1), receipt of FDA-approved pharmacological prophylaxis (OR = 0.5; 95% CI: 0.3-0.8), and ambulation by postoperative day 2 (OR = 0.3; 95% CI: 0.1-0.9). Obesity was neither a significant confounder nor a modifier of these effects. CONCLUSIONS: Severe obesity was not a significant independent predictor for VTE and did not modify the beneficial effect of FDA-approved pharmacological thromboprophylaxis. Bilateral TKA and failure to ambulate by the second day after surgery were significant risk factors.