Below‐knee cast immobilization and the risk of venous thrombosis: results from a large population‐based case–control study

Background From the available evidence, the risk of venous thrombosis in patients with below‐knee cast immobilization remains unclear. Objectives To estimate the risk of venous thrombosis after below‐knee cast immobilization and to identify high‐risk groups. Patients and methods We used data from a large population‐based case–control study (MEGA study) on the etiology of venous thrombosis (4418 cases; 6149 controls). Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated, and adjusted for age, sex, body mass index, and regular exercise. Absolute risks were estimated from the ORs. Results One hundred and thirty‐four patients and 23 controls had below‐knee plaster cast immobilization in the year before the index date, resulting in an eight‐fold increased risk (OR 8.3 [95% CI 5.3–12.9]). Traumatic indications led to a higher risk than non‐traumatic indications: OR 12.7 (95% CI 6.6–24.6) vs. OR 7.6 (95% CI 0.9–66.4). An additionally increased risk was found for combinations with genetic or acquired risk factors: oral contraceptives (OR 18.2 [95% CI 6.2–53.4]); obesity (OR 17.2 [95% CI 5.4–55.2]); factor V Leiden, factor II 20210A mutation, and/or non‐O blood group (OR 23.0 [95% CI 11.5–46.0]); all for a period of 1 year. Ninety per cent of the events occurred in the first 3 months after cast application. This led to a 56‐fold increased risk (OR 56.3 [95% CI 17.9–177.3]) in this period. Conclusions Below‐knee cast immobilization strongly increases the risk of venous thrombosis. We found distinct differences in intrinsic risk between individuals with respect to indication for cast immobilization and the presence of genetic or acquired risk factors.