Introduction Patients with lower limb injuries are commonly discharged from the ED with the affected area immobilised. Rigid casting of the lower limb is known to be a risk factor for the development of venous thromboembolism (VTE), making thromboprophylaxis in this population an important consideration for clinicians in the ED. The use of structured risk assessment methods (RAMs) to evaluate VTE risk and recommend thromboprophylaxis to those at higher risk is widespread in the UK. However, the evidence informing this practice is nearly exclusively based on studies of patients with rigid lower limb casts but many patients with knee injuries, including some with significant thrombotic risk factors, are managed in semi-rigid (‘cricket’) knee splints. These are both removable and allow free movement of the ankle, but the baseline risk of VTE and the performance of different RAMs in this population are not known.

Methods Consecutive patients (≥14 years) discharged from the ED at Aberdeen Royal Infirmary, between 1 January 2010 and 31 December 2021, in a semi-rigid knee splint were identified retrospectively and followed up to 3 months after splint removal for the development of symptomatic VTE. Secondarily, data permitting the assessment of five different RAMs (NICE, GEMNet, an Aberdeen tool, the Plymouth score (V.2) and the L-TRiP(cast) score) were extracted systematically and compared.

Results In 510 patients (mean age 32 (SD 16) years, 62% male) none received thromboprophylaxis and all completed follow-up. Two patients developed symptomatic VTE (0.4%, 95% CI 0.1% to 1.4%). The different RAMs varied considerably in the proportions identified for thromboprophylaxis from GEMNet (47%) to the L-TRiP(cast) score (2%), but no RAM was able to identify the two patients who progressed to VTE.

Conclusions In our cohort of patients managed in semi-rigid removable knee splints, the risk of symptomatic VTE was low, about 1 in 250, and current methods of VTE risk assessment did not prove clinically useful.