Stable ankle fractures can be successfully treated non-operatively with a below knee plaster cast. In some European centres it is standard practice to administer thromboprophylaxis, in the form of low molecular weight heparin, to these patients in order to reduce the risk of deep venous thrombosis (DVT).

The aim of our study was to assess the incidence of DVT in such patients in the absence of any thromboprophylaxis. We designed a prospective study, which was approved by the local ethics committee. We included 100 consecutive patients with ankle fractures treated in a below knee plaster cast. At the time of plaster removal (6 weeks), patients were examined for signs of DVT. A colour doppler duplex ultrasound scan was then performed by one of the two experienced musculoskeletal ultrasound technicians.

We found that 5 patients developed a DVT. Two of these were above knee, involving the superficial femoral vein and popliteal vein respectively. The other three were below knee. None of the patients had any clinical symptoms or signs of DVT. None of the patients developed pulmonary embolism. Of these five patients, four had some predisposing factors for DVT.

The annual incidence of DVT in the normal population is about 0.1%. This can increase to about 4.5% by the age of 75. DVT following hip and knee replacement can occur in 40-80% of cases. Routine thromboprophylaxis may be justified in these patients. However, with a low incidence of 5% following ankle fractures treated in a cast, we believe that routine thromboprophylaxis is not justified.