The purpose of this study is to evaluate the efficacy and safety of fondaparinux in comparison with a heparin (nadroparin) in preventing deep vein thrombosis (blood clots in the leg veins), whether symptomatic or detected by ultrasound, and pulmonary embolism (blood clots that migrate to the lungs) in patients with leg injuries below the knee that require a cast or other type of immobilization but not surgery.

The study is designed to evaluate the efficacy and safety of fondaparinux sodium 2.5 mg (1.5 mg in patients with a creatinine clearance between 30 and 50 mL/min) once daily versus Low-Molecular Weight Heparin (nadroparin 2850 anti-Xa IU, 0.3 mL, once daily), with respect to the occurrence of venous thromboembolism, death and bleeding complications in patients requiring rigid or semi-rigid immobilization for at least 21 days and up to 45 days because of isolated nonsurgical below-knee injury. Treatment will be continued up to complete mobilization, e.g. plaster cast or brace removal, for a maximum of 45 days. The study will be a European, multicentre, randomized, open-label, controlled, two-parallel-group, phase III study in 1350 male and female patients 18 years of age or older, presenting with at least one additional major risk factor for VTE. After randomization (Day 1), subjects will receive subcutaneously, once daily, either fondaparinux or nadroparin up to complete mobilization. After cast or brace removal, a systematic, bilateral compression ultrasound will be done in all patients. Patients will be contacted five weeks (± one week) after complete mobilization. All suspected venous thromboembolic events, including asymptomatic deep vein thrombosis, all deaths, and all bleeding events (with the exception of certain types of minor bleeding events defined in the protocol) will be reviewed by an independent adjudication committee blind to treatment assignment.

Inclusion Criteria:

• Requiring rigid or semi-rigid immobilization (e.g. with a plaster cast or brace) for at least 21 days and up to 45 days because of isolated non-surgical below-knee injury
• With a no weight-bearing recommendation at the time of inclusion (partial weight bearing is permitted e.g. crutches, walking cast, relief shoes),
• Presenting at least one of the following risk factors for venous thromboembolism: below-knee fracture or Achilles tendon rupture, age ≥40 years, body mass index > 30 kg/m2, oestrogen-containing hormonal replacement therapy or oral contraception, active cancer (treatment ongoing or stopped for less than one year), history of VTE, congenital or acquired hypercoagulable state,
• Requiring thromboprophylaxis according to the Investigator's judgement up to complete mobilization (corresponding to cast or brace removal)
• Able and willing to provide written informed consent

Exclusion Criteria:

• Delay between injury and randomization greater than two days,
• Treatment with antithrombotic or anticoagulant therapy, including low-dose anticoagulation, for more than 2 days prior to randomization,
• Anticoagulant therapy required or likely to be required during the study period for another reason (e.g. planned surgery justifying pharmacological thromboprophylaxis, curative dose for treatment of VTE, etc.)
• Known hypersensitivity to fondaparinux or nadroparin or their excipient,
• Known history of heparin-induced thrombocytopenia,
• Women of childbearing potential not using a reliable contraceptive method throughout the study period,
• Women pregnant or breast-feeding during the study period.
• Active, clinically significant bleeding,
• Clinically significant bleeding within the past six months,
• Major surgery within the previous three months,
• Intraocular (other than cataract), spinal, and/or brain surgery within the previous twelve months,
• Haemorrhagic stroke within the previous twelve months,
• Severe head injury within the previous three months,
• Documented congenital or acquired bleeding tendency/disorder(s),
• Previous (within 12 months) or active or currently treated peptic ulcer disease,
• Uncontrolled arterial hypertension (systolic blood pressure over 180 mm Hg or diastolic blood pressure over 110 mm Hg),
• Treatment with more than one antiplatelet agents (e.g. clopidogrel and aspirin) at any dose,
• Need for chronic aspirin at doses≥ 325 mg or chronic NSAIDs,
• Bacterial endocarditis,
• Severe hepatic impairment,
• Calculated creatinine clearance < 30 mL/min,
• Thrombocytopenia ( <100x10_9/L)
• Body weight < 50 kg.
• Any condition that could prevent the patient from providing written informed consent or from adhering to study treatment,
• Life expectancy under six months,
• Participation in any study using an investigational drug during the previous three months,
• Patient in whom V3 is unlikely to be feasible (e.g. patient moving house),
• In France, a subject will not be eligible for inclusion in this study if not either affiliated to or a beneficiary of a social security system. This is an additional exclusion criterion only applying to subjects enrolled in France.