Eligibility Criteria:
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.