Acute ankle sprain is the most frequent orthopaedic injury. Up to 4% of patients admitted to the emergency room will present with an acute ankle sprain. The lateral ligaments are involved at various degrees (anterior talo-fibular and calcaneo-fibular ligaments). Grade I acute ankle sprains are well treated with a compression bandage or an Aircast brace. Grade II and III acute ankle sprains are best treated with a below-knee cast. The Aircast brace is the next best treatment for such lesions (grades II and III) and will provide satisfactory support, but has been shown to be not as effective as the below-knee cast in terms of short to mid-term patients outcome (pain and function).