Osteonecrosis (ON) of the knee is second to the hip as the most common location [ 1 ] and presents in two major forms: spontaneous and secondary ON. Spontaneous ON of the knee (SPONK) was fi rst identifi ed as a separate entity by Ahlback et al. in 1968 [ 2 ] with no identifi able cause. Secondary ON is due to identifi able causes most commonly steroids and alcohol. Some authors include arthroscopicassociated ON with secondary causes, while others identify it as a separate disease entity. Despite the different clinical presentations of each of these forms, disease severity is classifi ed using the same systems and treatment algorithms composed of the same approaches.
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