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Objectives: To understand the difficulties studing risk and protective factors (here pre-filler injection ultrasonography) in rare diseases as vascular occlusion after filler injections Introduction: Vascular occlusions after filler injections are rare events. The incidene is estimated to range between 1:5000 to 1:6000. Pre-filler injection ultrasonography (US) is advocated to reduce this risk. So far there no good clinical data to support pre-filler injection ultrasonography to reduce adverse events as vascular occlusion. And due to the rarity of these reactions it is really difficult to obtain good evidence. Materials / method: Rough literature search Results: The best study design would be a registry of filler occlusions that allows a nested case-control study. This would require huge efforts. Furthermore - say when pre-filler injection US would be done on a regular base - how much risk could be reduced? Assuming US would reduce the number of vascular events from 1:5000 to 1:10000, so the absolute risk reduction would be 0.01%. The relative reduction For the present data, the relative risk reduction is 0.01÷ = 0.01, which is 1%. Thus, US would reduce the risk of a vascular event by 1%. Conclusion: The discussion on the merit of US is comparable to the discussion on aspiration. Therefore, there is no harm in doing an US before filler injection but it is not clear if this really will reduce the risk of vascular events.