How do you determine when to replace your running shoes—mileage, months, appearance or technology? Ask 10 different runners that question, and you’ll probably get 10 different answers. Conventional wisdom states that runners need the cushioning and support that shoes provide because it decreases joint stress. That said, running in worn-out shoes puts you at risk for injury to your ankles, knees, legs, hips and back. Some sources recommend you replace shoes based on mileage, time, examining inner cushion (EVA) or outer treads.   And yes, there’s even an app available to help runners decide when to say sayonara to their shoes.