What Happens When the ACL Tears? It’s one of the most dreaded injuries in sports—the ACL tear. But what is actually happening inside the joint when that "pop" occurs? 🦵💥 The Anterior Cruciate Ligament (ACL) is the primary stabilizer of your knee, preventing the tibia (shin bone) from sliding out in front of the femur (thigh bone). When it tears—usually during a sudden pivot, jump, or hard stop—the knee loses its "anchor." 1 In this illustration, you can see: The Instability: Without the ACL, t