Any trauma to the acl can make the acl to be considered "torn". The acl is made of 2 bundles which are comprised of fibers. When trauma occurs fibers are tearing hence "torn acl". It's just the degree which determines surgical route or no surgery at all. The fibers of the acl typical avulse from the attachment on the femur which may be the separation you are thinking of. Also the fibers pull away from each other when trauma occurs as the ligament pulls away from the femur or "ruptures".