The anterior cruciate ligament (ACL) starts in the front of the lower leg bone (tibia) and attaches to back of the thighbone (femur).  The ACL resists forward slippage of the lower leg bone under the thighbone.  The posterior cruciate ligament (PCL) starts in the back of the lower leg bone (tibia) and attaches to the front portion of the thighbone (femur).  The PCL resists backward slippage of the lower leg bone (tibia) under the thighbone (femur).  The ACL is the most commonly injured ligament of the knee.  A strain or partial tear might heal and only require physical therapy.  Complete tears often require surgical repair, which is commonly done arthroscopically.