There are two cruciate ligaments in the knee.  The word cruciate means “cross” and that is what the two cruciate ligaments do.   They form a cross or “X” as they pass through the middle of the knee.  They go from front to back (or back to front).  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).