Ligaments are tough, ropelike fibers that connect one bone to another. The four ligaments of the knee connect the upper leg bone (femur) to the lower leg bone (tibia) and stabilize knee movement.

• The anterior cruciate ligament (ACL) and the posterior cruciate ligament cross inside the center of the knee joint and stabilize front-to-back knee movements.
• The medial collateral ligament and the lateral collateral ligament connect the leg bones at the sides of the knee and stabilize side-to-side movements.