How do ligaments help the knees move safely?

Ligaments are tough, fibrous tissues that connect bones or cartilage at a joint, allowing movement within a safe range. On the inner side of the knee, a large ligament called the medial collateral ligament, or MCL, connects the femur to the tibia on the inside (big-toe side) of the knee joint, limiting sideways motion. The lateral collateral ligament does the same on the outside (little-toe side), connecting the femur to the small bone of the calf, or fibula. Deep within the joint, the anterior cruciate ligament, or ACL, connects the femur to the tibia in the center of the knee; it keeps the joint from rotating too far or letting your shin get out in front of your thighbone. Crossing behind that ligament is the posterior cruciate ligament, which keeps the shin bone from falling out of place behind the knee.