Cruciate ligaments are named after the Latin word for 'cross' because they intersect inside the knee to form an 'X'.

The term 'cruciate' is derived from the Latin word 'crux', meaning cross. This anatomical naming convention dates back to early medical descriptions where physicians noted the distinct intersection of the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). These two ligaments are located within the intracapsular space of the knee joint but remain extrasynovial, meaning they sit outside the fluid-filled joint sac.The ACL and PCL function as a four-bar linkage system, a mechanical concept studied extensively by researchers like J.J. O'Connor at Oxford University in the 1980s. This geometric arrangement allows the knee to maintain a moving center of rotation rather than a fixed one. By crossing, the ligaments provide stability in multiple planes, specifically controlling anterior and posterior translation of the tibia relative to the femur.The ACL typically measures about 33 millimeters in length and is responsible for resisting 85% of anterior tibial displacement. In contrast, the PCL is slightly thicker and stronger, serving as the primary restraint against posterior displacement. This 'X' configuration is essential for bipedal locomotion, as it allows humans to decelerate and change direction rapidly without the knee joint buckling under the force of body weight.