| Injuries to these ligaments are far less common, but potentially far more serious. The posterior cruciate ligament lies in the center of the knee where it crosses behind its anterior counterpart, the ACL. It shares some functions with the ACL to the extent that it helps provide smooth, controlled motion of the knee. It does so, however, in a manner just opposite the ACL, which is understandable considering that one goes from top right to bottom left, so to speak, and the other goes from top left to bottom right. The PCL prevents the thighbone from sliding forward when one goes down steps, and, as with the ACL, it prevents abnormal twisting.
Isolated PCL Tears
Isolated PCL tears are quite rare and the necessity to address this problem surgically is more controversial. Many patients with an isolated PCL tear do not even realize that they have a torn ligament. An isolated PCL tear can occur when the tibia is forcibly pushed backwards. Landing with your shin on a log, for example, will push the shin bone backwards relative to the knee.
Lateral Collateral Ligament (LCL)
The lateral collateral ligament (LCL), also called fibular collateral ligament, is the equivalent of the MCL: it lies relatively close to the skin along the outer aspect of the knee. In a manner similar to the MCL, it takes its origin on the femur, but instead of inserting on the upper tibia, as does the MCL, it inserts on top of the fibula, the skinny bone on the outer aspect of the leg.
Whereas the MCL is the most commonly injured ligament, the LCL is arguably the least commonly injured ligament. This is fortunate, because when the LCL is torn, the PCL is commonly torn too (and vice-versa), and this combination always mandates surgery: it will not heal on its own, and left untreated it will lead to a very unstable knee.
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