There are four ligaments in the knee: the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). The ACL and PCL stabilize front-to-back knee movements, while the MCL and LCL stabilize side-to-side movements.
The ACL can be sprained or torn if the knee is straightened beyond its normal limits (hyperextended), twisted, or bent side-to-side. A sprained or torn ACL is common in sports and usually results from a hard stop or aggressive twisting of the knee.
The PCL is the least common ligament to be injured.