The collateral ligaments act like guy ropes at the sides of the knee, protecting the knee against forces which would tend to prise it open sideways.
The collateral ligaments on either side of the knee differ in their anatomy. The lateral collateral ligament is a cord-like structure. The medial collateral ligament (MCL) is more of a band, and has both a deep and a superficial component. The MCL is the more commonly injured, although the injuy is more often a sprain than a complete rupture.