The collateral ligaments are the 'guy ropes' on the sides of the knee.
What do the collateral ligaments do?
The collateral ligaments restrain the joint from forces that would open it up on the side, ie varus or valgus forces.
Medial collateral ligament injuries of the knee: current treatment concepts. Chen L, Kim PD, Ahmad CS and N. Levine WN. Curr Rev Musculoskelet Med. 2008 Jun; 1(2): 108–113.