Ottawa knee rules

The Ottawa knee rules are a set of rules used to help physicians determine whether an x-ray of the knee is needed.

They state that an x-ray is required only in patients who have an acute knee injury with one or more of the following:
 * Age 55 years or older
 * Tenderness at head of fibula
 * Isolated tenderness of patella
 * Inability to flex to 90°
 * Inability to bear weight both immediately and in the emergency department (4 steps)