Two decision rules that we doctors use to help decide if a knee x-ray is needed after a knee injury are the Ottawa Knee Rules and the Pittsburgh Decision Rules*.
The Ottawa Knee Rules suggests knee x-rays if any of the following are present:
–Age 55 years or older
–Tenderness at the head of the fibula
–Isolated tenderness of the patella
–Inability to flex the knees to 90 degrees
–Inability to walk four weight bearing steps immediately after the injury and in the emergency dept.
The Pittsburgh Decision Rule suggests knee x-rays if:
–There is blunt trauma or a fall as the mechanism of injury plus either of the following:
a. Age younger than 12 years or older than 50 years
b. Inability to walk four weight bearing steps in the emergency department.
Even if no x-ray is suggested by the decision rule the physician using, his or her clinical judgment may still lead the ordering of knee x-rays. Both of the decision rules are very sensitive for detecting knee injuries in which a knee fracture is possible. But neither are 100% sensitive.
*Acute Knee Injuries: Use of Decision Rules for Selective Radiograph Ordering, Am Fam Physician. 1999 Dec 1;60(9):2599-2608. Available at http://www.aafp.org/afp/1999/1201/p2599.html.