If any of the five criteria are positive — age 55 or older, isolated patellar tenderness, fibular head tenderness, inability to flex the knee to 90 degrees, or inability to bear weight for four steps — then knee radiography is recommended to evaluate for fracture. The specific combination of positive criteria can help guide clinical suspicion: fibular head tenderness may suggest a proximal fibula fracture or tibial plateau fracture, while isolated patellar tenderness raises concern for a patellar fracture.

If none of the five criteria are present, the probability of a clinically significant knee fracture is extremely low. The Ottawa Knee Rules have approximately 99% sensitivity for significant fractures, meaning a negative result provides strong reassurance. The patient most likely has a soft tissue injury such as a ligamentous sprain, meniscal injury, or contusion, which can be initially managed conservatively. However, a negative result on these rules does not exclude soft tissue injuries that may require further workup (e.g., MRI for suspected ACL or meniscal tears).

As with all clinical decision rules, the result should be interpreted within the full clinical context. If the clinician has strong clinical suspicion despite a negative screen, imaging should still be obtained.

Apply the Ottawa Knee Rules to any adult patient presenting to an emergency department or urgent care setting with acute knee pain following a traumatic mechanism. The rules are intended for initial evaluation of knee injuries to determine the need for plain radiographs. They are most useful within the first 7 days of injury, before swelling and tenderness patterns evolve.

The rules are particularly valuable in high-volume emergency departments where knee injuries are common but only about 6% of those injuries involve fractures. By identifying patients who do not need X-rays, the rules reduce unnecessary imaging by over 25%, saving time, cost, and radiation exposure. They have been validated in multiple large prospective studies across different countries and healthcare settings.

The Ottawa Knee Rules were developed and validated in adults 18 years and older. They should not be applied to pediatric patients, though studies suggest reasonable performance in children over age 5 with appropriate modifications. The rules are not valid for patients with altered consciousness, intoxication, paraplegia, multiple trauma, or decreased leg sensation, as these conditions compromise the reliability of the physical examination.

The rules are designed to detect bony fractures only. They do not evaluate for ligamentous injuries (ACL, MCL, PCL, LCL), meniscal tears, or tendon injuries. A patient who passes the Ottawa Knee Rules may still have a significant soft tissue injury requiring further evaluation with MRI or orthopedic referral. If the patient has mechanical symptoms such as locking, catching, or giving way, further workup is warranted regardless of the Ottawa Knee Rules result.

The specificity is approximately 49%, meaning that more than half of patients who meet criteria for X-ray will not have a fracture. This is an acceptable trade-off for a screening tool designed to prioritize sensitivity. The rules also do not account for stress fractures, which may not be visible on initial plain radiographs.