This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Fewer unnecessary X-rays
Knee injuries from twists, falls and sports are among the most common reasons for an emergency visit, yet fewer than 1 in 10 involve a fracture. In the early 1990s, Ian Stiell and colleagues in Ottawa, already known for the Ottawa ankle rules, developed a rule to identify which adults with an acute knee injury need an X-ray. Published in 1995 and validated in 1996, the Ottawa knee rule safely reduces the number of knee X-rays.
The five criteria
A knee X-ray is indicated if any of these is present:
- Age 55 or older.
- Tenderness at the head of the fibula, the bony bump on the outside of the knee just below the joint.
- Isolated tenderness of the kneecap, with no other bone tenderness of the knee.
- Inability to bend the knee to 90 degrees.
- Inability to bear weight for four steps, both immediately after the injury and in the emergency room. Limping counts as bearing weight.
If none is present, a fracture is very unlikely and an X-ray is usually not needed.
Example
A 34-year-old twists her knee playing soccer. She can walk with a limp, bends her knee past 90 degrees, and there is no tenderness over the kneecap or the fibular head. None of the criteria applies: the rule says no X-ray is needed. The doctor will still examine the ligaments and menisci, which an X-ray does not show.
How reliable is it?
Studies, including a meta-analysis of more than 4,000 patients, found a sensitivity of about 99% for fractures and a specificity around 49%. Using the rule cut knee X-rays by about a quarter without missing fractures, saving time, radiation and cost. Studies in children aged about 5 and older suggest it also works in that group.
When the rule does not apply
The rule was designed for injuries less than 7 days old. It does not apply to:
- Isolated skin injuries, such as cuts.
- Patients referred with X-rays already taken.
- Patients with altered consciousness, intoxication, paraplegia or multiple injuries.
- Patients returning for reassessment of the same injury.
After a negative rule
Most knee injuries without fracture are sprains, ligament tears or meniscus injuries. Rest, ice, compression and elevation help in the first days, with weight-bearing as tolerated. See a doctor again if the knee locks, gives way, swells a lot within hours, or if you still cannot walk normally after a week. An MRI may be needed to assess ligaments or cartilage.