Emergency Medicine
Ottawa Knee Rule
Whether a knee X-ray is needed after injury.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The Ottawa Knee Rule is a validated decision tool that identifies which patients with an acute knee injury need an X-ray. A radiograph is indicated if any one of five criteria is present: age 55 or over, isolated tenderness of the patella, tenderness at the head of the fibula, inability to flex the knee to 90 degrees, or inability to bear weight for four steps both immediately after the injury and during assessment. The rule is designed for very high sensitivity, accepting some unnecessary imaging to avoid missing fractures. It applies to acute, isolated knee trauma.
Interpreting the result
If none of the five criteria are present, a clinically significant fracture is very unlikely and an X-ray can usually be safely avoided. If one or more criteria are present, a knee X-ray is indicated. The rule is a tool to rule out fracture rather than a guarantee, so persistent clinical concern can still justify imaging despite a negative result.
Worked example
A 60-year-old who fell awkwardly cannot bear weight for four steps and cannot flex the knee to 90 degrees. With age 55 or over and two other criteria present, a knee X-ray is indicated. A younger patient with full flexion, no bony tenderness and the ability to weight-bear could avoid imaging.
Pearls / pitfalls
- It applies to acute, isolated knee injury; reliability falls with delayed presentation, intoxication, distracting injuries or impaired sensation.
- It is validated in adults; apply local guidance in children, in whom evidence is more limited.
- 'Unable to bear weight' means cannot take four steps regardless of limping, both at the time and during assessment.
- A negative rule excludes most fractures but not all ligamentous or meniscal injury, so safety-net advice and review for persistent symptoms still apply.
Evidence & validation
Derived and prospectively validated by Stiell and colleagues in the 1990s, the rule has near-complete sensitivity for clinically significant knee fractures across multiple studies and meta-analyses, reducing knee radiography substantially. It is endorsed in emergency-medicine practice as a safe means of limiting unnecessary imaging.
Frequently asked questions
When does the Ottawa Knee Rule say an X-ray is not needed?
When none of the five criteria are present: under 55, no isolated patellar or fibular-head tenderness, able to flex to 90 degrees and able to bear weight for four steps. A significant fracture is then very unlikely.
What are the five criteria?
Age 55 or over, isolated patellar tenderness, tenderness at the fibular head, inability to flex the knee to 90 degrees, and inability to bear weight for four steps both immediately and during assessment. Any one indicates an X-ray.
Can the rule be used in children?
It was validated in adults, and although some studies support its use in older children, evidence is more limited. Apply local paediatric guidance, especially in younger children or where growth-plate injury is suspected.
Does a negative result exclude all knee injury?
No. It reliably excludes most significant fractures but not ligament or meniscal injuries. Persistent pain or clinical concern should prompt review or further assessment.
How is the Ottawa Knee Rule different from the ankle rules?
They share the same high-sensitivity, weight-bearing approach but use different anatomical criteria specific to the knee, including the age threshold of 55 and inability to flex to 90 degrees.
References
- Stiell IG, Greenberg GH, Wells GA, et al. Prospective validation of a decision rule for the use of radiography in acute knee injuries. JAMA. 1996;275(8):611–615.
- Royal College of Emergency Medicine. Knee injury guidance / Ottawa Knee Rule.
About the creator
Stiell IG et al.
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