How it's calculated
If any one of five clinical criteria is present, knee X-ray is indicated. If none are present, X-ray can be safely withheld.
Criteria
| Criterion |
|---|
| Age ≥ 55 years |
| Tenderness at the head of the fibula |
| Isolated tenderness of the patella |
| Inability to flex knee to 90° |
| Inability to bear weight 4 steps, immediately and in the ED |
Clinical use
- Applies only to acute knee injuries (generally within 7 days) — not intended for chronic knee pain, follow-up of a previously imaged injury, or isolated soft-tissue complaints without these criteria.
- Highly sensitive for clinically significant fractures — a negative rule (no criteria met) reliably excludes a fracture requiring imaging.
- Pairs well with the Ottawa Ankle Rules, which apply the same clinical decision-rule logic to acute ankle/midfoot injuries.
- Meant to reduce unnecessary radiography, not to override clinical judgement in atypical presentations (e.g. penetrating trauma, suspected dislocation).
Frequently asked questions
What is the Ottawa Knee Rule used for?
It helps decide whether an X-ray is needed after an acute knee injury, reducing unnecessary imaging in low-risk patients.
What happens if none of the criteria are met?
X-ray can generally be safely withheld — the rule has high sensitivity for excluding clinically significant fractures when negative.
Does this rule apply to chronic knee pain?
No — it is validated only for acute knee injuries, generally within 7 days of the injury.
Is this the same as the Ottawa Ankle Rules?
No, but it follows the same type of clinical decision-rule approach — the Ottawa Ankle Rules apply to acute ankle and midfoot injuries specifically.
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.
- Bachmann LM, Haberzeth S, Steurer J, ter Riet G. The accuracy of the Ottawa knee rule to rule out knee fractures: a systematic review. Ann Intern Med. 2004;140(2):121-124.