For healthcare professionals

Ottawa Knee Rule

The Ottawa knee rule helps decide which acute knee injuries need an X-ray. It uses five findings: age, two bone tenderness sites, knee flexion and the ability to walk four steps. Stiell and colleagues published it in 1995 and validated it in 1996. This tool is written for clinicians and is not a diagnosis.

Last reviewed: 2026-11-02 By the uncovermyrisk.com editorial team Educational only, not medical advice.

A leg that is cold, pale or numb below the knee, a badly misshapen knee, or bone through the skin: call 911 now.General safety advice (not part of the published rule). The rule is for patients already being assessed; it does not replace urgent care.

Patient findings

Tick each finding that applies. Nothing you enter leaves this page.

Does the rule apply?

Acute knee injury (blunt or twisting)

Findings

Age 55 years or olderIsolated tenderness of the patella (no bone tenderness of the knee other than at the patella)Tenderness of the head of the fibulaUnable to flex the knee to 90 degreesUnable to bear weight (four steps) both immediately after the injury and in the emergency department

Result appears here

What you will see

Whether the rule calls for a knee X-ray, and which findings triggered it.

Educational only, not medical advice. Not a diagnosis. Imaging decisions belong to the treating team.

Source

Stiell IG and colleagues, Annals of Emergency Medicine 1995 and JAMA 1996. The criteria are taken from the open-access (CC BY) introduction of Mohamed and colleagues, Cureus 2020, and match the wording in Sims and colleagues, Journal of Medical Radiation Sciences 2020 (CC BY). The tests check each of the five findings and that the rule only applies after an acute knee injury.

Validated in

Patients with an acute knee injury from blunt trauma or twisting. Sims and colleagues studied adults aged 18 or older and excluded children, multiple trauma and injuries more than 7 days old. Mohamed and colleagues excluded open fractures and penetrating injury.

Limits

The rule is built to rule out fractures, not to find them: a positive result means an X-ray is indicated, not that a fracture is present. Sensitivity has varied between studies: 100% in 110 patients in Ireland (Mohamed 2020) but 71% in an Australian audit of 149 adults where referrers did not always apply the rule (Sims 2020).

The rule

After an acute knee injury, a knee X-ray is indicated if any one of the following is present (Mohamed 2020):

Age 55 years or older
Isolated tenderness of the patella (no bone tenderness of the knee other than at the patella)
Tenderness of the head of the fibula
Unable to flex the knee to 90 degrees
Unable to bear weight (four steps) both immediately after the injury and in the emergency department

The same five criteria are described by Sims and colleagues (Sims 2020). The rule was first derived by Stiell and colleagues (Stiell 1995) and then validated prospectively (Stiell 1996).

Worked example: a 30-year-old who twisted a knee playing football, has no bone tenderness and can bend the knee to 90 degrees, but could not walk four steps after the injury or in the emergency department, meets one finding, so the rule calls for a knee X-ray.

What the studies found

In 110 patients with acute knee injury at a teaching hospital in Ireland, 12 had a fracture. The rule identified all 12 (sensitivity 100%), with a specificity of 39% (Mohamed 2020).

In an Australian audit of 149 adults, sensitivity was 71% and specificity 46%. Only 85 requests were positive by the rule, so following it would have reduced knee X-rays by 43% (Sims 2020).

In a UK emergency department audit, compliance with the rule for knee X-rays rose from 74.6% to 89.9% after teaching and other changes (Dalla Ali 2024).

Related calculators

For a knee rule based on age and walking after a fall, see the Pittsburgh Knee Rule. For ankle and foot injuries, see the Ottawa Ankle Rules. For head injury, see the Canadian CT Head Rule. Other bedside scores are listed under For healthcare professionals.

Sources

Mohamed A, Babikir E, Mustafa MKE. Ottawa Knee Rule: Investigating Use and Application in a Tertiary Teaching Hospital. Cureus. 2020;12(6):e8812. doi:10.7759/cureus.8812. PMID 32724756. Open access (CC BY). Introduction: the five Ottawa knee rule criteria; Results: 110 patients, 12 fractures, sensitivity 100%, specificity 39%. pmc.ncbi.nlm.nih.gov/articles/PMC7381883/
Sims JI, Chau M, Davies J. Validation of the Ottawa knee rule in adults: A single centre study. J Med Radiat Sci. 2020;67(3):193-198. doi:10.1002/jmrs.411. PMID 32567156. Open access (CC BY 4.0). 149 adults aged 18 or older; sensitivity 71%, specificity 46%; 43% fewer radiographs if the rule were followed. pmc.ncbi.nlm.nih.gov/articles/PMC7476189/
Dalla Ali S, Alhiraki OA, Naeem T. Evaluating Compliance With the Ottawa Rules: A Retrospective Clinical Audit at a District General Hospital in the UK. Cureus. 2024;16(7):e65115. doi:10.7759/cureus.65115. PMID 39171035. Open access (CC BY 4.0). Knee X-ray compliance 74.6% to 89.9%. pmc.ncbi.nlm.nih.gov/articles/PMC11338357/
Stiell IG, Greenberg GH, Wells GA, et al. Derivation of a decision rule for the use of radiography in acute knee injuries. Ann Emerg Med. 1995;26(4):405-413. PMID 7574120. Original paper (cited; the criteria used here are taken from Mohamed 2020, CC BY). pubmed.ncbi.nlm.nih.gov/7574120/
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. PMID 8594242. Original paper (cited; the criteria used here are taken from Mohamed 2020, CC BY). pubmed.ncbi.nlm.nih.gov/8594242/

Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.