Ottawa Knee Rules

Evaluates whether knee x-ray imaging is required based on pain and lack of mobility in the limb.

You can read more about the criteria used and about the original study in the text below the calculator.

The Ottawa knee rules calculator determines whether the patient requires knee x-ray for knee injury. It consists of 5 criteria highly correlated with knee fracture status.

The rule has been validated in numerous studies both in children (although not recommended as sole decision rule) and adults.

When at least one criteria are met, the Ottawa knee rule states that knee x-ray is necessary.

When neither of the criteria is met, the rule is specific enough so that the patient can avoid imagistic as long as it continues to be monitored for signs of fracture.

Because this knee rule takes in consideration the main factors consistent with serious knee injury, it is proven accurate in around 98% of cases of knee fractures, with satisfactory interobserver agreement.


Age above 55 years


Unable to bear weight both immediately and in ED


Unable to flex knee to 90 degrees


Isolated tenderness of the patella


Tenderness at the fibular head

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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


The method explained

This is a clinical decision making tool that checks whether the patient with trauma to the knee requires knee x-ray or not.

It helps reduce unnecessary radiation where knee trauma is not consistent with fracture signs.

The five criteria in the rule are:

1. Age above 55 – because age increases likelihood of fracture.

2. Isolated tenderness of the patella (significant when no other bone tenderness).

3. Tenderness at the fibular head.

4. Unable to flex knee to 90 degrees.

5. Unable to bear weight both immediately and in ED (inability to take four steps, for example two steps on each leg, regardless of limping).

There is also an equivalent rule for trauma of the ankle: the Ottawa ankle rule. Both rules have been validated in subsequent studies, however, clinical judgment should prevail.

The rule can be applied to children, however, there are some contraindications in injuries that are older than 7 days, in patients with altered levels of consciousness or who simply don’t collaborate or in case of several other fractures or injuries of both the articulation and the soft tissues.

The main limitation of the model is its low specificity and the possibility of false positives proved in several occasions.

Knee anatomy

Picture source: Ottawa Hospital Research Institute.


Rule interpretation

The Ottawa knee rule provides information as to whether the patient requires knee imagistic or can be monitored without that.

When one of the five criteria are met, the model recommends that radiography might prove necessary. As the factors taken in consideration are consistent with serious injuries, the score is accurate in around 98% of cases with relevant knee fractures.

If neither of the criteria is being met, the possibility that the patient suffers from a significant fracture is highly unlikely.

Acute knee injuries are a common presentation in emergency rooms and thus the application of this knee rule can significantly reduce unnecessary radiology (both costly and with patient effects) and contribute to an efficient streamline of patients.

The Ottawa knee rule has now become a routine procedure in the management of knee injuries.


About the study

The Ottawa knee rule was constructed following a study on 127 patients, supervised by Stiell et al. in 1995.

The study aimed to derive a highly sensitive decision rule for the selective use of radiography in acute knee injuries.

The patients were examined independently by two physicians to determine interobserver agreement.

It was found that the application of the rule would have led to a 28.0% relative reduction in the use of radiography in the study population.

The Ottawa knee rule is a reliable clinical decision making tool for establishing need for radiography in acute knee injuries.


Original source

Stiell IG, Greenberg GH, Wells GA, McKnight RD, Cwinn AA, Cacciotti T, McDowell I, Smith NA. Derivation of a decision rule for the use of radiography in acute knee injuries. Ann Emerg Med. 1995; 26(4):405-13.


Stiell IG, Greenberg GH, Wells GA, McDowell I, Cwinn AA, Smith NA, Cacciotti TF, Sivilotti ML. Prospective validation of a decision rule for the use of radiography in acute knee injuries. JAMA. 1996; 275(8):611-5.

Other references

1. Stiell IG, Wells GA, Hoag RH, Sivilotti ML, Cacciotti TF, Verbeek PR, Greenway KT, McDowell I, Cwinn AA, Greenberg GH, Nichol G, Michael JA. Implementation of the Ottawa Knee Rule for the use of radiography in acute knee injuries. JAMA. 1997; 278(23):2075-9.

2. 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):121-4.

Specialty: Radiology

System: Musculoskeletal

Objective: Evaluation

Type: Calculator

No. Of Criteria: 5

Year Of Study: 1995

Article By: Denise Nedea

Published On: May 19, 2017

Last Checked: May 19, 2017

Next Review: May 19, 2023