Ottawa knee rules: the five findings and how they combine

The Ottawa knee rule combines age and four examination findings after an acute knee injury: age 55 or older, tenderness at the head of the fibula, isolated tenderness of the kneecap (patella), inability to bend the knee to 90 degrees, and inability to bear weight both right after the injury and at the examination. The rule is met when one or more of the five is present. A 40-year-old with none of them doesn't meet it; a 55-year-old with nothing else does.

Not a medical device. Not a diagnosis or dosing recommendation.

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What are the Ottawa knee rules?

The Ottawa knee rule is a clinical decision rule from Ian G. Stiell and colleagues, published in Annals of Emergency Medicine in 1995. It's also searched as the Ottawa knee rules (plural), the Stiell knee rule or OKR.

The 1995 abstract states its aim as "To derive a highly sensitive decision rule for the selective use of radiography in acute knee injuries." It lists the five findings that make up the rule.

This page explains the findings and how they combine. It doesn't say what to do when the rule is met, and it doesn't diagnose a fracture.

The Ottawa knee rule logic

rule met = age >= 55
        OR fibular_head_tenderness
        OR isolated_patellar_tenderness
        OR flexion_less_than_90
        OR (unable_to_bear_weight_immediately AND unable_to_bear_weight_in_ed)

Exact expression the calculator uses:

Knee x-ray indicated iff age >= 55 or fibular_head_tenderness or isolated_patellar_tenderness or flexion_less_than_90 or (unable_to_bear_weight_immediately and unable_to_bear_weight_in_ed)

In words: any one of the five findings is enough. The weight-bearing finding is the one that has two parts, and both parts have to be true for it to count.

What each input means

How the logic works

1. Age. Age 55 or older meets the rule on its own.
2. Fibular head. Bone tenderness at the head of the fibula meets the rule.
3. Patella. Isolated tenderness of the patella meets the rule. "Isolated" means the kneecap is the one tender bone. The person using the calculator decides this; Magent doesn't work it out from the other answers.
4. Flexion. Being unable to bend the knee to 90 degrees meets the rule.
5. Weight bearing. This counts when the person couldn't take four steps both right after the injury and in the emergency department. One without the other doesn't count.
6. If any of steps 1–5 is yes, the rule is met. If none is, it isn't.

What the calculator returns

Magent's calculator returns true or false for the rule, the age it used, and a line for each finding showing whether it was met. The two weight-bearing answers appear as a single combined line.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: age 40, no findings

1. Age 40 is under 55.
2. No fibular head or patellar tenderness, can flex to 90 degrees, can bear weight.
3. Rule: not met.

Example 2: age 55, nothing else

1. Age 55 meets the age finding.
2. Rule: met.

Example 3: age 54, nothing else

1. Age 54 doesn't meet the age finding.
2. Rule: not met.

Example 4: fibular head tenderness

1. Age 40; fibular head tender: yes.
2. Rule: met.

Example 5: unable to bear weight right after the injury, able in the emergency department

1. Age 40; couldn't take four steps immediately, but could in the emergency department.
2. The weight-bearing finding needs both, so it isn't met.
3. Rule: not met.

Example 6: unable to bear weight both times

1. Age 40; couldn't take four steps immediately or in the emergency department.
2. Rule: met.

Ottawa knee rule combination table

Each row is one of Magent's expected values for the calculator. Unlisted findings are "no".

History of the Ottawa knee rules

The rule comes from Ian G. Stiell, Gary H. Greenberg, George A. Wells and colleagues, Derivation of a decision rule for the use of radiography in acute knee injuries, Annals of Emergency Medicine, 1995 (volume 26, issue 4, pages 405–413).

The abstract describes a prospectively administered survey in the "Emergency departments of two university hospitals," with a "Convenience sample of 1,047 adults with acute knee injuries." Attending emergency physicians assessed each patient for 23 standardized clinical findings. The authors kept the findings that were both reliable between examiners and strongly associated with fracture, then used a recursive-partitioning method to build the rule.

The abstract lists the resulting variables: "(1) age 55 years or older, (2) tenderness at the head of the fibula, (3) isolated tenderness of the patella, (4) inability to flex to 90 degrees, and (5) inability to bear weight both immediately and in the ED (four steps)." Those are the same five findings Magent's calculator uses, including age, unlike the Ottawa ankle calculator, which has no age input.

When it's used

The 1995 study covered adults with acute knee injuries. Magent's record describes the calculator as the adult rule as published, for blunt knee injury, with an age input from 18 to 120.

Where it's used

The original study took place in the emergency departments of two university hospitals.

Who uses it

In the 1995 study, attending emergency physicians assessed the findings. The calculator itself takes answers from whoever examined the knee.

Limits of the calculation

• It combines age and yes/no findings. It doesn't examine the knee and doesn't diagnose a fracture.
• It's the Ottawa knee rule. It isn't the Ottawa ankle rule, the Ottawa foot rule or the Pittsburgh knee rule.
• "Isolated" patellar tenderness is the caller's call; Magent doesn't infer it.
• Weight bearing counts when it failed both immediately and in the emergency department, not one or the other.
• Age has to be 18 to 120, matching the adult study.
• This page doesn't say what to do when the rule is met.

Clinical decisions belong to a qualified clinician who knows the patient.

How it differs from related calculators

• Ottawa ankle rule is the same research group's rule for ankle injuries. It uses malleolar-zone pain, malleolus tenderness and the same two-part weight-bearing finding. Magent's version has no age input.

Glossary

• Fibular head: the top end of the fibula, the thinner lower-leg bone, felt just below the outer side of the knee.
• Patella: the kneecap.
• Flexion: bending the knee.
• Bear weight: stand and walk on the injured leg; here, take four steps.
• ED: emergency department.

Common mistakes

• Counting one weight-bearing failure. It has to fail both immediately and in the emergency department.
• Treating any kneecap tenderness as "isolated." Isolated means no other bone tenderness of the knee.
• Leaving out age. Unlike Magent's Ottawa ankle calculator, the knee rule includes age.

Source

Stiell IG, Greenberg GH, Wells GA, et al. Derivation of a decision rule for the use of radiography in acute knee injuries70106-0). Ann Emerg Med. 1995;26(4):405-413. DOI: 10.1016/S0196-0644(95)70106-070106-0)

Frequently asked questions

What are the Ottawa knee rules?

A 1995 decision rule with five findings: age 55 or older, fibular head tenderness, isolated patellar tenderness, inability to flex to 90 degrees, and inability to take four steps both right after the injury and in the emergency department.

When is a knee x-ray needed under the Ottawa knee rules?

This page doesn't give disposition advice. It shows how the findings combine; what to do next depends on the clinical context.

How many findings have to be present?

One. Any single finding meets the rule.

Does age 54 count?

No. The age finding is 55 or older.

What does "isolated patellar tenderness" mean?

Tenderness of the kneecap with no other bone tenderness of the knee.

Is the Ottawa knee rule the same as the Pittsburgh knee rule?

No. They're separate rules with different findings and different age lines.

Who developed the Ottawa knee rules?

Ian Stiell and colleagues, in a 1995 paper in Annals of Emergency Medicine.

Related

• Emergency calculators
• Ottawa ankle rule
• Pittsburgh knee rule
• All calculator explainers