Ottawa Knee Rule Calculator - Knee X-Ray Decision Tool
Free Ottawa Knee Rule calculator with all 5 criteria plus Pittsburgh Knee Rules comparison. Instant X-ray recommendation for acute knee injuries. Sensitivity 98–100%, validated in 7,000+ patients.
Ottawa Knee Rule Assessment
5 bedside criteria for acute knee injury — ANY positive criterion indicates X-ray.
Tenderness on palpation of the patella with NO other bony tenderness of the knee.
Point tenderness on palpation of the fibular head (lateral proximal fibula, NOT the joint line).
Active flexion assessed during examination. Can the patient actively bend the knee to at least 90 degrees?
Unable to take 4 steps (2 per foot) BOTH immediately after injury AND in the ED. Limping IS acceptable — patient does NOT need to walk normally.
Key Definitions
Do NOT Apply If:
Ottawa Knee Rule
Complete all 5 criteria to see result
About This Calculator
What is the Ottawa Knee Rule?
The Ottawa Knee Rule is a clinical decision rule developed by Dr. Ian Stiell in 1995 to determine whether X-ray imaging is needed after an acute knee injury. It uses 5 simple bedside criteria to safely rule out clinically significant knee fractures. With pooled sensitivity of 98–100% and negative predictive value approaching 100%, a negative result reliably excludes fracture without imaging.
The 5 Criteria
A knee X-ray is indicated ONLY if the patient has ANY ONE of: (1) age ≥55, (2) isolated patellar tenderness, (3) fibular head tenderness, (4) inability to flex to 90°, or (5) inability to weight-bear for 4 steps. If ALL five are negative, X-ray is not needed.
Why does it matter?
- 92% of knee X-rays ordered after trauma do NOT show fractures — only ~6% of acute knee trauma patients actually have one
- The Ottawa Knee Rule reduces unnecessary knee X-rays by 20–30%
- This saves ~33 minutes per ED visit and ~$34 per patient
- Proper documentation of OKR findings provides strong medicolegal protection
Clinical Impact
The Ottawa Knee Rule is part of the "Ottawa Rules" family alongside the Ottawa Ankle Rules, all developed by Dr. Stiell. Together they represent the most validated clinical decision rules in emergency medicine, studied in over 20,000 patients across multiple countries.
Formula
Knee X-ray indicated if ANY ONE of: age ≥55, isolated patellar tenderness, fibular head tenderness, inability to flex to 90°, or inability to weight-bear 4 steps.Binary decision rule — not a point score. Any single positive criterion indicates X-ray. All five criteria must be negative to safely exclude fracture. Weight-bearing is assessed at two time points: immediately after injury AND at evaluation.
Clinical Considerations
- •Applies ONLY to acute traumatic knee injuries — NOT for non-traumatic knee pain.
- •Limping IS considered weight-bearing — patient must be completely UNABLE to take 4 steps.
- •Assess weight-bearing at TWO time points: immediately after injury AND at evaluation.
- •"Isolated" patellar tenderness means NO other bony knee tenderness — palpate other structures first.
- •The fibular head is on the lateral proximal leg, NOT at the joint line.
- •Does NOT assess ligamentous injuries — ACL, MCL, meniscus tears require separate evaluation.
Limitations
- •Moderate specificity (~49%) — roughly half of patients meeting criteria will have negative X-rays.
- •Does not detect ligament, meniscal, or other soft tissue injuries.
- •Clinician compliance is only ~63% in academic ED settings.
- •Rare false negatives possible — pooled sensitivity 98–100%, not absolute 100%.
- •Not validated for non-traumatic knee pain or injuries >7 days old.
- •Weight-bearing can be difficult to assess in very anxious or uncooperative patients.
- •Pittsburgh Knee Rules may have higher specificity (~60%) for blunt trauma/fall mechanisms.
Interpretation Guide
| Range | Classification | Recommendation |
|---|---|---|
| <-0 | X-Ray NOT Indicated | Knee fracture can be safely excluded per Ottawa Knee Rule (sensitivity 98–100%). Treat conservatively with RICE protocol. Follow up if not improving in 5–7 days. |
| 1-5 | X-Ray Indicated | Obtain knee radiograph series (AP, lateral views; consider sunrise/patellar view). Ottawa Knee Rule criteria met. |
Frequently Asked Questions
What are the Ottawa Knee Rules?
The Ottawa Knee Rules are a clinical decision rule developed by Dr. Ian Stiell in 1995 to determine whether X-ray imaging is needed after an acute knee injury. The rule uses 5 bedside criteria: age ≥55, isolated patellar tenderness, fibular head tenderness, inability to flex to 90°, and inability to weight-bear for 4 steps. If ANY one criterion is positive, knee X-ray is indicated. If ALL are negative, fracture can be safely excluded.
What are the 5 criteria of the Ottawa Knee Rules?
The 5 criteria are: (1) Age ≥55 years, (2) Isolated tenderness of the patella — patellar tenderness with no other bony knee tenderness, (3) Tenderness at the head of the fibula — the bony prominence on the lateral proximal leg, (4) Inability to flex the knee to 90 degrees — active flexion, and (5) Inability to weight-bear for 4 steps — both immediately after injury and at evaluation. Limping is acceptable for weight-bearing.
How accurate are the Ottawa Knee Rules?
The Ottawa Knee Rules have pooled sensitivity of 98–100% and negative predictive value approaching 100% for clinically significant knee fractures. A 2020 meta-analysis of 7,385 patients found sensitivity of 99% (95% CI: 97–100%). Specificity is moderate at ~49%, meaning roughly half of patients who meet criteria will not have a fracture. The negative likelihood ratio is 0.07.
Do I need a knee X-ray after a fall?
According to the Ottawa Knee Rules, you need a knee X-ray after a fall only if any of these apply: you are 55 or older, you have tenderness when pressing on the kneecap (with no other knee bone tenderness), you have tenderness at the fibular head (outer upper shin bone), you cannot bend your knee to 90 degrees, or you could not take 4 steps both right after the fall and now. If none apply, X-ray is not needed.
Can the Ottawa Knee Rules be used in children?
Yes. The Ottawa Knee Rules were validated in children ages 2–16 by Bulloch et al. (2003) in a multicenter study of 750 children. Sensitivity was 100% (95% CI: 94.9–100%) and specificity was 42.8%. The rules could have avoided 31.2% of radiographs in children. The original adult studies excluded patients under 18, but the pediatric validation confirmed applicability from age 2.
What is the difference between Ottawa Knee Rules and Pittsburgh Knee Rules?
The Ottawa Knee Rules use 5 criteria (age ≥55, patellar tenderness, fibular head tenderness, flexion, weight-bearing) and apply to all acute knee trauma. The Pittsburgh Knee Rules first require a blunt trauma or fall mechanism, then check age (<12 or >50) and weight-bearing. Pittsburgh has higher specificity (~60% vs ~49%) and better inter-rater reliability, while Ottawa is more extensively validated. Both have similar sensitivity (~98–99%).
What does isolated patellar tenderness mean?
Isolated patellar tenderness means the patient has pain when the patella (kneecap) is palpated, but has NO tenderness when any other bony structure of the knee is palpated. If the patient has patellar tenderness AND tenderness elsewhere on the knee bones, the patellar tenderness is not "isolated" and this specific criterion is negative. The examiner must palpate other knee structures to determine isolation.
How do you test weight bearing for Ottawa Knee Rules?
Weight-bearing is tested by asking the patient to take 4 steps (transferring weight twice to each foot). This must be assessed at TWO time points: immediately after injury and at the time of evaluation. Limping IS considered weight-bearing — the patient does NOT need to walk normally. The criterion is positive only if the patient is completely unable to take 4 weight-bearing steps.
Where is the fibular head located for the Ottawa Knee Rules?
The fibular head is the bony prominence on the lateral (outer) side of the proximal leg, just below the knee joint. It is located approximately 1–2 cm below the joint line on the outer side. It is NOT at the knee joint line itself. The fibular head can be identified by following the lateral knee down to the first prominent bony bump. Point tenderness here satisfies criterion #3.
Can the Ottawa Knee Rules miss a fracture?
Rarely. The pooled sensitivity is 98–100%, so 0–2% of fractures may be missed. Most missed fractures are clinically insignificant (small avulsion fragments that do not change management). Tibial plateau fractures may occasionally be missed. The rule is designed to detect clinically significant fractures requiring intervention. It does NOT assess ligamentous injuries (ACL, MCL, meniscus).
How much do the Ottawa Knee Rules reduce X-ray ordering?
The Ottawa Knee Rules reduce knee X-ray orders by 20–30% when properly applied. This saves approximately 33 minutes per ED visit and ~$34 per patient. Given that only ~6% of acute knee trauma patients have fractures, the vast majority of X-rays ordered without the rule are negative. National implementation could save an estimated $250 million annually in the US.
Do the Ottawa Knee Rules apply to non-traumatic knee pain?
No. The Ottawa Knee Rules apply ONLY to acute traumatic knee injuries (falls, blunt trauma, twisting mechanisms). They are NOT validated for non-traumatic knee pain such as gout, septic arthritis, overuse injuries, osteoarthritis flares, or atraumatic effusions. Imaging decisions for non-traumatic knee pain require different clinical reasoning.
References
1. Stiell IG, Greenberg GH, Wells GA, et al.. Derivation of a decision rule for the use of radiography in acute knee injuries. Annals of Emergency Medicine. 1995
2. 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
3. Stiell IG, Wells GA, Hoag RH, et al.. Implementation of the Ottawa Knee Rule for the use of radiography in acute knee injuries. JAMA. 1997
4. Bulloch B, Neto G, Plint A, et al.. Validation of the Ottawa Knee Rule in children: a multicenter study. Annals of Emergency Medicine. 2003
5. Bachmann LM, Haberzeth S, Steurer J, ter Riet G. The accuracy of the Ottawa knee rule to rule out knee fractures: a systematic review. Annals of Internal Medicine. 2004
View Source →Last updated: 2026-02-25
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