Ottawa Knee Rules

A validated clinical decision rule to determine whether radiography is required for acute knee injuries. Reduces unnecessary X-rays by approximately 25-30% whilst maintaining near-perfect sensitivity for fractures.

Important Clinical Disclaimer: The Ottawa Knee Rules is intended for use by qualified healthcare professionals only. It is for educational and informational purposes and should not replace clinical judgement. Always interpret results alongside the full clinical picture and follow your local guidelines and protocols.
Ottawa Knee Rules Assessment
Indication: Apply to patients with acute knee injury who have pain in the knee and have fallen or had a direct blow to the knee.

A knee X-ray is required if ANY of the following criteria are present:

Patient is 55 years old or older
Bony tenderness at the proximal fibula (lateral)
Bony tenderness at the patella with no other bony tenderness of the knee
Cannot bend knee to 90 degrees
Unable to take 4 steps immediately after injury AND in the emergency department (limping is acceptable)
About the Rules

The Ottawa Knee Rules were developed by Stiell et al. in 1995, following the success of the Ottawa Ankle Rules. They identify patients who require knee radiography after acute injury.

Performance:
  • Sensitivity: 98.5-100% for knee fractures
  • Specificity: ~49%
  • Reduces knee X-rays by 25-30%
  • Validated in multiple international cohorts
Key Points:
  • Any ONE positive criterion means X-ray is indicated
  • Rules have excellent sensitivity - virtually no fractures missed
  • Does NOT assess soft tissue injuries (ligaments, menisci)
Examination Guide
Palpation Sites:
  • Patella: Palpate entire patella - record if tenderness is ISOLATED to patella (no other bony tenderness)
  • Fibula head: Palpate lateral proximal fibula
Flexion Test:

Ask the patient to actively bend the knee. Document if they can achieve 90° flexion.

Weight Bearing:

Patient must be able to take 4 steps (transfer weight to each foot twice). Limping is acceptable.

When to Apply the Ottawa Knee Rules

Apply Rules If:
  • Acute knee injury (within 7 days)
  • Pain in the knee following trauma
  • Fall, direct blow, or twisting injury
  • Patient is alert and cooperative
  • No other distracting painful injuries
  • Not intoxicated
Do NOT Apply If:
  • Injury >7 days old
  • Isolated skin injuries
  • Patient intoxicated or uncooperative
  • Diminished sensation (e.g., paraplegia)
  • Multiple injuries (distracting injuries)
  • Reduced GCS / head injury
  • Returning for reassessment of same injury

Ottawa Knee Rules Criteria

Criterion How to Assess Clinical Rationale
Age ≥55 years Document patient's age Increased fracture risk with age, especially patella and proximal tibia
Tenderness at fibula head Palpate the head of the fibula (lateral, proximal to joint line) May indicate avulsion fracture or proximal fibula injury
Isolated patella tenderness Palpate entire patella - must be ONLY bony tender point Suggests patella fracture; "isolated" excludes generalised tenderness
Inability to flex to 90° Ask patient to actively bend knee to 90° Inability to flex suggests effusion/haemarthrosis from fracture
Inability to weight bear × 4 steps Assess immediately post-injury AND in department Inability to bear weight suggests significant structural injury

Frequently Asked Questions

No. The Ottawa Knee Rules are designed to detect fractures only. They do not reliably identify:

  • ACL tears
  • PCL tears
  • Collateral ligament injuries
  • Meniscal tears

If soft tissue injury is suspected, appropriate clinical examination (Lachman test, McMurray's, etc.) and potentially MRI should be considered, regardless of the Ottawa Knee Rules result.

The Ottawa Knee Rules have been studied in children with mixed results:

  • Original validation was in adults
  • Some paediatric studies suggest reasonable performance in children >5 years
  • Skeletally immature patients may have growth plate injuries
  • Use with caution in children - clinical judgement is important

Consider a lower threshold for X-ray in children, particularly if examination is difficult.

If the patient has generalised tenderness (not isolated to patella or fibula head), the "isolated patella tenderness" and "fibula head tenderness" criteria may not be met.

However:

  • Still assess the other criteria (age, flexion, weight bearing)
  • Generalised tenderness with effusion may warrant clinical concern
  • If there is a large effusion, consider aspiration (if lipohemarthrosis, fracture is likely)

Clinical judgement should guide the decision if there is significant swelling/effusion.

Patella dislocation is a clinical diagnosis. Many centres routinely X-ray after patella dislocation because:

  • Osteochondral fractures can occur during dislocation/relocation
  • Patella may have an associated avulsion fracture
  • X-ray helps exclude other injuries

The Ottawa Knee Rules were not specifically designed for this scenario. Most clinicians would X-ray a confirmed or suspected patella dislocation.

NICE Clinical Knowledge Summaries (CKS) on knee pain recommends:

  • Use the Ottawa Knee Rules to guide the need for X-ray
  • If no criteria are met, fracture is unlikely and X-ray is not routinely needed
  • Provide advice on PRICE and safety-netting
  • Consider soft tissue injuries separately (clinical examination, possible MRI)

The rules are endorsed by NICE, RCEM, and widely used in UK emergency departments.

Disclaimer

This calculator is provided for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment.

  • Always seek the advice of a qualified healthcare provider with any questions regarding a medical condition.
  • Never disregard professional medical advice or delay seeking it because of information from this tool.
  • Clinical decision-making should always incorporate the full clinical context, patient preferences, and local protocols.
  • The creators of this tool accept no liability for decisions made based on its output.

If you are a patient: please discuss any results with your healthcare provider. This tool is designed for use by medical professionals and may not be appropriate for self-assessment.

References

  1. Stiell IG, et al. (1995). Derivation of a decision rule for the use of radiography in acute knee injuries. Annals of Emergency Medicine, 26(4), 405-413.
  2. Stiell IG, et al. (1996). Prospective validation of a decision rule for the use of radiography in acute knee injuries. JAMA, 275(8), 611-615.
  3. Stiell IG, et al. (1997). Implementation of the Ottawa Knee Rule for the use of radiography in acute knee injuries. JAMA, 278(23), 2075-2079.
  4. Emparanza JI, Aginaga JR. (2001). Validation of the Ottawa Knee Rules. Annals of Emergency Medicine, 38(4), 364-368.
  5. NICE CKS (2022). Knee pain - assessment. National Institute for Health and Care Excellence.