Ottawa Knee Rule: Criteria for X-ray in Knee Injuries
The Ottawa Knee Rule states that patients 18 years of age or older with acute knee pain should have knee radiographs if they meet ANY of the following criteria: age 55 years or older, palpable tenderness over the head of the fibula, isolated patellar tenderness, inability to flex the knee to 90°, inability to bear weight immediately following the injury, or inability to walk in the emergency room (after taking 4 steps). 1
Detailed Criteria of the Ottawa Knee Rule
The Ottawa Knee Rule is a validated clinical decision tool designed to reduce unnecessary radiography in acute knee injuries. Patients require knee X-rays if they meet ANY of these criteria:
- Age ≥ 55 years
- Tenderness at the head of the fibula
- Isolated patellar tenderness
- Inability to flex the knee to 90°
- Inability to bear weight immediately after injury
- Inability to take 4 steps in the emergency department
Evidence Supporting the Ottawa Knee Rule
The Ottawa Knee Rule has been extensively validated and demonstrates:
- 100% sensitivity for detecting knee fractures in multiple studies 2
- Potential to reduce knee radiography by 26.4% in clinical practice 3
- Reduced waiting times and lower healthcare costs without missed fractures 3
Clinical Application and Limitations
When NOT to Apply the Rule
The Ottawa Knee Rule should not be applied in patients with:
- Gross deformity
- Palpable mass
- Penetrating injury
- Prosthetic hardware
- Unreliable clinical history/examination due to multiple injuries
- Altered mental status (head injury, intoxication, dementia)
- Neuropathy (paraplegia, diabetes)
- History suggesting increased fracture risk 1
Age Considerations
- The Ottawa Knee Rule applies to patients ≥18 years
- For patients <18 years, consider the Pittsburgh Decision Rule or clinical judgment
- For patients between 5-12 years, radiographs may be beneficial despite lack of clinical symptoms 1
Ottawa Knee Rule vs. Pittsburgh Decision Rule
The Pittsburgh Decision Rule is an alternative clinical decision tool that states radiographs should be obtained for:
- Patients <12 years or >50 years
- Patients who cannot take four weight-bearing steps in the emergency department 1
Comparative studies show:
- Both rules have high sensitivity (97-99%) for detecting fractures 4
- The Pittsburgh rule demonstrates higher specificity (60% vs 27%) 4, 5
- The Pittsburgh rule has better interobserver agreement (κ=0.71 vs 0.51) 5
Implementation Benefits
Proper implementation of the Ottawa Knee Rule leads to:
- Decreased use of knee radiography without patient dissatisfaction 3
- Reduced waiting times in the emergency department (85.7 vs 118.8 minutes) 3
- Lower healthcare costs ($80 vs $183 per patient) 3
Recommended Imaging When X-rays Are Indicated
When the Ottawa Knee Rule criteria are met, obtain:
- Minimum of two radiographic views (anteroposterior and lateral)
- Consider additional views (patellofemoral, internal oblique, external oblique) for suspected patellar fractures 1
If radiographs are negative but clinical suspicion remains high, consider MRI as the preferred second-line imaging modality for detecting occult fractures and soft tissue injuries 6.