Ottawa Knee Rule for Ordering Knee X-rays
Order knee radiographs in adults ≥18 years with acute knee injury if ANY of the following Ottawa Knee Rule criteria are present: age ≥55 years, isolated patellar tenderness, fibular head tenderness, inability to flex knee to 90°, or inability to bear weight (take 4 steps) both immediately after injury and in the emergency department. 1
Clinical Decision Criteria
The Ottawa Knee Rule applies to patients 18 years and older with acute knee pain following trauma. 1 The rule demonstrates 100% sensitivity for detecting knee fractures when properly applied, with potential to reduce unnecessary radiographs by 23-35%. 1
Specific Indications for Radiography
Order knee X-rays if the patient meets any one of these criteria: 1
- Age ≥55 years or older
- Palpable tenderness over the fibular head
- Isolated patellar tenderness (no other bony tenderness)
- Inability to flex the knee to 90 degrees
- Inability to bear weight immediately after injury
- Inability to walk in the emergency department (cannot take 4 weight-bearing steps)
Alternative: Pittsburgh Decision Rule
The Pittsburgh Decision Rule may be used as an alternative, particularly in specific age groups. This rule indicates radiographs for patients <12 years or >50 years old, as well as any patient unable to take four weight-bearing steps in the emergency department. 1 The Pittsburgh rule demonstrates higher specificity (60%) compared to Ottawa (27%) without loss of sensitivity. 2
Critical Exceptions - Always Order Radiographs
Clinical decision rules should NOT be applied in the following situations—radiographs must be obtained regardless of rule criteria: 1
- Gross deformity of the knee
- Palpable mass
- Penetrating injury
- Prosthetic hardware present
- Multiple injuries making examination unreliable
- Altered mental status (head injury, intoxication, dementia)
- Neuropathy (paraplegia, diabetes)
- History suggesting increased fracture risk
Physician judgment must supersede clinical guidelines in all cases. 1
Special Age Considerations
Children Ages 5-12 Years
This age group falls outside both Ottawa and Pittsburgh rule validation ranges. Radiographs may be beneficial despite lack of clinical symptoms in children ages 5-12 years. 1 The Ottawa Knee Rule showed only 92% sensitivity in pediatric populations, missing one fracture in an 8-year-old. 3
Children Under 5 Years
Clinical decision rules have not been validated for children under 5 years of age. 1
Imaging Technique When Indicated
When Ottawa or Pittsburgh criteria are met, obtain minimum of two radiographic views (anteroposterior and lateral) of the affected knee. 1 The lateral view should be obtained with the knee at 25-30 degrees of flexion in lateral decubitus position, demonstrating the patella in profile to evaluate for joint effusion. 1
A cross-table lateral view with horizontal beam enables visualization of lipohemarthrosis, frequently seen with intra-articular fractures. 1
Clinical Impact
Application of these decision rules can reduce radiographs by 46-53% in appropriate populations while maintaining high sensitivity for fracture detection. 1 Only 5.2% of knee radiographs in acute trauma settings demonstrate fractures, making selective imaging based on validated criteria both cost-effective and radiation-sparing. 1
Common Pitfall
Interobserver agreement between providers can be moderate (kappa 0.41-0.52), particularly for subjective findings like inability to bear weight and isolated patellar tenderness. 4, 5 Age criteria show almost perfect agreement (kappa 0.94-0.97), while physical examination findings show more variability. 4, 5 Ensure consistent application by carefully assessing each criterion independently.