What are the criteria for applying the Ottawa knee rule to determine if a patient with a knee injury requires an X-ray?

Ottawa Knee Rule Criteria for X-ray in Knee Injuries

According to the Ottawa Knee Rule, 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 Ottawa Knee Rule Criteria

The Ottawa Knee Rule includes six specific criteria that indicate the need for radiography:

1. Age ≥ 55 years
2. Fibular head tenderness - Palpable tenderness over the head of the fibula
3. Isolated patellar tenderness - Tenderness limited to the patella
4. Inability to flex knee to 90° - Due to pain or mechanical restriction
5. Inability to bear weight immediately after injury
6. Inability to walk 4 steps in the emergency department (regardless of limping)

Clinical Application and Effectiveness

• The Ottawa Knee Rule has demonstrated 100% sensitivity for detecting knee fractures in multiple studies, with the potential to reduce radiography by 25-35% 1, 2, 3
• Implementation of the rule has been associated with reduced waiting times and lower healthcare costs without compromising patient safety 4

Pittsburgh Decision Rule Alternative

The Pittsburgh Decision Rule is an alternative clinical decision rule that states:

• Radiographs should be obtained for patients <12 years old or >50 years old
• Radiographs should be obtained for patients who cannot take four weight-bearing steps in the emergency department 1

In comparative studies, the Pittsburgh rule has shown higher specificity (60% vs 27%) with comparable sensitivity (99% vs 97%) when compared to the Ottawa rule 5.

Important Exceptions and Considerations

The clinical decision rules should NOT be applied in the following situations:

• Patients with gross deformity
• Presence of a palpable mass
• Penetrating injury
• Prosthetic hardware
• Unreliable clinical history or physical examination due to multiple injuries
• Altered mental status (e.g., head injury, drug/alcohol use, dementia)
• Neuropathy (e.g., paraplegia, diabetes)
• History suggesting increased risk of fracture 1

Radiographic Views When Indicated

When radiographs are indicated based on the Ottawa Knee Rule:

• Minimum of two views (anteroposterior and lateral)
• Consider additional views (patellofemoral, internal oblique, external oblique) for specific clinical scenarios
• A cross-table lateral view with horizontal beam can help visualize lipohemarthrosis, which is often associated with intra-articular fractures 1

Clinical Pearls

• Physician judgment should supersede clinical guidelines in uncertain cases
• Fair inter-observer reliability exists between physicians and triage nurses in applying the Ottawa Knee Rule, though nurses tend to overestimate the presence of criteria 6
• The Ottawa Knee Rule may also be applicable to the pediatric population, though it was originally designed for adults 2
• Weight-bearing radiographic views are essential when evaluating for osteoarthritis 7

By systematically applying these criteria, clinicians can effectively identify patients who require radiographic evaluation while reducing unnecessary imaging, waiting times, and healthcare costs.