Ottawa Knee Rules Calculator
Determine whether knee X-rays are necessary using the Ottawa Knee Rules clinical criteria. Enter values for instant results with step-by-step formulas.
Formula
X-ray needed if ANY criterion is positive
The Ottawa Knee Rules recommend knee radiography if any one of five criteria is met: age 55 or over, tenderness at the fibular head, isolated patellar tenderness, inability to flex the knee to 90 degrees, or inability to bear weight for four steps both immediately after injury and in the emergency department.
Worked Examples
Example 1: Positive Ottawa Knee Rules Assessment
Problem: A 60-year-old patient presents after falling on ice. They have tenderness over the fibular head and cannot flex their knee past 70 degrees. Can they bear weight? Yes, with a limp.
Solution: Criterion 1: Age >= 55? YES (age 60)\nCriterion 2: Fibular head tenderness? YES\nCriterion 3: Isolated patellar tenderness? NO\nCriterion 4: Unable to flex to 90 degrees? YES (only 70 degrees)\nCriterion 5: Unable to bear weight (4 steps)? NO (can walk with limp)\nPositive criteria: 3 of 5
Result: X-ray RECOMMENDED: 3 positive criteria (age, fibular head tenderness, flexion deficit)
Example 2: Negative Ottawa Knee Rules Assessment
Problem: A 28-year-old soccer player twists their knee. They have general knee swelling and pain but no point tenderness at fibula or patella. They can flex to 90+ degrees and walked off the field.
Solution: Criterion 1: Age >= 55? NO (age 28)\nCriterion 2: Fibular head tenderness? NO\nCriterion 3: Isolated patellar tenderness? NO\nCriterion 4: Unable to flex to 90 degrees? NO (full flexion achieved)\nCriterion 5: Unable to bear weight? NO (walked off field)\nPositive criteria: 0 of 5
Result: X-ray NOT NEEDED: 0 positive criteria. Sensitivity 98.5% — fracture extremely unlikely.
Frequently Asked Questions
What are the Ottawa Knee Rules and when should they be applied?
The Ottawa Knee Rules are a set of validated clinical decision rules developed at the Ottawa Civic Hospital in 1995 by Dr. Ian Stiell and colleagues to help emergency physicians determine whether a patient with acute knee injury needs radiographic imaging. The rules apply to adults (over 18 years) presenting with acute knee trauma. They should be applied when a patient has knee pain or tenderness following an injury. The rules are not intended for patients under 18, pregnant women, patients with altered consciousness, paraplegia, obvious open fractures, or those returning for reassessment of the same injury. When all five criteria are negative, the probability of a clinically significant fracture is extremely low, and X-rays can safely be avoided.
How accurate are the Ottawa Knee Rules at detecting fractures?
The Ottawa Knee Rules demonstrate excellent diagnostic accuracy with a sensitivity of 97-100% for clinically significant knee fractures across multiple validation studies conducted in different countries and healthcare settings. The pooled sensitivity is approximately 98.5%, meaning the rules correctly identify nearly all patients who actually have fractures. The specificity is lower at around 48.6%, meaning about half of patients without fractures will still be referred for X-rays. However, the negative predictive value exceeds 99.5%, making it extremely safe to forgo imaging when all criteria are negative. Implementation of these rules has been shown to reduce unnecessary knee X-rays by 26-32% without missing clinically important fractures.
What constitutes inability to bear weight in the Ottawa Knee Rules?
In the context of the Ottawa Knee Rules, inability to bear weight is defined as the inability to take four full weight-bearing steps immediately after the injury AND upon presentation to the emergency department. Both time points must be assessed. A patient satisfies this criterion if they could not complete four steps at either time point, regardless of limping. This is an important distinction because many patients with knee injuries limp but can still bear weight. The criterion captures patients who physically cannot put any weight on the injured leg to complete the steps. Emergency physicians should directly observe the patient attempting to walk four steps in the examination room rather than relying solely on patient self-report.
Why is fibular head tenderness included in knee rules?
Tenderness at the fibular head is included because proximal fibula fractures are clinically significant injuries that can be missed during standard knee evaluation. The fibular head is located on the lateral side of the knee joint, and its fracture can result from direct impact, twisting mechanisms, or valgus stress injuries. Importantly, proximal fibula fractures may be associated with ligamentous injuries to the knee, peroneal nerve damage, and in severe cases, compartment syndrome. The fibular head is specifically palpated during Ottawa Knee Rules assessment because isolated tenderness at this point without other knee pathology may indicate a fracture that would require different management. Missing this injury could lead to delayed treatment and complications.
Can the Ottawa Knee Rules be used for children or repeated injuries?
The Ottawa Knee Rules were specifically validated in adults aged 18 years and older and should not be applied to pediatric patients without modification. Children have open growth plates (physes) that are more vulnerable to injury than adult bone, and fracture patterns differ significantly. Pediatric-specific decision rules exist but are less well-validated. For repeated or chronic injuries, the Ottawa Knee Rules are also not applicable because they were designed for acute traumatic presentations within seven days of injury. Patients returning for reassessment of the same injury, those with pre-existing knee conditions like rheumatoid arthritis, and those with superficial skin injuries overlying the knee should be evaluated using clinical judgment rather than the Ottawa Knee Rules decision framework.
How accurate are the results from Ottawa Knee Rules Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.