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Printed on 6/29/2026
For informational purposes only. This is not medical advice.
The Ottawa Knee Rules are a clinical decision tool to determine the need for knee radiography after acute knee injury. Developed in 1995, they have approximately 99% sensitivity for detecting clinically significant knee fractures. A knee X-ray is only required if any of the following are present: age 55 years or older, isolated tenderness of the patella (no other bone tenderness), tenderness at the head of the fibula, inability to flex the knee to 90°, or inability to bear weight for 4 steps both immediately and in the ED. Implementation can reduce knee X-rays by over 25%. For ankle injuries, apply [Ottawa Ankle Rules](/tools/ottawa-ankle-rules). In elderly patients with knee fractures, assess osteoporosis risk with [Fracture Risk FRAX Calculator](/tools/fracture-risk-frax). For polytrauma, assess overall severity with [Revised Trauma Score](/tools/revised-trauma-score).
Formula: X-ray indicated if: age ≥55 OR isolated patella tenderness OR fibular head tenderness OR inability to flex to 90° OR inability to bear weight 4 steps.
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Begin with a targeted history: How did the injury occur (mechanism)? Can the patient walk? Was the patient able to walk immediately after injury? Now systematically examine the five criteria: (1) Age ≥55: Note patient's age — this criterion is met automatically without examination. (2) Isolated patella tenderness: Palpate the patella and all bony structures around the knee. Isolated patella tenderness = pain only on the patella itself, with no tenderness elsewhere on the knee bony structures. If there is bony tenderness elsewhere in addition to the patella, this criterion is NOT met (the tenderness is not isolated). (3) Fibular head tenderness: Palpate the fibular head — the bony prominence on the lateral proximal lower leg, about 1 cm below the joint line. (4) Inability to flex to 90°: Ask the patient to bend the knee as far as possible. Can they reach 90° of flexion? Inability to achieve 90° due to pain counts as positive. (5) Inability to bear weight: Ask the patient to take 4 steps, transferring weight to the injured knee. Limping is acceptable; the criterion is inability to take 4 steps at all.
The Ottawa Knee Rules use an OR logic: if ANY of the 5 criteria is present, knee radiography is indicated. This differs from some scoring systems where multiple criteria must be positive. Assessment: Age ≥55 is positive (criterion met regardless of exam findings) → X-ray needed. Isolated patella tenderness is positive → X-ray needed. Fibular head tenderness is present → X-ray needed. Patient cannot flex knee to 90° → X-ray needed. Patient cannot bear weight 4 steps → X-ray needed. If ALL five criteria are negative (age <55, no isolated patella tenderness, no fibular head tenderness, can flex to 90°, can bear weight 4 steps), then knee X-rays are NOT required. Document each criterion explicitly in the medical record as positive or negative.
Ottawa Knee Rules NEGATIVE (all criteria absent): The probability of clinically significant knee fracture is <0.5%. No knee X-ray is required. Manage conservatively as likely soft tissue injury (ligament sprain, meniscal injury, contusion). Treatment: RICE (rest, ice, compression, elevation), NSAIDs, weight-bearing as tolerated. If clinical suspicion for ligament or meniscal injury exists (instability, locking, effusion), separate MRI may be indicated regardless of Ottawa result. Provide return precautions. Ottawa Knee Rules POSITIVE (any criterion present): Obtain AP, lateral, and possibly sunrise (patellofemoral) views of the knee. X-ray results guide further management: negative X-ray with ongoing concern = consider MRI, CT, or orthopedic referral. Tibial plateau fracture or patella fracture = orthopedic referral. Never use Ottawa rules alone to rule out ligament injury (ACL, PCL, collateral ligament) — these require clinical examination and sometimes MRI regardless of X-ray results.
Emergency physicians, PAs, and NPs
Apply Ottawa Knee Rules as standard protocol for all adult patients presenting to the ED with acute knee injuries. The rules allow confident deferral of knee X-rays in approximately 35–45% of patients with acute knee trauma, reducing unnecessary radiation, cost, and ED length of stay. A negative Ottawa exam reliably identifies patients who can be managed conservatively without imaging. Many high-volume EDs incorporate Ottawa Knee criteria into nursing triage protocols or electronic order sets to guide X-ray ordering at the point of first assessment. This is one of the most validated clinical decision rules in emergency medicine, with multiple large prospective studies confirming near-perfect sensitivity.
Family medicine physicians, urgent care providers, and NPs
For patients presenting to urgent care or primary care with acute traumatic knee pain, Ottawa Knee Rules help determine whether on-site or referred X-rays are necessary. A negative Ottawa exam allows initiation of conservative management (RICE, NSAIDs, follow-up in 1–2 weeks) without imaging, saving the patient time and cost. Positive Ottawa criteria support X-ray ordering at the visit. This is particularly valuable in urgent care settings where immediate radiograph capability exists — Ottawa rules help appropriately select patients for imaging. For patients where X-ray is negative but clinical concern persists (suspected ACL tear, large effusion, mechanical symptoms), refer to orthopedics for MRI consideration.
Team physicians, athletic trainers, and sports medicine clinicians
When an athlete sustains a knee injury during practice or competition, Ottawa Knee Rules provide a structured framework for rapid sideline assessment to determine whether urgent imaging is needed or the athlete can receive initial conservative management. An Ottawa-negative knee with full weight-bearing and near-normal range of motion supports a likely soft tissue injury, allowing for sideline or training room management with potential same-day return to limited activity. Ottawa-positive criteria (especially inability to bear weight or inability to flex to 90°) indicate the athlete requires imaging before any return-to-play decision. Protect athletes from returning to activity with unrecognized tibial plateau or patella fractures.
All emergency and urgent care providers
Explicit documentation of Ottawa Knee Rule application provides strong medicolegal protection for the decision to defer knee X-rays in low-risk patients. When documenting a negative Ottawa exam, record each criterion explicitly: 'Ottawa Knee Rules: age <55 (negative), no isolated patella tenderness (negative), no fibular head tenderness (negative), able to flex knee to 90° (negative), able to bear weight 4 steps (negative). All criteria negative — X-ray not indicated per Ottawa criteria. Diagnosis: knee sprain.' This documentation demonstrates systematic clinical reasoning and reduces malpractice exposure for missed fractures in Ottawa-negative patients. Published meta-analyses confirm <0.5% fracture rate in Ottawa-negative patients.
Hospital administrators and quality improvement teams
Implementing Ottawa Knee Rules protocols reduces healthcare costs while maintaining quality of care. Studies show 25–35% reduction in knee radiograph utilization without any increase in missed fractures. In a busy ED seeing 150 acute knee injuries per month, Ottawa implementation can eliminate 50–60 X-rays monthly (~600–700 annually). At approximately $100–200 per radiograph series, this represents $60,000–140,000 in annual cost savings per institution. Track Ottawa protocol compliance and X-ray utilization rates as quality metrics. Consider integrating Ottawa criteria into CDS (clinical decision support) alerts in the EHR to prompt clinicians to apply the rule before ordering knee radiographs.
The 'isolated patella tenderness' criterion is frequently misapplied. For this criterion to be positive, the patella must be the ONLY site of bony tenderness — if there is also tenderness at the fibular head, medial joint line, proximal tibia, or any other bony structure, the tenderness is not isolated and this criterion is technically not met. However, in practice, if there is patella tenderness PLUS other bony tenderness, the other areas of bony tenderness likely trigger their own positive criteria (e.g., fibular head tenderness). Document which bony structures are tender to facilitate correct rule application.
The weight-bearing criterion requires the patient to take FOUR steps, transferring weight to the injured knee twice. Limping significantly is acceptable — the patient can grimace, hold the wall, or walk very slowly, as long as they actually transfer weight to the knee and take 4 steps. If the patient refuses to walk or takes only 1–2 steps before stopping due to pain or fear, that counts as inability to bear weight. Also assess weight-bearing BOTH immediately after injury (from the patient's history) AND in the ED (observed). If they bear weight at either time point, the criterion may be negative — check if the protocol requires BOTH or EITHER to be negative.
The fibular head is located just below the lateral joint line of the knee, about 1–2 cm distal to the lateral femoral condyle. It is a distinct bony prominence that can be palpated from the lateral side. Do not confuse with the lateral malleolus (ankle fibula). To find it: place your finger on the lateral joint line of the knee, then move inferiorly 1–2 cm — the prominent bony bump you feel is the fibular head. Fibular head fractures occur in varus stress injuries and in tibial plateau fractures with associated lateral ligament complex injury. Tenderness here is often overlooked when clinicians focus primarily on the knee joint itself.
A critical limitation: Ottawa Knee Rules are designed exclusively to identify fractures that require immediate radiography. They have NO ability to exclude soft tissue injuries — ligament tears (ACL, PCL, MCL, LCL), meniscal injuries, or tendon injuries. A patient who is Ottawa-negative (no fracture risk) may still have a complete ACL rupture, a bucket-handle meniscal tear, or posterior cruciate ligament injury. If the patient has an effusion (hemarthrosis), instability on valgus/varus testing, Lachman test positive, or mechanical symptoms (locking, giving way), refer for MRI or orthopedic assessment regardless of Ottawa result. Ottawa rules and ligament/meniscal assessment are separate clinical questions.
Tibial plateau fractures are the most clinically important injury ruled out by Ottawa Knee Rules. In older adults with osteoporosis, tibial plateau fractures can occur with relatively minor trauma (standing pivot, low-energy fall) and may present with surprisingly mild tenderness and preserved weight-bearing ability. If you have high clinical suspicion for tibial plateau fracture (significant valgus mechanism, older patient, large effusion, joint line tenderness) despite Ottawa criteria being met and X-rays appearing negative, consider CT of the knee — tibial plateau fractures are often poorly visualized or missed on plain radiographs, particularly in the sagittal plane.
Ottawa Knee Rules were validated for acute injuries presenting within 7 days. After 7 days, the physical examination changes (swelling may resolve, tenderness patterns shift, range of motion improves as soft tissue reaction decreases), and the clinical question shifts from 'is there an acute fracture?' to 'is there a stress fracture, incomplete fracture, or evolving soft tissue injury?' For subacute presentations (7 days to several weeks), Ottawa criteria are no longer reliable guides to imaging — use clinical judgment, patient trajectory, and orthopedic consultation if needed. For chronic knee pain (>3 weeks), Ottawa rules do not apply at all.
Ottawa Knee Rules were not validated in patients under 18 years of age. Children have open growth plates (physes) that are weaker than ligaments, meaning forces that cause ligament sprains in adults cause physeal fractures (Salter-Harris injuries) in children. These fractures can be subtle on X-ray or missed entirely. For children with acute knee injuries, have a lower threshold for imaging than Ottawa criteria suggest. Modified pediatric decision rules exist but are less validated. In children, Ottawa Knee Rules can serve as a guide but should not be used as strict criteria to exclude imaging when clinical suspicion is present.
When applying Ottawa Knee Rules and deciding against imaging, document each criterion explicitly in the medical record: 'Ottawa Knee Rules: (1) Age <55 — negative, (2) Isolated patella tenderness — negative (no bony tenderness), (3) Fibular head tenderness — negative, (4) Knee flexion to 90° — achieved, (5) Weight-bearing 4 steps — observed in ED. All 5 criteria negative. Clinical decision: no radiograph indicated per Ottawa criteria. Diagnosis: lateral knee sprain. Plan: RICE, NSAIDs, crutches for comfort, follow-up in 1–2 weeks.' Explicit documentation demonstrates systematic clinical reasoning and reduces medicolegal risk from patient dissatisfaction.
Ottawa Knee Rules were developed by Stiell et al. (JAMA 1996) from 1054 patients with acute knee injuries. Sensitivity 97%, specificity 27%, reducing X-rays by 28% in validation. A subsequent meta-analysis confirmed sensitivity 98.5% (95% CI 93–100%) across 13 validation studies. The rules are included in the American College of Emergency Physicians (ACEP) Clinical Policy for acute knee injuries. Age, isolated patella tenderness, and inability to bear weight are the most frequently positive criteria.
If any of the five criteria are positive — age 55 or older, isolated patellar tenderness, fibular head tenderness, inability to flex the knee to 90 degrees, or inability to bear weight for four steps — then knee radiography is recommended to evaluate for fracture. The specific combination of positive criteria can help guide clinical suspicion: fibular head tenderness may suggest a proximal fibula fracture or tibial plateau fracture, while isolated patellar tenderness raises concern for a patellar fracture.
If none of the five criteria are present, the probability of a clinically significant knee fracture is extremely low. The Ottawa Knee Rules have approximately 99% sensitivity for significant fractures, meaning a negative result provides strong reassurance. The patient most likely has a soft tissue injury such as a ligamentous sprain, meniscal injury, or contusion, which can be initially managed conservatively. However, a negative result on these rules does not exclude soft tissue injuries that may require further workup (e.g., MRI for suspected ACL or meniscal tears).
As with all clinical decision rules, the result should be interpreted within the full clinical context. If the clinician has strong clinical suspicion despite a negative screen, imaging should still be obtained.
Apply the Ottawa Knee Rules to any adult patient presenting to an emergency department or urgent care setting with acute knee pain following a traumatic mechanism. The rules are intended for initial evaluation of knee injuries to determine the need for plain radiographs. They are most useful within the first 7 days of injury, before swelling and tenderness patterns evolve.
The rules are particularly valuable in high-volume emergency departments where knee injuries are common but only about 6% of those injuries involve fractures. By identifying patients who do not need X-rays, the rules reduce unnecessary imaging by over 25%, saving time, cost, and radiation exposure. They have been validated in multiple large prospective studies across different countries and healthcare settings.
The Ottawa Knee Rules were developed and validated in adults 18 years and older. They should not be applied to pediatric patients, though studies suggest reasonable performance in children over age 5 with appropriate modifications. The rules are not valid for patients with altered consciousness, intoxication, paraplegia, multiple trauma, or decreased leg sensation, as these conditions compromise the reliability of the physical examination.
The rules are designed to detect bony fractures only. They do not evaluate for ligamentous injuries (ACL, MCL, PCL, LCL), meniscal tears, or tendon injuries. A patient who passes the Ottawa Knee Rules may still have a significant soft tissue injury requiring further evaluation with MRI or orthopedic referral. If the patient has mechanical symptoms such as locking, catching, or giving way, further workup is warranted regardless of the Ottawa Knee Rules result.
The specificity is approximately 49%, meaning that more than half of patients who meet criteria for X-ray will not have a fracture. This is an acceptable trade-off for a screening tool designed to prioritize sensitivity. The rules also do not account for stress fractures, which may not be visible on initial plain radiographs.
For related assessments, see Ottawa Ankle Rules and Revised Trauma Score.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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Use the Ottawa Ankle Rules to determine if an ankle X-ray is needed after injury. A validated clinical decision rule with ~98% sensitivity for fractures.
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