Ottawa Knee Rule

The OKR was designed to detect clinically significant knee fractures — those requiring specific orthopaedic management. The most common fractures identified when the rule is positive include patellar fractures (most frequent), proximal tibial plateau fractures, fibular head fractures, and distal femoral fractures. Avulsion fractures at ligament attachment sites (e.g., tibial spine) may also be detected.

The rule is not designed to detect small cortical avulsions, osteochondral fragments, or stress fractures, which may not be visible on plain radiography regardless. If clinical suspicion for occult fracture remains high despite negative X-rays, consider CT or MRI as a next step.

The Ottawa Knee Rule addresses the need for bony imaging only. It does not assess or exclude soft tissue injuries, which are far more common than fractures after knee trauma. Key soft tissue injuries to consider include anterior cruciate ligament (ACL) tears, medial and lateral collateral ligament sprains, meniscal tears, patellar tendon rupture, and quadriceps tendon rupture.

Patients who are OKR-negative may still have significant ligamentous or meniscal injuries requiring further evaluation. Clinical examination — including Lachman test, McMurray test, varus/valgus stress testing, and assessment for effusion — remains essential even when radiography is not indicated by the rule.

The mechanism of injury does not form part of the Ottawa Knee Rule criteria, but it provides important clinical context. High-energy mechanisms (motor vehicle collisions, falls from height, dashboard injuries) carry a higher risk of complex fracture patterns — including tibial plateau fractures, supracondylar femoral fractures, and knee dislocations — that may warrant imaging regardless of the OKR result.

Knee dislocations, even when spontaneously reduced, represent a vascular emergency due to the risk of popliteal artery injury. Clinicians should maintain a low threshold for vascular assessment (ankle-brachial index, CT angiography) when the mechanism suggests possible dislocation, regardless of the OKR assessment.

The presence of a knee effusion is not a criterion in the Ottawa Knee Rule. However, a tense haemarthrosis following acute knee injury is clinically significant. In the original Stiell studies, effusion alone was not sufficiently specific to be included as a predictive criterion.

A large, tense effusion developing rapidly (within 2 hours) after injury is associated with intra-articular fracture or ACL rupture in up to 70% of cases. If aspiration reveals frank blood with fat globules (lipohemarthrosis), this is strongly suggestive of an intra-articular fracture and should prompt imaging regardless of the OKR result.

An alternative decision rule — the Pittsburgh Knee Rule — was developed by Seaberg and Jackson (1994). This rule requires knee radiography only if the mechanism was a fall or blunt trauma AND either the patient’s age is under 12 or over 50, or the patient cannot take four weight-bearing steps.

The Pittsburgh rule has shown comparable sensitivity to the OKR (~99%) with potentially higher specificity in some studies, meaning fewer X-rays ordered. However, it has been less extensively validated across multiple centres and populations. The Ottawa Knee Rule remains the most widely adopted and recommended tool in international emergency medicine guidelines.

The patellar tenderness criterion requires that the tenderness is isolated to the patella — meaning the patella is tender, but there is no bony tenderness elsewhere around the knee. This is the most commonly misinterpreted criterion. If the patient has tenderness at both the patella and the tibial plateau, the patellar tenderness is not “isolated” — the criterion refers specifically to patella-only bony tenderness. Examiners must palpate all bony landmarks systematically before determining this criterion.

Weight-bearing must be assessed as four full weight-bearing steps — not simply standing or taking one or two steps. The definition includes limping: a patient who limps through four steps is considered able to weight-bear. The assessment must be performed both at the time of injury (obtained by history) and in the emergency department. Failure to assess both time points is a common source of error. If the patient could walk immediately after the injury but cannot in the ED (or vice versa), the criterion is positive.

The OKR should not be applied in patients with altered consciousness, intoxication, polytrauma with distracting injuries, peripheral neuropathy, or pregnancy (where the risk-benefit of imaging differs). Applying the rule in these populations may produce falsely reassuring negative results because the patient cannot reliably report tenderness or perform the weight-bearing assessment. Clinicians should default to clinical judgement and a low threshold for imaging when the patient cannot fully participate in the assessment.

The Ottawa Knee Rule and the Ottawa Ankle Rule are separate decision instruments with different criteria. While both include age and weight-bearing assessments, the bony landmarks and thresholds differ. Clinicians should ensure they are applying the correct rule to the correct joint. In patients with combined knee and ankle injuries (e.g., following a fall), both rules should be applied independently to their respective joints.

A negative Ottawa Knee Rule suggests that a clinically significant fracture is unlikely — it does not exclude all knee pathology. Ligamentous injuries (ACL, MCL, LCL), meniscal tears, tendon ruptures, and other soft tissue injuries are not assessed by the rule. Patients discharged without imaging should receive clear safety-net advice: return if symptoms worsen, if they develop locking or giving way, or if they are unable to weight-bear after 5–7 days. Appropriate follow-up arrangements should be documented.