Ottawa Knee Rules

Who it is for. The rule is built for the acute, isolated, blunt knee injury in an alert patient. It also applies to children, on the strength of a separate paediatric validation discussed under limitations.

When it does not apply. Set the rule aside in:

• Pregnancy — avoid ionising radiation where possible and image on clinical judgement
• Obvious deformity or a clinically evident fracture
• Penetrating injury — open wounds follow a different pathway
• Polytrauma or a distracting injury — ATLS governs
• Isolated superficial skin injury
• Re-assessment of a knee already imaged in this episode

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The criteria. A knee radiograph series is indicated if any one of the five is positive. Do not wait for several before imaging.

Isolated means only. Isolated patellar tenderness means the patella is the only tender bony point. Joint-line tenderness does not satisfy the rule: it is a soft-tissue sign, not a criterion. Palpate the patella deliberately, because a vertical or osteochondral patellar fracture is easily missed otherwise.

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Four steps, both times. Weight-bearing is the most mis-applied criterion. The test is four steps, two onto each foot, assessed at both time points: immediately after the injury and in the department. A limp counts as weight-bearing. Ask about the scene as well as watching the patient walk in the department.

A negative rule. A negative rule makes a clinically significant fracture very unlikely. It does not exclude internal derangement (ACL, meniscal, osteochondral) or a non-displaced patellar or tibial-plateau fracture.

A positive rule. Specificity is intentionally low: 48.6% pooled by Bachmann, 54% in the adult derivation and 42.8% in the paediatric validation, roughly 43–54% across the studies cited here. Roughly half the patients who trigger a criterion turn out to have no fracture, so a positive rule means "this knee needs a radiograph", never "this knee is fractured". The rule over-images deliberately: it is a rule-out instrument, and the false positives are the price paid for missing nothing important. Use it to decide whether plain films are needed, not to "clear" a knee for sport.

A fracture rule, not a soft-tissue rule. The rule will not detect an isolated ACL rupture, meniscal tear or patellar dislocation; these need examination and frequently MRI. Examiners push on what it misses: the acute tense haemarthrosis and the subtle plain-film signs of occult injury. A tense haemarthrosis in a young athlete is one of the following until proven otherwise, regardless of normal films:

• ACL rupture, the commonest cause at around 70–75%
• A peripheral meniscal tear
• A patellar dislocation, often self-reduced, with an MPFL or medial retinacular tear
• An osteochondral fracture
• A PCL injury or an intra-articular fracture

The fat-fluid level. A lipohaemarthrosis, the "FBI" sign, is a fat-fluid level from marrow fat escaping into the joint. On a horizontal-beam (cross-table) lateral it is pathognomonic of an intra-articular fracture even when no fracture line is visible, often an occult tibial plateau fracture. That is the reason to obtain the horizontal-beam lateral and look hard.

The avulsion clues. Spotting any of these on a film that is otherwise negative for obvious fracture changes the whole work-up towards MRI and ligament assessment:

• Segond fracture — a small avulsion off the lateral tibial rim (anterolateral ligament/lateral capsule), strongly associated with an ACL rupture
• Reverse Segond — the medial rim, pointing to a PCL/medial-meniscus injury
• Arcuate sign — avulsion of the fibular head/styloid, signalling a posterolateral corner injury

When the films are normal. CT and MRI are more sensitive than plain films for occult tibial-plateau, osteochondral and Segond-type injuries. Consider them when the films are normal but the patient still cannot weight-bear. After a negative rule, persistent inability to weight-bear beyond 48 hours, an effusion or mechanical symptoms is a legitimate indication for cross-sectional imaging.

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Children. The rule has been validated in children aged 2–16 with 100% sensitivity. Most guidelines use about age 5 as the practical lower limit, so apply it with caution in pre-school children.

Reliability. Inter-observer agreement is good, with kappa around 0.77–0.91. The age and weight-bearing items still depend on the history given, and isolated patellar tenderness on careful palpation, so document what you asked and found.

Impact. The cost-saving holds across health systems: the rule consistently reduces knee radiography by ~20–30% without an increase in missed fractures.

The Pittsburgh Knee Rules. The examiner often asks for an alternative, and the main one is the Pittsburgh rule, which applies specifically to a knee injury from blunt trauma or a fall. Radiograph if that mechanism is present and either of:

• Age younger than 12 or older than 50 years
• Inability to walk four weight-bearing steps in the emergency department

It excludes an injury more than ~6 days old, prior surgery or fracture on that knee, and re-assessment.

Ottawa or Pittsburgh. Both have very high (near-100%) sensitivity for fracture, but Pittsburgh is generally reported to have higher specificity, so it sends fewer uninjured knees for radiographs. The trade-off is that Pittsburgh requires a qualifying mechanism and an age cut-off at both extremes, whereas Ottawa adds the patellar, fibular-head and flexion criteria. Ottawa is the more widely taught and used, and Pittsburgh the credible, more specific alternative worth naming. Either way the same safety nets apply: acute injury only, and a negative rule excludes a fracture, not internal derangement.