Pittsburgh Knee Rules
The Pittsburgh Knee Rules
The Pittsburgh Knee Rules are a clinical decision instrument derived to reduce unnecessary knee radiographs after acute knee trauma. They apply to blunt trauma or fall mechanisms only — penetrating injuries are excluded.
- Positive finding
- Younger than 12 years
- Clinical detail
- Immature skeleton — higher suspicion for physeal injuries and occult fractures in children
- Positive finding
- Older than 50 years
- Clinical detail
- Osteopenia and lower-energy fracture risk in older adults
- Positive finding
- Unable to take four weight-bearing steps
- Clinical detail
- Tested in the ED; limping counts as ambulation — true inability means the patient cannot take four steps at all
Under 12, Over 50, or Cannot Walk FourThe three Pittsburgh criteria
Hook:Under 12, Over 50, or can't walk Four - any one means X-ray the blunt-injured knee.
Weight-bearing ability is assessed in the emergency department, not at the scene. The criterion is met only if the patient is unable to take four steps (transfer weight twice onto each leg). A patient who limps but completes four steps is weight-bearing — radiographs are not indicated on this criterion alone. This is the single most common point candidates get wrong.
Applying the rule
- 1Check the mechanismThe rule applies only to a fall or blunt-trauma mechanism. Penetrating trauma, or an injury more than ~7 days old, falls outside the rule — image at clinical discretion.
- 2Apply the three criteriaAge under 12, age over 50, or inability to take four weight-bearing steps in the ED. Any one positive → obtain AP and lateral knee radiographs.
- 3Clear if all negativeAge 12–50 and able to walk four steps → radiographs can be safely omitted (near-100% negative predictive value for clinically significant fracture).
- 4Let judgement overrideSevere deformity, an open wound, neurovascular compromise or a high-energy mechanism → image regardless of the rule.
Clinical judgement overrides any decision rule. If the picture is concerning — severe swelling, deformity, neurovascular compromise, or a high-energy mechanism — obtain radiographs regardless of whether the Pittsburgh criteria are met. Decision rules assist, they do not replace, clinical assessment.
If the rule sends the patient for radiographs, look for a lipohaemarthrosis: a fat-fluid level (marrow fat floating on blood) seen on a horizontal-beam (cross-table) lateral knee film. It signals an occult intra-articular fracture releasing marrow fat into the joint - even when no cortical fracture line is visible - and should prompt CT and a search for a tibial plateau or osteochondral fracture. The catch is technique: a standard vertical-beam supine lateral will not show the fat-fluid level, so the cross-table lateral is what reveals it. It is the radiographic counterpart to the rule's clinical caution - a 'normal-looking' film with a lipohaemarthrosis is not normal.
Pittsburgh vs Ottawa Knee Rules
Comparing the two major knee decision rules is a common exam question. Both aim to reduce unnecessary radiographs while keeping near-perfect sensitivity for clinically significant fractures.
- Pittsburgh Knee Rules
- 3
- Ottawa Knee Rules
- 5
- Pittsburgh Knee Rules
- Younger than 12 years
- Ottawa Knee Rules
- Not included
- Pittsburgh Knee Rules
- Older than 50 years
- Ottawa Knee Rules
- 55 years or older
- Pittsburgh Knee Rules
- Yes — four steps in the ED
- Ottawa Knee Rules
- Yes — unable to take four steps (immediately and in the ED)
- Pittsburgh Knee Rules
- No
- Ottawa Knee Rules
- Isolated patellar tenderness; fibular head tenderness
- Pittsburgh Knee Rules
- No
- Ottawa Knee Rules
- Inability to flex the knee to 90°
- Pittsburgh Knee Rules
- Blunt trauma or fall only
- Ottawa Knee Rules
- Any acute knee injury
- Pittsburgh Knee Rules
- ~99% (Seaberg 1998)
- Ottawa Knee Rules
- ~97–99% (pooled 98.5%, Bachmann 2004)
- Pittsburgh Knee Rules
- Higher (~60%) — fewer films
- Ottawa Knee Rules
- Lower (~27–49%) — more films
- Pittsburgh Knee Rules
- Fewer external validations
- Ottawa Knee Rules
- Extensive international validation
Pittsburgh Simple (3), Ottawa Thorough (5)Pittsburgh vs Ottawa — quick distinguisher
Hook:Pittsburgh is Simple (3 criteria, more specific); Ottawa is Thorough (5 criteria, more validated).
- Penetrating trauma — the rule is not validated for this mechanism.
- Altered consciousness or intoxication — cannot reliably assess weight-bearing.
- Non-ambulatory at baseline — the weight-bearing test is meaningless.
- Injury more than ~7 days old — the rule is for acute presentations.
- Concerning examination (deformity, open wound, neurovascular compromise) — image regardless.
A frequent step-back question: why are these rules built the way they are? Decision rules like Pittsburgh and Ottawa are deliberately calibrated for very high sensitivity / negative predictive value - the aim is to safely rule out a clinically significant fracture, so missing one is the unacceptable error and the price paid is lower specificity (some unnecessary films). They are worth having because most post-trauma knee films are normal, so safely reducing imaging saves radiation, cost and emergency-department time. Crucially, a rule must climb a hierarchy of evidence before it changes practice: derivation (identify the predictors) → validation (prospective, then broad multicentre) → impact analysis (does using it actually reduce imaging, safely and cost-effectively?). Ottawa has been validated across many centres, pooled in meta-analysis and impact-analysed; Pittsburgh, though more specific, has not reached that level - which (rather than its accuracy) is the real reason Ottawa dominates guidelines.
Limitations & special populations
- Fewer validation studies than Ottawa. Pittsburgh was derived at a single centre and externally validated in a limited number of studies; the Ottawa rule has been validated in dozens of centres and pooled in meta-analysis — the main reason Ottawa dominates guidelines.
- The paediatric cut-off (under 12) is blunt. It does not account for skeletal maturity; younger adolescents with open physes may still warrant imaging.
- Intoxicated or cognitively impaired patients cannot be reliably assessed for weight-bearing — default to imaging.
- Pre-existing non-ambulators cannot be assessed for the weight-bearing criterion; age alone drives the decision.
- The rule detects clinically significant fractures only. Minor avulsions, subtle osteochondral injuries and purely ligamentous/meniscal pathology may be missed and may need MRI if clinically suspected.
- Both rules assume a focused knee examination has been done — a deformed knee, open wound or pulseless foot mandates imaging regardless.
Exam & revision
Everything below condenses the Pittsburgh Knee Rules for revision and viva practice.
- Entry = blunt trauma or fall; then X-ray if age under 12, age over 50, or unable to walk four steps.
- No criterion met (age 12–50, walks four steps) → no radiograph (near-100% NPV for significant fracture).
- Limping counts as ambulation — true inability means cannot take four steps at all.
- Pittsburgh (3 criteria) is simpler and more specific than Ottawa (5); equally sensitive, but Ottawa is far more validated.
- PANIC = when not to apply (Penetrating, Altered, Non-ambulatory, Injury over 7 days old, Concerning exam).
- Clinical judgement overrides the rule — image any deformity, neurovascular compromise or high-energy mechanism.
PANICWhen NOT to apply
Hook:Don't PANIC-apply the rule: Penetrating, Altered, Non-ambulatory, Injury over 7 days, Concerning exam.
Exam viva
Practise clinical reasoning and management decisions out loud
“A 35-year-old man presents to the emergency department after a fall onto his flexed right knee while playing basketball. He is tender over the patella but is able to walk four steps, albeit with a limp. Do you need to order knee radiographs? Discuss your clinical decision-making.”
“A 9-year-old boy falls off a trampoline and lands on his right knee. He is in significant pain and refuses to walk. Discuss the role of clinical decision rules in his management and any paediatric caveats.”
The three criteria (any positive = X-ray)
- Age younger than 12 years
- Age older than 50 years
- Inability to walk four weight-bearing steps (tested in ED; limping counts as ambulating)
Pittsburgh vs Ottawa at a glance
- Pittsburgh: 3 criteria, blunt trauma/fall only, age under 12 or over 50, weight-bearing
- Ottawa: 5 criteria, any acute knee injury, age ≥55, tenderness (patella, fibular head), flexion under 90°, weight-bearing
- Both ~near-100% sensitive for clinically significant fractures
- Pittsburgh more specific (fewer films); Ottawa far more validated
When NOT to apply (PANIC)
- Penetrating trauma — not validated
- Altered consciousness/intoxication — cannot assess weight-bearing
- Non-ambulatory at baseline — weight-bearing test meaningless
- Injury more than 7 days old — rule is for acute presentations
- Concerning exam (deformity, open wound, neurovascular compromise) — image regardless
Evidence
Clinical decision rule for knee radiographs
- Original derivation of the Pittsburgh Knee Rules: retrospective chart review (201 patients) then prospective validation (133 patients).
- The combination of fall/blunt mechanism with either inability to ambulate or age (under 12 or over 50) was 100% sensitive with 79% specificity in the prospective phase.
- In this single-centre derivation, the rule was estimated to reduce knee radiographs by up to 78% — and the authors called for a larger multicentre validation before widespread use.
Multicenter comparison of two clinical decision rules for the use of radiography in acute, high-risk knee injuries
- Prospective, blinded, multicentre comparison of Pittsburgh and Ottawa rules in 934 patients with acute knee injuries.
- Pittsburgh rule (745 applicable patients, 91 fractures): sensitivity 99%, specificity 60%, missing one fracture.
- Ottawa rule (750 patients, 87 fractures): sensitivity 97%, specificity 27% — the Pittsburgh rule was more specific without loss of sensitivity.
According to PubMed, the Pittsburgh derivation and its criteria/performance come from Seaberg & Jackson 1994 (DOI); the direct Pittsburgh-vs-Ottawa comparison (Pittsburgh 99%/60%, Ottawa 97%/27%) from Seaberg et al. 1998 (DOI); the pooled Ottawa accuracy (sensitivity 98.5%) from Bachmann et al. 2004 (DOI); and the Ottawa validation (100% sensitivity, 28% radiograph reduction) from Stiell et al. 1996 (JAMA 1996;275:611-5 - pre-DOI). Clinical judgement overriding the rule is standard emergency-medicine teaching.