Pittsburgh Knee Rules
The Pittsburgh Knee Rules
What the rule is for. 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
- Assessed in the emergency department, not at the scene
What "unable to weight-bear" means. The criterion is met only if the patient is unable to take four steps at all, four steps meaning weight transferred twice onto each leg. A patient who limps but completes four steps is weight-bearing, and 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. Outside it, including the exclusions listed under limitations below, image at clinical discretion.
- 2Apply the three criteriaAny 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 overrideA concerning clinical picture means imaging regardless of the rule.
Clinical judgement overrides any decision rule. Severe swelling, deformity, an open wound, neurovascular compromise, a pulseless foot or a high-energy mechanism means radiographs regardless of whether the Pittsburgh criteria are met. Both Pittsburgh and Ottawa assume a focused knee examination has been done: decision rules assist clinical assessment, they do not replace it.
When you do image, look for a lipohaemarthrosis. A fat-fluid level, marrow fat floating on blood, signals an occult intra-articular fracture releasing marrow fat into the joint, even when no cortical fracture line is visible. It should prompt CT and a search for a tibial plateau or osteochondral fracture. It is the radiographic counterpart to the rule's clinical caution: a normal-looking film with a lipohaemarthrosis is not normal.
The catch is technique. The fat-fluid level is seen on a horizontal-beam (cross-table) lateral knee film. A standard vertical-beam supine lateral will not show it.
Pittsburgh vs Ottawa Knee Rules
Pittsburgh or Ottawa. Comparing the two major knee decision rules is a common exam question: name both, list their criteria, and say which you would use and why. 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
Why the rules are built this way. This is a frequent step-back question. Decision rules like Pittsburgh and Ottawa are deliberately calibrated for very high sensitivity and 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.
The hierarchy of evidence. A rule must climb this ladder 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?
Why Ottawa dominates guidelines. Pittsburgh was derived at a single centre and externally validated in a limited number of studies. Ottawa has been validated in dozens of centres, pooled in meta-analysis and impact-analysed. Pittsburgh, though more specific, has not reached that level, and that, rather than its accuracy, is the real reason Ottawa dominates guidelines.
Limitations & special populations
Patients the rule cannot test. Intoxicated or cognitively impaired patients cannot be reliably assessed for weight-bearing, so default to imaging. In a patient who was non-ambulatory before the injury the weight-bearing criterion cannot be assessed, and age alone drives the decision. The PANIC mnemonic lists when not to apply the rule.
PANICWhen NOT to apply
Hook:Don't PANIC-apply the rule: Penetrating, Altered, Non-ambulatory, Injury over 7 days, Concerning exam.
What the rule detects. The rule detects clinically significant fractures only. Minor avulsions, subtle osteochondral injuries and purely ligamentous or meniscal pathology may be missed, and may need MRI if clinically suspected.
Children. The paediatric cut-off (under 12) is blunt. It does not account for skeletal maturity, and younger adolescents with open physes may still warrant imaging.
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.
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.