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X-rays are required for those with a positive Ottawa Knee Rule or a tense haemarthrosis.

The Ottawa Knee Rule applies to individuals that are 5 years of age or older, present within seven days of injury, are not intoxicated, can communicate appropriately, have normal sensation in their legs and do not have ‘distracting’ injuries.


A landmark study was published in 1995 that investigated the use of X-rays in an Accident and Emergency (A&E) department. 80% of patients that attended A&E with a knee injury were X-rayed but only 6.6% were found to have a fracture. This highlighted the importance of ordering X-rays selectively and a set of clinical prediction rules (Ottawa Knee Rule) were developed to guide clinicians on the appropriate use of imaging.

When to X-ray

X-rays are indicated when the Ottawa Knee Rule (OKR) is positive, or the individual presents with a tense swelling of the knee joint that developed within 2 hours injury (tense haemarthrosis).

Ottawa Knee Rule:

The Ottawa Knee Rule consists of five “yes–no” items. If any of the items can be answered ‘yes’ then plain X-rays (AP and lateral views) are required to check for evidence of a clinically important fracture (images 1-8). If all answers are ‘no’, a clinically important fracture is extremely unlikely and X-ray is not indicated.  Each individual item of the OKR and the rationale for ordering X-ray is presented in table 1.

Ottawa Knee RuleRationale
Patient is 55 years of age or olderIncreased prevalence of fractures in the ≥55 age group is likely associated with osteoporosis.
Isolated tenderness of the patella (knee cap)Isolated patellar tenderness is specific for a fracture of the patella (images 1-2)
Tenderness at the head of the fibulaTenderness at the head of the fibula is specific for a fracture of the fibula (images 3-4)
Unable to bend the knee to 90˚65% of patients with a knee fracture are unable to bend to 90 degrees.
Patient is unable to bear weight for 4 steps, both immediately and in A&E (limping is allowed).Inability to bear weight is one of the most reliable variables indicating a possible knee fracture.

Table 1. Ottawa Knee Rule and rationale for ordering X-rays.

Image 1: side (lateral) view of patellar fracture.

Image 2: front (AP) view of patellar fracture.

Image 3: side (lateral) view of displaced fracture of the neck of fibula.

Image 4: front (AP) view of displaced fracture of the neck of fibula.

Image 5: posterior cruciate ligament avulsion fracture.

Image 6: tibial eminence fracture (anterior cruciate ligament avulsion fracture).

Image 7: tibial plateau fracture (black arrow) and lipohaemarthrosis (white arrow).

Image 8: tibial plateau fracture on CT.

The OKR can only be applied to individuals that:

  • are 5 years of age or older
  • present within seven days of injury
  • are not intoxicated
  • can communicate appropriately
  • have normal sensation in their legs
  • do not have ‘distracting’ injuries.

If there is evidence of fat and blood within the joint (lipohaemarthrosis) on X-ray (image 7), but no obvious fracture, a computed tomography (CT) scan (image 8) may be required for more detailed information.

If there is a history suggestive of a heamarthrosis, but no evidence of fracture on X-ray, the individual is likely to have sustained a significant soft tissue injury, such as a cruciate ligament rupture, patellar dislocation or traumatic meniscal tear.

Diagnostic ability

The OKR can be used to rule out a knee fracture when negative. For details on the diagnostic accuracy and reliability of the OKR, please visit the statistics section.


Written by: Richard Norris, The Knee Resource

Reviewed by: Professor Ian G. Stiell, MD, MSc, FRCPC
Department of Emergency Medicine
University of Ottawa

Senior Scientist, Ottawa Hospital Research Institute



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