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. If all answers are ‘no’, a clinically important fracture is extremely unlikely and X-ray is not indicated.

1. Patient is 55 years of age or older
   Increased prevalence of fractures in the ≥55 age group is likely associated with osteoporosis.
2. Isolated tenderness of the patella (knee cap)
   Isolated patellar tenderness is specific for a fracture of the patella
3. Tenderness at the head of the fibula
   Tenderness at the head of the fibula is specific for a fracture of the fibula
4. Unable to bend the knee to 90˚
   65% of patients with a knee fracture were unable to bend to 90 degrees.
5. 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.

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, but no obvious fracture, a computed tomography (CT) scan 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.