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Ottawa knee rule OK

In previous issues of Bandolier we have given information about the Ottawa ankle rules - a set of clinical decision rules for helping to decide when someone with an ankle injury needed an X-ray to see if there was a fracture. That the ankle rule worked was demonstrated in a number of studies. Evidently working up the body, they now give us the Ottawa knee rule to determine whether a knee injury needs an X-ray. The knee rule is given in below.

A knee X-ray series is only required for knee injury patients with any of these findings:
  1. Age 55 or older
  2. Isolated tenderness of the patella (that is, no bone tenderness of the knee other than the patella).
  3. Tenderness at the head of the fibula.
  4. Inability to flex to 90 degrees.
  5. Inability to bear weight both immediately and in the emergency department (4 steps; unable to transfer weight twice onto each lower limb regardless of limping).

Making the knee rule

The way the rule was determined is interesting. Over 1000 knee injury patients were assessed for 23 standardised clinical findings. Those variables found to be most reliably and strongly associated with a fracture were subjected to statistical analysis. These were then incorporated into the rule.

Testing the knee rule

Four hospitals in Ontario were chosen. Two were control hospitals where no intervention was made. Two were intervention hospitals. The knee rule was introduced by means of a brief lecture, a pocket card, and wall posters in the emergency department. Staff were given regular updates of progress and difficulties in implementing the rule.

Eligible patients were those seen in the twelve months before and after the introduction on the rule. Clearly not all patients would be eligible and, for example, patients younger than 18 or those with major trauma were not regarded as suitable candidates for use of the decision rule.


About four thousand people (3,907) were seen with knee injury in the four hospitals over the two periods. The main result was the proportion sent for X-ray. This was an average of 26% lower in the intervention hospitals after the introduction of the knee rule compared with previously (Figure). Control hospitals showed no change.

The knee rule correctly predicted all the fractures. No patient who did not have an X-ray was found subsequently to have a fracture.

Time spent in the emergency department, time spent off work, and overall medical costs were lower (by US$103 per patient) in those who did not have an X-ray than those who did.


Yet another superb example of how to do a diagnostic test and show it works. The process they describe is so simple (compared with a typical randomised trial of a therapy, for instance), that it is perplexing that it is not used more frequently in clinical (and clinical plus laboratory) diagnosis. Because the knee rule is tested (as here), or as the ankle rule has been tested more than once, and shown to work, we have excellent evidence that we are doing the right thing. And while each in themselves makes only a small change in practice, the combined effect adds up to make a real difference.


  1. IG Stiell, GA Wells, RH Hoag et al. Implementation of the Ottawa knee rule for the use of radiography in acute knee injuries. JAMA 1997 278: 2075-9.

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