When are Knee Radiographs Recommended?

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When are Knee Radiographs Recommended?

When evaluating injuries to the knee it is usually helpful to know something about the mechanism of injury, the environment, and the footwear.  The timing/location of the pain and the timing/magnitude of the swelling can yield valuable information about the injury.  We know that some structures are highly vascularized and swell quickly, whereas others are poorly vascularized and swell slowly.  So the timing of the swelling can help us appreciate what type of tissue may be injured. 

As with the examination of any joint, ruling out a fracture is important.  For the knee, osseous concerns can be assessed via any one of several “knee rules.”  These include the Ottawa, Pittsburgh, Weber, and Fagan and Davies Rules. These can all be used to identify the potential for a fracture. 

In the interest of clarity, the two charts below summarize the criteria for the various “rules” and provide the statistics associated with each of the evaluative criteria.  One key point when applying these rules is the precise palpation of bony structures.  One must be sure she/he is palpating bone and not ligament or tendon. 

Comparison of the criteria reveals several similarities.  Thus, it should not be a surprise to see similar statistical values.  All of these “rules” demonstrate much better sensitivity than specificity.  Consequently, these criteria are very good at ruling out the possibility of a fracture but if the criteria are positive, they are not good at making the diagnosis of a fracture.  The objective of the rules are to identify the need for a radiograph (not to diagnosis a fracture).  So a positive test should be interpreted as the need for a radiograph. 

  

When the Ottawa Knee Rules are applied to children, the statistics are similar (sensitivity=100%, specificity=43%).  A study by Bulloch et al (2003) examined children 2-16 years of age.  Of 750 children enrolled, 670 radiographs were performed.  If the Ottawa Knee Rules had been applied, only 460 would have required a radiograph.  That value represents a 31.2% reduction in x-rays and demonstrates the usefulness of applying the criteria.

As a strong screening tool, a study by Jackson (2003) found that the Ottawa Knee Rules have the capacity to reduce the use of radiographs by 25-28% if they are utilized in the emergency room.   Thus, clinicians have an excellent tool in the Knee Rules to determine if a referral for a radiograph is needed and to render evidence-based treatment.

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  • Bachmann LM, Haberzeth S, Steurer J, ter Riet G. The accuracy of the Ottawa knee rule to rule out knee fractures A Systematic Review. Annals of Internal Medicine. 2004;140(2):121-124
  • Bulloch B, Neto G, Plint A, Lim R, Lidman P, Reed M, Nijssen-Jordan C, Tenenbein M, Klassen TP. Validation of the Ottawa knee rules in children: A multicenter study. Annals of Emergency Medicine. 2003; 42(1):48-55
  • Emparanza JI, Aginaga JR: Validation of the Ottawa knee rules, Annals Emergency Medicine 2001;38:364-368
  • Gulick DT. iOrtho+ Mobile App. DTG Enterprises LLC. 2020
  • Gulick, DT. OrthoNotes, 4th FA Davis Publishing, Philadelphia. 2018
  • Richman PB. More on the Ottawa knee rules. Annuals of Emergency Medicine. 1999;33(4):476
  • Stiell IG. Clinical decision rules in the emergency department. Canadian Medical Association Journal. 2000;163(11):1465-1466
  • Vijayasankar D, Boyle AA, Atkinson P. Can the Ottawa knee rule be applied to children? A systematic review & meta-analysis of observational studies. Emergency Medical Journal 2009;26:250-253

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