Radiology teaching during medical school is variable, ranging from informal teaching to required clerkships [1].​​ Many of us likely received an approach to a chest x-ray, but approaches to other studies may or may not have not been taught. We can do better! Enter EM:Rad, a series aimed at providing “just in time” approaches to commonly ordered radiology studies in the emergency department. When applicable, it will provide pertinent measurements specific to management, and offer a framework for when to get an additional view, if appropriate. We recently covered the elbow, wrist, shoulder, ankle, and foot. Next up: the knee.

Learning Objectives

  1. Interpret traumatic knee x-rays using a standard approach
  2. Identify clinical scenarios in which an additional view might improve pathology diagnosis

Why the knee matters and the radiology rule of 2’s

The Knee

  • 6 million ED visits for knee injuries between 1999 and 2008 [2].
  • Most commonly injured joint of adolescent athletes [2].
  • Does this patient need an x-ray? Consider applying the Ottawa Knee Rules.

Before we begin: Make sure to employ the rule of 2’s [3]

  • 2 views: One view is never enough.
  • 2 abnormalities: If you see one abnormality, look for another.
  • 2 joints: Image above and below (especially for forearm and leg).
  • 2 sides: If unsure regarding a potential pathologic finding, compare to another side.
  • 2 occasions: Always compare with old x-rays if available.
  • 2 visits: Bring patient back for repeat films.

An approach to the traumatic adult knee x-ray

  1. Adequacy
  2. Bones
    • Femur
    • Patella
    • Tibia and Tibial Plateau
    • Fibula
  3. Cartilage/joints
    • Patella Tendon
    • Effusions
  4. Consider an additional view

1.    Adequacy

    • The AP view should be centered on the joint space with a slight overlap of the lateral tibia and head of fibula. Patella should be visible in midline [4].

Figure 1: Normal AP knee x-ray. Case courtesy of Dr Andrew Dixon,, annotations by Stephen Villa MD.

    • On the lateral view, the fibular head should overlap the tibia and the femoral condyles should be superimposed.

Figure 2: Normal lateral knee x-ray. Case courtesy of Dr Andrew Dixon,, annotations by Stephen Villa MD.

2.    Bones

    • Femur
    • Patella
    • Tibia and Tibial Plateau
    • Fibula

3.    Cartilage and Joints

    • Patellar tendon length measurement:
      • The Insall-Salvati ratio should be roughly equal to 1 or within 20% of the contralateral side.
        • To measure, use the lateral view with the knee at 30 degrees of flexion.
          • Measure the patellar tendon length: The distance from the inferior pole of the patella to the superior aspect of the tibial tuberosity
          • Measure the greatest patellar length: The distance from the superior pole of the patella to the inferior pole of the patella.
          • Divide the patellar tendon length by the greatest patellar length
        • Patella “baja” or complete quadriceps rupture is consistent with Insall-Salvati ratio of less than 0.8.
        • Patella “alta” or complete patellar tendon tear is consistent with Insall-Salvati ratio of greater than 1.2
      • Pearl: There is considerable variance in normal values. Strongly consider either imaging the contralateral side or performing an ultrasound before making a definitive diagnosis [5].
knee radiology

Figure 3: Case courtesy of Dr Wael Nemattalla,

    • Effusions: indicate occult fracture or significant ligamentous or cartilaginous injury.
      • The knee has 3 fat pads:
        • Prefemoral fat pad
        • Anterior suprapatellar fat pad
        • Infrapatellar fat pad (aka Hoffa fat pad)
      • Fat pads are best seen on the lateral view.
      • In order to assess for enlarged fat pads, measure the suprapatellar recess.
        • The suprapatellar recess lies between the prefemoral and the anterior suprapatellar fat pads, and can become enlarged in the case of knee effusion.
        • A suprapatellar recess of < 5 mm has been found to be 100% sensitive for ruling out effusion [6].

Figure 4: Knee effusion. Prefemoral, anterior suprapatellar, and infrapatellar fat pads are shaded above. Note the prominent suprapatellar recess between the anterior suprapatellar and prefemoral fat pads. Case courtesy of Dr Henry Knipe, Annotations by Stephen Villa MD.

Figure 5: Lipohemarthosis. Case courtesy of Gerry Gardner,

4. Consider an additional view

Oblique Views

  • When: If there is clinical concern for tibial plateau fracture but minimal access to CT.
  • Why: Additional view allows one to appreciate subtle fractures otherwise obscured by AP and Lateral.
knee radiology

Figure 6: Oblique view. Note, this x-ray has a fibular head fracture. Case courtesy of Dr Maulik S Patel,

Sunrise View/Skyline View/Merchant View

  • When: High concern for patella fracture possibly missed by AP and Lateral.
  • Why: Axial projection of patella improves sensitivity to detect fracture.
knee radiology

Figure 7: Skyline view. Case courtesy of Dr Ian Bickle,

Read more about positive findings in our SplintER Series: Common ED Splint Techniques.

Afraid you might miss something commonly missed or catastrophic? Can’t miss knee injuries.

Normal x-ray? Check out EMDocs’wonderful review of occult knee injuries. EMCases has a good review of occult knee injuries as well.

Dealing with a polytrauma? Check out EM:Rad’s approach to the elbow, wrist, shoulder, ankle, and foot.


  1. Schiller, P. et al. Radiology Education in Medical School and Residency. The views and needs of program directors. Academic Radiology, Vol 25, No 10, October 2018. PMID: 29748056
  2. Gage, BE et al. Epidemiology of 6.6 million knee injuries presenting to United States emergency departments from 1999 through 2008. Acad Emerg Med. Volume 19. Issue 4. April 2012.  PMID 22506941
  3. Chan, Otto. Introduction: ABCs and Rules of Two. ABC of Emergency Radiology, Third Edition. Edited by Otto Chan. 2013 John wiley & Sons, Ltd. Published 2013.
  4. Sanville, P. et al. ABC of Emergency Radiology. The Knee. BMJ. Volume 308. January 1994. PMID 8298387
  5. Knutson, T. et al. Evaluation and management of traumatic knee injuries in the emergency department. Emerg Med Clin North Am. Volume 33. Issue 2. May 2015. PMID 25892726
  6. Tai, AW. et al. Accuracy of cross-table lateral knee radiography for evaluation of joint effusions. AJR Am J Roentgenol. Volume 194. Issue 4. Oct 2009. PMID 19770305.



Stephen Villa, MD

Stephen Villa, MD

Medical Education Fellow
Department of Emergency Medicine
University of California, Los Angeles