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
- Interpret traumatic knee x-rays using a standard approach
- 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
- Adequacy
- Bones
- Femur
- Patella
- Tibia and Tibial Plateau
- Fibula
- Cartilage/joints
- Patella Tendon
- Effusions
- 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].
-
- On the lateral view, the fibular head should overlap the tibia and the femoral condyles should be superimposed.
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
- To measure, use the lateral view with the knee at 30 degrees of flexion.
- 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].
- The Insall-Salvati ratio should be roughly equal to 1 or within 20% of the contralateral side.
- Patellar tendon length measurement:
-
- 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].
- The knee has 3 fat pads:
- Effusions: indicate occult fracture or significant ligamentous or cartilaginous injury.
-
- Lipohemarthrosis
- If present, strongly consider possible undiagnosed tibial plateau fracture [6].
- Lipohemarthrosis
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.
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.
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.
References
- 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
- 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
- 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.
- Sanville, P. et al. ABC of Emergency Radiology. The Knee. BMJ. Volume 308. January 1994. PMID 8298387
- 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
- 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.
Author information
The post EMRad: Radiologic Approach to the Traumatic Knee appeared first on ALiEM.