knee radiology

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

Have you ever been working a shift at 3am and wondered, “Am I missing something? I’ll just splint and instruct the patient to follow up with their PCP in 1 week.” This is a reasonable approach, especially if you’re concerned there could be a fracture. But we can do better. Enter the “Can’t Miss” series: a series organized by body part that will help identify injuries that ideally should not be missed. This list is not meant to be a comprehensive review of each body part, but rather to highlight and improve your sensitivity for these potentially catastrophic injuries. Now: the knee.

The Knee

  • 6 million ED visits for knee injuries between 1999 and 2008 [1].
  • Most commonly injured joint of adolescent athletes [1].
  • Epidemiology: High risk for vascular injury to popliteal artery, peroneal nerve injury, and compartment syndrome.
  • Mechanism: Usually high speed MVC or sports injury but can also be seen in low velocity falls, especially in patients with larger body habitus [2].
  • Symptoms: Knee dislocations are obvious but note that many spontaneously reduce up to 50% of the time [2].
  • Physical Exam: The knee may likely be unstable as a knee dislocation usually requires at least 3 ligaments to be completely ruptured [3]. Lifting the leg up by the heel while the patient is lying supine may result in hyperextension. You may see foot drop if the common peroneal nerve is injured. Check posterior tibialis and dorsalis pedis pulses as the popliteal artery is injured in ⅓ of cases [2].
  • Diagnostic Imaging: Obtain AP and lateral knee x-ray if diagnosis is in doubt. You may only see an effusion if the joint has spontaneously reduced.
  • Treatment: Immediate reduction by applying longitudinal traction followed by knee immobilization in 20 degrees of flexion [3]. Neurovascular status of the extremity should be documented and the patient should be hospitalized with orthopedics consultation [2]. Controversy exists regarding when to perform arteriography [2]. Some argue serial Ankle Brachial Indices are sufficient while others argue angiography should be performed [2]. Have a low threshold to consult your orthopedic surgeon.

Figure 1: AP and lateral of knee dislocation. Case courtesy of Dr Domenico Nicoletti,

  • Epidemiology: This pathology is relatively rare, but knee extensor injuries require a high index of suspicion as initial miss rates have been as high as 10-50% [3]. Quadriceps injuries are more common in patients over the age of 40 [2].
  • Mechanism: These injuries typically occur as a result of sudden, violent contract of knee extensor, usually with the foot planted and knee partially flexed [3]. For quadriceps injuries, a patient may miss a step or step into an unseen hole and report sudden pain or stumbling injury. Patellar injuries may occur as a result of a direct blow [3].
  • Symptoms: Patients may experience a “pop” with either injury [3].
  • Physical Exam: Tenderness and swelling at the site of injury. May feel a palpable defect. The inability to raise a straightened leg while fully supine, complete inability to extend the knee, or to maintain position after passive extension suggests rupture of either tendon.
  • Diagnostic Imaging: Lateral x-ray of the knee will best demonstrate. Depending on which tendon is ruptured, will see either a high-riding or low-riding patella.
  • Treatment: If complete rupture is expected, the knee should be immobilized in full extension and patient should be made non-weight bearing [3]. Typically, surgical repair should be performed within the first 7-10 days [2].

Figure 2: Quadriceps rupture. Note the “low-riding” patella. Case courtesy of Radswiki,

  • Epidemiology: 1% of skeletal injuries [4].
  • Mechanism: Typically result from a direct blow to the knee.
  • Symptoms: Tenderness and swelling at patella but are often still ambulatory [3].
  • Physical Exam: Swelling, ecchymosis, and tenderness over patella.
  • Diagnostic Imaging: AP, lateral, and sunrise view.
  • Treatment: Knee immobilizer in full extension, rest, ice, non weight bearing [2]. Make sure to note if there is significant displacement > 3 mm as patient should have early referral for operative repair [2].

Figure 3: Patellar fracture with significant displacement. Case courtesy of Dr Jeremy Jones,

  • Epidemiology: Very difficult to detect and more often seen in elderly population [2].
  • Mechanism: Can be caused by combination of axial, compressive load and a varus or valgus force [3]. May be seen in ground level falls in elderly. Sixty percent of the time, they involve the lateral tibial condyle [3]. Bicondylar lesions make up 25% of tibial plateau fractures [3].
  • Symptoms: Lateral knee pain. Usually unable to bear weight on affected extremity.
  • Physical Exam: Tenderness at proximal tibia [3]. Inability to walk.
  • Diagnostic Imaging: Oblique knee views may increase sensitivity from 79 to 85% [5]. May only see Lipohemarthrosis. Have a low threshold for CT If your x-rays are negative but you maintain high pretest probability for tibial plateau fracture.
  • Treatment: Although rare, make sure to assess for compartment syndrome and neurovascular status. One study found non-contiguous tibial fracture patterns or knee dislocations and higher grade AO/OTA classification were associated with compartment syndrome [6]. Consult with Orthopedics. If minimally displaced, management may be non-operative and non-weight bearing.

Figure 4: AP and lateral demonstrating tibial plateau fracture. Case courtesy of Dr Derek Smith,

  • Epidemiology: Associated with Anterior Cruciate Ligament (ACL) injury.
  • Mechanism: Similar to ACL injury pattern. Non contact pivot injury or lateral blow to knee. May describe a “pop”.
  • Symptoms: Diffuse knee pain.
  • Physical Exam: Knee effusion.
  • Diagnostic Imaging: See below. Note: A normal knee injury does not rule out a ACL injury but a “Segond Fracture” may indicate an injury has occured
  • Treatment: Rest, ice, analgesia. Close orthopedics follow up. Crutches and knee immobilizer for comfort, though it is not necessary to give knee immobilizer. If a knee immobilizer is given, instruct the patient to perform daily range of motion exercises to avoid contracture and maintain mobility [2].

Figure 5: Segond fracture. Case courtesy of Gerry Gardner,

  • Epidemiology: Indicative of injury to the posterolateral corner and associated with cruciate ligament injury. Undiagnosed injuries can lead to unstable knee or early osteoarthritis [7].
  • Mechanism: Direct blow with the tibia in external rotation or sudden hyperextension of the knee with the tibia internally rotated [8]. May also occur as a result of “Dashboard” injury [9].
  • Symptoms: Posterolateral knee pain.
  • Diagnostic Imaging: “Arcurate Sign” best seen on AP view.
  • Treatment: Knee immobilization and close orthopedic follow up.

Figure 6: Case courtesy of Dr Haytham Bedier,

Don’t forget to examine the hip! Hip fractures can often present as knee pain. If the patient reports no trauma, or minimal trauma that does not explain their level of pain or disability, consider septic joints.

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

Want a basic approach to traumatic knee imaging? Check out EMRad’s approach to the traumatic knee x-ray. 


  1. 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
  2. Glaspy, J et al. Chapter 271. Knee Injuries. In: Tintinalli’s Emergency Medicine. A Comprehensive Guide, 8th edition. New York: McGraw-Hill Education, 2016.
  3. 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
  4. Gwinner, C. Current concept Reviews: Fractures of the patella. GMS Interdiscip Plast Reconstr Surg DGPW. Volume 18. Issue 5. Jan. 2016 PMID 26816667
  5. Gray, S. et al. Acute knee trauma: how many plain film views are necessary for the initial examination. Skeletal Radiol. Volume 5. May 1997. PMID 9194231
  6. Gamulin, A, et al. Clinical and radiographic predictors of acute compartment syndrome in the treatment of tibial plateau fractures: a retrospective cohort study. BMC Musculoskeletal Disorders. Volume 18. Issue 1. July 2017. PMID 28720096 
  7. Markeson, CD et al. Arcuate Sign in posterolateral corner injury of the knee. J Emerg Med. Vol 19. Jan 2020.PMID: 32001118
  8. Strub, W. The Arcuate sign. Radiology. Volume 244. No. 2. August 2007. PMID 17641383
  9. Nannaparaju, M. Posterolateral corner injuries: Epidemiology, anatomy, biomechanics and diagnosis. Injury. Volume 49. Issue 6. June 2018. PMID 29254623
Stephen Villa, MD

Stephen Villa, MD

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