Knee radiograph (an approach)

Last revised by Daniel J Bell on 12 Sep 2022

Knee radiographs are common and often a quick and easy diagnostic exam in the emergency setting. An efficient approach to them requires a good understanding of anatomy with review strategies to ensure an accurate diagnosis.

Choosing a search strategy and utilizing it consistently is a helpful method to overcome common errors seen in diagnostic radiology. The order in which you interpret the radiograph is a personal preference. A recommended systematic checklist for reviewing musculoskeletal exams is: soft tissue areas, cortical margins, trabecular patterns, bony alignment, joint congruency, and review areas. Review the entire radiograph, regardless of perceived difficulty. Upon identifying an abnormality, do not cease the review, put it to the side and ensure to complete the checklist.

Assess all soft tissue structures for any associated or incidental soft tissue signs. In the case of the knee, it will involve the detection of secondary signs such as effusion or soft tissue swelling. 

Check for an effusion on the horizontal beam lateral:

Trace the cortex of each bone paying particular attention to regions that are superimposed such as the fibular head or patella. 

  • carefully look for a proximal tibial fracture
  • pay particular attention to:
    • tibial spine: avulsion
    • lateral tibial plateau: small avulsion (Segond fracture)
    • areas of increased density may point to underlying fracture
  • medial epicondyle: don't overcall calcification adjacent to the medial femoral epicondyle (Pellegrini-Stieda lesion)
  • fractures are usually easy to spot, often transverse
  • don't call a bipartite patella or tripartite patella fractures: well-corticated unfused center(s) at the superolateral pole
  • subtle avulsion fractures can be hard to spot ensure to check the origins of the:
    • medial collateral ligament
    • lateral collateral ligament
    • anterior cruciate ligament
    • posterior cruciate ligament
  • trace the articular surface keeping in mind the chance of an osteochondral defect

One should inspect for smooth, concurrent bony alignment in all views.

Small avulsion fractures of the knee more often than not are associated with instability and internal derangement. Careful scrutinisation of the origins and insertion points of ligaments is a must. From lateral to medial, superior to inferior, ensure to inspect 1:

  • origin of the lateral collateral ligament at the lateral femoral condyle
  • origin of the anterior cruciate ligament at the posterior-lateral portion of the intercondylar notch femoral condyle
  • insertion of the lateral capsule at the lateral tibia (at the joint line)
  • insertion of the arcuate ligament at the fibular styloid
  • insertion of the lateral collateral ligament and the bicep femoris tendon (conjoint tendon) at the fibular head
  • insertion of the iliotibial band at Gerdy tubercle of the tibia
  • origin of the medial collateral ligament at the medial femoral condyle
  • origin of the posterior cruciate ligament at the anterior middle portion of the medial condyle
  • insertion of the deep meniscofemoral ligament of the medial collateral ligament at the medial tibia (at the joint line)
  • insertion of the superficial fibers of the medial collateral ligament at the anteromedial tibia 5 cm distal to the tibiofemoral joint
  • anterior cruciate ligament insertion at the medial portion of the tibial spine
  • posterior cruciate ligament insertion at the posterior medial portion of the tibial plateau

It is worth spending extra time scrutinising areas of superimposition such as the fibular head and patella. 

  • fat and blood from bone marrow collect in suprapatellar bursa
  • a fat-fluid level may be the only sign of intra-articular fracture
  • associated with tibial plateau or distal femoral fractures
  • more: lipohemarthrosis
  • 80% involve the lateral plateau
  • fall from height or car bumper impact
  • associated significant cruciate and medial collateral ligament damage
  • more: tibial plateau fracture
  • avulsion fracture; bony fragment adjacent to lateral tibial plateau
  • internal rotation and varus stress; falls or sports
  • 75% associated with anterior cruciate ligament injury
  • more: Segond fracture
  • typically avulsion fracture of tibial attachment of anterior cruciate ligament
  • mechanism: rapid deceleration or hyperextension of the knee
  • most common in adolescents
  • more: intercondylar eminence fracture
  • majority transverse, also vertical or comminuted
  • direct blunt force or violent contraction of quadriceps
  • oblique or skyline views will confirm fractures
  • more: patella fracture
  • 6% of all femur fractures
  • bimodal distribution
  • high energy blunt trauma; falls in elderly
  • more: distal femoral fracture
  • typically occur with lateral tibial plateau fractures, but may be isolated
  • varus force
  • associated with lateral collateral ligament damage
  • more: proximal fibula fracture
  • post-traumatic soft-tissue calcification adjacent to medial epicondyle of femur
  • ossification following injury to medial collateral ligament
  • do not misdiagnose as a fracture
  • more: Pellegrini-Stieda lesion

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Cases and figures

  • Figure 1: effusion ?
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  • Case 1: lipohemarthrosis
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  • Figure 2: tibiofemoral alignment
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  • Case 2: lateral tibial plateau fracture
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  • Figure 3: plateau review
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  • Case 3: Segond fracture
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  • Figure 4: patella
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  • Case 4: tibial eminence fracture
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  • Figure 5: radiographic anatomy - AP view
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  • Case 5: patella fracture
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  • Figure 6: knee radiograph checklist (illustration)
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  • Figure 6: radiographic anatomy - lateral view
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  • Case 6: distal femoral fracture
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  • Case 7: fibular head fracture
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  • Case 8: Pellegrini-Stieda lesion
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