Trauma X-ray - Lower limb
Knee

Key points

  • The Horizontal Beam Lateral view allows identification of a knee joint effusion or lipohaemarthrosis (fat and blood in the joint)
  • Tibial plateau fractures can be very subtle and lipohaemarthrosis may be the only visible sign

Standard views

Anterior-Posterior (AP) and Lateral. In the context of trauma the Lateral view is acquired with the patient lying supine and with a horizontal X-ray beam. This allows effusions to be visualised in the suprapatellar pouch.

Knee - Normal AP

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Knee - Normal AP

  • The patella is often not clearly seen on this view

Knee - Normal Lateral (Horizontal Beam)

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Knee - Normal Lateral (Horizontal Beam)

  • The Horizontal Beam Lateral view is useful for assessing soft tissues as well as bones
  • The quadriceps and patellar tendons are visible
  • Note the normal suprapatellar pouch between fat pads above the patella (asterisks) - widening of these fat pads or increased density in this area can indicate a knee joint effusion

Skyline view

A 'Skyline' or 'Sunrise' view is rarely indicated in the context of trauma. This view is only necessary if the standard views are normal and a patellar fracture is still suspected, or to assess patellar dislocation. A skyline view can only be acquired if the patient can tolerate knee flexion.

Knee - Normal 'Skyline' view

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Knee - Normal 'Skyline' view

  • Not usually indicated in the context of trauma
  • More helpful to assess knee pain due to suspected patellofemoral compartment osteoarthritis
  • Normal patellofemoral compartment spacing (arrowheads)

Tibial plateau fractures

Fractures of the tibial plateau can be subtle with little displacement, or can be widely displaced, with varying degrees of comminution. There may be depression of the plateau surface, displacement of a fracture fragment, or a combination of both.

Not infrequently the formation of a lipohaemarthrosis is the only radiological sign. Lipohaemarthrosis is a layered effusion of fat and blood which has 'leaked' from the bone following a fracture.

Tibial plateau fracture - AP

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Tibial plateau fracture - AP

  • (Same patient as below)
  • Lateral tibial plateau fracture
  • The fracture fragment is displaced and depressed from its normal position (dotted line)

Tibial plateau fracture - Lateral

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Tibial plateau fracture - Lateral

  • (Same patient as above)
  • No visible fracture line
  • Depressed tibial plateau contour (arrow)
  • Lipohaemarthrosis (fat and blood in the joint)

Patellar injury

Fractures of the patella may only be visible on 1 of the 2 standard views, more often the lateral view.

Patellar fracture - Lateral

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Patellar fracture - Lateral

  • Increased density separating the fat pads indicates a joint effusion due to leakage of blood (haemarthrosis)

Patellar dislocation - Skyline view

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Patellar dislocation - Skyline view

  • The patella is grossly displaced
  • The roll over image shows its normal position

Normal variants

A fabella and a bipartite patella are 2 common normal variants that should not be mistaken for a fracture.

Knee - Fabella

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Knee - Fabella

  • A fabella is a normal sesamoid bone of the lateral head of gastrocnemius tendon - not to be mistaken for a fracture or loose body

Bipartite patella

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Bipartite patella

  • The patella is bipartite (in 2 parts) - a common normal variant
  • Note: Injury to the interface of the 2 components is possible which may be symptomatic