Knee (horizontal beam lateral view)

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The horizontal beam lateral view (cross-table lateral) is an orthogonal view of the AP view of the knee requiring little to no patient movement and is hence the lateral projection of choice for acute knee injuries.

Indications

This view is the ideal projection to assess for lipohemarthrosis as it demonstrates the region of subpatellar bursa and associated fat pads for possible displacement or presence of fluid levels from knee pathology . Knee pathology can include fracture or dislocation of the femur, tibia, fibula or patella.

Patient position

  • the patient is supine on the table/bed 
  • affect knee is flexed slightly ≈ 30° (to the best of patient's ability)
  • the detector is placed against the medial side of the knee running parallel to the affected leg, often held by the patient or sandbags 
  • the long axis of the femur is running perpendicular to the beam 

Technical factors

  • lateromedial projection
  • centering point
    • center to the knee joint 1.5-2.0 cm distal to the apex of the patella or at the tibial tuberosity if the patella is affected by certain injury patterns
  • collimation
    • superior to include the distal femur
    • inferior to include the proximal tibia/fibula
    • anterior to include the skin margin 
    • posterior to include skin margin
  • orientation  
    • landscape
  • detector size
    • 35 cm x 43 cm
  • exposure
    • 60-70 kVp
    • 7-10 mAs
  • SID
    • 100 cm
  • grid
    • no

Image technical evaluation

A true horizontal beam lateral projection will have the following characteristic:

  • superimposition of the medial and lateral condyles of the distal femur 
  • an open patellofemoral joint space 
  • slight superimposition of the fibular head with the tibia 

Practical points

It is easy to describe how a horizontal beam knee should turn out, that is everything should superimpose. To achieve this can be technically demanding.

Correcting rotational errors 

The distal femoral condyles have distinct features that can be used for differentiation and hence positional errors that can be corrected. The medial condyle has a medial adductor tubercle whilst the lateral condyle has a lateral condylopatellar sulcus.

When the resultant image does not demonstrate superimposition of the two condyles in the rotational plane, look out for these anatomical landmarks to determine if the knee needs to be externally or internally rotated.

  • figure 2 demonstrates the medial condyles adductor tubercle free from superimposition in the posterior portion of the image, and this means the leg is internally rotated too much. Correct this by externally rotating the leg
  • when the medial adductor tubercle is projected overly anterior to the lateral condyle (figure 6) the leg can be internally rotated to adjust it

To summarize, if the medial adductor tubercle is not superimposed, projecting posteriorly in the image rotate the knee externally.

If the lateral condyle significantly superimposes the medial adductor tubercle the knee must be internally rotated.

Correcting tube angle errors 

When the femoral condyles are projected unevenly in the inferior-superior plane, this is due to tube angle. This can be challenging to correct, but it's best only to change one factor; modify the tube angle do not move the patient and vice versa.

Using the anatomical landmarks discussed above find the medial adductor tubercle, and establish the medial condyle.

  • when the medial condyle is projected superior to the lateral condyle, the tube should be angle cephalad  
  • medial condyle is project inferior (figure 3) to the lateral condyle a caudal angle is required