Knee (skyline Merchant view)

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The knee skyline Merchant view is a superior-inferior projection of the patella. It is one of many different methods to obtain an axial projection of the patella.

Clinical Indications

This view is used in trauma patients to assess for a patellar fracture or subluxation and in orthopedics for patellofemoral joint disease. This is an ideal projection for patients that are better suited to the supine position.

Patient position

  • patient is supine on the table with both knees flexed at roughly 45°
  • patient's feet should be very close to the detector end of the bed (see technical factors)

Technical factors

  • superior-inferior axial projection
  • centering point
    • the central ray will be angled 160° from the vertical axis (or 30° from horizontal), shooting superior-inferior towards the base of the patella. This will require the tube to sit close to the patient's upper body.
    • the detector will be angled at roughly 30° at the foot end of the bed in line with the tube, ensuring the toes are not in the way
  • collimation
    • laterally to include the skin margins of the knee 
    • inferior to include the femoropatellar joint space 
    • superior to include skin margin
  • orientation  
    • landscape
  • detector size
    • 18 cm x 24 cm
  • exposure
    • 60-70 kVp
    • 7-10 mAs
  • SID
    • 100-120 cm
  • grid
    • no

Image technical evaluation

  • patella should be free from superimposition of all bony structures
  • clear visualization of the patellofemoral joint space 

Practical points

This projection is one of the more technically demanding projections of the lower limb. Hence it being one of seven techniques (that the author can find) to achieve it.

This particular method has a high yield if your patients can tolerate the position. Some points to consider when performing this projection:

  • dose
    • this projection often requires more dose than conventional knee radiographs due to tube angulation and, more often than not, a larger FFD/SID
  • tube angle 
    • 30° from horizontal is the academically acceptable angle for this technique, however, assessing the lateral projection and working out the optimal angle from the inferior-superior approach can be beneficial
  • patient's toes
    • the patient's toes will be at the end of the table and often, if not careful, can get in the way of the projection
    • if the patient cannot move their feet to be flat (this is common) you may adjust the tube angle to compensate (within reason)
  • detector 
    • often this projection requires the use of either an angled wall detector or a free-standing detector in a detector holder; if this is not possible, see the Laurine method
    • if the patient is on a trolley bed with bed ends, it is possible to rest the detector on the bed end, ensuring that it aligns with the tube angle
  • the pen test 
    • turning the collimator light on, hold the other end of a pen and place it on the lateral border of the patella, if the patient is positioned correctly, the pen will cast a shadow on the detector
  • anatomical presentation
    • remember to flip the image to display it as standard radiographic anatomy convention or as per department's protocol