Knee (skyline Laurin view)

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

Indication

This view is used in trauma to assess for a patellar fracture or subluxation and in orthopedics for patellofemoral joint disease.¹ It is best suited to patients able to maintain a semi-recumbent position on the examination table.

Patient position

• the patient is semi-recumbent on the table holding a detector superior of the patella in the landscape orientation 
• patient's feet should be very close to the tube side of the bed (see technical factors)
• the knee is bent close to 30°
• often a pillow or cushion should be placed behind the patient to assist them in maintaining this position 

Technical factors

• inferior-superior axial projection²
• centering point
  • the central ray will be angled 160° from the vertical axis (or 30° from horizontal), shooting inferior-superior towards the patella. This will require the tube to lay below the level of the examination table; hence the patient should be as close to this end of the table as possible. 
  • the apex of the patella
• collimation
  • laterally to include the skin margins of the knee 
  • inferior to include the femoropatellar joint space 
  • superior to include medial 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 the 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, by assessing the lateral projection and working out the optimal angle from the inferior-superior approach can be beneficial
• patients feet
  • the patient's feet will be at the end of the table and often if not careful; the skyline projection may also be a heavily magnified projection of the distal phalanges; ensure the patient's feet are plantar-flexed/out of the primary beam
• detector 
  • it is possible to use a detector stand rather than asking the patient to hold the detector; this alleviates the risk of motion artefact
• 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