Thumb (AP/PA view)

The thumb AP (anteroposterior) view is one of the standard views for assessment of the thumb. The PA (posteroanterior) view can be utilized when the patient is unable to achieve the position required for the AP view. They are one part of the three view thumb series.

There will be some magnification with the PA view.

Indications

The AP thumb can be requested for many reasons including trauma, suspected arthritic changes, or foreign body to name a few. It is an ideal view to inspect the metacarpal joint spaces, as well as the regions of ligamentous attachments, it may be the only view in which one could observe a collateral ligamentous injury such as a gamekeeper's thumb.

Patient position

AP thumb

• patient is seated alongside the table
• the arm is extended and medially rotated so as bring the dorsal aspect of thumb in contact with the cassette (see Figures 1 and 2)

PA thumb

• patient is seated alongside the table
• the thumb rests on a sponge, ensuring that the entire thumb is in the same horizontal plane.

Technical factors

• posteroanterior/anteroposterior projection
• centering point
  • base of the thumb
• collimation
  • laterally to the skin margins
  • distal to skin margin
  • proximal to the carpometacarpal joint 
• orientation  
  • portrait
• detector size
  • 18 cm x 24 cm
• exposure
  • 50-60 kVp
  • 3-5 mAs
• SID
  • 100 cm
• grid
  • no

Image technical evaluation 

1st digit is positioned PA, evidenced by the symmetric appearance of the concave aspects of the phalanges and metacarpals. Correct alignment in the horizontal plane is evidenced by open carpometacarpal and interphalangeal joint spaces.

Practical points 

A true AP projection can be very difficult to achieve with an acutely injured patient, although the PA view does result in magnification it can be the only option when the patient is in pain.