The stability of C1-2 depends mainly on the transverse ligament. When looking at peg view, watch out for these three things:
1. Vertical Alignment of lateral margins of C1 and C2 (Vertical white lines in the image below) - If there is vertical misalignment of the masses then consider four possibilities - ligament injury, burst fracture of C1 (Jefferson #), rotation or developmental variation.
Slight neck rotation can often lead to unequal appearing spaces on either side but if this is the case, then lateral masses of C1 and C2 remain normally aligned.
2. Spaces on both sides of Peg should be approximately equal (Horizontal white lines in the image below) - Normal distance between peg and the lateral masses on each side is between 3-5mm.
|Laterally displaced lateral masses (Jefferson #))|
|Rotated film leading to unequal distance on one side of peg|
3. Look if there is a fracture on the base of the peg
|Make a note of the horizontal black line crossing the base of peg - MACH EFFECT|
Images taken from: