The wrist is one of the most commonly requested X-Rays in the children’s emergency department. Wrist views are requested when injury to the distal radius/ulna or carpal bones are suspected. Below is a systematic approach to interpretation.
The wrist series examines the carpal bones (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate and hamate), the radiocarpal joint and the distal radius and ulna.
There are eight carpal bones present and each one is named according to its shape:
- Scaphoid (boat-shaped)
- Lunate (crescent moon-shaped)
- Triquetrum (pyramidal)
- Pisiform (pea-shaped)
- Trapezium (irregular trapezium-shaped)
- Trapezoid (wedge-shaped)
- Capitate (head-shaped) – *the largest of the carpal bones
- Hamate (wedge-shaped with a bony extension, or ‘hook’)
|Proximal carpal row
|Distal carpal row
How to best remember the carpal bones
There are many mnemonics around – some too rude for mention here! You will need to find the one that works for you… here’s one that’s super suited for clinicians working with kids:
Sam Likes To Push The Toy Car Hard
Failing that, save an image to your phone for quick reference!
The carpal bones are formed entirely from cartilage at birth – this is important from a radiological viewpoint as it means they are not visible on x-ray initially. They begin to ossify from about 1-2 months of age and are fully developed by the age of 8-12 years. Although there is variability in the timing, the order is always the same.
- Capitate 1-3 months
- Hamate 2-4 months
- Triquetrum 2-3 years
- Lunate 2-4 years
- Scaphoid 4-6 years
- Trapezium 4-6 years
- Trapezoid 4-6 years
- Pisiform – 8-12 years
Generally, on x-ray, one carpal bone is visible every year until full development – this acts as a handy (pun intended) ageing tool!
On requesting wrist X-Rays, most commonly you will receive posteroanterior and lateral projections, with oblique views forming part of the series usually when carpal injury is suspected.
1. Check the soft tissues
Look for signs of swelling or any incidental findings.
2. Trace the bony cortices
Trace each bone in turn to look for breaks or irregularities in the cortex.
Look closely at the distal radius, proximal carpal row (especially the scaphoid) and the proximal metacarpals. Disruptions in the cortex may be very subtle as in the case of this torus fracture (aka a buckle fracture)
3. Check bony alignment
On the AP view:
The distal radial articular surface should curve round the carpals with the articular surface getting more distal towards the ulnar styloid. The articular surfaces of the proximal and distal carpal rows should form three smooth arcs – these can be traced on the AP film.
The spacing between all carpal bones should be 1-2mm.
If the arc is broken or there is widening or lack of uniformity between the spaces, think about carpal dislocation.
The articular cortex at the base of each metacarpal parallels the articular surface of the adjacent carpal bone.
The carpo-metacarpo (CMC) joint spaces should be clearly seen and of uniform width (1-2mm).
The 2nd to 5th CMC joints are visualised as a zigzag tram line – on a normal view, there will always be the “light of day” seen between the bases of the 4th and 5th metacarpals and the hamate bone. If this is narrowed, think dislocation of the 4th or 5th metacarpal.
On the lateral view:
The distal radius, lunate and capitate should articulate with each other in a straight line on the lateral x-ray – the apple, cup, saucer analogy – the cup of the lunate should never be empty.
If the cup is empty, this suggests a perilunate dislocation.