A patient falls on his outstretched hand and has normal appear xrays but tenderness in the "anatomic snuff box" (between extensor pollicis longus and abductor pollicis longus/extensor pollicis brevis).
Why might such a patient be placed in a cast despite the normal x-ray?
Two anatomy pictures to keep in mind: first, the scaphoid bone, shown in red
next, the anatomic snuff box denoted by the arrow on the right---the point on the surface at which the scaphoid is palpated.
A fall on an outstretched hand can fracture the scaphoid bone.
Yet (probably owing to the scaphoid's geometry and location), a non-displaced fracture of this bone may not be discernable on a plain x-ray.
(Below is the MRI of a patient with a normal x-ray)
A patient with a fall and snuff box tenderness may have a non-displaced fracture.
In the event of a suspected scaphoid injury, it is imperative to prevent displacement of the fracture and disruption of the blood supply.
Blood supply to the scaphoid bone is tenuous as most of the scaphoid surface is cartilage--this leaves only a small area for arterial blood supply to enter the bone from the dorsal carpal branch of the radial artery and feed the bone in retrograde fashion.
If the fracture displaces, the blood supply may be interrupted and avascular necrosis of the scaphoid may result.
Preventative casting (to prevent fracture displacement) and follow up radiograph imaging is the standard of care following a scaphoid injury. (In the alternative, an MRI may be used to exclude a non-displaced fracture as shown above. This indeed may be cheaper in the long run, to say nothing of kinder: an unnecessary cast is a burden).