Why should a patient with negative x-rays but snuff box tenderness be placed in a cast?

 

A patient falls on his outstretched hand and has normal appearing x-rays but tenderness in the “anatomic snuff box.” Why might such a patient be placed in a cast despite the normal x-ray?


With any fall on an outstretched hand, there is concern for a scaphoid fracture (see Figure 1). 

Figure 1: Scaphoid fracture, outlined in red. (modified from radiopaedia.org/cases/scaphoid-fracture-6)


The scaphoid bone is located on the thumb side of the wrist (see Figure 2) and is palpable in the anatomic snuff box (see arrow, in the figure below). Thus, a fracture of the scaphoid will produce tenderness near the anatomic snuff box.

 Figure 2: Scaphoid outlined in red

Figure 3: the white arrow points to the center of “anatomic snuff box.” The lateral (radial/thumb sided) border is formed by the extensor pollicis brevis and abductor pollicis longus tendons. The medial (ulnar) border is the extensor pollicis longus tendon. The scaphoid lies directly below the skin here.


Because the scaphoid is not perfectly parallel with the wrist and palm, it can be difficult to detect nondisplaced fractures on routine radiographs. In cases where the x-ray is negative but there is snuff box tenderness, the wrist should be immobilized. 


Immobilization of the wrist is crucial to prevent displacement of any fracture that may be present, and in turn, prevent osteonecrosis of the scaphoid.


Clearly, nondisplaced fractures are at risk of displacement if they are not immobilized. Displacement is bad in and of itself, but in the case of a displaced scaphoid fracture, there is an increased risk of osteonecrosis because of the scaphoid’s (so-called) retrograde blood supply.


The scaphoid’s surface is largely covered in cartilage, leaving only a small area on the scaphoid for arterial blood to enter the bone via the dorsal carpal branch of the radial artery. (The talus, another cartilage-covered bone, is also one prone to osteonecrosis.)


The scaphoid receives its primary blood supply from the dorsal scaphoid branches of the radial artery, which enter the scaphoid at a point near the distal pole. As such, the scaphoid is perfused in the distal to proximal direction. (Because of this, the blood supply to the scaphoid is termed "retrograde.”)


Because of this retrograde blood supply, displacement of a fracture across the scaphoid waist (i.e. the middle third of the bone) will interrupt blood supply to the proximal pole. The loss of blood supply will of course deprive the bone of oxygenation and nutrients.


The standard of care for a potential scaphoid fracture is preventative casting for 2-3 weeks followed by repeat radiographic imaging. (The logic is that if a fracture were present, either a fracture line or a healing response will be seen.)


In some cases, a patient will initially undergo an MRI to rule out a displaced scaphoid fracture following a negative x-ray. While use of MRI is not the standard of care across the country, it can eliminate the need for unnecessary casting and potentially prove to be more cost effective than repeat visits and x-rays. (Moreover, an immediate MRI can spare one the hassles of wearing a cast for weeks for what turns out to be a bruise.)