Excision of the Proximal Pole of the Scaphoid Affects the Midcarpal Joint

Excision of the proximal pole of the scaphoid causes wrist instability. The often small proximal fragment can be challenging to retain, and multiple procedures are used to preserve the proximal pole. However, the mechanical implications of a proximal pole loss remain unclear. Possibly, under certain circumstances, the proximal pole can be excised without causing instability.

This study aimed to use a validated finite element model (FEM) of normal Type 1 and Type 2 wrists to examine the effect of removal of the proximal scaphoid pole on forces moving through the midcarpal joint. We hypothesize that the amount of bone removed alters the transfer of forces through the midcarpal joint and that this effect will differ between Type 1 and Type 2 wrists.

Fourteen wrist computed tomography (CT) scans were converted to .stl files: five Type 1 and nine Type 2. Using an established wrist FEM model, the proximal scaphoid was sequentially excised up to 50%. A 100-N computer-generated load was applied to the dorsal crests of the trapezoid and capitate. Carpal bone displacement was recorded in the x, y, and z directions. The percentage change from intact was calculated for Type 1 and 2 wrists, the direction of bony translation was recorded, and an index quantifying instability based on translation upon loading was calculated.

The excision of the proximal pole of the scaphoid caused significant translation of the carpal bones in both wrist types, p  < 0.001. Type 2 wrists reached significant displacement after 15% excision, compared to 25% for Type 1 wrists. The direction of translation differed between Type 1 and 2 wrists.In general, Type 1 wrists seem to be less stable than Type 2 wrists.

This study supports the importance of scaphoid proximal pole for wrist stability and the difference in carpal-bone translation between midcarpal joint types.This can be used in surgical planning when faced with fractures, nonunion, and complex fracture-dislocations.