Excision of the Distal Pole of the Scaphoid and the Midcarpal Joint

Background  Excision of the distal pole of the scaphoid is used to treat arthritis of the scaphotrapezial trapezoid (STT), radioscaphoid joint, and arthritis following scaphoid nonunion. Some patients develop midcarpal instability limiting utilization of this technique. Why some wrists develop postoperative instability while others do not, remains unclear. Purpose  To identify the wrists prone to developing midcarpal joint instability we evaluated the effect of midcarpal joint structure on force transfer through the wrist, we hypothesized that the force transfer will be further altered when a distal pole excision is performed and that midcarpal joint structure will affect force transfer. Materials and Methods  We used finite element analysis based on 19 computer tomography wrist scans. Nine type 1 (lunate has a facet with the capitate alone) and 10 type 2 (lunate has facets with both the capitate and hamate) models were prepared. A 200 N force was evenly split and applied to the dorsal crests of the trapezoid and capitate (100 N along each crest) to replicate the performance of a knuckle push-up. Displacement of the trapezoid, trapezium, scaphoid, capitate, and hamate was measured along each axis after the applied load. The simulation model was used to predict motion at the capitate and STT joint with excision of the distal pole. Results  Excision of the distal pole of the scaphoid affected the transfer of forces significantly (∼200% all bones in all directions) in all wrists. There are significant differences in force transfer between type 1 and type 2 wrists in the amount of force transferred (type 1 > type 2), in the percent difference from an intact wrist (type 1 > type 2) and in the direction of displacement (type 1 the bones moved in different directions while type 2 wrists moved as one block). Conclusion  This study suggests that midcarpal joint structure affects force transfer through the wrist and may predict wrist behavior following excision of the distal pole of the scaphoid. Specifically, type 1 wrists may be more prone to midcarpal joint collapse after excision. Level of Evidence : Level 1.