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The Mechanical Effect of Scaphoid Distal Pole Excision
Ronit Wollstein, MD
1; Nicholas Parody, Bsc
2; Sallie Yassin, MS
3; Paul Izard, BS
3; Steven Z Glickel, MD
41UAB Heersink School of Medicine, Birmingham, AL; 2NYU Grossman School of Medicine, New York, NY; 3NYU Langone Health, New York, NY; 4New York University School of Medicine, New York, NY
Introduction: One surgical option to treat arthritis following scaphoid nonunion is excision of the distal (ununited) pole of the scaphoid. This procedure can remove the arthritic surfaces without the need for a salvage procedure such as a four-corner fusion. However, some patients develop midcarpal instability thus limiting use of this technique. We aimed to try and identify those wrists prone to developing midcarpal-joint instability by evaluating the effect of midcarpal joint structure on force- transfer through the wrist. We hypothesized that midcarpal joint structure will affect force-transfer through the midcarpal joint following excision of the distal pole. Materials and Methods: Using a model based on CT scans of the wrist, finite element analysis was performed on 19 CT wrist scans. Ten type 2 and 9 type 1 models were prepared.
A 100 Newton (N) force was applied to the dorsal crests of the trapezoid and capitate (along each crest) using data from 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 model was then 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 when compared to a wrist with an intact scaphoid (about 200% all bones in all directions). There are significant differences in force-transfer between type 1 and type 2 midcarpal joint structure in the amount of force transferred (type1>type 2), in the percent difference from an intact wrist (type1>type 2) and in the direction of displacement (type 1 the bones have a different pattern of displacement than type 2 midcarpal joint wrists). Conclusions:
This study suggests that midcarpal-joint structure affects the way force is transferred through the wrist and may therefore affect stability after excision of the distal scaphoid. Specifically type 1 wrists may be more prone to midcarpal joint collapse after excision.
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