Distal Pole Excision for Scaphoid Nonunion Advanced Collapse Wrist

Abstract Excision of the distal pole of the scaphoid is a surgical procedure utilized to treat scaphoid nonunion advanced collapse (SNAC) wrist as well as scaphotrapezial trapezoidal (STT) joint osteoarthritis. Clinically, some patients treated with this method develop midcarpal joint instability and possibly symptomatic arthritis. Some studies suggest that the amount of distal pole removed affects the tendency toward collapse. However, there may be other factors affecting the collapse of the midcarpal joint, such as ligamentous injury and the location of the nonunion. Our purpose was to determine the resulting displacements through the midcarpal joint after distal pole excision for a SNAC wrist. We hypothesized that the clinical cases of midcarpal joint collapse will demonstrate changes in the direction of force transfer through the midcarpal joint following excision of the distal pole, and that these results will differ from excision of the distal pole in a normal joint. This study analyzed three anonymized wrist computed tomography (CT) images. Two of the samples had a scaphoid nonunion treated with excision of the distal pole of the scaphoid with consequent midcarpal joint collapse, and one had a normal wrist CT. Using finite element methods, displacement and stress were predicted. Model-based intracarpal angles pertaining to the midcarpal joint were measured. There was an increased overall translation of the carpal bones with excision of the distal pole of the scaphoid in the SNAC and normal wrists. With distal pole excision, the carpal bones moved volarly, proximally, and radially. The collapsed SNAC wrists demonstrated preoperative changes of the capitate position on the lunate. This study supports indicating excision of the distal pole of the scaphoid for a SNAC wrist if the wrist has a close-to-normal capito–lunate angle and scapho–lunate angle at presentation, with a distal nonunion location, and a type-2 (not type-1) midcarpal joint. However, there may not be a direct association between clinical results and these mechanical predictions. Therefore, though this information may be used to counsel patients when treating a symptomatic SNAC wrist, further study is needed to better define the factors affecting instability following scaphoid distal pole excision. 5.