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Determining the Accuracy of Scaphoid Screw Position with Fluoroscopy and Computed Tomography
Marissa Rae Matarrese, MD; J. Andrew Grotting, MD; Christopher Cerniglia, DO; Marci D. Jones, MD
University of Massachusetts, Worcester, MA

Introduction: Longer screws positioned centrally within the scaphoid are biomechanically advantageous. Accurate measurement of screw length is critical, as a screw that is too long can protrude into the joint and cause pain, but one that is too short may not adequately purchase the fractured fragment. The unique anatomy of the scaphoid makes intraoperative and postoperative imaging of the hardware challenging and can cause the surgeon to wonder if fracture fixation is adequate. The purpose of this cadaveric study was to determine whether fluoroscopic and computed tomographic images accurately determine the distance from the end of a headless cannulated screw to the subchondral cortex of the scaphoid.
Methods: Cannulated headless screws were placed into ten cadaveric wrist specimens. Fluoroscopic and computed tomographic (CT) images of each wrist were obtained. The distance between the screw and the subcortical bone was then measured. CT images were measured using two different window level/window width (WL/WW) settings: the standard bone window (WL 300/WW 500) and a customized hardware window (WL 2,000/ WW 3,500), chosen to better represent metal density. The scaphoids were then removed from each wrist and the distances from the screw to the proximal and distal cortices were measured. The differences between the radiographic measurements and the anatomic measurements were then calculated and the data compared.
Results: X-ray measurements overestimate the distance between the screw and the subcortical bone more than half the time (59% distally, 55% proximally), except in distal measurements in the PA plane, which underestimate 73% of the time. CT measurements underestimate the distance in 68% of specimens. Also, CT scan consistently determines screw position far better than fluoroscopy (R=0.8-0.9 for CT, 0.2-0.9 for fluoroscopy views). The CT customized hardware window measurements provide a more accurate measurement both distally and proximally than the bone window measurements (p≤0.05) because this window better represents the density of metal.
Conclusion: No one x-ray window reliably predicts screw position; however, critical examination of views in the PA plane prevent placement of a screw that is too long distally. Also, CT measurements are more reliable and more accurate than x-ray measurements. Lastly, a customized WL/WW setting should be used when analyzing the position of a screw inside the scaphoid, not a standard bone window.


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