Spatial measurement for safe placement of screws within the scaphoid using three-dimensional analysis.

The placement of a headless screw within the scaphoid is important for successful fixation of fractured scaphoids. We ascertained the safe position of a screw using virtual collision experiments on a three-dimensionally reconstructed scaphoid bone and screw model, and applied measured angles to simple radiographs. We used images from computed tomograms of the wrists taken from 13 men to reconstruct a three-dimensional cortical image of the scaphoid. Inside the bone, we inserted a virtually reconstructed Herbert screw model and calculated the safe angle along the long axis of the scaphoid at which the screw did not collide with the bone in any sample, then applied measured safe angles to simple anteroposterior and lateral radiographs. In all experiments, the safe angle between the screw and the anteroposterior scaphoid line (defined from the most distal portion of the scaphoid where it met the capitate to the proximal position where the lunate and capitate meet) ranged from 8° to 27° in the anteroposterior view, and the safe angle between the screw and the lateral scaphoid line (defined from the most volar portion of the scaphoid tuberosity to the volar position where the scaphoid faced the radius) ranged from 2° to 26° in the lateral view. These ranges can be used as a guide for a safe position of the screw within the scaphoid in volar percutaneous fixation of screws in scaphoid fractures.