Fixation Methods for Replacement Screws in Scaphoid Fractures: A Biomechanical Study

Abstract

To investigate the changes in biomechanical fixation of an initial and replacement headless compression screw by quantifying interfragmentary compression and shear displacement. A bone model was created with 2 blocks of rigid polyurethane foam to simulate a fracture site. A 24-mm headless compression screw was inserted into the model at a perpendicular or oblique angle, removed, and replaced with the 22-mm screw. The 22-mm screw replaced the 24-mm screw in either the same or reverse direction of initial insertion. All 4 methods of initial and replacement screw fixation were trialed 15 times for a total of 60 trials. Two button load cells measured compression. A digital microscope recorded shear displacement. Statistical analysis was performed to determine the differences in compression and sheer between the initial and replacement screw for each angle and screw insertion group with a 2-sided paired Student t test. In comparison with the average compression force achieved by placement of the primary screw, there was a significant loss of average compression force when the secondary screw was placed in the same direction and perpendicular to the fracture plane (50 N, P < .001), in the reverse direction and perpendicular to the fracture plane (9 N, P= .049), and in the same direction and 60º to the fracture plane (11 N, P < .001). A significantly greater average shear displacement was noted for initial screw insertion at oblique compared with perpendicular orientations (0.03 mm, P < .001). This study demonstrates loss of compression when replacing screws despite varying orientations. Additionally, shear displacement in obliquely placed screws increased compared with perpendicularly placed screws. This study provided quantitative evidence to support the importance of establishing the correct screw length initially to avoid compression loss caused by replacing a primary screw.