Finite Element Analysis of Different Screw Designs and Lengths in Scaphoid Waist Fracture Fixation

This study investigates the impact of screw designs and lengths on biomechanical performance and stress distribution in scaphoid waist fracture fixation, comparing centrally- threadless and fully- threaded headless compression screws, considering a computer- based finite element analysis (FEA). A three-dimensional model of an adult scaphoid was constructed from computed tomography data using Mimics software. This model was uploaded into multiphysics software to enable the application of physiological enhancements, followed by the mesh modelling technique. All the models were registered as homogenous and isotropic materials. A transverse waist fracture was simulated following Herbert B2 type fracture and fixed with either a fully- threaded compression screw, or centrally threadless non-variable pitch screw. Simplified models of both screws were implanted into the scaphoid models simulating bicortical, unicortical and subchondral fixation. Screws were applied in retrograde and antegrade direction. All screws were positioned either centrally or eccentrically perpendicular to fracture plane. Determining the different lengths, direction and screw position, models were divided into 4 groups: Group 1 (bicortical fixation- full length), Group 2 (unicortical fixation – 75% of full length), Group 3 (unicortical fixation >75% but not 100%, and Group 4 (subchondral fixation). Interfragmentary compression, gap formation, displacement (sliding distance), rotation and stress distribution (von Mises) between bone-bone and bone-materials were compared between groups. Less than 5% differences were considered as similar. In neutral wrist position, under 100N compressive load, 96 models were simulated and analyzed. von Mises stress values were concentrated more on the ulnar side in those had centrally threadless screw when screw engaged 90% or more of the entire length of scaphoid. Fully threaded design tends to bear the entire load with no further stress distribution across the fracture side, regardless of position of the screw (Figure 1 and 2). Furthermore, rotational stability was stronger in the fully threaded design. The results of this FEA study suggest that a fully threaded screws acts as a load bearing device, regardless of its length and position (³ 75% of entire scaphoid length), and may add more stability against bending moment, rotation or further gapping/ displacement compared to centrally threadless design. In contrast, centrally threadless design behaves as a load shearing device, depending on its position and length. For any figures or tables, please contact the authors directly.