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Finite Element Analysis Of Different Screw Designs And Lengths In Scaphoid Waist Fracture Fixation
Maxwell Campbell, PhD
1, Amin Kheiran, MD, MSc, FRCS
1, Jimmy Tat, MD, MSC, FRCSC
1, Daniel Langohr, PhD
2, Ruby Grewal, MD, MSc
3; Assaf Kadar, MD
4(1)Western University, London, ON, Canada, (2)University of Western Ontario, London, ON, Canada, (3)Hand and Upper Limb Center, Western University, London, ON, Canada, (4)St. Joseph's Health Care Center, London, ON, Canada
Background: 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).
Methods: A 3D model of eight 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 isotropic materials. A transverse waist fracture was simulated corresponding to 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 - 70% of full length), Group 3 (unicortical fixation >70% but not 100%), and Group 4 (subchondral fixation). Interfragmentary gap formation/ displacement, and stress distribution (von Mises) within bone-bone and bone-screw interface were compared between groups. ANOVA was utilised to compare variables, and <5% differences were considered as similar.
Results: In neutral wrist position, under 100N tensile loads, 640 models were simulated and analyzed. Stress at bone-screw interface was significant (p=0.014) in eccentric screw placement (2.918 mPa) compared to centric (2.096 mPa), in both screw designs. Overall, fully threaded screws demonstrated less stress at bone-screw interface compared to centrally threadless (p=0.006). Stress was significantly higher within the radial-dorsal followed by ulnar-dorsal regions in centrally threadless screws (p<0.05). The stress along bone-screw interface were significantly higher at the end of screws in centrally threadless compared to fully threaded. The latter tends to bear the entire load with no further stress distribution across the fracture side, especially once placed within subchondral zone (Figure 1 and 2).
Conclusion: The results of this FEA study suggest that a fully threaded screws acts as a load bearing device, when fixed within subchondral zone and engaged 60% or above of its length, compared to centrally threadless design. In contrast, centrally threadless design behaves as a load shearing device, and undergoes more stress at bone-screw and bone-bone interfaces, depending on its position and length.
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