Finite Element Analysis of Dual Screw Configurations for Enhanced Stability in Scaphoid Fracture Fixation

Internal fixation with a headless compression screw has become the gold standard for the surgical treatment of scaphoid fractures. Some authors reported on the theoretical benefit of dual screw fixation. Our aim was to investigate the optimal dual screw configuration for fixing scaphoid fractures with regard to biomechanical stability.

In a previously established finite element model of the scaphoid, a fracture plane at the scaphoid waist was modeled. Two 1.7 mm cannulated, partially threaded compression screws were positioned in parallel. The distance between the screws and their angulation relative to each other were systematically varied, using multiples of the screw diameter for the interscrew distance (3.4 mm, 5.1 mm, and 6.8 mm) and 5° increments for angle variation (5°, 10°, 15°, 15°, and 20°). For each scenario, maximum interfragmentary displacement (IFD in mm) and maximum von Mises stress (MPa) within the fracture planes of the distal and proximal fragments were compared.

IFD values were 0.057, 0.031, and 0.0227 mm for screw distances of 3.4, 5.1, and 6.8 mm, respectively. Von Mises stress was 105.6 MPa, 105.36 MPa, and 84.84 MPa in the proximal fragment and 86.3 MPa, 73.68 MPa, and 56.6 MPa in the distal fragment, for the same distance variants. For angles of 5°, 10°, 15°, and 20° between the screws, the IFD was 0.0217 mm, 0.0219 mm, 0.0221 mm, and 0.0224 mm, respectively. The corresponding von Mises stress was 84.84 MPa, 83.95 MPa, 82.85 MPa, 82.78 MPa, and 82.09 MPa for the proximal fragment and 56.6 MPa, 56.14 MPa, 60.04 MPa, 58.84 MPa, and 58.56 MPa for the distal fragment for screw angulations of 5°, 10°, 15°, and 20°.

Despite morphologic limitations of the scaphoid to place two screws maximally apart, a larger distance is biomechanically advantageous and provides more rigid fracture fixation. Angulation, however, has no relevant biomechanical influence, although it may facilitate easier placement of the two screws.

The study provides a basis for before surgery planning to optimize biomechanical stability in dual screw fixation of scaphoid fractures.