gms | German Medical Science

Objective: The effect of fixation stiffness on fracture healing has been demonstrated in numerous studies. In locking plate osteosynthesis, the plate working length (PWL) is the factor which can be most easily manipulated by the surgeon in order to affect fixation stiffness significantly. Insufficient control and inconsistent reporting of mechanical boundary conditions have hampered the development of a more comprehensive fracture fixation mechanotherapy. This study aimed to define necessary boundary conditions such that future studies could be more comparable to each other, thereby potentially providing significantly higher clinical relevance.

Methods: Firstly, we conducted a systematic literature search through PubMed to evaluate the frequency of reporting of PWL. Secondly a finite element analysis (FEA) in Abaqus 2019 and in vitro validation was performed to describe the effect of PWL more precisely. A cylinder (simulated cortical bone with 25/30mm inner/outer Diameter) and a locking compression plate and screws (9-hole, 4.5mm, material properties of titanium) was modelled. To model the deformation of the plate around the screw upon loading appropriately, we developed and validated a new modelling method for the screw-plate interface: an isotropic ring structure with elastic properties, placed between the head of the screw and the plate with a tie constraint to both surfaces.Validation was performed in vitro by cyclic loading of the locking screw with the same boundary conditions as the FEA. Six groups were defined with varying the number and allocation of screws (4 vs 6 screws; PWL: 36,72, 108 mm).Within each group, 4 different models were calculated with different boundary conditions as defined by Grant et al. (2015). Models were loaded axially with 500N and deformation of the plate around the screws was recorded.

Results and Conclusion: 138 studies (1984-2020) could be included in the literature review. 56% did not report any PWL, and of the remaining only 4% reported PWL (mean and variation) with a clear definition.

The new ring structure's modelling of the screw-plate interface allows for micromovements, resulting in a more realistic deformation of the plate around the holes. Plate deformation was mapped orthogonally to the loading axis. This showed a point of inflexion around the screw holes. This inflexion point was demonstrated as a peak in the plate deformation's derivative and projected most closely onto the center of the screw hole of the screw closest to the fracture gap. This was consistent between the models with varying boundary conditions, although the peak in the derivative was twice as high in models with less degrees of freedom, indicating a steeper gradient in the deformation.

We propose to define PWL as the distance between the center of the two screws next to the fracture gap proximally and distally. It should be reported in a standardized manner, including mean and variation in future publications, along with material and type of plate, thereby enabling universal data translatability.