We thank Dr. Beaupre for his kind comment.
We applied a friction coefficient of 0.3 with the gap element between a bone and a plate in our finite element analysis (FEA) and hypothesized that the bone and the thread of the screw were completely fixed with no motion between them. In our analysis, the friction coefficient between the bone and the screw could be a factor inducing slippage as you mentioned. However, we think that the friction coefficient between the bone and the plate was only a minor factor and that the results of our analysis would not change depending on the coefficient.
As to why we found a bit lower stiffness in the experimental model than in the FEA, in the experimental study we tried to reduce the osteotomy site as perfectly as possible. But even though all the procedures were done by the first author, who is an experienced trauma surgeon, we noticed subtle variations in the quality of reduction at the time of compression. So these experimental constructs should be inferior compared to the FE model in which the quality of reduction and compression were set ideally. Another reason is related to the mechanical properties of the artificial bone that was used in the experiment. The artificial polymeric bone has material properties similar to those of natural bone, but it is more brittle. We believe that the brittleness led to a crack between the screw and bone as the load increased in the experimental test, leading to a decrease in the stiffness of bone-plate fixation system.
We are preparing further studies, including fatigue loading of a bone-plate fixation system. We also intend to proceed with the study to improve consistency with the FEA results considering your comment. Thanks again for your interest in our paper and your kindness in providing additional literature references.