Loosening of bone screws caused by stress shielding, and subsequent unbalanced bone remodeling processes, results in bone loss around the screws, which can ultimately lead to bone fixation failure. Predicting the effects of the structural and material properties of screws on bone remodeling is crucially important in screw design and fixation reliability. Many studies have attempted to assess the significance of orthopedic screw parameters on screw pullout strength, but few have considered bone remodeling processes around the screw threads. The main objective of this study was to investigate the effects of various engineering designs of bone screws on stress shielding and screw loosening in a bone-screw system using a two-dimensional finite element model (FEM), including cortical and cancellous bone, and an orthopedic screw. In order to do so, an adaptive bone strain energy density algorithm was combined with FEM to study a 4-year follow-up of bone remodeling around the screw implant. A dimensionless set of stress transfer parameters (STP), and a newly defined parameter, called the strain energy density transfer parameter (SEDTP), were developed to quantify the screw-bone load sharing patterns. Lower STP and SEDTP values indicate a weaker transfer of mechanical stimuli to the neighboring bone. The results of this study show that increasing the major diameter of the screw, increasing the number of threads, and decreasing the minor screw diameter, pitch, and elastic modulus decrease the probability of screw loosening. The approach used here is not limited to the analysis of orthopedic screws, but may be used for other implant devices.

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