Finite element analysis of bone remodeling induced by swelling anchors considering heterogeneous properties

This study explored the biomechanical behavior of co-polymeric swelling bone anchors and their bone remodeling induction using finite element analysis of a model with heterogeneous properties. First, a hygro-elastic finite element framework was developed to capture the swelling of the bone anchors over time by moisture gain, validated against the data from free swelling experiments. Afterward, finite element models were developed using micro-CT data to capture heterogeneous material properties, and finally, bone remodeling induced by the swelling, acting as a mechanical stimulus, was investigated. The study examined three co-polymeric ratios of methyl methacrylate and acrylic acid (MMA/AA)-80/20, 85/15, and 90/10-and assessed the impact of their associated swelling ratios on bone remodeling and fixation strength. Moreover, in parallel with the numerical investigations, an in vivo study using a sheep model was conducted to evaluate the biocompatibility of these anchors and bone remodeling response to the swelling. The numerical findings highlighted the importance of optimizing swelling ratios to enhance mechanical engagement without causing adverse resorption. More specifically, the results demonstrated that bone regeneration in the region of interest is highly sensitive to the swelling ratio. When the swelling is maintained within an optimal range-such as in the 85/15 composition-favorable densification occurs at the bone-implant interface, enhancing osteointegration. In contrast, excessive swelling (e.g., the 80/20 composition) induces localized overload resorption due to elevated stress concentrations at the interface, which may compromise implant success. Additionally, correlations found between the numerical and in vivo study outcomes supported the notion of an optimal swelling threshold and confirmed the predictive capabilities of the developed hygro-elastic finite element framework. To underscore the importance of favorable bone remodeling in the interface, a push-out study was performed to analyze the fixation strength prior and subsequent to bone remodeling. The significant difference in push-out forces before and after remodeling demonstrates that bone densification at the interface can substantially enhance fixation strength.