Numerical Investigation Into Bone Remodeling Around Different Co-Polymeric Swelling Bone Anchors

In this study, a hygro-elastic finite element framework, along with a strain-energy-density based bone remodeling framework, was developed and used to simulate the swelling of co-polymeric bone anchors to investigate their hygro-mechanical response. To validate the numerical results, free swelling and in vivo experiments were conducted as well. The free swelling experiments were conducted on co-polymeric porous bone anchors (composed of cross-linked poly [methyl methacrylate-co-acrylic acid]) with two ratios of 80/20 and 90/10 to investigate their swelling characteristics in bovine serum, mimicking in vivo conditions. Subsequently, the swelling of bone anchors was simulated embedded in bone regions with different densities. The radial stresses induced in the interface were extracted to examine the mechanical response of the surrounding bone. According to Wolff's law, such mechanical loads can be regarded by bone mechanotransducers as stimuli for remodeling. The bone remodeling framework evaluated the impact of the radial force induced by the swelling of the bone anchor on the surrounding bone. The radial stress induced by the controlled swelling ratio of 90/10 composition resulted in favorable bone densification in the region of interest (approximately between 17.5% and 54% depending on the density of the region). However, the excessive swelling of 80/20 composition caused radial stresses to go beyond the threshold of 31 MPa, causing overload resorption in the interface (especially in high-density regions, where there was total resorption in the interface) and jeopardizing the success of the bone anchor and osteointegration. It was discovered that the swelling ratio plays an important role in bone remodeling, and that it must be controlled within a certain threshold to ensure bone densification and prevent overload resorption. The results of the in vivo sheep study also confirmed these findings.