Numerical design and analysis of swelling bone anchors
Authors:   , Sadighi Amirreza , Najafi Ahmad Raeisi 
Published: February 2025

In medical procedures, various types of anchors and screws are commonly used to achieve stable and long-lasting fixation for tasks like reattaching soft tissues, reconstructing tissues, and addressing bone issues such as fractures and defects. However, these traditional methods may encounter challenges, particularly in low-density bones like osteoporotic bone. The low shear resistance between the bone and the threads of these devices can lead to failures. This failure not only complicates the repair or reattachment process but can also cause significant damage to the bone. Additionally, the materials used in these devices, often superior in mechanical properties to the bone (e.g., titanium), can lead to long-term issues such as anchor loosening and stress shielding, further contributing to the potential for failure. Due to challenges with traditional anchors and screws in medical procedures, materials like PEEK (polyetheretherketone) and other resorbable polymers have been explored. These materials have an elastic modulus closer to bone, but they still rely on shear resistance, which may lead to failure and bone damage. Addressing these issues, a novel swellable bone anchor was developed, expanding radially for fixation through an expansion-fit mechanism rather than shear resistance. This expansion generates radial stresses on the bone, promoting frictional resistance and a locking mechanism, contributing to fixation. In addition to superior fixation, these anchors potentially stimulate bone regeneration, increase bone density, and avoid resorption. However, the optimal cross-linked poly (MMA-AA) ratio impacting fixation strength and bone damage needs further investigation. Despite previous research, there's a lack of detailed mechanical response studies, finite element modeling, and computational analysis on these swellable implants, providing opportunities for future exploration. The goal of this research is to develop a multi-physics hygro-elastic finite element model that can investigate the hygro-mechanical behavior of porous swellable implants and bone anchors in different bone regions and densities. The main focus is on understanding how these anchors, once implanted in bone, respond mechanically, along with studying how the surrounding bone reacts to the radial stresses induced in them by the anchors. This is important because the presence of pores in the implant affects its mechanical strength. Moreover, the swelling properties of these implants and bone anchors, which are made of cross-linked poly (MMA-AA), are highly dominated by the ratio of the monomers. Therefore, it is crucial to investigate their hygro-mechanical response and find the optimal combination of porosity and material. Additionally, a strain energy density (SED) based bone remodeling framework is created to explore how the bone adapts to the radial stresses caused by swelling. The study also looks into how bone grows into the porous implant and how the size of the pores and their distribution influence this process. By combining all this information, an advanced finite element tool is developed with which the design of swellable anchors, including the pore size and distribution as well as material composition (cross-linked poly (MMA-AA) ratio), can be optimized. The objective is to achieve a controlled swelling ratio that promotes favorable bone remodeling, avoids stress shielding, encourages bone growth into the implant, and ensures optimal fixation strength.