A finite element model for evaluating the effectiveness of the Advanced System for Implant Stability Testing (ASIST).

The Advanced system for Implant Stability Testing (ASIST) was developed to evaluate the stability of osseointegrated implants. ASIST matches the physical response with an analytical model's prediction to determine the stiffness of the bone implant interface (BII) which is then used to calculate the ASIST Stability Coefficient (ASC). In this investigation, a 3D dynamic finite element (FE) model of the ASIST experimental impact technique for bone anchored hearing aids was created. The objectives were to evaluate the analytical model's ability to capture the behavior of the implant system and to assess its effectiveness in minimising the effects of the system's geometry on the ASC scores. The models were developed on ABAQUS®, they consisted of the implant, abutment, screw, base support and impact rod. The models relied on frictional contact definitions between the system's components. The simplified "three-part" model had the implant, abutment and screw merged as one part while the "five-part" model treated them as separate components. Different interface conditions were simulated (friction coefficient range: 0-0.9) for three abutment lengths (6, 9 and 12 mm). The simulation output was the average nodal acceleration response of the rod, which was imported to the custom ASIST program in Mathematica® to obtain the ASC scores. The overall quality of the curve fits indicate that the analytical model is capable of representing the system's behavior. Moreover,ASC scores provide a reliable assessment of implant stability as they are sensitive to interface conditions and are minimally influenced by the system's geometry.