Validation of a screw implantation safety index for bone conduction implants

Bone conduction implants are used to treat hearing loss by transmitting mechanical vibrations to the inner ear via bone screws anchored in the temporal bone. Secure screw fixing is essential for an efficient signal transmission and implant stability. We hypothesized that regions of higher bone density should be prioritized for screw placement to optimize fixing. This ex-vivo study aimed to validate the Screw Implantation Safety Index (SISI), computed as the percentage of the insertion path characterized by dense bone, as a preoperative marker for safe and effective screw placement. We analyzed 24 bone samples (1 cm³) extracted from the temporal bones of two Thiel-preserved cadaver heads. Each specimen underwent quantitative computed-tomography (CT) and micro-CT ($\mu$CT) imaging to determine SISI values. Self-tapping titanium screws (5 mm) were inserted, and pull-out testing was conducted to assess maximum pull-out force, stiffness, and strain energy at the bone-screw interface. Linear regression models were used to evaluate associations between SISI and mechanical stability. SISI values ranged from 29% to 81% ($\mu$CT) and 39% to 93% (clinical CT). The average pull-out force was 535 N (standard deviation of 151 N), with weak but statistically significant correlations between CT-based SISI scores and pull-out force ($R^2=0.24$, $p=.01$), as well as strain energy ($R^2=0.23$, $p=.01$). Comparison with $\mu$CT data validated the clinical CT results. Our findings suggest that the SISI score could enhance preoperative planning for bone conduction implants by guiding optimal screw placement. Further research is needed to assess its clinical feasibility and impact on long-term implantation outcomes.