The Relationship Between Body Mass Index and Skull Soft Tissues: Implications for Bone Conduction Coupling

Objective: Quantify mastoid soft-tissue thickness on temporal bone computed tomography (CT) and evaluate associations with body mass index (BMI), age, and sex. Study design: Retrospective cross-sectional study. Setting: Tertiary referral center. Patients: Adults with temporal bone CT and a recorded BMI within 12 months. Main outcome measures: Mean mastoid soft-tissue thickness measured at 6 sites (3 per ear) on multiplanar reconstructions. Results: One hundred forty-seven CT scans (mean age: 57.4±22.0 y; 46.3% male; mean BMI: 30.2±9.9 kg/m 2 ). The mean mastoid thickness was 9.7±4.1 mm (range: 3.4 to 27.0 mm). BMI was associated with greater mastoid thickness (adjusted β =0.30 mm per kg/m 2 , 95% CI: 0.25-0.34; P <0.001). Male sex was associated with thicker tissue (adjusted difference: 1.37 mm, 95% CI: 0.46-2.27; P =0.003), whereas age was not independently associated. Interaural means were highly correlated ( r =0.89), with a negligible paired difference. Interrater ICC for mean mastoid thickness was 0.995 (n=59). Conclusions: Mastoid soft-tissue thickness varies widely and increases predictably with BMI. BMI may serve as an accessible clinical proxy for the soft-tissue envelope at bone conduction coupling sites, with potential implications for the interpretation of bone conduction audiometry and the planning of skin-drive and magnet-retained bone conduction hearing devices.