A multi-scale finite element framework for evaluating buttocks–support surface interactions under static and dynamic loading

The prevention of pressure injury (PI) remains challenging due to limited clinical evidence, while existing biomechanical studies, as complementary approaches, are largely confined to static conditions, simplified models, and insufficient representation of dynamic mattress behavior. Therefore, this study developed a multi-scale biomechanical framework to evaluate buttocks–support surface interactions under both static and dynamic loading conditions and to inform surface selection and design. A subject-specific buttocks model reconstructed from MRI/CT was combined with three support surfaces modeled in CAD—standard foam mattress (SFM), strip air-filled mattress (ST-AFM), and spherical air-filled mattress (SP-AFM)—to generate three distinct biomechanical models. Soft tissues and SFM were modeled using hyperelastic Ogden formulations, while air-chamber membranes were represented by calibrated Mooney–Rivlin models with internal ideal gas. Finite element simulations were conducted under static and alternating-pressure conditions (24/12 kPa), evaluating stiffness, reaction force, interface stress, and tissue strain. Under static loading, SFM exhibited the highest stiffness (7.69 N/mm), whereas SP-AFM showed lower stiffness (5.35 N/mm) and a lower proportion of simulated high-stress regions (> 16 kPa) by up to 0.53% under the prescribed-displacement condition. Strain responses were region-specific: SP-AFM produced lower compressive and shear strain magnitudes in the ischial tuberosity (IT) region (LE22: − 0.489; LE12: 0.506), while ST-AFM produced lower strain magnitudes in the sacrococcygeal (SC) region (LE22: − 1.183; LE12: 0.805). Under cyclic loading, stiffness decreased and strain exhibited phase-dependent variations. In the IT region, SP-AFM strain decreased from 0.4430 to 0.363 (− 17.8%), whereas ST-AFM increased to 0.712, exceeding SFM baseline by 13.5%. This study developed a multi-scale finite element framework for evaluating buttocks–support surface interactions and demonstrates that the mechanical performance of support surfaces is governed by the coupled interaction between structural configuration, anatomical heterogeneity, and temporal load modulation. These findings may guide pressure injury prevention by supporting region-specific chamber design and phase-aware pressure-control strategies to reduce sustained compression and transient tissue overloading in pressure-injury-prone areas.