The influence of the tertiary bronchi on dynamic lung deformation.

Finite element models of thoracic injury often treat the lung as a bulk homogeneous and isotropic material, which reduces the computational costs associated with such investigations. Ignoring the heterogeneous structure of the lung may be computationally expedient, but this simplification may inadvertently fail to capture the true lung strain dynamics. In the present work, a series of direct impact experiments were performed on porcine lungs, inflated to a relevant expiratory pressure, and monitored using high-speed X-ray imaging. The lungs were instrumented with radiopaque markers within the parenchyma and tertiary bronchi to monitor the resulting deformation mechanics. The deformation mechanics demonstrate a high degree of strain localization related to the structural heterogeneity of the lung. The relative motion of the tertiary bronchi was measured during the impact event, and used to estimate the parenchyma tissue strains in the inter-bronchial regions. These were shown to exceed the trans-lobe strains by a factor 3 to 5 times higher in their tensile, compressive, and shear strain responses. Our results demonstrate that the lung parenchyma and bronchial tissues form a heterogeneous structure with a substantial stiffness differential that cannot be appropriately modelled as a homogeneous and isotropic monolithic mass without loss of accuracy and predictive relevance.