Material properties of arachnoid trabeculae in situ using inverse finite element analysis

The pia-arachnoid complex (PAC), composed of pia mater, arachnoid mater, and arachnoid trabeculae, plays a critical role in transmitting mechanical forces between the brain and skull during head trauma. Despite its importance, the material properties of the PAC microstructure are unknown, limiting the accurate representation of the brain-skull interface in simulations of traumatic brain injuries. To address this unmet need, this study combined in situ, pressure-controlled subarachnoid inflation with optical coherence tomography imaging and inverse finite element (FE) analysis to calculate the material properties of individual arachnoid trabeculae. Individual FE models were created for 37 locations across piglets, pigs, and sheep brains. The elastic modulus was iteratively varied until the simulation matched the experimental arachnoid membrane displacement measured during inflation. The optimized models were validated by comparing individual trabeculae strains in the simulation to experimentally measured trabeculae strains, showing good agreement on average (7% difference). This indicates that material properties calculated from inverse FE analysis capture, on average, local single-trabecula behavior. The optimized inverse FE analysis yielded an average elastic modulus of 13.4 ± 13.6 kPa (median: 8.44kPa), with a linear elastic model adequately representing the experimental arachnoid membrane displacements (R² = 0.96 ± 0.04). The elastic modulus of trabeculae across piglets, pigs, and sheep was not significantly different (p = 0.17). This work provides the first estimate of the microstructural-level mechanics of individual trabeculae in the PAC, opening the door to the development of microscale PAC models to support traumatic brain injury modeling.