Biomechanics of Blunt Liver Injury:Relating Internal Pressure to Injury Severity and Developing a Constitutive Model of Stress-Strain Behavior

Abstract

Research suggests that in certain types of blunt liver trauma the mechanism of injury is linked to rapid increases in internal pressure within the liver. The objectives of this study were (1) to characterize the relationship between impact-induced pressures and blunt liver injury in an ex vivo organ experimental model; (2) to compare human liver intra-parenchymal pressure and vascular pressure with other biomechanical variables as predictors of liver injury risk; (3) to investigate the feasibility of measuring liver vascular pressure in impacts to pressurized full body post-mortem human subjects (PMHS); and (4) to develop a constitutive model of the mechanical behavior of human liver tissue in blunt impact loading. Test specimens included 19 ex vivo porcine livers, 14 ex vivo human livers, and 2 full body PMHS. Specimens were perfused with normal saline solution at physiological pressures, and a drop tower applied blunt impact at varying energies. Impact-induced pressures were measured by transducers in the hepatic veins and parenchyma (caudate lobe) of ex vivo specimens. Binary logistic regression demonstrated that tissue pressure measured in the parenchyma was the best indicator of serious liver injury risk (p = .002, Pseudo R-square = .78). A peak tissue pressure of 48 kPa was correlated to 50% risk of serious (AIS ≥ 3) liver injury. A burst injury mechanism directly related to hydrostatic pressure is postulated for the ex vivo liver loaded dynamically in a drop test experiment. A constitutive model previously developed for finite strain behavior of amorphous polymers was adapted to model liver stress-strain behavior observed in the ex vivo human liver impacts. The model includes six material properties and captures three features of liver stress-strain behavior in impact loading: (1) a relatively stiff initial modulus; (2) a rate-dependent yield or rollover to viscous “flow” behavior; and (3) strain hardening at large strains. Results of this research could be applied to improve the abdominal injury assessment capabilities of both anthropomorphic crash dummies and finite element human body models used in vehicle safety research.