A finite element-based injury metric for pulmonary contusion: investigation of candidate metrics through correlation with computed tomography

Pulmonary contusion (PC) is the most common thoracic soft tissue injury following non-penetrating blunt trauma and has been associated with mortality rates as high as 25%. This study is part of an ongoing effort to develop a finite element based injury criteria for PC. The aims of this study are two fold. The first is to investigate the use of computed tomography (CT) to quantify the volume of pathologic lung tissue in a prospective study of PC. The second is to use a finite element model (FEM) of the lung to investigate several mathematical predictors of contusion to determine the injury metric that best matches the spatial distribution of contusion obtained from the CT analysis. PC is induced in-situ utilizing male Sprague Dawley rats (n = 24) through direct impact to the right lung at 5.0 ms(-1). Force vs. deflection data are collected and used for model validation and optimization. CT scans are taken at 24 hours, 48 hours, 1 week, and 1 month post contusion. A numerical simulation is performed using an FEM of the rat lung and surrounding structures. Injury predictors investigated include maximum first principal strain, maximum shear strain, triaxial mean strain, octahedral shear stress, and maximum shear stress. Strain rate and the product of strain and strain rate are evaluated for all listed strains. At each post-impact time point, the volume of contused lung is used to determine the specific elements representing pathologic lung. Through this method, a threshold is determined for all listed metrics. The spatial distribution of the elements exceeding this threshold is compared to the spatial distribution of high-radiopacity lung tissue in the CT through a three dimensional registration technique to determine the predictor with the best correlation to the outcome. Impacts resulted in a mean energy input to the lung of 8.74 +/- 2.5 mJ. Segmentation of the imaging data yielded a mean unilateral high-radiopacity tissue estimate of 14.5% by volume at 24 hours with decreasing high-radiopacity lung volume at later time points. Significant differences in the volume of high radiopacity (HR) tissue were noted at 1 week and 1 month post contusion. The best fit injury metric at the 24 hour time point was the product of maximum principal strain and strain rate, with a value of 28.5 sec(-1). The maximum principal strain, and the maximum principal strain rate also characterized the distribution of pathology well with the proposed threshold for injury at 24 hours post-insult being 15.4 % and 304 sec(-1) respectively. This study quantifies pulmonary contusion via CT, an attractive modality due to its sensitivity to this lesion and widespread use in trauma centers. The study also presents a novel spatial validation technique for correlation of mathematical predictors to volumetric contusion data; a technique that may be extended to other injuries and modalities as well.