Evaluation and Validation of Thorax Model Responses: A Hierarchical Approach to Achieve High Biofidelity for Thoracic Musculoskeletal System

Abstract

As one of the most frequently occurring injuries, thoracic trauma is a significant public health burden occurring in road traffic crashes, sports accidents, and military events. The biomechanics of the human thorax under impact loading can be investigated by computational finite element (FE) models, which are capable of predicting complex thoracic responses and injury outcomes quantitatively. One of the key challenges for developing a biofidelic FE model involves model evaluation and validation. In this work, the biofidelity of a mid-sized male thorax model has been evaluated and enhanced by a multi-level, hierarchical strategy of validation, focusing on injury characteristics, and model improvement of the thoracic musculoskeletal system. At the component level, the biomechanical responses of several major thoracic load-bearing structures were validated against different relevant experimental cases in the literature, including the thoracic intervertebral joints, costovertebral joints, clavicle, sternum, and costal cartilages. As an example, the thoracic spine was improved by accurate representation of the components, material properties, and ligament failure features at tissue level then validated based on the quasi-static response at the segment level, flexion bending response at the functional spinal unit level, and extension angle of the whole thoracic spine. At ribcage and full thorax levels, the thorax model with validated bony components was evaluated by a series of experimental testing cases. The validation responses were rated above 0.76, as assessed by the CORA evaluation system, indicating the model exhibited overall good biofidelity. At both component and full thorax levels, the model showed good computational stability, and reasonable agreement with the experimental data both qualitatively and quantitatively. It is expected that our validated thorax model can predict thorax behavior with high biofidelity to assess injury risk and investigate injury mechanisms of the thoracic musculoskeletal system in various impact scenarios. The relevant validation cases established in this study shall be directly used for future evaluation of other thorax models, and the validation approach and process presented here may provide an insightful framework toward multi-level validating of human body models.

Keywords: biofidelity; finite element method; injury biomechanics; musculoskeletal system; thoracic spine; thorax model; validation.

FIGURE 1

FE modeling of intervertebral discs and ligaments: (A) IVD details (T7-T8) and intervertebral ligaments, and (B) lateral view of overall thoracic spine model.

FIGURE 2

Model setup for flexion bending of FSU and rear hub impact test: (A) flexion bending test of FSU T7-T9, and (B) hub-impact tests performed on the back surface of the mid-thorax.

FIGURE 3

Model setup for clavicle fracture and sternum bending tests: (A) three-point bending test (the green rectangle showed the position where the strain gages were attached), (B) axial compression test, and (C) sternum bending test.

FIGURE 4

Model setup for costal cartilage bending test.

FIGURE 5

Model setup for point loading simulation: (A) illustration of the loading points at the upper and lower sternum levels and at the costochondral junction (CCJ) of rib levels 1, 3, 4, 6, and 9, and (B) a close-up at a loading site in the FE model (Kindig et al., 2015).

FIGURE 6

Model setup for frontal pendulum impact simulation: (A) illustration of the impact condition. The impact force was measured as the contact force between the impactor and the chest, and (B) chest deflection was defined as the change of distance between the center of impactor surface and a node taken on the skin at the T8 level.

FIGURE 7

Model setup for shoulder pendulum impact simulation: (A) illustration of the impact condition (padded impact), and (B) illustration of shoulder deflection measured between points A and B for unpadded and padded simulation cases.

FIGURE 8

Model setup for table-top loading cases: (A,B) showed the single belt and double belts loading (belts were modeled as a shell layer with 2-mm thickness), (C) hub loading by a cylindrical rigid hub with a diameter of 152 mm, and (D) distributed loading by an extra-wide belt (203-mm width) simulated by a layer of shell elements with 2-mm thickness.

FIGURE 9

Flexion bending validation results of FSU: (A) intervertebral ligaments rupture (FSU T7-T9), and (B,C) showed the moment-angle response obtained from FSU T2-T4 and T7-T9.

FIGURE 10

Extension bending of spine under rear blunt impact on full torso: (A) a deformed configuration of the model under the blunt rear impact, (B) response of the chest deflection, and (C) response of the change in spine extension angle.

FIGURE 11

Force-deflection response for the denuded ribcage under point loading on different loading sites: (A) upper sternum, (B) lower sternum, and (C–F) displayed costochondral junction (CCJ) of the rib levels 1, 3, 4, and 6.

FIGURE 12

Simulation results of thorax model under Kroell et al. (1971) frontal pendulum impact: (A) superior view of the cross-section of the compressed thorax at the mid-sternum, (B) deflection-time response, and (C) force-time response.

FIGURE 13

Deflection and force responses under Koh et al. (2005) shoulder pendulum impact: (A) and (B) 4.4 m/s padded impact, (C,D) 4.5 m/s unpadded impact, (E,F) 6.4 m/s padded impact, and (G,H) 6.8 m/s unpadded impact.

FIGURE 14

The reaction force versus chest compression responses produced by the table top tests: (A) single diagonal belt loading, (B) double diagonal belts loading, (C) hub loading, and (D) distributed loading.