Investigation of fracture mechanisms in rib cortical bone

Predicting rib fractures remains an important challenge for the automotive safety community. While current rib computational models can successfully be used to determine the load and deflection required to cause rib fracture, they fail to properly predict the fracture location. It is assumed that a better understanding of bone fracture mechanism will help to improve the biofidelity of rib computational model. The goal of this study was to perform tensile tests of rib bone coupons to determine the fracture characteristics of the rib cortical bone. Thirty seven bone coupons were machined and tested under dynamic tensile load. An optical system was used to measure the in-plane displacements and strains on the outermost surface of each coupon. The Young’s modulus, Poisson’s ratio and failure strain were calculated based on the gathered data. An in-depth analysis of the strain distribution on the coupon surface was performed with the view to identify the fracture mechanisms. This study suggests that tension is not the main phenomenon that explains the onset of fracture in the rib cortical bone, and that properly predict rib bone fracture may require to account for the bone anisotropy and heterogeneity.. INTRODUCTION redicting rib fractures created by a dynamic event remains an important challenge for the automotive safety community and injury biomechanics at large. It is clear that the mechanical response of the human thorax can be greatly influenced by the number of fractured ribs. To determine rib injury mechanisms under dynamic loading the behavior of the ribs and the entire rib cage have been extensively investigated (Kent et al., 2004; Song et al. 2009; Shigeta et al., 2009; Lessley et al., 2010; Hallman et al. 2010). The rib bone material properties for bovine (Ferreira et al., 2006; Adharapurapu et al., 2006) and human bone (Keller et al., 1990; Kemper et al., 2005; Hansen et al., 2008; Subit et al., 2011) have been reported by many researchers. The available data can be utilized to help to understand the rib mechanical behavior using modern engineering tools – e.g. finite element method (FEM) models. While current rib computational models can successfully determine the load and deflection required to cause the rib to fracture, they fail to properly predict the fracture location (Li et al., 2010). This indicates that further research is needed to P improve the knowledge of the fracture mechanisms in the rib, especially in its cortical bone. The cortical bone has voids and is a composite-like material that makes it non-homogenous. The bone material properties determined assuming homogeneity of the bone, are likely to inadequately describe the mechanical response of the rib. Therefore the goal of this study was to perform tensile tests of rib cortical bone coupons and analyze their deformation using a digital image correlation (DIC) method in order to determine the fracture characteristics of the rib cortical bone. A protocol was developed to machine and test rib bone coupons of constant thickness under dynamic loading. All the samples were imaged prior to testing using a microcomputed tomograph (microCT). METHODS The bilateral 6 and 7 ribs were harvested from three post mortem human subjects (PMHS; Table 1) in accordance with the ethical guidelines and research protocol approved by the Human Usage Review Panel and the University of Virginia institutional review board. Table 1. PMHS information Subject Age at time of death Couse of death Body mass (kg) Stature (cm) 510 69 Chronic obstructive pulmonary disease 93.4 168 511 49 Brain injury 98 175 518 70 Heart failure 67.6 165 Thirty seven bone coupons that were 25.4-mm long, 2.5-mm wide in the gage area, and 0.5 mm thick (fig. 1) were machined as it was described by Subit et al., 2013. Figure 1: Dimensions of the rib cortical bone coupon A hydraulic tensile machine (Model 8874, Instron Inc, Norwood, MA, USA), with a specially designed aluminum clamping system, was used to test the coupons under tensile loading (at constant velocity of 24 mm/s up to fracture). The clamping system consisted of two low-mass clamps (10.9 grams each, fig. 2). To prevent slippage of the coupon while avoiding bone crushing, a torque of 5 Nm was applied to the clamping screws prior to testing. The pins that go through the rod-end ball joints were utilized to connect the clamps to the base of the tensile machine and the end of the piston (fig. 3). After installing the clamps in the machine, all the samples were preloaded (between 1 N and 4 N) to ensure that there was no clearance between the pins and the clevises. Figure 2: Detailed view of the clamps Figure 3: Clamping system for the coupons The tensile load was measured by a three axis loadcell (model 6085, Denton Inc, Plymouth, MI, USA) located underneath the bottom clamp, connected to a standard data acquisition system (DEWE-2010, Dewton GmbH, Gratz, Austria). The displacement of the top clamp with reference to the tensile machine base was measured using a displacement potentiometer. The load and displacement were sampled at 100 kHz. An optical system was used to measure the in-plane displacements and strains of each coupon (Aramis, GOM, Germany). The bone coupons were stored in a saline solution until the black-and-white paint pattern (fig. 4a) was applied to the outermost surface of the coupon. The optical system was comprised of one high-speed imager (NAC GX-1, NAC Image Technology, Simi Valley, CA) with a 100 mm macro lens. The average strains (ε11 and ε12; fig. 4b) in the coupon gage area were calculated based from the Aramis results, and the tensile stress was estimated by dividing the tensile force by the cross-sectional area of the coupon gage area. The Young’s modulus, Poisson’s ratio and failure strain were also calculated. The (effective) Young’s modulus was defined as the slope of the stress-strain curve between 0 and 0.5%. The bone microstructure and porosity were analyzed based on the microCT images, with the use of the MATLAB software (The Mathworks, Natick, MA, USA), and an in-depth analysis of the strain distribution on the coupon surface was performed with the view to identifying the fracture mechanisms.