Abdominal injuries sustained during impact events such as motor vehicle crashes are noteworthy in their severity. A study by Klinich et al. [1
] that examined data from a National Automotive Sampling Study of motor vehicle crashes in the United States from 1998–2004 reported that although abdominal organ injuries make up only a small percentage of overall traumas, they account for 13% of critical injuries. Amongst abdominal injuries, two of the most commonly injured organs from impact forces are the kidneys and liver; whether it be from a car accident, explosion, or impact from a projectile [2
]. Due to the quantity of injuries, and the life-threatening impact of these injuries, studies investigating the injury mechanism have become increasingly common to improve understanding in a variety of fields such as safety, forensics, diagnostic medicine, etc.
Although crash test dummies are often used to model human response during motor vehicle accidents and other impacts, including physical abdominal organ models capable of measuring and predicting injuries is difficult and requires a large amount of resources [5
]. Instead, a tool that is commonly used to gain insight into the mechanism of abdominal injury is finite element simulation. Many models have been created to aid in investigating abdominal tissue trauma [6
]. The model results are dependent on the tissue material properties, and models in the literature use properties derived from a variety of methods not always reflective of the model application. Since the tissue mechanical properties are dependent on the testing methodology, using properties from incompatible testing could lead to inaccurate results. For example, many organs exhibit differing properties when tested in tension versus compression [15
], and most human tissue shows dependence on the loading rate for the elastic modulus (E
) and failure properties [5
It has been reported that, during blunt traumatic injury, the liver and kidneys are placed in compression at dynamic rates [19
]. However, much of the current literature utilizes methods other than whole organ compression, which is the loading condition experienced in-vivo
, such as tension testing, which involves dissecting the whole organ into several pieces in order to standardize the specimens and to control the location where nonlinear behavior and/or failure occurs [5
]. Other methods include probing, which utilizes a compression testing protocol with a loading apparatus that is significantly smaller than the surface area of the organ [5
]. Probing methodologies do not reflect the loading scenario caused by blunt traumatic injury, and thus, material properties derived using such methods might not be appropriate. However, probing at low levels of force and deformation can be used on live specimens [32
]. The underlining assumption of tension testing or probing studies are that the properties measured for a portion of the organ are representative of those for a whole intact organ. Comparison of the results from previous studies that used different testing methods reveals variation in failure stress and strain up to 300% [5
]. The wide range of published results supports the statement that utilizing a testing methodology that is reflective of the loading scenario being modeled is important to obtaining accurate results.
Only a select few studies have investigated the material properties of the liver and kidney under compression and/or at dynamic rates [31
], but they did not investigate the properties of intact organs. Instead, they used partial specimens, which has been shown to affect the measured properties [16
]. Snedeker [34
] measured the elastic modulus of the parenchyma of human and porcine kidneys under quasi-static compressive loading. Umale [5
] also performed dynamic compression testing on the porcine kidney parenchyma at rates of 1.5 to 2 m/s but did not report any of the material parameters, but rather provided fit data for an Ogden model. Indentation (probing) testing was performed by Umale [5
] and Lu [29
] on human and porcine kidneys at quasi-static loading rates. Previous studies of the liver followed the same testing procedures and loading rates, with most testing performed on partial specimens under tension. Of the few compression tests, the majority were at quasi-static rates [5
]. Only a single study [36
] tested the liver under dynamic compression using a Kolsky Bar technique, and did not report the material properties directly but only through a custom mathematical model.
Furthermore, several studies utilize organs from non-human hosts, such as porcine, bovine, or monkeys, as it is often difficult to obtain human specimens [5
]. Very few studies directly compare the results from human and non-human hosts [17
], and only one study for kidney and another for liver tissue has investigated the feasibility of using porcine tissue properties as a substitute for human tissue parameters [34
]. The study involving the kidney only investigated the tissue from the two hosts using quasi-static tension testing of the kidney capsule. It was found that the elastic modulus differed significantly, but the failure properties did not, and thus porcine kidney tissue is a justifiable surrogate for human tissue [34
]. It is still unknown whether these findings hold true at dynamic rates or when tested on an intact organ. The study that compared human and porcine liver failure stress also investigated bovine liver properties and the effect of loading rate through comparing results with two similar studies [38
]. It was found that all three hosts demonstrated strain-rate dependent characteristics but that the specific results varied greatly between hosts. Moreover, the findings were compared to other published results that did not use the same methods and all the studies used partial specimens rather than intact livers.
There remain several gaps in knowledge regarding the response of the kidneys and liver to compressive loading. First, the stress-strain behavior, including failure properties, has not been extensively studied and reported for full organ testing. Second, the strain rates at which testing has been performed are limited, and little or no data is available at higher strain rates. Finally, the suitability of using porcine organ results as a substitute for human organ properties has not been fully explored, particularly under compression and at the strain rates of interest for many blunt trauma incidents. This study is the first to investigate these topics in depth and to compare the results using multiple testing methodologies.
The goals of this research are to characterize the material properties of the intact liver and kidney in compression using two protocols, full unconfined compression and probing, at varying strain rates for human and porcine tissues. Specifically, the aims are to determine the feasibility of using porcine tissue as a model for human tissue, compare the results using the two testing protocols, evaluate the impact of using intact organs, and quantify the relationship between strain rate and the elastic modulus (E), failure stress (σf), and failure strain (ef) of the liver and kidney individually. These parameters were chosen since they are required for developing numerical models and for interpreting the results regarding injury. It is hypothesized that there will be no differences between the material properties of human and porcine tissues, and also that increasing strain-rate will increase the E, σf and εf. Further characterizing the material properties of these organs will lead to improved computational and physical models for a range of dynamic loading conditions.