Experimental in vitro mechanical characterization of porcine Glisson's capsule and hepatic veins

Abstract

Understanding the mechanical properties of human liver is the most critical aspect of numerical modeling for medical applications and impact biomechanics. Many researchers work on identifying mechanical properties of the liver both in vivo and in vitro considering the high liver injury percentage in abdominal trauma and for easy detection of fatal liver diseases such as viral hepatitis, cirrhosis, etc. This study is performed to characterize mechanical properties of individual parts of the liver, namely Glisson's capsule and hepatic veins, as these parts are rarely characterized separately. The long term objective of this study is to develop a realistic liver model by characterizing individual parts and later integrating them. In vitro uniaxial quasi-static tensile tests are done on fresh unfrozen porcine hepatic parts for large deformations at the rate of 0.1 mm/s with a Bose Electroforce 3200 biomaterials test instrument. Results show that mean values of small strain and large strain elastic moduli are 8.22±3.42 and 48.15±4.5 MPa for Glisson's capsule (30 samples) and 0.62±0.41 and 2.81±2.23 MPa for veins (20 samples), respectively, and are found to be in good agreement with data in the literature. Finally, a non-linear hyper-elastic constitutive law is proposed for the two separate liver constituents under study.

Introduction

The liver is the largest solid organ in the abdomen, which when subjected to blunt trauma is associated with the highest morbidity and mortality rates (Nahum and Melvin, 1993). Liver lacerations (punctured liver or ruptured liver) and liver hematomas (blood vessel injury) are often observed in car accidents due to blunt or penetrating trauma. The mechanical properties of Glisson's capsule and hepatic veins can be useful to simulate liver laceration and hematoma, respectively. According to the “Handbook of Human Tolerance” (Elhaney et al., 1976), the liver is the most injured organ by the vehicle interior components such as seat belts, steering wheel, dashboard, armrests, etc. So, mechanical properties of the liver and liver constituents are of great interest for researchers working on biomedical applications, virtual surgery simulators and impact biomechanics.

Qualitative comparison indicates that human and porcine livers have almost indistinguishable mechanical properties. Due to similarities in structure and function with the human liver, the porcine liver acts as a surrogate to the human liver (Kim et al., 2008). Whereas along with human liver, detailed porcine liver characterization and modeling is also of great interest, due to the use of pigs as human surrogates in medical and impact biomechanics investigations.

In a study by Yeh et al. (2002) it is proposed that Young's modulus of the liver can be used to predict the degree of fibrosis in a liver, by demonstrating that Young's modulus of the liver increases with growing grades of fibrosis. It is usually caused by diseases such as hepatitis B, hepatitis C, cirrhosis, hepatocellular carcinoma, etc. Also, various noninvasive techniques, such as Ultrasound Imaging, Magnetic Resonance Elastography (MRE; Klatt et al., 2010) and Transient Elastography using Fibroscan^® (Sporea et al., 2008) are used to characterize liver properties and/or to determine liver stiffness for the calibration of the cirrhosis or fibrosis level in the liver. Indentation tests (in vivo, in situ and in vitro tests) (Carter et al., 2001, Kim et al., 2008) and in vitro techniques such as shear, compression and tension tests (Gao et al., 2010, Yeh et al., 2002), are done on the liver to characterize its mechanical properties. In this study, in vitro tensile tests are preferred, which can be justified by the fact that during liver laceration and hematoma, Glisson's capsule and hepatic vessels, respectively, fail due to local tensile stress.

A capsule in general is a protective tissue sheath covering delicate organs, such as liver, kidney, spleen, eye lens, etc. It plays an important role in maintaining the integrity of the shape of organs and protecting them from shocks and impacts. A few studies are reported in the literature characterizing mechanical properties of capsules of different organs of the body. Snedeker et al. (2005) studied human and porcine kidney capsules in tension and estimated elastic moduli for small and large deformations. Similar results were obtained by Hollenstein et al. (2006) for a bovine liver capsule tested in tension. Brunon et al. (2010) performed a quasi-static tensile test on samples consisting of parenchyma and capsule of porcine and human liver tissue, in order to study the phenomenon of capsule and parenchyma failure at the time of liver laceration during the impact.

To the best of our knowledge there are currently no attempts made to study or characterize liver vessels, probably because it is difficult to locate and extract vessels in the liver. A few attempts are made by researchers to extract mechanical properties of arteries and veins from different parts of the body. Pukacki et al. (2000) carried out tensile experiments on the fresh and cryopreserved arteries and veins of human whereas Balazs et al. (2010) studied porcine coronary veins in tension to determine elastic modulus of the vein.

In this study, elastic moduli obtained for porcine hepatic veins and Glisson's capsule tested in quasi-static tension, are compared with the literature data and a non-linear hyper-elastic Ogden model constitutive law is proposed for both hepatic tissues. This data can further be integrated to develop a complete numerical liver model, which can be used as a healthy human liver model for impact biomechanics and biomedical applications.