Hyperelastic Mechanical Properties of Ex Vivo Normal and Intrauterine Growth Restricted Placenta

Abstract

Intrauterine Growth Restriction (IUGR) is a serious and prevalent pregnancy complication that is due to placental insufficiency and IUGR babies suffer significantly higher risks of mortality and morbidity. Current detection rate for IUGR is generally poor and thus an alternative diagnostic tool is needed to improve the IUGR detection. Elastography, a non-invasive method that measures the tissue stiffness, has been proposed as one such technique. However, to date, we have limited information on the mechanical properties of IUGR placenta. In this study, we investigated the mechanical properties of normal and IUGR placentae and prescribed a suitable hyperelastic model to describe their mechanical behaviors. A total of 46 normal and 43 IUGR placenta samples were investigated. Results showed that placenta samples were isotropic, but had a high spatial variability of stiffness. The samples also had significant viscoelasticity. IUGR placenta was observed to be slightly stiffer than normal placenta but the difference was significant only at compression rate of 0.25 Hz and with 20% compression depth. Three simple hyperelastic models-Yeoh, Ogden and Fung models, were found to be able to fit the experimentally measured mechanical behaviors, and Fung model performed slightly better. These results may be useful for optimizing placenta elastography for the detection of IUGR.

A Review of Biomechanics Analysis of the Umbilical–Placenta System With Regards to Diseases

“…In vitro experiments had shown that hypoxic condition was shown to lead to increased production of collagen (Agocha et al, 1997;Chen and Aplin, 2003;Kowalski et al, 2015). IUGR placenta, in fact, was found to have a greater amount of collagen to elastin ratio as compared to that of in normal (Saw et al, 2018b), which corroborated with the in vitro experiment results (Agocha et al, 1997;Chen and Aplin, 2003;Kowalski et al, 2015). The increased amount of collagen in IUGR suggested that the IUGR placenta was in hypoxic condition and thus increased in its stiffness.…”

“…For example, a recent study developed a FE model of the uterus, placenta, fetus and amniotic fluid, and investigated the biomechanical effects of a blunt trauma on the pregnant abdomen (Irannejad Parizi et al, 2020 ). A hyperelastic model such as the Ogden model was commonly used in the past to describe the mechanical properties of the placenta during loading conditions (Hu et al, 2009 ; Pérès et al, 2014 ; Saw et al, 2018b ). Recently, models were developed by taking the viscoelastic properties of the placenta into account and such models were able to describe the stress-strain relationship of the placenta during loading and unloading conditions well (Lau et al, 2016 ; Panda and Buist, 2018 ).…”

“…Histological studies had shown abundant shreds of evidence that the diseased placenta had considerably alteration in placental microstructures in which diseased placenta had increased syncytiotrophoblast apoptosis, fibrosis (Allaire et al, 2000 ; Roberts and Post, 2008 ; Scifres and Nelson, 2009 ; Güven et al, 2018 ), thicker trophoblast epithelium (Langheinrich et al, 2008 ), reduced elastin, and increased collagen (Macara et al, 1996 ; Wilhelm et al, 2002 ), suggesting that the mechanical properties of the placenta during disease could have been altered but no direct proof about the placenta mechanical properties was shown. Direct mechanical testing of normal and IUGR placenta was recently conducted and the mechanical properties of IUGR placenta were found to be stiffer than that of the normal placenta and the difference could be captured from constitutive models' parameters (Saw et al, 2018b ). However, the placenta stiffness difference was only significant at certain compression settings, suggesting that the use of non-invasive ultrasound elastography could be used in detecting disease placenta but the setting of placental elastography should be controlled wisely to enhance its' accuracy.…”

“…Such efforts have been made in tumors (Wang and Insana, 2013 ) and breast cancer (Goenezen et al, 2012 ) elastography but not on the human placenta. Another noteworthy issue is that the placenta has been found to have substantial spatial heterogeneity in its mechanical properties (Saw et al, 2018b ). This would thus require measurements to be performed over several locations of the placenta during evaluation.…”