A finite porous-viscoelastic model capturing mechanical behavior of human cervix under multi-step spherical indentation

Fatigue loading and volumetric microscopy demonstrate changes to the mouse cervix throughout and after pregnancy

INTRODUCTION

The cervix plays important mechanical roles in pregnancy and regulating the timing of parturition. Dysfunction of the cervix is implicated in disorders of parturition including spontaneous preterm birth, failed induction of labor and post term pregnancies. To address these disorders, it is imperative to first understand the function of the cervix throughout a normal pregnancy. However, current knowledge on the response of the cervix to mechanical fatigue and the underlying microstructural changes throughout a pregnancy is lacking.

METHODS

In this study, mechanical fatigue properties were measured at different stages of pregnancy using uniaxial fatigue testing that simulated circumferential hoop stresses in the cervix. Collagen microstructure was quantified using second harmonic generation imaging and three-dimensional orientation analysis.

RESULTS

The stiffness and modulus of the cervix during fatigue testing were dramatically reduced in all stages of pregnancy, and pregnant samples experienced greater peak strain before failure. All mechanical properties recovered postpartum despite persistent changes in cervix size. Microstructural analysis demonstrated increased local collagen alignment in postpartum samples, which may indicate a mechanism that serves to improve material properties after childbirth.

DISCUSSION

Altogether, conclusions from this study enhance our understanding of how properties of the cervix change with pregnancy and lay the foundation for future work investigating how alterations from this healthy function can lead to spontaneous preterm birth and other reproductive complications.

Time-Dependent Material Properties and Composition of the Nonhuman Primate Uterine Layers Through Gestation

The uterus is central to the establishment, maintenance, and delivery of a healthy pregnancy. Biomechanics is an important contributor to pregnancy success, and alterations to normal uterine biomechanical functions can contribute to an array of obstetric pathologies. Few studies have characterized the passive mechanical properties of the gravid human uterus, and ethical limitations have largely prevented the investigation of mid-gestation periods. To address this key knowledge gap, this study seeks to characterize the structural, compositional, and time-dependent micro-mechanical properties of the nonhuman primate (NHP) uterine layers in nonpregnancy and at three time-points in pregnancy: early 2nd, early 3rd, and late 3rd trimesters. Distinct material and compositional properties were noted across the different tissue layers, with the endometrium-decidua being the least stiff, most viscous, least diffusible, and most hydrated layer of the NHP uterus. Pregnancy induced notable compositional and structural changes to the endometrium-decidua and myometrium, but no micro-mechanical property changes. Further comparison to published human data revealed notable similarities across species, with minor differences noted for the perimetrium and nonpregnant endometrium. This work provides insights into the material properties of the NHP uterus and demonstrates the validity of NHPs as a model for studying certain aspects of human uterine biomechanics.

Equilibrium Mechanical Properties of the Nonhuman Primate Cervix

Cervical remodeling is critical for a healthy pregnancy. Premature tissue changes can lead to preterm birth (PTB), and the absence of remodeling can lead to post-term birth, causing significant morbidity. Comprehensive characterization of cervical material properties is necessary to uncover the mechanisms behind abnormal cervical softening. Quantifying cervical material properties during gestation is challenging in humans. Thus, a nonhuman primate (NHP) model is employed for this study. In this study, cervical tissue samples were collected from Rhesus macaques before pregnancy and at three gestational time points. Indentation and tension mechanical tests were conducted, coupled with digital image correlation (DIC), constitutive material modeling, and inverse finite element analysis (IFEA) to characterize the equilibrium material response of the macaque cervix during pregnancy. Results show, as gestation progresses: (1) the cervical fiber network becomes more extensible (nonpregnant versus pregnant locking stretch: 2.03 ± 1.09 versus 2.99 ± 1.39) and less stiff (nonpregnant versus pregnant initial stiffness: 272 ± 252 kPa versus 43 ± 43 kPa); (2) the ground substance compressibility does not change much (nonpregnant versus pregnant bulk modulus: 1.37 ± 0.82 kPa versus 2.81 ± 2.81 kPa); (3) fiber network dispersion increases, moving from aligned to randomly oriented (nonpregnant versus pregnant concentration coefficient: 1.03 ± 0.46 versus 0.50 ± 0.20); and (4) the largest change in fiber stiffness and dispersion happen during the second trimester. These results, for the first time, reveal the remodeling process of a nonhuman primate cervix and its distinct regimes throughout the entire pregnancy.