Equilibrium mechanical properties of the human uterus in tension and compression

A successful pregnancy relies on the proper cellular, biochemical, and mechanical functions of the uterus. A comprehensive understanding of nonpregnant and pregnant uterine mechanical properties is key to understanding different obstetric and gynecological disorders such as preterm birth, placenta accreta, uterine rupture, leiomyoma, adenomyosis, and endometriosis. This study sought to characterize the macro-scale equilibrium material behaviors of the human uterus in nonpregnancy and late pregnancy under both compressive and tensile loading. Forty four human uterine specimens from 16 patients (8 nonpregnant [NP] and 8 pregnant [PG]) were tested using spherical indentation and uniaxial tension coupled with digital image correlation (DIC). A three-strain level incremental load-hold protocol was applied to both tests. A microstructurally-inspired material model considering fiber architecture was applied to this dataset. Inverse finite element analysis (IFEA) was then performed to generate a single set of mechanical parameters to describe compressive and tensile behaviors. The freeze-thaw effect on uterine mechanical properties was also evaluated. For this cohort of tissue samples, the fiber network of the PG uterus was more extensible than in the NP tissue. The initial fiber stiffness and ground substance compressibility were similar between NP and PG uterine tissue. Lastly, a single freeze-thaw cycle did not systematically alter the mechanical behavior of the human uterus under indentation.