Characterization of ovine utero-placental interface tensile failure

Data on the strength of the utero-placental interface (UPI) would help improve understanding of the mechanisms of placental abruption (premature separation of the placenta from the uterus) during motor-vehicle crashes involving pregnant occupants. An ovine model was selected for study because like the human, its placenta has a villous attachment structure. Uteri with intact placentas were obtained from three sheep as by-products of another research study. The samples were harvested between 102 and 119 days of the 145-day gestational period. Rectangular specimens with areas measuring 15 mm × 5 mm were cut through the thickness of the placenta and uterus. Each subject provided eight samples, of which four were tested at a nominal strain rate of 0.10 strains/sec and the remainder was tested at a nominal strain rate of 1.0 strains/sec. Sutures were used to secure the uterine side of the specimens to the test fixture, while mechanical clamps were used to attach the placenta side. A FARO arm scanner recorded the initial geometry of the tissue, and a random dot pattern applied to the placenta and uterus tissue allowed visualization of displacement. For the structure of the UPI, mean tensile failure strain and standard deviations are 0.37 (0.11) and 0.37 (0.18) for the 0.10 and 1.0 strain rates, respectively (p-value = 0.970) while the associated failure stresses are 6.5 (1.37) and 15.0 (5.08) kPa, (p-value = 0.064). The results from sheep UPI testing provide the first estimate of the human UPI structural failure tolerance.