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.

External landmark, body surface, and volume data of a mid-sized male in seated and standing postures.

The purpose of this study was to acquire external landmark, undeformed surface, and volume data from a pre-screened individual representing a mid-sized male (height 174.9 cm, weight 78.6 ± 0.77 kg) in the seated and standing postures. The individual matched the 50th percentile value of 15 measures of external anthropometry from previous anthropometric studies with an average deviation of 3%. As part of a related study, a comprehensive full body medical image data set was acquired from the same individual on whom landmark data were collected. Three dimensional bone renderings from this data were used to visually verify the landmark and surface results. A total of 54 landmarks and external surface data were collected using a 7-axis digitizer. A seat buck designed in-house with removable back and seat pan panels enabled collection of undeformed surface contours of the back, buttocks, and posterior thigh. Eight metrics describing the buck positioning are provided. A repeatability study was conducted with three trials to assess intra-observer variability. Total volume and surface area of the seated model were found to be 75.8 × 10(3) cm(3) and 18.6 × 10(3) cm(2) and match the volume and surface area of the standing posture within 1%. Root mean squared error values from the repeatability study were on average 5.9 and 6.6 mm for the seated and standing postures respectively. The peak RMS error as a percentage of the centroid size of the landmark data sets were 3% for both the seated and standing trials. The data were collected as part of a global program on the development of an advanced human body model for blunt injury simulation. In addition, the reported data can be used for many diverse applications of biomechanical research such as ergonomics and morphometrics studies.

Development and Testing of a Prototype Pregnant Abdomen for the Small-Female Hybrid III ATD.

A new prototype pregnant abdomen for the Hybrid III small-female ATD is being developed and has been evaluated in a series of component and whole-dummy tests. The new abdomen uses a fluid-filled silicone-rubber bladder to represent the human uterus at 30-weeks gestation, and incorporates anthropometry based on measurements of pregnant women in an automotive driving posture. The response of the new pregnant abdomen to rigid-bar, belt, and close-proximity airbag loading closely matches the human cadaver response, which is thought to be representative to the response of the pregnant abdomen. In the current prototype, known as MAMA-2B (Maternal Anthropomorphic Measurement Apparatus, version 2B), the risk of adverse fetal outcome is determined by measuring the peak anterior pressure within the fluid-filled bladder. Peak internal bladder pressures measured in a series of sled-test simulations of frontal crashes of different severities and occupant-restraint conditions have been correlated to the likelihood of adverse fetal outcome based on risk curves developed from in-depth investigations of real-world crashes involving pregnant occupants. Compared to the original pregnant abdomen, the new prototype has improved geometry and improved impact response to a range of potential in-vehicle loading conditions, However, additional instrumentation development and more rigorous testing are needed before the MAMA-2B can be confidently used to assess restraint system performance with regard to reducing the likelihood of adverse fetal outcome in motor-vehicle crashes.

A comprehensive program to improve safety for pregnant women and fetuses in motor vehicle crashes: a preliminary report.

OBJECTIVE: A program was developed to study the mechanisms of abruptio placentae and pregnancy loss caused by motor vehicle crashes. The results were intended to be used to develop strategies to improve protection of the fetus in this setting. STUDY DESIGN: Four integrated projects were conducted: (1) seated anthropometric measurements and belt fit determination during pregnancy, (2) development of new models of traumatic abruptio placentae, (3) investigations of crashes involving pregnant women, and (4) the development of the second-generation pregnant crash dummy from these data and others. RESULTS: Twenty-two different pregnant subjects in five different height groups underwent serial measurements of abdominal surface contours, seat belt fit, and distances between the subjects and various landmarks in the automobile interior with a laboratory-designed "automobile seat" (seating buck). The abdomen was significantly closer to the steering wheel in the shorter stature group than among the taller women. Beginning at approximately 20 weeks' gestation the fundus of the uterus was above the lower rim of the steering wheel. Lap belts fit properly over the anterior superior iliac spine throughout gestation, but the lap belt overlapped the uterus in the midsagittal plane. Two separate mechanisms for traumatic abruptio placentae were tested: shear failure and tensile failure. In the shear failure model large circumferential strains in the uterine wall induce a shear strain across the uteroplacental interface, and the model predicts placental separation at a mean circumferential strain of -58% +/- 8%. By means of finite-element modeling, it was demonstrated that tensile failure might also be a mechanism that causes abruptio placentae during rapid deceleration of the uterus. Crash investigations were performed in 43 cases involving pregnant women beyond 20 weeks' gestation. There were a total of 8 fetal losses and 8 major complications (fetal survival with abruptio placentae, direct fetal injury, or preterm delivery before 34 weeks' as a result of the accident). The best predictors of fetal loss or adverse outcome were impact severity and proper seat belt use. With these newly acquired data a second-generation crash dummy, known as the Maternal Anthropomorphic Measurement Apparatus version 2b (MAMA-2b), is being developed. It incorporates strain gauges in the fundal region of the fluid-filled uterus plus pressure transducers in both the anterior and posterior uterus. Criteria are being developed to associate the likelihood of abruptio placentae with measurements from these instruments that correspond to the two major hypothesized mechanisms of abruptio placentae. CONCLUSION: An improved understanding of the elements of automobile crashes that cause fetal loss and other major pregnancy complications has been gained through this series of investigations.

Investigations of crashes involving pregnant occupants.

Case reports of 16 crashes involving pregnant occupants are presented that illustrate the main conclusions of a crash-investigation program that includes 42 crashes investigated to date. Some unusual cases that are exceptions to the overall trends are also described. The study indicates a strong association between adverse fetal outcome and both crash severity and maternal injury. Proper restraint use, with and without airbag deployment, generally leads to acceptable fetal outcomes in lower severity crashes, while it does not affect fetal outcome in high-severity crashes. Compared to properly restrained pregnant occupants, improperly restrained occupants have a higher risk of adverse fetal outcome in lower severity crashes, which comprise the majority of all motor-vehicle collisions.