Dynamic material properties of the pregnant human uterus.

Given that automobile crashes are the largest single cause of death for pregnant females, scientists are developing advanced computer models of pregnant occupants. The purpose of this study is to quantify the dynamic material properties of the human uterus in order to increase the biofidelity of these models. A total of 19 dynamic tension tests were performed on pregnant human uterus tissues taken from six separate donors. The tissues were collected during full term Cesarean style deliveries and tested within 36 h of surgery. The tissues were processed into uniform coupon sections and tested at 1.5 strains/s using linear motors. Local stress and strain were determined from load data and optical markers using high speed video. The experiments resulted in a non-linear stress versus strain curves with an overall average peak failure true strain of 0.32±0.112 and a corresponding peak failure true stress of 656.3±483.9 kPa. These are the first data available for the dynamic response of pregnant human uterus tissues, and it is anticipated they will increase the accuracy of future pregnant female computational models.

Comparison of risk factors for cervical spine, head, serious, and fatal injury in rollover crashes.

Previous epidemiological studies of rollover crashes have focused primarily on serious and fatal injuries in general, while rollover crash testing has focused almost exclusively on cervical spine injury. The purpose of this study was to examine and compare the risk factors for cervical spine, head, serious, and fatal injury in real world rollover crashes. Rollover crashes from 1995-2008 in the National Automotive Sampling System-Crashworthiness Data System (NASS-CDS) were investigated. A large data set of 6015 raw cases (2.5 million weighted) was generated. Nonparametric univariate analyses, univariate logistic regression, and multivariate logistic regression were conducted. Complete or partial ejection, a lack of seatbelt use, a greater number of roof inversions, and older occupant age significantly increased the risk of all types of injuries studied (p<0.05). Far side seating position increased the risk of fatal, head, and cervical spine injury (p<0.05), but not serious injury in general. Higher BMI was associated with an increased risk of fatal, serious, and cervical spine injury (p<0.05), but not head injury. Greater roof crush was associated with a higher rate of fatal and cervical spine injury (p<0.05). Vehicle type, occupant height, and occupant gender had inconsistent and generally non-significant effects on injury. This study demonstrates both common and unique risk factors for different types of injuries in rollover crashes.

Head and neck loading in everyday and vigorous activities.

The purpose of this study was to document head and neck loading in a group of ordinary people engaged in non-injurious everyday and more vigorous physical activities. Twenty (20) volunteers that were representative of the general population were subjected to seven test scenarios: a soccer ball impact to the forehead, a self-imposed hand strike to the forehead, vigorous head shaking, plopping down in a chair, jumping off a step, a seated drop onto the buttocks, and a vertical drop while seated supine in a chair. Some scenarios involved prescribed and well-controlled stimuli, while others allowed the volunteers to perform common activities at a self-selected level of intensity. Head accelerations up to 31 g and 2888 rad/s(2) and neck loads up to 268 N in posterior shear, 526 N in compression, and 36 Nm in extension were recorded. Most head and neck injury criteria predicted a low risk of injury in all activities. However, rotational head accelerations and Neck Injury Criterion (NIC) values were much higher than some proposed tolerance limits in a large number of tests, all of which were non-injurious. The data from this study help us to establish an envelope of head and neck loading that is commonly encountered and presents a minimal risk of injury.

Effect of strain rate on the tensile material properties of human placenta.

Automobile crashes are the largest cause of injury death for pregnant females and the leading cause of traumatic fetal injury mortality in the United States. Computational models, useful tools to evaluate the risk of fetal loss in motor vehicle crashes, are based on a limited number of quasistatic material tests of the placenta. This study presents a total of 64 uniaxial tensile tests on coupon specimens from six human placentas at three strain rates. Material properties of the placental tissue were evaluated at strain rates of 0.07/s, 0.70/s, and 7.00/s. The test data have average failure strains of 0.34, 0.36, and 0.37, respectively. Failure stresses of 10.8 kPa, 11.4 kPa, and 18.6 kPa correspond to an increase in strain rate from 0.07/s to 7.0/s. The results indicate rate dependence only when comparing the highest strain rate of 7.0/s to either of the lower rates. There is no significant rate dependence between 0.07/s and 0.70/s. When compared with previous testing of placental tissue, the current study addresses the material response to more strain rates as well as provides a much larger set of available data. In summary, tensile material properties for the placenta have been determined for use in computational modeling of pregnant occupant kinematics in events ranging from low impact activities to severe impacts such as in motor vehicle crashes.

Dynamic material properties of the human sclera.

As a result of trauma, approximately 30,000 people become blind in one eye every year in the United States. A common injury prediction tool is computational modeling, which requires accurate material properties to produce reliable results. Therefore, the purpose of this study was to determine the dynamic material properties of the human sclera. A high-rate pressurization system was used to create dynamic pressure to the point of rupture in 12 human eyes. Measurements were obtained for the internal pressure, the diameter of the globe, the thickness of the sclera, and the changing coordinates of the optical markers using high-rate video. A relationship between true stress and true strain was determined for the sclera tissue in two directions. It was found that the average maximum true stress was 13.89+/-4.81 MPa for both the equatorial and meridional directions, the average maximum true strain along the equator was 0.041+/-0.014, and the average maximum true strain along the meridian was 0.058+/-0.018. Results show a significant difference in the maximum strain in the equatorial and meridional directions (p=0.02). In comparing these data with previous studies, it is concluded that the human sclera is both anisotropic and viscoelastic. The dynamic material properties presented in this study can be used for advanced models of the human eye to help prevent eye injuries in the future.

Dynamic biaxial tissue properties of pregnant porcine uterine tissue.

Automobile crashes are the largest single cause of death for pregnant females and the leading cause of traumatic fetal injury mortality in the United States. Current research for pregnant occupant safety utilizing computational models is limited by available pregnant tissue data. The purpose of this study is to collect experimental data from biaxial tissue tests on pregnant uterine tissue at a dynamic rate. Experimental tests were completed on pregnant porcine uterus which was chosen as a surrogate for the human pregnant uterus given its similarity and availability. Biaxial dynamic tensile tests were performed using a custom designed system of linear motors to pull a cruciform shaped specimen in tension simultaneously with four tissue clamps. The test series included 23 tests with corresponding peak stress and strain measurements of the central region of the specimen where optical markers tracked local displacements. The specimen was loaded at a rate of 0.7 strains per second to match the uterine strain rate in a motor vehicle crash. Experimental results include peak stresses and peak strains for the pregnant uterine tissue in tension. When loaded biaxially, the circumferential peak stress is 500 +/- 219 kPa with a corresponding peak true strain 0.30 +/- 0.09 and the longitudinal peak stress is 320 +/- 176 kPa with a corresponding peak true strain 0.30 +/- 0.09. This material information can be implemented in pregnant occupant models to evaluate the uterine tissue response to impact loading scenarios.

The influence of arm position on thoracic response in side impacts.

The purpose of this study was to investigate the influence of arm position on thoracic response and injury severity in side impacts. A total of sixteen non-destructive side impact tests and four destructive side impact tests were preformed using four human male cadavers. Single-axis strain gages were placed on the lateral and posterior regions of ribs three through eight on the impacted side, and the lateral region of ribs three through eight on the non-impacted side. Thoracic rods attached to ribs five, seven, and nine were used to measure lateral rib deflection. For the non-destructive tests, four test conditions with different arm positions were evaluated for each cadaver by performing displacement-controlled, low-energy, lateral impacts, 16 kg at 3 m/s, with a pneumatic impactor. The results of these tests showed that the highest average peak forces, peak rib deflections, and peak rib strains were observed when only the ribs were impacted and lowest when the shoulder was impacted. In addition, higher average peak forces, peak rib deflections, and rib strains were observed when the arm was placed parallel with the thorax versus 45 degrees. For the destructive tests, two test conditions were evaluated by performing high-energy lateral impacts, 23.4 kg at 12 m/s, with a pneumatic impactor. Only one destructive test was performed per cadaver with the arm placed at either 45 degrees or parallel with the thorax. Using rib fractures as the parameter for AIS, both tests conducted with the arm at 45 degrees resulted in an AIS = 4 due to the large number of ribs with multiple fractures which resulted in a flail chest. Conversely, both tests conducted with the arm parallel with the thorax resulted in an AIS=3. The analysis of thoracic strain gage time histories showed that, in general, the rib fracture timing varied with respect to thoracic region. Using scaled rib 5 deflection, lateral fractures on the impacted side occurred between 4.2 mm and 34.9 mm, posterior fractures on the impacted side occurred between 19.0 mm and 37.8 mm, and lateral fractures on the non-impacted side occurred between 60.2 mm and 74.3 mm of deflection. It was found that AIS = 1 injuries occurred at scaled rib deflections of 4.2 mm to 8.6 mm (2% to 3% compression), AIS = 2 at 9.6 to 17.4 mm (4% to 7% compression), and AIS = 3 at 13.1 mm to 20.1 mm (5% to 9% compression) measured at rib five. In conclusion, the results of the current study show that in low-energy side impacts both the arm and shoulder reduce impactor force, rib deflection, and rib strain. In high-energy side impacts, the position of the arm has a considerable effect on both the total number and distribution of rib fractures.

Dynamic tensile properties of human placenta.

Automobile crashes are the largest cause of injury death for pregnant females and the leading cause of traumatic fetal injury mortality in the United States. Computational models, useful tools to evaluate the risk of fetal loss in motor vehicle crashes, are based on a limited number of quasi-static material tests of the placenta. This study presents a total of 20 dynamic uniaxial tensile tests on the maternal side of the placenta and 10 dynamic uniaxial tensile tests on the chorion layer of the placenta. These tests were completed from 6 human placentas to determine material properties at a strain rate of 7.0 strains/s. The results show that the average peak strain at failure for both the maternal portion and the chorion layer of the placenta are similar with a value of 0.56 and 0.61, respectively. However, the average failure stress for the chorion layer, 167.8 kPa, is much higher than the average failure stress for the placenta with the chorionic plate removed, 18.6 kPa. This is due to differences in the structure and function of these layers in the placenta. In summary, dynamic loading data for the placenta have been determined for use in computational modeling of pregnant occupant kinematics in motor vehicle crashes. Moreover the computational model should utilize the material properties for the placenta without the chorion layer.

Evaluation of pregnant female injury risk during everyday activities.

Exercise is encouraged for the pregnant female, but there are no data available indicating the risk of fetal loss associated with the level of exercise. The purpose of this study was to assess the risk of fetal loss by simulating exercises using the pregnant computational model. A previously validated MADYMO computer model of a 30-week pregnant female has proven a useful tool in calculating the risk of adverse fetal outcome. Four small female nonpregnant volunteers performed six activities including sitting in a chair normally, walking at 1.3 m/s, running at 2.7 m/s, performing jumping jacks, achieving maximum vertical leap in place, and jumping off of a step 20 cm high. The results for this study are 12 simulations with an average risk of fetal loss equal to 10.0 +/- 4.1%. The minimum risk from the simulations is 3.1% for walking and the maximum risk is 18.8% for running. The low impact exercises have effectively no risk when taken in context with the validation of the computational model from motor vehicle crash data. However, the pregnant female can have an appreciable risk of adverse fetal outcome for the higher impact activities. In conclusion, this study confirms a low risk of adverse fetal outcome for a healthy pregnant female during low impact exercise events.

Pregnant occupant injury risk in severe frontal crashes using computer simulations.

Automobile crashes are the largest cause of death for pregnant females and the leading cause of traumatic fetal injury mortality in the United States. A previously validated MADYMO computer model of a 30-week pregnant occupant was used in this study to investigate the pregnant occupant response in a severe frontal motor vehicle crash. This study presents simulations of 26 different severe car crash tests, encompassing nine vehicle models that represent the compact, medium, and sport utility vehicle classes during the years 1996 to 2006. With the pregnant occupant in the passenger seat, these tests involve a vehicle with an initial velocity of 35 mph into a fixed barrier with the full width of the front of the vehicle. Uterine strain from the computational model indicates the risk of adverse fetal outcome for a pregnant occupant in each vehicle. The average risk of fetal loss associated with these frontal crashes is 85 +/- 13% with a minimum risk of 55% and a maximum risk of 100%. This high risk of fetal loss is consistent with published pregnant occupant case studies that have an equivalent change in velocity. When compared to testing for the average male, this study suggests that current safety standards do not accurately address the risk to a pregnant occupant in a severe frontal crash.

Tensile material properties of human tibia cortical bone effects of orientation and loading rate.

The purpose of this study was to quantify effects of both specimen orientation and loading rate on the tensile material properties for human tibia cortical bone in a controlled study. This study presents 25 human tibia cortical bone coupon tests obtained from the mid-diaphysis of two fresh frozen male human cadavers: 11 axial and 14 lateral. The primary component for the tension coupon testing was a high rate servo-hydraulic Material Testing System (MTS) with a custom slack adaptor. The specimen were loaded at a constant strain rate of approximately 0.05 strains/s, 0.5 strains/s, or 5.0 strains/s. Axial specimens were found to have a significantly larger ultimate stress and ultimate strain compared to lateral specimens for all loading rates, and a significantly larger modulus for low and high loading rates. This finding illustrates the anisentropic behavior of bone over a range of strain rates, which is attributed to the microstructure of the bone and the osteon orientation along the long axis of the bone. With respect to loading rate, both axial and lateral specimens showed significant increases in the modulus and significant decreases in ultimate strain with increased loading rate. Although not significant, axial specimens showed another traditional viscoelastic trend, with ultimate stress increasing with increased loading rate.

The effects of extraocular muscles on static displacements of the human eye.

More than 30,000 people lose sight in at least one eye every year in the United States. Globe rupture is one of the most severe injuries and can often result in the loss of an eye. Previous studies to determine the injury tolerance of the human eye to globe rupture have not investigated the effects of extraocular muscles on the response of the eye. The purpose of the current study is to quantify the effects of the extraocular muscles using quasistatic displacement tests of the human eye in situ. A total of three post-mortem human heads were used for the matched pair tests designed to elucidate any differences in the force-deflection response of the human eye with the extraocular muscles intact and transected. Computed tomography imaging was utilized to observe the deformation of the eye within the orbit for each displacement, from 0 mm to 30 mm. Slight differences in the force-deflection response are observed; however, it is not clear how these differences will influence impact response at a dynamic rate. It was also observed that under quasistatic displacements that the eye is able to translate out of the way of the impactor assembly, even under large deflections, and without globe rupture. Additional dynamic tests are recommended to determine the effects of the extraocular muscles on eye impact response.

Predicting neck injuries due to head-supported mass.

BACKGROUND: Technological advances in military equipment have resulted in more devices being mounted on the helmet to enhance the capability of the soldier. The soldier's neck must bear this head-supported mass (HSM) and the resulting dynamic characteristics of the head and neck system are changed. The purpose of this study was to vary the conditions of impact as well as the design criteria to quantify the effect of HSM on neck injury risk through computational modeling. METHODS: The TNO MADYMO detailed neck model was used for a matrix of 196 simulations designed to vary the impact conditions and HSM properties added to the model. These parameters included seven impact directions, three impact magnitudes, nine mass locations, and three mass magnitudes. The data collected from these simulations were evaluated for injury risk using the lower neck beam criterion equation. RESULTS: The results from these simulations provide detailed information regarding the risk of injury based on a particular HSM configuration and the acceleration of the body. The predominant factor in increasing risk in the lower neck is the increase in pulse magnitude. The effect of pulse magnitude is more dominant in the directions that create a flexion or lateral bending moment. CONCLUSION: HSM increases the level of injury, but the impact level that the subject is exposed to is a more dominating factor in determining injury risk.

Analysis of real-time head accelerations in collegiate football players.

OBJECTIVE: To measure and analyze head accelerations during American collegiate football practices and games. METHODS: A newly developed in-helmet 6-accelerometer system that transmits data via radio frequency to a sideline receiver and laptop computer system was implemented. From the data transfer of these accelerometer traces, the sideline staff has real-time data including the head acceleration, the head injury criteria value, the severity index value, and the impact location. Data are presented for instrumented players for the entire 2003 football season, including practices and games. SETTING: American collegiate football. SUBJECTS: Thirty-eight players from Virginia Tech's varsity football team. MAIN OUTCOME MEASUREMENTS: Accelerations and pathomechanics of head impacts. RESULTS: : A total of 3312 impacts were recorded over 35 practices and 10 games for 38 players. The average peak head acceleration, Gadd Severity Index, and Head Injury Criteria were 32 g +/- 25 g, 36 g +/- 91 g, and 26 g +/- 64 g, respectively. One concussive event was observed with a peak acceleration of 81 g, a 267 Gadd Severity Index, and 200 Head Injury Criteria. Because the concussion was not reported until the day after of the event, a retrospective diagnosis based on his history and clinical evaluation suggested a mild concussion. CONCLUSIONS: The primary finding of this study is that the helmet-mounted accelerometer system proved effective at collecting thousands of head impact events and providing contemporaneous head impact parameters that can be integrated with existing clinical evaluation techniques.

Material properties of human rib cortical bone from dynamic tension coupon testing.

The purpose of this study was to develop material properties of human rib cortical bone using dynamic tension coupon testing. This study presents 117 human rib cortical bone coupon tests from six cadavers, three male and three female, ranging in age from 18 to 67 years old. The rib sections were taken from the anterior, lateral, and posterior regions on ribs 1 through 12 of each cadaver's rib cage. The cortical bone was isolated from each rib section with a low speed diamond saw, and milled into dog bone shaped tension coupons using a small computer numerical control machine. A high-rate servo-hydraulic Material Testing System equipped with a custom slack adaptor, to provide constant strain rates, was used to apply tension loads to failure at an average rate of 0.5 strains/sec. The elastic modulus, yield stress, yield strain, ultimate stress, ultimate strain, and strain energy density were determined from the resulting stress versus strain curves. The overall average of all cadaver data gives an elastic modulus of 13.9 GPa, a yield stress of 93.9 MPa, a yield strain of 0.88 %, an ultimate stress of 124.2 MPa, an ultimate strain of 2.7 %, and a strain energy density of 250.1 MPa-microstrain. For all cadavers, the plastic region of the stress versus strain curves was substantial and contributed approximately 60 % to the strain energy and over 80 % in the tests with the 18 year old cadaver. The rib cortical bone becomes more brittle with increasing age, shown by an increase in the modulus (p < 0.01) and a decrease in peak strain (p < 0.01). In contrast to previous three-bending tests on whole rib and rib cortical bone coupons, there were no significant differences in material properties with respect to rib region or rib level. When these results are considered in conjunction with the previous three-point bending tests, there is regional variation in the structural response of the human rib cage, but this variation appears to be primarily a result of changes in the local geometry of each rib while the material properties remain nearly constant within an individual.