Biofidelity Assessment of the WIAMan Thorax by a Comparative Study With Hybrid III, THOR, and PMHS in Frontal Sled Testing.

The Warrior Injury Assessment Manikin (WIAMan) anthropomorphic test device (ATD) has been originally developed to predict and prevent injuries for occupants in military vehicles, in an underbody blast environment. However, its crash performance and biofidelity of the thoracic region have not been explored. The aim of this study was to determine and evaluate the WIAMan thoracic responses in a typical frontal sled test. The 40 kph frontal sled tests were conducted to quantify the WIAMan thoracic kinematics, chest deflection, and belt loads. Comparative biofidelities of the WIAMan thorax and other surrogates, including postmortem human surrogates (PMHSs), Hybrid III, and test device for human occupant restraint (THOR) ATDs, were assessed under comparable testing conditions. The similarities and differences between WIAMan and the other surrogates were compared and analyzed, including the motion of bilateral shoulders and T1, time histories of chest deflections, and belt loads. The CORrelation and Analysis (CORA) ratings were used to evaluate the correlations of thoracic responses between the ATDs and PMHS. Compared to the PMHS and THOR, the WIAMan experienced a similar level of left shoulder forward excursions. Larger chest deflection was exhibited in WIAMan throughout the whole duration of belt compression. Differences were found in belt loads between subject types. Overall, WIAMan had slightly lower CORA scores but showed comparable overall performance. The overall thoracic responses of WIAMan under the frontal sled test were more compliant than HIII, but still reasonable compared with PMHS and THOR. Comprehensive systematic studies on comparative biofidelity of WIAMan and other surrogates under different impact conditions are expected in future research.

Evaluating the Limits in the Biomechanics of Blunt Lung Injury.

Thoracic blunt trauma is evident in up to one-fifth of all hospital admissions, and is second only to head trauma in motor vehicle crashes. One of the most problematic injury mechanisms associated with blunt thoracic trauma is pulmonary contusion, occurring in up to 75% of blunt thoracic trauma cases. The source and effects of pulmonary contusion caused by blunt lung injury are not well defined, especially within the field of continuum biomechanics. This, paired with unreliable diagnostics for pulmonary contusion, leads to uncertainty in both the clinical entity and mechanics of how to predict the presence of injury. There is a distinct need to combine the clinical aspects with mechanical insights through the identification and mitigation of blunt lung trauma and material testing and modeling. This is achieved through using the mechanical insights of lung tissue behavior in order to better understand the injurious mechanisms and courses of treatment of blunt-caused pulmonary contusion. This paper hopes to act as a step forward in connecting two perspectives of blunt lung injury, the clinical entity, and mechanical testing and modeling, by reviewing the known literature and identifying the unknowns within the two related fields. Through a review of related literature, clinical evidence is correlated to mechanical data to gain a better understanding of what is being missed in identification and response to blunt lung injury as a whole.

Optimization of Underbody Blast Energy-Attenuating Seat Mechanisms Using Modified MADYMO Human Body Models.

Though energy attenuating (EA) seats for air and spacecraft applications have existed for decades, they have not yet been fully characterized for their energy attenuation capability or resulting effect on occupant protection in vertical underbody blast. EA seats utilize stroking mechanisms to absorb energy and reduce the vertical forces imparted on the occupant's pelvis and lower spine. Using dynamic rigid-body modeling, a virtual tool to determine optimal force and deflection limits was developed to reduce pelvis and lower spine injuries in underbody blast events using a generic seat model. The tool consists of a mathematical dynamic model (MADYMO)-modified human body model (HBM), basic EA seat model, and an optimizing sequence using modefrontier software. This optimizing tool may be shared with EA seat manufacturers and applied to military seat development efforts for EA mechanisms for a given occupant and designated blast severity. To optimally tune the EA seat response, the MADYMO human body model was first updated to improve its fidelity in kinematic response data for high rate vertical accelerative loading relative to experimental data from laboratory simulated underbody blast tests using postmortem human surrogates (PMHS). Subsequently, using available injury criteria for underbody blast, the optimization tool demonstrated the ability to identify successful EA mechanism critical design value configurations to reduce forces and accelerations in the pelvis and lower spine HBM to presumed noninjurious levels. This tool could be tailored by varying input pulses, force and deflection limits, and occupant size to evaluate EA mechanism designs.

An Injury Risk Function for the Leg, Foot, and Ankle Exposed to Axial Impact Loading Using Force and Impulse.

Most injury risk functions (IRFs) for dynamic axial loading of the leg have been targeted toward automotive applications such as predicting injury caused by intrusion into the occupant compartment from frontal collisions. Recent focus on leg injuries in the military has led to questions about the applicability of these IRFs shorter duration, higher amplitude loading associated with underbody blast (UBB). To investigate these questions, data were collected from seven separate test series that subjected post-mortem human legs to axial impact. A force and impulse-based Weibull survival model was developed from these studies to estimate fracture risk. Specimen age was included as a covariate to reduce variance and improve survival model fit. The injury criterion estimated 50% risk of injury for a leg exposed to 13 N s of impulse at peak force and 8.07 kN of force for force durations less than and greater than half the natural period of the leg, respectively. A supplemental statistical analysis estimated that the proposed IRF improves injury prediction accuracy by more than 9% compared to the predictions from automobile-based risk functions developed for automotive intrusion. The proposed leg IRF not only improves injury prediction for higher rate conditions but also provides a single injury prediction tool for an expanded range of load durations ranging from 5 to 90 ms, which spans both automotive and military loading environments.

Comparison of Hybrid-III and postmortem human surrogate response to simulated underbody blast loading.

Response of the human body to high-rate vertical loading, such as military vehicle underbody blast (UBB), is not well understood because of the chaotic nature of such events. The purpose of this research was to compare the response of postmortem human surrogates (PMHS) and the Hybrid-III anthropomorphic test device (ATD) to simulated UBB loading ranging from 100 to 860 g seat and floor acceleration. Data from 13 whole body PMHS tests were used to create response corridors for vertical loading conditions for the pelvis, T1, head, femur, and tibia; these responses were compared to Hybrid-III responses under matched loading conditions.

Development of a biofidelic computational model of human pelvis for predicting biomechanical responses and pelvic fractures.

BACKGROUND AND OBJECTIVE: The pelvis, a crucial structure for human locomotion, is susceptible to injuries resulting in significant morbidity and disability. This study aims to introduce and validate a biofidelic computational pelvis model, enhancing our understanding of pelvis injury mechanisms under lateral loading conditions. METHODS: The Finite Element (FE) pelvic model, representing a mid-sized male, was developed with variable cortical thickness in pelvis bones. Material properties were determined through a synthesis of existing constitutive models, parametric studies, and multiple validations. Comprehensive validation included various tests, such as load-displacement assessments of sacroiliac joints, quasi-static and dynamic lateral compression on the acetabulum, dynamic side impacts on the acetabulum and iliac wing using defleshed pelvis, and lateral impacts by a rigid plate on the full body's pelvis region. RESULTS: Simulation results demonstrated a reasonable correlation between the pelvis model's overall response and cadaveric testing data. Predicted fracture patterns of the isolated pelvis exhibited fair agreement with experimental results. CONCLUSIONS: This study introduces a credible computational model, providing valuable biomechanical insights into the pelvis' response under diverse lateral loading conditions and fracture patterns. The work establishes a robust framework for developing and enhancing the biofidelity of pelvis FE models through a multi-level validation approach, stimulating further research in modeling, validation, and experimental studies related to pelvic injuries. The findings are expected to offer critical perspectives for predicting, preventing, and mitigating pelvic injuries from vehicular accidents, contributing to advancements in clinical research on medical treatments for pelvic fractures.

A Novel Paradigm to Develop Regional Thoracoabdominal Criteria for Behind Armor Blunt Trauma Based on Original Data.

INTRODUCTION: For behind armor blunt trauma (BABT), recent prominent BABT standards for chest plate define a maximum deformation distance of 44 mm in clay. It was developed for soft body armor applications with limited animal, gelatin, and clay tests. The legacy criterion does not account for differing regional thoracoabdominal tolerances to behind armor-induced injury. This study examines the rationale and approaches used in the legacy BABT clay criterion and presents a novel paradigm to develop thoracoabdominal regional injury risk curves. MATERIALS AND METHODS: A review of the original military and law enforcement studies using animals, surrogates, and body armor materials was conducted, and a reanalysis of data was performed. A multiparameter model analysis describes survival-lethality responses using impactor/projectile (mass, diameter, and impact velocity) and specimen (weight and tissue thickness) variables. Binary regression risk curves with ±95% confidence intervals (CIs) and peak deformations from simulant tests are presented. RESULTS: Injury risk curves from 74 goat thorax tests showed that peak deflections of 44.7 mm (±95% CI: 17.6 to 55.4 mm) and 49.9 mm (±95% CI: 24.7 to 60.4 mm) were associated with the 10% and 15% probability of lethal outcomes. 20% gelatin and Roma Plastilina #1 clay were stiffer than goat. The clay was stiffer than 20% gelatin. Penetration diameters showed greater variations (on a test-by-test basis, difference 36-53%) than penetration depths (0-12%) across a range of projectiles and velocities. CONCLUSIONS: While the original authors stressed limitations and the importance of additional tests for refining the 44 mm recommendation, they were not pursued. As live swine tests are effective in developing injury criteria and the responses of different areas of the thoracoabdominal regions are different because of anatomy, structure, and function, a new set of swine and human cadaver tests are necessary to develop scaling relationships. Live swine tests are needed to develop incapacitation/lethal injury risk functions; using scaling relationships, human injury criteria can be developed.

Pulmonary injury risk assessment for long-duration blasts: a meta-analysis.

BACKGROUND: Long-duration blasts are an increasing threat with the expanded use of thermobaric and other novel explosives. Other potential long-duration threats include large explosions from improvised explosive devices, weapons caches, and other explosives including nuclear explosives. However, there are very few long-duration pulmonary blast injury assessments, and use of short-duration exposure injury metrics is inappropriate as the injury mechanism for long-duration exposures is likely different from that of short-duration exposures. METHODS: This study develops an injury model for long-duration (>10 milliseconds positive overpressure phase) blasts with sharp rising overpressures. For this study, data on more than 2,730 large animal experiments were collected from more than 55 experimental studies on blast. From this dataset, nearly 850 large animal experiments were selected with positive phase overpressure durations of 10 milliseconds or more. Various models were evaluated to determine the best fit of injury risk as a function of pressure and duration. A linear logistic regression was performed on the experimental data for threshold injury and lethality in terms of pressure and duration. The effects of mass, pressure, and duration scaling were all evaluated, and two goodness-of-fit indicators were used to assess the different models. RESULTS AND CONCLUSIONS: New injury risk assessment curves were determined for both incident and reflected pressure conditions for reflecting surface and free-field exposures. Position dependent injury risk curves were also determined. The resulting curves are an improvement to existing assessments, because they use actual data to demonstrate theoretical assumptions on the injury risk.

Drosophila melanogaster larvae as a model for blast lung injury.

BACKGROUND: Primary blast injuries, specifically lung injuries, resulting from blast overpressure exposures are a major source of mortality for victims of blast events. However, existing pulmonary injury criteria are inappropriate for common exposure environments. This study uses Drosophila melanogaster larvae to develop a simple phenomenological model for human pulmonary injury from primary blast exposure. METHODS: Drosophila larvae were exposed to blast overpressures generated by a 5.1-cm internal diameter shock tube and their mortality was observed after the exposure. To establish mortality thresholds, a survival analysis was conducted using survival data and peak incident pressures. In addition, a histologic analysis was performed on the larvae to establish the mechanisms of blast injury. RESULTS: The results of the survival analysis suggest that blast overpressure for 50% Drosophila survival is greater than human threshold lung injury and is similar to human 50% survival levels, in the range of overpressure durations tested (1-5 ms). A "parallel" analysis of the Bass et al. 50% human survival curves indicates that 50% Drosophila survival is equivalent to a human injury resulting in a 69% chance of survival. Histologic analysis of the blast-exposed larvae failed to demonstrate damage to the dorsal trunk of the tracheal system; however, the presence of flocculent material in the larvae body cavities and tracheas suggests tissue damage. CONCLUSIONS: This study shows that D. melanogaster survival can be correlated with large animal injury models to approximate a human blast lung injury tolerance. Within the range of durations tested, Drosophila larvae may be used as a simple model for blast injury.

Kinematic response of the spine during simulated aircraft ejections.

INTRODUCTION: Military aviators are susceptible to spinal injuries during high-speed ejection scenarios. These injuries commonly arise as a result of strains induced by extreme flexion or compression of the spinal column. This study characterizes the vertebral motion of two postmortem human surrogates (PMHS) during a simulated catapult phase of ejection on a horizontal decelerator sled. METHODS: During testing, the PMHS were restrained supinely to a mock ejection seat and subjected to a horizontal deceleration profile directed along the local z-axis. Two midsized males (175.3 cm, 77.1 kg; 185.4 cm, 72.6 kg) were tested. High-rate motion capture equipment was used to measure the three-dimensional displacement of the head, vertebrae, and pelvis during the ejection event. RESULTS: The two PMHS showed generally similar kinematic motion. Head injury criterion (HIC) results were well below injury threshold levels for both specimens. The specimens both showed compression of the spine, with a reduction in length of 23.9 mm and 45.7 mm. Post-test autopsies revealed fractures in the C5, T1, and L1 vertebrae. DISCUSSION: This paper provides an analysis of spinal motion during an aircraft ejection.The injuries observed in the test subjects were consistent with those seen in epidemiological studies. Future studies should examine the effects of gender, muscle tensing, out-of-position (of head from neutral position) occupants, and external forces (e.g., windblast) on spinal kinematics during aircraft ejection.

Biomechanics of the Human Brain during Dynamic Rotation of the Head.

Traumatic brain injuries (TBI) are a substantial societal burden. The development of better technologies and systems to prevent and/or mitigate the severity of brain injury requires an improved understanding of the mechanisms of brain injury, and more specifically, how head impact exposure relates to brain deformation. Biomechanical investigations have used computational models to identify these relations, but more experimental brain deformation data are needed to validate these models and support their conclusions. The objective of this study was to generate a dataset describing in situ human brain motion under rotational loading at impact conditions considered injurious. Six head-neck human post-mortem specimens, unembalmed and never frozen, were instrumented with 24 sonomicrometry crystals embedded throughout the parenchyma that can directly measure dynamic brain motion. Dynamic brain displacement, relative to the skull, was measured for each specimen with four loading severities in the three directions of controlled rotation, for a total of 12 tests per specimen. All testing was completed 42-72 h post-mortem for each specimen. The final dataset contains approximately 5,000 individual point displacement time-histories that can be used to validate computational brain models. Brain motion was direction-dependent, with axial rotation resulting in the largest magnitude of displacement. Displacements were largest in the mid-cerebrum, and the inferior regions of the brain-the cerebellum and brainstem-experienced relatively lower peak displacements. Brain motion was also found to be positively correlated to peak angular velocity, and negatively correlated with angular velocity duration, a finding that has implications related to brain injury risk-assessment methods. This dataset of dynamic human brain motion will form the foundation for the continued development and refinement of computational models of the human brain for predicting TBI.

Ejection injury to the spine in small aviators: sled tests of manikins vs. post mortem specimens.

INTRODUCTION: This study presents the results of seven aerospace manikin and three post mortem human surrogate (PMHS) horizontal deceleration sled tests. The objective of this study was to establish a body of baseline data that examines the ability of small (fifth percentile) manikins to predict whole-body kinematics associated with aircraft ejection, and whether currently available head and neck injury criteria are applicable in these situations. METHODS: Subjects were exposed to a short-duration local z-axis sled pulse while horizontally seated and restrained in an ejection seat. Test subjects included instrumented fifth percentile female and male manikins, and two small (163.8 cm, 48.3 kg; 143.5 cm, 48.6 kg) female and one small (166.2 cm, 54.3 kg) male PMHS. RESULTS: The anterior (local x-axis) translations of the PMHS heads were less than those observed in the manikin tests, but the local z-axis translations of the PMHS heads were greater than those of the manikins. Z-axis translations of the manikins' T1 were generally similar to those of the PMHS T1, but the anterior x-axis translations of T1 were greater in the PMHS. The neck injury criterion (Nij) tended to under-predict observed injury (primarily ruptures of the posterior ligaments at C4-5, T2-3), and the Beam Criterion (BC) tended to over-predict observed injury for small occupants. The USN/USAF neck injury criteria (NIC) performed best in predicting the observed injuries. DISCUSSION: Present manikin designs do not predict the kinematics of PMHS in ejection tests. Further refinement of existing injury criteria is required to accurately predict location and severity of ejection-induced injuries.

Viscoelastic properties of the cervical spinal ligaments under fast strain-rate deformations.

The mechanical response of ligaments under fast strain-rate deformations is a necessary input into computational models that are used for injury assessment. However, this information frequently is not available for the ligaments that are routinely injured in fast-rate loading scenarios. In the current study, experiments were conducted at fast strain rates for the cervical spinal ligaments: the anterior longitudinal ligament, the posterior longitudinal ligament and the ligamentum flavum. Bone-ligament-bone complexes at three spine levels were harvested for mechanical testing. Displacement-controlled sub-failure uniaxial tensile tests were performed in both load-relaxation and sinusoidal conditions. A nonlinear (separable) viscoelastic model was used to examine the experimental data. An unexpected result of the modeling was that the instantaneous elastic functions could be approximated as linear for these strain rates. A five-parameter model was sufficient to characterize the ligament viscoelastic responses and had good predictive capacity under different applied loading conditions.

Pulmonary injury risk assessment for short-duration blasts.

BACKGROUND: Blast injuries are becoming more common in modern war and terrorist action. This increasing threat underscores the importance of understanding and evaluating blast effects. METHODS: For this study, data on more than 2,550 large animal experiments were collected from more than 50 experimental studies on blast. From this dataset, over 1,100 large animal experiments were selected with positive phase overpressure durations of 30 milliseconds or less. A two variable nonlinear logistic regression was performed on the experimental data for threshold injury and lethality in terms of pressure and duration. The effects of mass, pressure, and duration scaling were all evaluated. RESULTS: New injury risk assessment curves were analyzed for both incident and reflected pressure conditions. Position dependent injury risk curves were also analyzed and were found to be unnecessary, at least for prone and side on conditions. CONCLUSIONS: The injury risk assessment showed good correlation to some of the existing injury assessments. It also showed good correspondence to a reported human case of blast exposure. Pressure scaling was analyzed to be unnecessary for these short duration exposures. Recommended injury assessments for various orientations relative to the incoming blast wave are included.

A Novel Method for Quantifying Human In Situ Whole Brain Deformation under Rotational Loading Using Sonomicrometry.

Traumatic brain injuries (TBI) are one of the least understood injuries to the body. Finite element (FE) models of the brain have been crucial for understanding concussion and for developing injury mitigation systems; however, the experimental brain deformation data currently used to validate these models are limited. The objective of this study was to develop a methodology for the investigation of in situ three-dimensional brain deformation during pure rotational loading of the head, using sonomicrometry. Sonomicrometry uses ultrasonic pulses to measure the dynamic distances between piezoelectric crystals implanted in any sound-transmitting media. A human cadaveric head-neck specimen was acquired 14 h postmortem and was instrumented with an array of 32 small sonomicrometry crystals embedded in the head: 24 crystals were implanted in the brain, and 8 were fixed to the inner skull. A dynamic rotation was then applied to the head using a closed-loop controlled test device. Four pulses with different severity levels were applied around three orthogonal anatomical axes of rotation. A repeated test of the highest severity rotation was conducted in each axis to assess repeatability. All tests were completed within 56 h postmortem. Overall, the combined experimental and sonomicrometry methods were demonstrated to reliably and repeatedly capture three-dimensional dynamic deformation of an intact human brain. These methods provide a framework for using sonomicrometry to acquire multidimensional experimental data required for FE model development and validation, and will lend insight into the deformations sustained by the brain during impact.

Preliminary Investigation of Skull Fracture Patterns Using an Impactor Representative of Helmet Back-Face Deformation.

Military combat helmets protect the wearer from a variety of battlefield threats, including projectiles. Helmet back-face deformation (BFD) is the result of the helmet defeating a projectile and deforming inward. Back-face deformation can result in localized blunt impacts to the head. A method was developed to investigate skull injury due to BFD behind-armor blunt trauma. A representative impactor was designed from the BFD profiles of modern combat helmets subjected to ballistic impacts. Three post-mortem human subject head specimens were each impacted using the representative impactor at three anatomical regions (frontal bone, right/left temporo-parietal regions) using a pneumatic projectile launcher. Thirty-six impacts were conducted at energy levels between 5 J and 25 J. Fractures were detected in two specimens. Two of the specimens experienced temporo-parietal fractures while the third specimen experienced no fractures. Biomechanical metrics, including impactor acceleration, were obtained for all tests. The work presented herein describes initial research utilizing a test method enabling the collection of dynamic exposure and biomechanical response data for the skull at the BFD-head interface.

Development of an injury risk curve for pelvic fracture in vertical loading environments.

OBJECTIVE: Pelvis injury mechanisms are dependent upon loading direction (frontal, lateral, and vertical). Studies exist on the frontal and lateral modes; however, similar studies in the vertical mode are relatively sparse. Injury risk curves and response corridors are needed to delineate the biomechanical responses. The objective of the study was to derive risk curves for pelvis injuries using postmortem human subjects (PMHSs). METHODS: Published data from whole-body PMHSs loaded axially through the pelvis were analyzed. Accelerometers were placed on the pelvis/sacrum and seat. Specimens were loaded along the inferior to superior direction using a horizontal sled or a vertical accelerator device. Specimens were positioned supine in the horizontal sled and seated upright on the vertical accelerator. Pre- and posttest images were obtained and autopsies were completed to document the pathology. Variables used in the development of risk curves included velocity, acceleration, time to peak acceleration, pulse duration of acceleration, and jerk for the seat and sacrum. Survival analysis was used for risk curves. To determine the best predictor of pelvis injury, the Brier Score metric (BSM) was used. The best parametric distribution was determined using the corrected Akaike information criterion (AICc). Injury data points were treated as either uncensored or left/interval censored. Noninjury data points were treated as right censored. RESULTS: Twenty-four PMHS specimens were identified from 3 published data sets. Fifteen PMHS specimens sustained injuries and 9 remained intact. The BSM ranged from 1.24 to 24.75 and, in general, the BSMs for the seat metric-related scores were greater than the sacrum data. The sacrum acceleration was the optimal metric for predicting pelvis tolerance (lowest BSM). The Weibull distribution had the lowest AICc, with right and left/interval-censored data. This was also true when injury data were treated as exact (uncensored) observations. The 50% probability of injury was associated with 229 G for the uncensored analysis and 139 G for the censored analysis, and the quality indices in both cases were in the "good" range. CONCLUSIONS: Statistical determination of the best injury metric will help improve the accuracy of injury prediction, prioritize instrumentation choice in dummy development, and improve design criteria for crash mitigation. The present study showed that injury risk curves using response data are better biomechanical descriptors of human responses than exposure data. These data are important in automotive safety because complex loading of the pelvis, including submarining, occurs in frontal car crashes.

Experimental Investigation of Cavitation as a Possible Damage Mechanism in Blast-Induced Traumatic Brain Injury in Post-Mortem Human Subject Heads.

The potential of blast-induced traumatic brain injury from the mechanism of localized cavitation of the cerebrospinal fluid (CSF) is investigated. While the mechanism and criteria for non-impact blast-induced traumatic brain injury is still unknown, this study demonstrates that local cavitation in the CSF layer of the cranial volume could contribute to these injuries. The cranial contents of three post-mortem human subject (PMHS) heads were replaced with both a normal saline solution and a ballistic gel mixture with a simulated CSF layer. Each were instrumented with multiple pressure transducers and placed inside identical shock tubes at two different research facilities. Sensor data indicates that cavitation may have occurred in the PMHS models at pressure levels below those for a 50% risk of blast lung injury. This study points to skull flexion, the result of the shock wave on the front of the skull leading to a negative pressure in the contrecoup, as a possible mechanism that contributes to the onset of cavitation. Based on observation of intracranial pressure transducer data from the PMHS model, cavitation onset is thought to occur from approximately a 140 kPa head-on incident blast.

Influence of bone microstructure on the mechanical properties of skull cortical bone - A combined experimental and computational approach.

The strength and compliance of the dense cortical layers of the human skull have been examined since the beginning of the 20th century with the wide range in the observed mechanical properties attributed to natural biological variance. Since this variance may be explained by the difference in structural arrangement of bone tissue, micro-computed tomography (µCT) was used in conjunction with mechanical testing to study the relationship between the microstructure of human skull cortical coupons and their mechanical response. Ninety-seven bone samples were machined from the cortical tables of the calvaria of ten fresh post mortem human surrogates and tested in dynamic tension until failure. A linear response between stress and strain was observed until close to failure, which occurred at 0.6% strain on average. The effective modulus of elasticity for the coupons was 12.01 ± 3.28GPa. Porosity of the test specimens, determined from µCT, could explain only 51% of the variation of their effective elastic modulus. Finite element (FE) models of the tested specimens built from µCT images indicated that modeling the microstructural arrangement of the bone, in addition to the porosity, led to a marginal improvement of the coefficient of determination to 54%. Modulus for skull cortical bone for an element size of 50µm was estimated to be 19GPa at an average. Unlike the load bearing bones of the body, almost half of the variance in the mechanical properties of cortical bone from the skull may be attributed to differences at the sub-osteon (< 50µm) level. ANOVA tests indicated that effective failure stress and strain varied significantly between the frontal and parietal bones, while the bone phase modulus was different for the superior and inferior aspects of the calvarium. The micro FE models did not indicate any anisotropy attributable to the pores observable under µCT.

Development of injury criteria for pelvic fracture in frontal crashes.

OBJECTIVE: This article assesses the position-dependent injury tolerance of the hip in the frontal direction based on testing of eight postmortem human subjects. METHODS: For each subject, the left and right hemipelvis complex was axially loaded using a previously developed test configuration. Six positions were defined from a seated femur neutral condition, combining flexed, neutral, and extended femur positions with abducted, neutral, and adducted positions. RESULTS: Axial injury tolerances based on peak force were found to be 6,850 +/- 840 N in the extended, neutral position and 4,080 +/- 830 N in the flexed, neutral position. From the flexed neutral orientation, the peak axial force increased 18% for 20 degrees abduction and decreased 6% for 20 degrees adduction. From the extended, neutral orientation, the peak axial force decreased 4% for 20 degrees abduction and decreased 3% for 20 degrees adduction. However, as there is evidence that increases in loading may occur after the initiation of fracture, the magnitude of the peak force is likely related to the extent of injury, not to the initial tolerance. Using the axial femur force at the initiation of fracture (assessed with acoustic crack sensors) as a potentially more relevant indicator of injury may lower the existing injury criteria. This fracture initiation force varied by position from 3,010 +/- 560 N in the flexed, neutral position to 5,470 N in the extended, abducted position. Further, there was a large position-dependent variation in the ratio of fracture initiation force to the peak axial force. The initiation of fracture was 83% of the peak axial force in the extended, abducted position, but the ratio was 34% in the extended, adducted position. CONCLUSIONS: This may have significant implications for the development of pelvic injury criteria by automobile designers attempting to mitigate pelvis injuries.