The Development of a Tympanic Membrane Model and Probabilistic Dose-Response Risk Assessment of Rupture Because of Blast.

INTRODUCTION: There is no dose-response model available for the assessment of the risk of tympanic membrane rupture (TMR), commonly known as eardrum rupture, from exposures to blast from nonlethal flashbangs, which can occur concurrently with temporary threshold shift. Therefore, the objective of this work was to develop a fast-running, lumped parameter model of the tympanic membrane (TM) with probabilistic dose-dependent prediction of injury risk. MATERIALS AND METHODS: The lumped parameter model was first benchmarked with a finite element model of the middle ear. To develop the dose-response curves, TMR data from a historic cadaver study were utilized. From these data, the binary probability response was constructed and logistic regression was applied to generate the respective dose-response curves at moderate and severe eardrum rupture severity. RESULTS: Hosmer-Lemeshow statistical and receiver operation characteristic analyses showed that maximum stored TM energy was the overall best dose metric or injury correlate when compared with total work and peak TM pressure. CONCLUSIONS: Dose-response curves are needed for probabilistic risk assessments of unintended effects like TMR. For increased functionality, the lumped parameter model was packaged as a software library that predicts eardrum rupture for a given blast loading condition.

Anthropomorphic Blast Test Device for Primary Blast Injury Risk Assessment.

INTRODUCTION: Blast overpressure health hazard assessment is required prior to fielding of weapon systems that produce blast overpressures that pose risk of auditory and nonauditory blast lung injuries. The anthropomorphic blast test device (ABTD) offers a single device solution for collection of both auditory and nonauditory data from a single blast at anthropometrically correct locations for injury risk assessment. It also allows for better replication of personnel positioning during weapons firings. The ABTD is an update of the blast test device (BTD), the current Army standard for collection of thoracic blast loading data. Validation testing of the ABTD is required to ensure that lung injury model validated using BTD collected test data and sheep subjects is still applicable when the ABTD is used. METHODS: Open field validation blast tests were conducted with BTD and ABTD placed at matching locations. Tests at seven blast strength levels were completed spanning the range of overpressures for occupational testing. RESULTS: The two devices produced very similar values for lung injury dose over all blast levels and orientations. CONCLUSION: The ABTD was validated successfully for open field tests. For occupational blast injury assessments, ABTD can be used in place of the BTD and provide enhanced capabilities.

Incident Angle Correction Algorithm For Impulse Noise Injury Assessment.

Objectives: We developed an empirical algorithm to account for the effect of the change in the A-weighted sound exposure level (SELA) as a result of the change in angle of incidence (AoI) of the impulse noise on the prediction of hearing loss. The product is the upgraded software tool, Auditory 4.5 that incorporates the incident angle correction algorithm. Methods: The SELA calculated from free-field pressure data is used as the dose metric that was corrected for AoI. The angle-dependent eardrum pressure was measured by performing shock tube tests with the Acoustical Testing Fixture varied over a wide range of orientation angles. The yaw angle was varied from 0 to 360° and the pitch angle from -60° to +90° in 15° steps. The algorithm was constructed by calculating a correction factor, ΔSELA for any given AoI at the ear relative to the SELA at normal incidence. The ΔSELA values were applied to correct the dose values to predict injury for all AoI. Results: A three-dimensional contour of ΔSELA as a function of the AoI was produced. The largest ΔSELA was 9.81 dB at pitch = -15° and yaw = 255°. ΔSELA values compared well against available benchmark data. Conclusions: A new capability has been incorporated in Auditory 4.5 to predict the effects of AoI on impulse noise injury.

The Development of a Probabilistic Dose-Response for a Burn Injury Model.

OBJECTIVE: The objective was to augment a burn injury model, BURNSIM, with probabilistic dose-response risk curves. METHODS: To develop the dose-response, we drew on a considerable amount of historical porcine burn injury data collected by U.S. Army Aeromedical Research Laboratory in the 1970s. The experimental parameters of each usable data point served as inputs to BURNSIM to calculate the burn damage integral (i.e., the internal dose) for 4 severities (mild, intermediate, deep second- and third-degree burns). The binary probability response was constructed and logistic regression was applied to generate the respective dose-response. Historic data collected at the University of Rochester in the 1950s were used for validation. RESULTS: Four dose-response curves were generated, ranging from mild to third degree, with tight 95% confidence bands for mild to deep second degree, and slightly wider bands for third degree. Parametric sensitivity analysis revealed that epidermal and whole skin thicknesses, skin temperature, and blood flow rate have a large effect on predicted outcomes. CONCLUSIONS: Addition of dose-response curves provides a critical augmentation to BURNSIM to improve operational risk assessments of burn hazard. Future recommendations for BURNSIM include the use of body location- and gender-specific parameters with coupling to a thermoregulatory model.

An Interim LAeq8 Criterion for Impulse Noise Injury.

OBJECTIVES: We present a method to account for the effects of the hearing protection devices (HPDs) for use with the 8 hours equivalent A-weighted energy (LAeq8) criterion. The method involves the calculation of the LAeq8 equivalent unprotected free-field dose (LAeq8EUFF), which is obtained by using the insertion loss (IL) data of the HPD together with free-field pressure measurements. METHODS: The method was validated against the historical the U.S. Army Medical Research and Materiel Command walk-up study data with volunteers exposed to simulated large weapon noise wearing a range of HPDs. The IL data were obtained using standard acoustical test fixtures fitted with the matching HPDs in replicated field tests and using shock tubes at conditions comparable to the actual exposure intensities. Logistic regression calculations were performed to correlate the LAeq8EUFF values against the walk-up study outcomes to determine the L(95,95) threshold for the protection of 95% of the population with 95% of the time. RESULTS: Data comparison shows that L(95,95) is 89 dBA, which is slightly higher than the 85 dBA criterion but falls in the 80 to 90 dBA range as used by various NATO nations. CONCLUSIONS: Therefore, considering the limitation of the walk-up dataset, it is conservative to adopt the 85 dBA threshold for general application.

Impulse Noise Injury Model.

OBJECTIVES: The new Auditory 4.0 model has been developed for the assessment of auditory outcomes, expressed as temporary threshold shift (TTS) and permanent threshold shift (PTS), from exposures to impulse noise for unprotected ears, including the prediction of TTS recovery. METHODS: Auditory 4.0 is an empirical model, constructed from test data collected from chinchillas exposed to impulse noise in the laboratory. Injury outcomes are defined as TTS and PTS, and Auditory 4.0 provides the full range of TTS and PTS dose-response curves with the risk factor constructed from A-weighted sound exposure level. Human data from large weapons noise exposure was also used to guide the development of the recovery model. RESULTS: Guided by data, a 28-dBA shift was applied to the dose-response curves to account for the scaling from chinchillas to humans. Historical data from rifle noise tests were used to validate the dose-response curves. New chinchilla tests were performed to collect recovery data to construct the TTS recovery model. CONCLUSIONS: Auditory 4.0 is the only model known to date that provides the full TTS and PTS dose-response curves, including a TTS recovery model. The model shows good agreement with historical data.

Impulse noise injury prediction based on the cochlear energy.

The current impulse noise criteria for the protection against impulse noise injury do not incorporate an objective measure of hearing protection. A new biomechanically-based model has been developed based on improvement of the Auditory Hazard Assessment Algorithm for the Human (AHAAH) using the integrated cochlear energy (ICE) as the damage risk correlate (DRC). The model parameters have been corrected using the latest literature data. The anomalous dose-response inversion behavior of the AHAAH model was eliminated. The modeling results show that the annular ligament (AL) parameters are the dominant cause of the non-monotonic dose-response behavior of AHAAH. Based on parametric optimization analysis, a 40% reduction of the AL compliance from the AHAAH default value removed the dose-response inversion problem, and this value was found to be within the physiological range when compared with experimental data. The transfer functions from the new model are in good agreement with those of the human ear. A dose-response curve based on ICE was developed using the human walk-up temporary threshold shift (TTS) data. Furthermore, the ICE values calculated for the German rifle noise tests show excellent comparison with the injury outcomes, hence providing a significant independent validation of the improved model. The ICE was found to be the best DRC to both large weapons and small arms noise injury data, covering both protected and unprotected exposures, respectively. The new AHAAH model with ICE as the dose metric is adequate for use as a medical standard against impulse noise injury.

Finite element modeling of blast lung injury in sheep.

A detailed 3D finite element model (FEM) of the sheep thorax was developed to predict heterogeneous and volumetric lung injury due to blast. A shared node mesh of the sheep thorax was constructed from a computed tomography (CT) scan of a sheep cadaver, and while most material properties were taken from literature, an elastic-plastic material model was used for the ribs based on three-point bending experiments performed on sheep rib specimens. Anesthetized sheep were blasted in an enclosure, and blast overpressure data were collected using the blast test device (BTD), while surface lung injury was quantified during necropsy. Matching blasts were simulated using the sheep thorax FEM. Surface lung injury in the FEM was matched to pathology reports by setting a threshold value of the scalar output termed the strain product (maximum value of the dot product of strain and strain-rate vectors over all simulation time) in the surface elements. Volumetric lung injury was quantified by applying the threshold value to all elements in the model lungs, and a correlation was found between predicted volumetric injury and measured postblast lung weights. All predictions are made for the left and right lungs separately. This work represents a significant step toward the prediction of localized and heterogeneous blast lung injury, as well as volumetric injury, which was not recorded during field testing for sheep.

A model for predicting primary blast lung injury.

BACKGROUND: This article presents a model-based method for predicting primary blast injury. On the basis of the normalized work injury mechanism from previous work, this method presents a new model that accounts for the effects of blast orientation and species difference. METHODS: The analysis used test data from a series of extensive experimental studies sponsored by the US Army Medical Research and Materiel Command. In these studies, more than 1200 sheep were exposed to air blast in free-field and confined enclosures, and lung injuries were quantified as the percentage of surface area contused. Blast overpressure data were collected using blast test devices placed at matching locations to represent loadings to the thorax. Adopting the modified Lobdell model with further modifications specifically for blast and scaling, the thorax deformation histories for the left, chest, and right sides of the thorax were calculated for all sheep subjects. Using the calculated thorax velocities, effective normalized work was computed for each test subject representing the irreversible work performed on the lung tissues normalized by lung volume and ambient pressure. RESULTS: Dose-response curves for four categories of injuries (trace, slight, moderate, and severe) were developed by performing log-logistic correlations of the computed normalized work with the injury outcomes, including the effect of multiple shots. A blast lethality correlation was also established. CONCLUSION: Validated by sheep data, the present work revalidates the previous understanding and findings of the blast lung injury mechanism and provides an anthropomorphic model for primary blast injury prediction that can be used for occupational and survivability analysis. LEVEL OF EVIDENCE: Economic and decision analysis, level III.

Evaluation of the biofidelity of FMVSS No. 218 injury criteria.

OBJECTIVE: The biofidelity of the injury criteria used by Federal Motor Vehicle Safety Standards (FMVSS) No. 218 was examined against biomechanically based injury metrics. METHODS: An experimental method was developed to measure the helmet contact pressure distribution on a headform during an impact attenuation test. The headform pressure data from eighty impact tests to the front, crown, and side of a helmet were used in finite element model simulations to predict skull fracture. Using headform acceleration data as inputs, the Simulated Injury Monitor software package (SIMon) was used to predict brain injuries for concussion, brain contusion, and subdural hematoma. RESULTS: It was found that FMVSS No. 218 headform peak acceleration is the best correlate with injury metrics. Dwell times over 150 and 200 g both had poor correlation with injury metrics. The failure probability for skull fracture agrees with published results at similar linear accelerations. Concussion results were inconclusive. CONCLUSIONS: This research has shown that peak head acceleration can be an acceptable injury metric for the FMVSS No. 218 test method. However, the current 400 g allows for a high probability of head injury. An adjusted linear head acceleration limit of 210 g predicts a 15 percent skull fracture probability. The FMVSS No. 218 test method is adequate for predicting skull fracture based on peak head acceleration limits. However, due to the use of the rigid head/neck assembly that restricts rotation, the test method is likely inadequate for predicting brain injuries.

Correlates to traumatic brain injury in nonhuman primates.

BACKGROUND: Traumatic brain injury (TBI) is a major health problem, both in terms of the economic cost to society and the survivor's quality of life. The development of devices to protect against TBI requires criteria that relate observed injury to measurements of head kinematics. The objective of this study is to find the best statistical correlates to impact-induced TBI in nonhuman primates using a qualified, self-consistent set of historical kinematic and TBI data from impact tests on nonhuman primates. METHODS: A database was constructed and qualified from historical head impact tests on nonhuman primates. Multivariate logistic regression analysis with backwards stepwise elimination was performed. Variables considered are the peak rotational acceleration (Omegamax), the peak linear acceleration (Amax), and the number of impacts (N). RESULTS: Bivariate combinations of angular acceleration and the number of impacts are the best correlates to all modes of TBI considered, i.e., concussion, subarachnoid hemorrhage, brain contusion, and subdural hematoma. For a nonhuman primate with 100-g brain mass, the criteria that the probability of TBI is less than 10% by injury mode are:Concussion: OmegamaxN(0.84) < 70 krad/s/s SAH: OmegamaxN(0.70) < 160 krad/s/s Contusion: Omegamax N(0.35) < 160 krad/s/s SDH: Omegamax N(0.60) < 280 krad/s/s CONCLUSIONS: Based on this dataset, the best statistically based risk factor for all modes of TBI in nonhuman primates is the bivariate combination of rotational acceleration and number of impacts.

A new biomechanically-based criterion for lateral skull fracture.

This work develops a skull fracture criterion for lateral impact-induced head injury using postmortem human subject tests, anatomical test device measurements, statistical analyses, and finite element modeling. It is shown that skull fracture correlates with the tensile strain in the compact tables of the cranial bone as calculated by the finite element model and that the Skull Fracture Correlate (SFC), the average acceleration over the HIC time interval, is the best predictor of skull fracture. For 15% or less probability of skull fracture the lateral skull fracture criterion is SFC < 120 g, which is the same as the frontal criterion derived earlier. The biomechanical basis of SFC is established by its correlation with strain.