Prognosticative Extensions and Enhancements to Existing Practices: a Biomechanical PEEEP Show into the Future.

With the rapidly expanding capabilities and sophistication of experimental and analytical techniques that can be applied toward biomechanical endeavors, it would appear to be a useful exercise to review current practice and discuss what might be the form and function of future research that could make substantial improvements in the ability to detect and evaluate the potential for automotive impact trauma. To accomplish this, an exercise, dubbed Prognosticative Extensions and Enhancements to Existing Practices: a Biomechanical PEEEP Show into the Future, will be pursued and presented. It presents the author's impressions of what are the prevalent injury mechanisms active in each major body region, his technical evaluation of the efficacy of the currently accepted injury criteria being applied to detect and evaluate the consequences of those injury mechanisms, and presents his vision of the form and function of future biomechanical capabilities that could, using both enhanced analytical and experimental research techniques, have the potential for greatly improving both our understanding of impact injuries and our ability to prevent them.

Ranking of NASS injury codes by survivability.

A ranking system is established whereby injury codes in the National Automotive Sampling System (NASS) are ordered by survivability based on actual mortality rates. Special provisions are made for cases in which injuries are coded as "not further specified" and "severity unknown." Once the ranking system is established, an injury analysis is carried out in which NASS crash victims are characterized by their two highest-ranking injuries. Then, each victim's probability of survival is estimated using a new "primary/secondary" fatality prediction procedure. When deviance statistics are considered, the new procedure predicts fatalities better than the Injury Severity Score, a commonly applied metric that is based on the Abbreviated Injury Scale. Ultimately, the new rankings - which single out specific injuries - provide a means to improve benefits analyses used to support crash injury research.

On the Development of the SIMon Finite Element Head Model.

The SIMon (Simulated Injury Monitor) software package is being developed to advance the interpretation of injury mechanisms based on kinematic and kinetic data measured in the advanced anthropomorphic test dummy (AATD) and applying the measured dummy response to the human mathematical models imbedded in SIMon. The human finite element head model (FEHM) within the SIMon environment is presented in this paper. Three-dimensional head kinematic data in the form of either a nine accelerometer array or three linear CG head accelerations combined with three angular velocities serves as an input to the model. Three injury metrics are calculated: Cumulative strain damage measure (CSDM) - a correlate for diffuse axonal injury (DAI); Dilatational damage measure (DDM) - to estimate the potential for contusions; and Relative motion damage measure (RMDM) - a correlate for acute subdural hematoma (ASDH). During the development, the SIMon FEHM was tuned using cadaveric neutral density targets (NDT) data and further validated against the other available cadaveric NDT data and animal brain injury experiments. The hourglass control methods, integration schemes, mesh density, and contact stiffness penalty coefficient were parametrically altered to investigate their effect on the model's response. A set of numerical and physical parameters was established that allowed a satisfactory prediction of the motion of the brain with respect to the skull, when compared with the NDT data, and a proper separation of injury/no injury cases, when compared with the brain injury data. Critical limits for each brain injury metric were also established. Finally, the SIMon FEHM performance was compared against HIC15 through the use of NHTSA frontal and side impact crash test data. It was found that the injury metrics in the current SIMon model predicted injury in all cases where HIC15 was greater than 700 and several cases from the side impact test data where HIC15 was relatively small. Side impact was found to be potentially more injurious to the human brain than frontal impact due to the more severe rotational kinematics.

Development of Side Impact Thoracic Injury Criteria and Their Application to the Modified ES-2 Dummy with Rib Extensions (ES-2re).

Forty-two side impact cadaver sled tests were conducted at 24 and 32 km/h impact speeds into rigid and padded walls. The post-mortem human subjects were instrumented with accelerometers on the ribs and spine and chest bands around the thorax and abdomen to characterize their mechanical response during the impact. Load cells at the wall measured the impact force at the level of the thorax, abdomen, pelvis, and lower extremities. The resulting injuries were determined through detailed autopsy and radiography. Rib fractures with or without associated hemo/pneumo thorax or flail chest were the most common injury with severity ranging from AIS=0 to 5. Full and half thorax deflections were computed from the chest band data. The cadaver test data was analyzed using ANOVA and logistic regression. The age of the subject at the time of death had influence on injury outcome while gender and mass of the subject had little or no influence on injury outcome. Existing side impact injury criteria were evaluated such as Thoracic Trauma Index (TTI), Average Spinal Acceleration (ASA), full and half thorax deflections, chest velocity and viscous criterion, and contact force. The analysis results indicate that maximum normalized average half thorax deflection was the best predictor of AIS>/=3 and AIS>/=4 thoracic injury. TTI and upper spine accelerations were also good predictors of thoracic injury. Sixteen side impact sled tests were also conducted with the modified ES-2 dummy with rib extensions (ES-2re) under similar impact conditions as the cadaver tests. The rib extensions were added to the original ES-2 dummy ribs to prevent the "seat grabbing" action of the back plate that was noticed in some side impact vehicle crash tests with the ES-2 dummy. A separate analysis was conducted using the injury response and subject characteristics from the cadaver tests and the physical parameters derived from measurements on the ES-2re dummy in sled tests under similar conditions as the cadaver tests. This analysis provided thoracic injury criteria that could be directly applied to the ES-2re dummy. This analysis indicated subject age to have significant influence on injury outcome. Maximum rib deflection and ASA of the ES-2re were the best predictors of thoracic injury. A 50% risk of AIS>/=3 thoracic injury for a 45 year old corresponds to 44 mm (standard error range: 32 to 54 mm) of ES-2re maximum rib deflection and ASA of 46 gs (standard error range: 34 -58 gs).

Cervical vertebral motions and biomechanical responses to direct loading of human head.

There is little known data characterizing the biomechanical responses of the human head and neck under direct head loading conditions. However, the evaluation of the appropriateness of current crash test dummy head-neck systems is easily accomplished. Such an effort, using experimental means, generates and provides characterizations of human head-neck response to several direct head loading conditions. Low-level impact loads were applied to the head and face of volunteers and dummies. The resultant forces and moments at the occipital condyle were calculated. For the volunteers, activation of the neck musculature was determined using electromyography (EMG). In addition, cervical vertebral motions of the volunteers have been taken by means of X-ray cineradiography. The Ethics Committee of Tsukuba University approved the protocol of the experiments in advance. External force of about 210 N was applied to the head and face of five volunteers with an average age of 25 for the duration of 100 msec or so, via a strap at one of four locations in various directions: (1) an upward load applied to the chin, (2) a rearward load applied to the chin without facial mask, (3) a rearward load applied to the chin with the facial mask, and (4) a rearward load applied to the forehead. The same impact force as those for the human volunteers was also applied to HY-III, THOR, and BioRID. We found that cervical vertebral motions differ markedly according to the difference in impact loading condition. Some particular characteristics are also found, such as the flexion or extension of the upper cervical vertebrae (C0, C1, and C2) or middle cervical vertebrae (C3-C4), showing that the modes of cervical vertebral motions are markedly different among the different loading conditions. We also found that the biofidelity of dummies to neck response characteristics of the volunteers at the low-level impact loads is in the order of BioRID, THOR, and HY-III. It is relevant in this regard that the BioRID dummy was designed for a low-severity impact environment, whereas THOR and HY-III were optimized for higher-severity impacts.

The axial injury tolerance of the human foot/ankle complex and the effect of Achilles tension.

Axial loading of the foot/ankle complex is an important injury mechanism in vehicular trauma that is responsible for severe injuries such as calcaneal and tibial pilon fractures. Axial loading may be applied to the leg externally, by the toepan and/or pedals, as well as internally, by active muscle tension applied through the Achilles tendon during pre-impact bracing. The objectives of this study were to investigate the effect of Achilles tension on fracture mode and to empirically model the axial loading tolerance of the foot/ankle complex. Blunt axial impact tests were performed on forty-three (43) isolated lower extremities with and without experimentally simulated Achilles tension. The primary fracture mode was calcaneal fracture in both groups. However, fracture initiated at the distal tibia more frequently with the addition of Achilles tension (p < 0.05). Acoustic sensors mounted to the bone demonstrated that fracture initiated at the time of peak local axial force. A survival analysis was performed on the injury data set using a Weibull regression model with specimen age, gender, body mass, and peak Achilles tension as predictor variables (R2 = 0.90). A closed-form survivor function was developed to predict the risk of fracture to the foot/ankle complex in terms of axial tibial force. The axial tibial force associated with a 50% risk of injury ranged from 3.7 kN for a 65 year-old 5th percentile female to 8.3 kN for a 45 year-old 50th percentile male, assuming no Achilles tension. The survivor function presented here may be used to estimate the risk of foot/ankle fracture that a blunt axial impact would pose to a human based on the peak tibial axial force measured by an anthropomorphic test device.

The effects of axial preload and dorsiflexion on the tolerance of the ankle/subtalar joint to dynamic inversion and eversion.

Forced inversion or eversion of the foot is considered a common mechanism of ankle injury in vehicle crashes. The objective of this study was to model empirically the injury tolerance of the human ankle/subtalar joint to dynamic inversion and eversion under three different loading conditions: neutral flexion with no axial preload, neutral flexion with 2 kN axial preload, and 30 degrees of dorsiflexion with 2 kN axial preload. 44 tests were conducted on cadaveric lower limbs, with injury occurring in 30 specimens. Common injuries included malleolar fractures, osteochondral fractures of the talus, fractures of the lateral process of the talus, and collateral ligament tears, depending on the loading configuration. The time of injury was determined either by the peak ankle moment or by a sudden drop in ankle moment that was accompanied by a burst of acoustic emission. Characteristic moment-angle curves to injury were generated for each loading configuration. Neutrally flexed ankles with no applied axial preload sustained injury at 21 +/- 5 Nm and 38 degrees +/- 8 degrees in inversion, and 47 +/- 21 Nm and 28 degrees +/- 4 degrees in eversion. For ankles tested in neutral flexion with 2 kN of axial preload, inversion failure occurred at 77 +/- 27 Nm and 40 degrees +/- 12 degrees , and eversion failure occurred at 142 +/- 100 Nm and 41 degrees +/- 14 degrees . Ankles dorsiflexed 30 degrees and axially preloaded to 2 kN sustained inversion injury at 62 +/- 31 Nm and 33 degrees +/- 4 degrees , and eversion injury at 140 +/- 53 Nm and 40 degrees +/- 6 degrees . Survival analyses were performed to generate injury risk curves in terms of joint moment and rotation angle.

Response corridors of human surrogates in lateral impacts.

Thirty-six lateral PMHS sled tests were performed at 6.7 or 8.9 m/s, under rigid or padded loading conditions and with a variety of impact surface geometries. Forces between the simulated vehicle environment and the thorax, abdomen, and pelvis, as well as torso deflections and various accelerations were measured and scaled to the average male. Mean +/- one standard deviation corridors were calculated. PMHS response corridors for force, torso deflection and acceleration were developed. The offset test condition, when partnered with the flat wall condition, forms the basis of a robust battery of tests that can be used to evaluate how an ATD interacts with its environment, and how body regions within the ATD interact with each other.

Development of a new biofidelity ranking system for anthropomorphic test devices.

A new biofidelity assessment system is being developed and applied to three side impact dummies: the WorldSID-alpha, the ES-2 and the SID-HIII. This system quantifies (1) the ability of a dummy to load a vehicle as a cadaver does, "External Biofidelity," and (2) the ability of a dummy to replicate those cadaver responses that best predict injury potential, "Internal Biofidelity." The ranking system uses cadaver and dummy responses from head drop tests, thorax and shoulder pendulum tests, and whole body sled tests. Each test condition is assigned a weight factor based on the number of human subjects tested to form the biomechanical response corridor and how well the biofidelity tests represent FMVSS 214, side NCAP (SNCAP) and FMVSS 201 Pole crash environments. For each response requirement, the cumulative variance of the dummy response relative to the mean cadaver response (DCV) and the cumulative variance of the mean cadaver response relative to the mean plus one standard deviation (CCV) are calculated. The ratio of DCV/CCV expresses how well the dummy response duplicates the mean cadaver response: a smaller ratio indicating better biofidelity. For each test condition, the square root is taken of each Response Comparison Value (DCV/CCV), and then these values are averaged and multiplied by the appropriate Test Condition Weight. The weighted and averaged comparison values are then summed and divided by the sum of the Test Condition Weights to obtain a rank for each body region. Each dummy obtains an overall rank for External Biofidelity and an overall rank for Internal Biofidelity comprised of an average of the ranks from each body region. Of the three dummies studied, the selected comparison test data indicate that the WorldSID-alpha prototype dummy demonstrated the best overall External Biofidelity although improvement is needed in all of the dummies to better replicate human kinematics. All three dummies estimate potential injury assessment with similar levels of Internal Biofidelity.