Occupant Injury and Response on Oblique-Facing Aircraft Seats: A Computational Study.

Increased interest in the airline industry to enhance occupant comfort and maximize seating density has prompted the design and installation of obliquely mounted seats in aircraft. Previous oblique whole-body sled tests demonstrated multiple failures, chiefly distraction-associated spinal injuries under oblique impacts. The present computational study was performed with the rationale to examine how oblique loading induces component level responses and associated injury occurrence. The age-specific human body model (HBM) was simulated for two oblique seating conditions (with and without an armrest). The boundary conditions consisted of a 16 g standard aviation crash pulse, 45 deg seat orientation, and with restrained pelvis and lower extremities. The overall biofidelity rating for both conditions ranged from 0.5 to 0.7. The validated models were then used to investigate the influence of pulse intensity and seat orientation by varying the pulse from 16 g to 8 g and seat orientation from 0 deg to 90 deg. A total of 12 parametric simulations were performed. The pulse intensity simulations suggest that the HBM could tolerate 11.2 g without lumbar spine failure, while the possibility of cervical spine failure reduced with the pulse magnitude <9.6 g pulse. The seat orientation study demonstrated that for all seat angles the HBM predicted failure in the cervical and lumbar regions at 16 g; however, the contribution of the tensile load and lateral and flexion moments varied with respect to the change in seat angle. These preliminary outcomes are anticipated to assist in formulating safety standards and in designing countermeasures for oblique seating configurations.

An investigation of elderly occupant injury risks based on anthropometric changes compared to young counterparts.

OBJECTIVE: The objective of the study was to investigate the difference between elderly and young occupant injury risks using human body finite element modeling in frontal impacts. METHODS: Two elderly male occupant models (representative age 70-80 years) were developed using the Global Human Body Consortium (GHBMC) 50th percentile as the baseline model. In the first elderly model (EM-1), material property changes were incorporated, and in the second elderly model (EM-2), material and anthropometric changes were incorporated. Material properties were based on literature. The baseline model was morphed to elderly anthropometry for EM-2. The three models were simulated in a frontal crash vehicle environment at 56 km/h. Responses from the two elderly and baseline models were compared with cadaver experimental data in thoracic, abdominal, and frontal impacts. Correlation and analysis scores were used for correlation with experimental data. The probabilities of head, neck, and thoracic injuries were assessed. RESULTS: The elderly models showed a good correlation with experimental responses. The elderly EM-1 had higher risk of head and brain injuries compared to the elderly EM-2 and baseline GHBMC models. The elderly EM-2 demonstrated higher risk of neck, chest, and abdominal injuries than the elderly EM-1 and baseline models. CONCLUSIONS: The study investigated injury risks of two elderly occupants and compared to a young occupant in frontal crashes. The change in the material properties alone (EM-1) suggested that elderly occupants may be vulnerable to a greater risk of head and thoracic injuries, whereas change in both anthropometric and material properties (EM-2) suggested that elderly occupants may be vulnerable to a greater risk of thoracic and neck injuries. The second elderly model results were in better agreement with field injury data from the literature; thus, both anthropometric and material properties should be considered when assessing the injury risks of elderly occupants. The elderly models developed in this study can be used to simulate different impact conditions and determine injury risks for this group of our population.

Comparison of small female occupant model responses with experimental data in a reclined posture.

Objective: The objective of the current study was to compare the GHBMC female model responses with in-house sled test data for three small female post mortem human surrogates (PMHS) at 32 km/h and a seatback recline angle of 45 degrees. The kinematics and the seatbelt forces were used to compare the female PMHS and model responses. The study aimed to identify updates that may be needed to the model.Methods: In-house experimental sled test kinematic and seatbelt response data for the small females were obtained. The 5th female GHBMC was simulated with the same boundary conditions as in the experiments. In addition, using the PMHS computed tomography (CT) and test environment scans, the female model geometry was updated to a subject-specific model for one of the specimens, and the models were simulated to obtain 5th female and subject-specific model responses. The kinematic response and the seatbelt forces for the two models were compared with the average of the three experimental data.Results: The head, T8 and L4 excursions, head and pelvis accelerations and seatbelt forces for the two female models were compared with the experimental data. The model responses were in agreement with the PMHS; however, the subject-specific model showed a closer agreement with the kinematic response. The subject-specific model did not submarine as in the experiments, whereas the 5th female model submarined. However, the subject-specific model showed 20% higher seatbelt forces than the PMHS.Conclusion: This study showed that anthropometric differences may significantly alter occupant kinematics in reclined posture and need to be incorporated to investigate kinematics and injury mechanisms. The next step of the study involves incorporating age-specific material changes and investigating the subject-specific injury mechanisms. The results will be useful to develop countermeasures for autonomous vehicles.

A biomechanical investigation of lumbar interbody fusion techniques.

The anterior, posterior, transforaminal, and circumferential lumbar interbody fusions (ALIF, PLIF, TLIF, CLIF/360) are used to treat spondylolisthesis, trauma, and degenerative pathologies. This study aims to investigate the biomechanical effects of the lumbar interbody fusion techniques on the spine. A validated T12-sacrum lumbar spine finite-element model was used to simulate surgical fusion of L4-L5 segment using ALIF, PLIF with one and two cages, TLIF with unilateral and bilateral fixation, and CLIF/360. The models were simulated under pure-moment and combined (moment and compression) loadings to investigate the effect of different lumbar interbody fusion techniques on range of motion, forces transferred through the vertebral bodies, disc pressures, and endplate stresses. The range of motion of the lumbar spine was decreased the most for fusions with bilateral posterior instrumentations (TLIF, PLIF, and CLIF/360). The increase in forces transmitted through the vertebrae and increase in disc pressures were directly proportional to the range of motion. The discs superior to fusion were under higher pressure, which was attributed to adjacent segment degeneration in the superior discs. The increase in endplate stresses was directly proportional to the cross-sectional area and was greater in caudal endplates at the fusion level, which was attributed to cage subsidence. The response of the models was in line with overall clinical observations from the patients and can be further used for future studies, which aim to investigate the effect of geometrical and material variations in the spine. The model results will assist surgeons in making informed decisions when selecting fusion procedures based on biomechanical effects.

Development and validation of an elderly human body model for frontal impacts.

OBJECTIVE: The study aims to develop an elderly model occupant representative of 50th percentile 75-year-old male using the younger 50th percentile Global Human Body Models Consortium Human Body Model. METHODS: The 50th percentile base model was morphed to elderly anthropometry. The material properties of tissues were updated according to the aging functions from the literature. The elderly model was simulated for thoracic impact, abdomen impact, and frontal impact sled tests. The model-predicted contact force-displacement, regional body excursion, acceleration, and seatbelt force responses were compared with matched elderly postmortem human surrogate experimental data. RESULTS: The force-displacement responses for the thorax and abdomen impacts were within the experimental corridors. The head excursion in the z-direction was within the mean ± one standard deviation experimental corridors. The correlation analysis values of the head, T1 vertebra, pelvis acceleration, and seatbelt forces signals for the frontal sled tests were 0.62, 0.72, 0.63, and 0.78, respectively, and the overall mean value was 0.69. CONCLUSIONS: The developed model with the morphological and material changes representing an elderly occupant is considered to be validated under three experimental scenarios, and it can be used for crashworthiness applications (develop countermeasures) with a focus on elderly occupants. The process used in the development of the elderly model can also be used to understand the responses of elderly occupants with different postures.

Development and validation of osteoligamentous lumbar spine under complex loading conditions: A step towards patient-specific modeling.

Finite-element models are used to investigate the biomechanics of normal, diseased and surgically fused spines. Generally, nominal spine geometries are used to understand the biomechanics, which has created a need for a technique that develops patient-specific lumbar spine geometries. In the current study, a lumbar spine (T12-Sacrum) was developed using a technique that facilitates geometrical morphing, which assists in incorporating patient-specific morphologies into the model. The model evaluations can be used to propose a biomechanically suitable lumbar spine fusion procedure for patients. This study focuses on the validation of the base model under pure-moment, pure-compression and combined-compression-and-moment loadings. Experimental data from the literature were used to validate the response of the model. The L1-L2, L2-L3, L3-L4, L4-L5 and L5-sacrum segments demonstrated a range of motion of 4.5, 4.0, 5.4, 5.0 and 8.9° in flexion; 3.0, 2.5, 3.6, 3.1 and 5.2° in extension; 6.2, 5.8, 6.4, 5.0 and 6.1° in right and left lateral bending; and 2.9, 3.0, 2.9, 1.9 and 2.5° in right and left axial rotation, all under 10 Nm pure-moment loading. The L1-L2, L2-L3, L3-L4, L4-L5 and L5-sacrum discs demonstrated compressions of 1.1, 1.4, 1.6, 1.4 and 0.9 mm under 1200 N follower- or pure-compression loading. With the combined loading of 280 N follower and 7.5 Nm moment, the L1-L5 model demonstrated 11.7, 7.2, 18.3 and 10.4 degrees of range of motion in flexion, extension, bending and rotation, respectively. The model results were in good agreement with corridors from six different experimental studies and can be used for future clinical studies.

Effectiveness of center-mounted airbag in far-side impacts based on THOR sled tests.

Objective: The study aimed to evaluate the protection offered by a center-mounted airbag in far-side impacts using the Test device for Human Occupant Restraint (THOR) anthropometric test device (ATD). Methods: A rigid buck was designed based on a production vehicle. The buck consisted of a rigid seat, center console, dash, and far-side door structure. The center console and dash were covered with paper honeycomb (152 kPa), and the far-side door structure was covered with Ethafoam 220 padding material. The airbag was mounted on the seat, to the right of the occupant. The THOR-M50 ATD was positioned according to the standard seating procedure and restrained using a standard 3-point seat belt with a pretensioner and retractor. The buck was mounted on an acceleration sled in 2 orientations. Four tests at 45° (oblique) and 2 tests at 90° (lateral) orientations were conducted. Tests were performed with and without an airbag at 30 km/h delta-V and 14 g acceleration. The head accelerations, neck forces and moments, thoracic accelerations and forces, pelvis accelerations, anterior superior iliac spine (ASIS) forces and moments, and belt webbing loads were obtained from sensors, and the external kinematics was obtained using an optical motion capture system and high-speed digital cameras. Results: With the center-mounted airbag, in 90° and 45° tests, reductions were observed for the following parameters: head lateral excursions by 6% and 11%, head vertical excursions by 19% and 26%, and peak head resultant accelerations by 36% and 11%. Other regional accelerations, forces, and moments were also reduced for both impact angles. A reduction in seat belt forces with the airbag was observed in 90° tests. Conclusion: The center-mounted airbag reduced the ATD excursions and accelerations in the 45° and 90° tests, thus reducing the risk of injury due to contact with the intruding structure. The results of this study may assist in designing countermeasures for vehicles in far-side impact.

Mechanisms of Cervical Spine Disc Injury under Cyclic Loading.

STUDY DESIGN: Determination of human cervical spine disc response under cyclic loading. PURPOSE: To explain the potential mechanisms of intervertebral disc injury caused by cyclic loading. OVERVIEW OF LITERATURE: Certain occupational environments in civilian and military populations may affect the cervical spine of individuals by cyclic loading. Research on this mechanism is scarce. METHODS: Here, we developed a finite element model of the human C4-C5 disc. It comprised endplates, five layers of fibers, a nucleus, and an annulus ground substance. The endplates, ground substance, and annular fibers were modeled with elastic, hyperviscoelastic, and hyper-elastic materials, respectively. We subjected the disc to compressive loading (150 N) for 10,000 cycles at frequencies of 2 Hz (low) and 4 Hz (high). We measured disc displacements over the entire loading period. We obtained maximum and minimum principal stress and strain and von Mises stress distributions at both frequencies for all components. Further, we used contours to infer potential mechanisms of internal load transfer within the disc components. RESULTS: The points of the model disc displacement versus the loading cycles were within the experimental corridors for both frequencies. The principal stresses were higher in the ground matrix, maximum stress was higher in the anterior and posterior annular regions, and minimum stress was higher along the superior and inferior peripheries. The maximum principal strains were radially directed, whereas the minimum principal strains were axially/obliquely directed. The stresses in the fibers were greater and concentrated in the posterolateral regions in the innermost layer. CONCLUSIONS: Disc displacement was lower at high frequency, thus exhibiting strain rate stiffening and explaining stress accumulation at superior and interior peripheries. Greater stresses and strains at the boundaries explain disc injuries, such as delamination. The greater development of stresses in the innermost annular fiber layer (migrating toward the posterolateral regions) explains disc prolapse.

Factors influencing the effectiveness of occupant retention under far-side impacts: A parametric study.

The occupant retention and injuries under far-side impact are invariably dependent upon the effectiveness of the seatbelt restraint system, which is largely driven by parameters such as seatbelt pre-tensioner limiting load, D-ring position above and behind the shoulder, and friction coefficient between the torso and the seatbelt. The cumulative effect of systematic variation of these parameters on occupant kinematics under far-side is rarely studied in the literature. In this study, a systematic and detailed analysis was performed to understand the effect of these parameters on occupant retention. A rigid buck assembly with Global Human Body Model Consortium Human Body Model, validated with post mortem human surrogate experiments was used under two different impact scenarios-lateral and oblique. A simulation matrix of 16 cases was designed by varying the magnitude of the parameters for each impact scenario. Each case was graded as good, moderate, or poor retention based on the position of the shoulder seatbelt at the time of rebound. Head accelerations and excursions, chest compression, rib fractures, and neck moments of the HBM were analyzed to understand the effect of improved retention on occupant kinematics. Results showed that higher pre-tensioner limiting load, higher seatbelt friction, and backward position of D-ring improved retention in both lateral and oblique scenarios. Head acceleration, and excursions and chest compression decreased from poor retention cases to good retention cases for both impact scenarios. Rib fractures were higher in cases with poor retention as compared to those with good retention. The peak lateral neck moments changed marginally from poor to good retention; however, the rate of loading of the neck was significantly higher in good retention. Thus, the current study suggested that the backward D-ring position coupled with higher pretensioner limiting load and friction is likely to improve retention in far-side impacts and prevent injuries from the occupant slipping out of the restraint system. Better retention reduced occupant acceleration, excursion, chest compression and number of rib fractures, on the contrary it might instill higher injury vulnerability to neck and brain.

Fatigue responses of the human cervical spine intervertebral discs.

Numerous studies have been conducted since more than fifty years to understand the behavior of the human lumbar spine under fatigue loading. Applications have been largely driven by low back pain and human body vibration problems. The human neck also sustains fatigue loading in certain type of civilian occupational and military operational activities, and research is very limited in this area. Being a visco-elastic structure, it is important to determine the stress-relaxation properties of the human cervical spine intervertebral discs to enable accurate simulations of these structures in stress-analysis models. While finite element models have the ability to incorporate viscoelastic material definitions, data specific to the cervical spine are limited. The present study was conducted to determine these properties and understand the responses of the human lower cervical spine discs under large number of cyclic loads in the axial compression mode. Eight disc segments consisting of the adjacent vertebral bodies along with the longitudinal ligaments were subjected to compression, followed by 10,000 cycles of loading at 2 or 4Hz frequency by limiting the axial load to approximately 150 N, and subsequent to resting period, subjected to compression to extract the stress-relaxation properties using the quasi-linear viscoelastic (QLV) material model. The coefficients of the model and disc displacements as a function of cycles and loading frequency are presented. The disc responses demonstrated a plateauing effect after the first 2000 to 4000 cycles, which were highly nonlinear. The paper compares these responses with the "work hardening" phenomenon proposed in clinical literature for the lumbar spine to explain the fatigue behavior of the discs. The quantitative results in terms of QLV coefficients can serve as inputs to complex finite element models of the cervical spine to delineate the local and internal load-sharing responses of the disc segment.

Evaluation of kinematics and injuries to restrained occupants in far-side crashes using full-scale vehicle and human body models.

OBJECTIVE: The objective of the current study was to perform a parametric study with different impact objects, impact locations, and impact speeds by analyzing occupant kinematics and injury estimations using a whole-vehicle and whole-body finite element-human body model (FE-HBM). To confirm the HBM responses, the biofidelity of the model was validated using data from postmortem human surrogate (PMHS) sled tests. METHODS: The biofidelity of the model was validated using data from sled experiments and correlational analysis (CORA). Full-scale simulations were performed using a restrained Global Human Body Model Consortium (GHBMC) model seated on a 2001 Ford Taurus model using a far-side lateral impact condition. The driver seat was placed in the center position to represent a nominal initial impact condition. A 3-point seat belt with pretensioner and retractor was used to restrain the GHBMC model. A parametric study was performed using 12 simulations by varying impact locations, impacting object, and impact speed using the full-scale models. In all 12 simulations, the principal direction of force (PDOF) was selected as 90°. The impacting objects were a 10-in.-diameter rigid vertical pole and a movable deformable barrier. The impact location of the pole was at the C-pillar in the first case, at the B-pillar in the second case, and, finally, at the A-pillar in the third case. The vehicle and the GHBMC models were defined an initial velocity of 35 km/h (high speed) and 15 km/h (low speed). Excursion of the head center of gravity (CG), T6, and pelvis were measured from the simulations. In addition, injury risk estimations were performed on head, rib cage, lungs, kidneys, liver, spleen, and pelvis. RESULTS: The average CORA rating was 0.7. The shoulder belt slipped in B- and C-pillar impacts but somewhat engaged in the A-pillar case. In the B-pillar case, the head contacted the intruding struck-side structures, indicating higher risk of injury. Occupant kinematics depended on interaction with restraints and internal structures-especially the passenger seat. Risk analysis indicated that the head had the highest risk of sustaining an injury in the B-pillar case compared to the other 2 cases. Higher lap belt load (3.4 kN) may correspond to the Abbreviated Injury Scale (AIS) 2 pelvic injury observed in the B-pillar case. Risk of injury to other soft anatomical structures varied with impact configuration and restraint interaction. CONCLUSION: The average CORA rating was 0.7. In general, the results indicated that the high-speed impacts against the pole resulted in severe injuries, higher excursions followed by low-speed pole, high-speed moving deformable barrier (MDB), and low-speed MDB impacts. The vehicle and occupant kinematics varied with different impact setups and the latter kinematics were likely influenced by restraint effectiveness. Increased restraint engagement increased the injury risk to the corresponding anatomic structure, whereas ineffective restraint engagement increased the occupant excursion, resulting in a direct impact to the struck-side interior structures.

Rate-dependent fracture characteristics of lumbar vertebral bodies.

Experimental testing incorporating lumbar columns and isolated components is essential to advance the understanding of injury tolerance and for the development of safety enhancements. This study incorporated a whole column axial acceleration model and an isolated vertebral body model to quantify compression rates during realistic loading and compressive tolerance of vertebrae. Eight lumbar columns and 53 vertebral bodies from 23 PMHS were used. Three-factor ANOVA was used to determine significant differences (p<0.05) in physiologic and failure biomechanics based on compression rate, spinal level, and gender. Results demonstrated a significant increase in ultimate force (i.e., fracture) from lower to higher compression rates. Ultimate stress also increased with compression rate. Displacement and strain to failure were consistent at both compression rates. Differences in ultimate mechanics between vertebral bodies obtained from males and females demonstrated non-significant trends, with female vertebral bodies having lower ultimate force that would be associated with decreased injury tolerance. This was likely a result of smaller vertebrae in that population. Combined with existing literature, results presented in this manuscript contribute to the understanding of lumbar spine tolerance during axial loading events that occur in both military and civilian environments with regard to effects of compression rate and gender.

Experimental mechanical characterization of abdominal organs: liver, kidney & spleen.

Abdominal organs are the most vulnerable body parts during vehicle trauma, leading to high mortality rate due to acute injuries of liver, kidney, spleen and other abdominal organs. Accurate mechanical properties and FE models of these organs are required for simulating the traumas, so that better designing of the accident environment can be done and the organs can be protected from severe damage. Also from biomedical aspect, accurate mechanical properties of organs are required for better designing of surgical tools and virtual surgery environments. In this study porcine liver, kidney and spleen tissues are studied in vitro and hyper-elastic material laws are provided for each. 12 porcine kidneys are used to perform 40 elongation tests on renal capsule and 60 compression tests on renal cortex, 5 porcine livers are used to perform 45 static compression tests on liver parenchyma and 5 porcine spleens are used to carry out 20 compression tests. All the tests are carried out at a static speed of 0.05 mm/s. A comparative analysis of all the results is done with the literature and though the results are of same order of magnitude, a slight dissonance is observed for the renal capsule. It is also observed that the spleen is the least stiff organ in the abdomen whereas the kidney is the stiffest. The results of this study would be essential to develop the FE models of liver, kidney and spleen which can be further used for impact biomechanical and biomedical applications.

Experimental in vitro mechanical characterization of porcine Glisson's capsule and hepatic veins.

Understanding the mechanical properties of human liver is the most critical aspect of numerical modeling for medical applications and impact biomechanics. Many researchers work on identifying mechanical properties of the liver both in vivo and in vitro considering the high liver injury percentage in abdominal trauma and for easy detection of fatal liver diseases such as viral hepatitis, cirrhosis, etc. This study is performed to characterize mechanical properties of individual parts of the liver, namely Glisson's capsule and hepatic veins, as these parts are rarely characterized separately. The long term objective of this study is to develop a realistic liver model by characterizing individual parts and later integrating them. In vitro uniaxial quasi-static tensile tests are done on fresh unfrozen porcine hepatic parts for large deformations at the rate of 0.1mm/s with a Bose Electroforce 3200 biomaterials test instrument. Results show that mean values of small strain and large strain elastic moduli are 8.22 ± 3.42 and 48.15 ± 4.5 MPa for Glisson's capsule (30 samples) and 0.62 ± 0.41 and 2.81 ± 2.23 MPa for veins (20 samples), respectively, and are found to be in good agreement with data in the literature. Finally, a non-linear hyper-elastic constitutive law is proposed for the two separate liver constituents under study.