Historical human data will be selected from the Wright-Patterson Collaborative Biodynamics Network (CBDN) and National Highway Traffic Safety Administration (NHTSA) databases. The data will be selected based on loading dynamics and subject demographics. Once these data are selected, the THOR ATD will be tested in identical conditions. A Bayesian analysis along with survival analysis will be used to relate the resulting THOR responses to improve injury risk predictions. The results of the Occupant Protection Data Mining and Modeling Task will be used as prior distributions.
Matched- pair tests between postmortem human surrogates (PMHS) and each ATD are used to determine ATD- specific injury criteria. The merit of the matched-pair design is the one-to-one correlation of the results from external loads to both surrogates.
To achieve specific aim 1, historical PMHS test cases for the five areas of the body focused on in this experiment were selected from the Medical College of Wisconsin (MCW) database for matched-pair testing. Selection of these cases were made based on their similarity to spaceflight loading dynamics and astronaut demographics.
The first area of the body that was tested to evaluate a unique injury response mechanism was lower neck . Lower neck injury in rearward loading was tested when two different PMHS historical data sets were used for this study. A group of five intact PMHS were seated and belted in a custom metal seat on an accelerator sled and were subjected to low or high velocities. A group of 13 PMHS were isolated at the T2 level. The PMHS in the second group were exposed to 4 different severities.
A matched-pair test for each PMHS was conducted for both the THOR and Hybrid-III 50th percentile male ATDs. For the intact PMHS test conditions, the full ATD was placed in the same custom chair and subjected to the same input conditions. For the head-neck complex PMHS tests, the head and neck were removed from the rest of the ATD and tested on the same sled-pendulum test rig. Internal accelerometers and neck load cells were used to capture dynamic data.
Vertical neck loading was tested by using PMHS from multiple studies. They were organized into 2 groups: upright and inverted; denoted by groups A and B respectively. In total, there were 42 subjects (26 males and 16 females). The upright subjects were isolated at T2-T3 and the applied loading rates ranged from 0.05 to 8.9 m/s. The specimens for the inverted subjects were raised approximately 0.53 m and dropped to the platform of a drop apparatus with velocities of 2.3 to 3.3 m/s. Matched pair tests could not be completed because the necks of both the Hybrid III and THOR are much less compliant than the human neck, and therefore could not match the posture and alignment of PMHS.
Upper and Lower neck were tested under lateral loading. Biomechanical data were gathered for 11 subjects and organized into 3 groups: : (1) fully restrained torso with belts securing both the torso and extremities, (2) torso secured with a three-point center-mounted buckle, and (3) a different design of the three-point belt in which the specimens were positioned on a custom-designed seat rigidly fixed to the platform of a sled. The subjects were seated facing forward and upright. Tension force and extension bending moment data at the upper and lower neck were analyzed in this report to develop neck IRCs. Matched-pair tests were conducted for Groups A and B. Both the THOR and Hybrid-III were placed in the same seat and belt setup as each PMHS group and given the same sled dynamics.
Thorax was tested in lateral loading conditions. Seventeen PMHS were used in this part of the study, each instrumented with 3 chest band strain gauge suites wrapped around the circumference of the thorax at the level of rib 4, xiphoid process, and rib 10. The PMHS was seated on a custom seat which was fixed to an accelerator sled, and each test was conducted at 6.7 m/s. The Hybrid-III lacks instrumentation on the ribs to calculate chest deflection and therefore could not be used for matched-pair testing. The THOR was also not used for matched- pair testing for this injury metric because its internal rib sensors demonstrated very little deflection in side impacts.
Pelvis was tested in lateral loading when biomechanical data was gathered from 2 different data sets. A total of 63 tests were performed, split between two groups. The first data set included 10 PMHS, with thirty -seven tests performed, seated in a driving posture and unbelted without lateral support. The second group had 12 subjects tested in a similar manner. The Hybrid III-ATD does not have acetabulum/ hip internal load cells and hence is incapable for matched-pair tests in this study.
Injury risk correlations derived from PMHS and ATD comparisons were used to improve upon the injury metrics previously developed under the Occupant Protection (OP) Data Mining and Modeling Task. For upper and lower neck assessments, forces and interaction criteria had differences of risk levels between the magnitudes of metrics. Forces curves were noted to be similar to the upper and lower necks, while the moment curves were amplifies at the lower neck due to the kinematics of the cervical column during lateral acceleration.
The inverted drop test series indicated that age of the specimen is a significant covariate. It was noted to be negatively associated with the risk of injury. A comparison between the forces at all risk levels indicated a greater magnitude of force with the upright than the inverted model, reflecting the differences between the bony in the former versus soft tissue and distractive nature of injuries sustained in the latter set up.
All injury risk methods had an additional calculation at 35-45 years of age, since age was found to be a significant covariate. The results from the upright experimental model indicated that age is a significant variable and increasing age was associated with higher risk of injury.
These results suggest that for compression- related injuries, specimen age should be used as a covariate or individual specimen data may be scaled to derive risk curves. Results for distraction or extension-related injuries showed that specimen age does not need to be used as a covariate in the statistical analysis. The findings from these tests and survival analysis indicate that age factor modulates human cervical spine tolerance to impact injury.
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