Biomechanical response targets for physical and computational models of the pediatric trunk.

OBJECTIVES: This paper quantifies pediatric thoracoabdominal response to belt loading to guide the scaling of existing adult response data and to assess the validity of a juvenile porcine abdominal model for application to the development of physical and computational models of the human child. METHODS: Table-top belt-loading experiments were performed on 6, 7, and 15 year-old pediatric post-mortem human subjects (PMHS). Response targets are reported for diagonal belt and distributed loading of the anterior thorax and for horizontal belt loading of the abdomen. RESULTS: The pediatric PMHS exhibited abdominal response similar to the swine, including the degree of rate sensitivity. The thoraces of the PMHS were as stiff as, or slightly more stiff than, published adult corridors. CONCLUSIONS: An assessment of age-related changes in thoracic stiffness suggests that the effective stiffness of the chest increases through the fourth decade of life and then decreases, resulting in stiffness values similar for children and elderly adults.

The biomechanics of the pediatric and adult human thoracic spine.

A growing body of literature points out the relevance of the thoracic spine dynamics in understanding the thorax-restraint interaction as well as in determining the kinematics of the head and cervical spine. This study characterizes the dynamic response in bending of eight human spinal specimens (4 pediatric: ages 7 and 15 years, 4 adult: ages 48 and 52 years) from two sections along the thoracic spine (T2-T4 and T7-T9). Each specimen consisted of three vertebral bodies connected by the corresponding intervertebral discs. All ligaments were preserved in the preparation with the exception of the inter-transverse ligament. Specimens were exposed to a series of five dynamic bending ramp-and-hold tests with varying amplitudes at a nominal rate of 2 rad/s. After this battery of tests, failure experiments were conducted. The 7-year-old specimen showed the lowest tolerance to a moment (T2-T4: 12.1 Nm; T7-T9: 11.6 Nm) with no significant reduction of the relative rotation between the vertebrae. The 15-year-old failure tolerance was comparable to that of the adult specimens. Failure of the adult specimens occurred within a wide range at the T2-T4 thoracic section (23.3 Nm- 53.0 Nm) while it was circumscribed to the interval 48.3 Nm-52.5 Nm for the T7-T9 section. The series of dynamic ramp-and-hold were used to assess two different scaling methods (mass scaling and SAE scaling). Neither method was able to capture the stiffness, peak moment and relaxation characteristics exhibited by the pediatric specimens.

Response of the Worldwide Side Impact Dummy (WorldSID) to Localized Constant-Speed Impacts.

The objective of this study was to evaluate WorldSID constant-speed shoulder and thorax impact responses in terms of impact force, external and internal deflection (1D and 2D IR-Tracc response) for two velocities (1 m/s and 3 m/s), at three impact levels (shoulder, upper thorax and mid thorax) in three impact directions (lateral, +15° posterolateral, -15° anteraolateral). In addition, the impact force and external deflection were compared to previously published cadaver data. Each impact condition was repeated twice. A total of 42 tests were performed. The WorldSID's lowest peak impact force and external deflection were found for impact at shoulder level regardless of impact direction. Maximum force and deflection were found for impact at mid thorax. Comparison between WorldSID and PMHS showed similar external chest deflections for impacts at 3 m/s. The peak impact force response with respect to impact level was found to be reversed for the WorldSID compared to the PMHS, for which shoulder impact resulted in the highest peak force. External time history responses for the WorldSID compared to the one PMHS impacted at 1 m/s in lateral impact direction showed a significant difference in both timing and magnitude. External deflections at upper and mid thorax were approximately twice as high as the internal 1D deflection measured by the IR-Tracc. However, taking into account the rotation of the rib, the calculated 2D deflection response at the posterior impact direction was closer to the external deflection, and thus also to the PMHS deflection response at 3 m/s. These findings emphasize the need of 2D deflection measurement.

Analysis of spinal motion and loads during frontal impacts. Comparison between PMHS and ATD.

Quantifying the kinematics of the human spine during a frontal impact is a challenge due to the multi-degree-of-freedom structure of the vertebral column. This papers reports on a series of six frontal impacts sled tests performed on three Post Mortem Human Surrogates (PMHS). Each subject was exposed first to a low-speed, non-injurious frontal impact (9 km/h) and then to a high-speed one (40 km/h). Five additional tests were performed using the Hybrid III 50(th) percentile male ATD for comparison with the PMHS. A 3D motion capture system was used to record the 6-degree-of-freedom motion of body segments (head, T1, T8, L2, L4 and pelvis). The 3D trajectories of individual bony structures in the PMHS were determined using bone-mounted marker arrays, thus avoiding skin-attached markers and their potential measurements artifacts. The PMHS spines showed different behavior between low and high speed. While at low speed the head and upper spinal segments lagged the lower portion of the spine and pelvis in reaching their maximum forward displacement (time for maximum forward head excursion was 254.3±31.9 ms and 140.3±9 ms for the pelvis), these differences were minimal at high speed (127±2.6 ms for the head vs. 116.7±3.5 ms for the pelvis). The ATD did not exhibit this speed-dependant behavior. Furthermore, the ATD's forward displacements were consistently less than those exhibited by the PMHS, regardless of the speed. Neck loads at the atlanto-occipital joint were estimated for the PMHS using inverse dynamics techniques and compared to those measured in the ATD. It was found that the axial and shear forces and the flexion moment at the upper neck of the PMHS were higher than those measured in the ATD.

Expansion and evaluation of data characterizing the structural behavior of the pediatric abdomen.

Despite the importance of abdominal injuries in children involved in motor vehicle collisions, only two papers have reported experimental data quantifying the pediatric abdominal response to belt loading. One developed and characterized a porcine model of the pediatric abdomen and the other presented a series of tests performed on a single pediatric (7-year-old female) post-mortem human subject (PMHS) and used the data to evaluate the efficacy of the porcine model. The current paper presents the results from an additional pediatric (6-year-old female) PMHS test series and an expanded evaluation of the porcine model using the combined PMHS data. The two PMHS exhibited remarkably similar abdominal stiffness, both by level (upper and lower) and by rate (quasi-static and ∼2 m/s dynamic). Both PMHS and swine exhibited the same stiffness trend by abdominal level (lower stiffer than upper: 3444 N reaction force at 30.5 mm of displacement compared to 1756 N in the 6-year-old dynamic tests). The magnitude of lower abdomen stiffness was slightly less in the swine than in the PMHS (the average dynamic PMHS response was 1086 N greater than the porcine envelopes at 30.5 mm displacement) while the upper abdomen PMHS responses fit within the porcine response envelope.