The response of pediatric ribs to quasi-static loading: mechanical properties and microstructure.

Traumatic injury is a major cause of death in the child population. Motor vehicle crashes account for a large portion of these deaths, and a considerable effort is put forth by the safety community to identify injury mechanisms and methods of injury prevention. However, construction of biofidelic anthropomorphic test devices and computational models for this purpose requires knowledge of bone properties that is difficult to obtain. The objective of this study is to characterize the relationship between mechanical properties and measures of skeletal development in the growing rib. Anterolateral segments of 44 ribs from 12 pediatric individuals (age range: 5 months to 9 years) were experimentally tested in three-point bending. Univariate mixed models were used to assess the predictive abilities of development-related variables (e.g., age, stature, histomorphometry, cross-sectional geometry) on mechanical variables (material and structural properties). Results show that stature, in addition to age, may be a reliable predictor of bone strength, and that histomorphometry has potential to explain bone properties and to further our understanding of fracture mechanisms. For example, percent secondary lamellar bone (%Sd.Ar) successfully predicts peak force (F P) and Young's modulus (E). Application of these findings is not restricted to injury biomechanics, but can also be referenced in forensic and anthropological contexts.

Using pressure to predict liver injury risk from blunt impact.

Liver trauma research suggests that rapidly increasing internal pressure plays a role in causing blunt liver injury. Knowledge of the relationship between pressure and the likelihood of liver injury could be used to enhance the design of crash test dummies. The objectives of this study were (1) to characterize the relationship between impact-induced pressures and blunt liver injury in an experimental model to impacts of ex vivo organs; and (2) to compare human liver vascular pressure and tissue pressure in the parenchyma with other biomechanical variables as predictors of liver injury risk. Test specimens were 14 ex vivo human livers. Specimens were perfused with normal saline solution at physiological pressures, and a drop tower applied blunt impact at varying energies. Impact-induced pressures were measured by transducers inserted into the hepatic veins and the parenchyma (caudate lobe) of ex vivo specimens. Experimentally induced liver injuries were consistent with those documented in the Crash Injury Research and Engineering Network (CIREN) database. Binary logistic regression analysis demonstrated that injury predictors associated with tissue pressure measured in the parenchyma were the best indicators of serious liver injury risk. The best injury predictor overall was the product of the peak rate of tissue pressure increase and the peak tissue pressure, P T max * P T max (pseudo-R2 = .82, p = .001). A burst injury mechanism directly related to hydrostatic pressure is postulated for the ex vivo liver loaded dynamically in a drop test experiment.

Oblique and lateral impact response of the PMHS thorax.

This study characterizes the PMHS thoracic response to blunt impact in oblique and lateral directions. A significant amount of data has been collected from lateral impacts conducted on human cadavers. Substantially less data has been collected from impacts that are anterior of lateral in an oblique direction. In the past, data collected from the handful of oblique impact studies were considered to be similar enough to the data from purely lateral impacts such that the oblique data were combined with data from lateral impacts. Defining the biomechanical response of the PMHS thorax to oblique impact is of great importance in side impact vehicle crashes where the loading is often anterior-oblique in direction. Data in this study was obtained from a chestband placed on the thorax at the level of impact to measure thoracic deflection. Two low energy impacts were conducted on each of seven subjects at 2.5 m/s, with one lateral impact and one oblique impact to opposite sides of each PMHS. Data was normalized using the Mertz-Viano method for a two mass system to allow for inter-subject comparisons. Force versus deflection response corridors were generated for the two impact types using an objective mathematical approach and compared to one another. Results were also compared to existing data for oblique and lateral thoracic impacts. The oblique thoracic response in low speed pendulum impacts was found to be different than the lateral thoracic response, in terms of force and deflection. Specifically, the lateral force was greater than the oblique force, and oblique deflection greater than lateral deflection for equal energy impacts.

Shoulder impact response and injury due to lateral and oblique loading.

Little is known about the response of the shoulder complex due to lateral and oblique loading. Increasing this knowledge of shoulder response due to these types of loading could aid in improving the biofidelity of the shoulder mechanisms of anthropomorphic test devices (ATDs). The first objective of this study was to define force versus deflection corridors for the shoulder corresponding to both lateral and oblique loading. A second focus of the shoulder research was to study the differences in potential injury between oblique and lateral loading. These objectives were carried out by combining previously published lateral impact data from 24 tests along with 14 additional recently completed lateral and oblique tests. The newly completed tests utilized a pneumatic ram to impact the shoulder of approximately fiftieth percentile sized cadavers at the level of the glenohumeral joint with a constant speed of approximately 4.4 m/sec. Of the 14 tests, four of them were conducted lateral to the shoulder along the subject's y-axis, four of them were conducted 15 anterior to this axis, and six were conducted 30 anterior to the subject's y-axis. As in the previous testing, the first thoracic vertebrae and both shoulders of the subject were instrumented with tri-axial linear accelerometers on the sternum, clavicle, acromion process, and inferior angle of the scapula. The impacting mass was instrumented with an accelerometer and displacement transducer. In addition to this instrumentation, the tests were documented by high-speed digital imagery. Radiographs (x-rays), magnetic resonance images (MRIs), and autopsies were used to document injury to the subjects. The results from the tests revealed differences between the stiffness of the shoulder when loaded laterally to that when it is loaded obliquely. The shoulder was found to deflect twice as much medially when loaded obliquely then when it is loaded laterally. This can be attributed to the ability of the scapula to slide posteriorly around the thoracic cage. The ability of the shoulder to displace medially while simultaneously deflecting posteriorly in oblique impact is important to replicate in the ATDs because it results in the load being transmitted to the upper thoracic cage.

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.