Viscoelastic Characterization of Parasagittal Bridging Veins and Implications for Traumatic Brain Injury: A Pilot Study.

Many previous studies on the mechanical properties of Parasagittal Bridging Veins (PSBVs) found that strain rate had a significant effect on some mechanical properties, but did not extensively study the viscoelastic effects, which are difficult to detect with uniaxial simple tensile tests. In this study, relaxation tests and tests under cyclic loading were performed, and it was found that PSBVs do indeed exhibit clear viscoelastic effects. In addition, a complete viscoelastic model for the PSBVs is proposed and data from relaxation, cyclic load and load-unload tests for triangular loads are used to find reference values that characterize the viscoelastic behavior of the PSBVs. Although such models have been proposed for other types of blood vessels, this is the first study that clearly demonstrates the existence of viscoelastic effects from an experimental point of view and also proposes a specific model to explain the data obtained. Finally, this study provides reference values for the usual viscoelastic properties, which would allow more accurate numerical simulation of PSBVs by means of computational models.

Mechanical Behavior of Blood Vessels: Elastic and Viscoelastic Contributions.

The mechanical properties of the cerebral bridging veins (CBVs) were studied using advanced microtensile equipment. Detailed high-quality curves were obtained at different strain rates, showing a clearly nonlinear stress-strain response. In addition, the tissue of the CBVs exhibits stress relaxation and a preconditioning effect under cyclic loading, unequivocal indications of viscoelastic behavior. Interestingly, most previous literature that conducts uniaxial tensile tests had not found significant viscoelastic effects in CBVs, but the use of more sensitive tests allowed to observe the viscoelastic effects. For that reason, a careful mathematical analysis is presented, clarifying why in uniaxial tests with moderate strain rates, it is difficult to observe any viscoelastic effect. The analysis provides a theoretical explanation as to why many recent studies that investigated mechanical properties did not find a significant viscoelastic effect, even though in other circumstances, the CBV tissue would clearly exhibit viscoelastic behavior. Finally, this study provides reference values for the usual mechanical properties, as well as calculations of constitutive parameters for nonlinear elastic and viscoelastic models that would allow more accurate numerical simulation of CBVs in Finite Element-based computational models in future works.

Influence of anthopometric variables on the mechanical properties of human rib cortical bone.

Objective. The mechanical properties of ribs from a large number ofpost-mortemhuman subjects (PMHS) were analyzed to search for variation according to age, sex or BMI in the sample. A large sample of specimens from different donors (N= 64) with a very wide range of ages and anthropometric characteristics was tested.Methods. Uniaxial tensile tests were used for a sample of coupons machined from cortical bone tissue in order to isolate the purely mechanical properties from the geometrically influenced properties of the rib. Each coupon is about 25 mm long and has a thickness of about 0.5 mm. The mechanical properties measured for each specimen/coupon include YM, yield stress, ultimate stress (maximum failure stress), ultimate strain, and resilience (energy to fracture of SED). The study provides new methodological improvements in DIC techniques.Results. This study is notable for using an atypically large sample of number of PMHS. The size of the sample allowed the authors to determine that age has a significant effect on failure stress (p< 0.0001), yield stress (p= 0.0047), ultimate strain (p< 0.0001) and resilience (p< 0.0001) [numbers in parentheses represent the correspondingp- values]. Finally, there is a combined effect, so that for a given age, an increase of BMI leads to a decrease of the maximum strain (i.e. cortical bone is less stiff when both age and BMI are higher).

Prediction of mechanical properties of human rib cortical bone using fractal dimension.

A large number of post mortem human subjects was used to investigate the relation between the micro-structure of rib cortical bone and the mechanical properties using Fractal Dimension. Uniaxial tensile tests were performed on coupons of rib cortical bone. Tensile strength, yield stress, Young's Modulus, maximum strain, and work to fracture were determined for each coupon. Fractal dimension was computed using CT images and Digital Image Correlation procedures. A highly significant effect of fractal dimension in the mechanical properties was found. In addition, the variation in mechanical properties was found to be adequately represented by Generalized Extreme Value type distributions.

[Pedestrian head injury biomechanics and damage mechanism. Pedestrian protection automotive regulation assessment]. / Biomecánica y mecanismo de producción del traumatismo cráneo-encefálico en el peatón atropellado. Evaluación de la normativa actual en la automoción.

INTRODUCTION: Pedestrian-vehicle collisions are a leading cause of death among motor vehicle accidents. Recently, pedestrian injury research has been increased, mostly due to the implementation of European and Japanese regulations. This research presents an analysis of the main head injury vehicle sources and injury mechanisms observed in the field, posteriorly the data are compared with the current pedestrian regulations. METHODS: The analysis has been performed through an epidemiologic transversal and descriptive study, using the Pedestrian Crash Data Study (PCDS) involving 552 pedestrians, sustaining a total of 4.500 documented injuries. RESULTS: According to this research, the hood surface is responsible for only 15,1% of all the head injuries. On the other hand, the windshield glazing is responsible for 41,8%. In case of sedan vehicles the head impact location exceeds what is expected in the current regulation, and therefore no countermeasures are applied. From all the head injuries sustained by the pedestrians just 20% have the linear acceleration as isolated injury mechanism, 40% of the injuries are due to rotational acceleration. CONCLUSIONS: In this research, the importance of the rotational acceleration as injury mechanism, in case of pedestrian-vehicle collision is highlighted. In the current pedestrian regulation just the linear acceleration is addressed in the main injury criteria used for head injury prediction.

New insights in the analysis of blunt force trauma in human bones. Preliminary results.

Determining the time of injury is an important but still a challenging task in forensic anthropology. In literature, many descriptions can be found to make a distinction between perimortem and postmortem fractures. Characteristics that are more related to fractures in fresh conditions, however, are not extensively investigated. This study compared 28 perimortem fractures from autopsies and 21 both fresh and dry experimentally reproduced human bone fractures. Preliminary results showed the following five distinct traits that might be related to perimortem conditions: layered breakage, bone scales, crushed margins, wave lines and flakes with matching flake defect. These distinct traits might not only be good estimators of perimortem trauma but also may be an indicator of trauma in intra vitam conditions, especially related with muscular reaction to injury. Furthermore, layered breakage seems to be a good trait to infer the biomechanics of trauma.

Injury pattern in lethal motorbikes-pedestrian collisions, in the area of Barcelona, Spain.

INTRODUCTION: There are several studies about M1 type vehicle-pedestrian collision injury pattern, and based on them, there has been several changes in automobiles for pedestrian protection. However, the lack of sufficient studies about injury pattern in motorbikes-pedestrian collisions leads to a lack of optimization design of these vehicles. The objective of this research is to study the injury pattern of pedestrians involved in collisions with motorized two-wheeled vehicles. METHODS: A retrospective descriptive study of pedestrian's deaths after collisions with motorcycles in an urban area, like Barcelona was performed. The cases were collected from the Forensic Pathology Service database of the Institute of Legal Medicine of Catalonia. The selected cases were categorized as pedestrian-motorcycle collision, between January 1st, 2005 and December 31st, 2014. Data were collected from the autopsy, medical, and police report. The collected information was then analyzed using Microsoft Excel statistical functions. RESULTS: Traumatic Brain Injury is the main cause of death in pedestrian hit by motorized two-wheeled vehicles (62.85%). The most frequent injury was the subarachnoid hemorrhage, in 71.4% of cases, followed by cerebral contusions and skull base fractures (65.7%). By contrast, pelvic fractures and tibia fractures only appeared in 28.6%. CONCLUSIONS: The study characterizes the injury pattern of pedestrians involved in a collision with motorized two-wheeled vehicles in an urban area, like Barcelona, which has been found to be different from other vehicle-pedestrian collisions, with a higher incidence of brain injuries and minor frequency of lower extremities fractures in pelvis, tibia and fibula.

A review of pelvic fractures in adult pedestrians: experimental studies involving PMHS used to determine injury criteria for pedestrian dummies and component test procedures.

OBJECTIVES: Perform a systematic review for the most relevant pelvic injury research involving PMHS. The review begins with an explanation of the pelvic anatomy and a general description of pelvic fracture patterns followed by the particular case of pelvic fractures sustained in pedestrian-vehicle collisions. Field data documenting the vehicle, crash, and human risk factors for pedestrian pelvic injuries are assessed. METHOD: A summary of full-scale PMHS tests and subsystem lateral pelvic tests is provided with an interpretation of the most significant findings for the most relevant studies. CONCLUSIONS: Based on the mechanisms of pedestrian pelvic injury, force, acceleration, and velocity and compression have been assessed as predictive variables by researchers although no consensus criterion exists.

Indentation response of human patella with elastic modulus correlation to localized fractal dimension and bone mineral density.

The goal of this study was to determine material properties for the anterior cortex and subcortical regions of human patellae and relate those properties to mineral density and fractal dimension of the bone. Ten human patellae were obtained from eight fresh frozen human cadavers and subjected to anteriorly-directed spherical indentation-relaxation experiments using two different sized indenters to two different indentation depths. Response data were fit to a three-mode viscoelastic model obtained through elastic-viscoelastic correspondence of the Hertzian contact relation for spherical indentation. A location-specific effective bone density measurement that more heavily weighted bone material close to the indentation site (by von Mises stress distribution) was determined from micro-computed tomography (38µm resolution) data captured for each specimen. The same imagery data were used to compute location specific fractal dimension estimates for each indentation site. Individual and averaged patella material models verified the hypothesis that when the larger indenter and greater indentation depth is used to engage the surface and deeper (trabecular) bone, the bone exhibits a more compliant response than when only the surface (cortical) bone was engaged (instantaneous elastic modulus was 325MPa vs. 207MPa, p<0.05). Effective bone mineral density was shown to be a significant predictor of the elastic modulus for both small and large indentation types (p<0.05) despite relatively low correlations. Exponential regressions of fractal dimension on elastic modulus showed significant relationships with high correlation for both the small (R(2)=0.93) and large (R(2)=0.97) indentations.

Side impact PMHS thoracic response with large-volume air bag.

OBJECTIVE: The objective of this study is to assess the response of postmortem human subjects (PMHS) to a large-volume side air bag in a fully instrumented and well-controlled side impact test condition. METHODS: Three adult male PMHS were subjected to right-side pure lateral impacts. Each stationary seated subject was struck at 4.3 ± 0.1 m/s by a rigid wall installed on a 1700-kg rail-mounted sled. Each subject was held stationary by a system of tethers until immediately prior to being impacted by the moving wall. A large side air bag was mounted to the wall and deployed so that it was fully inflated at the time it contacted the subject's right side. The load wall consisted of an adjustable matrix of 15 individual plates, each supported by a 5-axis load cell that recorded the interaction between the subject and impacting wall. Two-dimensional (external) torso deformation was provided by a chest band that encircled the torso at the level of the sixth rib laterally. Triaxial acceleration was measured at the head, spine, and sacrum via 3 orthogonal accelerometers mounted to the same bone-mounted hardware that held the marker clusters used for kinematic analysis. RESULTS: Peak pelvic load normal to the wall averaged 6.8 kN, which was over 5 times that recorded for the shoulder (1.3 kN) and the thorax (1.2 kN). Lateral chest deflection ranged from 9 to 21 mm. Two of the 3 subjects sustained 2 and 9 fractures, respectively. CONCLUSIONS: Two of the 3 PMHS sustained rib fractures despite low levels of thorax deflection. We attribute this finding to individual variability in subject injury tolerance. Other response parameters exhibited lower levels of variability and characterize PMHS response to a potentially beneficial side impact countermeasure. Supplemental materials are available for this article. Go to the publisher's online edition of Traffic Injury Prevention to view the supplemental file.

Increased risk of driver fatality due to unrestrained rear-seat passengers in severe frontal crashes.

While belt usage among rear-seat passengers is disproportionately lower than their front-seat counterpart, this may have serious consequences in the event of a crash not only for the unbelted rear-seat passenger but also for the front-seat passengers as well. To quantify that effect, the objective of the study is to evaluate the increased likelihood of driver fatality in the presence of unrestrained rear-seat passengers in a severe frontal collision. U.S.-based census data from 2001 to 2009 fatal motor vehicle crashes was used to enroll frontal crashes which involved 1998 or later year vehicle models with belted drivers and at least one adult passenger in the rear left seat behind the driver. Results using multivariate logistic regression analysis indicated that the odds of a belt restrained driver sustaining a fatal injury was 137% (95% CI=95%, 189%) higher when the passenger behind the driver was unbelted in comparison to a belted case while the effects of driver age, sex, speed limit, vehicle body type, airbag deployment and driver ejection were controlled in the model. The likelihood of driver fatality due to an unrestrained rear left passenger increased further (119-197%) in the presence of additional unrestrained rear seat passengers in the rear middle or right seats. The results from the study highlight the fact that future advances to front row passive safety systems (e.g. multi-stage airbag deployment) must be adapted to take into account the effect of unrestrained rear-seat passengers.

Fractal dimension and mechanical properties of human cortical bone.

Fractal dimension (FD) can be used to characterize microstructure of porous media, particularly bone tissue. The porous microstructure of cortical bone is observable in micro-CT (µCT) images. Estimations of fractal dimensions of µCT images of coupons of human cortical bone are obtained. The same samples were tested on a tensile test machine and Young's modulus (YM) and Failure stress were obtained. When both types of measures were compared, a clear correlation was found (R=-81%, P<0.01). Young's modulus of each sample and the FD of its µCT images are correlated. From the assumption that cortical bone is approximately a fractal set, a non-linear constitutive relation involving FD is obtained for YM. Experimental results show good agreement with this constitutive relation. Additional parameters in the non-linear relation between YM and FD have been estimated from experimental results and related to physical parameters.

Driving position field study, differences with the whiplash protocol and biomechanics experimental responses.

Rear-impact collisions at low speed are a leading cause of economic costs among motor vehicle accidents. Recently, EuroNCAP has incorporated in its protocol the whiplash test, to reproduce a low-speed rear impact. This paper presents a field driving study to assess the potential differences between the EuroNCAP dummy tests and actual drivers in the field, focusing on occupant position and biomechanics experimental results. A total of 182 drivers were randomly selected in two geographical areas in Spain. The driving position of each driver was recorded with a focus on the most relevant measurements for rear impact. Statistical analysis was performed to obtain means, standard deviations and density functions to compare observational seating position with that of the EuroNCAP testing protocol. The observational data showed a similar seatback angle to that used in the EuroNCAP protocol (24° in front of 25° for the protocol), a greater distance between the head vertex and the top of the head restraint (53mm compared to 39.5mm), and less distance between the occipital bone of the head and the headrest (67.9 compared to 89.3mm). Based on these data, 4 dummy tests were conducted using the dummy BioRID IIg. The baseline test was designed to reproduce the dummy position according to EuroNCAP 3.0 whiplash protocol. Three different additional tests were defined to reproduce the actual observed driving position as well as to assess a "worst case" scenario in terms of reduced seatback angle. These variations in initial driver position, comparing the EuroNCAP protocol to the observational study results, were not observed to cause significant differences in the biomechanical values measured in the BioRID IIg, The T1 acceleration was reduced less than 8%, the NIC was increased about 8%, and the NKm presented a reduction of 20%. Reducing the seat angle was observed to be more harmful in terms of NIC.

A parametric study of hard tissue injury prediction using finite elements: consideration of geometric complexity, subfailure material properties, CT-thresholding, and element characteristics.

OBJECTIVE: The objectives of this study were to examine the axial response of the clavicle under quasistatic compressions replicating the body boundary conditions and to quantify the sensitivity of finite element-predicted fracture in the clavicle to several parameters. METHODS: Clavicles were harvested from 14 donors (age range 14-56 years). Quasistatic axial compression tests were performed using a custom rig designed to replicate in situ boundary conditions. Prior to testing, high-resolution computed tomography (CT) scans were taken of each clavicle. From those images, finite element models were constructed. Factors varied parametrically included the density used to threshold cortical bone in the CT scans, the presence of trabecular bone, the mesh density, Young's modulus, the maximum stress, and the element type (shell vs. solid, triangular vs. quadrilateral surface elements). RESULTS: The experiments revealed significant variability in the peak force (2.41 +/- 0.72 kN) and displacement to peak force (4.9 +/- 1.1 mm), with age (p < .05) and with some geometrical traits of the specimens. In the finite element models, the failure force and location were moderately dependent upon the Young's modulus. The fracture force was highly sensitive to the yield stress (80-110 MPa). CONCLUSION: Neither fracture location nor force was strongly dependent on mesh density as long as the element size was less than 5 x 5 mm(2). Both the fracture location and force were strongly dependent upon the threshold density used to define the thickness of the cortical shell.

Pedestrian injuries in eight European countries: an analysis of hospital discharge data.

Out of the 50,000 yearly road traffic deaths in the European Union (formed by 27 European countries and commonly designated as EU-27), some 8500 are pedestrians. While some studies focus on the increased risk for pedestrian mortality compared to other road users, there is a dearth of information on injury patterns that could be used to prioritize injury prevention measures. Hospital discharge data from eight European countries have been used in this study. Injury information from 10,341 pedestrians sustaining 19,424 injuries has been analyzed. Data have been augmented with Abbreviated Injury Scale, Functional Capacity Index and Injury Severity Score codes, and have been categorized into the Barell Matrix. Fractures (51.1%, 50.3-51.8) and internal injuries (21.3%, 20.7-21.9) are the most frequently found in the data; however, blood vessel injuries and internal injuries are the ones associated with the highest risk of death. Head and lower extremities account for 26% of the injuries each, being spinal and thoracic injuries those showing the highest threat to life risk. Hip and lower extremities injuries are the most frequent cause of functional limitation 1 year after discharge. Due to its intrinsic importance, different injury causation mechanisms for head injuries have been analyzed. Though current standards and regulations consider Head Injury Criterion (HIC) as the only tool to assess the risk of injuries to the head, real world injury data show that only 12.1% (11.0-13.2) of these injuries can be attributed to a pure translational mechanism and therefore susceptible to be predicted by HIC. Design of prevention strategies, particularly from the engineering point of view, should benefit from this information.

Failure of the lumbar pedicles under bending loading - biomed 2010.

The purpose of this study was to investigate the magnitude of bending moment that results in fracture of the pedicles when lumbar vertebrae are loaded in four-point bending. Nine human second lumbar vertebrae (L2) were harvested from donors aged 59-75 years. The specimens were potted and then subjected to quasi-static sagittal-plane four-point bending, which allowed for a constant bending moment applied over a 3.8 cm span centered on the vertebral pedicles until fracture. The failure bending moment calculated for the pedicles varied widely (30.7 +/- 12.3 Nm) and was poorly correlated with subject age (y = -0.91x + 91.5, R(2) = -0.27). With increasing displacement, the bending moment applied to the pedicles increased, first linearly, followed by a non-linear portion, prior to specimen fracture. In general, the specimens failed at the interface of the pedicles and vertebral bodies, but failures were observed elsewhere as well. These data provide sufficient response and boundary condition information for finite element modeling and model validation.

Characterization of the transverse and spinous vertebral processes: fracture forces under quasi-static and dynamic loading - biomed 2010.

The purpose of this study was to investigate the response and failure properties of the transverse and spinous vertebral processes under cantilever bending. Twelve human third lumbar vertebrae (L3) were harvested from donors aged 56-79 years, and the processes were loaded in either dynamic (1000 mm/s) or quasi-static (1 mm/s) cantilever bending. All of the spinous processes were loaded dynamically, and transverse process loading was alternated between dynamic and quasi-static and between right and left. Peak forces measured for the transverse processes were 252 +/- 77 N and 234 +/- 50 N in the dynamic and quasi-static tests, respectively, which corresponded to lack of rate sensitivity in the transverse process peak force for the range of loading rates considered (p=0.6). Peak forces measured for the spinous processes were 1179.6 +/- 587.1 N, and both subject mass (p=0.038) and subject age (p=0.006) were found to be significant predictors of peak force.

Endplate indentation of the fourth lumbar vertebra - biomed 2010.

This study presents the results of indentation tests on the superior vertebral endplate of the 4th lumbar vertebra (L4) of eleven male cadaveric subjects (65 +/- 7 years). Three locations on the superior endplate surface were loaded with a 7.9 mm spherical indentor at either a low (1 mm/s) or high (1000 mm/s) rate. Anterior midline and posterior right and left indentation locations were chosen to prevent local deformations and fractures from influencing the results of subsequent and preceding tests. Peak forces were higher in the dynamic tests (498 +/- 261 N) than in the quasi-static tests (451 +/- 256 N) on the posterior side, although the difference was not significant (p = 0.139). However, the peak forces in the anterior tests (304 +/-166 N) were significantly lower (p =0.0157) than in the posterior tests with the same loading rate. The variation in failure forces in the current study correlates with the variation in thickness of endplate cortical bone (between specimens and between anterior and posterior locations on the same specimen) as measured from small field of view computed tomography scans.

Response of the sternum under dynamic 3-point bending - biomed 2010.

The goal of this study was to investigate the response and failure properties of the human sternum under bending loading. Nine sternum specimens from post mortem human surrogates (n=7 male, n=2 female, age: 62.7 +/- 10.9 years) were extracted and potted in a three point bending test setup. Specimens were loaded to failure at their center points in bending at 1100 mm/s, with some specimens previously loaded in a non-failure quasi-static loading test. In two cases, the non-failure test was repeated to show that specimens were not damaged during non-failure testing. The sternum specimens were found generally to be unable to support shear forces in the anterior-posterior direction and as a result had relatively low failure moments (24.1 Nm +/- 20.1 Nm). While two of the specimens did fail in bending, the remaining specimens failed as a result of the high tensile forces introduced by the bending loads. These specimens first experienced compressive loads, and then, as the potted ends continued to rotate, tensile loads, which resulted in failure of the specimens (400-800 N).

Injury biomechanics as a necessary tool in the field of forensic science: a pedestrian run-over case study.

A 49-year-old male pedestrian was fatally injured when an overloaded truck backed over him and two of the truck's rear wheels rolled over his chest. An analysis is presented to estimate whether or not the subject would have been severely injured if the truck had been loaded to the maximum-permitted weight. The magnitude of compression of the subject's chest is predicted both for the case weight and the maximum-permitted weight of the vehicle. These predicted magnitudes of chest compression are then used to predict the probability of thoracic injury in both cases. The analysis suggests that loading by either the case weight or the maximum-permitted weight of the vehicle would have caused very severe compressions of the chest, likely resulting in multiple rib fractures, collapse of the ribcage and injury to the thoracic organs. Thus, this analysis suggests that severe, possibly life-threatening, thoracic injury would have occurred if the vehicle was loaded to its maximum-permitted weight.