A microcontinuum model for mechanical properties of esophageal tissue: experimental methodology and constitutive analysis.

Accurate material properties of tissues are a key factor for the improvement of medical procedures and treatments. Experimental data are essential in order to formulate and validate a useful constitutive model for predicting the mechanical behavior of tissues in these procedures. This study develops a comprehensive experimental protocol at multiple length scale levels in order to obtain stress-strain curves for esophagus tissue. This paper compares two different models: a conventional, non-linear elastic model, and a microcontinuum model based on fiber rearrangement. Also, a detailed description of the experimental procedure is provided. While the focus was on esophageal tissues, the experimental procedure and microcontinuum are considered widely applicable to other samples of soft tissue.

Thoracic response to shoulder belt loading: comparison of tabletop and frontal sled tests with PMHS.

OBJECTIVE: The recent refinement of high-rate optical tracking allows dramatically detailed thoracic deformation measurements to be taken during postmortem human subject (PMHS) sled tests. These data allow analysis of restraint belt geometry and the 3-dimensional thoracic deformations generated by belt impingement. One consequence of this new capability is a better understanding of complementary thoracic characterization experiments such as tabletop tests and how the thoracic response can be interpreted for applications involving more complex loading mechanisms. METHODS: This article reports a detailed evaluation of the timing, magnitude, and direction of the applied belt forces and the resulting thoracic deformations in 2 previously performed tests series involving frontal sled tests and tabletop belt-loading tests. RESULTS: In the sled tests, the posteriorly directed component (SAE x) of the belt tension (F(B)) was F(Bx) = 0.70 F(B) at the shoulder but only F(Bx) = 0.14 F(B) where the belt engaged the anterolateral torso inferiorly. The corresponding components on the tabletop were F(Bx) = 0.60 F(B) (shoulder) and F(Bx) = 0.48 F(B) (lower). CONCLUSIONS: When these components are cross-plotted with chest deflection, pronounced consequences of thoracic anterior wall deformation patterns due to flexion of the thoracic spine and the internal viscera's inertia can be seen in the effective thoracic stiffness. Supplemental materials are available for this article. Go to the publisher's online edition of Traffic Injury Prevention to view the supplemental file.

Minimization of analytical injury metrics for head impact injuries.

OBJECTIVE: To compare the predictions of the head injury criterion (HIC), currently used to predict the risk of traumatic brain injury in frontal vehicle impact and pedestrian impact tests, with the predictions of other empirical and analytical injury metrics. METHODS: The appropriateness of different criteria relative to injury metrics derived from a head finite element (FE) model is investigated for different deceleration pulses in this research. Empirical injury metrics are computed by direct calculation for different analyzed pulses. In addition, for each pulse full FE model simulations of a complete human head were performed by means of the SIMon model. The computations are used to calculate the analytical injury metrics. RESULTS: This article shows that an optimal head deceleration curve based on HIC does not minimize other analytical injury metrics. The results obtained in this study suggest that the HIC criterion does not necessarily provide the same severity ranking for different external loadings to the head as the injury metrics derived from the FE models. CONCLUSION: Countermeasures designed based only on HIC could differ significantly from those based on analytical injury measures computed by FE models. The use of multiple injury metrics is recommended given that no scalar measure seems to be positively and strongly correlated with relevant injury metrics.

Human surrogates for injury biomechanics research.

This article reviews the attributes of the human surrogates most commonly used in injury biomechanics research. In particular, the merits of human cadavers, human volunteers, animals, dummies, and computational models are assessed relative to their ability to characterize the living human response and injury in an impact environment. Although data obtained from these surrogates have enabled biomechanical engineers and designers to develop effective injury countermeasures for occupants and pedestrians involved in crashes, the magnitude of the traffic safety problem necessitates expanded efforts in research and development. This article makes the case that while there are limitations and challenges associated with any particular surrogate, each provides a critical and necessary component in the continued quest to reduce crash-related injuries and fatalities.

Comparison of Hybrid III child test dummies to pediatric PMHS in blunt thoracic impact response.

The limited availability of pediatric biomechanical impact response data presents a significant challenge to the development of child dummies. In the absence of these data, the development of the current generation of child dummies has been driven by scaling of the biomechanical response requirements of the existing adult test dummies. Recently published pediatric blunt thoracic impact response data provide a unique opportunity to evaluate the efficacy of these scaling methodologies. However, the published data include several processing anomalies and nonphysical features. These features are corrected by minimizing instrumentation and processing error to improve the fidelity of the individual force-deflection responses. Using these data, biomechanical impact response corridors are calculated for a 3-year-old child and a 6-year-old child. These calculated corridors differ from both the originally published postmortem human subject (PMHS) corridors and the impact response requirements of the current child dummies. Furthermore, the response of the Hybrid III 3-year-old test dummy in the same impact condition shows a similar deflection but a significantly higher force than the 3-year-old corridor. The response of the Hybrid III 6-year-old dummy, on the other hand, correlates well with the calculated 6-year-old corridor. The newly developed 3-year-old and 6-year-old blunt thoracic impact response corridors can be used to define data-driven impact response requirements as an alternative to scaling-driven requirements.

Influence of pre-collision occupant parameters on injury outcome in a frontal collision.

Optimal performance of adaptive restraint systems in the vehicle requires an accurate assessment of occupant characteristics including physical properties and pre-collision response of the occupant. To provide a feasible framework for incorporating occupant characteristics into adaptive restraint schemes, this study evaluates the sensitivity of injury risk in frontal collisions to four occupant parameters: mass, stature, posture and bracing level. The numerical approach includes using commercial multi-body software to develop occupant models that span a range of occupant parameters representative of the real-world driver population. Coupled with a multi-body model of the vehicle interior and standard restraint system, risk of occupant injuries within specific body regions are predicted through numerical simulations in conjunction with established injury risk functions. The results show occupant posture to be the most significant parameter affecting the overall risk of injury in frontal collisions. The causal relationship as predicted using the numerical model has been compared to the traffic injury epidemiology findings, and the feasibility of an analytical methodology to provide real-time estimates of injury severity has been discussed. Preliminary estimates from the study indicate that the proposed methodology will provide a framework to optimize restraint performance and potentially reduce the risk of injuries up to 35% (based on parameter-specific optimization), using accurate information regarding the pre-collision occupant characteristics.

Re-evaluating the neck injury index (NII) using experimental PMHS tests.

OBJECTIVE: The neck injury index, NII, developed in ISO 13232 (2005) as a testing and evaluation procedure for assessing the risk of injury to the AO/C1/C2 region of the cervical spine in motorcycle riders is reevaluated using an existing postmortem human subjects (PMHS) data set and resulting in a reformulated NII criterion applicable to PMHS tests. METHODS: A recent series of 36 PMHS head/neck component tests was used to examine the risk of neck injury in frontal impacts and to assess the predictive capability of NII for impacts of various orientations. Using force and moment load cell PMHS experimental data, injury risk was assessed using NII evaluated with the ISO 13232-5 algorithms. RESULTS: The injury risk predictions are compared with the injury outcomes from the head/neck PMHS. The NII criterion underestimated the injury incidence of the PMHS experimental group. The average predicted risk of injuries for the experimental injury tests based on NII across the MAIS levels was 0.7 percent, though there were 11 AIS 3+ injuries observed in the actual testing (30.6%). Using the experimental injury outcomes and the experimental force and moment time histories, the normalizing coefficients from NII are reevaluated to minimize the difference between NII risk assessment and the experimental injury outcome in the least squares (L(2)) basis. This reanalysis is compared with existing human and PMHS neck injury criteria. CONCLUSIONS: By reanalyzing the NII formulation using an existing PMHS injury data set with known forces and moments and known injury outcomes, a new NII(PMHS) is developed that uses PMHS loads to predict injury. This reformulation removes the dependency of the original NII formulation on the forces and moments from motorcyclist anthropomorphic test device (MATD) experiments and simulations yet retains the advantages of the multi-axial neck injury criterion.

Viscoelastic response of the thorax under dynamic belt loading.

OBJECTIVE: Three postmortem human surrogates (PMHS) were positioned and rigidly mounted through the spine to a tabletop test fixture for the purpose of characterizing thoracic response to diagonal belt loading with well-defined boundary conditions. METHODS: These PMHS were mounted to a stationary apparatus that supported the spine and shoulders in a configuration comparable to that seen in a 48 km/h automobile sled test at the time of maximum chest deformation. A belt restraint was positioned across the anterior torso with attachments at D-ring and buckle locations based on the geometry of a mid-sized sedan. The belt was attached to a trolley driven by a hydraulic ram linked to a universal test machine. Ramp and hold experiments were conducted at rates of 0.5, 0.9, and 1.2 m/s and hold times of 60 s. Ramp-hold displacement waveforms of up to 20 percent of the chest depth were applied to the chest while the resulting belt loads and spinal reaction loads were recorded. These data were used to identify parameters in a seven-parameter thoracic structural model mathematically analogous to a viscoelastic material model. A final test with 40 percent deflection was performed at the completion of the loading sequence. RESULTS: Model fits to ramps of different magnitudes indicated that the assumption of temporal linearity was reasonable over the range of inputs in this study. In agreement with previous studies, the spatial (force-deflection) response was only slightly nonlinear, indicating that a fully linear model would be reasonable up to the deflection levels used here. CONCLUSIONS: Pronounced variability in the instantaneous elastic behavior was observed among the three test subjects, whereas the relaxation behavior exhibited less variability.

Car safety seats for children: rear facing for best protection.

OBJECTIVE: To compare the injury risk between rear-facing (RFCS) and forward-facing (FFCS) car seats for children less than 2 years of age in the USA. METHODS: Data were extracted from a US National Highway Traffic Safety Administration vehicle crash database for the years 1988-2003. Children 0-23 months of age restrained in an RFCS or FFCS when riding in passenger cars, sport utility vehicles, or light trucks were included in the study. Logistic regression models and restraint effectiveness calculations were used to compare the risk of injury between children restrained in RFCSs and FFCSs. RESULTS: Children in FFCSs were significantly more likely to be seriously injured than children restrained in RFCSs in all crash types (OR=1.76, 95% CI 1.40 to 2.20). When considering frontal crashes alone, children in FFCSs were more likely to be seriously injured (OR=1.23), although this finding was not statistically significant (95% CI 0.95 to 1.59). In side crashes, however, children in FFCSs were much more likely to be injured (OR=5.53, 95% CI 3.74 to 8.18). When 1 year olds were analyzed separately, these children were also more likely to be seriously injured when restrained in FFCSs (OR=5.32, 95% CI 3.43 to 8.24). Effectiveness estimates for RFCSs (93%) were found to be 15% higher than those for FFCSs (78%). CONCLUSIONS: RFCSs are more effective than FFCSs in protecting restrained children aged 0-23 months. The same findings apply when 1 year olds are analyzed separately. Use of an RFCS, in accordance with restraint recommendations for child size and weight, is an excellent choice for optimum protection up to a child's second birthday.

Quantification of ankle articular cartilage topography and thickness using a high resolution stereophotography system.

OBJECTIVE: To describe the topography and to measure thicknesses, surface areas and volumes in the cartilage layers of the ankle. METHODS: Twelve cadaveric ankle joints were disarticulated and the cartilage surfaces of each bone were imaged with a highly accurate (+/-2 microm) stereophotography system (ATOS). The cartilage was then dissolved and the subchondral bone imaged. The geometric data were then used to measure the quantitative parameters in each cartilage layer. RESULTS: The mean cartilage volume across the 12 specimens ranged from 0.32+/-0.08 ml for the fibula to 2.44+/-0.48 ml for the talus. The mean thickness of both the talar (1.1+/-0.18 mm) and tibial (1.16+/-0.14 mm) cartilage was significantly thicker than the fibula (0.85+/-0.13 mm). The talus had the greatest mean maximum cartilage thickness (2.38+/-0.4 mm). CONCLUSIONS: The reported stereophotographic technique may be used as an independent gold standard for validation of the accuracy of quantitative cartilage measurements made using magnetic resonance imaging. The thickness distribution maps show that the thickest articular cartilage occurs over the talar shoulders where osteochondral lesions commonly occur and not in the centre of the talar dome as commonly believed.

Frontal sled tests comparing rear and forward facing child restraints with 1-3 year old dummies.

Although most countries recommend transitioning children from rear facing (RF) to forward facing (FF) child restraints at one year of age, Swedish data suggests that RF restraints are more effective. The objective of this study was to compare RF and FF orientations in frontal sled tests. Four dummies (CRABI 12 mo, Q1.5, Hybrid III 3 yr, and Q3) were used to represent children from 1 to 3 years of age. Restraint systems tested included both 1) LATCH and 2) rigid ISOFIX with support leg designs. Rear facing restraints with support legs provided the best results for all injury measures, while RF restraints in general provided the lowest chest displacements and neck loads.

Quantifying the relationship between vehicle interior geometry and child restraint systems.

The prevention of interactions of children or child restraints with other vehicle structures is critical to child passenger safety. Fifteen current vehicles and seven rear and forward facing child restraint systems were measured in an attempt to quantify the available distance between child restraints and these vehicle structures. Rear facing child restraints exhibited such small amounts of clearance that contact would be expected in the majority of frontal crashes. Upper tethers are critical in the prevention of head contact, while head contact is likely when the upper tether is not used.

Nonlinear viscoelastic behavior of human knee ligaments subjected to complex loading histories.

The nonlinear viscoelastic structural response of the major human knee ligaments when subjected to complex loading histories is investigated, with emphasis on the collateral ligaments. Bone-ligament-bone specimens are tested in knee distraction loading, where the ligaments are in the anatomical position corresponding to a fully extended knee. Temporal nonlinearities for time scales in the range of 1

Vehicle interior interactions and kinematics of rear facing child restraints in frontal crashes.

The performance of rear facing child restraints in frontal crashes can be determined by controlling a) the child's kinematics and b) interactions with vehicle structures. Twelve sled tests were performed to analyze the effect of the location and structural properties of vehicle interior components. The role of restraint kinematics was studied by developing computational models which underwent idealized motions. Stiff structures originally offset from the restraint, but which contact the restraint late in the test, cause increased injury values. Attachment methods which reduce child restraint rotation and more rigidly couple the restraint to the vehicle result in the best safety performance.

Pedestrian injuries: viscoelastic properties of human knee ligaments at high loading rates.

OBJECTIVE: Accidents involving pedestrians are very common, and often lead to severe injuries to the lower extremities. In a large portion of pedestrian-automobile collisions, knee ligament injuries are sustained. In this study, the viscoelastic properties of the four major human knee ligaments were investigated at loading rates representative for pedestrian-automobile collisions. METHODS: Bone-ligament-bone specimens were tested in knee distraction loading. The collateral ligaments and the separate functional bundles of the cruciate ligaments were tested in the anatomical position corresponding to a fully extended knee. A series of step-and-hold tests and ramp tests at different rates were conducted to characterize the time-dependent behavior of the knee ligaments for deformation rates associated with the pedestrian impact loading environment. The quasi linear viscoelastic (QLV) theory was used to describe the structural response of the knee ligaments and averaged parameters for this model were determined. RESULTS: The QLV theory was found to be applicable for the time range that is relevant for pedestrian-automobile collisions. The structural behavior of the knee ligaments was found to be particularly rate-sensitive for high elongation rates, as occur during these collisions. The ligament stiffness was found to increase with age for both the collateral ligaments and with weight for the medial collateral ligament. CONCLUSIONS: For the loading conditions that are relevant for pedestrian-automobile collisions, the use of the QLV model for the description of the mechanical behavior of knee ligaments is appropriate. The rate-sensitivity is particularly important for these extreme loading conditions. The relaxation behavior was found to be consistent between different ligament types and samples. Variations due to donor anthropometry were found predominantly for the instantaneous elastic behavior.

Dynamic bending tolerance and elastic-plastic material properties of the human femur.

The objective of this study was to provide data on the structural tolerance and material properties of the human femur in dynamic bending. Fifteen (15) isolated femurs from eight (8) males were tested in either posterior-to-anterior or lateral-to-medial three-point bending. The failure moment was 458 +/- 95 Nm and did not differ significantly with loading direction. A method was developed to estimate the elastic-plastic material properties of the bone using both force-deflection data and strain gauge measurements. The bone material appeared to yield at about one third of the ultimate strain level prior to fracture. It is hoped that these data will aid in the development of injury criteria and finite element models for predicting injuries to pedestrians and vehicle occupants.

Retrospective analysis of malleolar fractures in an impact environment.

To protect against malleolar fractures in frontal crashes it is important to understand the mechanisms of injury. We have investigated the accuracy of Orthopaedic Specialists in deducing the injury mechanisms of experimentally generated malleolar fractures from radiographs; and the applicability of classic descriptors of injury mechanisms, such as the Lauge-Hansen classification system, in analysing impact induced trauma. Orthopaedic Specialists did not consistently deduce the mechanism of ankle injuries suggesting there may not be a unique fracture pattern for every injury mechanism and that the Lauge-Hansen classification system does not reliably describe ankle fractures created in the impact environment.

The performance of various rear facing child restraint systems in a frontal crash.

Current forward facing (FF) child restraint designs use LATCH and ISOFIX systems to couple the restraint to the vehicle. Rear facing (RF) child restraints, however, have multiple coupling methods that vary by manufacturer and country of origin. Sled tests were performed with the CRABI 12 month dummy in six different RF attachment conditions. The performance of the rear facing child restraints (restraint kinematics, head accelerations, and neck loads) was highly dependent on the coupling method used. The results were also compared to a FF LATCH restraint.

Prediction of cervical spine injury risk for the 6-year-old child in frontal crashes.

This article presents a series of 49 km/h sled tests using the Hybrid III 6-year-old dummy in a high-back booster, a low-back booster, and a three-point belt. Although a 10-year review at a level I trauma center showed that noncontact cervical spine injuries are rare in correctly restrained booster-age children, dummy neck loads exceeded published injury thresholds in all tests. The dummy underwent extreme neck flexion during the test, causing full-face contact with the dummy's chest. These dummy kinematics were compared to the kinematics of a 12-year-old cadaver tested in a similar impact environment. The cadaver test showed neck flexion, but also significant thoracic spinal flexion which was nonexistent in the dummy. This comparison was expanded using MADYMO simulations in which the thoracic spinal stiffness of the dummy model was decreased to give a more biofidelic kinematic response. We conclude that the stiff thoracic spine of the dummy results in high neck forces and moments that are not representative of the true injury potential.