Fundamentals of impact biomechanics: Part 2--Biomechanics of the abdomen, pelvis, and lower extremities

Development of a biofidelic computational model of human pelvis for predicting biomechanical responses and pelvic fractures

BACKGROUND AND OBJECTIVE

The pelvis, a crucial structure for human locomotion, is susceptible to injuries resulting in significant morbidity and disability. This study aims to introduce and validate a biofidelic computational pelvis model, enhancing our understanding of pelvis injury mechanisms under lateral loading conditions.

METHODS

The Finite Element (FE) pelvic model, representing a mid-sized male, was developed with variable cortical thickness in pelvis bones. Material properties were determined through a synthesis of existing constitutive models, parametric studies, and multiple validations. Comprehensive validation included various tests, such as load-displacement assessments of sacroiliac joints, quasi-static and dynamic lateral compression on the acetabulum, dynamic side impacts on the acetabulum and iliac wing using defleshed pelvis, and lateral impacts by a rigid plate on the full body's pelvis region.

RESULTS

Simulation results demonstrated a reasonable correlation between the pelvis model's overall response and cadaveric testing data. Predicted fracture patterns of the isolated pelvis exhibited fair agreement with experimental results.

CONCLUSIONS

This study introduces a credible computational model, providing valuable biomechanical insights into the pelvis' response under diverse lateral loading conditions and fracture patterns. The work establishes a robust framework for developing and enhancing the biofidelity of pelvis FE models through a multi-level validation approach, stimulating further research in modeling, validation, and experimental studies related to pelvic injuries. The findings are expected to offer critical perspectives for predicting, preventing, and mitigating pelvic injuries from vehicular accidents, contributing to advancements in clinical research on medical treatments for pelvic fractures.

Injuries caused by defensive bullet launchers and resource utilization during the French yellow vests protests: A retrospective study

BACKGROUND

In 2018, the French "Yellow Vest" social protest movement spread with weekly demonstrations resulting in confrontations between protesters and law enforcement. Non-lethal weapons, such as defensive bullet launchers (DBL) were used, and significant injuries have been reported through media, leading to public controversy regarding their use. These injuries are not well-known to civilian emergency physicians. The aim of this study is to describe the injuries caused by DBL among Emergency Department (ED) patients during these demonstrations and to identify the characteristics that required specialized care and hospital admission.

METHODS

A multicenter retrospective study was conducted in 7 EDs of academic hospitals in Paris, France. Adult ED patients who presented with DBL injuries during "yellow vest" strikes between November 2018 and May 2019 were included. The primary outcome was the rate of DBL patients requiring hospital admission. We also compared the characteristics of the injuries and the care provided between the admitted patients and other DBL patients.

RESULTS

152 patients were included. 17% were admitted to hospital, with 19% of them being transferred to intensive care units. 49% of all patients had head, face, eye or neck injuries including 4 cases of intracranial hemorrhage, 1 carotide dissection, 1 laryngeal edema, 1 pneumencephalus. 11% of all patients presented with multiple wounds, and 28% had fractures (77% of admitted patients vs 18%, p < 0.001). Surgery was required for 20% of all patients (62% of admitted patients vs 10%, p < 0.001). Maxillofacial surgery was performed on 38% of admitted patients, orthopedic surgery on 25%, and neurosurgery on 13%. No death were reported.

CONCLUSION

The use of DBL during the "yellow vest" civil strikes was associated with a high rate of head, face, eye or neck injuries among injured ED patients. Hospital admission was associated with a higher rate of fractures, with most of them requiring maxillofacial, orthopedic and neurosurgeries.

Impact attenuation provided by older adult protective headwear products during simulated fall-related head impacts

INTRODUCTION

While protective headwear products (PHP) are designed to protect older adults from fall-related head injuries, there are limited data on their protective capacity. This study's goal was to assess the impact attenuation provided by commercially available PHP during simulated head impacts.

METHODS

A drop tower and Hybrid III headform measured the decrease in peak linear acceleration (g atten ) provided by 12 PHP for front- and back-of-head impacts at low (clinically relevant: 3.5 m/s) and high (5.7 m/s) impact velocities.

RESULTS

The range of g atten across PHP was larger at the low velocity (56% and 41% for back and frontal impacts, respectively) vs. high velocity condition (27% and 38% for back and frontal impacts, respectively). A significant interaction between impact location and velocity was observed (p < .05), with significantly greater g atten for back-of-head compared to front-of-head impacts at the low impact velocity (19% mean difference). While not significant, there was a modest positive association between g atten and product padding thickness for back-of-head impacts (p = .095; r = 0.349).

CONCLUSION

This study demonstrates the wide range in impact attenuation across commercially available PHP, and suggests that existing products provide greater impact attenuation during back-of-head impacts. These data may inform evidence-based decisions for clinicians and consumers and help drive industry innovation.

Assessment of lumbar spinal disc injury in frontal crashes

Frontal vehicle crashes have been a leading cause of spinal injuries in recent years. Reconstruction of frontal crashes using computational models and spinal load analysis helps us understand the patterns of injury and load propagation during frontal crashes. By reconstructing a real crash test and using a viscoelastic crash dummy model, spinal injury patterns were analyzed. The results indicated that a moderate crash with an impact speed of 56 km/h leads to injuries in L1-L2 and L5-S1 levels (L for lumbar and S for sacral vertebrae). The largest spinal loads and injuries were mainly observed immediately after the airbag deployment when the peak of the crash acceleration transpires. Also, the effects of impulse magnitude on the spinal loads and head injury criterion (HIC) showed that HIC is more sensitive than compressive forces to the magnitude of impulse. Moreover, the effects of disc preconditioning as a major factor in the risk of injury was evaluated. The results demonstrate that as the lumbar spine is subjected to a longer preloading, it will be more vulnerable to injury; preconditioning of the discs more adversely affected the risk of injury than a 10% increase in the crash impulse. Overall the results highlight the importance of spinal injury prevention in frontal crashes.

Multibody Models for the Analysis of a Fall From Height: Accident, Suicide, or Murder?

The final subject position is often the only evidence in the case of the fall of a human being from a given height. Foreseeing the body trajectory and the respective driving force may not be trivial due to the possibility of rotations and to an unknown initial position and momentum of the subject. This article illustrates how multibody models can be used for this aim, with specific reference to an actual case, where a worker fell into a stair well, prior to stair mounting, and he was found in an unexpected posture. The aim of the analysis was establishing if this worker was dead in that same place, if he had been pushed, and which was his initial position. A multibody model of the subject has been built ("numerical android"), given his stature and his known mass. Multiple simulations have been performed, following a design of experiments where various initial positions and velocity as well as pushing forces have been considered, while the objective function to be minimized was the deviation of the numerical android position from the actual worker position. At the end of the analysis, it was possible to point how a very limited set of conditions, all including the application of an external pushing force (or initial speed), could produce the given final posture with an error on the distance function equal to 0.39 m. The full analysis gives a demonstration of the potentiality of multibody models as a tool for the analysis of falls in forensic inquiries.

Force, impulse and energy during falling with and without knee protection: an in-vitro study

The mechanics of protective knee padding mitigating injury from a high-force fall have not been investigated in real-life scenarios to date. This study compares the effect of wearing knee pads to unprotected impact on a hard surface. We hypothesized that knee pads reduce the force and energy transmitted to the bony structures of the knee cap compared with unprotected conditions. Eight human knee cadaver specimens were embedded and fixed with a flexion angle of 100 degrees in a custom-made drop testing device (75 kg including the knee). The usage of a knee pad led to an average peak force attenuation on impact of 15% (no pad: 5932 N SD: 2472 N; pad: 4210 N SD: 2199 N; p < 0.001). Contact time on the plate was higher with a knee pad (no pad: 0.015 s SD: 0.009 s; pad: 0.028 s SD: 0.014 s; p < 0.001). Therefore, the observed impulse was also increased (no pad: 62.2 Ns SD: 17.8 Ns; pad: 74.6 Ns SD: 18.6 Ns; p < 0.001). This effect diminished as drop height was increased. Energy dissipation, defined as the difference between kinetic energy pre-impact and peak potential energy post-impact, was higher without a knee pad (no pad: 10.5 J SD: 6.2 J; pad: 4.2 J SD: 5.0 J; p < 0.001). The results from this study illustrate the magnitude of influence that knee pads have on peak forces, transmitted impulse, and energy transfer from a high-force impact in real-life scenarios. Contrary to expectations, the knee pad did not act as a mechanical damper. The mechanical behavior more closely resembled a spring that temporarily stores energy and consequentially reduces peak forces upon impact. Based on this study, future developments in padding might benefit from focusing on the aspect of energy storage and temporarily delayed energy dissipation.

Biomarkers affected by impact velocity and maximum strain of cartilage during injury

Osteoarthritis is one of the most common, debilitating, musculoskeletal diseases; 12% associated with traumatic injury resulting in post-traumatic osteoarthritis (PTOA). Our objective was to develop a single impact model with cartilage "injury level" defined in terms of controlled combinations of strain rate to a maximum strain (both independent of cartilage load resistance) to study their sensitivity to articular cartilage cell viability and potential PTOA biomarkers. A servo-hydraulic test machine was used to measure canine humeral head cartilage explant thickness under repeatable pressure, then subject it (except sham and controls) to a single impact having controlled constant velocity V=1 or 100mm/s (strain rate 1.82 or 182/s) to maximum strain ε=10%, 30%, or 50%. Thereafter, explants were cultured in media for twelve days, with media changed at day 1, 2, 3, 6, 9, 12. Explant thickness was measured at day 0 (pre-injury), 6 and 12 (post-injury). Cell viability, and tissue collagen and glycosaminoglycan (GAG) were analyzed immediately post-injury and day 12. Culture media were tested for biomarkers: GAG, collagen II, chondroitin sulfate-846, nitric oxide, and prostaglandin E2 (PGE2). Detrimental effects on cell viability, and release of GAG and PGE2 to the media were primarily strain-dependent, (PGE2 being more prolonged and sensitive at lower strains). The cartilage injury model appears to be useful (possibly superior) for investigating the relationship between impact severity of injury and the onset of PTOA, specifically for discovery of biomarkers to evaluate the risk of developing clinical PTOA, and to compare effective treatments for arthritis prevention.

Blast Wave Loading Pathways in Heterogeneous Material Systems–Experimental and Numerical Approaches

Blast waves generated in the field explosions impinge on the head-brain complex and induce mechanical pressure pulses in the brain resulting in traumatic brain injury. Severity of the brain injury (mild to moderate to severe) is dependent upon the magnitude and duration of the pressure pulse, which in turn depends on the intensity and duration of the oncoming blast wave. A fluid-filled cylinder is idealized to represent the head-brain complex in its simplest form; the cylinder is experimentally subjected to an air blast of Friedlander type, and the temporal variations of cylinder surface pressures and strains and fluid pressures are measured. Based on these measured data and results from computational simulations, the mechanical loading pathways from the external blast to the pressure field in the fluid are identified; it is hypothesized that the net loading at a given material point in the fluid comprises direct transmissive loads and deflection-induced indirect loads. Parametric studies show that the acoustic impedance mismatches between the cylinder and the contained fluid as well as the flexural rigidity of the cylinder determine the shape/intensity of pressure pulses in the fluid.

An evaluation of applied biomechanics as an adjunct to systematic specific causation in forensic medicine

Biomechanical tests of post hoc probability have been proposed by prior authors as reliable tests of causation in forensic settings. Biomechanical assessment of injury kinetics and kinematics is a potentially important tool in forensic medicine, but there is also the potential for misapplication. The most reliable application is when biomechanical analysis is used to explain injury mechanisms, such as how an injury may have occurred. When a biomechanical analysis is used as a means of determining whether, rather than how an injury has resulted from a traumatic exposure, then a lack of reliability of the methodology limits its application in forensic medicine. Herein, we describe a systematic assessment of causation by adapting established general causation principles to specific causation scenarios, and how biomechanical analysis of injury mechanics is properly used to augment such an approach in conjunction with the principles of forensic epidemiology. An example calculation of relative risk associated with cervical spine injury is provided as a representative probabilistic metric for assessing causation. The statistical benefits and limitations of biomechanical analysis are discussed as an adjunct to forensic medicine.

Non-collision injuries in urban buses--strategies for prevention

Public transport is a potentially important part of independent living for older people, but they are over-represented in non-collision bus injuries. This paper reports on a computational modelling approach to addressing this problem: the Madymo human model validated for simulating passive, seated vehicle occupants was adapted to simulate a standing passenger in an accelerating bus. The force/deformation characteristics of the bus were measured and the human model was expanded to include a validated active hand grip. Real world urban bus acceleration profiles were measured and used as inputs for the simulations. Balance loss could not be predicted, but injuries from contact with the vehicle floor following a fall were evaluated. Results show that peak bus accelerations measured (+/-0.32g) exceed reported acceleration thresholds for balance loss for a standing passenger using a handgrip (0.15g). The maximum predicted probability of knee and head injuries arising from impact with bus seats, handrails and walls were 53% and 35%, respectively. The stairwell and horizontal seatback handles were particularly hazardous and the latter should be replaced with vertical handrails. Driver training should be expanded to include video training based on multibody simulations to highlight the risks for standing passengers induced by harsh braking and acceleration.

Effect of Upper and Lower Extremity Control Strategies on Predicted Injury Risk During Simulated Forward Falls: A Study in Healthy Young Adults

Fall-related wrist fractures are common at any age. We used a seven-link, sagittally symmetric, biomechanical model to test the hypothesis that systematically alterations in the configuration of the body during a forward fall from standing height can significantly influence the impact force on the wrists. Movement of each joint was accomplished by a pair of agonist and antagonist joint muscle torque actuators with assigned torque-angle, torque-velocity, and neuromuscular latency properties. Proportional-derivative joint controllers were used to achieve desired target body segment configurations in the pre- and∕or postground contact phases of the fall. Outcome measures included wrist impact forces and whole-body kinetic energy at impact in the best, and worst, case impact injury risk scenarios. The results showed that peak wrist impact force ranged from less than 1kN to more than 2.5kN, reflecting a fourfold difference in whole-body kinetic energy at impact (from less than 40J to more than 160J) over the range of precontact hip and knee joint angles used at impact. A reduction in the whole-body kinetic energy at impact was primarily associated with increasing negative work associated with hip flexion. Altering upper extremity configuration prior to impact significantly reduced the peak wrist impact force by up to 58% (from 919Nto2212N). Increased peak wrist impact forces associated greater shoulder flexion and less elbow flexion. Increasing postcontact arm retraction can reduce the peak wrist impact force by 28% (from 1491Nto1078N), but postcontact hip and knee rotations had a relatively small effect on the peak wrist impact force (8% reduction; from 1411Nto1303N). In summary, the choice of the joint control strategy during a forward fall can significantly affect the risk of wrist injury. The most effective strategy was to increase the negative work during hip flexion in order to dissipate kinetic energy thereby reducing the loss in potential energy prior to first impact. Extended hip or elbow configurations should be avoided in order to reduce forearm impact forces.

Effect of pre-impact movement strategies on the impact forces resulting from a lateral fall

Approximately 90% of hip fractures in older adults result from falls, mostly from landing on or near the hip. A three-dimensional, 11-segment, forward dynamic biomechanical model was developed to investigate whether segment movement strategies prior to impact can affect the impact forces resulting from a lateral fall. Four different pre-impact movement strategies, with and without using the ipsilateral arm to break the fall, were implemented using paired actuators representing the agonist and antagonist muscles acting about each joint. Proportional-derivative feedback controller controlled joint angles and velocities so as to minimize risk of fracture at any of the impact sites. It was hypothesized that (a) the use of active knee, hip and arm joint torques during the pre-contact phase affects neither the whole body kinetic energy at impact nor the peak impact forces on the knee, hip or shoulder and (b) muscle strength and reaction time do not substantially affect peak impact forces. The results demonstrate that, compared with falling laterally as a rigid body, an arrest strategy that combines flexion of the lower extremities, ground contact with the side of the lower leg along with an axial rotation to progressively present the posterolateral aspects of the thigh, pelvis and then torso, can reduce the peak hip impact force by up to 56%. A 30% decline in muscle strength did not markedly affect the effectiveness of that fall strategy. However, a 300-ms delay in implementing the movement strategy inevitably caused hip impact forces consistent with fracture unless the arm was used to break the fall prior to the hip impact.

Influence of stress rate on water loss, matrix deformation and chondrocyte viability in impacted articular cartilage

The biomechanical response of articular cartilage to a wide range of impact loading rates was investigated for stress magnitudes that exist during joint trauma. Viable, intact bovine cartilage explants were impacted in confined compression with stress rates of 25, 50, 130 and 1000 MPa/s and stress magnitudes of 10, 20, 30 and 40 MPa. Water loss, cell viability, dynamic impact modulus (DIM) and matrix deformation were measured. Under all loading conditions the water loss was small (approximately 15%); water loss increased linearly with increasing peak stress and decreased exponentially with increasing stress rate. Cell death was localized within the superficial zone (< or =12% of total tissue thickness); the depth of cell death from the articular surface increased with peak stress and decreased with increasing stress rate. The DIM increased (200-700 MPa) and matrix deformation decreased with increasing stress rate. Initial water and proteoglycan (PG) content had a weak, yet significant influence on water loss, cell death and DIM. However, the significance of the inhomogeneous structure and composition of the cartilage matrix was accentuated when explants impacted on the deep zone had less water loss and matrix deformation, higher DIM, and no cell death compared to explants impacted on the articular surface. The mechano-biological response of articular cartilage depended on magnitude and rate of impact loading.

Mechanical characterization of porcine abdominal organs

Typical automotive related abdominal injuries occur due to contact with the rim of the steering wheel, seatbelt and armrest, however, the rate is less than in other body regions. When solid abdominal organs, such as the liver, kidneys and spleen are involved, the injury severity tends to be higher. Although sled and pendulum impact tests have been conducted using cadavers and animals, the mechanical properties and the tissue level injury tolerance of abdominal solid organs are not well characterized. These data are needed in the development of computer models, the improvement of current anthropometric test devices and the enhancement of our understanding of abdominal injury mechanisms. In this study, a series of experimental tests on solid abdominal organs was conducted using porcine liver, kidney and spleen specimens. Additionally, the injury tolerance of the solid organs was deduced from the experimental data.