Age-Dependent Fracture Characteristics of Rigid and Compliant Surface Impacts on the Infant Skull-A Porcine Model*,†

Modeling, experiment, and validation of a piglet head

INTRODUCTION

Traumatic brain injury (TBI) is a prevalent type of disabling and fatal injury in infants/toddlers, which is usually caused by falls or traffic accidents. Considering that it is difficult to collect realistic material properties and validation data of child heads due to ethical reasons, experiments on the piglet heads and the finite element (FE) models are generally used as a substitute for the investigations of child TBI.

METHODS

In this study, first, a high-quality FE model of a 4-week-old piglet head, including brain (cerebrum, cerebellum, brainstem), skull, soft tissue, cerebrospinal fluid, dura matter, pia matter and mandible, is developed. Then, test for the material properties of the piglet head and that for the global validation data are conducted. For the former, the mechanical properties of the brain, overlying soft tissue and skull of the 4-week-old piglet head are tested, and the constitutive models and corresponding parameters are further defined. For the latter, the quasi-static compression test and dynamic impact test (free-fall drop impact test, drop-hammer impact test) are performed on the piglet head. Finally, the piglet head FE model was validated against tests in terms of the contact force and intracranial pressure (ICP) under eight conditions (one for the compression condition, four for the free-fall impact condition, and three for the drop-hammer impact condition).

RESULTS

The trends of simulated curves are consistent with the experimental results under all conditions. For the contact force, the average error of the peak values between simulations and tests is about 12.9 %, and the average error of time durations is about 6.8 %. For the ICP, the average errors of peak values and time durations between simulations and tests are about 8.9 % and 9.9 %.

CONCLUSIONS

The results show that the piglet head model has high bio-fidelity, which can be used to predict the head global response and the ICP, and further to assist the investigation of child TBI. The model provides another effective way to evaluate the modeling strategies and material constitute models suitable for child head FE model, and can better to understand the inducement and mechanism of child TBI under different external loading conditions.

Experimental investigation of brain contusion characteristics and dynamic response in low-age children using an animal model

INTRODUCTION

Brain contusion is a prevalent traumatic brain injury (TBI) in low-age children, bearing the potential for coma and fatality. Hence, it is imperative to undertake comprehensive research in this field.

METHODS

This study employed 4-week-old piglets as surrogates for children and introduced self-designed devices for both free-fall drop impact tests and drop-hammer impact tests. The study explored the characteristics of brain contusion and dynamic responses of brain under these distinct testing conditions.

RESULTS

Brain contusions induced by free-fall and drop-hammer conditions both were categorized as the coup injury, except that slight difference in the contusion location was observed, with contusion occurring mainly in the surrounding regions beneath the impact location under free-fall condition and the region just right beneath the impact location under drop-hammer condition. Analysis of impact force and intracranial pressure (ICP) curves indicated similar trends in impact forces under both conditions, yet different trends in ICPs. Further examination of the peak impact forces and ICPs elucidated that, with increasing impact energy, the former followed a combined power and first-order polynomial function, while the latter adhered to a power function. The brain contusion was induced at the height (energy) of 2 m (17.2 J), but not at the heights of 0.4, 0.7, 1, 1.35 and 1.7 m, when the vertex of the piglet head collided with a rigid plate. In the case of a cylindrical rigid hammer (cross-sectional area constituting 40 % of the parietal bone) striking the head, the brain contusion was observed under the energy of 21.9 J, but not under energies of 8.1 J, 12.7 J and 20.3 J. Notably, the incidence of brain contusion was more pronounced under the free-fall condition.

CONCLUSIONS

These findings not only facilitate a comprehensive understanding of brain contusion dynamics in pediatric TBIs, but also contribute to the validation of theories and finite element models for piglet heads, which are commonly employed as surrogates for children.

Understanding Bilateral Skull Fractures in Infancy: A Retrospective Multicenter Case Review

BACKGROUND

Bilateral skull fractures in infancy often raise suspicion for abuse. Nevertheless, literature suggests that they may occur accidentally. However, empiric data are lacking.

OBJECTIVE

This multicenter retrospective review aimed to characterize bilateral skull fractures in a large sample.

PARTICIPANTS AND SETTING

Medical records for infants younger than 24 months with bilateral skull fractures involving hospital consultation with a child abuse pediatrician (CAP) were reviewed from 2005 to 2020 at 13 nationally represented institutions.

METHODS

Standardized data collection across institutions included historical features, fracture characteristics, and additional injuries, as well as the CAP's determination of accident versus abuse. Pooled data were analyzed for descriptive and bivariate analyses.

RESULTS

For 235 cases, 141 were accidental, and 94 abuse. The majority occurred in young infants, and a history of a fall was common in 70% of cases. More than 80% involved both parietal bones. Bilateral simple linear fractures were more common in accidental cases, 79% versus 35%, whereas a complex fracture was more frequent in abuse cases, 55% versus 21% ( P < 0.001). Almost two thirds of accidental cases showed approximation of the fractures at the sagittal suture, compared with one third of abuse cases ( P < 0.001). Whereas focal intracranial hemorrhage was seen in 43% of all cases, diffuse intracranial hemorrhage was seen more in abuse cases (45%) than accidents (11%). Skin trauma was more common in abusive than accidental injury (67% vs 17%, P < 0.001), as were additional fractures on skeletal survey (49% vs 3%, P < 0.001).

CONCLUSIONS

A fall history was common in bilateral skull fractures deemed accidental by a CAP. Most accidental cases involved young infants with biparietal simple linear fractures, without skin trauma or additional fractures. A skeletal survey may aid in the determination of accidental or abusive injury for unwitnessed events resulting in bilateral skull fractures in infants.

Development and global validation of a 1-week-old piglet head finite element model for impact simulations

PURPOSE

Child head injury under impact scenarios (e.g. falling, vehicle crashing, etc.) is an important topic in the field of injury biomechanics. The head of piglet was commonly used as the surrogate to investigate the biomechanical response and mechanisms of pediatric head injuries because of the similar cellular structures and material properties. However, up to date, piglet head models with accurate geometry and material properties, which have been validated by impact experiments, are seldom. We aimed to develop such a model for future research.

METHODS

In this study, first, the detailed anatomical structures of the piglet head, including the skull, suture, brain, pia mater, dura mater, cerebrospinal fluid, scalp, and soft tissue, were constructed based on CT scans. Then, a structured butterfly method was adopted to mesh the complex geometries of the piglet head to generate high-quality elements and each component was assigned corresponding constitutive material models. Finally, the guided drop tower tests were conducted and the force-time histories were ectracted to validate the piglet head finite element model.

RESULTS

Simulations were conducted on the developed finite element model under impact conditions and the simulation results were compared with the experimental data from the guided drop tower tests and the published literature. The average peak force and duration of the guide drop tower test were similar to that of the simulation, with an error below 10%. The inaccuracy was below 20%. The average peak force and duration reported in the literature were comparable to those of the simulation, with the exception of the duration for an impact energy of 11 J. The results showed that the model was capable to capture the response of the pig head.

CONCLUSION

This study can provide an effective tool for investigating child head injury mechanisms and protection strategies under impact loading conditions.

Diagnostic accuracies of CTs, X-rays and Lodox to detect blunt force trauma in adults, using a pig model

The accurate radiological detection of skeletal trauma is crucial for the investigation of blunt force trauma (BFT) cases. The sensitivities of CTs, X-rays and Lodox in identifying BFT fractures and minimum number of impacts in pigs, used as proxies for adult humans, were assessed. Ten human sized pig carcasses were struck with a mallet and scanned, after which the number of fractures and minimum number of impacts detected radiologically were recorded. Pig carcasses were then macerated, and the defleshed, skeletonized remains were considered the gold standard as far as number and location of fractures were concerned. CTs were most sensitive in identifying fractures and impacts in all body regions, with overall sensitivities of 55.4% and 71.5% respectively, while X-rays and Lodox had sensitivities of 25.8% and 29.3% for fractures, and 43.5% and 41.1% for impacts, respectively. All modalities were highly specific for identifying fractures (CT: 99.1%; X-ray: 98.9%; Lodox: 99.4%). CTs should be used to analyze blunt force trauma when a radiological assessment is required, but an examination of the defleshed bones remains the gold standard for the deceased whenever feasible. X-rays and Lodox have limited diagnostic value in these cases and relying on them to detect fractures may compromise the accurate forensic investigation of blunt trauma victims. However, the use of Lodox for initial screening of major trauma is still of value. Sensitivities are generally higher for detecting fractures in pigs compared to piglets, indicating that increased diagnostic value is achieved when imaging pigs compared to piglets.

Preliminary observations of the sequence of damage in excised human juvenile cranial bone at speeds equivalent to falls from 1.6 m

There is much debate within the forensic community around the indications that suggest a head injury sustained by a child resulted from abusive head trauma, rather than from accidental causes, especially when a fall from low height is the explanation given by a caregiver. To better understand this problem, finite element models of the paediatric head have been and continue to be developed. These models require material models that fit the behaviour of paediatric head tissues under dynamic loading conditions. Currently, the highest loading rate for which skull data exists is 2.81 ms^-1. This study improves on this by providing preliminary experimental data for a loading rate of 5.65 ± 0.14 ms^-1, equivalent to a fall of 1.6 m. Eleven specimens of paediatric cranial bone (frontal, occipital, parietal and temporal) from seven donors (age range 3 weeks to 18 years) were tested in three-point bending with an impactor of radius 2 mm. It was found that prompt brittle fracture with virtually no bending occurs in all specimens but those aged 3 weeks old, where bending preceded brittle fracture. The maximum impact force increased with age (or thickness) and was higher in occipital bone. Energy absorbed to failure followed a similar trend, with values 0.11 and 0.35 mJ/mm³ for age 3 weeks, agreeing with previously published static tests, increasing with age up to 9 mJ/mm³ for 18-year-old occipital bone. The preliminary data provided here can help analysts improve paediatric head finite element models that can be used to provide better predictions of the nature of head injuries from both a biomechanical and forensic point of view.

Evaluating the evidentiary value of the analysis of skeletal trauma in forensic research: A review of research and practice

Understanding bone trauma characteristics is a fundamental component of forensic investigations that can assist in understanding the nature of blunt trauma related deaths. The variation of each blunt force trauma (BFT) injury is dependent on a magnitude of factors including, age, sex, health, angle of impact, impact mechanism, impact force and clothing, making BFT one of the more difficult area of trauma to interpret solely based on skeletal fractures. A detailed literature review was performed to assess the value and scientific rigour of the current research in forensic anthropology, forensic pathology and biomechanics that attempts to provide an objective framework in which forensic practitioners can assess and interpret BFT injuries. Four areas of research which investigate the analysis of BFT are examined. These included research involving experiments on animal models; experiments on human models (and synthetic models); computer modelling, and research/publications including 'mild', 'moderate' and 'severe' as descriptions of impact force, resulting from trauma. Also briefly discussed is how BFT research is framed within medicolegal contexts. While many published works have contributed to the understanding of the biomechanics of BFT, more research that can provide an objective means to accurately assess and interpret BFT injures is required.

Undetected traumatic diastasis of cranial sutures: a case of child abuse

Traumatic diastasis of cranial sutures is a type of bone fracture more common in children than in adults, but little attention has been paid to this skull damage. Differentiation between inflicted and accidental traumatic head injury is still a challenge in forensic pathology, particularly in pediatric population. In fact, diastasis of cranial sutures may occur with or without other skull fractures and may be the only evidence of an abusive head trauma (AHT). This is a case study dealing with undetected traumatic diastasis of cranial sutures in child abuse. The skeletonized juvenile remains were found inside a suitcase. A diastasis of the coronal and sagittal sutures was the only finding recorded at the autopsy with no other relevant bone defects. The diastasis was originally attributed by the medical examiner to a physiological unfused stage of the calvarial bones. Therefore, the cause of death was undetermined. Twelve years later an anthropological revision of the cold case showed that diastasis of the coronal and sagittal sutures was assessed as the evidence of an AHT. Analysis of skull fractures in child abuse can be challenging as normal skull suture variants mimicking intentional injury are reported. Diastasis of the cranial sutures can be also a post-mortem effect of burning or freezing. Therefore, a differential diagnosis between natural, accidental or inflicted skull defects is mandatory in death investigation. A multidisciplinary approach in such circumstances is strongly recommended in order to reduce the risk of misdiagnosis.

The importance of nonlinear tissue modelling in finite element simulations of infant head impacts

Despite recent efforts on the development of finite element (FE) head models of infants, a model capable of capturing head responses under various impact scenarios has not been reported. This is hypothesized partially attributed to the use of simplified linear elastic models for soft tissues of suture, scalp and dura. Orthotropic elastic constants are yet to be determined to incorporate the direction-specific material properties of infant cranial bone due to grain fibres radiating from the ossification centres. We report here on our efforts in advancing the above-mentioned aspects in material modelling in infant head and further incorporate them into subject-specific FE head models of a newborn, 5- and 9-month-old infant. Each model is subjected to five impact tests (forehead, occiput, vertex, right and left parietal impacts) and two compression tests. The predicted global head impact responses of the acceleration-time impact curves and the force-deflection compression curves for different age groups agree well with the experimental data reported in the literature. In particular, the newly developed Ogden hyperelastic model for suture, together with the nonlinear modelling of scalp and dura mater, enables the models to achieve more realistic impact performance compared with linear elastic models. The proposed approach for obtaining age-dependent skull bone orthotropic material constants counts both an increase in stiffness and decrease in anisotropy in the skull bone-two essential biological growth parameters during early infancy. The profound deformation of infant head causes a large stretch at the interfaces between the skull bones and the suture, suggesting that infant skull fractures are likely to initiate from the interfaces; the impact angle has a profound influence on global head impact responses and the skull injury metrics for certain impact locations, especially true for a parietal impact.

Biomechanical Studies on Patterns of Cranial Bone Fracture Using the Immature Porcine Model

This review was prepared for the American Society of Mechanical Engineers Lissner Medal. It specifically discusses research performed in the Orthopaedic Biomechanics Laboratories on pediatric cranial bone mechanics and patterns of fracture in collaboration with the Forensic Anthropology Laboratory at Michigan State University. Cranial fractures are often an important element seen by forensic anthropologists during the investigation of pediatric trauma cases litigated in courts. While forensic anthropologists and forensic biomechanists are often called on to testify in these cases, there is little basic science developed in support of their testimony. The following is a review of studies conducted in the above laboratories and supported by the National Institute of Justice to begin an understanding of the mechanics and patterns of pediatric cranial bone fracture. With the lack of human pediatric specimens, the studies utilize an immature porcine model. Because much case evidence involves cranial bone fracture, the studies described below focus on determining input loading based on the resultant bone fracture pattern. The studies involve impact to the parietal bone, the most often fractured cranial bone, and begin with experiments on entrapped heads, progressing to those involving free-falling heads. The studies involve head drops onto different types and shapes of interfaces with variations of impact energy. The studies show linear fractures initiating from sutural boundaries, away from the impact site, for flat surface impacts, in contrast to depressed fractures for more focal impacts. The results have been incorporated into a “Fracture Printing Interface (FPI),” using machine learning and pattern recognition algorithms. The interface has been used to help interpret mechanisms of injury in pediatric death cases collected from medical examiner offices. The ultimate aim of this program of study is to develop a “Human Fracture Printing Interface” that can be used by forensic investigators in determining mechanisms of pediatric cranial bone fracture.

Skeletal Trauma: An Anthropological Review

As anthropologists take on a larger role in medical examiner's offices, the incorporation of bone trauma analysis into the autopsy increases. The purpose of this invited review is to summarize recent anthropological literature that exemplifies the value of forensic anthropology in medicolegal death investigation, concentrating in the area of skeletal trauma analysis. Forensic anthropologists have a strong understanding of bone's response to trauma, gained through research and case studies. With this body of knowledge they are able to examine and interpret skeletal injury resulting from blunt, sharp, firearm, and thermal trauma. For example, toolmark class characteristics are recognized through sharp force injury examination, and fracture pattern analysis provides details of the impacting surface area. Interpretation of skeletal trauma allows for reconstruction of events surrounding death, and may inform the manner of death classification.

The Value of Anthropology in Medicolegal Death Investigation of Pediatric Nonaccidental Injury

Forensic anthropologists have made remarkable contributions to the medicolegal investigation of nonaccidental injury in pediatric cases. They have created standard nomenclature for fracture descriptions. Anthropologists have developed novel techniques that increase the sensitivity of the pediatric autopsy. They have performed biomechanical research that enables reconstruction of events surrounding death. Also, anthropology practitioners have developed several reference guides on the subject of nonaccidental injury that are of value to forensic pathologists. These advancements assist forensic pathologists in the accurate classification of cause and manner of death in pediatric cases.

The Role of Interface Shape on the Impact Characteristics and Cranial Fracture Patterns Using the Immature Porcine Head Model,

The forensic literature suggests that when adolescents fall onto edged and pointed surfaces, depressed fractures can occur at low energy levels. This study documents impact biomechanics and fracture characteristics of infant porcine skulls dropped onto flat, curved, edged, and focal surfaces. Results showed that the energy needed for fracture initiation was nearly four times higher against a flat surface than against the other surfaces. While characteristic measures of fracture such as number and length of fractures did not vary with impact surface shape, the fracture patterns did depend on impact surface shape. While experimental impacts against the flat surface produced linear fractures initiating at sutural boundaries peripheral to the point of impact (POI), more focal impacts produced depressed fractures initiating at the POI. The study supported case-based forensic literature suggesting cranial fracture patterns depend on impact surface shape and that fracture initiation energy is lower for more focal impacts.