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Research Papers

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

[+] Author and Article Information
Roger C. Haut

Orthopaedic Biomechanics Laboratories,
Department of Radiology,
College of Osteopathic Medicine;
Department of Mechanical Engineering,
College of Engineering,
Michigan State University,
A407 East Fee Hall,
East Lansing, MI 48824
e-mail: haut@msu.edu

Feng Wei

Orthopaedic Biomechanics Laboratories,
Department of Radiology,
College of Osteopathic Medicine;
Department of Mechanical Engineering,
College of Engineering,
Michigan State University,
A-414B East Fee Hall,
East Lansing, MI 48824
e-mail: weifeng@msu.edu

Manuscript received June 14, 2016; final manuscript received August 2, 2016; published online January 19, 2017. Assoc. Editor: David Corr.

J Biomech Eng 139(2), 021001 (Jan 19, 2017) (11 pages) Paper No: BIO-16-1255; doi: 10.1115/1.4034430 History: Received June 14, 2016; Revised August 02, 2016

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.

Trauma is a leading cause of hospitalization for children worldwide [1]. Three quarters of these cases and 70% of injury-related deaths are the result of head trauma. Falls are the leading cause of childhood head trauma and the third leading cause of death in children aged 1–4 years [25]. Head injury accounts for 80% of mortalities in battered children [6]. Child abuse is the leading cause of head injuries in infants [7] and trauma-related death in children under 4 years of age [8]. Every year, state and local child protective services receive more than 3 × 106 reports of children being abused or neglected, and nearly 60% of these reports are for children under the age of 3 [8].

Fractures are a common childhood injury, occurring in 8–12% of all pediatric injuries [911]. In infants and toddlers, abuse is reported in 12–20% of these cases [12]. One study of 567 abuse cases showed 55% having fractures [13]. Long bone fractures were present in 26% of cases. Twenty-four percent of cases involved the skull, while fractures of the rib cage and clavicle had frequencies of 14% and 4.2%, respectively. While such skeletal injuries often suggest child abuse, the same bones can be injured in accidental events.

Forensic investigators examine the characteristics of bone fracture to help determine injury causation. Inconsistencies between fracture characteristics and testimony are often a basis for litigation. While it is the responsibility of forensic pathologists to determine the cause of death, forensic anthropologists play a role in determining the manner of death. In the late 1970s and early 1980s, forensic anthropologists began working closely with local medical examiner offices to consult on skeletal trauma. During this period, most publications by forensic anthropologists were case-based [1417]. In the late 1980s, skeletal trauma research began to take shape in forensic science [18]. Today, forensic anthropologists and biomechanical engineers often collaborate during criminal litigation, as much basic science on skeletal trauma has been developed by engineers for automobile safety standards. For example, in the early 1950s, Lissner teamed with Gurdjian to investigate head injury using adult human cadaver specimens [1922]. Since concussion was often observed with skull fracture, cranial fracture tolerance was used as a basis for brain injury [23]. Later studies using animal models and human volunteers expanded the time domain of these data, which ultimately lead to the head injury criterion (HIC). Additional studies with animals addressed rotational head accelerations that cause diffuse brain injury and subdural hematoma [24,25]. While biomechanical studies with human volunteers and cadavers have yielded information on the tolerance and patterns of traumatic bone fracture for adults, only limited data exists for the pediatric victim. With reference to the automotive industry, scaling rules have been offered based on differences between pediatric and adult anatomy [26]. Such work has led to development of anthropomorphic dummies that are used as human surrogates in studies of traumatic injury for both the automotive industry [27] and in forensics [28]. While these surrogates as important tools in the determination of pediatric injury, there is still an issue of biofidelity, as few data exist for validation studies [2932]. In medicolegal death investigations, forensic biomechanists and anthropologists often attempt to use fracture patterns to help determine the circumstances involved in skeletal trauma. The hallmark study on cranial fracture patterning was conducted on adult cadavers [22], but few studies document the patterns of pediatric cranial fracture [33]. Such data can prove helpful in determining the potential for abuse based on defendant testimony. In this regard, the Orthopaedic Biomechanics Laboratories (OBL) at Michigan State University (MSU) has teamed with the Forensic Anthropology Laboratory (FAL) at MSU to investigate patterns of cranial fracture for the pediatric trauma victim.

Forensic case studies involving pediatric cranial fracture often rely on injury biomechanics research to help determine injury causation. This mechanics-based field of study relies on analytical, experimental, and numerical simulation techniques to address questions as to how an injury has occurred. Most of the current biomechanical research can be classified into four primary categories: (1) case-based investigation or assessment [34]; (2) test dummy or human surrogate experiments [35]; (3) computer modeling or simulation [36]; and (4) animal injury models [37]. While some laboratories use anthropomorphic dummies to assess potential trauma, the models are not suited for the analysis of fracture patterns. And, there are ethical as well as practical issues with experiments on human pediatric cadavers. Computer modeling still lacks sufficient data for accurate and practical simulations, as there are limited pediatric data available, specifically on the material properties of pediatric tissues. As an alternative to the use of human pediatric tissues for study, animal models have been used in trauma research. In particular, the porcine animal model has been suggested in this literature. In one study on porcine infant cranial bone and suture, it has been shown that “the elastic modulus, rupture modulus, and energy absorbed to failure of infant (2–3 day old) cranial (parietal) bone are similar to those of the human infant in three-point bending” [38]. The most common site of skull fracture in abuse and nonabuse cases is the parietal bone [39]. While the study is an important step to correlate the mechanical behavior of human infant cranial bone to porcine infant cranial bone, the ages and number of specimens were limited. Margulies and colleagues used beam specimens oriented perpendicular to and across the sutures [38]. Since the mechanical properties of prenatal human infant parietal bone are nonisotropic [40], early studies by the OBL involved bending experiments on porcine parietal bone parallel and perpendicular to the coronal suture, as well as across it [41].

Porcine specimens aged 3–21 days were obtained from a local supplier and stored at −20 °C before testing. Three beam specimens were cut from each skull: one across the coronal suture and two from the parietal bone: one parallel to and one perpendicular to the suture. The specimen width was approximately 5 mm. Three thickness measurements were taken at equally spaced intervals along the beam.

The specimens were loaded to failure in four-point bending. Each specimen was mounted between two aluminum sleeves using air-hardening cement (Technovit, Jorgensen Laboratories, Inc., Loveland, CO). The potted specimens were mounted in a custom-designed four-point bending fixture attached to a servohydraulic testing machine (Model 1331, Instron Corp., Canton, OH).

The force and actuator displacement were recorded to failure during each experiment. From these data, initial specimen stiffness, ultimate stress, ultimate strain, bending modulus, and strain energy to failure were determined. For each orientation of parietal bone specimen, there was a significant increase in bending stiffness with age. The stiffness was not statistically dependent on orientation. There was no change with age in the bending modulus of the bone (6.01 ± 1.73 GPa) and bone-suture-bone specimens (2.73 ± 0.84 GPa). The bending modulus of bone-suture-bone specimens was significantly lower than that of bone specimens up to 14 days, but similar thereafter suggesting maturation of the cranial sutures. Using bending modulus data from Ref. [30] and thickness values obtained from Ref. [42], the study suggested a good correlation in bending rigidity between days of porcine age and months of human age (Fig. 1).

While several studies document information on infant skull and brain under blunt impact, the data are limited in scope. While the head of an infant is smaller and geometrically unlike an adult [43], scaling techniques have met with limited success in predicting impact responses of pediatric skulls [44]. Using adult data to predict pediatric skull fracture patterns, however, may be even more problematic due to the differences in structural and mechanical properties [4547]. As a result of the above study showing good correlation between the mechanical properties of human and porcine skulls with age [41], our next study used porcine specimens aged from 2 to 28 days to study fracture patterns based on defined types of input loads [48]. An entrapped head was used to help control impact characteristics, as well as simulate cases where heads may be supported rigidly during trauma [49]. In the study, soft tissue, other than the periosteum, was removed from the left side of the skull. This side of each skull was placed in a bed of air-hardening epoxy (Fiber Strand, Martin Senour Corp., Cleveland, OH). Each skull was impacted in the center of the right parietal bone. The impact location was controlled by supporting the head in a four degrees-of-freedom, lockable fixture (Fig. 2). Each specimen was subjected to a single impact with a gravity-accelerated mass (GAM).

Input energy was adjusted by changing the drop height of a 1.67 kg GAM for young and a 1.92 kg GAM for older specimens. The input energy required to cause fracture increased from 1.5 J at 2 days of age to 11.5 J at 28 days of age. The impacting head was covered with either a rigid, aluminum interface or a compliant interface (1.10 MPa crush strength Hexcel, Hexcel Corp., Stamford, CT).

An important finding of the study was that linear bone fractures initiated at a bone–suture interface and propagated toward the impact site (Fig. 3). Frequently, the fractures were accompanied by diastatic fractures at the bone–suture interface. Diastatic fractures were typically not seen with the rigid interface until 10 days of age, while these fractures were seen as early as 4 days of age with the compliant interface. Neither interface showed suture damage after 17 days of age. A two-factor (age, interface) ANOVA performed on total fracture length (bone and diastatic combined) revealed more damage caused by the compliant than rigid interface on skulls less than 17 days of age, similar amounts of damage from both interfaces between 17 and 22 days of age, and significantly more damage with the rigid than compliant interface for specimens aged 24–28 days.

The above study showed that low energy impacts initiated fractures at a bone–suture boundary. In many pediatric death cases, however, there are multiple skull fractures that cross suture boundaries [39,50]. Multiple, wide or cross-suture fractures are likely indicative of high energy trauma [4]. The next study therefore investigated high energy impacts and involved specimens aged 2–28 days [51]. The same impact location and interfaces were again used for these high energy experiments. Energy levels at each specimen age were double those of the previous study [48] by raising the impact height. After impact and removal of soft tissues, fracture diagrams were constructed for each specimen on a standard skull template.

To compare the patterns of fracture between specimens and interfaces, a Geographic Information System (GIS) method of analysis was utilized in this study [51]. The patterns of fracture for each interface were separated into two age groups (2–9 and 19–28 days) and superposed onto a projected view of the porcine cranium. A second view of the posterior aspect of the cranium was also included, as many high energy fractures involved occipital bone fractures. Fracture data from Ref. [48], described above, were also revisited with these GIS plots to compare low energy experimental data to the current high energy data. The GIS model analysis then counted the number of overlaid fractures on each cranial grouped template, generating a map of where fractures appeared most frequently for each particular age, interface, and energy level. The GIS fracture maps showed that the length of fractures was greater for rigid than compliant interface impacts for the younger age group in the current study (Fig. 4). For the compliant interface experiments, the pattern maps showed fractures primarily initiating at four sites along sutures. For the rigid interface, however, there were more initiation sites. There was also significantly more diastatic fracturing in the rigid than compliant interface experiments, specifically along the coronal suture. Interestingly, in these high energy impacts to the parietal bone, significant fracturing was also documented in the occipital bone for both age groups and interfaces.

In the older group of specimens (19–28 days), more fracturing was again confirmed in the rigid than compliant interface experiments (Fig. 5). No diastatic fractures, however, were noted. Sites of fracture initiation were evident in the parietal bone along the coronal and lambdoid sutures for the compliant interface experiments. These sites were also noted in the rigid interface experiments; however, there were more propagated fractures. Again, under high energy impact onto the center of the parietal bone, significant fracturing was documented in the occipital bone for both interfaces.

The GIS maps of the revisited Baumer et al. [48] data showed three primary areas of fracture initiation, regardless of interface. For the younger age group (2–9 days old), the compliant interface produced more fractures of the skull than the rigid for the same impact energy (Fig. 6).

For a low energy of impact, there was little to no skull fracture with the compliant interface for the older age group (Fig. 7). Two fracture initiation sites were noted along the coronal and lambdoid sutures. The rigid impacts produced more propagated fractures initiating at approximately the same locations as in the compliant interface experiments.

The fracture pattern characteristics were described by assessing the frequency of fracture on each GIS map for a given set of impact conditions. For example, high energy impacts in the younger age group (2–9 days) tended to produce significantly more occipital bone fractures for both interfaces than for the low energy impacts. The rigid interface also generated much diastatic fractures for the higher impact energy, whereas the compliant interface did not. These findings contrast with those noted in the low energy study where the compliant interface produced more diastatic fractures than the rigid interface for the younger aged specimens. One observation from the studies was that if a given fracture pattern for a younger aged victim involved occipital and diastatic fracture, the causation of injury may have been due to a high energy, rigid impact.

Often, children sustain head trauma from accidental falls during development of walking skills or from child support devices, such as highchairs [52,53]. Pediatric falls typically result in impacts to the head due to the increased weight ratio of the head to the body [5457]. Falls are the third leading cause of death in infants aged 1–4 years of age [5]. However, in a study of 89 children under 2 years of age, 19 of the 20 fatalities were attributed to physical abuse rather than accidental falls [4].

To study infant cranial fracture patterns due to falls, the next study used porcine heads ranging 2–17 days of age [58]. Each head was fastened to a mounting plate with Velcro straps. A four degrees-of-freedom clamp attached to the plate was used to orient the parietal bone normal to the impact interface. The mounting plate was then fastened to a hollow aluminum rod. The rod was clamped with an electromagnetic solenoid, acting as a catch and release mechanism, and attached to a gravity-dropped trolley (Fig. 8). Upon impact, the trolley was stopped and the porcine skull was allowed to continue its downward movement until it impacted a large, rigid, aluminum interface. The impact energy for each age of specimen was matched to that of the previous high energy, entrapped head study by weighing each head and mount assembly and adjusting the drop height.

Diastatic fractures were documented in specimens as early as 3 days of age. No diastatic fractures, however, were noted in specimens older than 14 days of age. In the younger age group (2–9 days old), extensive diastatic fracturing was documented along the coronal suture (Fig. 9). Several times, bone fractures extended across the coronal suture into the frontal bone, but much fewer than for the entrapped case. Bone fractures also tended to initiate only along the coronal suture, in contrast to multiple areas around the parietal bone in the high energy, entrapped head experiments. Importantly, there were no documented occipital fractures, in contrast to the extensive occipital fracturing for entrapped heads at the same impact energy.

In the older age group (10–17 days old), fracture initiation was again documented primarily along the anterior parietal bone at the coronal suture but with a few cases of fracture initiating at the intersection of the lambdoidal and squamosal sutures (Fig. 10). There were no occipital bone fractures for this group. This was in contrast to the extensive pattern of occipital and parietal bone fractures resulting from the head being entrapped.

Diastatic fracture along the coronal suture was again recorded in the older age group. However, the degree of diastatic fracturing was significantly less in the older age group than in the younger group. Several of the older specimens had fracture initiation on either side of the parietal bone at locations remote from the point of impact. The same was true in the high energy entrapped head study described earlier.

These results documented that free-falling produced a different pattern of fracture, for a given impact energy, than on an entrapped head. Importantly, there was a lack of occipital bone fracture in these free fall experiments.

We have sought computational modeling to help explain mechanisms of fracture initiation on the immature porcine skull [59]. The hypothesis of our studies has been that the orientations and locations of principal tensile stresses in the parietal bone under a centralized load would help explain the sites of fracture initiation. This strategy was used previously by Baumer et al. [36] in simulations of infant crushing injuries of the cranial base during documented clinical cases. That study showed excellent correlation between the orientations and locations of principal tensile stresses and the sites and orientations of basilar cranial bone fracture for several of these clinical cases.

The model of the porcine cranium was created in Abaqus CAE (v6.11, Dassault Systemes; Vélizy-Villacoublay, France) using dimensions from CT scans of impacted crania [60]. Most elements of the cranial geometry were accounted for, although some regions were simplified due to the distance from the area of interest and complexity of meshing (Fig. 11). The average parietal bone thickness was taken from specimens tested in four-point bending, described above, and was accordingly altered with age. The sutures were given a more pronounced width than observed on the actual cranium, since the material properties used in the model were not directly obtained from the suture, but involved bone on either side of the sutures.

Analysis of the principal stress distributions in this simplified model under a centralized pressure on the parietal bone showed large tensile stresses located around the periphery of the bone that often coincides with the locations of linear fracture initiation (Fig. 12).

Joint simulation studies have also been conducted with Wayne State University [61]. Without going into details, the geometry for the model was based on a CT scan of a typical 21 day old specimen. The images were imported into Mimics 13 (Materialise, Leuven, Belgium). Thresholding, region-growing, and other segmentation techniques were applied to reconstruct the skull. The outer shell was then exported to Hypermesh 9.0 (Altair, Troy, MI), where the triangular surface mesh was manually converted into four-node quadrilateral shell elements. The Hughes–Liu shell element formulation was used in the model. Material properties were partially derived from the MSU–OBL beam studies described above [41]. A strain energy-based element deletion technique was used to help predict the fracture patterns using ls-dyna software (Livermore Software Technology Corporation, Livermore, CA). While fracture initiation sites were reasonably well predicted along the sutures, a further refinement of the constitutive models to include fracture mechanics theory and crack propagation algorithms were found to be needed to more accurately model crack propagation in most cases.

Early studies of Weber [62,63] are particularly valuable to the forensics community as they represent situations in which intact human pediatric heads were dropped onto the parieto-occipital region from a given height against various types of surfaces, and cranial fracture patterns were documented. In Ref. [62], 15 cadaver bodies aged approximately 0–9 months were dropped in free-fall from a height of 0.82 m onto a stone tile surface, a carpeted floor, and a foam-supported linoleum surface. A pattern of linear fractures was shown in each experiment with three cases having fractures across sutures. In another study [63], 35 heads were dropped, again from 0.82 m, onto softly cushioned ground with a 2-cm thick foam rubber mat and a double-folded (8 cm thick) camel hair blanket. In these low energy head drops onto the stone tile surface, a carpeted floor, and a foam-supported linoleum floor, cranial fractures were observed in every case. For drops onto the softer surfaces, however, minimal fracturing was indeed noted. The frequency of cranial fractures has been also reduced when the fall energy is decreased [33,64]. Thus, the degree of cranial fracturing may be dependent on both drop energy and impact interface characteristics.

In the next study, porcine heads aged 2–19 days old were used [65]. The previously described free-fall head drop fixture was slightly modified to accommodate the need for more impact energy onto more realistic compliant surfaces, especially for the older aged specimens. When the required drop height exceeded the height of the tower, a spring at the top of the tower was used to increase the velocity of the trolley and, thus, increase the impact energy of the dropped heads. Specimens 2–9 days old were impacted with approximately 23 J of drop energy, and specimens 10–19 days old were impacted with approximately 33 J. These energy levels were set to be approximately twice those of the earlier Powell et al. [58] study. The rigid surface was a 1.5 cm thick aluminum disk 20 cm in diameter. A more compliant interface was provided by covering the disk with thin, loop-pile commercial-grade carpet, referred to as carpet 1. A second interface was provided by covering the disk with a carpet underlayment (6-lb, 7/16-in. virgin foam rebond, Carpenter) with an overlying cut-pile polyester carpet (1/2-in. pile height, 29.1 oz/sq yard, Frontier Park), referred to as carpet 2. Carpet 3 was composed of another carpet underlayment (8-lb, 7/16-in. virgin foam rebond, Carpenter) covered with a Tactesse (soft fine fiber) pile carpet (Supreme Splender, style 58870, Stainmaster). These three types of carpeted surfaces were chosen to represent typical field conditions and allow a range of impact stiffness values which was measured using a Clegg Impact Tester (Model 95049A, Lafayette Instrument Co.; Lafayette, IN). This device measures the peak deceleration of a 2.25 kg steel missile dropped from a height of 45 cm onto the surface lying on a steel plate.

Head drops onto the rigid interface and carpet 1 resulted in cranial fracturing in every experiment. In contrast, impacts onto carpets 2 and 3 resulted in cranial fractures for 65% and 37.5% of experiments, respectively. The GIS maps of cranial fractures from impacts onto the various surfaces showed a similar distribution of fracture initiation sites, but as the compliance of the interface increased (lower Clegg scores), the degree of fracturing was reduced. Impacts onto all the surfaces resulted in fracture initiation sites that were concentrated around the edges of the right parietal at the coronal, squamosal, and lambdoid sutures.

Many fractures were located in the right parietal and frontal bones adjacent to the coronal suture (Fig. 13). For these high-energy impact experiments onto the right parietal bone, the occipital bone was also often fractured, especially for impacts onto the more rigid interfaces, with many fractures passing through the center of this bone (Fig. 14). Importantly, at these energy levels, a number of specimens also suffered fractures to the left or opposite-side parietal bone (Fig. 15). These opposite-side fractures appeared to initiate along the lambdoid and coronal sutures and rarely from the sagittal suture. In fact, only one case showed a fracture that spanned the sagittal suture.

Cranial fracture characteristics from the carpet impacts were also compared to the previously published lower energy, rigid interface head drop data of Powell et al. [58], described above. The low energy head drops produced fewer fractures in bones other than the impacted right parietal than the higher energy rigid impacts of the current study. On the other hand, there were statistically more diastatic fractures produced in the lower energy, rigid interface experiments of Powell et al. [58] than in the current higher energy experiments conducted on carpets 2 and 3.

Interestingly, the maximum contact forces on the skulls were consistently higher during head drops onto carpets 2 and 3 than onto the rigid interface or carpet 1, where there was more extensive cranial fracturing. This translates into relatively higher head decelerations generated during impacts onto the more compliant carpets than rigid surfaces. This data may suggest that while head impacts onto highly carpeted interfaces reduce the extent of cranial fracturing, they may increase the potential for traumatic brain injury due to higher contact forces and therefore head decelerations [34].

Pediatric deaths involving blunt force cranial trauma are difficult cases, as it may often be difficult to determine if the fracture pattern is consistent with a single or multiple impacts, or even from an impact onto a flat or shaped surface. Both experimental and clinical studies show confusing and often contradictory conclusions, leaving the medicolegal death investigator without clear guidance. For example, it has been reported that falls need to be from over 1 m to produce skull fracture; however, a fall from a lesser height onto a small area impact point, such as an edged surface, can also produce a fracture [66]. Furthermore, while early studies by Weber [62,63] indicate that complex cranial fractures can regularly occur in falls from 0.82 m onto a rigid flat surface, a more recent clinical study indicates that children less than 18 months of age generally have only simple linear skull fractures in falls from high heights [67]. Furthermore, while it has been reported that depressed skull fractures rarely occur in accidental falls [68], Williams [69] has reported small, depressed skull fractures in children less than 3 years of age from accidental falls against an object or a cornered edge. Wheeler and Shope [70] rightfully suggest that in attempting to determine accidental from abusive trauma, “multiple factors need to be considered including the age of patient, presence of an edged surface, and the quality of the contact surface.”

In this next study, porcine heads were used with ages of 1–20 days [71]. The free fall drop tower was modified to generate single impacts against shaped impact surfaces. Four impact surface shapes were made from aluminum stock: a 90 deg edged interface, a 2 in. dia. spherical shape, a 5/8 in. dia. spherical shape, and a 1/4 in. dia. flat-ended cylinder.

A detailed analysis of the cranial fracture patterns focused on the frequencies of experiments that produced the following variables: (1) fractures that initiated away from the point of impact at suture boundaries adjacent to the impacted parietal bone; (2) fractures that initiated at the point of impact; (3) curvilinear fractures; (4) depressed fractures; and (5) linear “crease” fractures (resulting from impact against the 90 deg edged surface). Fracture diagrams were scanned (Model SE A3 USB 1200 Pro, Mustek Systems, Inc., Hsin-Chu, Taiwan) and entered into a Geographic Information System (GIS), esri arcgis, software program that allowed fracture patterns using a particular impact surface shape to be overlaid onto a single image [72].

While the drop impact energy required to initiate cranial fracture across all age groups was lowest for the 1/4 in. flat-ended cylinder and the 5/8 in. dia. spherical surfaces (1.1 ± 0.6 J), there were no significant differences between the other impact surface shapes (1.9 ± 1.8 J), except for the flat impact surface (13.3 ± 3.3 J) [58], which was significantly higher than all other shapes.

Upon visual inspection, it was apparent that the characteristic patterns of fracture were dependent on the shape of the impact surface (Fig. 16). A correspondence analysis demonstrated significant differences in fracture characters with impact interface shape. Specifically, peripheral linear fractures were significantly noted with the 2 in. dia. spherical shape, the 5/8 in. dia. spherical shape, and the flat surface. Frequencywise, peripheral linear fractures were generated in 75% of cases with the 90 deg edged shape, 100% of cases with the 2 in. dia. spherical shape, 90% of cases with the 5/8 in. dia. spherical shape, and 60% of cases with the 1/4 in. flat-ended cylinder shape. Fractures at the point of impact were the next most common fracture type and were recorded at the highest frequency with the 1/4 in. dia. flat-ended cylinder in 80% of cases. Peripheral linear and point of impact fractures appeared together with the highest frequency on the 2 in. dia. spherical shape with 50% and 40% for both the 5/8 in. dia. spherical shape and the 1/4 in. dia. flat-ended cylinder shape.

As hypothesized, when the impact surface became more focal, the frequency of depressed fractures increases, as was apparent in 100% of cases with the 1/4 in. flat-ended cylinder and 80% of cases with the 5/8 in. dia. spherical shape, while only 40% of cases had depressed fractures with the 2 in. dia. spherical shape and no cases with the flat surface. Curvilinear type fractures were most common with the 5/8 in. dia. spherical shape (80% of cases) and the 1/4 in. dia. flat-ended cylinder (60% of cases). The curvilinear fractures tended to extend around the periphery of a depressed area and mimicked the edge of the impact interface. Lastly, linear crease patterns were present in 55% of cases with the 90 deg edged surfaces, with a distribution of 4/10 when oriented parallel to the coronal suture and 7/10 cases for transversely oriented specimens.

The above-described infant porcine head model, developed by joint association of the OBL and the FAL at MSU, has been utilized in a series of studies to investigate the effects of impact energy level, impact surface type, and head constraint condition on the patterns of cranial fracture. These data have shown complex fracture patterns due to various impact categories indicating that there is a need to develop techniques for a statistical discrimination of porcine cranial fracture patterns, with the goal that these techniques ultimately will have direct utility in guiding pediatric cranial fracture pattern recognition. The purpose of this next study was to use the porcine cranial impact data to develop an automated pattern recognition method to classify cranial fracture patterns associated with impact energy level (high or low), impact surface type (rigid or compliant), and impact constraint condition (entrapped or free-fall) for a given aged specimen [73].

A total of 354 porcine cranial fracture patterns were generated in the above studies [48,51,58]. Data in the form of cranial fracture diagrams were uploaded to a user-friendly fracture printing interface (FPI). The FPI was developed in matlab (MathWorks, Inc., Natick, MA) and enables one to call classification functions from the WEKA toolbox [74] to execute on the automatically extracted porcine fracture characteristic parameters (features) and categorize the patterns. The FPI provides users a selection of 20 different classifiers that come from five different families of classification methods: (i) decision tree, (ii) discriminant function, (iii) Bayesian methods, (iv) lazy learners, and (v) ensemble methods [73].

Performance of the FPI was evaluated for different subtasks. Each subtask was created by choosing a subset of fracture diagrams that share one or more class labels (e.g., head constraint condition in free-fall) and performing classification for another class label (e.g., predicting the impact energy level) for different age groups (young, old, or all). The experimental data enabled the use of classification accuracy as a performance measure. As an example, one task was to predict impact energy level given the impact constraint condition and surface type using a decision tree classifier (Fig. 17).

Another classification aimed at predicting the head constraint condition given the level of impact energy and the type of impact surface, while a third classification was on surface type prediction, including test data from heads free-falling onto a carpeted surface. As these particular impacts did not cause the same number of fractures as those onto a rigid surface, thus appearing often like a low energy impact, the classifying setting was modified for the free-fall head impact case and a two-phase algorithm was built. In the first phase, a binary classification problem was designed with one class being high energy free-fall head impacts onto a rigid surface and a second class being the union of high energy impacts onto a carpeted surface and low energy impacts onto a rigid surface. Then, if a query skull fracture was categorized as the second class, another classification problem was designed (the second phase), in which the goal was to determine if the fracture pattern was caused by a low-energy impact onto a rigid surface or by a high-energy impact onto a carpeted surface.

Often, key features play a particularly important role in the classification procedure. For example, the length of fracturing due to a rigid impact surface has been shown to be significantly greater than that from a compliant surface. Therefore, total fracture length was used as a key feature to predict a rigid versus compliant surface type. Similarly, total fracture length and new fracture initiation sites (in this case, the occipital region) were generally used together to predict a high versus low energy impact. And, total number of diastatic fractures was a key feature to differentiate low energy free-falling onto a rigid surface versus high energy free-falling onto a carpeted surface.

For each class label (energy level, constraint condition, or surface type) and each age group, accuracies of prediction were compared between three scenarios, in which 2, 1, or 0 labels were used as input, respectively, (Fig. 18). In general, the accuracy of prediction significantly decreased with the number of known labels, i.e., the available information. Over all evaluations, the average accuracy provided by each classifier showed a range of 72.9–78.1%. Furthermore, the average accuracy across the 20 classifiers was 75.6%.

The ultimate aim of this research program is to use the algorithm, based currently only on porcine cranial fracture pattern data, to develop a computer program that can automatically generate a fracture feature set based on human pediatric fracture pattern inputs that can be compared to a known database, therefore facilitating the process of predicting the most likely cause of a particular fracture print in a forensic case.

To date, the research team has been able to collect case files on pediatric deaths that exhibited cranial fracture patterns from 15 medical examiner's offices around the country. Information on the decedent's age, descriptions of cranial injuries, cause of death, manner of death, history of injury and reliability of the history provided has been collected, along with photographs and/or diagrams of the cranial fractures. The dataset currently contains 314 cases. There are currently 176 cases ruled as accidents, 126 cases ruled homicides, and 12 cases classified as undetermined.

A recent pilot study was conducted on 106 human infant skull fracture patterns from the above database that dealt specifically with homicide cases and automobile accidents. The study has shown that the fracture printing interface developed from the porcine data can be transformed to classify cranial fracture patterns from human infant skulls [75]. In the study, the key feature used in the algorithm was energy level, as homicide cases were typically thought to involve a relatively low energy level compared to car accidents which are more likely high energy events. The current algorithm, with a slight modification to account for the human infant skull shape, was able to achieve an accuracy of 84% for categorizing the cause of a specific fracture pattern as being due to a homicide or an automobile accident.

In a more recent study [76], the above database of human pediatric homicide cases has been examined for specific cases that may compare with the porcine data in which various shaped implements were utilized to develop cranial fracture patterns, as described above. The impetus for this research was case-based and derived from questions that have arisen when asked to consult on pediatric forensic cases that involve cranial fractures. Specifically, the questions have been surrounding the mechanism of injury that caused the fractures, documented during autopsy, which would ultimately aid in the medical examiner's decision whether the incident was accidental or intentional. The problem is that today these decisions are primarily based on each examiner's previous experience, which does not meet the Daubert standard for inclusion of expert testimony [77].

In the current investigation, 86 homicide cases were analyzed. Twenty-seven cases (31%) were shown to be consistent with large contact area impacts, those showing peripheral linear fractures emanating from sutures and propagating into a cranial bone. In 24 cases (28%), the cranial fracture patterns were consistent with focalized impacts with 14 cases (16%) having areas of depression, 13 cases (15%) having a clear point of impact fracture pattern, and eight cases (9%) having curvilinear fractures. While many of these case files were able to be classified based on the current porcine dataset, 35 cases (41%) were classified as being undetermined with very complex fracture patterns with mixed characteristics, and possibly due to more than a single impact. While the above study has not yet attempted to correlate the proposed mechanism of proposed injury with testimony, it may suggest that future interpretations of pediatric cranial fracture patterns may no longer solely have to be based on a medicolegal death investigator's previous experience, but rather be supplemented by a machine learning algorithm developed from ground truth: experimental data generated with an animal model in the laboratory.

The controlled trauma experiments with the porcine model provide a large dataset to investigators in order to fill a current gap in knowledge and ensure best practice for the interpretation of cranial injuries. The results gathered in years of controlled experiments on the mechanisms and characteristics of cranial fracture patterns using the porcine model may prove to be useful in the analysis of pediatric homicide cases. The fracture characteristics from shaped implements, first recognized in the porcine experimental models and now apparent in pediatric forensic cases, will soon be added into a human fracture printing interface. While this work is still in its infant stages, the plan is that once validated by the generation of a large dataset of human case files in which “truth of testimony” might be able to be extracted, the human pediatric fracture printing interface will provide medicolegal death investigators with an analytical tool to more effectively interpret cranial fracture patterns in pediatric deaths, based on the ability to establish a statistical level of scientific certainty about injury causation. This will help close a major gap in current best practice and provide an avenue to satisfy some of the Daubert criteria for the admissibility of forensic evidence in the courtroom.

The authors would like to thank Dr. Todd W. Fenton, Mr. Clifford Beckett, and many graduate students from the OBL and the FAL who have contributed to the above-mentioned studies. The research has been supported by the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice (Award Nos. 2007-DN-BX-K196 and 2011-DN-BX-K540).

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References

Lam, W. H. , and MacKersie, A. , 1999, “ Paediatric Head Injury: Incidence, Aetiology and Management,” Paediatr. Anaesth., 9(5), pp. 377–385. [CrossRef] [PubMed]
Reece, R. M. , and Sege, R. , 2000, “ Childhood Head Injuries: Accidental or Inflicted?,” Arch. Pediatr. Adolesc. Med., 154(1), pp. 11–15. http://archpedi.jamanetwork.com/article.aspx?articleid=348423 [PubMed]
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Barber, I. , Perez-Rossello, J. M. , Wilson, C. R. , and Kleinman, P. K. , 2015, “ The Yield of High-Detail Radiographic Skeletal Surveys in Suspected Infant Abuse,” Pediatr. Radiol., 45(1), pp. 69–80. [CrossRef] [PubMed]
Kerley, E. R. , 1978, “ The Identification of Battered-Infant Skeletons,” J. Forensic Sci., 23(1), pp. 163–168. [CrossRef] [PubMed]
Kerley, E. R. , 1976, “ Forensic Anthropology and Crimes Involving Children,” J. Forensic Sci., 21(2), pp. 333–339. [CrossRef] [PubMed]
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Maples, W. R. , 1986, “ Trauma Analysis by the Forensic Anthropologist,” Forensic Osteology: Advances in the Identification of Human Remains, K. J. Reichs , ed., Charles C. Thomas, Springfield, IL, pp. 218–228.
Passalacqua, N. V. , and Fenton, T. W. , 2012, “ Developments in Skeletal Trauma: Blunt Force Trauma,” A Companion to Forensic Anthropology, D. C. Dirkmaat , ed., Blackwell, Oxford, UK.
Gurdjian, E. S. , and Lissner, H. R. , 1945, “ Deformation of the Skull in Head Injury; A Study With the Stress Coat Technique,” Surg. Gynecol. Obstet., 81, pp. 679–687. [PubMed]
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Figures

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Fig. 1

Bending rigidity of human and porcine specimens versus age (human—months; porcine—days). (Figure adapted from Ref. [41].)

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Fig. 2

Electronically controlled drop test fixture. (Figure adapted from Ref. [48].)

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Fig. 3

Representative fracture sites for (a) rigid and (b) compliant interface impacts (five-day-old specimens shown). Fractures initiated at the bone–suture boundaries. (Figure adapted from Ref. [48].)

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Fig. 4

GIS maps of 2–9 day old rigid (a) and compliant (b) impacts of high energy. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [51].)

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Fig. 5

GIS maps of the 19–28 day old rigid (a) and compliant (b) impacts of high energy. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [51].)

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Fig. 6

GIS maps of the 2–9 day old rigid (a) and compliant (b) impacts of low energy. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [51].)

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Fig. 7

GIS maps of the 19–28 day old rigid (a) and compliant (b) impacts of low energy. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [51].)

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Fig. 8

Schematic of the free fall drop tower assembly. (Figure adapted from Ref. [58].)

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Fig. 9

GIS maps of the 2–9 day old age group for the free fall (a) and entrapped (b) head impacts. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [58].)

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Fig. 10

GIS maps of the 10–17 day old age group for the free fall (a) and entrapped (b) head impacts. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [58].)

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Fig. 11

Simplified model of the porcine cranium with overlays of modeled structures on true structures. Coronal cut of CT used for thickness comparison. *Feature not modeled. Feature simplified. #Suture width altered for consistency with material property data obtained previously. Dotted line represents location of frontal bone removal. (Figure adapted from Ref. [60].)

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Fig. 12

Overlay of distribution of the top 80% principal tensile stresses from FE models on representative experimental cranial fractures for (a) rigid interface impact to a young specimen, (b) rigid interface impact to an old specimen, (c) compliant interface impact to a young specimen, and (d) compliant interface impact to an old specimen. Longer lines represent relatively higher stresses. (Figure adapted from Ref. [60].)

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Fig. 13

GIS maps of the impacted, right parietal bone fracture patterns for (a) rigid, (b) carpet 1, (c) carpet 2, and (d) carpet three interfaces. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [65].)

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Fig. 14

GIS maps of the posterior, occipital bone fracture patterns for (a) rigid, (b) carpet 1, (c) carpet 2, and (d) carpet three interfaces. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [65].)

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Fig. 15

GIS maps of the left or opposite-side parietal bone fracture patterns for (a) rigid, (b) carpet 1, (c) carpet 2, and (d) carpet three interfaces. Frequency means number of fracture occurrences at the same location. (Figure adapted from Ref. [65].)

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Fig. 16

Specimens dropped against (a) the 2 in. diameter spherical shape, (b) the 5/8 in. diameter spherical shape, (c) the 90 deg edged interface, and (d) the 1/4 in. diameter flat-ended cylinder. (Figure adapted from Ref. [71].)

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Fig. 17

A sample decision tree to classify data from the porcine head onto a rigid surface into high and low energy impacts

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Fig. 18

Predicting accuracy as a function of the number of known class labels for prediction of energy level (a), constraint condition (b), and surface type (c). Two-input means that users provide 2 of 3 class labels as input. Similarly, one-input and zero-input mean that users provide 1 and 0 (cannot provide any) of three labels as input, respectively. (Figure adapted from Ref. [73].)

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