Experimental Study of Cranial Injuries Due to
Blunt Force Trauma: Sus scrofa domestica Model
Felipe Otero1* and Marien Béguelin1,2
1 Universidad Nacional de Río Negro, Argentina
2 Anexo del Museo de La Plata, Universidad Nacional de La Plata, Argentina
Submission:November 01, 2019; Published: November 26, 2019
*Corresponding author:Felipe Otero, Universidad Nacional de Río Negro, Instituto de Investigación en Paleobiología y Geología, Av. Roca 1242, (8332) General Roca, Río Negro. Argentina.
How to cite this article:Felipe Otero, Marien Béguelin. Experimental Study of Cranial Injuries Due to Blunt Force Trauma: Sus Scrofra Domestica Model.
J Forensic Sci & Criminal Inves. 2019; 13(2): 555856. DOI: 10.19080/JFSCI.2019.13.555856.
Cranial blunt force trauma is of major concern in forensic sciences. The aim of this study is to shed light on cranial bone trauma caused by blunt weapons from an experimental perspective. The experimentation involved the production of blunt injuries to 21 pig skulls with different objects: metal hammer, wooden club (baseball bat), stone and boleadora. These blunt objects were chosen because they can be easily used as homicidal weapons since they are common elements that can be seen in daily situations today. The marks produced were recorded and analyzed using quantitative and qualitative variables. The results showed that it is possible to identify the blunt weapon that caused the injury through the analysis of variables such as maximum diameter or depth of the bone injury. Furthermore, it was observed that depending on the damage, the injury could be associated with a blunt object with specific characteristics.
Keywords:Forensic anthropology; Interpersonal violence; Blunt force trauma; Experimental design; Sus scrofa domestica
The analysis of trauma in human skeletal remains involves the study of injury patterns [1-3]. Blunt force trauma may result from impact with any hard surface. Some marks on bone may result from the application of a force onto an area of impact caused by hard elements with a rounded or blunt surface, such as a stone, a baseball bat, a boleadora or a hammer. Different weapons can produce different tool marks and injury patterns, which can be identified by using different variables. These marks are also known as blunt injuries [4,5]. Research and analysis of these types of injuries in skeletal remains help address forensic contexts with abundant evidence accounting for conflicting situations. In forensic cases, human bones frequently exhibit blunt injuries [6,7], but it is difficult to establish a correlation between the marks and the injury-causing weapon [7-10]. This is because a lot of factors that contribute to injury production such as the characteristics of the person who effects the blows (their physical conditions), the biomechanical properties of the bone, the weight and speed reached by the weapon, the material with which it is made, its contact surface, the size of the area impacted, the direction of impact and the thickness of the scalp
and hair. The thickness of the bone also has an influence since the bone material tends to break mainly in the thinnest areas and therefore less resistant. The bones with greater volume need to be impacted with a lot of energy to cause their fragmentation [7-9].
In addition, there is a limited possibility of bone’s response to blunt force trauma: different instruments generate similar injuries and the same instrument can generate different injuries. Also, different causal agents (e.g. falls or accidental impacts) converge in blunt injuries similar to those produced by situations of violence [3,10-13]. Therefore, it is necessary to develop frames of reference that can establish a cause-effect association between the injury morphology and the weapon using a probabilistic model, and experimental studies have the potential to achieve this [1-3,10,14-16]. However, there are very few experimental studies, especially those aimed at identifying the weapon that caused the blunt injury [15,17]. The aim of this paper is to contribute to knowledge of injuries on cranial bones (e.g. frontal, parietal (left and right), temporal (left and right), occipital, sphenoid and ethmoid) made by blunt weapons and identify the injury patterns produced by each object. Through
an experimental study, this work is based on the production of
blunt injuries to 21 pig skulls (Sus scrofa domestica) with a metal
hammer, a wooden club (baseball bat), a stone and a boleadora.
Subsequently, univariate and multivariate statistical methods
were used to analyze the quantified data set.
Adopting Gordon’s Model , the experimental research
consisted in striking pig heads previously prepared and then
analyzing the resulting marks on bone. A total of 21 skulls
from subadult pigs (Sus scrofa domestica) of undetermined
sex were used for this study. The age of death of all the pigs is
similar (24 weeks (+/- 2)). The specimens were assigned a Pig
Identification Number (PIN) from 0 to 20. The weapons used
were metal Hammer (H), wooden club (baseball bat) (C), stone
(S) and boleadora (B) (Figure 1). These objects were chosen for
this experiment because they are frequently reported in forensic
cases. A hammer, a stone or a wooden club can be easily used
as homicidal weapons since they are common blunt objects that
can be seen in every human daily situation. A boleadora is an
object commonly found in rural areas of Patagonia (Argentina),
which can be used as a weapon of interpersonal violence. It
is a throwing hunting weapon composed of a stone ball (e.g.
andesite), with a groove carved around the circumference and
with a rope or strip of leather tied to the stone [18-20]. The
strikes were executed without releasing the rope. Furthermore,
weapons with varying characteristics (e.g. size, weight and
material) were chosen to produce different injury patterns and
make associations between the morphology of the injury and
the instrument. The first skull (PIN 0) was impacted only to test
the different instruments and experimental device (i.e. force and
impact site) and ensure that the blow produced a potentially
recordable effect. The other 20 skulls were divided into 4 groups
of 5 animal subjects. Each group was assigned a blunt object.
The pig heads were distributed randomly into the groups so that
the varying characteristics of the pig skulls, such as size, did not
influence the results of the weapon blows.
The skin of the pig heads was removed with knives and
scalpel. A thin layer of soft tissue (i.e. muscle, fat, cartilage)
was preserved to mimic the human scalp. In this way, the
interface between the bone and the surface of each head was
represented by 3 to 5 mm of muscle mass and connective tissue.
The pig Sus scrofa domestica was chosen as a human proxy for
this experiment following the criteria already established and
evaluated by different authors [1,2,21-24]. In comparative
terms, this species shares several physical traits such as bone
and skin with Homo sapiens making it useful mainly in forensic
contexts. The pig heads were placed in such a way that they kept
limited mobility in relation to a vertical axis, i.e. they neither
hang freely when they were impacted nor were, they completely
stuck, since none of these situations resemble natural mobility
resulting when striking a living individual. The device designed
for this purpose consisted of a base on which the heads were
placed in order to mimic the support that the spine provides to
the human skull (Figure 2). Such device consisted of two buckets
filled with sand placed on the floor, side by side, with one end of a
wooden stick inserted into each bucket. The opposite ends were
introduced into the zygomatic arches, extending to the orbital
cavity. In turn, the zygomatic arches were secured to plastic pull
tight seals attached to an elastic cord which dangled from a rail
on the roof, thus allowing the desired mobility.
One of the authors of this study delivered all the blows with
a hammer, club and stone, whereas in the case of the boleadora,
an expert in the art of employing it inflicted the blows. The
entire experimental process was filmed and photographed.
The subsequent cleaning of the material was done by boiling
the remains with detergent for domestic use, removing the
remaining tissue and leaving them to dry at room temperature.
In order to quantify the damage, depth (D), maximum (MD) and
perpendicular (PMD) (orthogonal to the maximum diameter)
diameters of each injury were measured with a digital caliper
with an accuracy of 0.01 mm. Besides, the data obtained from
fresh bones injuries (perimortem) was analyzed using a set
of qualitative variables. Firstly, the presence of fractures was
recorded. If it was positive, then fractures produced were
registered as linear (LF), concentric (CF) and/or stellate (SF).
Subsequently, the presence or absence of fragments (small
flakes of bone) attached to Fracture edges (FA) was recorded.
Furthermore, the presence or absence of Raised Edges (RE),
which are produced when the bone material is plastically
deformed. Finally, the presence or absence of Bone Loss (BL), i.e.
the lack of anatomical units or their fragments [25-28] (Figure
3). These variables were examined with the naked eye and/
or with a 10X magnifying glass. The data were analyzed with
uni-, bi-, and multivariate methods, and with parametric and
nonparametric methods, depending on the nature of the data
A total of 64 blows were delivered to 20 experimental skulls
of Sus scrofa domestica, using 5 skulls per weapon: 19 blows with
a hammer, 15 with a wooden club, 15 with a stone and 15 with
a boleadora. Each skull was struck 3 times, except for PIN 2, 3, 4 and 5 (corresponding to the hammer), which were impacted 4
times each. However, to perform the analysis, only 61 blows (18
with hammer, 15 with club, 15 with stone and 13 with boleadora)
were considered, as some of them impacted exactly on the same
area, thus preventing an accurate recording of each injury. These
redundant blows (RB) were counted as one provided that it was
not possible to distinguish one blow trace from another. Nine of
the 61 blows recorded for the analysis did not affect the bone
material, i.e., they did not cause injuries. This indicates that
14.75% of the blows delivered were absorbed by the soft tissue.
For the quantitative analysis, 12 of the 61 blows were not
considered relevant for this study since they affected other
statistical population: two cases correspond to situations in
which more than one blow impacted the same place; the other
case involved one blow to the zygomatic bone, which is not
included in this work. Therefore, in total, only 49 blows were
considered for these analyses.
Before analyzing the variables, it was explored graphically if
the application of three (or four) impacts on each experimental
head (in different places) influenced the bone resistance and,
therefore, the pattern of injuries produced (i.e. if each blow
caused more damage than the previous one). The sizes of the
injuries inflicted were illustrated in a multiple graph (the three
variables plotted on the same figure) following the chronological
order of the blows to each skull, separated by the weapon used.
The pattern expected if each blow influenced the following one
is an increase in the size of every variable in the same cranium.
(Figure 4) shows that there is no pattern of increase in the size
of the injury between one blow and the next, for any of the
weapons. For example, blow 4.3 made with hammer decreases
the size of the three variables but in Cranium 2 the opposite
occurs. This indicates the independence of the blows, meaning
that no relationship was found between the size of the injury and
the order of the blows.
Fig. 5 (A, B and C) shows the distribution of the medians for
the injuries per weapon in relation to the variables MD, PMD
and D, respectively. As a result of the Kruskal-Wallis test, no
significant differences were observed between the weapons for
the variable PMD (p= 0.089). However, there were differences for
the variable’s MD (p= 0.029) and depth (p= 0.002). Therefore,
a posteriori tests were carried out to identify which objects
differed. In relation to MD pairs, it was observed that medians
for boleadora and stone differed significantly (p= 0.024), and in
relation to depth, the hammer showed significant differences
with all the weapons: boleadora (p= 0.019), club (p= 0.012), and
stone (p= 0.002).
To analyze the data through bivariate methods, log(x) was
applied and then the outliers were eliminated. Table 1 shows
the results of the Spearman correlation coefficient among all
the combinations of metric variables, for all the instruments
used. Figure 6 shows regression lines of the variables MD, PMD
and D for all the weapons. It was observed that for hammer
(r2=0.745-p=0.008) and for boleadora (r2=0.636-p=0.026), the
relationship between PMD and MD is positive. Therefore, when
the maximum diameter increases, the perpendicular diameter to
the maximum also increases. It was also observed that MD and D
correlate with each other for the stone (r2=0.665-p=0.013) and
the boleadora (r2=0.651-p=0.030).
Permutational Multivariate Analysis of Variance
(PERMANOVA) was performed to compare the effects among
weapons based on all metric variables. It showed significant
differences between the weapons (p= 0.017). A posteriori tests
for the analysis between pairs showed significant differences
between boleadora-hammer (p= 0,041), and boleadora-stone
(p= 0,009). In this analysis, the data was transformed into
natural logarithm of each value plus one [ln (x+1)] .
The associations between weapons and qualitative variables
(i.e. LF, CF, SF, FA, RE and BL, expressed as frequencies of presences
and absences) were evaluated by contingency tables (chi-square
test) for each variable independently. Null hypothesis states
that presence/absence ratio (P/A ratio) is homogeneous across weapons. Rejection of null hypothesis implies that at least one
weapon produces a different P/A ratio. Table 2 summarizes the
information collected from the experimental marks. Statistically
significant differences were found for LF and RE. Conversely, the
other variables did not show significant differences between the
weapons as a result of the statistical tests.
Pig bones are suitable analogous for experimental studies of
fracture patterns [7,10] because of their similarities with human
bones: both are made of external and internal tables and a diploe
[28,30]. However, pigs’ skulls differ in having thinner tables,
thicker diploe, and greater total thickness of the three layers
if compared with human skulls. These differences might make
pig cranial bones more resistant to blunt force trauma than
human ones. There are also differences in the anatomy of the
skulls: in the case of pigs the parietal bone is small, square and
flat, while in human skulls are large and convex laterally .
Notwithstanding this considerations, the pigs’ skulls are still
convenient proxies for human skulls, moreover in countries were
laws do not allow the use of corpses with experimental purposes
(e.g. body farms). In most experimental studies, only one blow is
applied to each skull based on the assumption that the second
blow will cause greater damage [6,28]. However, in this work
evidence showed that each blow causes injuries independently
of the others (a maximum of four blows). Therefore, it is possible
to deliver more than one blow in different areas of a pig skull
without affecting its resistance. Through the univariate analysis,
the results showed that it is possible to recognize the marks on
pig skulls made by the different blunt weapons using metric and
categorical variables. The univariate analysis of morphometric
variables of the marks is useful in finding differences between
the patterns of injury produced by the different weapons. Similar
results were found by Small , who concluded that the depth
and the maximum diameter served to find differences between
the instruments, although the perpendicular diameter to the
maximum was not useful by itself to identify the weapon. In
addition, the multivariate analysis showed significant differences
between the marks associated with the different objects when
analyzing the three metric variables together.
For MD, differences between B and S were identified. The
scatterplot of marks by weapons (Figures 5.a) shows that B
varies in a smaller range than S. This may be due to the spherical
shape of B which always generates the same impact area. In
contrast, S has an irregular shape, so the impact area can vary
depending on the side of the impact. This observation is relevant
because when the weapon is irregularly shaped it may create
different marks. Therefore, an irregularly shaped weapon could
be associated with a range of possibilities. In other weapons, the
morphology of the injury is more limited and subject to the shape
of the impact surface of the weapon. The strong correlation
that exits between the diameters (e.g. MD and PMD) of B and
H supports the hypothesis that the surface impacted by these
weapons is always similar.
In addition, significant differences were observed between
H and the other effectors (B, S and C) for the variable depth,
which helps infer that the mark left by H is characterized by
having a greater depth. This may be since H hits a small area and
the energy of the blow is concentrated causing greater damage.
Blows to a smaller focal area of bone tend to cause a high stress
level in the damaged area and, conversely, as the area increases,
the tension decreases [8,9]. The latter would support the fact
that C and S have fewer presence observations in the variables
that qualitatively account for the damage than B and H (Table 2).
In addition, the hardness of the raw material directly influences
the energy transfer at the time of injury production. While metal
largely transfers the energy of the impact, wood tends to absorb
the blow . It should be noted that the depth increases when
the maximum diameter of the stone and boleadora increases
as well. This could be because the diameter of the weapons
increases towards the center or equator. If these weapons hit
with more force, they would cause a greater depression and
therefore a greater area of impact. Moreover, the material of the
weapon (metal, wood or rock in this case) is a factor that could
be influencing this variable. However, the data obtained from this
study is not enough to perform further tests which can confirm
this hypothesis. Finally, with respect to the qualitative variables,
it was observed that the presence/absence of linear fractures
as well as raised edges would be useful by themselves to find
differences between the effectors. The other variables (e.g. CF,
SF, FA, BL) would not contribute individually to the identification
of an effector. However, the whole data corresponding to all
the qualitative variables help quantify a percentage of damage
caused by each instrument. Thus, it would be possible to classify
the various weapons into categories based on the damage
This work represents an experimental approach to the
study of injuries caused by blunt weapons. The goal is to
provide forensic science with information on which to base
interpretations of the weapon used to produce injuries, at the
bone level, in a victim. The results have raised new questions
that should be examined in depth in future studies where the
hypotheses can be refined, and the number of factors involved
can be reduced (for example, tests with weapons having the
same characteristics, different sizes, or areas of impact, etc.).
Furthermore, it is expected that further studies will determine
the impact energy through the analysis of the film recorded and
include other variables (e.g. area and perimeter) and/or other
We thank Fernando Archuby by for his cooperation on the
statistical analysis and Antonio Flores for striking the pig heads
with the boleadora. Diego and Verónica García gently donated
the pig heads from their farm. This study was funded by PI-
40-A-613 UNRN grant.
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