On the effects of preservation, blade angle and intra- and inter-individual differences on the identification of tool class characteristics retained on human costal cartilage in cut

Master in Forensic Sciences

University of Porto

On the effects of preservation, blade angle and intra-

and inter-individual differences on the identification of

tool class characteristics retained on human costal

cartilage in cut marks analysis

Katerina Puentes

Porto, 2011

ORIGINAL ARTICLE

A Dissertation presented for the

Master in Forensic Sciences Degree

On the effects of preservation, blade angle and intra- and inter-individual differences on

the identification of tool class characteristics retained on human costal cartilage in cut

marks analysis

Post-graduate student: Katerina Puentes, MD

Adviser: Hugo Filipe Violante Cardoso, PhD (Faculty of Medicine - University of Porto)

AKNOWLEDGMENTS

To Steven A. Symes, who was the beginning of everything:

THANK YOU Steve…

To my beloved family, Marigula, Sofia, Karina and Martijn,

who were the engine behind my strength to walk this path…

To my adviser, Prof. Hugo Cardoso, for his guidance,

patience and understanding…

To Dr. Luis Coelho, for his priceless support and help, and

above all, for his friendship during this process…

To Mr. Amílcar Freitas da Rocha, for his precious

collaboration with the graphic aspects of this work…

To Mrs. Elisa Duarte, for her invaluable help with the

statistical analysis along the process…

To Prof. Agostinho Santos and Prof. Teresa Magalhães for

their continuous support and help…

INDEX

I. Abstract ....................................................................................................... 1

II. Introduction ................................................................................................... 3

III. Materials and Methods ................................................................................. 9

IV. Results ...................................................................................................... 17

V. Discussion .................................................................................................. 21

VI. Conclusions ............................................................................................... 27

VII. References ............................................................................................... 29

VIII. Appendix 1 .............................................................................................. 31

1

ABSTRACT

Identification of tool class characteristics from cut marks in either bone or

cartilage is a valuable source of data to the forensic scientist. Various animal

models have been used in experimental studies for the analysis of individual

and class characteristics. However, human tissue has seldom been utilized and

it is likely to differ from that of non-humans in key aspects. This study wishes to

assess how the preservation method, the knife’s blade angle, and both intra-

and inter-individual differences in cartilage samples affect the ability of costal

cartilage to retain the original class characteristics of the knife, as measured by

the distance between consecutive striations in cut mark analysis. The 160

cartilaginous samples used in this study originated from the ribcage of 7 male

cadavers that underwent autopsy at the North Branch of the National Institute of

Legal Medicine, in Portugal, and three different serrated knives were purchased

from a large department store, to be used in the study. Samples of costal

cartilage from 2 individuals were assigned to each knife. Each individual

provided 20 cartilage samples. Cartilage samples were manually cut using each

of the three knives, following two motions: one parallel and one perpendicular to

the blade’s teeth long axis. Casts of the samples were made with Mikrosil®.

Image capture and processing were performed with an Olympus

stereomicroscope and its software. Direct image superimposition was used to

test how the preservation method used for the cartilage samples (formalin 10%)

affects preservation of cut marks on the cartilage surface. The distance

between striations in the acquired image was measured. No significant

distortion or shrinkage of the striation pattern was caused by preservation of

samples in formalin 10% solution. The blade’s penetration angle and the inter-

2

individual differences were shown to affect the identification of the tool class

characteristics from the striation pattern observed in a kerf wall, although this

fact seem to be related only to the degree of calcification of the costal cartilage.

Intra-individual differences do not seem to be relevant enough as to affect in a

significant way the identification of the tool class characteristics from the

striation pattern observed in a kerf wall, for the same knife following the same

motion. The degree of calcification of the cartilage is a source of great variation

regarding the interpretation of striations pattern in cartilage.

3

INTRODUCTION

Cutting lesions are defined, from a medico-legal point of view, as those in

which the length is greater than the depth of the wound [1]. Cutting lesions are

also tool marks on human tissue and the American Association of Firearm and

Tool Mark Examiners defines a tool mark as a mark produced when a tool, or

object, is placed against another object and force enough is applied to the tool

or object, so that it leaves an impression [2]. Cut wounds involve the use of an

instrument whose action is exerted through at least one sharp edge, and the

instruments most frequently used are knives and saws, although cleavers, axes

and swords among others, can also be used. Nevertheless, the last three types

of weapon more frequently exert their action through a cutting (sharp) and

striking (blunt) combined mechanism. Injuries caused by sharp instruments or

mechanisms can result from interpersonal violence, both ante-mortem and peri-

mortem, which can cause serious damage or even the death of the victim. This

type of trauma can also result from acts of post-mortem mutilation or

dismemberment, often following a homicide.

The great value for forensic doctors, as well as for other forensic

scientists, of the analysis of cut marks and particularly the identification of the

features of marks caused by sharp instruments in either bone or cartilage,

regardless of whether they are peri-mortem or post-mortem inflicted criminal

acts can be twofold [3, 4]. Firstly, the potential information which can be

extracted through the detailed characterization of these tool marks, allows the

recognition of the weapon/instrument class that was used and, secondly, in

special circumstances, even identify the specific individual blade that created

the cut mark. Consequently, the proper documentation and analysis of knife and

4

saw marks do have the potential to contribute significantly to the interpretation

of the criminal acts involved [3,4].

Forensic scientists are usually asked to determine the type of weapon

used to produce a sharp force trauma defect and sometimes, even to match the

defect to a specific weapon. Tool class characteristics are characteristics that

are common to all tools that are of the same “type” (i.e. shape: a W-shaped kerf

suggests a cross-cut saw, a U-shaped kerf suggests a rip-cut saw, while a V-

shape kerf suggests a beveled blade – knife -) as opposed to individual

characteristics which are characteristics that are unique to one particular tool.

The “type” of weapon used is determined through the tool class characteristics

which do not establish tool mark uniqueness. These also include, mean

distance between teeth in serrated knives or instruments.

The great variety of sharp instruments that may eventually be used in

acts of violence is such that sometimes the tool marks that they produce have

very few common characteristics. Even the same instrument, depending on how

it is used, may cause different types of injuries. Such is the case of a knife,

which can cause stab wounds (in which the depth is greater than the length of

the wound), or merely cut or incised wounds if used only through a “sliding”

movement of the blade.

The identification of distinctive features in tool marks reveals particular

complexities [5]. The difficulty and complexity of classifying knife wounds are

demonstrated by examining a single knife wound to the chest affecting two

consecutive ribs [4] where the examination of the two concurrently cut ribs

would produce two different morphological results. While the rib defect

5

attributed to the spine of the knife cannot be classified conclusively as a wound

created by a knife, the incised wound in the other rib can due to its class

characteristics (i.e., V-shaped kerf). Thus, it is the proximity of the wounds that

conservatively suggests a single weapon, making possible for this knife’s blade

to be described as a single beveled-edge knife.

Although knife wounds are second only to ballistic injuries as the major

cause of violent death in homicides, knife wound analysis has also received

little attention in forensic investigation. Frustration and confusion often arises

with regard to analysis and examination of tool marks while common

misconceptions regarding the analysis procedures and the disagreement

between forensic scientists show the need for a standardized protocol for

analysis of tool marks in bone and cartilage so as to meet the current most

demanding evidentiary standards [6].

Several experimental studies have been conducted in cut mark analysis

in order to extrapolate the results and allow its application in the forensic

context [2,4,7,8,9,10,11,12,13]. Some of these experimental studies have

focused on macroscopic or microscopic perspective of the cut mark analysis,

while others used both perspectives. Microscopic observations usually showed

a greater ability to reliably identify the class of the instrument used to produce

the tool mark, i.e., the gross appearance of some of the tool marks made with a

machete in bone was similar to the tool marks produced by a knife, and only the

microscopic appearance of a set of striations in each tool mark allowed the

distinction between the instruments [13]. However, a review of the published

literature showed that a careful macroscopic observation and detailed

6

description of the observed tool marks is considered essential before their

microscopic analysis.

Although various animal models are widely used in experimental studies

of trauma analysis, the human osseous and cartilaginous tissue is significantly

different from the non-human. In particular, it clearly differs from the animal

models most commonly used, namely porcine and bovine. These species are

fast-growing animals when compared to humans, and that fact is reflected in the

microscopic structure of the cartilage. Porcine cartilage has an increased cell

density when compared to human cartilage as well as a higher proteoglycans’

concentration, thus being denser. This higher density is reflected in a different

collagen network architecture which influences its role as a modulator of the

tissue’s mechanical properties [14]. Moreover, the animal pieces used in

experimental studies are commercially obtained and often belong to young

individuals, in which the cartilaginous tissue structure reflects a relatively

immature stage of development and therefore its non-adult biomechanical

characteristics, further compromising the results of a study of this nature. Due to

all these facts, it becomes challenging to interpret the results acquired from

animal models in order to extrapolate them to a human model.

Despite an increasing interest in tool mark analysis, attempts at it have

become a dismal scientific endeavor. Many forensic anthropologists and

pathologists have performed multiple saw and knife mark analyses in

experimental studies using mostly animal models [15,16,17,18] in an attempt to

extrapolate the results to the human model. Due to the previously explained

differences between the human and non-human cartilage structure, it is

7

believed to be of great interest and pertinence to use human cartilaginous

tissue in an experimental model study.

In the literature, whenever a preservation method was used for the

cartilage samples, it was a 10% formalin solution. It is believed to be so due to

the systematic use of this solution for tissue samples’ preservation in a medical

examiner’s general context. Formalin is known to act on tissue through its

dehydration, nonetheless, no study attempting to establish whether this

dehydration caused any shrinkage or distortion of the tool marks under

examination is known to the author.

It is also noticeable from the literature, that cut marks were produced

holding the blade perpendicular to the sample, or in a 90º angle to the sample.

Nonetheless, none of the studies refer if and how the researchers ensured that

the blade, while penetrating into the sample, stayed “perpendicular” or at a

“90º”. Thus, the assessment of the influence of the variation in the blade’s

penetration angle would be as important in a forensic context as the evaluation

of the striations produced while trying to keep the blade in a specific angle

during its penetration into the cartilage sample.

The present research was designed in order to address the issue of the

misidentification of a blade when differentiating cut marks on cartilage produced

by differently serrated blades. The goals of this study are 1) determining how

the preservation method used for the costal cartilage samples (formalin 10%)

affects preservation of cut marks, 2) asses how the blade’s penetration angle,

as well as the inter- and intra-individual differences, affect the identification of

the tool class characteristics from the striation pattern observed in a kerf wall.

8

This study wishes to assess how the preservation of cartilage, the knife’s blade

angle, and both intra- and inter-individual differences in cartilage samples affect

the ability of costal cartilage to retain the original class characteristics of the

knife, as measured by the distance between consecutive striations in cut mark

analysis.

9

MATERIAL AND METHODS

The 160 cartilaginous samples used in this study originated from the

ribcage of 7 male cadavers, with ages between 20 and 60 years that underwent

autopsy at the North Branch of the National Institute of Legal Medicine, in

Portugal. Death from traumatic causes was an exclusion criterion, as well as

any history of bone or connective tissue disease.

To ensure that the study would be carried out in accordance with ethical

rules, the project was submitted for ethical approval to the regional Ethical

Commission (Ethical Commission of the São João Health Center). Ethical

approval was granted (Appendix 1), provided that the National Registry for Non-

Donors (RENNDA) would be checked for each cadaver previous to the

samples’ collection, thus ensuring this collection would be performed in

accordance with Act 274/99 (July 22nd) which regulates the use of cadavers for

teaching purposes and scientific research.

Three different serrated knives (Fig. 1) were purchased from a large

department store. Knife standard terminology with respect to knife blade

anatomy is described according to [18]. Knife 1 was a straight spine, left

grounded, mixed pattern finely serrated knife (Fig. 2). Knife 2 was a straight

spine, right grounded, coarsely serrated knife (Fig. 3). Knife 3 was a straight

spine, left grounded, finely serrated knife (Fig. 4). None of the knives was used

previously to the study.

10

Figure 1. Photograph of the three knives used in this study

Figure 2. Detail of knife 1, a straight spine, left grounded, mixed pattern finely serrated

knife

11

Figure 3. Detail of knife 2, straight spine, right grounded, coarsely serrated knife

Figure 4. Detail of knife 3, a straight spine, left grounded, finely serrated knife

12

In the first stage of the study, the costal cartilage of both 4th, 5th and 6th

ribs was dissected during the autopsy of one adult healthy male individual with

a non-traumatic cause of death. Immediately after the dissection, 40 samples of

dissected costal cartilage were manually cut using the same knife, following a

motion parallel to the long axis of the teeth in the serrated edge. Casts of the

“fresh” cut surface were made using Mikrosil Casting Material®. The 40

samples of the dissected costal cartilage were then placed in a formalin 10%

solution for 7 days, after which were all re-casted. Both “fresh” and “preserved”

casts from each of the 40 samples were observed and photographed using an

Olympus SZX10 stereomicroscope at a 0.63 magnification. The “fresh” casts

images were then compared with the “preserved” casts images by direct image

superimposition.

In the second stage of this study costal cartilage samples were collected

from 6 “healthy” male individuals, with ages between 20 and 60 years and with

a non-traumatic cause of death. Samples of costal cartilage from 2 individuals

were assigned to each knife. Each individual provided 20 cartilage samples.

Ten of those samples were cut following motion A and ten following motion B.

Motion A (Fig. 5) runs parallel to the long axis of the knife’s teeth, whereas

motion B (Fig. 6) runs perpendicular to the long axis of the knife’s teeth.

Figure 5. Direction of Motion A is indicated by the arrow

13

Figure 6. Direction of Motion B is indicated by the arrow

In total, 120 samples of cartilage were cut using the three knives,

following the two different motions. All cuts were done manually by the same

investigator (Fig. 7).

Figure 7. A group of 10 of the 120 cartilage samples obtained illustrating the type of

samples analyzed in this study

14

The 120 samples of the costal cartilage were then placed in formalin 10%

solution for 7 days, and the cut surface was casted using Mikrosil Casting

Material® (Fig. 8).

Figure 8. A group of 10 of the 120 casts obtained from the cartilage samples illustrating

the type of casts produced

All casts were observed under an Olympus SZX10 stereomicroscope.

Image capture and processing, as well as the measurements of the distance

between the striations in the kerf wall were performed using Olympus “cell^B”

software. Microscopic analysis, image capture and processing were easier with

the casts than with the cartilage, as reported by other studies [17,18]. Distances

between striations were measured by drawing a transect across the kerf wall

cast, perpendicular to the striations and intersecting as many as possible (Fig.

15

9). Distances were measured in millimeters as the length between consecutive

striations, using the transect as reference (Fig. 9).

Figure 9. Distances (in millimeters) between striations were measured by drawing a

transect (long inferior line) across the kerf wall cast, perpendicular to the striations and

intersecting as many as possible

Mean distances between consecutive striations were first compared

between casts of the same individual (N=10), for each knife and each motion,

using an ANOVA or a Kruskal-Wallis test. Mean distances between striations in

one individual were then compared to those of the other individual, using an

independent samples t-test or a Mann-Whitney test. Finally, mean distances

between striations of each individual were compared to mean distances

between teeth of each knife, for motion A only, using an independent samples t-

test. The choice of parametric or non-parametric tests was dependent on

16

assumptions of normality and heteroscedasticity of data. In some occasions,

non-equal variances t-tests were used. Given that cuts produced by motion B

will not reflect the actual distances between the knife’s teeth, the proportion in

the decrease of the distance between consecutive striations was calculated for

these samples. Statistical analysis of data was performed using SPSS 17.0.

17

RESULTS

No significant distortion or shrinkage of the striation pattern was

observed when comparing the 2 groups of casts (20 “fresh” versus 20

“preserved” casts). Figure 10 illustrates complete superimposition of the same

set of striations of the same cut in both “fresh” and “preserved” casts. The

preservation of the cartilage samples in formalin 10% also improved the visual

contrast and quality of the striation pattern observed (Fig. 10).

Figure 10. Superior half of the image shows the “preserved” cartilage cast. Inferior half

of the image shows the “fresh” cartilage cast.

Regarding the second phase of the study, all 120 cuts made with the 3

serrated blades produced striations on the kerf walls casted. The striations were

easily visible to the naked eye on both the cartilage and the casts, sometimes

with improved visibility on the cartilage compared to the casts. The striations in

the cut samples produced by the 3 knives using motion A and B tended to be

18

parallel between them (Fig. 11 and 12). However, cuts following motion B

showed a fan-shaped pattern at the entrance point of the knife into the cartilage

(Fig. 12).

Figure 11. Example of one of the casts produced with knife 2 following Motion A in

which striations appear to be quite parallel between them

19

Figure 12. Example of one of the casts produced with knife 2 following Motion B in

which striations appear to be parallel between them, although in the left side of the cast,

striations appear to be more distanced between them than in the right side of the cast

(fan-shaped pattern). Left side of the cast corresponds to the entrance point of the blade

into the cartilage

ANOVA / Kruskal-Wallis results showed that mean distances between

striations, for each knife and for each motion, were similar in all casts of the

same individual (Table 1). When comparing mean distances between striations

between individual 1 and 2, for each knife and each motion, only knife 1 for

motion A showed statistically significant differences between individuals (Table

2). Mean distances between the knife’s teeth did not differ significantly from

mean distances between striations produced by that knife in motion A, except

for one individual sampled for knife 1 (Table 3). When analyzing the decrease in

20

the distances between consecutive striations for motion B, as a proportion of

the distances in motion A, knife 1 showed a mean reduction of 67% (individual

1) and 70% (individual 2), knife 2 showed a mean reduction of 61% (individual

1) and 64% (individual 2), and knife 3 showed a mean reduction of 60%

(individual 1) and 87% (individual 2).

Table 1 – Results for comparisons between casts of the same individual in mean distances

between consecutive striations.

Knife 1 Knife 2 Knife 3

Motion Indiv. 1 Indiv. 2 Indiv. 1 Indiv. 2 Indiv. 1 Indiv. 2

A F= 1.136 df= 97 p= 0.347

F= 0.626 df= 90 p= 0.772

F= 0.141 df= 18 p= 0.979

F= 1.230 df= 29 p= 0.332

F= 0.402 df= 25 p= 0.868

F= 0.185 df= 38 p= 0.994

B F= 0.940 df= 39 p= 0.506

F= 1.116 df= 39 p= 0.382

F= 0.637 df= 29 p= 0.753

F= 0.558 df= 28 p= 0.814

KW=3.739 df= 9 p= 0.928

KW= 7.366 df= 8 p= 0.498

F (ANOVA), KW (Kruskal-Wallis), df (degrees of freedom), p (significance)

Table 2 – Results for comparisons between individuals (1 versus 2) in mean distances between

consecutive striations.

Motion Knife 1 Knife 2 Knife 3

A t= -2.552 df= 187 p= 0.012

t*= 0.587 df= 47 p= 0.562

t= -0.539 df= 63 p= 0.591

B t= -0.530 df= 78 p= 0.598

t= 0.078 df= 57 p= 0.938

MW= 431.5 df= 17 p= 0.153

t (equal variances t-test), t* (unequal variances t-test), df (degrees of freedom), p (significance),

MW (Mann-Whitney)

Table 3 – Results for comparisons between mean distances between consecutive striations in

individual 1 and 2 and mean distances between teeth in each of the knives, for motion A.

Individual Knife 1 Knife 2 Knife 3

1 t*= -2.894 df= 137 p= 0.004

t*=0.649 df= 29 p= 0.601

t= -4.550 df= 39 p= 0.651

2 t*= -0.018 df= 130 p= 0.986

t*= 0.049 df= 40 p= 0.961

t= -0.081 df= 52 p= 0.936

t (equal variances t-test), t* (unequal variances t-test), df (degrees of freedom), p (significance)

21

DISCUSSION

Assessing the influence of formalin preservation on the striation pattern,

direct and complete image superimposition, when comparing the 2 groups of

casts (20 “fresh” versus 20 “preserved” casts) showed that no significant

distortion or shrinkage of the striation pattern occurred by preservation of the

cartilage samples in a 10% formalin % solution for 7 days. The preservation of

the cartilage samples in a 10% formalin solution also improved the visual

contrast and quality of the striation pattern observed, being this improvement

most probably an effect of the dehydrating proprieties of the formalin. Taking

into consideration that the sharp force trauma analysis SWGANTH

recommendations state that “cartilage should be analyzed for the presence of

sharp force defects before removal of soft tissues”, this dehydrating proprieties

of the formalin solution appear to be very useful in order to facilitate the

examination.

Analysis of the factors influencing cut marks showed that mean distances

between striations, for each knife and for each motion, are similar in all casts of

the same individual. This means that the same knife, in the same cutting

motion, cutting different cartilage samples in the same individual, produces a

similar and repeatable striation pattern, which can be measured consistently. In

addition to intra-individual consistency in striation pattern, results also show that

this is also the case for inter-individual comparisons. There was only one

exception when comparing distances between individual 1 and 2, for knife 1 and

motion A, where these individuals differed. Since only individual 1 showed

differences between the distances measured in the casts and the distances

between the knife’s teeth, this suggested a possible problem with this

22

individual’s costal cartilage samples. In fact, this individual is the oldest of the 6

cadavers utilized in the second phase of the study (60 years-old) and showed,

by far, the most calcified cartilage of the entire sample. Although the degree of

calcification of each cartilage sample was never quantified, a qualitative

assessment was sufficient to reveal important differences between subjects. In

comparison, mean distances between striations produced by knife 2 and 3 did

not seem to differ from distances between the knife’s teeth. When analyzing the

decrease in the distances between consecutive striations for motion B, as a

proportion of the distances in motion A for the same knife and the same

individual, the greatest reduction noticed was in individual 2 for knife 3 (an 87%

decrease). This was the younger individual (20 years-old) of the 6 cadavers

used in the second phase of the study, and coincidentally it was also the

individual who provided the least calcified cartilage samples. In all three knifes

utilized, a reduction of 60-70% in mean distances between striations can be

expected, when the blade is progressing on a plane that is perpendicular to that

of the teeth’s axis.

Striations produced following cutting motion B, using all three knifes,

showed a consistent fan-shaped striation pattern, which was always observed

at the entrance point of the knife, and it seems to be a defining morphological

feature of these casts. It was also noticed that the more calcified the cartilage

was, the more angular was the curve of the striations at the base of the fan-

shaped pattern before the striations become parallel. This pattern is also

described in a study using porcine cartilage by Pounder, 2011, and it is

explained by the fact that the knife used had a drop point blade and unlike a

straight spine blade, the drop point blade cannot enter the cartilage in a simple

23

vertical manner and the blade moves sideways until the drop point has passed

through the firm cartilage. This explanation, however, does not seem to justify

the presence of a fan-shaped pattern in the striations produced by the 3 knives

in this study, following motion B, since all knives had straight spine blades. It

was rather noticed that this fan-shaped pattern was related to a change in the

penetration angle of the knife while progressing into the cartilage. The

morphological differences (more or less angular) in the fan-shaped pattern were

also noticed to be related to the degree of calcification of the cartilage, since the

more calcified the cartilage was, the more “unsteady” was the blade’s

penetration in the cartilage, thus producing a more angular fan-shaped pattern.

One of the main factors affecting the ability of cartilage to retain the

class characteristics of the knife seems to be the amount of calcification. This

was rather clear when comparing the more calcified cartilages of individual 1

with that of individual 2, under knife 1 and motion A, as well as when comparing

the reduction in striations of individual 1 with that of individual 2, under knife 3

and motion B. The more calcified the cartilage is, the more “resistance” seems

to offer to the passage of the blade making its penetration into the cartilage

“unsteady”. On the other hand, the less calcified the cartilage is, the less

“resistance” seems to offer to the passage of the blade, making its penetration

into the cartilage much more “steady”. Other experimental studies conducted in

this area using non-human cartilage samples [15,16,17,18] from young age

commercially acquired animals eliminate this variation introduced by the

influence of calcified cartilaginous tissue from mature and older individuals.

Consequently, this study cautions against the fact that cartilage from

24

commercially acquired animals may not reflect human cartilage in numerous

ways, in addition to the obvious human/non-human divide.

There are some potential limitations to this study, one of which is the

variations eventually introduced by the investigator while manually producing

the cuts. Although a mechanically controlled machine may eliminate this source

of error, it has little to resemble actual cut marks in a real forensic scenario.

Sample size also be other important limitations, as the study was only able to

compare between two different individuals, across the three different knives.

The number and diversity in class of knives may be another limitation, but these

only call attention for future research. In particular, this study’s research design

did not assess the influence of different degrees of calcification in the striation

pattern in detail. Future research is needed to falsify, confirm or complete the

results in this study.

The present study wished to contribute with new data concerning the

analysis of cut marks on human costal cartilage. Results suggest that the class

characteristics of the knife can be retained rather faithfully in costal cartilage

when the cut is parallel to the teeth’s axis, with exceptions resulting from the

use of calcified cartilage. However, in an actual forensic case there may be

situations when the expert knows the penetrating angle and other when he/she

does not. Depending on the class of the knife utilized, striations produced by a

perpendicular cutting motion of a certain knife may mimic the striations

produced by a parallel cutting motion of another knife with a much smaller

distance between the teeth’s axis. This is an unavoidable fact. However, results

in this study also suggest that whenever the cutting motion is not parallel to the

teeth’s axis, a fan-shaped pattern, with striations at a greater or lesser angle,

25

can be identified at the point of the blade’s entry. The exact angle at which the

blade actually penetrated is, unfortunately, impossible to determine at the

moment.

26

27

CONCLUSIONS

This study wished to assess how certain variables influence the ability of

human cartilage to retain the class characteristic of the blade in sharp force

trauma. Cartilage samples preservation in formalin 10% solution not only do not

caused significant distortion or shrinkage of the striation pattern, but improved

the visual contrast and quality of the striation pattern observed. With important

exceptions, this study was able to show that cartilage is able to retain the class

characteristic of the blade (mean distance between teeth) used to cut it, quite

faithfully, when the cutting motion is parallel to that of the axis of the knife’s

teeth. However, The blade’s penetration angle and inter-individual differences in

costal cartilage affect the identification of the tool class characteristics from the

striation pattern observed in a kerf wall, although this fact seem to be intimately

related to the degree of calcification of the costal cartilage of the individual

under analysis.

28

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