In this study, 720 cut marks, on 240 defleshed pig ribs, using 3 different types of knifes, were made using manual infliction by a single human. The choice of kitchen knives in this study were those commonly found to have been used in violent cases [2, 4, 5, 45]. Newly purchased kitchen knives were used to minimise the chance of blade defects imparting additional features in the cut mark [10, 21, 49, 50]. Despite trying to ensure that a consistent force and angle were applied each time, a variety of sizes and shapes of cut marks were observed even when the same weapon was used. This is an inevitable consequence of manual infliction, but also the natural variation between ribs, resulting in a variation in the blade/rib interaction. These findings are in line with previous studies [21, 49]. Further studies using a cutting rig with defined force and angles may be conducted to reduce the human variations.
It is important to note that a variety of cut mark dimensions and morphological characteristics are expected when using different knife blade types; the difference in the blades’ cutting edge characteristics is reported earlier in Figure 1; cut marks created and examined in this study have characteristics which are in general agreement with previous studies [8, 51,52,53]. The profile of the cut marks inflicted by a non-serrated blade was found to be linear-shaped, a narrow cross-section, and with no striations which are indications of this knife type. Conversely, a coarse-serrated blade normally produces an elliptical V- or U-shaped cross-sectional cut mark with a raised or smooth margin and striated wall. A fine-serrated blade knife produces an elliptical shape with a V-shaped cross-section and smooth or raised margins with a striated kerf wall. The exact relationship between the knife and cut mark cross-section and dimensions needs more detailed investigation in future studies, with profiles and measurements of the cross-sections of the blade edges correlated to the cut marks.
Previous studies have shown that a cut mark inflicted by a non-serrated blade tends to be narrower than those made by a serrated blade [18, 50, 53]. The results of this study were consistent and showed a significant statistical distribution (p<0.001) of the kerf width differences from non-serrated, coarse-serrated, and fine-serrated blades (Table 3). It was observed that the order of increasing cut mark width was the same as the cutting edge width of the blade. In relation to the cutting edge angle of the blades, there were differences. While the non-serrated blade had the smallest blade edge angle and the narrowest cut mark, the cut mark from the coarse blade knife was wider than that for the fine-serrated blade. This is despite the cutting edge angle of the fine-serrated blade being larger than that for the coarse-serrated blade. This indicates that the larger serrations and wider blade of the coarse-serrated knife influenced the cut mark width beyond that of the cutting edge angle. It is however noted that, due to manual infliction used in this and other studies, variations in the force and angle of the blade could produce wider cut marks due to the angle of impact and force applied [54, 55]. Therefore, further study still needs to be conducted with different types of knife blade to determine the kerf width with weapon type. In contrast, the kerf lengths from the different blade types were found to be similar, with variations likely due to small fluctuations in the knife force/angle and/or rib shape. Thus, no association between the kerf length and knife blade type has been proven in this study in contrast to Humphrey et al (2017) , indicating further research is needed.
Several characteristics of kerf shape were described comprising linear, elliptical, rectangular, and irregular shapes. Statistical significant differences (p<0.05) between the different knife types and kerf shapes were observed (Table 3) indicating that the kerf shape is specifically related to the size and morphology of the blade (blade characteristics are given in Figure 1). Briefly, all cut marks produced by a non-serrated blade displayed a linear and narrow kerf shape due to the narrow knife edge, whereas the shape of cut marks inflicted by coarse-serrated and fine-serrated blades was predominantly elliptical, 80% and 92.5% of marks, respectively. An elliptical shape arises from the geometry of a V-shaped blade and a cylindrical surface. However, some variation is observed. A large serrated blade produces mostly elliptical or rectangular shapes, the latter due to the deeper penetration into the bone. A fine-serrated blade usually produces a cut mark with an elliptical shape because its small serration makes good contact when cutting through the bone surface.
Normally, a V shape is the most common characteristic of the cross-sectional shape observed in knife cut marks [14, 56]. Lewis (2008)  advised using this feature to distinguish knife type. Nonetheless, Cerutti et al. (2016)  stated that this feature could not be used to differentiate between different types of knife blade because a V-shaped feature can be produced from any type of a knife blade. This study showed a variety of cross-sectional kerf mark shapes, but there were relationships between the cross-sectional shape and the blade type. Specifically, some cross-sectional shapes can be found more frequently with a specific type of knife blade. Most of the cut marks made by non-serrated blade knives displayed narrow cross-sections, with the remaining 13.7% being V-shaped. Around 55% of cut marks inflicted by a coarse-serrated blade had a V-shaped cross-section, while the remaining had a U-shaped cross-section. In addition, 95% of cut marks inflicted by fine-serrated blade produced V-shaped marks, with the remaining having U-shaped cross-sections. These results support a trend that the definition of the V shape varies depending on the size and shape of the blade [8, 13, 18, 21, 49, 54,55,56,57]. A U-shaped cross-sectional shape can be formed by a saw and other types of broad blade weapons such as a stone tool [10, 13, 56]. A coarse-serrated blade used in this study produced a U-shaped cross-section in 45% of samples (55% V-shaped), a much greater variation in cross-sectional shape than the other blade types. This finding can be explained by the cutting mechanism of this blade type, which is like a handsaw. When a coarse-serrated blade cuts bone, it can cut smoothly, skip over the bone surface, and/or change in cutting direction. This variation in the blade/bone interaction then explains the greater variation in the cross-sectional shape of cut marks inflicted by this blade type. For all three knife types, the degree to which the variations in kerf shape result solely from differences in the angle and/or force of the blade (inevitable with the manual infliction) during the cutting action is unclear, but it should be noted that knife impact differences could be a contributing factor.
A raised kerf margin is defined as having ragged and deformed margins along cut mark edges. In this study, the kerf margin showed a correlation to the cutting edge feature of the inflicted blade. The cut marks made by a coarse-serrated blade had the most frequent incidence of raised margin (58.3%), followed by those inflicted by a fine-serrated blade (43.3%). Conversely, the cut marks inflicted by a non-serrated blade showed only smooth margins. These findings are consistent with previous works suggesting cut marks with raised margins are made by serrated knives [7, 14, 20]. Tennick (2012)  stated that a raised margin tends to be the result of the interaction between a blade edge and a bone surface. Chattering, scraping, and skipping are commonly found in the case of cutting with serrated blade teeth, and it is these mechanisms which result in the raised margin. Likewise, the number and pattern of serrated blade teeth may play a role in the likelihood of these cutting artefacts. A fine-serrated blade has smaller and compact teeth providing better grip on a bone surface, and therefore a fine-serrated blade is more likely to create a smooth and regular kerf margin. Conversely, a coarse-serrated blade with fewer and larger teeth provides a poor grip on a bone surface resulting in greater skip and chatter.
In this study, an increased percentage with kerf striations in the cut marks inflicted by fine-serrated (67%) and coarse-serrated (60%) blades and the lack of this characteristic in the cut marks inflicted by a non-serrated blade indicate that kerf striations are useful to distinguish knife class, as found by others [20, 50]. In addition, the morphology of the striations varied with the two different types of serrated knives, indicating that the blade/teeth size and thickness influence the striations, as also found in other studies [14, 53]. A fine-serrated blade usually produces smaller and more delicate kerf striations because it interacts closely with a bone surface. However, in this study, not all cut marks from serrated blades contained visible kerf striations, particularly those produced by the fine striated blade; hence, the lack of striations cannot be used to rule out a serrated blade. Although this may be a result of the cutting, it is highly likely that a major factor is due to the difficulty of visualising the fine detail on the partially translucent/light coloured bone as reported by others, supporting the need to use casting/oblique lighting to enhance surface topography visualisation [48, 58]. As previously described, the decision was made not to cast the cut marks as the fine striations were only a small portion of a much larger study into the persistence of cut mark characteristics.
For forensic investigations, it is important to be able to detect a traumatic lesion on a burned bone and attempt to classify the type of weapon used. Previous literature showed that sharp force trauma on a bone could survive the bone being burned [26, 32,33,34, 38, 59]. It is highlighted that morphological changes to the trauma could occur because of the burning; hence, further research is necessary [30, 32,33,34,35,36]. Similarly, all the cut marks in this study were grossly identifiable after burning, despite the bone sample fracturing, with a small number of fractures intersecting cut marks.
A skeletal element is subjected to an intense loss of water and organic matter during the burning process. Theoretically, these should lead to a marked decrease in cut mark dimensions from a combination of collagen and moisture loss as well as hydroxyapatite recrystallisation [26, 28, 31]. Previous literature showed that heat-induced bone shrinkage has an effect on all bone dimensions especially at high temperature [26, 27, 30, 31, 59,60,61,62,63]. A burned bone starts to shrink at 200°C, but a temperature of around 800°C is a critical temperature at which the extent of heat-induced dimensional changes increases significantly [31, 39, 64].
All samples in this study were burned at 850°C for 30 min. A statistically significant decrease in the kerf length around 10.8–17.6% and the kerf width of 28.5–34.9% compared with the original dimension was observed. Consequently, cut mark dimensional changes are non-uniform and have a directional dependency. The alignment of collagen fibres may play an important role to explain this characteristic. In this study, cut marks were perpendicular to the lengthwise direction of collagen fibres which are aligned along the shaft of the ribs [64,65,66]. As a result, degradation and shrinkage of collagen fibres from the burning process, contracting the length of the rib, thereby reduced the kerf width more than the kerf length. In addition, warping deformation may influence the kerf dimensions by either pulling or pushing the cut mark walls into the other one .
The use of juvenile bone in this study may have affected the degree of heat-induced dimensional changes. If collagen changes are associated with burned bone’s dimensional changes, age should be an important factor too because the proportion of collagen fibre changes during life [67, 68]. Compared to an adult bone, the juvenile skeleton has a higher collagen and lower mineral content. Therefore, upon burning a greater contraction in the direction aligned with the collagen fibres could be expected compared with that in an adult bone. Hence, the age of the individual, as well as the alignment of cut marks with the collagen fibre direction, is likely to affect the cut mark dimensional change after the burning process. This has been reported by some but warrants further investigation [31, 36].
In this study, heat-induced fractures intersected and therefore damaged a minority of cut marks, but most of the cut marks remained clearly defined after the burning process. However, some heat-induced morphological changes were statistically significant. After the bone samples were recovered from the furnace, ellipse-shaped cut marks from coarse-serrated and fine-serrated blades changed to either a rectangular or irregular shape. In addition, raised margins of cut marks inflicted by a coarse-serrated blade are transformed into smooth margins (Table 4).
The burning-induced changes in the kerf shape of cut marks inflicted by coarse-serrated and fine-serrated blades, as opposed to the stable cut mark from a non-serrated blade, are likely due to the variation in the cutting edge/bone interaction. The teeth on the blade of coarse-serrated and fine-serrated knives cut and chatter over the bone surface, therefore imparting greater mechanical trauma localised around, as well as finer detail within, the cut mark. Thus, the raised kerf margins, seen with the coarse-serrated blade, would be such damaged bone and would be more susceptible to alteration during a burning event. In this study, the raised kerf margins were commonly absent after the burning process although deformed, blackened, and eroded margins were also observed.
Almost all kerf striations could be recognised after the burning event, meaning that whereas fire affects some cut mark characteristics, it does not necessarily destroy them all. The definition of the kerf striations depends not only on the morphological characteristics of the knife but also the ability of the bone sample to receive the cut mark [40, 41]. In comparison with the raised kerf margin, formed from displaced and damaged bone and therefore susceptible to heat effects, this suggests that the more robust striations are not formed in a similar manner, instead are surface topography resulting from the cutting action of the serrated knife blade with the bone.
A distinction between a skeletal traumatic lesion and a heat-induced fracture has been identified in this study. In addition, it was seen that cut marks did not act as nucleation sites or crack paths for fracture hence had a good chance of surviving and being observable post burning. Visually, all cut marks could be recognised and identified by their linear, narrow features with a smooth or raised margin. In contrast heat-induced damages had well-defined borders and a random crack path. Microscopically, burning of the bones resulted in the complete or partial erosion of raised kerf margins associated with cut marks, although fine detail within the marks such as striations was unaltered (Fig. 8), whereas smooth and clean borders were observed in heat-induced fractures (Fig. 9). These findings correspond with advice by Pope and Smith (2004)  who stipulate using microscopic analysis of defect borders to distinguish between heat-induced damage and a traumatic fracture. They recommended that reconstruction of a suspected lesion should be first carried out, and this should be followed by microscopic examination [32,33,34]. Generally, the bone lesions from heat-induced damages and perimortem fractures are the outcomes of the type of loading force and the property of skeletal tissue involved [28, 32, 34, 59, 69]. A heat-induced fracture formed in a brittle bone element does not have as good energy-absorbing property as a normal bone. Therefore, it is dissimilar to a traumatic fracture formed in ductile, fresh bone material. Herrmann and Bennett (1999)  summarised that a mechanism of the heat-induced fracture in a burned bone is very similar to a fracture from a high-velocity injury such as a gunshot wound.