Forensic Science, Medicine, and Pathology

, 4:212

Experimental simulation of non-ballistic wounding by sharp and blunt punches

Authors

  • Brittany Wong
    • Department of Oral Sciences, Faculty of DentistryUniversity of Otago
    • Department of Oral Sciences, Faculty of DentistryUniversity of Otago
  • Ionut Ichim
    • Department of Oral Rehabilitation, Faculty of DentistryUniversity of Otago
  • Michael Swain
    • Department of Oral Rehabilitation, Faculty of DentistryUniversity of Otago
  • Vicki Livingstone
    • Department of Preventive and Social Medicine, Dunedin School of MedicineUniversity of Otago
  • Neil Waddell
    • Department of Oral Rehabilitation, Faculty of DentistryUniversity of Otago
  • Michael Taylor
    • Institute for Environmental Science and Research
Original Paper

DOI: 10.1007/s12024-008-9042-z

Cite this article as:
Wong, B., Kieser, J.A., Ichim, I. et al. Forensic Sci Med Pathol (2008) 4: 212. doi:10.1007/s12024-008-9042-z

Abstract

Despite a long history of gross and microscopic descriptions of blunt and sharp force injury to the dermal tissues, few have addressed the mechanisms underlying such trauma. The need to develop an understanding of how non-ballistic injury occurs calls for an ability to biomechanically model the process. We recently introduced a basic skin and subcutaneous model, which we used to investigate wounding from a spherical object. Here we employ the same model to examine wounding caused by a sharp wedge shaped object and a blunt rectangular object. Macroscopic examination and SEM views of the surface and cross sections of blunt and sharp force tears show that while in the former there is a clean cut through the skin into the underlying sponge, in the latter there is a tissue plug confined to the skin that is smaller than the impacting rectangle. Fracture initiation in the subdermal tissue occurs at the angles of the impacting object. In sharp force trauma, there is localized breaching of the skin layer coupled with the wedging action of the impacting object. Because the subdermal tissue, in this case the underlying hydrated foam, is attached to the base of the skin, it will contribute to further tearing of the foam beneath the line of contact.

Keywords

Forensic scienceTraumatologySkin wounding biomechanical modeling

Introduction

Understanding how a particular wounding event has taken place has proven to be a challenge for forensic scientists. Even anatomical features that can be directly related to the traumatic contact between a given weapon and skin, which intuitively should be expected to show major diagnostic features, have been shown to be inconclusive [1]. The purpose of this study is to investigate the utility of a synthetic skin/subcutaneous tissue model for interpreting non-ballistic blunt and sharp force trauma.

Non-ballistic wound analysis has traditionally occurred at two different but related levels. At one level, investigators have sought to describe wounds using ever more sophisticated techniques such as radiography, scanning electron microscopy, and 3D surface digitization [25]. They have also focused on molecular biological phenomena associated with wounding [6, 7], and have recently tried to quantify the forces required to penetrate tissues by studying the dynamics of experimental knife stabbing events [811]. In spite of this work, however, the anatomical complexity of human skin makes it difficult to draw direct inferences of actual wounding occurrences [10, 12]. Additionally, ethical considerations surrounding such studies in live animals have made verification difficult if not impossible.

At another level, researchers have sought to model wounding biomechanically. Shergold and Fleck [13, 14] constructed a micromechanical model of deep penetration of skin by small diameter sharp and blunt punches. This work showed that the penetration mechanism depended on the geometry of the punch tip. Thus a sharp tip penetrated the skin by wedging open a planar (mode I) crack and a blunt tip penetrated by a ring (mode II) crack. We recently described a simple simulation of blunt force injury in synthetic skin fused to subcutaneous tissue that allowed us to model the skin/subcutaneous complex as an integrated energy absorbing system [15, 16]. Our skin/subcutaneous model consisted of a silicone layer fused to a hydrated, open-cell foam. We found that at the moment of blunt force impact, kinetic energy was almost instantly transferred to the contact site, thus creating an area of high pressure both in the skin and in the subsurface fluid-filled tissue. The intense pressure generated resulted in penetration of the skin material by fracture and ruptured cellular components about and beneath the area of contact.

In the present study we apply our synthetic skin/subcutaneous model to the issue of wounding by sharp and blunt forms. Basic sharp (wedge) and blunt (rectangular) wounds were chosen for investigation as they represent the wide range of trauma typically encountered by forensic scientists. We expect the wounding architecture to reflect differences in wounding mechanism mainly through patterns of rupture of the silicon/hydrated foam complex.

Materials and method

As described previously [15], we used a drop tube device to deliver a 5 kg steel cylinder perpendicularly onto the target (Fig. 1). Two steel elements were welded onto the cylinder; firstly, a chisel tip (13 × 40 mm, 45°) to simulate sharp force injury and secondly, a rectangular shaped tip (13 × 40 mm) to simulate a punch type injury (Fig. 2). As in our previous study, we used three drop heights, 300, 400, and 500 mm. Our skin/subcutaneous target consisted of a silicone layer on foam. We used two open-celled foams with different hardness and similar medium density: Dunlop Enduro 38-200 and Dunlop Elephant 34-115®. Here the first number is the sponge density (weight of polyurethane in Kg/m3) and the second refers to its hardness (resistance to compression in N). To simulate the fluid-content of the subdermal tissues, we soaked each foam square in 500 ml water, until fully saturated immediately before placing it beneath the drop tube. For our skin simulation we used a uniform layer of Deguform® silicone, mixed to a 1:1 ratio in a Multivac® stirring unit and then poured into a flat mold. Two thicknesses were used −1.5 and 2.5 mm. Two different foams and two different thicknesses were used to test whether different strength foams were representative of a range of different tissue behaviors, as empirically suggested by the findings of Smalls et al. [17) on human skin from different locations. Before the layer had set, the sponge samples (each 25 mm thick, 150 × 150 mm) were gently placed onto the silicone, so as to ensure adhesion. SEM observations indicated that the silicone infiltrated the outer layers of the foam particles. Once set, each sample was peeled from the mold and the edges trimmed flush with the sponge. We found empirically that the bonding between “skin” and foam was always excellent because of the silicone infiltration and in all tests conducted it never delaminated. We used silicone because it has similar physical properties (tensile strength skin = 10–20 MPa, silicone = 3.5–15 MPa; tear strength skin = 2–20 kNm−1, silicone = 5–40 kNm−1).
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Fig. 1

The drop tube device to deliver a 5 kg steel cylinder perpendicularly onto the target. Two steel elements were welded onto the cylinder; firstly, a chisel tip (13 × 40 mm, 45°) to simulate sharp force injury and secondly, a rectangular-shaped tip (13 × 40 mm) to simulate a punch type injury

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Fig. 2

Simulated blunt force and sharp force injuries resultant from the rectangular-shaped tip and the chisel tip used. Left, the rectangular shape ruptures the silicone and compresses a plug beneath a mode I rectangular crack. Upon withdrawal of the force, the tissue plug is slightly raised above the surface. Right, the chisel tip ruptures the silicone layer and the underlying sponge by wedging open a planar mode II crack

Simple observations allowed us to record the shape of puncture patterns and measure their dimensions. Injuries to both the silicone layer (termed external tears) and foam layer (internal tears) were recorded and the latter were classified as follows; L-shaped, double L-shaped, square-ended, and rectangular (Fig. 3). Measurements of tears of the silicone surface were defined as follows (Figs. 4 and 5).
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Fig. 3

Patterns of internal tear of the underlying hydrated sponge; A = L shaped, B = double L, C = square-end, and D = rectangular

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Fig. 4

Measurements taken of lacerations of the silicone skin surface. Left, diagrammatic representation of penetration of rectangular shape into the silicone. Right, peripheral width A; external width B; internal width C

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Fig. 5

Superior view of measurements taken of tears of the silicone plug after impact by the rectangular shape. Left, external width and length; right, internal length and width

Peripheral length and width, defined as the maximum dimension of the footprint of the impacting object left on the silicone skin. This was best visualized at an angle against the light.

External length and width, defined as the maximum dimensions of the silicone plug left behind after impact.

Internal length and width, defined as the maximum internal dimensions of the silicone plug left behind after impact, measured beneath the fringe of detached silicone. Group mean values of the outcome variables (peripheral length and width, internal length and width, and external length and width) were compared using a 3-way ANOVA grouped on sponge type, sponge thickness, and height. The main effects and all interactions were included in the model. All statistical analyses were conducted using STATA 9.1.2 (StataCorp, Texas, USA). A p-value < 0.05 was considered to be statistically significant. Multinomial logistic regression was used to estimate the association between the outcome variable (shape of the subcutaneous tear: A, B, C, or D) and skin thickness, drop height, and foam type. The main effects were included in the model and laceration type A (L-shaped) was used as the reference category.

Results

Blunt force impact

A total of 71 (of 72) punctures were available for examination; a double impact resulted in the exclusion of one wound. All impacts resulted in tears of both the silicone and underlying foam layers. On the silicone skin, punctures were characterized by a footprint of the impacting object, inside which was a characteristic silicone plug with irregular, undermined margins with tissue bridges (Fig. 6).
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Fig. 6

Puncture to the overlying silicone layer showing tissue plug with irregular margins with associated tissue bridges

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Fig. 7

Scanning electron micrographs of wounds caused by rectangular (a and b) and wedge-shaped (c and d) impacts. Left surface view, right cross section showing silicone skin fused to sponge

For peripheral width (Fig. 4), there was a larger separation between measurements in thin-skinned, dense sponges (Table 1), with smallest measures found in lower drop heights. The main effects of skin and height were statistically significant (p = 0.028 and p < 0.001, respectively) as was the two-way interaction of sponge by height (p = 0.002). This was also the case for peripheral width (Table 1, p = 0.013, p = 0.009, and p = 0.012 for the main effects of skin and height and their interaction, respectively). For external length (Fig. 5), the three main effects were all statistically significant (p < 0.001 for all three). For external width, the main effects of skin and height (p < 0.001 for both) were statistically significant, as were the two-way interactions of sponge by height and height by skin (p = 0.005 and p = 0.003, respectively). Clearly, the size of external wound dimensions is more strongly related to the impact force than peripheral measurements.
Table 1

Means (SD) of length and width measurements for different drop heights on two different densities of sponge and skin thicknesses

 

Height (mm)

Enduro sponge n = 6

Elephant sponge n = 6

Skin thickness

Peripheral

Length

  

    1.5 mm

300

34.70 (2.20)a

36.83 (0.75)

400

38.33 (0.88)

36.67 (1.51)

500

37.08 (1.63)

36.67 (0.52)

    3.5 mm

300

35.92 (1.11)

36.92 (1.20)

400

37.67 (0.52)

37.83 (0.68)

500

37.58 (0.49)

37.92 (0.66)

Peripheral

Width

  

    1.5 mm

300

9.90 (1.47)a

11.00 (0.63)

400

11.92 (0.20)

9.42 (4.69)

500

11.50 (0.45)

12.75 (1.60)

    3.5 mm

300

11.17 (0.41)

11.58 (0.66)

400

12.50 (0.45)

11.75 (0.76)

500

12.33 (0.61)

12.83 (0.52)

Internal

Length

  

    1.5 mm

300

17.50 (16.02)a

30.67 (1.21)

400

31.50 (1.38)

29.92 (1.28)

500

29.67 (1.03)

30.75 (1.08)

    3.5 mm

300

30.00 (0.63)

30.42 (1.43)

400

31.58 (0.66)

30.50 (0.95)

500

31.33 (0.52)

30.33 (1.66)

Internal

Width

  

    1.5 mm

300

6.30 (1.15)a

5.75 (0.76)

400

6.58 (0.49)

6.00 (0.71)

500

5.83 (0.26)

6.42 (0.66)

    3.5 mm

300

6.00 (0.55)

5.58 (0.38)

400

6.83 (0.41)

5.75 (0.76)

500

7.00 (1.05)

6.42 (0.80)

External

Length

  

    1.5 mm

300

6.90 (5.72)a

13.50 (9.31)

400

17.42 (10.00)

21.67 (10.16)

500

18.92 (10.13)

29.83 (1.17)

    3.5 mm

300

0.00 (0.00)

5.42 (6.51)

400

5.67 (4.32)

11.17 (13.51)

500

10.50 (12.32)

23.08 (6.17)

External

Width

  

    1.5 mm

300

3.90 (2.30)a

4.50 (0.89)

400

5.25 (0.42)

4.92 (0.86)

500

5.08 (0.80)

5.83 (0.75)

    3.5 mm

300

0.00 (0.00)

1.92 (2.11)

400

5.67 (0.52)

3.17 (2.50)

500

4.50 (2.35)

5.50 (1.05)

an = 5

For internal length (Fig. 5), the main effects of skin and height were statistically significant (p = 0.025 and p = 0.008, respectively); all two-way interactions were statistically significant (p = 0.023 for sponge by skin, p = 0.005 for sponge by height, and p = 0.043 for height by skin). The three-way interaction was also statistically significant (p = 0.027). For internal width (Figs. 4 and 5), the main effects of sponge and height (p = 0.011 and p = 0.043, respectively) were statistically significant. In summary, the extent of lacerations for thin skin and denser sponge are significantly associated with drop height. The effect is, however, not linear. As expected, thicker skin tended to be less lacerated. This effect was, however, not universal. In some cases, thicker skin showed the same or even larger wounds than thinner skin (e.g., peripheral length and width, and internal length).

Types of internal laceration found for different skin thickness, drop heights, and foam are given in Table 2. There appears to be no consistent pattern in these data, which might be due to the fact that even small deviations from an absolutely vertical drop could alter the likelihood of resulting in a rectangular rather than an L-shaped laceration. Table 3 presents the results of the multinomial logistic regression analysis with skin thickness, drop height, and foam type included as main effects in the model. The relative risk ratios and their corresponding confidence intervals indicate that none of the main effects were statistically significant predictors of the subcutaneous laceration type.
Table 2

Types of internal tear found for different skin thicknesses, drop heights, and foams

 

Height (mm)

Enduro sponge n = 6

Elephant sponge n = 6

A

B

C

D

A

B

C

D

Skin thickness

1.5 mm

300a

1

2

2

0

3

0

1

2

400

6

0

0

0

2

2

0

2

500

1

2

2

1

3

2

0

1

3.5 mm

300

0

0

0

6

4

0

2

0

400

2

0

3

1

0

4

1

1

500

3

2

1

0

2

0

1

3

an = 5

Table 3

Results of multinomial logistic regression, relative risk ratio (95% confidence interval)

 

B vs. A

C vs. A

D vs. A

Sponge

    Enduro

1.00

1.00

1.00

    Elephant

1.27 (0.34–4.72)

0.57 (0.15–2.25)

1.02 (0.29–3.58)

Skin

    Thin

1.00

1.00

1.00

    Thick

1.07 (0.29–4.01)

2.35 (0.60–9.21)

2.73 (0.76–9.75)

Height

    300 mm

1.00

1.00

1.00

    400 mm

2.42 (0.38–15.44)

0.61 (0.12–3.15)

0.39 (0.08–1.82)

    500 mm

2.69 (0.42–17.37)

0.69 (0.13–3.58)

0.54 (0.12–2.43)

A, L shaped; B, double L; C, square-end; D, rectangular

Blunt and sharp force impact

Figure 7 shows SEM views of the surface and cross sections of blunt (top) and sharp force (bottom) lacerations. In the case of a sharp force impact, there is a clean cut through the skin into the underlying sponge, whose cells are sliced in a linear fashion. In blunt force impact, a tissue plug confined to the skin, but smaller than the impacting rectangle, is formed. Unlike sharp force injury, the underlying sponge is not torn linearly. Rather, it appears that individual cells rupture as a result of hydrostatic pressure increase in the water-filled sponge cells.
https://static-content.springer.com/image/art%3A10.1007%2Fs12024-008-9042-z/MediaObjects/12024_2008_9042_Fig8_HTML.jpg
Fig. 8

At impact, tension increases as the skin stretches at the edges of the rectangle (top). Because of frictional interlock between the surface of the skin and the bottom of the impacting object, sites of maximum tension are located at the angles where cracks are initiated (bottom). The crack follows a diagonal path (see also Fig. 7b)

Discussion

Blunt force impact

The impact of a flat rectangular-shaped or a sharp wedge-shaped object onto the skin model may be considered in terms of the associated contact mechanics. Let us initially consider contact with a blunt object. Here, the initial contact will generate a compaction of the elastic deformable skin layer as well as high pressure within the fluid-filled foam just below the contact area (Figs 2 and 8). The local contact pressures at the surface will be greatest at the edges of the flat punch [18], resulting in areas of high shearing stress in the skin layer adjacent to the contact edges. As the rectangle is driven further into the tissue, the skin layer will be subjected to even higher extensional strains. This will cause slippage between the surface of the blunt object and the skin surface as the strain within the skin attempts to equilibrate (provided the frictional interlock between the skin and blunt object does not prevent such extension of the skin layer). Because of the surface frictional constraint and the highly deformable nature of the skin, extensional strain will be greater on its underside, especially in the vicinity of the contact edges as schematically illustrated in Fig. 8. With increasing displacement of the blunt object into the surface, the strain within the skin layer will eventually exceed a critical value and it will begin to tear diagonally (see Figs 7b and 8—detail). Based upon the frictional constraint argument above, we propose that the skin layer failure is actually initiated from the underside, which will have undergone greatest extensional strains. The SEM images in Fig. 7 suggest that failure was initiated from the local interface between foam struts and silicone skin material and extends toward the surface as shown schematically in Fig. 8, resulting in a footprint of the impacting object, inside which is a characteristic silicone plug with irregular, undermined margins (Figs. 68). We observed this exact pattern in our earlier study of wounds generated by different screwdriver heads on pigskin [1].

When the skin layer has torn, a reduction in contact pressure would be anticipated as the resistance to penetration of the structure to the blunt object would reduce and also the fluid under high pressure within the foam would be able to spurt out through the breach in the skin surface. Upon rebound or when all the energy has been dissipated, the skin layer would attempt to return to its original shape because of the highly elastic nature of this material. Hence, the observation that the residual surface scar of the wound generated by the blunt impact is now much smaller than the dimensions of the impacting object.

Sharp and blunt force impact

Upon initial contact between a sharp edge and the outer skin layer of silicone a highly localized stress concentration occurs within the skin layer (Figs. 2 and 8). The highly elastic nature of this layer results in localized displacement of the contact region with resultant pressure generated in the fluid-filled foam substratum. This localized pressure within the fluid is caused by the incompressibility of the liquid and the limited permeability of the foam. In addition a slight pressure wave also causes the surface layer of the skin adjacent to the contact area to be upwardly displaced and strained.

When the strain in the silicone skin layer exceeds a critical value, it will tear at some point along the sharp contact edge and extend along the length of contact. This localized breaching of the skin layer coupled with the wedging action of the impacting object and the fact that the foam structure is attached to the base of the skin will contribute to further tearing of the foam beneath the line of contact. Skin rupture will logically be followed by a reduction in contact pressure as the resistance to penetration.

That the elasticity of skin and viscosity of underlying tissues are important determinants of blunt force injury has been highlighted by Mohajna et al. [18]. These authors examined blunt force injuries caused by rubber bullets during the Arab-Israeli disturbances of 2000 and found that tissue damage induced by these bullets was largely attributable to direct compression, resulting in a crushing effect compounded by the shockwave generated during impact. This is known as hydrodynamic cavitation, and results in a rapid release of energy [19]. It has also been suggested that the shock-disturbed region will further unload as the magnitude of the shock strength decreases during wave propagation through the tissues [20]. It should be noted, however, that the velocity of impact of these bullets would be appreciably greater than what we have investigated here. Additionally, the penetration velocity would be higher than the shock wave velocity for the rubber bullet. In our study, the pressure wave would be faster than the penetration velocity.

In a previous study, we used our synthetic skin model to investigate wounding caused by a round blunt object [15]. We found that tears (lacerations) of the silicone skin layer resulted from only 48.6% of impacts, yet most (96%) of the impacts produced “internal wounds” or subsurface cavitation. In the current study, all 71 impacts examined resulted in laceration of both the silicone and the underlying foam layers. In summary, our work suggests that because impact by a rectangular or wedge-shaped object will result in stress concentration occurring either at the sharp edge of the wedge or at the 90° angle of the rectangular object both will result in surface laceration. In contrast, round object impact results in peripheral stretching of the skin which may or may not lead to rupture. Subcutaneous wounds also differ in each case (Fig. 9). Impact by a spherical object results in large, cone-shaped subdermal cavitation and associated hydrostatic cellular damage (A), rectangular impacts result in substantive subsurface cellular damage and parallel rupture lines at the level of the angles of the impacting object (B), and wedge-shaped impacts result in a sharp notch or planar crack (C).
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Fig. 9

Damage caused by different impacting bodies. (a) An inverted cone of subsurface cavitation results from round body impact; (b) two subsurface rupture lines results from rectangular body impact; and (c), sharp notch results from wedge-shaped impact

Educational message

  1. 1.

    Forensic traumatology seeks to describe and understand the mechanisms involved in the generation of injury.

     
  2. 2.

    Our simple silicone skin fused to hydrated sponge model is a useful way of investigating the biomechanics of different types of trauma, such as that caused by spherical, rectangular, and wedge-shaped objects.

     
  3. 3.

    Our model suggests that while spherical impact is not always associated with skin rupture and may show substantial underlying cavitation, similar levels of impact from a rectangular or wedge shape always lead to surface laceration.

     
  4. 4.

    When a rectangular body impacts skin, extensional strains will concentrate at the contact edges, on the underside of the skin. This will result in diagonal tearing from the underside. Sharp force impacts always result in a planar crack of the skin and the subdermal tissue.

     

Copyright information

© Humana Press 2008