From the *Department of Pathology, London Health Sciences Centre; †Western University; and ‡School of Dentistry, Schulich School of Medicine and Dentistry, London, Ontario, Canada
Manuscript received July 24, 2013; accepted October 27, 2013.
Financial support was received through the Pathology Internal Funds for Academic Development.
The authors report no conflicts of interest.
Reprints: Michael Shkrum, MD, Department of Pathology, London Health Sciences Centre, University Hospital, 339 Windermere Rd, London, Ontario, Canada N6A 5A5. E-mail: firstname.lastname@example.org.
Tool mark analysis is a method of matching a weapon with the injury it caused. In a homicidal stabbing using a serrated knife, a stab wound that involves a cartilage may leave striations from the serration points on the blade edge. Assessing tissue striations is a means of identifying the weapon as having a serrated blade. This prospective study examines the possibility that similar striations may be produced in human soft tissues. Using tissues taken at the time of hospital-consented autopsies, stab wound tracks were assessed in a variety of human tissues (aorta, skin, liver, kidney, and cardiac and skeletal muscle). Stab wounds were produced postmortem with similar serrated and smooth-edged blades. The walls of the stab wounds were exposed, documented by photography and cast with dental impression material. Striations were identified by naked-eye examination in the skin and aorta. Photodocumentation of fresh tissue was best achieved in the aorta. Striations were not identified in wound tracks produced by the smooth-edged blade. Three blinded forensic pathologists were assessed for their ability to detect striations in photographs of wound tracks and had substantial interobserver agreement (κ = 0.76) identifying striations. This study demonstrates that tool mark striations can be present in some noncartilaginous human tissues.
Tool mark analysis plays an important role in the analysis of traumatic injury in forensic pathology as it is a way of matching a weapon with the injury it caused. Stab wounds produced by serrated blades are generally indistinguishable from stab wounds produced by smooth-edged blades, except when the serrated blade is drawn obliquely across the skin surface, creating parallel superficial cuts from the serrated points.1 Others have reported that stab wounds involving a cartilage can leave striations on the cut surface from the serration points on the blade edge.2,3 Infrequently, a serrated blade may result in a stab wound with ragged edges to the skin margins.3,4 The same findings have not been described in noncartilaginous human tissue. Recently, the presence of striations produced by a serrated blade has been reported in animal soft tissues.5 Assessing the walls of the stab wound for tissue striations may be a means of identifying the weapon as having a serrated blade. This study evaluated the presence of tool mark striations in stab wounds produced posthumously by serrated and smooth-edged blades in the following fresh human tissues: kidney, aorta, skin, liver, and skeletal and cardiac muscle obtained at the time of hospital-consented autopsies. These tissues have been chosen as there is a high frequency of involvement of the heart and aorta, liver, kidney, and limbs in homicidal stabbings.6–10
MATERIALS AND METHODS
Following ethics approval, a prospective analysis of stab wound track in fresh human tissues (aorta, skin of the anterior chest, liver, kidney, skeletal muscle, and left ventricular myocardium), from hospital-consented adult autopsies, was performed. During the autopsy, a portion of each tissue type was removed from the body and was pinned to a block of florist foam. Stab wounds were created using knives with similar blade dimensions and a straight spine. One knife had a finely serrated edge, and the other was smooth-edged (Fig. 1). The stab wounds were made by the author (R.J.) using a single in-and-out overhand thrusting motion that was perpendicular to the surface of the tissue and, in the case of muscle, at right angles to the muscle fibers. The total numbers of stab wound tracks produced by each blade were 10 for skin and aorta, 6 for kidney, and 5 for skeletal and cardiac muscle. Following the stabbing, the walls of the stab tracks were exposed with a scalpel blade and assessed by naked-eye examination. Then the wound track was documented by a 1500-pixel digital camera with a white color balance under oblique lighting. Following this, the tissue was cast with polyvinylsiloxane (PVS) dental impression material. Photographs of the walls of the wound tracks produced by both serrated and smooth-edged blades of all tissue types were randomly inserted into a PowerPoint presentation. Photographs that were not optimally focused were excluded. Using these photographs, 3 blinded forensic pathologists were assessed whether they could independently see striations in the various tissues.
The serrated blade consistently produced striations that were readily evident by naked-eye examination in the skin and aorta (Fig. 2A). Striations were not easily visualized in the skin when peripheral edema was noted. Striations were not identified in the liver, kidney, and skeletal and cardiac muscle. The smooth-edged blade did not produce striations (Fig. 2B). Photodocumentation of the skin striations was affected by both the obscuring nature of the natural lines/creases in the skin and its pale color. The forensic pathologists, who were blinded to the knife type, had substantial interobserver agreement (κ = 0.76) and accurately identified the striations in the aorta. The PVS casting technique was not successful in capturing striations. Only a single stab wound track of the skin produced by a serrated blade showed some faint cast markings. Many of the molds showed features intrinsic to the tissue type.
In this preliminary study of stab wounds using both serrated and smooth-edged knives, striations on the walls of stab tracks were produced using the serrated blade. The striations were evident by naked-eye examination most frequently in the skin and aorta. There was substantial interobserver agreement among the 3 blinded forensic pathologists regarding the assessment of striations in the wound tracks. In particular, the presence of striations produced by the serrated blade in the aorta was confirmed. However, wound tracks produced by the serrated blade did not show visible striations in the liver, kidney, and skeletal and heart muscle. The difference may reflect the degree of water content.11 Retention of striations in the skin was limited when peripheral edema was noted at the time of autopsy. The higher water content may have affected the ability to retain the fine compression of the subdermal tissue produced by the serrations.11 This factor may have been further exacerbated because the study tissues originated from patients who had been in the intensive care unit and consistently had either been resuscitated with large volumes of fluids or progressed to terminal multiorgan system failure. Selecting tissues from hospital cases without risk factors for edema such as congestive heart failure, renal failure, and resuscitation with massive volumes of fluids, and so on, would have been more representative of the real-world scenario of rapid death following a stab-related injury; however, there were no such cases during the restricted study period. In addition, the sample sizes of kidney, liver, and muscle tissues were less than the aorta and skin. In future studies, choosing tissue with less hydration, increasing the sample size of the other tissue types, and using different angles may provide more opportunities to identify striations.
The smooth-edged blade failed to produce identifiable striations in any of the tissue types. This supports the conclusion that the presence of striations in a wound track may be produced by a serrated blade but that the absence of striations does not ensure that the blade was smooth-edged. In particular, striations in the aorta and skin could identify the weapon as having a serrated blade in our study.
Skin striations on edematous and pale tissues were difficult to document photographically. Also, natural lines/creases in the skin obscured the striations. This may be overcome by allowing the tissue to fix in 10% buffered formalin following exposure of the wound track.12 Formalin may help firm and sharpen the edges of the striations and enhance capturing these patterns by photography. Although fixation may result in tissue shrinkage, the striations should still be readily identifiable and maintain their class characteristics.12 Using a high-quality macro lens on the digital camera may improve the capture of striations in less than optimal tissues. In the animal studies, a coarsely serrated blade was used.5 We opted to use a finely serrated knife because a comparable smooth-edged blade with dimensions similar to the serrated blade could be acquired. In future studies, using a coarsely serrated blade may provide easily detectable striations in the tissues. Finding a smooth-edged blade with similar dimensions as the coarsely serrated blade may be difficult. This can be resolved by having 2 similar serrated blades and altering the serrated edge into a smooth edge with a bench grinder and sharpening stone.
A permanent copy of the wound track using PVS and casting was challenging and technique dependent. The casting materials were chosen to maximize ease of manipulation and production of accurate impressions. Different casting materials could be used, but they would be required to produce a stable impression without dimensional change5 in a morgue environment.5 The combination of the hydrophobic qualities of the PVS material and the variable water content in the tissues may have contributed to the difficulties encountered.
Since homicidal stabbings have increased in some regions, even surpassing gun-related homicides,13 tool mark analyses of stab wound tracks at the time of autopsy have the potential for focusing investigative efforts in searching for more specific knives.13 Given that striations could be identified in the skin and aorta in this study, a search for these features in stabbing-related deaths can be of value in assessing whether the weapon used was serrated.
Although this study showed there are limitations in the postmortem tool mark assessment of stab wound tracks, continued research efforts in refining techniques in documenting serrations in stab wound tracks will contribute to enhancing the validity of this type of tool mark analysis. This study was faced with multiple technical challenges and was time consuming. In a busy forensic pathology service, the amount of time required to document striations may not be feasible until the technique is improved to allow its integration into daily practice.
The authors thank A-Z Microscopes and Dr D. Ramsay for photographic assistance; Mr Rejean Royer for assisting in troubleshooting technical problems with the knife selection; and Drs E. Tweedie and E. Tugaleva for participating as 2 of the blinded forensic pathologists.
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