1. Identifying Sharp-Force Trauma-Serrated vs. Non-Serrated Blades
Martha Kools, Taylor Maddox and Carlos Sanchez
Methods
Results
Abstract
The purpose of this study is to determine the accuracy of techniques devised to identify characteristics of knives used to inflict damage on bone. According to the FBI’s Unified Crime Report, of the nearly 12,000 people murdered in 2014, over 1,500 were killed by sharp force trauma with cutting instruments. While much research has been dedicated towards understanding the different effects on bone caused by sharp and blunt force trauma, forensic anthropology is only beginning to differentiate between individual weapons. Currently, perimortem trauma associated with knives is understood to present as predominantly clean, linear puncture damage, rarely exhibiting radiating or concentric fractures; however, it has been observed that different types of knives create distinctive features. Thompson and Inglis (2009) found that fracture patterns from stab marks could indicate the use of either a serrated or non-serrated blade. To test this hypothesis, this study will measure stab wounds inflicted by a third party on the ribs of a domestic pig, and identify them as either serrated or non-serrated. An error rate will then be calculated to determine the overall precision of the technique.
Table 1. Stab mark observations from the naked eye. Kerf damage: a subjective five-point scale ranging from zero (no damage) to five (extensive damage) as defined by Thompson and Inglis (2009).
For this study fifteen ribs were used to test the stab marks created by the five kitchen knives. Each knife was collected from a set of common kitchen knives, (Brand: Two VonChef knives both serrated/non serrated and one Rada Cutlery serrated steak knife) and given a number in order to differentiate the marks. We first gathering the dimensions of the knives as well as each rib (ribs were also given a number to identify with). Dr. Misty Weitzel then generated the stab wounds randomly on the ribs, making note of which knife was used on which rib for the team to review after the experiment, and compare results. This was a blind study, including two rounds of stab wounds in order to strive for more accurate results. The ribs were boiled in hot water for 1.5 hours to clean the tissue off the bone. The team examined each rib with the cut marks and gathered the dimensions of the damage. Using microscopic measurements, the team used techniques provided by Thompson and Inglis (2009), to determine the individual knife that inflicted the wound. Measurements included length, width, kerf damage, and a general description of the damage. The fracture pattern of the cut mark was also examined. This is due to the fact that you can also determine the weapon used with an in-depth analysis of the fracture patterns. Results of the examination were recorded in the table under the “results” section.
Rib No.
Observations
Length (mm)
Width (mm)
Kerf Damage 1
No trauma observed.
N/A 2
Single shallow fracture located on the anterior side of the costal neck.
3.0
< 0.5 3
Small puncture trauma located on the posterior-proximal side of the costal shaft.
2.5
0.5 4
Single shallow fracture located on the anterior-proximal side of the costal shaft.
4.5 5
Single interrupted fracture located on the superior-proximal side of the costal shaft.
12.5 6
Three fractures with varying traits:
Interrupted deep fracture starting on the superior-anterior and ending on the superior-posterior side of the costal neck.
Single shallow fracture located on the superior side of the costal shaft at the midline.
Deep fracture located on the superior-distal side of the costal shaft with a small parallel fracture on the posterior end.
Fracture #
1- 0.5
2- < 0.5
3- 1.5
1- 3
2- 1
3- 3 7
Two fractures with varying traits:
Single shallow fracture located on the superior-anterior side of the costal shaft at the midline.
Single deep fracture located on the superior-distal side of the costal shaft.
1- 2.0
2- 7.5
1- < 0.5
2- 0.5
1- 1
2- 2 8
Single shallow fracture located on the superior-posterior side of the costal shaft.
Small puncture trauma located on the posterior side of the costal shaft at the midline.
Single deep fracture located on the inferior-posterior side of the costal shaft.
1- 6.0
2- 2.0
3- 4.0
2- 1.5
3-1.0
2- 3
3- 2 9 10 11
Single deep fracture located on the superior side of the costal shaft at the midline.
5.5 12
Two shallow parallel fractures located on the posterior side of the costal shaft.
1- 3.0 13
Single deep fracture located on the superior-distal end of the costal shaft.
Single shallow fracture located on the inferior side of the costal shaft at the midline.
2- 5.5
3- 6.5
2- 1.0
3- 0.5
3- 1 14 15
Discussion / Conclusion
While the initial purpose of this study was to determine the accuracy of identifying sharp-force trauma as having originated from either serrated or non-serrated blades, as defined by Thompson and Inglis (2009), the results ended up revealing an unforeseen conclusion. Due to several mechanical issues on the day of data collection, length and width measurements had to be estimated and fracture patterns were unable to be observed at a microscopic level. Because of these limitations, calculating the accuracy of techniques used in differentiating knife characteristics was impossible. Instead an interesting factor became evident in comparing soft tissue trauma to the damage inflicted on the individual ribs.
As apparent in Figure 1, many of the stab marks initially impacted the bone, but then took the path of least resistance through the soft tissue into what would be the chest cavity in a live subject. This left a minimal amount of visible trauma on the bone itself, observed predominantly as shallow linear fractures, which did not indicate the possible magnitude of corresponding soft tissue injury. It could be deduced that in some situations, while skeletal sharp-force trauma may appear to be low, the associated soft tissue damage could be much greater, potentially even fatal. Further studies could examine this phenomenon by using complete specimens in testing, with the goal of measuring damage inflicted on organs and the vascular system. Findings would provide a crucial understanding of sharp-force trauma that could prevent a mistaken dismissal of possible causes of death in a forensic investigation of skeletal remains.
Introduction
Forensic anthropologists are becoming better at identifying the type of weapon used in a crime scene investigation. One of the many skills forensic anthropologists have is to determine the cause of death through identifying the magnitude of sharp force trauma. According to the FBI’s Uniform Crime Report, in 2014 over 1,500 people were killed by sharp force trauma with sharp cutting objects. Forensic Pathologist, Dr. Joseph Prahlow, has determined three different types of wounds found in sharp force trauma. They are stab, incise, and chop wounds. In our experiment we focused on stab marks left in the bone. Previous work has focused on interpreting sharp force trauma from bone in order to determine the cause of death in both forensic and archaeological contexts (Thompson and Inglis 2009). Currently forensic anthropologists are typically asked to determine the type of weapon used that produced a sharp force trauma defect in the bone, rather than match the morphology of a defect to a specific weapon (Crowder 2009).
Bone often preserves the evidence of sharp force trauma much longer than the soft tissues (Thompson and Inglis 2009). Cut marks can be seen by the naked eye, however microscopes have proven to be better at analyzing the cut marks (Thompson and Inglis 2009). The evidence of cut marks in the bone may provide a clue to investigators in identifying the weapon used. Therefore understanding the differences between serrated and non-serrated cut marks in the bone will lead forensic pathologists and investigators to not only determine the type of weapon, but also guide them to the specific weapon. The purpose of this research is to measure stab wounds inflicted by a third party on the ribs of a domestic pig, and identify them as either serrated or non-serrated.
. Figure 1. Significant soft tissue trauma observed between ribs #10 and #11 with minimal sharp force fractures apparent once soft tissue was removed
References
Thompson J. Inglis,“Differentiation of serrated and non-serrated blades from stab marks in bone” (2009):
Christian Crowder et al. “Microscopic Analysis of Sharp Force Trauma in Bone and Cartilage: A Validation Study” (2009): 3-7
Joseph A Prahlow, “Forensic Autopsy of Sharp Force Injuries,” (2014): 1-2.
Figure 2. Rib #10 showed no apparent damage and Rib #11 had short fracture on inferior surface.