Quantification of forces required for stabbing with screwdrivers and other blunter instruments

Low-velocity nail penetration response of muscle tissue and gelatin

Quantitative estimation of soft tissue injuries due to penetration of sharp objects is a challenging task for forensic pathologists. The severity of injury depends on the force required to penetrate the tissue. This study focuses on investigating the amount of force required to penetrate porcine muscle tissue and gelatin simulants (10 % wt) to mimic human muscle tissue when subjected to sharp objects like nail at velocities below 5 m/s. A custom-made experimental setup was used to examine the influence of penetration velocity and nail diameter on penetration forces. Images captured by a high-speed camera were used to track the position and velocity of the nail. A finite element (FE) model was established to simulate the penetration behavior of the tissue and gelatin. The FE simulations of the nail penetration were validated through direct comparison with the experimental results. In tissues as well as in the simulant, penetration forces were seen to increase with increasing nail diameter and velocity. Porcine muscle tissue showed 23.9-46.5 % higher penetration forces than gelatin simulants (10 % wt) depending on nail diameter and velocity; the difference being higher for higher nail diameter and velocity. The ranges of maximum penetration forces measured were 8.6-59.1 N for porcine muscle tissue and 6.8-34.9 N for gelatin simulant. This study helps to quantify injuries caused by sharp nails at low velocities and offers insights with potential applications in injury management strategies and forensic studies.

In vitro skin puncture methodology for material characterization

Quantifying the mechanical behavior of skin has been foundational in applications of cosmetics, surgical techniques, forensic science, and protective clothing development. However, previous puncture studies have lacked consistent and physiological boundary conditions of skin. To determine natural skin tension, excision of in situ porcine skin resulted in significantly different diameter reduction (shrinkage) in leg (19.5 %) and abdominal skin (38.4 %) compared to flank skin (28.5 %) (p = 0.047). To examine effects of initial tension and pre-conditioning, five conditions of initial tension (as percentage of diameter increase) and pre-conditioning were tested in quasistatic puncture with a 5 mm spherical impactor using an electrohydraulic load frame and custom clamping apparatus. Samples with less than 5 % initial tension resulted in significantly greater (p = 0.011) force at failure (279.2 N) compared to samples with greater than 25 % initial tension (195.1 N). Eight pre-conditioning cycles of 15 mm displacement reduced hysteresis by 45 %. The coefficient of variance was substantially reduced for force, force normalized by cutis thickness, displacement, stiffness, and strain energy up to 46 %. Pre-conditioned samples at physiological initial tension (14-25 %) resulted in significantly greater (p = 0.03) normalized forces at failure (278.3 N/mm) compared to non-conditioned samples of the same initial tension (234.4 N/mm). Pre-conditioned samples with 14-25 % initial tension, representing physiological boundary conditions, resulted in the most appropriate failure thresholds with the least variation. For in vitro puncture studies, the magnitude of applied initial tension should be defined based on anatomical location, through a shrinkage experimentation, to match natural tension of skin. Characterizing the biological behavior and tolerances of skin may be utilized in finite element models to aid in protective clothing development and forensic science analyses.

Skin simulants for wound ballistic investigation - an experimental study

Gunshot wound analysis is an important part of medicolegal practice, in both autopsies and examinations of living persons. Well-established and studied simulants exist that exhibit both physical and biomechanical properties of soft-tissues and bones. Current research literature on ballistic wounds focuses on the biomechanical properties of skin simulants. In our extensive experimental study, we tested numerous synthetic and natural materials, regarding their macromorphological bullet impact characteristics, and compared these data with those from real bullet injuries gathered from medicolegal practice. Over thirty varieties of potential skin simulants were shot perpendicularly, and at 45°, at a distance of 10 m and 0.3 m, using full metal jacket (FMJ) projectiles (9 × 19 mm Luger). Simulants included ballistic gelatine at various concentrations, dental silicones with several degrees of hardness, alginates, latex, chamois leather, suture trainers for medical training purposes and various material compound models. In addition to complying to the general requirements for a synthetic simulant, results obtained from dental silicones shore hardness 70 (backed with 20 % by mass gelatine), were especially highly comparable to gunshot entry wounds in skin from real cases. Based on these results, particularly focusing on the macroscopically detectable criteria, we can strongly recommend dental silicone shore hardness 70 as a skin simulant for wound ballistics examinations.

A probabilistic model for murder weapon identification using stab-marks in human ribs

The aim of this article is to provide a scientific and statistical basis to identify the murder weapon in stabbing cases from the geometric characteristics of the stab-marks left on human ribs. For this purpose, a quantitative predictive model is developed, based on geometric measurements of the stab-mark and its location along the rib. A general method based on Bayesian inference and probabilities is used for the model development, rather than a deterministic model given its inability in certain occasions to identify the murder weapon. Following the process explained in this article to collect the stab-mark information required, the complete probabilistic model exposed attained a high accuracy in the identification of the murder weapon between two macroscopically identical blades with a microscopic alteration in one of them (more than 90% of correct identification is achieved).

Patterned injuries from screwdrivers

A 40-year-old man was stabbed multiple times with a cross-tipped, Phillips head screwdriver with many of the puncture wounds characterized by a cruciate pattern consistently measuring approximately 5 × 5 mm corresponding to the shape of the weapon. Death was due to a single penetrating wound to the chest that had transfixed the aorta. This report characterizes the features of injuries that may be inflicted by Phillips head screwdrivers, contrasting this with injuries that may occur if the weapon is a flat or slotted head screwdriver. Given their ready availability, ease of handling, and sharpness, screwdrivers are surprisingly rarely used in fatal assaults.

Providing a Forensic Expert Opinion on the "Degree of Force": Evidentiary Considerations

Forensic pathologists and anthropologists are often asked in court for an opinion about the degree of force required to cause a specific injury. This paper examines and discusses the concept of 'degree of force' and why it is considered a pertinent issue in legal proceedings. This discussion identifies the implicit assumptions that often underpin questions about the 'degree of force'. The current knowledge base for opinions on the degree of force is then provided by means of a literature review. A critical appraisal of this literature shows that much of the results from experimental research is of limited value in routine casework. An alternative approach to addressing the issue is provided through a discussion of the application of Bayes' theorem, also called the likelihood ratio framework. It is argued that the use of this framework makes it possible for an expert to provide relevant and specific evidence, whilst maintaining the boundaries of their field of expertise.

An evaluation of human stabbing performance to inform the standardisation of textile damage examinations: Do simulation trials correlate to reported stabbings?

Forensic textile damage examinations are commonly requested in cases such as stabbings. These requests often involve the testing of knives or other weapons submitted to determine if they could have caused the damage to the evidential garment. Currently a forensic practitioner conducts this testing by manually performing the stabbing action. A biomechanics performance trial was conducted to evaluate how a range of human factors contribute to the creation of textile damage by stabbing actions. Surveys of sharp force fatalities and clinical penetrative injuries reported the chest and abdomen as the most frequent target location for stab wounds. The location of the cut-type damage recorded during the trial was found to correlate to the location of stab injuries incurred during actual stabbing cases. The type of weapon had an impact on the actions undertaken. Participants mostly utilised the smaller utility and hunting knives in underarm thrusting or overarm hacking actions, whereas an overarm hacking action, or combined hacking/slashing action was performed when using the machete. The familiarity of the knife, shape of the handle and perceived risk of injury determined how the handle was held. Participants frequently stabbed into the target immediately in front of their dominant hand, however care should be taken in interpreting this in a casework scenario. The machete was used with the highest mean velocity, and the utility knife the lowest.

Evaluating the evidentiary value of the analysis of skeletal trauma in forensic research: A review of research and practice

Understanding bone trauma characteristics is a fundamental component of forensic investigations that can assist in understanding the nature of blunt trauma related deaths. The variation of each blunt force trauma (BFT) injury is dependent on a magnitude of factors including, age, sex, health, angle of impact, impact mechanism, impact force and clothing, making BFT one of the more difficult area of trauma to interpret solely based on skeletal fractures. A detailed literature review was performed to assess the value and scientific rigour of the current research in forensic anthropology, forensic pathology and biomechanics that attempts to provide an objective framework in which forensic practitioners can assess and interpret BFT injuries. Four areas of research which investigate the analysis of BFT are examined. These included research involving experiments on animal models; experiments on human models (and synthetic models); computer modelling, and research/publications including 'mild', 'moderate' and 'severe' as descriptions of impact force, resulting from trauma. Also briefly discussed is how BFT research is framed within medicolegal contexts. While many published works have contributed to the understanding of the biomechanics of BFT, more research that can provide an objective means to accurately assess and interpret BFT injures is required.

Edged weapons awareness

Edged weapons (sometimes referred to as sharp weapons or blades) are an increasing threat to military personnel, the blue light community (police, ambulance, firefighters, other first responders) and the general public worldwide. The use of edged weapons in criminal and terrorist incidents internationally means the forensic community needs an awareness of the technology of edged weapons, how they are used, the damage (clothing and wounding) that might be caused and any other forensic implications. In this paper, the magnitude of the problem is presented, prior research summarised and implications for forensic investigations discussed.

Injury potential of thrown sharp kitchen and household utensils

We examined the possibility of inflicting serious injuries with sharp objects in an experimental setting by throwing four sharp objects from different distances and with different throwing techniques. Using an overarm-handle (OA/H), overarm-blade (OA/B), underarm-handle (UA/H), underarm-blade (UA/B) and thrust (T/H) throwing technique, 10 adults (sex ratio 1:1) threw a chef's knife, a skinning knife, a paring knife and office scissors from 4 m and 2 m distance at synthetic abdomen models made of 10% gelatin covered with synthetic skin. The amount of hits and penetrations of the target and their penetration depth were noted, as was the rotation of the blade tip towards the target along its flight trajectory. The chef's knife injury/hit ratio was 0.167/4 m and 0.160/2 m; the skinning knife recorded an injury/hit ratio of 0.087/4 m and 0.153/2 m; the paring knife of 0.087/4 m and 0.113/2 m; and the scissors 0.087/4 m and 0.067/2 m. Mean penetration depths were as follows: the chef's knife: 4 m, 4.42 cm, 2 m, 3.41 cm; the skinning knife: 4 m, 4.19 cm, 2 m, 4.11 cm; the paring knife: 4 m, 1.62 cm, 2 m, 1.55 cm; and the scissors: 4 m, 2.08 cm, 2 m, 0.95 cm. Handle-throw penetration-depths were: 4 m: 3.77 cm and 2 m: 2.86 cm; blade-throw depths were: 4 m: 3.14 cm and 2 m: 2.69 cm. Overarm-throw penetration-depths were: 4 m: 3.62 cm and 2 m: 3.25 cm; and underarm-throw penetration-depths were 4 m: 3.30 cm and 2 m: 2.30 cm. No thrust-throws with the paring knife and scissors could pierce the target. The tips pointed toward the target at angles of 60°-120°, earlier in handle-throws than blade-throws, especially with the paring knife and the scissors. When thrown, especially with a handle-held technique, heavier objects pierced more often and more deeply. Thrust-throws at short distances are unlikely to pierce a human.

Kinetic and kinematic analysis of stamping impacts during simulated rucking in rugby union

Laceration injuries account for up to 23% of injuries in rugby union. They are frequently caused by studded footwear as a result of a player stamping onto another player during the ruck. Little is known about the kinetics and kinematics of rugby stamping impacts; current test methods assessing laceration injury risk of stud designs therefore lack informed test parameters. In this study, twelve participants stamped on an anthropomorphic test device in a one-on-one simulated ruck setting. Velocity and inclination angle of the foot prior to impact was determined from high-speed video footage. Total stamping force and individual stud force were measured using pressure sensors. Mean foot inbound velocity was 4.3 m ∙ s^-1 (range 2.1-6.3 m ∙ s^-1). Mean peak total force was 1246 N and mean peak stud force was 214 N. The total mean effective mass during stamping was 6.6 kg (range: 1.6-13.5 kg) and stud effective mass was 1.2 kg (range: 0.5-2.9 kg). These results provide representative test parameters for mechanical test devices designed to assess laceration injury risk of studded footwear for rugby union.

Traumatic Brain Injury Due to Screwdriver Assaults: Literature Review and Case Report

Penetrating head injuries due to the use of screwdrivers as wounding agents in acts of interpersonal violence seldom occur. The aim of this article is to update and summarize the relevant literature on penetrating craniocerebral screwdriver stab wounds and to report a new case of screwdriver assault. A number of studies were reviewed to investigate the incidence, distribution, common findings, mechanism of injury, differential diagnostic criteria, complications, treatment, and prognosis of craniocerebral screwdriver stab injuries. It was observed that the degree of traumatic severity depends on the cross-sectional area of the screwdriver and the anatomical region of injury. Craniocerebral screwdriver injuries are mainly cases of interpersonal violence and the mortality rate is approximately 47.6%. In 23.8% of the incidents, the trauma is overlooked on admission because of the small entry wound and, thus, the severity of the injury is not initially appreciated.

Forensic Veterinary Pathology

Sharp-force injuries are injuries caused by a mechanical force using sharp objects against the skin. Sharp-force injuries are mainly classified as stab, incised, chop, and therapeutic wounds and are less frequent than blunt-force injuries in animals. The analysis of the edges of the wound is crucial, especially if more than one type of lesion is involved. It may be difficult to differentiate between sharp trauma and blunt trauma, because lacerations can resemble incised wounds. The accurate documentation and examination of these injuries may indicate the instrument involved, the relationship between the animal and the perpetrator, and the force of the stab. Situations in which this type of trauma occurs may involve social violence, accidents, hunting, veterinary medical management, and religious rituals. The causes of death related to this type of trauma include hypovolemic shock, pneumothorax, or asphyxiation due to aspiration of blood. Necropsy findings should provide objective and unbiased information about the cause and manner of death to aid the investigation and further judgment of a possible crime.

Investigation of dental materials as skin simulants for forensic skin/skull/brain model impact testing

PURPOSE

The purpose of this study was to measure the tear strength and hardness of four different dental silicones in comparison to that of porcine skin.

METHODS

Specimens were prepared (n = 20/group) according to ASTM D624-00, using three hydrophilic vinyl polysiloxane impression materials, one duplication silicone, and fresh porcine skin. A universal testing machine was used to strain each test specimen until complete rupture and calculate its tear strength (kNm(-1)). Failure analysis was then conducted using a stereoscopic zoom light microscope, as well as a scanning electron microscope (SEM). A shore A-type durometer was used to measure the hardness of all specimens.

RESULTS

The tear strength for the silicones ranged from 1.75 to 9.58 kNm(-1) and the pigskin from 3.65 to 56.40 kNm(-1). The mean shore hardness for the silicones ranged from 16.275 to 62.65DU and the pigskin had a mean shore hardness of 22.65DU, with p values <0.0125 (0.05/4). Failure analysis of the silicone materials showed the origin of failure being in the tension side of the specimens and typical failure patterns were observed. Examining the materials under a SEM revealed that materials with higher viscosity presented with a larger amount of filler particle content than silicones with low viscosity, with the duplication silicone having no filler content.

CONCLUSION

Dental silicones are a good alternative for skin in studies that require a skin simulant.

Quantification of subconcussive impact forces to the head using a forensic model

Concussive and subconcussive head injury is a global phenomenon that affects millions of people each year. Concussive injury has been extensively studied in sport, which has led to a greater understanding of the biomechanical forces involved and guidelines aimed at preventing athletes from playing while concussed. Subconcussive forces by definition do not meet the threshold for concussion but nonetheless may have significant long term consequences due to the repetitive pattern of injury to the head. Quantifying these impact forces using a forensic head model provides the groundwork for future studies by establishing a range or threshold of subconcussive impact forces that could be correlated with clinical assessments. The use of a forensic head model has distinct advantages in terms of ethics and safety.