A question frequently asked of forensic practitioners is ‘how much force was necessary to inflict this particular wound’, and as with other aspects of knife wound interpretation, this question is not straightforward to answer. #
A rough scale of ‘mild pressure’, ‘moderate force’ or ‘extreme force’ is often used, and the pathologist or forensic physician may utilise the presence of hilt-guard bruising, or the penetration of bone as a guide to their description of force.
The traditional teaching in forensic medicine, based on research by Knight (1975) and others, suggested that the only resistance offered by a body to a stabbing implement was that offered by the skin.
Once the skin surface had been breached, the knife would pass through the body with relative ease. It was therefore the sharpness of the knife tip that superseded all other factors (such as momentum and impact velocity) affecting the depth and extent of a stab injury.
Standards for the evaluation of the sharpness of a knife or similar instruments have been published (BS EN ISO 8442-5), and researchers (eg. Andersen et al 1999) have evaluated the puncture resistance of a variety of possible skin simulants, finding that pig skin is stronger than chamois leather under tensile load, but the puncture resistance for the two is almost identical.
These assertions were based on experiments on cadavers using spring-based instrumented knives measuring the amount of force required to pierce skin and various body parts.
The skin covering the chest wall, for example was easier to pierce than that of the abdomen, because of the tightness of the skin stretched over the chest wall like the ‘skin’ of a drum.
Knight found that as little as 5N of force was required to penetrate skin with a sharp knife, and Chadwick et al (1999) found that volunteers could generate up to 2000N of force along the long axis of a blade at impact (more in overhand stabs than thrusts).
The overhand style of stabbing generated more ‘cutting power’, as the knife was dragged down through the skin, and the ‘swipe/ slash’ style generated the largest lateral forces during simulated stabbing attacks.
A common defence assertion is that the victim ‘fell’ onto the knife or that it was an ‘accident’, and Knight’s research seemed to add weight to the validity of some of these claims.
The resistance offered by soft tissues/ muscle fascia etc is now thought to be more significant. Indeed, tissues other than skin can offer resistance enough to slow a moving knife down, requiring the assailant to continue to exert pressure in order to complete a thrust (Horsfall et al 1999; Chadwick et al 1999; O’Callaghan et al 1999).
O’Callaghan et al (1999) found that skin did indeed provide the greatest resistance to penetration (with a mean penetration force of 49.5 N), whilst significant secondary resistive forces were provided by subcutaneous muscle (but not adipose tissue).
They argued that the original research of Knight and Green was not quite accurate, as the spring based instrumented knives used would ‘store’ energy at the point of penetration, which would then be released after penetration, providing a significant amount of energy that would be more than enough to overcome the resistive forces of muscle and fatty tissue, thus giving the impression that the knife was able to pass through such tissues with no appreciable resistance.
The identification of such secondary resistive forces is an important consideration for defences involving thrown knives – the energy possessed by the knife is likely to be dissipated having overcome the resistive forces of skin (and clothing), such that deeper penetration through muscle is less likely.
The subject of the biomechanics in relation to stab injuries is of vital importance to those seeking to protect persons against such injuries, eg. Police services and manufactures of stab-proof clothing. Current research indicates a far more complicated situation than was perhaps previously appreciated.
Cheap kitchen knives can easily break with minimal force, and when such a knife tip strikes bone, the tip may break off, and remain embedded in the bone. X rays of knife wounds and surrounding body parts are thus useful in finding such foreign bodies.
If knife tips are recovered at surgery or during autopsy, they should be retained as trace evidence, and submitted to ‘tool mark’ specialists who may be able to match the fragment to a recovered suspect weapon.
Biomechanical factors that can be assessed in sharp force injuries: #
- The properties of the knife
- Movement of the knife up to the point of impact (affected by clothing and movement of the victim (with respect to the assailant), and from the skin to it’s termination within the body)
- The speed and direction of delivery of the blow
- Intrinsic properties of the knife – shape/ sharpness/ weight
- Delivery of the blow – velocity/ type of thrust and ‘follow through’ (‘over arm’ vs. ‘under arm’)
- Movement of the knife within the body – affected by skin/ organ resistance, movement of the victim and whether bone has been impacted
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Read more: https://forensicmed.webnode.com/wounds/sharp-force-trauma/stab-wounds/