Running with knives

Stabbing is the most common form of murder in the UK and Ireland. However, while forensic scientists understand the basics of the process – a sharp implement lethally penetrating flesh – little is known of the mechanical forces involved. A way to determine quantitatively how hard a victim was stabbed could provide a clear picture of the suspect’s intent to harm.

Now, deputy pathologist Michael Curtis, Marie Cassidy, who both have a high public profile in Ireland as well as being leaders in the international field of forensic science, engineers Michael Gilchrist and Michel Destrade, together with Stephen Keenan and Greg Byrne, have developed a device that can analyse the mechanics of knife stabbing. The team has now used four commonly available household knives with different geometries but all having single-edged, double-sided, non-serrated blade tips to test penetration of simulated flesh composed of polyurethane (skin), compliant foam (fat) and ballistic soap (cartilage).

Dr Michel Destrade

Their findings are rather surprising in that they reveal how skin tension and direction of blade penetration can affect the end result significantly. Less force and energy are also required to puncture the skin when the plane of the blade is parallel to a direction of greater skin tension than when perpendicular. This is consistent with the observed behaviour when cutting biological skin: less force is required to cut parallel to the Langer lines than perpendicularly and less force is required to cut when the skin is under a greater level of tension, the researchers say.

Moreover, even the same manufacturer and model of knife can produce very different results. The team have obtained evidence that the quality control processes used to manufacture knives fail to produce consistently uniform blade points in knives that are nominally identical. They say that the consequences of this are that the penetration forces associated with purportedly the same model of knife can vary by as much as 100%.

Knife (Photo by David Bradley)

Medical witnesses at murder trials involving a knife wound are usually asked what force would have been required to produce the given wound. Mild force would usually be used to describe penetration of skin and soft tissue whereas moderate force is needed to breach cartilage or rib bone. A severe force, that would lead to damage of the knife itself, is typical if dense bone, such as spine, is stabbed.

The work by Gilchrist and colleagues now provides a more quantitative measure for the force required to penetrate different tissues and protective clothing. So, that a less subjective measure of mild, moderate and severe can now be described. The researchers explain that their research could provide insight into the much used defence that the victim ran on to the knife!

M GILCHRIST, S KEENAN, M CURTIS, M CASSIDY, G BYRNE, M DESTRADE (2008). Measuring knife stab penetration into skin simulant using a novel biaxial tension device Forensic Science International DOI: 10.1016/j.forsciint.2007.10.010

Further reading

Suggested searches