Forensic Science, 6 (1975) 249-255 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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THE DYNAMICS OF STAB WOUNDS

BERNARD KNIGHT

Welsh National School of Medicine, University of Wales, Cardiff (Gt. Britain)

(Received November 10,1975; accepted January 2,1976)

INTRODUCTION

Knife stabbing is a relatively frequent cause of both serious wounding and homicide in Britain. It arises in two main situations, firstly in youthful gang fights, football hooliganism etc., where the stabbings are not often fatal; secondly, in domestic disputes between husband and wife, where death is often the final consequence.

When such cases come to trial in a criminal court, an almost inevitable question is asked of the medical witness by either advocate or judge. This question is “What degree of force would be necessary to inflict the given wound?“.

This question is almost impossible to answer, as an estimate of the degree of force is totally subjective. Excluding the obvious extremes of “a slight prick” and a “ferocious lunge”, a verbal description of the force required to penetrate the body with the weapon is meaningless, as every witness will have his own criteria of what constitutes varying degrees of pressure.

In the majority of stabbing homicides in which the author has been involved as an expert witness, a defence has been raised that the injured person fell upon the passively held knife because of some accidental tumble or trip. This possibility becomes a matter for discussion in the court and once again estimates of the dynamics of the penetration of the body become confused in verbal descriptions.

It is patently obvious that “degree of force’9 means different things to different people, not excluding pathologists. In an attempt to clarify some of the basic factors involved, some experiments were conducted under con- trolled conditions. This work was originally stimulated by a murder trial involving a domestic stabbing, in which the defence lawyers wished to pursue the possibility of a fall upon a knife by a drunken woman. The experiments made for this particular case were later extended into a more general inves- tigation. The object was not to attempt an exact mathematical exercise, but to provide generalisations that could be placed before judge, jury and advocates in court.

APPARATUS AND METHODS

In an attempt to remove subjective estimates a simple apparatus was constructed which would give a quantitative measure of the pressure applied

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to the knife. This consisted of a strong Perspex box, within which a smaller box would slide freely, similar to the piston in a syringe. The handles of a variety of knives could be rigidly wedged into the inner box. Two external springs resisted the relative movement of the two parts of the apparatus. To calibrate the device, weights were added to the outer box and a scale marked off to indicate the resistance in kilograms. In essence, the outer box formed an enlarged handle to the knife and provided a means whereby the minimum pressure for penetration of the skin could conveniently be measured.

Using this device, several hundred penetrating incisions were made in cadavers immediately before routine medicolegal autopsy. In order to avoid any unnecessary marking of the bodies, the experimental wounds were almost always made in the line of the routine autopsy incision: the most used position was in the mid-line of the upper abdomen, a common site for actual criminal stabbings. Other incisions were made down the mid-line of the abdomen and some were made in the intercostal spaces as near to the mid-line as possible, so that they could be incorporated in the routine autopsy incision.

Attempts were made to use a simulated model of skin and underlying tissue, by means of varying thicknesses of polyethylene “epidermis” over a plastic-foam “dermis”. However, this appeared to be unsatisfactory simula- tion of human tissue and was abandoned.

The majority of experimental incisions were made upon the supine cadaver on an autopsy table, but a series of experiments was conducted where the body, in rigor mortis, was allowed to fall forwards from the direct position upon the rigidly held blade.

RESULTS

The presentation of numerous quantitative measurements in differing circumstances would appear to be of little value, as the investigation revealed that wide variations in force arise mainly from differences in the cutting point of the weapon, which would not be reproducible from operator to operator. However, it can be said that with a very sharply-pointed knife, penetration of the abdominal skin and subcutaneous tissues usually required between a half and three kilograms pressure. The following findings were recorded:

Sharpness of the knife This factor over-shadowed all other considerations and ease of penetration

of the skin and subcutaneous tissues depended more upon the sharpness of the first few millimetres of the tip of the weapon than any other factor. Using a weapon typical of that seen in many actual homicides (a wooden- handled kitchen or butcher’s knife with one cutting edge) penetration could

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Fig. 1. Apparatus for measuring the maximum pressure applied to the handle of a knife penetrating the body surface; the black triangular indicator is moved by the inner spring- loaded handle-carrier and remains in position after the knife has returned to its original position.

be easily attained with the pressure of one little finger only, amounting to a force of only half a kilogram, as long as the tip was sharpened to the maximum possible degree. When this sharp edge was even slightly blunted, either by repeated use or by artificial means, the pressure required to insert the same knife rose rapidly to 3, 4 or 5 kilograms and if very blunt, could not be inserted through the skin at all with steady pressure of one hand.

Velocity and momentum Most of the tests were done in a virtually isometric situation, that is, with

the tip of the knife stationary and in contact with the skin. Gradually

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increasing pressure was applied, until penetration occurred. A series of ex- periments was also conducted in which a rapid lunge was made at the skin surface, the knife travelling at several feet per second. With a very sharp point, this dramatically altered the results, as with a really fast entry, no reading was recorded upon the scale. This was attributed to inertia in the spring system as penetration occurred so rapidly that the knife did not begin to move relative to the handle of the apparatus. On the “ mu2/2” principle, it is obvious that the velocity of the knife has a profound effect. However where the knife was changed for a less sharp one, a progressive halting at the skin surface was experienced and the pointer of the instrument began to record more and more pressure as the bluntness increased, until with extremes of bluntness, penetration could not be effected even with a rapidly moving weapon.

Region of the body The variations recorded with the same knife in differing parts of the body

were far less than variations due to differing degrees of sharpness. Though most tests were done on upper abdominal and thoracic areas, a few experi- ments were done elsewhere and it appeared that variations depended more upon the underlying tissues than upon the resistance of the skin. For instance, the intercostal spaces could be penetrated much more easily than the upper abdomen, apparently because the tissues were stretched tightly across ribs and the skin did not subside without penetration, as in the upper abdomen. Similarly, the force required to penetrate the abdominal wall was reduced to at least half, when the skin was put under tension before stabbing. The skin of large muscular areas such as the thigh was also more easily penetrated, due to the firm resistance beneath.

Age of the victim Though it is common knowledge to pathologists that skin resistance

varies considerably on making an autopsy incision, the differences are again far less than that due to sharpness of the knife. Age appeared to have a toughening effect upon the skin, but even apart from age, certain individuals appeared to have far more resistant skin than others. However, the variation was slight, compared to the far greater differences observed when knives of differing sharpness were used.

Shape of the blade The shape of the tip of the knife blade has considerable bearing upon the

ease of penetration, primarily because the configuration of the first few centimetres determines the sharpness which can be obtained on the distal vital millimetres. In other words, an acutely-pointed dart-like weapon can be introduced more easily than a knife with an obtusely-curved extremity, even though the cutting edge of the curve may be very sharp. The limiting factor is the cross-sectional area of the extreme tip, which delivers the thrust of the weapon over a much smaller area if extremely pointed. In the present

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experiments, several similar kitchen knives were used, a number of which were new and others which had been ground by repeated sharpenings so that the blade was more triangular than curved. Given an equally efficient sharpening, the older knives were much more effective in penetrating skin. It must be emphasised that it is the first few millimetres that is most impor- tant, and as soon as the tip of the knife has penetrated the dermis, the rest of the weapon follows it with virtually no added force.

A falling body upon fixed knife A common defence to charges of homicide and wounding consists of the

claim that the victim stumbled or fell upon a passively held knife. Experi- ments were therefore carried out to attempt to clarify this situation. Cadavers in rigor were supported in the upright position and allowed to lean forward upon a knife firmly held in the operator’s hand so that the point impinged upon the upper abdomen or intercostal spaces. With a very sharply- pointed weapon, a cadaver of average physique was transfixed upon the knife without difficulty, even when leaning only slightly out of the vertical against the point of the weapon. When the point was held initially only 10 eentimetres from the erect abdominal wall, the movement of the body for- ward was sufficient to drive the complete length of the blade into the skin, subcutaneous tissues, liver etc., so that the body became supported only by the hilt of the weapon against the skin. Again it must be emphasised that this occurred with a very sharply pointed weapon, such as the kitchen knife, ground to the best possible edge by means of a Carborundum abrasive stone. Intercostal spaces were penetrated with even greater ease. It is thus very obvious that the moving body can readily transfix itself upon the knife, provided that the knife point is very sharp.

Stages of penetration During the experimental work, it became apparent that the skin was by

far the most resistant tissue. Once a knife penetrates the skin, no further force need be applied to cause rapid penetration of the subcutaneous tissues and any underlying organ, except for bone or calcified cartilage. With a slow isometric application of pressure to the skin, the skin would dimple and the indicator gauge would rise to a threshold point when there was a sudden release of pressure and the knife plunged through the under- lying tissues with no additional application of force. When a knife was pressed with only a single finger, the passing of this threshold-point caused the knife to penetrate deeply into the body, even though the operator attempted to remove the pressure as soon as loss of resistance was felt. The stretching of the elastic skin in the depression below the knife point before penetration occurs, causes the tension of the assailant’s arm muscles to act as the reservoir of the necessary energy for subsequent deep penetra- tion. The resistance of solid organs like liver or rib cartilage is far less than that experienced at the tough collagenous layer of the upper dermis. In medicolegal terms, this appears to make it unsafe to assume that a deeply-

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penetrating wound was necessarily caused by more force than a shallow wound. It is the skin resistance that appears to determine how deeply the knife penetrates, unless there is further application of pressure once the threshold resistance of the skin is overcome.

CONCLUSIONS

Though most of the foregoing results are a matter of common knowledge and common sense, the attempt to put such observations on a semiquanti- tative basis seems to provide the following conclusions.

The sharpness of the tip of the weapon is paramount in all considerations of the amount of force needed to cause any given stab wound. The degree of sharpness obtainable can vary enormously, from a needle-like point which will enter the skin almost under the weight of the knife alone without added pressure, to a blunt knife which cannot be introduced through the skin even with maximal effort and which may cause bruising or abrasions before an actual incised wound can occur. Thus in the discussion of the force required for any particular wound, the degree of sharpness must be evaluated wherever possible. In many cases this is difficult because the state of the weapon may have been altered by subsequent handling or even the passage of time, which dulls the edge considerably even if no abuse has been com- mitted upon the metal.

The velocity of the weapon at the moment of impact is also extremely important, though is often unknown unless reliable eye-witnesses are available. The velocity and thus momentum is critical in assessing the ease of penetration, but this does not detract from the sharpness of the knife-point being a major factor.

The age of the person, the part of the body, and other such personal factors often offered in criminal trials, are of little consequence compared to the predominant factors of point sharpness and speed of entry. The only worthwhile observation in this respect is that intercostal spaces are particularly easy to puncture, due to the almost drum-like stretching of the skin and thin subcutaneous tissue across the bridges of the ribs.

Given a sharply pointed knife, there is no reason to deny or doubt the ability of a slowly moving body to impale itself upon a fixed knife, even to the extent of driving itself upon the blade up to the hilt. No forward movement of the knife is necessary, nor need the hilt of the knife be sup- ported upon any relatively rigid object such as the hip of the holder, as has been suggested in more than one criminal trial. However, this possibility declines rapidly with progressive blunting of the knife point and each individual case must be assessed on the cutting power of the tip of the weapon.

The “threshold-phenomenon” of skin resistance must be borne in mind, as once this is suddenly overcome, the knife can plunge deeply into the underlying tissues. Failure to appreciate this has given rise to sterile and mis-

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leading argument in criminal trials, postulating the deliberate continuance of pressure by the assailant where a deep wound has resulted. This certainly need not necessarily be the case.

Though this is a topic not readily amenable to quantitative measurement and reproducible results, the factors discussed make it obvious that much illogical and uninformed opinion has been ventilated in criminal courts in past years. Unless an evaluation of the sharpness of the weapon is attempted - preferably with some experimental work on the given knife - subjective guesses as to the amount of force required to produce a given wound cannot only be futile, but sometimes contrary to the best interests of justice.