18 February, 2002

Dear Shri Varma ji,

Many thanks for your phone call yesterday. The explanation regarding payment is like this.

1. On 15 February, 2002, you sent a check for Rs. 2750, I believe it was for the booklet and article of February 2002. Payment for March 2002 article was not given.

2. On 17 December, 2001, I received a check for Rs. 1250. I believe it was like this:

Payment for January 2002 article (1000)+ backlog for December 2001 article (250)

However for Booklet of November 2001 also, I was paid Rs. 250 less (only Rs 1500, although the payment settled for booklet was Rs. 1750).

3. The payment for March 2002 article is Rs. 1000.

4. So the total outstanding balance was (1000+250) or Rs. 1250/=

5. Rs. 500/= check given on 20 February, 2002.

6. Outstanding balance as on 25 February, 2002 is Rs. 750/= only.

So please adjust this amount also. It is a small amount, but I thought we should keep our records straight.

Thanks

Sincerely

Anil

For APRIL 2002 issue

SCIENCE IN CRIME DETECTION-105

MEASURING THE STING OF STAB WOUNDS

‑Dr. Anil Aggrawal

When stabbing cases come to trial in a criminal court, an almost inevitable question is put to the medical witness by either advocate or judge. This question is “what degree of force would be necessary to inflict this wound!” Such a question is difficult if not impossible to answer, as an assessment of the force in scientific terms of energy would be meaningless to the court. For instance I can say, that a force of 5 Newtons is enough to cause this knife to enter the abdomen, but the court may not be able to understand, what is the meaning of a Newton. And it could be rather futile to attempt to take lessons in physics in the court!

Any other attempt at description could be totally subjective. Every witness had his own criteria of what constitutes various degrees of force or pressure and what may be a “heavy blow” to one witness might be “moderate force” to another.

This question is usually raised in connection with the defense aspects of the trial. The readers may want to know why? And there is a very good reason. It is often assumed that a very forceful stab is associated with a greater degree of intent and similarly where accident is raised as the defense, a lesser degree of force is helpful to the legal representatives of the accused. In several stabbing homicides in which doctors have been involved as expert witnesses, the defense has been put forward that the victim fell upon the passively held knife, because of some accidental stumble or trip. This possibility becomes a matter for intense discussion in the court and once again estimates of the force needed for the penetration of the body becomes confused in subjective descriptions.

Some forensic medical research has been performed which attempts to replace such subjective opinions by hard physical measurements, but these are not always helpful in court as “dynes per square centimeter,” “foot-pounds” or other physical units are usually meaningless to both advocates, judge, and jury. In an attempt to offer some more readily understandable data to clarify some of the basic factors involved, scientists have conducted ingenious experiments under controlled conditions. The object was not to attempt an exact statistical exercise, but to provide generalizations that could be placed before a court composed of lay persons.

In these interesting experiments, a relatively simple device was constructed which would be attached to the handle of various test knives and which recorded the pressure applied to the handle directly in metric units (kilograms). This provided a means whereby the minimum pressure for penetration of the skin and tissues could conveniently be measured.

Using this apparatus, a large number of incisions were made in cadavers which were submitted to routine medicolegal autopsy. For ethical reasons, the experimenters made the incisions at a place which could be incorporated in the routine autopsy incision, so that no unnecessary marking of the body was inflicted. The most common site for the experiment was in the midline of the upper abdomen - a common site for actual criminal stabbings - and in the lower part of the chest. A smaller number of other wounds were made upon other parts of the body when the opportunity permitted such as thighs and the lateral sides of the chest.

The actual quantitative results are of little intrinsic value as it was shown by these experiments that the individual knife and circumstances cause wide variation in the actual pressure involved, but it could be said that with a very sharp knife, the amount of pressure necessary to penetrate the skin of the trunk varied between a half and three kilograms.

Much more important is an appreciation of the various factors which can influence the ease of penetration, some of which have profound medicolegal significance.

Sharpness of the Knife

This was easily the most potent of all factors and the facility with which the knife penetrated the skin depended more upon the sharpness of the tip of the weapon than any other factor. Employing a weapon similar to that seen in many actual homicides - a wooden-handled kitchen or chefs knife with one cutting edge - penetration of the chest or abdomen could easily be performed with the pressure of one little finger, amounting to a force of only half a kilogram. This could be attained only if the tip was sharpened to the maximum possible degree. When this sharp point was even slightly blunted, the pressure required to introduce the same knife rose rapidly to three, four, or even five kilograms and if very blunt would not penetrate the skin at all with the pressure of one hand. The criterion of tip sharpness overshadowed all other factors in this investigation.

Speed of Approach

Most of the tests were done in circumstances which eradicated any effect of velocity and momentum - the knife was placed in contact with the skin and the pressure gradually increased until penetration occurred. However, when a more realistic reenactment of the stabbing situation was simulated with a rapid lunge at the skin surface, a dramatic alteration of the usual results was seen. With the knife traveling at several feet per second, penetration occurred so readily that no reading was recorded upon the scale of the instrument. This was attributed to the Inertia of the spring system, so that the knife did not begin to move relative to the handle of the apparatus before deep penetration was achieved. This was seen with a sharp instrument, but where the knife was changed for a less effective weapon, 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. However, it is obvious that the velocity of the weapon has profound effects on ease of penetration.

Part of the Body Stabbed

This was much less important than the two factors previously mentioned. It appeared that the resistance depended more upon the underlying tissues than upon the skin itself. For instance, the spaces between the ribs could be penetrated much more easily than the upper abdomen, because the tissues were stretched tightly across the ribs and the skin did not subside before 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.

The age and other physical characteristics of the victim appear to be relatively unimportant in altering the ease with which a stab wound could be inflicted. Though older people have more fibrous tissue in their subcutaneous tissues, this factor made no difference using a really sharp weapon, compared with the overall importance of sharpness and speed of approach.

Shape of the Knife

The configuration of the blade has an appreciable bearing upon the ease of penetration. It is the first few millimeters of the tip that is important and a blade with a narrow termination will penetrate much more easily than a rounded blade, even if the edges are very sharp in each type of weapon. It is the vital few millimeters at the tip which determine the penetrating power of the knife and an acutely-pointed “dart like” weapon can be introduced much more easily than a knife with a curved extremity. This is an obvious observation, as the efficiency of any weapon is determined by the concentration of energy into the smallest cross-section area at the point of application. Many of the weapons seen in homicidal stabbings have been the domestic-type which has a straight back and a curved front edge. It was observed that those which had been considerably worn by repeated sharpening, tended to have a triangular rather than rounded shape. Given an equal degree of sharpening, these older knives were more efficient in penetrating the skin. It must be emphasized that it is the first few millimeters that is important, for as soon as the tip of the knife has penetrated the skin, the rest of the weapon follows it with virtually no added effort.

A Body Falling upon a Fixed Knife

This has considerable medicolegal relevance, as in a high proportion of stabbing cases that come to trial, the defense suggest that the victim accidentally fell upon a passively-held knife, allegedly being grasped either innocently, or in a position of mere self-defense. In order to clarify this situation, experiments were conducted in which a cadaver was allowed to fall forward from an upright position onto a fixed knife. With a very sharply-pointed weapon, a cadaver of average physique was transfixed upon the knife without any difficulty. This would occur even when the body was leaning only slightly out of the vertical, quite close to the weapon, no rapid movement between the two components being present. When the knife point was initially held only 10 centimeters from the upper abdominal wall, the movement of the body forward under gravity was sufficient to drive the complete length of the blade into the cadaver, so that the body became supported by the hilt of the weapon against the skin: It must be emphasized that this occurred with a very sharply pointed weapon, ground to the best possible point by means of a carborundum stone, but was still consistent with the degree of sharpness attained by various kitchen knives actually used in homicidal situations.

Intercostal spaces were penetrated with even greater ease and it is thus very obvious that a body can readily transfix itself upon a knife in this alleged situation, provided the point is very sharp.

During the experimental work it became apparent that the skin was by far the most resistant tissue, apart from bone and calcified cartilage. Once the knife penetrated the skin no further force needed to be applied to cause rapid penetration of the tissues beneath the skin. During the slow isometric application of pressure to the skin, the latter would dimple and the indicator gauge would rise to ,a threshold point when there was a sudden release of pressure, the knife plunging through the underlying tissues with no additional force. When a knife was pressed with only a single finger, the passing of this threshold point caused the knife to suddenly penetrate deeply into the body, even though the operator attempted to remove the pressure as soon as the 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 a reservoir of the necessary energy for subsequent deep penetration. Alternatively, if the body is falling against a fixed knife, the momentum of the body supplies the energy. The resistance of solid organs like the liver or even noncalcified 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-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.

Timing of Stab Wounds

Sometimes, the necessity of determining whether some or all of wounds were inflicted before or after death is important. Especially in frenzied sexual assaults, some or all of the stab wounds may be postmortem in nature, following death from some other cause, such as blunt injury or strangulation. This is especially so if the well-recognized signs of a sadistic sexual attack are present, notably on the breasts and lower abdomen. Where the stab wounds have occurred at or near the time of death, the only method of deducing the possibility of ante- or postmortem infliction is the amount of hemorrhage present, either externally or into body cavities such as cavities of the lungs or in the abdomen. Though there are new techniques, including histochemistry, for determining when the wounds were indicted, these are rarely of use in the circumstances where the injuries occurred in the perimortal period. Histochemistry is a rather complicated technique in which certain enzymes and other chemicals within the wound are studied. It is seen that if the wound was made during life, certain enzymes would appear in the wound, but if the wound was made after death, these enzymes would not appear in the wound. This fact is very important in deciding whether the wound was antemortem (made during life) or postmortem (made after death).

These simple experiments, based upon observation of common-sense factors, appear to have some useful relevance to common medicolegal situations. The following conclusions could be made from these interesting experiments.

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 greatly, from a needlelike point which would enter the skin almost under the weight of the weapon alone, 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 be made. In the discussion of the force required for any particular wound, the degree of sharpness of the particular knife must be evaluated where ever possible. In many cases this is difficult, because the state of the weapon may have been altered by subsequent handling between the wounding or homicide and the time of examination. Even the passage of time may dull the edge appreciably from the original state, even if no physical abuse has been made upon the metal.

The speed of approach of the weapon at the moment of impact is also extremely important, though it is often unknown unless reliable eye witnesses are available. The velocity and momentum is critical in assessing the ease of penetration, but this is closely related to the sharpness of the knife point, in assessing the overall facility with which penetration could have occurred.

The age of the person, the part of the body, and other personal factors often discussed in criminal trials, are of little consequence compared to the all-important factors of point sharpness and speed of entry. One point that might be mentioned, is that the intercostal spaces are particularly easy to puncture, due to the almost drum like stretching of the skin and thin subcutaneous tissues across the bridges of the ribs.

Given a sharply-pointed knife, there is no reason to deny or doubt the ability of even 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. This situation is often put forward as a defense to a criminal charge and if the knife is sharp, there are no medico pathologic reasons to dispute this possibility. No forward movement of the knife is necessary nor need the hilt of the knife be supported upon any relatively rigid object such as the hip of the holder, as has been suggested in more than one criminal trial. However, the possibility of easy passive penetration declines rapidly with progressive blunting of the knife-point. Each individual case must be assessed upon 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 misleading argument by advocates in criminal trials, who postulate 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 above make it obvious that much illogical and uninformed opinion has been ventilated in criminal courts in past years. Some sophisticated measurements have been made upon stab wounds, resulting in excellent academic results, but the objection to these is that such quantitative data, measured in various physical and mathematical units, mean very little to the judge, jury, and even advocates in a criminal trial. They do very little to assist the understanding of the circumstances, when placed before a lay audience. The relatively simple factors discussed above would appear to be of some use in clarifying such situations. Unless an evaluation of the sharpness of the weapon is attempted-preferably with some experimental work on a 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.

Captions to figures:

1. Typical Homicidal Stab wound, inflicted by a kitchen knife which was driven deeply into the chest. The wound is slightly narrower than the blade, due to gaping at right angles to the axis of the wound.

2. A simple device to measure the pressure applied to the handle of a test knife, which is attached to a springloaded slide inside the outer box. A self-retaining pointer indicates the maximum load on a precalibrated scale.

This article has (3088) words

Dr. Anil Aggrawal, M.D.

Professor of Forensic Medicine

Maulana Azad Medical College

New Delhi‑110002