Introduction
Sharp force injuries are distinguished from blunt force injuries and represent the second major class of injuries. They result from contact with keen edges or sharp points. Generally sharp force injuries are subclassified as either incised wounds (cuts and slashes) or puncture wounds (stabs). An incised wound is a breach of the skin, of variable depth, resulting from contact with a keen edge. Usually there is at least penetration of the full thickness of the skin (i.e. both the epidermis and the dermis) together with variable involvement of the deeper tissues, but shallow, partial-thickness skin cuts can occur, for example a common ‘paper cut’. Stab or puncture wounds are penetrating injuries with a depth greater than the wound dimensions on the skin surface. They are caused by long, thin objects, usually sharp-pointed, such as a knife.
Cuts and stabs may be assaultive, self-inflicted or accidental. The injury pattern assists in establishing the manner of infliction, but instances of single fatal stab wounds can be difficult to resolve. Incised wounds provide little information about the weapon, whereas stab wounds commonly permit at least a general reconstruction of weapon dimensions. Both incised wounds and stab wounds typically yield no trace evidence, but are associated with blood staining of the weapon, clothing and scene, opening up the possibility of crime scene reconstruction through bloodstain pattern interpretation. Questions which are often raised, but difficult to answer, include the degree of force required to inflict a stab wound, the order of infliction of the wounds, and the period of consciousness and level of activity possible following wounding.
Incised Wounds (Cuts)
The features of incised wounds reflect their production by keen-edged, rather than blunt, objects. Incised wounds have well-defined, cleanly severed margins in contrast to the ragged, torn edges of lacerations. Occasionally, incised wounds show some irregularity or notching of the wound margins resulting from cutting across skin folds, a feature most commonly seen in defensive injuries to the palms (see below). Typically incised wounds have no associated abrasion or bruising of the adjacent skin, and there is complete severance of all tissues within the depths of the wound, so that there are none of the ’tissue bridges’ which characterize lacerations. The distinction between incised wounds and lacerations is of considerable forensic importance. The diagnosis of an incised wound implies contact with a sharp-edged object or weapon, whereas the diagnosis of a laceration implies contact with a blunt object or weapon as the cause of the injury. Unfortunately, in clinical medicine, this clear forensic terminology is often not maintained, so that incised wounds and lacerations are both loosely referred to as lacerations. This may create confusion when medical records are used in legal proceedings.
Although the presence of an incised wound indicates that the causative object was keen-edged, the wound commonly provides no further information about the weapon. Most incised wounds are knife wounds, but this is not a conclusion that can be safely drawn from the examination of the wound alone. Cuts may also be produced by razor blades, shards of glass, sharp edges of machinery and tools, tin cans and similar objects. Whatever the nature of the keen-edged object producing the incised wound, it cleanly severs the tissues and typically leaves no trace evidence within the wound. However, the object will have traces of blood and tissue, which can be used to link a victim and a weapon through DNA fingerprinting. Cleanly severed blood vessels within the wound bleed profusely, and if large vessels are cut then the hemorrhage may be sufficient to kill. Such extensive hemorrhage commonly results in blood soaking of clothing together with blood staining and spattering of surroundings, opening up the possibility of reconstruction of the events through blood spatter analysis. When large veins are severed, particularly in the neck, air may be drawn into the circulation, obstructing the flow of blood through the heart and so killing by air embolism. The diagnosis of air embolism is easily missed at autopsy if it is not considered and actively sought at the outset.
Incised wounds inflicted with heavy-bladed weapons, such as an axe, adze, sharpened spade or machete, or by a boat propeller, characteristically show abrasion, and sometimes bruising, of the wound margin. This results when the edges of the first-formed incised wound are crushed by the forceful entry of the heavy blade following behind. The extent of the abrasion is highly variable and is sometimes only apparent on close inspection, perhaps aided by a hand-lens. However, the large size of the wounds and the commonly associated cuts to bone are more obvious pointers to the type of weapon. These wounds are commonly described as chop wounds.
Accidents
Accidentally inflicted incised wounds are common. They may result from the use of knives in the home or workplace, broken window glass (in burglaries), broken drinks glasses, and sharp-edged machinery or parts of vehicles. The commonest accidental incised wounds are probably the simple, shallow cuts produced by stiff paper or tough grasses.
Frequently seen is the pattern of multiple, incised wounds to the backs of the hands and to the face, caused by the shattering of the tempered-glass side windows of vehicles during collisions. The resultant dice-like fragments of glass produce the characteristic scattered, small, irregular incised wounds with associated abrasions. They involve the side of the body adjacent to the shattered window, so that it is possible to determine whether the individual was seated on the left or right side of the vehicle at the time of impact. In this way the injuries can be used to distinguish the driver from the front-seat passenger.
Self-infliction
The ready availability of knives and the relative pain-lessness of incised wounds have encouraged their use in suicide since antiquity. The Romans were said to have favored opening the veins of the arms whilst lying in a warm bath. Suicide by cutting the throat was far commoner in the years before the wide availability of relatively painless drugs and poisons, so that the best descriptions and illustrations are in the older texts. Suicidal incised wounds are typically directed towards sites, such as the neck, wrists and, less commonly, the elbows, knees and ankles, where large blood vessels are close to the skin surface. The cuts are made by first pressing the sharp edge against the skin and then running it across the skin surface, so that the shallower wounds are both of uniform depth and respect the general contours of the body. Self-inflicted incised wounds are commonly, but not invariably, accompanied by multiple, parallel, and sometimes superimposed shallow wounds, known as hesitation or tentative wounds. Such wounds reflect a testing of the blade as well as the indecision so often present in suicidal acts. Having produced an incised wound, the suicide may then repeat the cuts in the depths of the first-formed wound. This behavior is inferred from the multiple trailing cuts arising from the ends and sides of the principal wound. Occasionally these repeated cuts make the wound extremely deep, and death investigators may find it difficult to accept the possibility of self-infliction. For example, in suicide by cutting the throat there may be repeated cut marks on the exposed cervical spine associated with severing of all the anterior neck structures.
Parallel, shallowly incised wounds to the wrists and forearms, resulting from self-harm, may heal leaving multiple linear scars. Often they are most prominent on the nondominant arm and involve the extensor and flexor surfaces of the forearms and the volar surface of the wrists. If seen in clinical forensic practice these scars raise the possibility of a personality or psychiatric disorder, with the attendant risk of suicide in custody. If seen at autopsy, they raise the index of suspicion for suicide, alcohol and drug abuse, and risk-taking behavior leading to accidental death.
Self-inflicted cuts are sometimes seen in individuals who falsely allege that they have been assaulted. The motivation is most often the manipulation of personal relationships at times of stress, but may involve financial gain, such as claiming state compensation for victims of assault. The pattern of injury involves accessible sites, and reflects handedness, infliction by pressure rather than slashing, and the avoidance of sensitive or critical areas such as nipples, lips and eyes. Even so, the injuries can be extensive and mutilating.
Assaults
Defensive injuries to the hands and forearms are typical of knife attacks. They may be absent in sudden, overwhelming attacks (so called ‘blitz’ attacks), or if the victim is unable to offer a defense as a result of the effects of alcohol and drugs, unconsciousness, bindings, or other physical and emotional circumstances. Since defensive wounds reflect anticipation of injury and an attempt to ward off the harm, they may be seen in accidents as well as in assaults. For example, multiple cuts to the palms of the hands can be produced in a fall onto a glass-strewn floor, as well as during an attack with a broken bottle. In a knife attack, defensive cuts to the palms and fingers result from attempts to grab or to deflect the weapon, whereas slash and stab wounds to the backs of the hands and the forearms result from shielding movements. A victim on the ground, being attacked by a standing assailant, may have defensive wounds to the legs. In cases of suicide, typical defensive-type wounds to the hands and arms do not occur, but the occasional shallow cut to the fingertip may reflect testing the keenness of the blade.
Assaultive incised wounds inflicted on a resisting victim are typically scattered, include defensive-type wounds and involve the clothing, which, by contrast, is characteristically undamaged in self-infliction. Wounds to areas of the body that are difficult to access for self-infliction, e.g. the back, immediately raise the suspicion of assault. Although it is rarely, if ever, possible to determine the precise order of infliction of the assaultive wounds, as a general rule the scattered injuries are inflicted first during the phase of active resistance, whereas the closely grouped injuries, directed towards vital target sites, are inflicted later. In knife assaults it is usual for there to be both incised wounds and stab wounds, with death usually the result of the deeply penetrating stab wounds rather than the shallower cuts. A combination of incised and stab wounds may also be seen in suicides.
Stab Wounds
Stab wounds are penetrating injuries produced by a long, thin object, which is typically pointed. Most commonly, the instrument is flat with a sharp point, such as a knife, a shard of glass or a length of metal or wood. Other weapons may be long and thin with a sharp point, such as a skewer, hypodermic needle, ice pick, or old-fashioned hatpin. With sufficient force even long, rigid objects that are blunt-ended will produce puncture wounds, e.g. screwdriver, wooden stake or protruding parts of machinery or motor vehicles. The appearance and dimensions of the resulting wound often provides useful information about the object producing it. Stab wounds, like incised wounds, typically yield no trace evidence, are associated with blood and tissue staining of the weapon, as well as blood staining and damage to clothing, and offer the opportunity for scene reconstruction from blood spatter analysis.
Clothing
Observations and reports on stab wounds should always attempt to correlate damage to the clothing with wounds to the body, because this very much facilitates reconstruction of the incident. Aligning cut and stab holes to the clothing with cut and stab wounds to the body may help determine clothing position and body position at the time of stabbing. Some stabs or cuts to clothing may not penetrate to produce wounds, or there may be multiple holes in the clothing reflecting a single stab through folds. Some wounds may not be associated with damage to all layers of the clothing, for example sparing a coat or jacket. Suicides typically, but not universally, adjust their clothing to expose the bare skin before inflicting the injuries, so that the clothing is disturbed but undamaged. Any bloodstain pattern to the clothing is best interpreted in the light of the wounds to the body and the scene examination.
Wound description
If there are multiple wounds it is often useful to number them. Each wound should then be described giving details of its site relative to local anatomical landmarks as well as its distance from the midline and distance above the heel. Next the shape, size and orientation of the wound should be recorded and the dimensions assessed with the wound edges reap-posed. Unusual marks to the surrounding skin, such as bruises and abrasions need to be described since, however small, they are likely to prove significant. The wound should never be probed with a sharp object, and particularly not with an alleged weapon, but rather the direction should be assessed during dissection. Oblique penetration of the blade through the skin may leave one wound edge bevelled and the opposite edge overhanging, i.e. undercut, giving an indication of the direction of the underlying wound track. The depth of the wound track needs to be estimated, recognizing that it is subject to some considerable error. Damage to tissues and organs is described sequentially and thereafter the effects of the damage, e.g. hemorrhage, pneumothorax, air embolism, is recorded. A wound which passes completely through a structure is described as perforating or transfixing or ‘passing through and through’. If the wound track enters a structure but does not exit it then the wound is described as penetrating. Following this convention, a stab wound which passed through the front of the chest to end in the spinal column could be described as a penetrating stab wound of the chest which perforated the sternum, perforated the heart and penetrated the vertebral column. Systematic observation and recording of the site, shape and size, direction, depth, damage and effects of each wound is the hallmark of good practice. Photo documentation of the wounds with appropriate measuring scales is standard in suspicious death investigation, and advisable in any potentially contentious case.
The skin surface appearance of a stab wound is influenced both by the nature of the weapon and by the characteristics of the skin. The skin contains a large amount of elastic tissue which will both stretch and recoil. This elastic tissue is not randomly distributed but is aligned so as to produce natural lines of tension (Langer’s lines) which have been mapped out on the skin surface, and are illustrated in standard anatomy texts. The extent of wound gaping, and the extent of wound scarring in survivors of knife assaults, is influenced by the alignment of the wounds relative to Langer’s lines. Stab and incised wounds which are aligned with their long axis parallel with Langer’s lines gape only slightly, a fact made use of by surgeons who align their incisions in this way to promote healing and reduce scarring. Wounds aligned at right angles to Langer’s lines tend to gape widely, and scar prominently, because the natural lines of tension of the skin pull the wound open. Wound gaping is also influenced by the extent of damage to the underlying supporting fascia and muscles. Where a stab wound is gaping then the wound edges must be re-approximated at autopsy to reconstruct the original shape of the wound, something that is easily achieved with transparent tape. It is the dimensions of the reconstructed wound rather than the original gaping wound that are of interpretative value.
Weapon dimensions
If a stabbing with a knife is ‘straight in and out’ then the length of the stab wound on the skin surface will reflect the width of the knife blade. There are important qualifications that apply to this however. The skin wound length may be marginally (a few millimeters) shorter than the blade width as a result of the elastic recoil of the skin. If the knife blade has a marked taper and the entire length of the blade did not enter the body then the skin wound length will not represent the maximum width of the blade. If the blade did not pass ‘straight in and out’ but instead there was some ‘rocking’ of the blade, or if it was withdrawn at a different angle from the original thrust, then the skin wound will be longer than the inserted blade width. Consequently, the most reliable assessment of blade width is made from the deepest wound with the shortest skin surface length. A single weapon can produce a series of wounds encompassing a wide range of skin-surface lengths and wound depths. This is often seen in a multiple stabbing fatality and is consistent with the use of only one weapon. However, it is rarely possible to exclude any speculative suggestion of more than one weapon, and, by inference, more than one assailant.
The depth of the wound gives an indication of the length of the weapon. Clearly the wound track length may be less than the blade length if the entire blade did not enter the body. Less obvious is the fact that the wound track length may be greater than the blade length. This occurs if the knife thrust is forceful and the tissues are compressed, so that when the weapon is withdrawn the track length in the now decompressed tissues is greater than the blade length. This tissue compression effect is most marked in wounds to the anterior chest and abdomen, since the entire chest or abdominal wall can be driven backwards by the blow. A small pocket-knife, with a blade less than 5 cm (2 in) can cause, in a slim person, a fatal stab wound to the heart or one which penetrates the abdomen to transfix the aorta. An added complication in measuring the wound track length at autopsy is that the corpse is supine with the viscera in a slightly different relative position to a living person standing or sitting. For all of these reasons the wound track depth should be used with caution in predicting the blade length of the weapon. If by chance some fixed bone, such as a vertebra, is damaged at the end of the wound track, then the assessment of depth of penetration is easier, but still subject to inaccuracy.
As well as providing an indication of blade width and length, a stab wound may provide other useful information about the weapon. Wound breadth on the skin surface is a reflection of blade thickness and a typical small kitchen knife, with a blade thickness of 2 mm or less, produces a very narrow wound. The use of a thicker-bladed weapon may be readily apparent from the measured wound breadth on the skin surface. Most knives have a single-edged blade, that is one keen edge and one blunt edge to the blade. The resultant wound reflects the cross-sectional shape of the blade and, with the wound gaping, often appears boat-like with a pointed prow and a blunted stern. Sometimes the blunted stern shape is distorted into a double-pronged fishtail. The thicker the blade of the weapon, the more obvious is the blunting of one end of the wound when contrasted with the other pointed end. Knives with double-edged blades (daggers) are specifically designed for use as weapons and produce a wound that is pointed at both ends, but such a wound may not be distinguishable from one produced by a thin, single-edged blade. The cross-sectional shape of the blade of the weapon may be accurately reproduced if it passes through bone, e.g. skull, pelvis, sternum or ribs. At the same time, trace material, such as paint, present on the blade will be scraped off by the bone and deposited in the wound. Stab wounds in solid organs such as the liver may retain the profile of the weapon, and this can be visualized by filling the wound track with a radio-opaque contrast material and taking a radiograph.
Stab wounds inflicted during a struggle, with knife thrusts at awkward angles and with movements of both parties, may show characteristics reflecting this. Even so, it is rarely if ever possible to reconstruct the positions of victim and assailant from the location and direction of the wounds. A notch on the otherwise cleanly cut edge of the wound is a result of withdrawal of the blade at a different angle from the entry thrust. Exaggeration of this effect leads to a V-shaped, or even cruciate, wound when there is marked twisting of the blade or twisting of the body of the victim. A linear abrasion (scratch), extending from one end of the wound, results from the withdrawing blade tip running across the skin. A single stab hole on the skin surface may be associated with more than one wound track through the tissues, reflecting a knife thrust followed by continuing struggle or repeated thrusts of the weapon without complete withdrawal. If the entire knife blade is forcefully driven into the body then there may be bruising or abrasion of the surrounding skin from the hilt of the weapon. More commonly, there is a small rectangular abrasion at one end of the wound reflecting penetration of the blade beyond the notch at the base of the blade (the ‘kick’) which separates the sharpened edge from the rectangular portion of metal from which the blade was forged (the ‘tang’). Consequently the presence of this small abrasion is an indication that the blade penetrated to its full length.
Weapons other than knives may produce characteristic stab wounds. Bayonets, which have a ridge along the back of the blade and a groove along either side, to facilitate withdrawal of the weapon, may produce a wound like an elongated letter ‘T’. A pointed metal bar which is square in cross-section typically produces a cruciate wound, whereas one which is circular in cross-section, e.g. a pitchfork, produces an elliptical wound. A closed scissors produces a compressed Z-shaped wound, and a triangular file will produce a three-cornered wound. If the cross-sectional shape of the weapon varies along its length, e.g. a screwdriver, then the depth of penetration will affect the appearance of the wound. Relatively blunt instruments such as pokers, closed scissors and files, tend to bruise and abrade the wound margins, a feature not otherwise seen in stab wounds. The blunter the object and the thicker its shaft then the more likely is the skin surface wound to become a ragged, often cruciate, split. In cases where the wound appearance is unusual, it is helpful to conduct experiments with a duplicate suspect weapon in order to see whether the appearance of the wound can be reproduced.
Degree of force
A commonly asked question in the courts is the amount of force required to produce a specific stab wound. This is usually a difficult if not impossible question to answer. The sharpness of the point of the weapon is the most critical factor in determining the degree of force required to produce a stab wound. In general, relatively little force is required to produce a deeply penetrating stab wound using a sharply pointed weapon, and the amount of force is easily overestimated. The greatest resistance to penetration is provided by the skin and once this resistance is overcome the blade enters the tissues with greater ease. In this respect an analogy can be made with the stabbing of a ripe melon. The important implication is that the depth of the wound is not a measure of the degree of force applied. However, penetration of any bone or cartilage implies a significant degree of force, all the more so if the tip of the blade has broken off and remains embedded in the bone, something which is best identified by X-ray. Similarly a significant degree of force may be inferred from the presence of the hilt mark of the weapon on the skin surface, an uncommon finding, or a wound track significantly longer than blade length, suggesting forceful tissue compression during the stabbing (see above). Even so, the stabbing force may have been a combination of both the thrust of the weapon and also any forward movement of the victim, such as in a fall. This latter proposition is commonly raised by the defense, and is rarely possible to discount, in deaths from single stabs.
Homicide, suicide and accident
Most stabbing deaths are homicidal and the wounds are usually multiple, since the first wounds will usually leave the victim capable of some continuing resistance. The stab wounds are commonly associated with incised wounds and the typical pattern is of scattered wounds, many deeply penetrating, with wound tracks in different directions and several potentially lethal. Wounds to the back and other sites inaccessible to the victim all suggest homicide, as do a large number of scattered wounds. Defensive wounds to the hands and arms are common but their absence does not exclude homicide. Multiple grouped shallow wounds in a homicide may reflect threatening or sadistic behavior prior to the lethal blows, or alternatively postmortem piquerism.
In suicidal stabbings the wounds are usually multiple and closely grouped over the left breast or upper abdomen with no associated damage to the clothing which is characteristically removed or pulled aside, although this is not invariable. Occasionally suicidal stabs are inflicted on the neck. Some wounds are shallow representing tentative thrusts, and only a few are deeply penetrating and potentially lethal. Multiple wound tracks arising from a single stab hole reflect repeated thrusts following partial withdrawal of the blade. There may be associated incised wounds to the wrists and neck. A small percentage of suicides will use more than one method, so that a drug overdose is followed up with cutting and stabbing, or the knife wounds followed by hanging or drowning.
The most difficult cases are those with a single stab wound and no other injuries. Allegations of homicide are usually met with a defense of accident. In other circumstances distinguishing suicide from homicide may be difficult. Some suicides may accomplish the act with a single thrust of the weapon which is left protruding from the wound. Very occasionally the hand of the decedent may be gripping the weapon, in cadaveric spasm. Although this is proof that the victim was holding the weapon at the time of death, it is not conclusive proof of suicide.
Collapse and death
A lethal injury can lie beneath the most trivial looking skin wound, emphasizing the fact that the primary characteristic of a stab wound is its depth. Stab wounds kill mainly as a result of hemorrhage, which is usually internal rather than external. Indeed, there may be little bleeding onto the skin and clothing in a fatal stabbing. Whereas the loss of a large volume of blood will clearly account for death, a much smaller blood loss, of about one liter, may have a fatal outcome if it occurs very rapidly, as in cases with gaping wounds to the heart or aorta. As happens with incised wounds, death can also result from air embolism, in which case collapse usually occurs very rapidly. Both air embolism and pneumothorax are possibilities to be explored at the start of the autopsy. Those victims who survive the immediate trauma of a stabbing may succumb to infections after many days or weeks.
Initially the victim of a stabbing assault may be unaware of having received a wound. The adrenaline-surge, preoccupation with a struggle, intoxication with alcohol or drugs, the relative painlessness of sharp force injuries, and the tendency of stab wounds to bleed internally rather than externally, are some of the factors contributing to this lack of awareness. Consciousness and physical activity can be maintained for a variable period after the stabbing, depending on the rate of blood loss or the development of air embolism. Even after examining the wound it is difficult to give a useful estimate. Following a stab wound to the heart, at least a few minutes of activity and consciousness is common. Stabs to the brain can produce immediate incapacity, or even death, depending on the part damaged, and similarly stab wounds severing large nerves of the limbs can immediately paralyse that limb. Otherwise incapacitation reflects the rapidity and the extent of blood loss. Since the development of air embolism can be delayed, its presence, although typically associated with sudden collapse, is not necessarily an indicator of instant collapse.
