Stab wounds: visceral position, penetration and tissue resistance
THE CONTEXT
Assessment of a stab wound must account for changes in body position after death, the properties of the knife and resistance offered by tissues.
Changes in visceral position
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Visceral positions at autopsy differ from those during standing, bending or movement at the time of assault.
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Tension contracts the abdomen and reduces the distance between the abdominal wall and spine. Relaxation on the autopsy table increases this distance.
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Lung collapse makes the postmortem depth of an anterior chest stab wound greater than during life. For a posterior chest stab wound, the depth is less because the lungs collapse posteriorly.
Knife characteristics and force
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The force required to inflict a stab wound is subjective and can be expressed comparatively as slight, moderate, considerable or extreme.
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Sharpness of the extreme tip is the most important factor in skin penetration. Once the tip perforates the skin, the blade passes readily unless obstructed by bone or cartilage.
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With equal sharpness and applied force, a thin, slender, double-edged knife penetrates more deeply than a wide, single-edged blade.
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A blunt-pointed instrument requires considerable force to puncture skin and penetrate soft tissues.
Tissue resistance
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Apart from bone and calcified cartilage, skin offers the greatest resistance to knife penetration, followed by muscle.
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Uncalcified cartilage can be penetrated by a sharp knife, although more force is required.
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A forcible stab with a strong, sharp knife can penetrate a rib, sternum or skull.