Mechanical injury: responses of skin, soft tissues, bone and body fluids
THE CONTEXT
Different tissues respond differently to a blow because of their mechanical properties. Movement and the duration of energy discharge also influence injury.
Movement and duration of impact
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A blow to a freely moving head may cause little damage, whereas a similar blow to a head resting on the ground may cause marked skull injury.
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Increasing the time over which energy is discharged decreases the destructive effect. The source contrasts a quickly withdrawn punch with one in which the fist stays in place.
Skin and subcutaneous tissues
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Skin is very pliable, slightly elastic and resistant to traction. It may remain intact despite severe injury to underlying structures after a blunt blow.
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Skin may split when crushed against rigid bone.
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Subcutaneous tissues provide cushioning because of their fat content and pliable connective tissue.
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Severe blunt blows may crush and displace incompressible subcutaneous fat between skin and underlying structures, producing bruises.
Muscles and bones
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Muscles usually resist blows because of their plasticity, elasticity and strong encapsulating sheaths.
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Muscles may be crushed against bone, lacerated by displaced broken bone fragments or ruptured by excessive stretching.
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Bone may bend without breaking but fractures when bending exceeds its elastic limit.
Hydrostatic transmission through fluids
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Body fluid is incompressible but readily displaced. A blow to a fluid-containing hollow organ can generate hydrostatic forces transmitted equally in all directions.
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These forces may rupture anatomically distant, mechanically weak tissues. Sudden chest compression may rupture distal venules and capillaries, as in traumatic asphyxia.
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Violent fluid displacement in the gastrointestinal or urinary tract may cause distant ruptures within those tracts.