Different patterns of tissue compression, stretching and shearing produce distinct forms of laceration.
Split lacerations
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Split lacerations occur when skin is crushed between two hard objects. In the scalp, tissue is crushed between the skull and the ground or a blunt instrument.
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Split lacerations show tissue bridges and are not undermined.
Incised-looking wounds
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Lacerations without excessive skin crushing may have relatively sharp margins.
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Blunt impact where skin lies close to bone and subcutaneous tissue is scanty can stretch and split tissues linearly, producing an incised-looking wound.
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Listed sites include the scalp, eyebrows, cheekbones, lower jaw, iliac crest, perineum and shin.
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A fall onto a flexed knee or elbow, or a wound from broken glass or sharp stone, may also simulate an incised wound.
Stretch lacerations
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Overstretching of fixed skin causes laceration. Local pressure with increasing pull tears the skin and peels a flap from underlying bone or deep fascia.
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These injuries occur in motor vehicle run-over incidents, and the flap may indicate the vehicle's direction.
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They can also occur from kicking or when sudden bone deformity after fracture tears the skin, making the fracture compound.
Avulsion and flaying
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Avulsion is detachment of a traumatised surface or viscus from its attachments by sufficient shearing force delivered at an acute angle.
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Shearing and grinding by a heavy object, such as a lorry wheel passing over a limb, can separate skin from underlying tissues over a large area; this is called flaying.
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Underlying muscles are crushed, and bones may be fractured.