Mechanical Injuries — Abrasion, Contusion (Bruise), Laceration, Incised and Stab Wounds

A mechanical injury is damage to the body caused by physical force. Forensic medicine classifies it by the type of force, because the pattern of the wound points to the weapon and to how the injury happened — questions a court will ask.

An abrasion is the removal of the superficial layer of skin (mainly the epidermis) by friction or pressure. It bleeds little, and if it stays in the epidermis it heals without a scar, usually within about 2 weeks. Despite being minor clinically, it is one of the most informative injuries in forensic medicine.

• What an abrasion proves: an object touched the skin at an angle of less than 90°, with movement and friction between them.
• Patterned (imprint) abrasions reproduce the surface of the object and can be matched to a weapon.
• Trace evidence: soil, glass, fibres or paint in an abrasion are collected before cleaning.
• Antemortem vs postmortem: a postmortem abrasion is yellow-brown and parchment-like with no vital reaction; an antemortem one shows bleeding and inflammation. Histology of healing stages helps estimate age.

A contusion (bruise) is bleeding into tissues from small vessels ruptured by blunt force, with the skin surface intact. It can occur in skin, mucosa and internal organs (for example a cerebral or pulmonary contusion).

• May appear late: deep bruises may not be visible for days, so a normal first examination does not exclude blunt force. Re-examine after a day if the history suggests injury.
• May appear away from the impact: blood tracks along tissue planes. Raccoon (periorbital) eyes after a base-of-skull fracture are the classic example — they are not from a blow to the eye.
• Patterned bruises reproduce the object — a shoe sole, fingertip pressure or a tramline bruise (parallel linear bruises with a pale strip between them) from a single elongated weapon.
• Bruise vs postmortem lividity: in a bruise, blood has escaped into the tissues; in lividity it stays inside vessels.
• Bruises are harder to see on dark skin, so absence of visible bruising proves little.

A laceration is a tear of skin and underlying tissue caused by blunt force — a direct blow, crushing, or stretching of tissue. It is a blunt-force injury even if the doctor is tempted to call it a 'cut'.

• Margins: ragged, irregular and usually bruised and abraded.
• Base: irregularly torn, with strands of subcutaneous tissue bridging the wound — the key feature that separates a laceration from an incised wound.
• Scalp trap: on the scalp, a laceration can look like an incised wound. A hand lens shows the ragged, bruised margins and tissue bridges.
• Trace evidence (soil, glass, paint) may be found in the wound.

Sharp-force injuries are caused by objects with a cutting edge or a sharp point. An incised wound (cut) is made by the edge drawn across the skin; a stab wound is made by a point driven in.

• Stab wound shape gives the blade: both ends sharp suggests a double-edged blade; one sharp and one blunt (square or split 'fishtail') end suggests a single-edged blade.
• Hilt (guard) mark: a bruise or abrasion around the wound (sometimes showing the hilt pattern) means the blade went in up to the hilt.
• V-shaped notch: twisting of the knife or movement of the victim during withdrawal.
• Clothing: compare cuts in clothing with wounds; their absence over a wound can point to self-infliction.
• Every stab wound is a surgical emergency until deep injury is ruled out — the neck, chest, abdomen and groin are highest risk.

Defence wounds are injuries the victim gets while trying to ward off an attack. Their presence points to homicide and a conscious, resisting victim; their absence may mean a surprise attack, restraint or incapacitation.

• Site: hands, forearms and fingers. In one homicide series, defence injuries were on the hands in 80%, forearms in 65% and fingers in 40%.
• Sharp weapon: mostly incised wounds — classically on the fingers and palms and the back of the hand.
• Blunt weapon: contusions and fractures of the forearm and hand.

Hesitation (tentative, trial) cuts are the opposite clue — they point to suicide. They are found in about 70% of sharp-force suicides: multiple, superficial, parallel cuts near the main fatal wound, commonly on the forearms and the neck.

Most serious blunt trauma comes from motor vehicle crashes and pedestrian injuries; falls are the other major cause, especially in the elderly. Severity depends on the mechanism of injury and the patient's comorbidities, so the medico-legal report should record the mechanism as well as the wounds.

1. Number each injury separately — never write 'multiple injuries'.
2. Type (abrasion, contusion, laceration, incised, stab) using exact terms.
3. Site in relation to fixed anatomical landmarks.
4. Size — length, breadth and depth (depth of stab wounds is often known only at surgery).
5. Shape, margins, colour and pattern — patterned injuries can identify the weapon.
6. Direction of force (for example the heap of epidermis in a graze).
7. Trace evidence (glass, soil, fibres, hair) collected and sealed.
8. Photographs with a scale before and after treatment.
9. Age of injury stated only within honest limits; histology (vital reaction, inflammation, haemosiderin) is more reliable than colour.