Sharp force

• Incidents

  • 2013 12% of all US murders involved knives

  • 2011-12 knife related crimes in Scotland:

  –51% of all homicides attributed to sharp implements

  • 2011-12 knife related crimes in England and Wales:

  –39% of all homicides attributed to sharp implements
• Effects of Sharp Force on Bone

  •53% cases have bone/cartilage injuries (BCI)

  –BCI mostly related to cutting not stabbing

  •Thoracic injuries most common

  –Ribs/sternum involved in 56% of BCI lesions

  •Preferential anatomical sites ribs, laryngeal cartilages, vertebral  column

  •Rarely lower limbs & likely defensive if present

  •In fatally stabbed:

  –69% injuries on left side

  •R handed attackers

  –Suicide cases focussed on larynx; can be very minor so often  missed at PM
• Sharp Force Trauma

  •Result of narrowly focused, dynamic compression forces applied to the  surface of a bone

  –Puncture

  –Incision

  –Cleft

  •May result in:

  –Fracture lines

  –Hinge fractures

  –Wastage

  •Caused by:

  –Variety of implements with narrow focussed point or edge

  •E.g knife, axe, saw, syringe
• Sharp Force Trauma Features

  •Requires compressive force

  •Results in wound formation at point of impact

  –Dependant on:

  •Direction

  •Focus

  •Energy of force

  •Injury types:

  –Puncture – Cone shaped focus perpendicular to bone surface

  –Incision – Force applied over long, narrow surface area.

  –Cleft/notch – Dynamic force from a long, sharp edged implement  applied perpendicular to bone surface
• Sharp Force Trauma Features

  •Fracture lines (rare)

  –Same rules apply as with blunt and ballistic trauma

  •Spread outward in radiating lines from punctures and clefts

  –Concentric fractures rare

  •Hinge Fractures

  –‘green bone’ effect

  –Can vary in size depending on implement used

  •Axe vs knife

  •Elastic recoil with minimal plastic deformation

  –Compression followed by expansion of hard tissue  surrounding impact point

  –Bone bends away from force, then recoils back to  original position

  –Resultant wound can be smaller than instrument causing  it.
• Sharp Force Trauma Features

  •Wastage

  –Fragments of bone separated from main  section

  –Most commonly associated with clefts  from heavy chopping instruments

  •Striations

  –Present on walls of primary injury

  –Striations etched by passage of  implement

  –Run parallel to direction of applied force

  –Small striations = sharp blade

  –Large striations = dull or serrated blade
• Punctures

  •Caused by pointed instruments

  –Force directed vertically to bone surface

  •Forms indentation at point of contact

  –Small section of bone can be broken inwards

  –Thin bones particularly vulnerable to puncture

  •Cross-sectional morphology of puncture wound

  –Cone shaped pit with apex at base of injury
• Punctures

  •Depth of puncture dependant on:

  –Force

  –Implement

  •Heavy force can puncture the cortex of long bones

  •Striations visible with magnification

  –Appear perpendicular to surface of bone

  •Fracture lines and hinge fractures can occur in conjunction

  •Wastage is rare
• Incisions

  •Typically caused by sharp forces being drawn across bone surface

  –May be caused by forceful stabbing

  •Cuts can be so thin that they appear as lines

  •Cuts can be deep and have a V-shaped morphology

  –Can be mistaken for cleft

  •Characteristics dependant on:

  –Implement

  –Energy used

  –Area of bone contacted
• Incisions

  •General rule:

  –Long instruments contacting over large area cause long incisions

  –Short instruments contacting small bones will cause small incisions

  –More difficult to make relationships for thickness

  •Elastic recoil of bone

  •Movement of blade within incision (sawing action)

  •Fracture lines rare

  –Energy across rather than vertically oriented

  •Hinge fractures common in forceful attacks

  –Orange peel appearance

  •Striations appear parallel to long axis of cut
• Clefts

  •Caused by vertical forces applied by heavy instruments with  long sharp edges

  –More characteristic of hacking trauma

  –Extreme power required

  •Presents as V-shaped notch of varying sizes

  •Heavy instrument required (axe)

  •Accompanied by extensive fracture lines

  –Radiate from cleft

  •Hinging and wastage common

  •Striations appear vertical to the bone surface

  –Due to vertical origin of force.
• Characteristics of Sharp Force Trauma

  Eight characteristics of sharp force trauma

  –Cross sectional shape

  –Width

  –Depth

  –Length

  –Striations

  –Fracture lines

  –Hinge fractures

  –Wastage

  •Direction of Kerf (cut) ‘pull’ can sometimes be discerned

  •Magnification useful for identification of most features
• Sharp Force Wound Analysis

  Description of wound

  •Wound type (puncture, incision, cleft)

  •Associated features (e.g fracture lines)

  •Location of wound

  –Using anatomical terminology

  •Size (length, width and depth)

  •Number of wounds

  –Attempt to comment on sequence

  •Analysis equipment

  •Miscellaneous details

  •Describe