Sharp force - tools

• Tool Characteristics

  •Type and size of tool may be discerned

  –An emphasis is placed on the identification of sharp weapon “class” characteristics as  opposed to “type” characteristics.

  •Three categories of tool (defined by delivery method):

  –Stabbing

  –Cutting

  –Chopping

  •Presence of striations indicates toothed blade

  •Placement of wound also indicative of tool type

  –E.g wound on anterior vertebral body indicates long instrument
• Tool Characteristics

  •Size relates to blade characteristics

  –Length

  –Width

  –Surface contour (e.g serrated)

  •Sharpness

  •Plastic characteristics of bone often prevent the interpretation of a direct relationship between tool size and  wound size

  •Expert predictions of knife design and dimensions from wound defects are commonly attempted, though not so  commonly substantiated.
• Saw Wounds

  •Knives and saws have little in common

  •Saw marks involve a multiplicity of repetitive movements

  •Generally reserved for situations of post-mortem dismemberment and mutilation.

  •Presence of saw marks in bone is rare and commonly considered of little use forensically.

  –Seldom do saw marks get more notice than a presence or absence to  the forensic examiner.

  •What knife wounds and saw marks do have in common is their  potential to produce characteristics that may contribute to a  better understanding of a crime.
• Saw Characteristics

  •Crosscut saw

  –Cut across grain

  –Edges of teeth angle at ~ 70o to bone long axis

  •E.g carpenters saw

  •Rip Saw

  –Cut along grain

  –Edges of teeth angle at ~90o to long axis

  •Chisel action

  •Angle of teeth produces wider kerf (cut) than  blade width for both types

  •Number of teeth varies widely between saws
• Characteristics of Saw Wounds

  •Three types of marks caused by saws to bone:

  •Superficial false starts

  –Initial low pressure draw across bone

  –can be misidentified as knife cuts

  •False start kerfs

  –Shallow incomplete saw kerf

  –Caused by bouncing of saw blade during push stroke

  •Sectioned bone cuts

  –Deep kerfs

  –Indicate that a number of strokes have taken place
• Characteristics of Saw Wounds

  Striae

  •Kerf walls display details of saw blade

  •Formed by uneven action of teeth on wall

  –Small teeth (power saws) create fine striations

  –Large teeth/ few per cm (hand saws) create prominent striations.

  •Floor of kerf can display characteristics of teeth

  –Rough floor = course saw

  –Smooth floor = fine toothed saw

  •Kerf floor lost when bone is completely transverse

  –Only useful for false starts

  •Traits can remain visible on the breakaway spur

  
• Analysis of Saw Marks

  Direction of Cut

  •Direction of saw progress

  –e.g anterior to posterior shaft

  –Determined from location of false starts and  breakaway spurs

  •Direction of saw stroke

  –Identification of exit chipping

  –Initial contact surface sharp and well defined

  –Surface through which teeth emerge during  push stroke possess roughened flakes

  •Particularly prominent with rip saws
• Analysis of Saw Marks

  Blade width and shape

  •Blade width cannot exceed kerf width

  –Intact cut can be measured for approximation of blade width

  –Blade width generally slightly less than kerf width

  –General rule: blade width = kerf width/1.5

  •Striae left by blade can indicate blade shape

  •Fixed radius striae

  –Circular blades

  •Semi-circular lines on kerf wall

  •Uniform shape to striae throughout cut

  •Non-fixed radius striae

  –Straight, rigid and non-rigid blades

  •Rigid blade = straight striae

  •Deviation from cutting angle can be present
• Analysis of Saw Marks

  Source of saw energy

  •Hand saw

  –Back and forth movement = uneven kerf walls

  –Groove walls

  –More striae than power saws

  –Change in sawing direction can result in bevelled  kerf edges

  •Power saw

  –Smooth, polished kerf walls

  –Straighter cuts

  
• Direction of Force

  •Injuries to anterior skeleton in absence of posterior  injuries suggests anterior delivery

  –Reverse true for posterior injuries

  –Complicated when both ant and post injuries present

  •Entry wound is larger than exit wound in sharp  trauma

  –Reverse of ballistic trauma

  –Caution! Entry and exit wound could be two separate  events

  •Rare to be able to determine position of force origin  when multiple sharp force injuries are present

  •Victim protecting themselves from sharp force  attack results in multiple wound placement.
• Weapon direction and path

  •Spatial relationship between  attacker and victim

  •Handedness of attacker

  •Macroscopic evaluation may  be difficult

  •SEM more appropriate

  •Anatomical positioning

  •Knife cut wound(KCW) angle

  •Thick vs thin end

  •Hinge fracture

  •Edge/point terminal

  •Blade sheen

  •Remember bone is viscoelastic

  •And…SFT is combination of  energy forms
• Number and Sequence of Events

  •Simply count number of separate primary incidents

  •Simple with incisions and punctures

  •Clefts difficult due to overlapping blows being obscured  by wastage

  •Over and underestimation of sharp trauma incidents is  common

  –Single traumatic incident can cause two or more separate  wounds

  –Traumatic incident may not leave mark on bone

  •NEVER compromise the integrity of the evidence by  trying to fit the object into the hole it produced.

  •Sequence of sharp force blows cannot be determined

  –Only possible if fracture lines are present

  •Hacking trauma
• Animal Scavenging

  •Scavenging marks on bone can be  confused as sharp force trauma

  •Can manifest as puncture marks, pits  and scoring.
• Identification of Hacking Tools

  •Studies considering differential wounding characteristics of a meat cleaver,  machete and axe.

  •Wound characteristics enabled identification of implement

  •Pig limbs used

  –Human analogue?
• Sharp Force Trauma Analysis

  •Differences between hatchet and knife microscopic  wound characteristics

  •The bone lesions produced were macroscopically  almost identical exhibiting a linear and narrow  lesion.

  •Microscopic features identified using SEM

  •Knife

  –Straight edges

  –One side of kerf exhibited flaking and raising

  •Hatchet

  –Uneven edges

  –Compression of bone away from wound on both sides of  kerf
• Sharp Force Trauma Analysis

  •Analysis of striations caused by scalpel, paring  knife and kitchen knife using SEM

  •Major finding:

  –Same instrument can produce different striation  patterns

  –Overlap between weapons

  •Striations from pairing knife and scalpel similar

  •Conclusion:

  –Weapon ID cannot be specific (scalpel, vs small  blade knife) only general (e.g axe vs knife)