Forensics 101: Tool Marks in Bone

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When skeletonized human remains are recovered, sometimes the only evidence police and scientists have to determine cause of death is the bones themselves. The bones are examined to identify any remnants of tool marks—also called kerf marks—that might indicate a traumatic injury. If the body has been dismembered, those kerf marks can lead directly to the tool(s) used postmortem, even if they might not directly suggest a cause of death.

Any sharp implement applied to bone with sufficient force will leave a distinctive imprint, be it scavenger teeth or a cutting tool. Examination of the bone both macroscopically (using the naked eye) and microscopically (using a light microscope/scanning electron microscope for magnification) can provide crucial information, since each tool leaves a characteristic mark that can assist in its identification.

Types of cutting tools:

Knives: Knives are narrow bladed and leave a corresponding narrow ‘V’-shaped trough in the bone. They are single or double-bladed (double-bladed knives can leave an opposite ‘V’-shaped trough in surrounding bone) and tend to leave behind only microscopic striations.

 

Saw: Saw blades come in many different form factors, but uniformly leave a wider, square-bottomed trough in bone. They tend to leave distinguishable striations that are easily seen by the naked eye. Individual blade and tooth size can be identified based on the striations, as can the blade type—ie. straight or rotary. Saw kerfs often have characteristic accessory marks, such as false start notches, especially when manual saws are used.

 

Axe: Like a knife, an axe leaves a smooth ‘V’-shaped trough in the bone, but the defect is very wide and is often significantly deeper due to the lever action of swinging an axe. These kerf marks are often accompanied by microscopic or macroscopic impact fractures and/or flaking surrounding the contact site.

 

Besides providing information about the type of tool used, kerf marks also provide contextual information, including the handedness of the attacker, the relative positions of attacker and victim, whether the wound was self-inflicted, and the motion of the blade (cut vs. stab). With this information, police can determine not only the most likely weapon used, but how the murder was committed, and these details can often be used to definitely identify the murderer.