In television and movies, you’ll sometimes see a scene where the pathologist describes to a detective the specific dimensions of a knife used in an attack, all based on the autopsy findings. While this is no doubt extremely helpful to an investigation (and the limits of a 60-minute run time) it’s also impossible to provide this level of detail from an autopsy. Unfortunately the pervasiveness of such creative liberties in pop culture creates unrealistic expectations from juries, lawyers, and law enforcement, which we often need to gently correct. There are certain characteristics we can identify which may clarify what kind of blade was used, but conversely may not match the dimensions of the weapon at all.
While any object with a pointed or edged tip can be used to inflict sharp force injuries, we will focus on knives as the stereotypical ‘sharp force’ weapon. As a reminder, sharp force injuries have clean, neat edges and no tissue bridging in the wound depths. These are the features that are used to distinguish sharp force from lacerations, which are skin tears due to blunt force injury (see last month’s blog post for a refresher!). The most important first step for any instance of sharp force injury is to fully x-ray the body; this allows for identification of potential evidence to discover (such as a broken knife tip), and identifies potential sharp hazards for the prosector.
The first issue when interpreting sharp force injuries at autopsy is distortion of wounds by skin elasticity. Depending on the location and orientation of the wound, the edges may be pulled by tension along Langer’s lines. If this is the case, re- approximating the wound edges is needed before measuring for better accuracy. The ends of a stab wound may reflect whether a blade has one or two edges, depending on whether both ends are tapered (two-edged) or if one end is blunt and the other tapered (single-edged). The morphology isn’t always clear, though. Occasionally a double-edged blade will have a blunt-edged segment known as the “ricasso” just before the crossguard or handle. If the blade is fully inserted, the impression of the ricasso may make the wound appear to have two blunt edges.
The second difficulty faced is that knife wounds are rarely inflicted on a stationary body – there is typically (at least initially) a struggle involving motion of both the victim and the blade. This can result in wounds which are distorted from the dimensions of the weapon. These kinds of wounds also don’t necessarily imply that the person holding the knife was ‘torturing’ the victim – it just means that there was motion between the body and the blade. For example, a ‘dovetail’ or “V”-shaped wound can be created if the knife is inserted and removed at different angles – this could happen if either the victim is moving, if the assailant moves the angle of the knife, or both.
Another relatively frequent source of confusion is depth of wound relative to the length of the blade. All soft tissues are compressible, but certain parts of the body have a little more give. For example, the relatively soft abdominal pannus and viscera are easily compressed, whereas the chest is held in a rigid position by the rib cage. Therefore it’s very possible for a 5” blade to leave a wound 8” in depth if inflicted in a soft location like the abdomen. Additionally, if a blade isn’t inserted to the full length, the wound depth could also be shorter than the weapon.
One concerning trope on television is the immediate reconstruction of a crime based on autopsy findings. As one example, much emphasis is placed in the media on identifying ‘defensive wounds’. However, there is no way to tell at autopsy if a wound was sustained defensively or aggressively (and to be fair, even if the incident was caught on camera there could be differing interpretations). There are characteristics which are more commonly seen in ‘defensive’ wounds, such as locations on the hands and forearms as the victim attempts to block or grab the weapon. Hand wounds can also be seen on the assailant, though, as blood can make their hand slip onto the cutting blade during thrusts. Similarly, it is never possible to ascertain the “handedness” of an assailant from the wound pattern.
Serrated knives may leave a characteristic pattern of abrasions on the body but may leave no mark depending on the angle of the wound. So, if such marks are present, it’s distinctive of a serrated weapon – but if they’re absent, the weapon could be either a serrated or straight edged blade.
Contrary to what is shown on TV, there is no way to definitively ‘match’ one specific knife to a particular wound. The closest we could get would be finding a broken segment of the blade within the body – and even then, one must imagine there is more than one knife in the world that has a broken tip. While the world would be a much simpler place if such answers were possible, it’s our duty as scientists and physicians to be honest about the limitations of science and avoid speculation.
-Alison Krywanczyk, MD, FASCP, is currently a Deputy Medical Examiner at the Cuyahoga County Medical Examiner’s Office.
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