Methods of analysis of bite marks

Now, metric method analysis. Fourteen parameters are taken. Mesiodistal width of the twelve anterior teeth- six maxillary, six mandibular, two intercanine distances. And these are taken from the dentition of the suspect and positive impression bite mark and then they are compared. This comparison looks easy, but there are errors- subjective errors, observation errors. And this is a laboured, time consuming procedure. This, because of the errors, can give a varied response. Because the impression may have gone deeper and there might be swelling, inflammation, which can distort the results. So we may not get the correct response. In our study, many of the canines were not very clear also. That’s why we got some varied response also. So metric analysis,

various studies have been done worldwide and false matches are more, which is a serious, significant error. Studies have shown intercanine distance and canine are good in age and gender. There’s significant role. Facial reconstruction also intercanine distance plays a reference point. But when it comes to bite mark, our study and various other studies have shown intercanine distance do not play any role. They are insignificant. Canines also, as I said, play a excellent, significant role in age and gender identification. Bite marks, it’s insignificant. Next method is the hand tracing overlay method.

As shown in this figure, with the fine tip pen from the dentition cast, we are trying to mark it and the next picture shows you that with the ABFO scale, I have taken a scan of that dentition. Both maxillary as well as mandibular. Now this scanned picture is, using Adobe Photoshop, is laid over the bite mark making it one to one size. And then there is using the magic wand tool, we have tried to match the biting edges to the bite mark. Whether it is matching or not. As shown here, there is a little, on the top mark, there is slight deviation, but that could be, as I was talking about, the elasticity of the skin.

So when the bite mark was caused, it might have come down or it could have gone up. Now, probably that. But that with experience, we can say that. But that itself can become observer bias. If I think this is the probable accused, then I will take elasticity as a reason, and I’ll put this person as probable suspect. But if I am of the surety that this is not the suspect then I’ll say no no it is not matching and we move forward. The studies have shown that it is reliable but there are subjective errors and observer biases can be seen. That’s why it should be avoided. That is what some studies do mention.

Coming to the part, that is the dentition overlay. Now, we’ll take the dentition and we’ll overlay it to the bite mark. For that, there are three ways or three steps. One is only the maxillary. The second is only mandibular. Third is both. As shown in the picture the one is to one image size is done and anterior teeth are selected. That is overlaid on the bite mark photograph and seen whether there is matching or not, Just like what is on the hand tracing, the matching is there but in the peripheral, that is canines, it is not matching. Again, elasticity of skin could be the role play here. But we cannot confirmatory say that is the reason.

Similarly, we go to the other photograph. But again, the canines are not matching. So here we get this false, two people coming closer and only canines not matching so probabilities are high now that there are two suspects with similar bite marks. In our study, we saw that metric method showed no agreement. In hand tracing analysis there was a fair sort of agreement. But dentition showed substantial agreement, that is, there was dentition overlay when two investigators did it they showed substantial agreement between the two. But still, it was not 100% agreement. So to summarise, the bite marks, metric method is inaccurate. Canine, intercanine distance again inaccurate, unreliable and insignificant in bite marks.

But when it comes to hand tracing, it is reliable. But better to avoid it because it is more for exclusion criteria. Computer assisted also same as hand tracing, reliable as an exclusion criteria.

All of the following cases I will show you are real cases, most of which I’ve been asked to provide an opinion for. An example of the computer overlay method. The upper images show scanned casts with correct scales, lower image shows the injury also with the scale, although the scale is inadequate. Image of the completed computer overlay. Note that although this dentition did make this bite mark without any doubt, many dentitions could fit this injury in the same way. Also, note how imprecisely the dentition matches and the discrepancy is easily seen. This is due to 2-D, 3-D issues in the imaging and the fact that the skin has not accurately reproduced the shape and dimensions of the dentition in the injury.

Another image of a computer overlay. This instance, a lower arch compared with an injury. Note multiple areas of discrepancy. For all we know, my teeth could be made to match this injury just as easily. It is also difficult to see how the dentist managed to discern which part of the injury relates to the lower teeth. An example of metric analysis combined with the overlay method follows in the next three slides. Note here that there are actually two bite marks overlaying each other and that it is not really possible to tell which marks are made by which bite mark in the areas I’ve circled. This is important in what follows.

Here, we can say various linear dimensions marked on the injury and compared to the dental cast of the suspect. The punctate bruise located closest to the ‘B’ marked on the injury. This is the exact point where the two separate bites intersect. So how is the conclusion drawn that this small bruise belongs to this bite and not the other bite. A source of possible area added to all the other issues of distortion due to muscle tension, 3-D curvature and photographic error. The metric analysis combined with the overlay. Note that the two suspect dentitions are being compared to the same injury. Note also that it is impossible to say which of the two dentitions fits better.

It is just as likely that neither fit. You might as well toss a coin. And now we have the most basic technique. Placing the cast of a suspect directly onto the injury or comparing the cast and the injury side by side. Although this partially deals with the three dimensional issues and photographic measuring issues, it does not deal with the invariable distortion in the injury due to the nature of skin and underlying muscle. Nor does it deal with the fact that many dentitions are so similar in shape and dimension. These comparisons are no more valid than any other method.

This slide demonstrates the computerized version of direct comparison. It looks impressive, but similar to the computer overlay technique is really only a veneer of science applied to old techniques and does not deal with the fundamental issues inherent in the discipline.