Bitemarks are, sadly, often a common feature of child abuse cases. While there is an ongoing argument about the use of bitemarks, there is often a requirement to determine if an injury has been caused by an adult or a child. I am frequently asked this question, and, when looking at the medical reports in such cases I often see:
Suspected bitemark on thigh – looks small and therefore consistent with a child.
The first thing we need to consider is what is a child with respect to bitemarks? We normally consider if someone is an adult or a child based on the legal framework of the jurisdiction. For example in this might be 18 years old, or, in some countries 16. However, when considering bitemarks we need to think about the biological development of teeth. We normally consider that the canine to canine (upper six and lower six teeth) are shown in most bitemarks, although it is possible to have more of less depending on the nature of the biter’s dentition, the dynamics of the biting and the bitten substrate.
The eruption charts are shown below for primary and permanent teeth. This presupposes that we can tell the difference, from impressions or marks on skin, between permanent and primary teeth. Not an easy task. Looking at the eruption we can see that by 22 months most infants will have their anterior primary dentition and by 12 years most children will have an adult anterior dentition. So our question really is now focussed on this – can we tell the difference between a 22 month old and someone over 12 years old? A dental adult is therefore someone over 12 and if our case involves a 30 year old, a 20 year old and an 18 year old – then we cant say much.
However, the more likely scenario is one in which a small child is bitten and the question is – was this one of the parents or adult carers – or could it have been a younger sibling or a child at day care or similar? The issue of the size of the bite can be traced back to this paper by Wagner in 1986 which was cited in much read paper by the American Academy of Paediatrics. The crucial line is this:
If the bite is human, and the canine-to-canine distance is less than 3.0 cm, the bitemark in question likely belongs to the deciduous dentition of a child.
So this is a really simple way of determining if a bite is caused by an adult or a child? Simply measure it. Before we look at the evidence for this, lets consider the measurement of injuries from skin. In short, they are not very reliable, especially when one considers the difficulties in taking images of small children, some of whom will be distressed. Photographic and skin distortion will be present and depending on the anatomical location could result in distortions of over 5 mm – so this needs to be considered – especially in marginal cases.
Lets assume that we have an injury that we agree is a bitemark and have measured it – at its widest point, and we will make the assumption that this is the inter-canine width. How useful is this information going to be? Lets look at these data from the orthodontic literature (and you can view the full paper here) Our measurement indicates that the bite is 33mm across. Given the advice from Wagner – this might suggest an adult – but we are also aware of measurement error. The table below shows the problem with this approach – a child of 8 years will have an average intercanine distance of around 32.5 mm and 45 year old will have 33.7! Given the standard deviations (around 3 to 2mm) and the measurement error these data show that size alone cant help us discriminate between the age of a biter.
Even if we consider the suspected child is only 3 years old, the average of 28.8mm, and the SD of 2.5 places the bitemark within the possible range.
Size therefore should only be one factor that is considered in such cases and the overall morphology of the injury must be considered, for example the presence of interdental spacing. Another cautionary note about bitemarks.