bitemarks

Bitemark

Source: Dental Forensics

 

 

A bitemark has been defined as ‘a pattern produced by human or animal dentitions and associated structures in any substance capable of being marked by these means’ (Clark 1992).

This section will only be concerned with bitemarks made on human skin, and not inanimate objects. The analysis of bitmarks remains a considerable challenge to forensic odontologists (forensic dentists), and this section will provide only an outline of the subject.

When faced with a potential bitemark, it is important firstly to recognise it as such, so that steps can be taken to maximise the chances of collecting as much evidence as possible before the mark’s characteristics change, and for the appropriate personnel to be contacted so that expert examination of the mark takes place as soon as possible.

A bitemark can also be thought of as a form of ‘tool mark’ – a term used in forensic science to describe a mark made by an object or implement which can be analysed to provide some information about the characteristics of that which made it.

Other authors (Gall et al 2003) classify bitemarks as examples of ‘crush’ injuries, where each tooth compresses the skin and soft tissues, crushing them. This action leaves indentations and/ or breaks in the skin.

Marks made on skin (from teeth) during punching are termed ‘reverse bitmarks’. It is these wounds that carry a high risk of infection, and joint involvement, and must be thoroughly examined and irrigated prior to any definitive treatment.

Bitemarks can provide useful evidence in cases of assault (particularly in cases of Non Accidental Injury (NAI cases) – the evidence is of a comparative nature, and this section will outline the means by which this evidence can be collected and analysed.

Bitemarks may also provide a source of assailant DNA - assessment of these injuries must therefore take place after collection of biological trace evidence, where relevant (Sweet et al 1997).

 

epidemiology

 

Bitemarks are said to be seen relatively commonly (Knight 1996), and most commonly in the context of sexually motivated assault or NAI in children (Mason 2000). They are also seen in attacks on police officers, and during sporting activities, such as rugby football.

Bitemarks can also be self-inflicted, such as during a fall onto the face, or during an epileptic fit etc.

Where bitemarks have been inflicted in an attempt to draw suspicion on somebody else, the marks are to be distributed over areas of the body that are accessible to the biter’s mouth – and in common with other wounds of self-infliction, they are more likely to be superficial wounds.

Areas of the body most likely to bitten during assaults

(Adapted from Knight 1996; Clark 1992; Vale and Noguchi 1983; Freeman et al 2005)

  • Breasts (e.g. sexually motivated assault)
  • Arms
  • Legs
  • Face/ head
  • Abdomen
  • Back
  • Shoulder
  • Buttocks
  • Female genitalia
  • Hands/ fingers
  • Chest
  • Ears/ nose
  • Neck
  • Male genitalia

 

Children often bite each other, and in a nursery school setting, the teacher usually knows who the biting children are and will have experience of hearing a scream, and seeing the telltale rounded marks appear.

The Welsh Child Protection Systematic Review Group carried out a systematic review of the literature in order to answer the question 'can we identify abusive bites on children?'. They 5 out of 149 papers which met the inclusion criteria (confirmed abusive bites), and although these papers described the general characteristics of a bite, there was a paucity of data regarding the likely location and pattern of abusive bites in children (Kemp et al 2006).

The marks left by children’s teeth are much smaller than those made by adults in cases of NAI and the inter-canine distance is usually approximately >2.5 - 3.0 cm in children (2.5 - 4.5 cm in adults). (See Wikipedia for a discussion of the deciduous and permanent dentitions)

However, there is considerable variation between the ability of forensic odontologists/ dentists and lay personnel (such as social workers and police officers) to recognise bitemarks made by children as opposed to adults, and so this arbitrary size based evaluation of a mark is highly speculative (Nathanson 2000, Gall et al 2003, Wynne 2003 and Whittaker et al 1998).

pinterest - bitemarks

 

 

bitemark morphology

Dental characteristics of a human bite (Kemp et al 2006)

 

Bitemarks can be considered to be a spectrum of injuries ranging from a ‘suction’ mark, through marks indicating increasing violence ranging from bruising with no indentations to deep lacerations made by penetrating teeth (Knight 1996 and Clark 1992).

The 'bitemark severity index' has been developed by Pretty (2006 and 2007) in an attempt to standardise bitemark terminology. Bitemarks at each end of the scale are unlikely to possess unique identifying characteristics, whilst those in the middle of the scale are likely to present the 'highest level of significance', enabling the exclusion and inclusion of potential suspects.

 

 

Human bitemark severity and significance scale (Pretty 2006 and 2007)

 

 

Suction marks ('love bite' or 'hickey') are seen as a collection of punctate haemorrhages (small petechiae to larger bruises, depending upon the degree of suction applied). These are bruises resulting in leakage of blood from small vessels caused by the combination of suction and pressure of tissues being forced into the biters mouth and compressed against the palate and/ or incisors. Suction marks, when present are said to be diagnostic of a human bite (Clark 1992).

As well as bruising, a range of other injuries can be seen to make up a bitemark – abrasions or ‘parallel scrape’ marks from sharp or uneven teeth moving over the skin surface, and lacerations from deep tooth penetration into the epidermis/ dermis. Amputations of body parts (for example ears and noses) can also occur.

The human bitemark is classically a circular or oval mark (bruise) with central sparing, ranging from that made by a limited part of the dental arcade (for example marks left by the anterior teeth from canine to canine of the upper and lower jaws, separated by gaps at each side), or a more forcible bite leaving 2 concave ‘bows’ (with the concavities facing each other) with gaps at each end and having a suction mark in the middle.

Lower incisors tend to anchor the skin whilst upper anterior teeth bite down onto the tissue. There may therefore be a static line or curved mark left by the lower incisors and canines (these incisors are all the same width, unlike those of the upper jaw – a useful feature in distinguishing upper and lower jaws) with a more dynamic mark left by the teeth of the upper jaw, with scrape marks present.

Where the skin is bruised during the biting process, the marks made will distort over time, as the bruising diffuses into surrounding soft tissues. Speed of examination is therefore of the essence, before distortion obscures individual teeth marks within the bite. Abrasions within the mark, however, retain their morphology, and can be useful features for the forensic odontologist to find.

The clarity of the bitemark is affected by the location of the body part being bitten, in particular whether the area is curved, and its degree of flexibility.

Bitemarks are usually formed during a highly dynamic process – the biter and the person being bitten will be moving about during the assault, and this leads to a degree of distortion.

Bitemarks may therefore not present themselves as regular curved marks, but as a complex wound featuring multiple arcades, bruises and abrasions.

There are several well described mimics which should be borne in mind when faced with a potential bitemark, including defibrillator pad marks, the bottom of a glass bottle, the heel of a shoe, jewellery and children’s toys (Clement 2003 and Clark 1992).

bitemark analysis

 

The clinical history

When faced with a person who has allegedly been bitten, a history of the bite/ assault should be ascertained;

  • When was the bite inflicted?
  • Which part(s) of the body were bitten?
  • Which position(s) were the bitten parts in at the time?
  • Did the bite take place through clothing? Has this clothing been submitted for examination already?
  • Has the skin been washed since the assault?
  • Does the person suffer any from any condition liable to have influenced the appearance of a bite mark/ bruise? (e.g. Bleeding diatheses or clotting disorder etc)

 

The examination

As with all injuries, the wound should be described (areas of bruising, abrasions and lacerations etc) and should preferably be sketched.

Photographs should be taken with and without scales, and preferably with the bitten part being held in the same position as it was in when it was bitten.

At this point, if the history and examination do suggest a human bite (see below for the features of animal bites), a forensic physician should be contacted, and a request made for expert examination of the wound, together with a forensic odontologist (if available).

Most changes to the morphology of a bitemark occur in the first 24 hours, but indentations will be lost within hours, and so the maximum amount of evidence to be gained from a bite mark must be secured as quickly as possible after the assault (Clark 1992 p.157).

 

Trace evidence collection

The forensic physician can then supervise additional procedures, such as swabbing the wound for DNA (buccal cells)/ serological evidence (secretor status – ABO antigens corresponding to the blood group are secreted by 76% of the population in their saliva). Bowers (2006) points out that such evidence may be of vital importance where bite-mark evidence is subsequently challenged by defendants.

Swabs can either be taken by using sterile cotton-tipped swab sticks, or moistened cigarette papers placed between microscope slides prior to submission to a forensic science laboratory. Control swabs are taken from the opposite side of the body to that of the bite (Mason 2000, Knight 1996 and Girardin et al 2003).

 

Photography

The expert examination will also involve the photographing of the wound from several different angles, using a selection of scales (such as the American Board of Forensic Odontology (ABFO)  ABFO No. 2 scale), and with the body part in different positions.

Photographs taken from a distance to show the relationship of the mark with the rest of the body are useful, as well as the close up images.

Serial photographs are useful in illustrating the changes of the mark over time, as some aspects of the mark may become clearer (Wynne 2003).

 

Examination of victim's dentition

The odontologist will also supervise the examination of the victim’s dentition (with or without the taking of teeth impressions) and the taking of bitemark impressions (for example with fast setting rubber or silicone based materials) for later comparison with a suspect’s dentition.

 

Distortion of bitemarks

One of the main drawbacks with the analysis of bitemarks is the effect of distortion on the mark, and the subsequent difficulties matching the mark with a suspect’s dentition.

Distortion can occur at different stages of the bite, and it’s examination and evaluation (Sheasby and MacDonald 2001 and Clark 1992).

Distortion of the mark can occur during the biting process itself (primary distortion), which is a dynamic process, and thus the degree of distortion is proportional to the degree of movement.

Each episode of contact between the teeth of the biter and the victim’s skin represents a unique event, which will produce a unique mark. A single assault may therefore result in bitemarks that vary in appearance, even though they have been caused by the same dentition.

Because skin is elastic, and the amount of elasticity varies with age and body part, the second aspect of primary distortion is a reflection of the response of skin in various body locations to being bitten.

Not only is skin extensibility highly variable, but thickness varies from site to site, and the presence of support (i.e. bones) beneath the skin surface all act to modify the response of a particular body part to biting.

The response of the body to injury can also distort the appearance of the bitemark – for example bruising and swelling from oedema etc.

Bitemark distortion can also occur at later stages of the evaluation of the mark – for example, the process of photography can result in huge distortion of the appearance of the mark.

The position of the bitten body part during the assault should be replicated for the photography, and where this is not possible, some authors have even suggested that any further analysis of that mark is meaningless (Sheasby and MacDonald 2001).

The photography of bite marks and the use of various scales is a complex subject area, with no consensus as to the correct approach. Each forensic odontologist has to justify his/ her evaluation and conclusions, and will be open to cross-examination on the limitations of such an approach.

The general principle is to obtain a 1: 1 representation of the mark with minimum distortion.

Some authors favour 2 rigid scales to be used at right angles to each other, whilst others prefer the use of scales following natural curves of the bitten part (Robinson 2000 and Mason 2000). Others favour a mixture of rigid scales aligned to the plane of the camera film with a flexible scale following the curve of the bitten part (Clark 1992).

Ultraviolet photography may also be used to pick up detail not capable of being seen with the naked eye. This technique can demonstrate past injuries due to abnormal pigmentation at the site of a previous injury, but this ability can also cause problems of interpretation as well (Clark 1992).

 

Comparison of bite mark and suspect dentition

In order to make any useful interpretation of a bitemark, one must have something to compare it to. Where a suspect is identified, the forensic odontologist examines the suspect’s dentition and prepares impressions of the biting surfaces at different angles as well as models of the entire dentition.

The power to obtain such impressions is found in the Police and Criminal Evidence Act 1984 (PACE) as amended by the Criminal Justice and Public Order Act 1994.

Bitemark comparison evidence is highly controversial, and much of this stems from the lack of agreed standards of conformity (as exists for fingerprint evidence) and the potential for distortion at all stages of the evidence collection and evaluation process, as detailed above.

Assessment of the probability of a suspected person’s dentition making the bite mark involves a subjective judgement, and requires considerable experience and expertise (Whittaker et al 1998).

Some argue that the identification process is one of exclusion only – i.e. one can only state categorically that a particular suspect’s dentition could not have created the bitemark under investigation. A positive identification of a suspect from bitemark evidence thus falls into the realm of opinion evidence – a finding of conformity by the court therefore depends upon the credibility and weight attached to the evidence of the expert witness, and of course the skill of the advocacy (Mason 2000).

The process of bitemark comparison evaluations involves superimposition of the suspect’s dentition onto the bite mark. The aim is to identify sufficient correspondence between the sizes and shapes of the teeth of an accused with the features of the bitemark, taking account of the limitations caused by distortion.

Unique features such as missing teeth, abnormally aligned teeth or damaged teeth provide particularly useful reference points (Sheasby and MacDonald 2001 pp.77-78).

 

Features of a suspect’s dentition useful in bite mark analysis

(Adapted from Knight 1996 and Clark 1992)

  • Shape of the dental arch (any rotations, abnormal positions, gaps or missing teeth)
  • Number of teeth present in each jaw (odontogram)
  • Presence of dentures/ sharp denture clasps
  • Distortion of occlusive surfaces during biting (occlusal registration)
  • Occlusal level of teeth within the jaw
  • Broken/ fractured teeth (particularly incisal fractures that may be responsible for abrasions)
  • Any prominent teeth
  • Biting pattern at various angles including bite overhang

Traditionally, the process of superimposition involved inking the occlusal surfaces of the suspect’s teeth (on a model) and marking the teeth ‘bite’ pattern onto an acetate sheet. This sheet would then be placed over a true 1:1 photograph of the bitemark, and the sheet moved around to see if there was any match between the two (Clark 1992 and Knight 1996).

With the advent of faster computer processing ability, photographic software packages such as Adobe’s Photoshop® has taken centre stage in the ‘overlay’ process (Clement 2003). Scaled photographs of both suspect dentition and bitemark can be superimposed by a relative ‘fading in and out’ of one over the other. The process can be documented at each stage, and easily reproduced, without harming the evidence itself.

 

Reconstruction of bitemark

Source: Visible Proofs

 

These methods rely on the superimposition of 2D photographs representing 3D structures, and are therefore inherently inaccurate. Thali et al (2003) and Blackwell et al (2007) describe the use of a 3D-CAD approach to the superimposition process, where the bite mark is photographed from different angles and software used to build a 3D virtual model of the mark.

The same is carried out for the suspect dentition, and the 2 ‘virtual models’ can be manipulated in relation to each other in virtual space. The process of the bite being performed can be evaluated, as the relative positions of each tooth can be assessed at different biting depths.

From their work, it is clear that most important area of the bite mark for the evaluation process is that made by the anterior teeth, where there is the least amount of tissue expansion. As the bite progresses, there is increasing distortion and then the lateral teeth make their marks.

dog skull

 

Photo Credit: Fifi LePew via Compfight cc

 

Photo credit: Fifi LePew via Flickr CC

animal bitemarks

Dog attack

Source: Wikipedia

 

Having established that a mark is indeed a bitemark, one must also determine whether the mark was made by a human (child or adult) or an animal.

In a live victim, this matter will usually be simple to ascertain – from the history. However, in a young child, or unconscious person, the true nature of the injury may not be immediately apparent.

Domestic animals are implicated in the majority of bites – particularly large breeds of dog (pit bull terriers and German Shepherds etc) – and they are usually known to the victim (either a family pet or that of a neighbour).

Besser (2007) points out, however, that there is some evidence that most common dog bites are caused by Staffordshire bull terriers, Jack Russell terriers, medium sized mongrels as well as Alsatians (German Shepherds) - breeds of dog nor covered by the Dangerous Dogs Act 1991 - prohibiting pit bull terriers, the Japanese tosa, the Argentine dogo and the fila Braseleiro).

Morgan and Palmer (2007) indicate that each year, 250,000 people who have been bitten by dogs attend trauma units in the UK. Besser (2007) indicates that 70,000 people attended UK Accident and Emergency units for dog bites in 2002, and that many were the result of attacks on children by the family pet in the home. 4133 patients were admitted to hospital in England in 2006, as a result of injuries caused by dog bites.

There are approximately 1-2 million animal bites annually in the USA, and this is thought to be an underestimate (Clark et al 1991). Of these, approximately 10 to 20 dog bites are fatal (Brogan et al 1995).

Canine teeth 'anchor' the victim, whilst other teeth bite and tear tissues (the 'hole and tear' appearance).

De Munnynck and Van de Voorde (2002) reviewed fatal injuries caused by dogs, and indicated that the features to be considered 'pathognomonic' for dog bites are;

  • puncture wound(s) (caused by canine tooth)
  • wound with ragged and irregular edges - 'stretch lacerations' (caused by other teeth in the process of biting, shaking and tearing and sometimes including avulsed tissue with irregular borders resembling a dental arch outline)
  • claw marks (multiple, parallel, linear scratches or drying scuff abrasions)

Biting force is variable (depending upon the breed of dog), ranging from 310 kPa - over 30,000 kPa - resulting in potentially devastating injury and tissue devitalisation. De Munnynck and Van de Voorde (2002) advise that vertical forces exceeding 450 pounds per square inch (31 x 104 N/m2) have been measured during a dog attack - sufficient to penetrate sheet metal.

Children (particularly boys aged 1-6 years) and the elderly are most vulnerable, and the dog usually bites for territorial reasons.

The head and neck are targeted in particular, and Brogan et al (1995) identified a large percentage of child dog bite victims to have suffered serious head, neck and facial injuries including fractures and deep neck injuries requiring surgical exploration.

They urge hospital doctors to consider the possibility of underlying fractures and damage to deep structures whenever they are faced with a child dog bite victim.

In dog bites, the anterior segment of the dental arch is much narrower than in a human, (giving a ‘U’ shaped appearance as opposed to the rounded shape of the human bitemark) and the canines are more conical, curved and much larger (Clark 1992, Gall et al 2003).

Dogs (and cats) have asymmetric maxillary and mandibular arches, and the canine lower arch is narrower and shorter than the upper. However, the shapes of the arches are breed dependent, and so the shape of the wound will also vary according to breed (Clark et al 1991).

The feline bite is much shorter and more rounded than a dog bite, and they have small round puncture marks, and are often associated with parallel scratches from ‘clawing’. 

Rodent bitemarks consist of long grooves caused by thick ‘chisel’ shape cutting edges of the central incisors. These marks are often seen on bodies recovered after some time post-mortem rather than in live persons, but could conceivably be seen on the bodies of neglected infants living in unsanitary housing conditions.

Wounds caused by wild animals tend to be more severe – for example grizzly bears have been known to cause severe ‘scalping’ injuries and large cats severe neck wounds accompanied by deep incised wounds from claws (Wyatt 2003).

references

 

 

Additional references

Papers included in the The Welsh Child Protection Systematic Review Group abusive bites on children review;

3D CT/MRI visualisation of bitemarks

 

online resources

 

 

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