Forensic Dentistry

Forensic dentistry (odontology) is the field within the greater disciplines of dentistry and forensic science that evaluates, manages, and presents dental evidence in legal proceedings in the interest of justice.¹ Forensic dental casework often involves identification of unidentified or missing individuals, human remains, or victims of mass fatality incidents, including natural and accidental disasters. This is accomplished by comparison of a victim’s dentition and supporting structures with dental records of known individuals. The latter records may be obtained from private dental offices, prison or military dental databases, or records retained by the Federal Bureau of Investigation through its National Crime Information Center’s Missing, Unidentified, and Wanted Persons files in a Web environment.²

Since forensic odontology is one of several forensic specialties, the forensic dentist’s role often interfaces with those of the anthropologist, criminologist, toxicologist, pathologist, and law enforcement official involved with a case. Only a brief summary is given here with general references added to cited references for further reading.

JAWS AND TEETH

Unlike the static, genetically determined friction ridges found on the human hand and foot (commonly referred to as fingerprints and footprints) each individual’s dentition is fluid and changes throughout life as the deciduous teeth are exfoliated and the permanent dentition erupts. In addition, the teeth are subject to dental and periodontal disease, the oral manifestations of systemic diseases that may affect the individual, and alterations of dental structures related to techniques of restoration and replacement that are employed by the dental practitioner.

Historically, despite this potential for an ever-changing dentition, inspection of the teeth and jaws has been used as a legally accepted method of human identification. Significant cases in which identification has been resolved by means of examination of dental evidence include identification of Revolutionary War hero Dr. Joseph Warren, the prominent Boston murder victim Dr. George Parkman, and Adolph Hitler.³

Paramount to the success of a forensic dental comparison between antemortem and postmortem dental records is the requirement that both sets of records provide the maximum amount of dental information for analysis. Thus, dentists are encouraged to document all dental anomalies, existing restorations, and missing teeth in their written, photographic, and radiographic records.

CHRONOLOGICAL AGE

The determination of dental maturity and tooth development has been considered in Chapter 2. As indicated there, the determination of chronological age is possible with some reasonable degree of accuracy. When it is employed in conjunction with evaluation of the stage of development of osseous tissues,⁴ the two methods can be used to determine an estimated age for an individual less than 20 years old. This is accomplished by entering data from both methods into an electronic encyclopedia on maxillofacial, dental, and skeletal development.⁵

Chronological age determination is a central issue in population studies, and racial and sex differences in tooth development and eruption patterns are acknowledged.^6,⁷ Dental age assessment is also important in the forensic dental evaluation of human remains or living individuals. Aging of the dentition has been employed in forensic dental casework involving estimation of age of unidentified individuals.^8,⁹

Separation of victims of multiple fatality incidents by age facilitates the narrowing of searches for eventual identification by comparison of medical and dental records. Dental evaluation of illegal immigrants, who may present authorities with misinformation concerning their age, is important in cases in which protection of unaccompanied minors is a concern.²

A variety of dental age estimation procedures have been employed. Some are useful in the analysis of radiographic changes associated with development of the dentition of children.¹⁰ Others use dental radiographs to determine the dental chronological age in living or deceased adult individuals.^11,¹²

Estimation of the age of an adult may also be based on six characteristics first identified by Gustafson in 1950 ¹³ by his observation of ground sections of extracted teeth. These six variables include attrition of the occlusal or incisal surfaces, degree of deposition of secondary dentin (evaluation of the size of the pulp chamber and canal), deposition of apical cementum, attachment level of the periodontium, root resorption, and radicular translucency.

In addition, biochemical methods that assess the ratios of levorotatory and dextrorotatory isomers of the long-lived, metabolically stable amino acid aspartate have been employed in both living and deceased subjects to age tissues in which this substance is found. Through a racemization process the L form of aspartic acid is slowly transformed into its stereoisomer, which is the D form of the amino acid. As enamel and dentin age, levels of the D form of aspartic acid increase in these calcified dental structures. These values can be measured and related to known levels for age estimation.¹⁴

DENTAL DNA

Each individual’s unique genetic information is contained within the nuclear and mitochondrial deoxyribonucleic acid (DNA) molecules of their cells. Only identical twins share the same DNA. Nuclear DNA is transmitted from either parent, whereas mitochondrial DNA is derived only through a maternal route. As a unique biological molecule, DNA offers the forensic scientist a means of positively identifying an individual when this material can be obtained from tissues or bodily fluids recovered at a crime scene or from human remains or other forensic evidence.

The most widely used method for forensic analysis of DNA material is the restriction fragment length polymorphism (RFLP) technique. This laboratory procedure requires that large amounts (>100 ng) of DNA be used in the analysis. The polymerase chain reaction (PCR) technique is employed when this cannot be accomplished because of degradation of the DNA molecule submitted as evidence, when only small amounts (<100 pg) of DNA are available for analysis, or when the DNA sample is fragmented.¹⁵ The PCR method amplifies the amount of DNA available for analysis by copying a specific locus of genetic material.¹⁶ Since only small amounts of DNA evidence need be evaluated using PCR technology, a positive identification may still be effected when human remains have been left unburied for long periods or have been incinerated, or when DNA trace evidence is obtained from saliva, blood, or fluid samples.

The calcified and pulp tissues within a tooth often present the forensic scientist with the most uncontaminated and protected DNA samples for analysis. Thus, even small amounts of DNA recovered from these tissues often may be analyzed using the PCR method when other means of identification have been lost or degraded as evidence.^17,¹⁸

In addition, the sex of a decedent can be determined through DNA analysis of the sex-linked amelogenin gene. A DNA sample from the victim can be compared directly with one obtained from the clothing or other personal effects of a putative subject of interest or indirectly with a sample from the parents or siblings of the individual one is attempting to identify.¹⁹

BITE MARKS

When the dentition of a human or animal impresses the surface of an object during the act of biting, a pattern mark is often imprinted on that surface (Figure 4-1, A). Like a tool mark left as forensic evidence, the pattern left by the teeth can be evaluated and compared to the dentition that allegedly caused it (Figure 4-1, B). Whether the bite mark pattern involves the skin of a victim or suspect or remains on the surface of an inanimate object, to be probative, it must have class and individual characteristics consistent with a mark caused by teeth.

Figure 4-1 Forensic bite marks. A, Forehead bite marks. B, Bite marks related to maxillary dental cast.

(Courtesy Dr. David Ord.)

Class characteristics of a bite mark include the size and shape of the pattern. In most cases this should be consistent with the dental arch size of the suspected biter (human or animal) and retain a circular shape consisting of two half arches (maxilla and mandible) separated by a space (temporomandibular joint). One arch should be larger in its greatest dimension, representing the greater arch length of the maxilla.

Individual characteristics associated with a bite mark involve the patterns routinely made by specific teeth. In the human dentition these include the following:

Maxillary central incisor—large rectangle

Mandibular incisor and maxillary lateral incisor—small rectangle

Cuspid—point or triangle (when there is incisal wear)

Maxillary cuspid—figure-eight pattern directed buccal to lingual

Mandibular cuspid—point representing the buccal cusp

Molar—not routinely seen in the patterns left by human biters

It is generally accepted that no two individuals have an identical dentition based on variations in the arrangement, spacing, size, and shape of specific teeth and dental arches. Currently, however, neither quantitative values nor databases have been established for the dentition that are similar to those described for fingerprint and DNA analysis and comparison. Thus, although bite mark evidence has been admissible in the federal and state courts of the United States based on the Frye rule and decisions related to the Federal Rules of Evidence,^20,²¹ this evidence is often used in an exculpatory manner.

Guidelines have been established and are continually reviewed and revised to provide the forensic odontologist with evidence gathering and analytical procedures to be followed to ensure that recovered bite mark evidence is admissible in court and supports corroborative evidence in the case.²²

Comparative Dental Anatomy

To understand the human dentition, it is helpful to compare the dentitions of other vertebrates. In doing so, it should become clear that the dentition in humans is different in many ways from that of other vertebrates in form and function. However, it should be equally clear that the presence of related characteristics in a wide range of vertebrates suggests a plan common to all.

Only a brief summary of the topic is presented here, starting with a simple form of tooth, the single cone or lobe, and then combinations of lobes forming more complicated teeth that are found in highly developed animals and in humans. Additional material on the subject may be found in the references and bibliography presented at the end of this chapter.

Figure 4-2 graphically illustrates a theory of the following four phylogenetic classes of tooth forms²³^–²⁵:

1. Single cone (haplodont)

2. Three cusps in line (triconodont)

3. Three cusps in a triangle (tritubercular molar)

4. Four cusps in a quadrangle (quadritubercular molar)

Figure 4-2 Classification of cusp forms. A, Haplodont. B, Triconodont. C, Tritubercular molar. D, Quadritubercular molar. The solid black dots represent upper molar cusps. The circles represent lower molar cusps.

(From Thompson AH: Comparative dental anatomy, ed 2, revised by M Dewey, St Louis, 1915, Mosby.)

The haplodont class is represented schematically by the simplest form of tooth, the single cone (Figure 4-2, A). This type of dentition usually includes many teeth in both jaws and is seen where jaw movements are limited to simple open and close (hinge) movements (Figure 4-3). No occlusion of the teeth occurs in this class, since the teeth are used mainly for prehension or combat.^26,²⁷ Their main function is the procurement of food. Jaw movements are related to and governed by tooth form in all cases.

Figure 4-3 The Mississippi alligator. An interesting commentary on the anatomy of the alligator: because of the alligator’s physical problems, the upper jaw is the mobile one. The lower jaw, closer to the ground, is static.

(From Kronfeld R: Dental histology and comparative dental anatomy, Philadelphia, 1937, Lea & Febiger.)

The triconodont class exhibits three cusps in line in posterior teeth as indicated in Figure 4-2, B. Anthropologically, the largest cusp is centered with a smaller cusp located anteriorly and another posteriorly. Purely triconodont dentitions are not seen, although certain breeds of dogs²⁸ and other carnivores have teeth reflecting the triconodont form (Figure 4-4). Nevertheless, dogs and other animals carnivorous by nature are considered to be in the third category (Figures 4-5 through 4-8), namely, the tritubercular class.^29,³⁰ The three-cusp arrangement of the triconodont class and the more efficient three-cornered tritubercular molar arrangement are both consistent with the teeth’s bypassing each other more or less when the jaw is opened or closed. However, the quadritubercular class reflects an occlusal contact relationship between the teeth of the upper and lower jaws. Articulation of the jaws and teeth is consistent with jaw movements and functions of these classes of teeth.