AN ASSESSMENT AND COMPARISON OF THIRD MOLAR DEVELOPMENT IN RELATION TO CHRONOLOGICAL AGE IN A WESTERN AUSTRALIAN AND A SOUTH INDIAN POPULATION

Transcription

1 AN ASSESSMENT AND COMPARISON OF THIRD MOLAR DEVELOPMENT IN RELATION TO CHRONOLOGICAL AGE IN A WESTERN AUSTRALIAN AND A SOUTH INDIAN POPULATION Geetha Govindaiah Varadanayakanahally Centre for Forensic Science University of Western Australia This thesis is presented in partial fulfilment of the requirements for the Master of Forensic Science 2011 Page i

2 Dedication I would like to dedicate this research to my husband, Harish and my parents without whose continuous support this all would have not been possible. Everything is achievable, the more you want it, sooner you will get it Author unknown Page ii

3 Declaration I declare that the research presented in this thesis, for the Master of Forensic Science at the University of Western Australia, is my own work. The results of the work have not been submitted for assessment, in full or part, within any other tertiary institute, except where due acknowledgement has been made in the text. Geetha Govindaiah Varadanayakanahally Page iii

4 Abstract In a forensic investigation the estimation of age at death is an important step towards the identification of unknown human skeletal remains. An accurate estimation of age will significantly narrow the field of possible matching identities. In order to achieve this, there are many skeletal methods available to the forensic odontologist and anthropologist, including assessment of skeletal and dental maturation (in the juvenile age range). However, the rate of skeletal maturation can be affected by environmental factors that include poor nutrition and illness. Dental development, however, is under strict genetic control and is strongly correlated to chronological age. This makes teeth a reliable age marker for assessment in forensic investigations. There are many published methods for evaluating and quantifying dental maturation in order to estimate personal age. One of the more widely applied methods was first described in 1973 by Demirjian and Goldstein, who studied French-Canadian children. The present study applies a modification of that method to statistically quantify the timing of third molar mineralization in a Western Australian and South Indian population. The primary aim is to evaluate how accurately age can be estimated using the third molars, to assess ethnic differences in mineralization rates, and to formulate population specific standards for age estimation using this tooth. Comparisons between sexes, upper and lower arches and side differences (within and between populations) are made to provide statistically usable reference data of mineralization rates in the third molars specific to Western Australia and South India. In addition, the degree of third molar agenesis is assessed in both populations. The sample comprises 561 conventionally taken orthopanthomographs (OPG s) representing 312 Western Australian (173 males and 139 females aged between Page iv

5 7 to 30 years) and 249 South Indian (124 males and 125 females aged between (10 to 30 years) individuals. Mineralization status was assessed in each third molar according to the eight stage (A H) tooth classification system proposed by Demirjian et al. (1973). Descriptive statistics, including the mean age and standard deviation for individual third molar mineralization stages, are presented for each population. Comparisons between sexes, upper and lower arches, and left-right sides were statistically quantified using the Mann Whitney U test. All statistical analysis was performed using the SPSS software package (SPSS Inc. Chicago, IL). In the Western Australian population it was found that in males the upper third molars complete development (stage H) by years and the lowers by years. In females the upper third molars are fully formed by years and the lowers by years. In the South Indian population, the upper and lower third molars complete development by and years respectively in the male sample. In females the upper and lower third molars complete development by and years respectively. Overall comparisons of third molar maturity between males and females in the Western Australian population showed that the former generally achieved maturity earlier than the latter; by approximately 7 to 11 months. In the South Indian population, male dental development occurred earlier, by approximately 10 to 13 months. Mineralization of the upper (maxillary) third molars in both populations occurred earlier than the mandibular dentition except for the Western Australian males which showed the reversal. There were no significant bilateral differences in the timing of third molar mineralization in either population. It was found that Western Australian males and females generally achieved third molar maturity earlier than the South Indians (by approximately 10 to 20 months). An important outcome of this research is a series of statistics useful for predicting dental age in both populations. Page v

6 Regarding the frequency of missing individual third molars, the upper right third molar were found to be the most commonly missing tooth in the Western Australian (40.4%) and South Indian (22.5%) population. As this variability in third molar development is mostly related to population differences, these findings should be taken into account in forensic examinations when assessing the viability of using this tooth. Page vi

7 Acknowledgements This project has become a dream come true for me as many goals were set and achieved. I was blessed with the support, guidance of many people in life. Though it will not be enough to express my gratitude on words to all those people who helped me, I would still like to take this opportunity to thank them. Professional Acknowledgement First and foremost, I would like to thank the University of Western Australia for offering me an ideal environment to continue my intellectual journey and challenge myself everyday. Dr. Daniel Franklin, my supervisor, I can never thank him enough for his guidance, inspiration and patience to get the best out throughout the entire research. I would also like to thank my co-supervisor Dr. Peter Mack for his continued support and guidance. The Centre for Forensic Science (CFS) Thank you to the Centre for Forensic Science for providing me an wonderful experience. My extended gratitude to Professor Ian Dadour (Director, Forensic Entomologist, CFS) for providing admittance to the forensic science program. As well thanks to Alexandra Knight (Administrative Assistant, CFS) for her patience in answering numerous queries and s. Princes Margaret Hospital for Children I would like to express my gratitude to all those who have offered me their time while collecting numerous data for my research. Firstly Princes Margaret Hospital for providing the Western Australian sample (OPG s), The Ramaiah Page vii

8 institute and KLE institute of Dental Sciences, Bangalore, India for providing the South Indian sample. Thank you all. Personal Acknowledgement I would like to thank my husband Harish, and my family as they form the backbone and origin of my happiness. Their love and support without any complaint or regret has enabled me to complete this project. Also my heartfelt gratitude to Professor Anil Sukumaran and Dr. Girish Kumar for helping hands in this project. Thank you all again as I am deeply indebted. Page viii

9 Table of Contents Chapter One Introduction Background to the study The importance and significance of population specific standards Aims of the project Sources of material Thesis format... 6 Chapter Two Dental Anatomy, Histology and Nomenclature Introduction The Dentition Primary dentition Permanent dentition Dentition periods Primary dentition period Mixed dentition period Permanent dentition period Tooth numbering systems Universal numbering system The Zsigmondy/Palmer Notation System Federation Dentaire International (FDI) General dental anatomy terminology Divisions of the tooth Tissues of the tooth Dental Nomenclature The development and eruption of the teeth Stages of tooth development Eruption Page ix

10 2.8. The anatomy of the primary dentition The primary maxillary dentition: The primary mandibular dentition: Importance of the primary teeth: Morphological differences between the primary and the permanent teeth The anatomy of the permanent third molars An overview of the maxillary third molars The morphology of the maxillary third molar Clinical consideration of the maxillary third molars An overview of the mandibular third molars The morphology of the mandibular third molars Clinical considerations of the Mandibular third molars Absence of the third molars Human molecular genetics: Histology of the dental tissues Enamel Dentin Dental pulp Chapter Three Literature Review Introduction Dental age estimation Dental age estimation methods Moorrees et al. (1963) Demirjian et al. (1973) Ubelaker (1999) Willems et al. (2001) Dental age estimation using the third molars Mincer et al. (1993) Chaillet et al. (2004) Page x

11 3.4.3.Arany et al. (2004) Prieto et al. (2005) Olze et al. (2006) Zeng et al. (2010) Brief review of other methods Gleiser and Hunt (1955) Gustafson and Koch (1974) Harris and Nortje (1984) Kullman et al. (1992) Australian Research Farah et al. (1999) McKenna (2002) Flood (2007) Blenkin and Evans (2011) Indian research: Koshy and Tandon (1998) Prabhakar et al. (2002) Hegde and Sood (2002) Rai et al. (2009) Summary Chapter Four Materials and Methods Materials Population Orthopantomographs (OPGs) Methods Introduction Data Collection Demirjian et al. (1973) stage descriptions Criterion for allocation of stages Assessment of intra-observer error Page xi

12 4.2.3.Data Analysis Statistical analyses Chapter Five Results Introduction Assessment of Intra-observer error Descriptive statistics: Western Australian population Upper Right Third Molar Upper Left Third Molar Lower right third molar Lower left third molar Overall development of all four third molars in males and females Comparative statistics: comparison of age of attainment of developmental stages A H between the upper (maxillary) and lower (mandibular) third molars Comparative statistics: comparison of age of attainment of developmental stages A H between the right and left third molars Descriptive statistics: South Indian population Upper Right Third Molar Upper Left Third Molar Lower right third molar Lower left third molar Overall development of all four third molars in males and females Comparative statistics: comparison of age of attainment of developmental stages A H between the upper (Maxillary) and lower (Mandibular) third molars Comparative statistics: comparison of age of attainment of developmental stages (A H) between the right and left third molars Comparative statistics: Comparison of age of attainment of developmental stages A H for the upper (Maxillary) and lower (Mandibular) third molars between Western Australian and South Indian males and females Upper Third Molars (Male) Page xii

13 5.9.2.Lower Third Molars (Male) Upper third molars (Female) Lower third molars (Female) Absence of the third molars in the Western Australian and South Indian sample Chapter Six Discussion and Conclusions Introduction Developmental age range, sex differences and population variability of the third molars Overall developmental age range of the third molars in the Western Australian and South Indian populations Overall sex differences in the third molar development in the Western Australian and South Indian population Comparisons of third molar development between Western Australian and South Indian individuals Comparisons of third molar development to other populations Jaw differences in third molar development in a Western Australian and a South Indian sample Bilateral variation in third molar development within Western Australian and South Indian individuals Absence of the third molars Forensic importance of the third molars Potential limitations of the present study Recommendation for future research Conclusions References Page xiii

14 List of Tables: Table 2.1: Sequence of chronology of tooth eruption of the primary dentition (Avery, 1992). Table 2.2: Sequence of chronology of tooth eruption of the permanent dentition (Avery, 1992). Table: 2.3: The development and eruption sequence of the Maxillary third molars (Wheeler, 2003) Table 2.4: The development and eruption sequence of the Mandibular third molars (Wheeler, 2003) Table 3.1: Description of the Demirjian et al. (1973) dental developmental stages (A H). Table 4.1: Age and sex distribution of the Western Australian population. Table 4.2: Age and sex distribution of the South Indian population. Table 4.3: Interpretation of Kappa Value (from Landis and Koch 1997). Table 5.1: Kappa values, indicating levels of agreement. Table 5.2: Descriptive values (including mean and standard deviation) of stages A - H for the upper right third molar in males and females. Table 5.3: Descriptive values (including mean and standard deviation) of stages A - H for the upper left third molar in males and females. Table 5.4: Descriptive values (including mean and standard deviation) of stages A - H for the lower right third molar in males and females. Table 5.5: Descriptive values (including mean and standard deviation) of stages A - H for the lower left third molar in males and females. Table 5.6: Overall descriptive values (including mean, standard deviation and significance) of stages A- H for all four third molars in males and females. Page xiv

15 Table 5.7: Comparative values (including mean, standard deviation and significance values) for the age of attainment of developmental stages (A H) for the upper and lower third molar. Table 5.8: Comparative values (including mean, standard deviation and significance values) for the age of attainment of the developmental stages (A- H) for the right and left third molars. Table 5.9: Descriptive values (including mean and standard deviation) of stages A - H for the upper right third molar in males and females. Table 5.10: Descriptive values (including mean and standard deviation) of stages A - H for the upper left third molar in males and females. Table 5.11: Descriptive values (including mean and standard deviation) of stages A - H for the lower right third molar in males and females. Table 5.12: Descriptive values (including mean and standard deviation) of stages A - H for the lower left third molar in males and females. Table 5.13: Overall descriptive values (including mean, standard deviation and significance) of stages A- H for all four third molars in males and females. Table 5.14: Comparative values (including mean, standard deviation and significance values) for the age of attainment of the developmental stages (A H) for the upper and lower third molars. Table 5.15: Comparative values (including mean, standard deviation and significance values) for the age of attainment of the developmental stages (A H) for the right and left third molars. Table 5.16: Comparative values (including mean, standard deviation and significance values) for the age of attainment of developmental stages A - H for the upper third molars in Western Australian and South Indian males Table 5.17: Comparative values (including mean, standard deviation and significance values) for the age of attainment of developmental stages A - H for the lower third molars in Western Australian and South Indian males Page xv

16 Table 5.18: Comparative values (including mean, standard deviation and significance values) for the age of attainment of developmental stages A - H for the upper third molars for Western Australian and South Indian females Table 5.19: Comparative values (including mean, standard deviation and significance values) for the age of attainment of developmental stages A - H for the lower third molars in Western Australian and South Indian females Table 5.20: Percentage of missing all four third molars in different age groups Table 5.21: Percentage of missing individual third molars in Western Australian and South Indian Sample Table 6.1: Mean age (in years) of attainment of Demirjian et al. (1973) stages (A, D and H) in several populations for males and females. Page xvi

17 List of Figures: Figure 2.1: Occlusal view of the primary dentition along with types of teeth (from Thomas et al. 2006). Figure 2.2: Occlusal views of permanent dentition along with types of teeth numbered using universal dental numbering system (from Thomas et al. 2006). Figure 2.3: Dentition Stages (from Fuller and Denehy, 1984). Figure 2.4: Tissues of the tooth (from Short and Levin-Goldstein, 2002). Figure 2.5: Illustration and definition of dental Nomenclature (from Wheeler, 2003) Figure 2.6: The deciduous dentition facial view (from Woelfel and Scheid, 1997) Figure 2.7: Various views of the Maxillary third molar (from Thomas et al. 2006). Figure 2.8: Crown form of the Maxillary right third molar (from Thomas et al. 2006). Figure 2.9: Various views of the Mandibular right third molar (from Thomas et al. 2006). Figure 2.10: Diagramatic representation of rods in enamel (from Orban, 1976). Figure 2.11: Composite diagram of a human tooth in cross-section illustrating the different types of dentin (from Nanci, 2008). Figure 2.12: Coronal section of a molar showing the dental pulp zones (from Orban, 1976). Figure 4.1: Age and sex distribution of the Western Australian population. Figure 4.2: Age and sex distribution of the South Indian population. Figure 4.3a-h: Stage A-H of the Demirjian et al. (1973) method. Page xvii

18 Figure 5.1: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the upper right third molar in males and females. Figure 5.2: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the upper left third molar in males and females. Figure 5.3: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the lower right third molar in males and females. Figure 5.4: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the lower left third molar in males and females. Figure 5.5: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the males and females. Figure 5.6: Mean age of attainment of Demirjian et al. (1973) stages (A H) for the upper and lower third molars Figure 5.7: Mean age of attainment of (A H) Demirjian et al. (1973) stages for the right and left third molars. Figure 5.8: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the upper right third molar in males and females. Figure 5.9: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the upper left third molar in males and females. Figure 5.10: Illustration of mean ages of attainment of Demirjian et al. (1973) stages for the lower right third molar in males and females. Figure 5.11: Illustration of mean age of attainment of Demirjian stages for the lower left third molar in males and females. Figure 5.12: Illustration of mean age of attainment of Demirjian et al. (1973) stages for the males and females. Figure 5.13: Mean age of attainment of Demirjian et al. (1973) stages (A H) for the upper and lower third molars. Page xviii

19 Figure 5.14: Mean age of attainment of Demirjian et al. (1973) stages (A H) for the right and left third molars. Figure 5.15: Mean age of attainment of Demirjian et al. (1973) stages (A H) in the Upper third molar in males for Western Australia and South India. Figure 5.16: Mean age of attainment of Demirjian et al. (1973) stages (A H) in the lower third molar in males for Western Australia and South India. Figure 5.17: Mean age of attainment of Demirjian et al. (1973) stages (A H) in the upper third molar in females for Western Australian and South India Figure 5.18: Mean age of attainment of Demirjian et al. (1973) stages (A H) in the lower third molar in females for Western Australia and South India. Page xix

20 Chapter One Introduction 1.1. Background to the study Forensic odontology, also known as forensic dentistry, is the application of dentistry for criminal justice purposes. It involves the proper collection, handling, examination, and evaluation of dental evidence (Neville et al. 2002). That evidence can then be used in criminal investigation and to identify human remains. The estimation of age at death is an important step towards the identification of human remains and has a long tradition in the field of forensic sciences. Forensic age estimation has been beneficial in assisting authorities in narrowing the search of possible matches for unknown victims, especially in the identification of mass disaster victims (Herschaft et al. 2006). Age estimation of living individuals is also an important current focus of forensic research, especially in multicultural societies where legal and illegal immigration is increasing and documentary evidence of age may be lacking (Bosmans et al. 2005). Depending on the stage of the life cycle (eg: juvenile or adult), specific methods are available for age estimation. Franklin (2010: 1-2) states that It is welldocumented that age estimation is usually most accurate in individuals still growing; in mature individuals however, most standards generally rely on the highly variable degeneration of bones (e.g. pubic symphysis; sacro-iliac joint; sternal rib ends). The latter characteristics are more influenced by environmental factors, as opposed to the more predictable and welldocumented developmental markers characteristic of juveniles (e.g. dental development; skeletal growth and maturation). Page 1

21 Dental emergence and mineralization are the main characteristics assessed for forensic age estimation in children and young adults (Olze et al. 2003). Tooth development is strongly correlated with chronological age, as the teeth consistently develop in relation to age (even under conditions of chronic illness and nutritional deficiency), which indicates that the process is under strict genetic control (Cardoso, 2007). This makes the teeth a reliable age marker to use in forensic anthropology and odontology. However, the reliability of age estimates based on dental development is not uniform from birth to adulthood. After the age of 14 years, when most of the teeth are in the process of completing apical closure (Kullman et al. 1992), dental age estimation becomes less accurate. The only teeth still forming after that age are the third molars, which are highly variable in their pattern of formation and the age of complete mineralization varies widely (Kullman et al. 1992, Bolanos et al. 2003). A further limitation is that this tooth is often congenitally absent. For example, Thompson et al. (1974) summarized published research on a variety of populations and found that the proportion of subjects with one to four missing third molars ranged from 9 to 35%. This issue is discussed further in Chapter Two. The teeth are the most durable element of the human skeleton and their structures provide an inherent resistance to erosion, deterioration, and fire long after death. The teeth, although resistant to most physical trauma, can become brittle and fragile when subjected to temperatures over 600 C (Karkhanis and Franklin, 2010). The teeth demonstrate a variety of morphologies and varied conditions of wear, trauma, disease, and professional manipulation. Thus an approximate age and useful indications of probable sex, ethnicity, occupation, personal habits, medical history, and environment can often be revealed through the analysis of teeth (Rogers, 1988). Dental techniques that use progressive morphological changes have proven to be the most accurate methods for estimating age in infants, children and adolescents (Senn and Stimson, 2010). Several methods for evaluating and quantifying dental development have been performed in order to establish dental age standards. One of the most widely applied of those methods was first Page 2

22 described in 1973 and was based on French-Canadian children (Demirjian et al. 1973). That method evaluates the development of seven mandibular teeth from panaromic radiographs (OPG) and outlines a technique for calculating dental age. Since then, numerous studies have been undertaken for other populations, which have demonstrated considerable variability in dental development and maturation. The focus of the present study is to evaluate how accurately age can be estimated using the third molar and to formulate population specific standards for a Western Australian and a South Indian population. Other issues including congenital variability are also considered (see section 1.3; page 5). This study will collect statistically quantified reference data relating to the mineralization status of the third molars specific to Western Australian and South Indian populations The importance and significance of population specific standards As shown by Nystrom et al. (1986) and Olze et al. (2003) the timing of attaining dental maturity varies between different population groups. In particular, the development of the third molars show remarkable diversity, as well as different frequencies of agenesis (congenital absence) among different ethnic groups (Clow, 1984; Uzamis et al. 2000). Thus appropriate quantitative methods and population specific standards should be applied when estimating dental age for forensic purposes (Liversidge et al. 2003). Age estimation using a population specific standard will obviously provide the greatest accuracy. At present there are no Western Australian and South Indian population specific standards for estimating age from third molar dental development. This study therefore will use the Demirjian et al. (1973) eight stage (A-H) dental development system to assess the third molars of Western Australian and South Indian individuals, which will provide specific reference data for forensic application in those populations. The specific aims of this project are detailed below. Page 3

23 1.3. Aims of the project 1. To evaluate the applicability of the Demirjian et al. (1973) method of age estimation to third molar development in a Western Australian and a South Indian population: The Demirjian et al. (1973) method of age estimation has been widely used for dental age estimation in different populations. However, to-date no previous research has assessed the accuracy of using the individual third molars to estimate age using this system. This study therefore, will focus upon evaluating the applicability of the Demirjian et al. (1973) method to all four third molars in a Western Australian and a South Indian population. The primary aim of this study is to establish age estimation standards which can be applied to both populations for estimating the age of an individual by assessing the development of their third molars. 2. To evaluate ethnic differences in the mineralization rate of the third molar: Olze et al. (2003) demonstrated clear ethnic differences in the chronology of third molar mineralization between Asian and European populations. In order to maximize the accuracy of age estimation, it is important that such findings are considered when examining individuals from different ethnic groups. The increasing volume of Indian immigrants and students settling in Australia, in the broader context of globalization, has led to a need for development of population specific age assessment standards. Since there is no such study on either the Australian or Indian populations, the present research will establish any differences in the chronology of third molar mineralization. The outcome of this study can then provide reference data for age estimation of unidentified deceased individuals, in addition to forensic age estimation in living persons. Results will also be evaluated in a broader global context (e.g: published data on other populations). Page 4

24 3. To evaluate if any statistically significant sex differences exist in the timing of mineralization of the third molar in both Western Australian and South Indian individuals: It is of at-most importance for forensic investigators to know the variation in age estimation accuracy between males and females, as this determines whether the sex of unknown remains must be known prior to applying any age estimation standard. This study will therefore, examine the accuracy of age estimation using Demirjian et al. (1973) method as applied to both sexes. The rate of error for the individual sexes will then be compared with previous studies and interpreted in the context of suitability for forensic use. 4. To evaluate the variability in the development and eruption of the third molars: For forensic purposes the reliability and adequate precision of age estimation using the third molars is crucially important. The main limitation of this study is the remarkable biological variability in the formation of third molars compared to all other permanent teeth. Differences in the developmental pattern and mineralization of this particular tooth vary widely. Discrepancies have also been observed between the development of the maxillary (upper) and the mandibular (lower) third molars (Garn et al. 1963). Furthermore Saito (1936) reported earlier mineralization and emergence on the right side than on the left in the mandibular third molars. Accordingly, this study will evaluate and compare the development and emergence of the maxillary and mandibular third molars, and also consider left and right side differences. 5. To evaluate the frequency of absence of the third molars: There is a high frequency of, and large variations in, the prevalence of third molar absence. Garn et al. (1963) demonstrated between 10-20% agenesis of this tooth in white American and European individuals older than 14 years of age. More recent studies have presented variable estimates, ranging from 7 to 10% to as high as 32.4% (Llarena and Nuno, 1990). Although this variability may mostly relate to population differences, other factors, such as Page 5

25 sex, age, and degree of dental maturation of the individuals play a major role (Bolanos et al. 2003). This issue is discussed further in Chapter Two Sources of material The OPG s (orthopantomographs) examined in this project were taken as part of routine therapeutic scans. OPGs are full mouth x-rays that show all the upper and lower teeth, including teeth that are unerupted. The OPGs from Southern India were obtained from diagnostic and radiography centres in Bangalore, India in accordance with established ethical guidelines. The OPGs from Western Australia were obtained through the Picture Archive and Communication System (PACS) from medical practices of various Western Australian hospitals (e.g. Charles Gairdner; Royal Perth) following established ethical guidelines. Ethics approval to undertake this project was granted by the Human Research Ethics Committee, of the University of Western Australia (Reference no. RA/4/1/4158) Thesis format The thesis is presented in six chapters. The first chapter is the introduction and outlines the aims of the project and sources of data. The second chapter concerns general dental anatomy, dental development, eruption sequences, mineralization and shedding of teeth. Chapter Three reviews the literature on dental age estimation methods. The fourth chapter outlines the materials and methods; followed by Chapter Five which outlines the results. The final chapter provides a discussion of the results and the final conclusions. Page 6

26 Chapter Two Dental Anatomy, Histology and Nomenclature 2.1. Introduction Dental anatomy is the study of the morphology of the various teeth in the human dentitions and knowledge of how their shape, form, structure, colour, and function relate to each other, both in the same dental arch, and to the teeth in the opposing arch. Thus the study of dental anatomy provides one of the basic components of the skills needed to practice all aspects of dentistry (Wheeler, 2003). This chapter presents an overview of tooth morphology, development, classification and nomenclature of the human dentition. This is then followed by a general outline of the deciduous and permanent dental development rates The Dentition The term dentition refers to all of the teeth in the upper jaw (maxilla) and the lower jaw (mandible) bones. Accordingly, the upper teeth are known as maxillary, and together form an arch shape known as the maxillary arch. In contrast, the lower teeth are mandibular and they collectively form the mandibular arch. Humans have two dentitions throughout life: one during childhood, known as the primary dentition; and one for most (or all) of adulthood, which is the permanent dentition (Woelfel and Scheid, 1997) Primary Dentition The first set of teeth in the mouth is the primary or deciduous dentition, which begins to form prenatally about 6 weeks in utero, and is completed postnatally at approximately 3 years of age. The first teeth in this dentition appear in the oral cavity at approximately 6 months of age; the last primary teeth generally emerge at 28(±4 months) years of age. The deciduous dentition remains intact until the child is about 6 years of age (Wheeler, 2003). There are only 20 teeth in Page 7

27 the primary dentition; 10 each in the maxillary and mandibular arches. This dentition is also known as the deciduous dentition, referring to the fact that these teeth are eventually shed or exfoliated by 12 to 13 years of age, having being replaced by the permanent dentition. The complete primary dentition has five teeth in each quadrant as shown in Figure 2.1. The two front teeth in each quadrant are the central and lateral incisor, followed posteriorly by one canine, then a first and second primary molar (Woelfel and Scheid, 1997). Figure 2.1: Occlusal view of the primary dentition along with types of teeth (from Thomas et al. 2006) Permanent dentition The permanent dentition is also known as the succedaneous dentition; it succeeds the primary dentition. It is composed of 32 teeth: 16 each in the maxillary and mandibular arches. The complete dentition has eight teeth in each quadrant as shown in Figure 2.2. The two front teeth in a quadrant are the central and lateral incisors, followed by one canine, a first and second premolar, then the first, second, and third molars. Between 6 and 6 years of age, the first permanent tooth (the mandibular first molar) erupts posterior to the second Page 8

28 deciduous molar. No deciduous tooth has exfoliated to provide space for this permanent tooth; the mandible has increased in length so that there is now space for an additional tooth (Short and Levin-Goldstein, 2002). Figure 2.2: Occlusal views of permanent dentition along with types of teeth numbered using universal dental numbering system (from Thomas et al. 2006) Dentition periods Although there are two dentitions, there are three dentitions periods (see Figure 2.3) because the two dentitions overlap; this overlap is known as the mixed dentition period (see below) Primary dentition period The primary dentition period is the first of the three periods. It begins with the eruption of the primary mandibular central incisors, generally at around 6 months of age, and is completed with the eruption of the second molar at around Page 9

29 3 years of age. Only the primary teeth are present during this time. This period usually ends with the eruption of the permanent mandibular first molar. The jaw bones are beginning to grow during this period to accommodate the larger permanent teeth Mixed dentition period The mixed dentition period follows the primary dentition period and occurs between approximately 6 to 12 years of age. Both the primary and permanent teeth are present during this transitional stage. In this period the shedding of both the primary teeth and the eruption of the permanent teeth occurs. This period thus begins with eruption of the first permanent tooth, a permanent mandibular first molar and usually ends at around age 12, with the exfoliation of the last deciduous tooth, normally the maxillary canine (Fuller and Denehy, 1984). Figure 2.3: Dentition Stages (from Fuller and Denehy, 1984) Permanent dentition period The permanent dentition period is the last of the three. This period begins with the shedding of the last primary tooth (canine or molar) at around 12 years of age. This period includes the eruption of all the permanent teeth, except for those teeth that are congenitally missing or impacted and cannot erupt (usually the third molars). The permanent teeth are usually the only teeth present during this period. Growth of the jawbones is not very noticeable as it slows and then eventually stops (Thomas et al. 2006). Page 10

30 2.4. Dental numbering systems Recording and storing accurate dental records is an important task in any dental practice. To do so expeditiously, it is necessary to adopt a type of code or numbering system for teeth. Uniformity in the methodology for maintaining dental records is thus essential so that they are understandable to all dental practitioners (Woelfel and Scheid, 1997). Several tooth numbering systems are in use globally Universal numbering system This system was first suggested by Parreidt in 1882 and was officially adopted by the American Dental Association (ADA) in It is accepted by third party providers and is endorsed by the American Society of Forensic Odontology. In this system, the primary teeth are designated in a consecutive arrangement by using capital letters (A through T) starting with the maxillary right second molar, moving clockwise, and ending with the mandibular right second molar(avery 1992). The Universal System notation for the entire primary dentition is as follows: A B C D E F G H I J T S R Q P O N M L K In this system the permanent dentition are numbered from 1 through 32. The maxillary teeth are numbered from 1 through 16, beginning with the right third molar. The mandibular teeth are numbered from 17 through 32, beginning with the mandibular left third molar, as shown below (Wheeler, 2003). For example, the right maxillary first molar is designated as 3, the left maxillary central incisor as 9 and right mandibular first molar as Page 11

31 The Zsigmondy/Palmer Notation System This system was introduced by Adolph Zsigmondy of Vienna in 1861 and then modified for the primary dentition in 1874 (Wheeler, 2003). The system utilizes simple brackets to represent the four quadrants of the dentition as if you are facing the patient: is upper right. is upper left, is lower right, and lower left (Woelfel and Scheid, 1997). For example left maxillary central incisor is designated as A and the right mandibular second molar as E. The primary dentition is as follows: E D C B A A B C D E E D C B A A B C D E The permanent dentition is a four quadrant symbolic system, in which beginning with the central incisors, the teeth are numbered 1 through 8 in each arch. For example the maxillary right first molar is designated 6 and the maxillary left central incisor as Federation Dentaire International (FDI) The two digit system proposed by the FDI (for both the primary and permanent dentitions) has been adopted by the World Health Organization (WHO) and accepted by other organizations such as the International Association of Dental Research (IADR) (Wheeler, 2003). In this system the first digit denotes the dentition, arch and side; the second digit denotes the tooth (1 to 8 for permanent and 1 to 5 for deciduous teeth). The first of the two digits used in this system are designated as follows. 1. Permanent dentition, maxillary, right side 2. Permanent dentition, maxillary, left side 3. Permanent dentition, mandibular, left side 4. Permanent dentition, mandibular, right side Page 12

32 5. Deciduous dentition, maxillary, right side 6. Deciduous dentition, maxillary, left side 7. Deciduous dentition, mandibular, left side 8. Deciduous dentition, mandibular, right side The FDI system of tooth notation for primary teeth is as follows: The FDI system of tooth notation for permanent teeth is as follows, for example the permanent upper right central incisor is designated as 11 (pronounced as one one, not eleven ) General dental anatomy terminology A brief definition and description of the various anatomical features of a normal tooth and its supporting structures are discussed below Divisions of the tooth Each tooth consists of a crown and one or more roots. The crown is that portion of the tooth which is normally visible in the mouth and covered with enamel. The teeth have differently shaped crowns, each adapted to perform a specific function. The roots located in the bone are not normally visible and are covered with cementum. The roots stabilize or support the teeth from the pressures of mastication (Short and Levin-Goldstein 2002; Fuller and Denehy 1984). Portions of the crown and roots of a tooth can also be defined in more specific ways. The anatomical crown is that portion covered by enamel which remains mostly constant throughout the life of the tooth. The clinical crown is the portion of the Page 13

33 anatomical crown that is visible and not covered by the gingiva. Similarly, the anatomical root is the portion of the root covered by cementum. The clinical root of a tooth is the portion of the anatomical root that is visible, subject to variability over time, again related to ginvival recession Tissues of the tooth There are four tissues of a tooth: enamel; dentin; cementum; and pulp (as shown in Figure 2.4). The crown of the tooth is covered with enamel which is the hardest tissue in the body. Enamel is made up of 96% inorganic and 4% organic matter and water. Dentin is the hard yellowish tissue underlying the enamel and cementum that constitutes the bulk of the tooth. Dentin is not normally visible except on a dental radiograph, a sectioned tooth, or on a badly worn (attrition) tooth. Dentin is made up of 70% inorganic and 30% organic matter and water. Cementum is the dull-yellow layer of hard, bone like tissue which covers the dentin of the anatomical root. The pulp is the soft tissue found in the centre part of the tooth. It contains the nutrient supply in the form of blood vessels and nerves. The pulp cavity is the internal cavity (surrounded by dentine), which contains the pulp. The pulp cavity consists of the pulp canal, which is the portion of the pulp cavity that is located in the root of the tooth while the pulp chamber is that portion which is found in the anatomical crown of the tooth (Woelfel and Scheid 1997; Thomas et al. 2006). Page 14

34 Figure 2.4: Tissues of the tooth (from Short and Levin-Goldstein, 2002) Dental Nomenclature The teeth in the maxillary arch of the upper jaw bone are the maxillary teeth. The teeth in the mandibular arch of the lower jaw bone are the mandibular teeth. Each dental arch has a midline; an imaginary vertical plane that divides the arch into two approximately equal halves (right and left). Thus each dental arch can be divided into two quadrants, with four quadrants in the entire oral cavity; the maxillary right and left quadrant, the mandibular right and left quadrant. Teeth can also be described according to their position in each dental arch in relation to the midline. The incisiors and canines are considered anterior teeth; in contrast the premolars and molars are considered posterior teeth because they are away from the midline (Wheeler, 2003). Each tooth has five surfaces: mesial; distal; facial; lingual; and occlusal (see Figure 2.5). The surface closest to the midline is the mesial surface and away from the midline is the distal surface. Tooth surfaces closest to the facial surface are considered the facial surface. Those facial tooth surfaces closest to the lips are termed labial, close to the inner cheek the surface is termed buccal. Those tooth surfaces closest to the tongue are the lingual surfaces and those closest to the palate are the palatal surfaces. The masticatory surface is the chewing Page 15

35 surface on the superior surface of the crown. This is the incisal surface for anterior teeth and the occlusal surface for the posterior teeth (Wheeler, 2003). Figure 2.5: Illustration and definition of dental Nomenclature (from Wheeler, 2003) The development and eruption of the teeth The knowledge of the development and emergence of the teeth into the oral cavity is not only applicable to clinical practice, but in forensics, bioarchaeology and paleoanthropology. Tooth development is initiated by the interaction of the oral epithelial cells with the underlying mesenchymal cells. From this interaction, a total of 20 primary and 32 permanent teeth normally develop. Each developing tooth grows as an anatomically distinct unit and the fundamental developmental process is similar for all teeth (Avery, 1992). Each tooth develops through successive bud, cap and bell stages. The various stages of tooth development and eruption are summarized below Stages of tooth development Although tooth formation is a continuous process, it is characterized by a series Page 16

36 of easily distinguishable stages known as the bud, cap and bell stages. These stages are defined according to the shape of the epithelial enamel organ segment of the developing tooth. During these early stages the tooth germs grow and expand, and the cells that are to form the hard tissues of the teeth then differentiate. The bud (or the first) stage is the rounded localized growth of the epithelial cells of the enamel organ. This happens at about six weeks of postconception. During this stage the 20 tooth buds begin to appear segmentally in the dental lamina in the approximate location of the corresponding primary teeth (Fuller and Denehy, 1984). As further development takes place, the generally round form of the bud gains a concave surface. The basal portion invaginates, and the structure thus formed gives the appearance of a cap, and hence this phase is termed the cap stage. This stage consists of an enamel organ, dental papilla, and dental follicle (Fuller and Denehy, 1984). As the concavity in the basal area of the cap continues to deepen, the development of the tooth enters the bell stage. At this stage the enamel organ has differentiated and the tooth s crown is identifiable. During this time most of the dentin and enamel of the crown is laid down. After this stage, when enamel and dentine deposition have formed, the bell stage is regarded as ending and the root development stage begins. After the crowns and roots of these teeth form and mineralize, the supporting tissues of the teeth, cementum, periodontal ligament, and alveolar bone begin to form. Subsequently, the completed tooth crown erupts into the oral cavity. Root formation and cementogenesis then proceed until a functional tooth and its supporting apparatus are fully developed (Avery, 1992) Eruption Tooth eruption is the process by which the developing teeth emerge through the bone and soft tissue of the jaws, and the overlying mucosa, to enter the oral cavity, contact teeth of the opposing arch, and function in mastication. The Page 17

37 movements related to tooth eruption begin during crown formation and require adjustments relative to the forming bony crypt. This is termed the pre-eruptive phase. Tooth eruption is also involved in the initiation of root development and continues until the emergence of the teeth into the oral cavity; this is the prefunctional eruptive phase. The teeth continue to erupt until they reach occlusal or incisal contact. They then undergo functional eruptive movements, which include compensation for jaw growth and occlusal wear of the enamel. This stage is known as the functional eruptive phase. Eruption is actually a continuous process, ending only with the loss of the tooth. The teeth differ extensively in their eruptive schedules as shown in Tables 2.1 and 2.2 (Avery, 1992). Table 2.1: Sequence of chronology of tooth eruption of the primary dentition (Avery, 1992). Table 2.2: Sequence of chronology of tooth eruption of the permanent dentition (Avery, 1992). Page 18

38 The eruption process can be divided into active and passive eruption. Active eruption is whereby the crown of the tooth first moves from within the jaw into the oral cavity, a process that continues until the tooth meets its antagonist in the opposite jaw. Active eruption begins when the crown of the tooth is complete and a portion of the root has started to form. Once active eruption is complete, other factors that occur during life, such as normal attrition or trauma can cause breakdown on the periodontium. This can then result in exposure of cementum, wearing of enamel, or gingival recession. The increase in the length of the clinical crown caused by gingival recession is referred to as passive eruption (Wheeler, 2003) The anatomy of the primary dentition The primary (or deciduous) dentition consists of 20 teeth; 10 each in the maxillary and mandibular arches. There are five teeth in each quadrant; a central and lateral incisor, canine, first and second molar. There are no premolars in the primary dentition. The primary teeth emerge in children between the ages of 6 months and 2 years. At around age 6, these teeth are gradually replaced by the permanent dentition. The primary teeth actually function in the mouth for an average of 8 and 7.6 years respectively for the maxillary and mandibular teeth. The primary teeth perform vital functions such as effective mastication, formulation of clear speech, and maintaining a normal facial appearance. The primary teeth also help in maintaining space and arch continuity for the eruption of the permanent dentition (Woelfel and Scheid, 1997) The primary maxillary dentition: Arch and side determination of primary teeth is based on the anatomical differences, which are briefly elaborated below and is illustrated in Figure 2.6. Page 19

39 i) Incisors: the labial crown is smooth with a straight incisal edge; there are no mamelons. The crown is wide with a cingulam and marginal ridges on the lingual. ii) Canine: a broad cervical ridge causes the cervix to appear constricted. The cusp tip is pointed, but short; the single root is long and slender. iii) First Molar: the number of cusps varies from two to four. There is no groove on the buccal surface to divide the cusps. The occlusal surface has a central fossa and a mesial triangular fossa molars, and the bifurcation of the two buccal roots begins almost apically to the cervix. iv) Second Molar: the anatomy is the same as that of the permanent maxillary first molar (see below). There are two buccal cusps divided by a buccal groove and two lingual cusps with a cusp of tubercle or fifth cusp groove, on the mesiolingual cusp. There are three roots; two buccal and one lingual (Short and Levin-Goldstein, 2002). Figure 2.6: The deciduous dentition facial view (from Woelfel and Scheid, 1997). Page 20

40 The primary mandibular dentition: i) Incisors: both labial and lingual surfaces are smooth, although there is a slight cingulam and marginal ridges on the lingual surface. ii) Canines: the buccal surface has a pronounced cervical ridge. The lingual surface has a cingulum and lingual ridges. iii) First Molar: as with the maxillary first molar, there is no definite anatomy. Usually there are two buccal cusps divided by a depression, rather than a groove, and two lingual cusps. There are two roots; both are long, slender and divergent. The occlusal surface has lingual groove and a central groove that is crossed by the buccal groove. iv) Second Molar: the anatomy is identical to that of the permanent mandibular first molar. Grooves divide the three buccal and the two lingual cusps. The occlusal groove pattern resembles the permanent mandibular first molar. Although there may be more supplemental grooves. There are two long, thin and divergent roots, which can be twice as long as the crown (Woelfel and Scheid, 1997) Importance of the primary teeth: The form and function of the primary dentition are both important. Each primary tooth has the same function as the permanent tooth that succeeds it, and each maintains a position for its permanent tooth replacement. If a primary tooth is prematurely lost, the permanent replacement may erupt too early or emerge in an incorrect position. This may result in improper alignment and malocclusion Morphological differences between the primary and the permanent teeth The morphological differences between the primary and permanent dentition with respect to the crown form, root and the pulpal cavity are discussed below. i) The primary teeth are generally smaller than their permanent counterparts. This size discrepancy exists for crown and root portions of both anterior and posterior teeth. Page 21

41 ii) The primary teeth are usually less pigmented and are whiter in appearance than the permanent teeth. iii) The crowns of the primary anterior teeth are larger mesiodistally in comparison with the permanent teeth. iv) The crowns of the primary teeth are more constricted at the cervix than those of the permanent teeth. v) The crowns of the primary teeth appear bulbous often having labial or buccal cingula. vi) The roots of the primary molars are much more flared or spread, than the roots of the permanent molars. This flare creates additional space for the permanent premolar crown to develop. vii) The cervical ridges of the enamel of the anterior teeth are more prominent. viii) The layers of enamel and dentin in the crowns of primary teeth are thinner when compared to the permanent teeth. ix) The pulp cavity is relatively larger in the deciduous teeth. The mesial pulp horns of the primary molars are especially large. x) The primary teeth have more consistent shapes than the permanent dentition. xi) All of the primary second molars resemble the first permanent molar (Woelfel and Scheid 1997; Wheeler 2003; Short and Levin-Goldstein 2002) The anatomy of the permanent third molars An overview of the maxillary third molars The maxillary third molar is the eighth and last maxillary tooth to erupt from from the midline; the sequence of eruption is shown in Table 2.3. If they erupt, it is distal to the permanent maxillary second molars. This tooth has mesial contact but not distal contact. The maxillary third molar often varies considerably in size, contour, and relative position to other teeth. If well developed, it often bears a resemblance to the maxillary second molar. The third Page 22

42 molar supplements the second molar in function, but the crown is smaller and the roots are shorter, there is also often fusion of one or more roots. The third molars show more variation in development than any other tooth in the dentition (Wheeler, 2003). Table: 2.3: The development and eruption sequence of the maxillary third molar (Wheeler, 2003). Stage Approximate Age First evidence of mineralization 7 to 9 years Enamel completed 12 to 16 years Eruption 17 to 21 years Root completed 18 to 21 years The morphology of the maxillary third molar The following describes the morphology of the maxillary third molar from various aspects. i) Buccal Aspect The crown is shorter cervico-occlusally and narrower mesio-distally. The roots are fused together (functioning as one large root), and are shorter cervicoapically. The fused roots end in a taper at the apex. The mesial outline of the roots have a more extreme slant to the distal root, giving the apices of the fused roots a more distil relationship to the center of the crown (Figure 2.7). Page 23

43 Figure 2.7: Various views of the Maxillary third molar (from Thomas et al. 2006). ii) Lingual aspect In addition to the differences mentioned above, there is just one large lingual cusp and therefore no lingual groove (Figure 2.7). iii) Mesial aspect Aside from metrical differences, the main mesial feature is the taper of the fused roots and a bifurcation in the region of the apical third of the root (Figure 2.7). iv) Distal aspect From this aspect most of the buccal surface of the crown is in view. Part of the occlusal surface may be seen because of the angulation of the occlusal surface in relation to the long axis of the root. The distance from the cervical line to the marginal ridge is short (Figure 2.7). Page 24

44 v) Occlusal aspect The occlusal aspect has a heart-shaped outline. The lingual cusp is large and well developed. There is no disto-lingual cusp, which gives a semi-circular outline to the tooth from one contact area to the other (Figure 2.7) (Wheeler, 2003). vi) Crown form The crown of the maxillary third molar is poorly developed compared with the other maxillary molars. The tooth is composed of four developmental lobes. There are two types of occlusal outlines for this tooth; the most common outline is heart-shaped, similar to the maxillary second molar. Generally with this outline the teeth has only three cusps; mesio-buccal, disto-buccal and mesiolingual. If a fourth cusp is present, the occlusal outline is a rhomboidal type, with a small and non-functioning disto-lingual cusp (see Figure 2.8). No oblique ridge is present. For both types of occlusal form, the disto-buccal cusp is much shorter than the mesio-buccal cusp, which helps to distinguish the right maxillary third molar from the left. Figure 2.8: Crown form of maxillary right third molar (from Thomas et al. 2006) vii) Root form Like crown form, root numbers and morphology are extremely variable. There are three roots which may be separated, but more commonly they are fused Page 25

45 most of their length, with the furcation extending only a short distance cervically from the apices of the roots. This results in a long fused root trunk that is often very crooked; the majority of the roots curve distally in their apical third (Woelfel and Scheid, 1997) Clinical consideration of the maxillary third molars The permanent maxillary third molars may fail to erupt and remain impacted within the alveolar bone. An impacted tooth is an unerupted or partially erupted tooth that is positioned against another tooth, bone, or even soft tissue in such a way that complete eruption is unlikely. This impaction usually occurs because the maxilla is under-developed and space (or arch length) is insufficient to accommodate these teeth. Surgical removable may be necessary if impacted (Thomas et al. 2006) An overview of the mandibular third molars The mandibular third molar is the eighth and last mandibular tooth from the midline. The development and eruption sequence is shown in Table 2.4. The mandibular third molars vary considerably in different individuals and present many anomalies both in form and position. This tooth supplements the second molar in function and has an irregularly developed crown with undersized roots that are more or less malformed. Occasionally, mandibular third molars are present that are comparable in size and development to the mandibular first molar. Table 2.4: The development and eruption sequence of the mandibular third molar (Wheeler, 2003). Stage Approximate Age First evidence of mineralization 8 to 10 years Enamel completed 12 to 16 years Eruption 17 to 21 years Root completed 18 to 25 years Page 26

46 The morphology of the mandibular third molars The following describes the morphology of the mandibular third molar from various aspects. i) Buccal aspect The crown is about the same length cervico-occlusally but is narrower mesiodistally. The roots are fused together (functioning as one large root) and they are shorter cervico-apically. The fused roots divide sufficiently at the apex to form two distinct apices. The outline mesially and distally of the fused roots have a more extreme slant distally, which places the apices of the roots in a more distal relationship to the centre of the crown (Figure 2.9). Figure 2.9: Various views of the Mandibular right third molar (from Thomas et al. 2006). ii) Lingual aspect There is no outstanding variation except for those mentioned above (see buccal aspect). Page 27

47 iii) Mesial aspect The distal root apex cannot be seen; the only variation in the morphology of this tooth (from that of the second molar) are odontometric. iv) Distal aspect The outline of the distal aspect is quite similar to that of the second molar, which makes allowances for a narrower crown bucco-lingually and shorter roots (Figure 2.9). v) Occlusal aspect The crown is shorter mesio-distally and narrower bucco-lingually, the crown tapers more distally, and the line angles are more rounded. A number of supplemental grooves are evident occlusally (Figure 2.9). vi) Crown form The crown is more oval than rectangular; the two mesial cusps are larger than the two distal cusps. The occlusal surface appears quite wrinkled, with an irregular groove pattern. Usually numerous occlusal pits are present; if an excess of these features exists the occlusal surface is described as crenulated. vii) Root form The mandibular third molar usually has two roots that are fused, irregularly curved, and shorter than those of a mandibular second molar. The roots show a marked distal inclination (Wheeler, 2003) Clinical considerations of the Mandibular third molars The permanent mandibular third molars may fail to erupt and remain impacted within the surrounding alveolar bone, which occurs more frequently than in the maxilla. This impaction typically occurs in 10% of the population, usually because the mandible is underdeveloped and space or arch length is insufficient to accommodate these teeth. Surgical removable may be necessary if they are impacted or partially erupted (Wheeler, 2003). Page 28

48 Absence of the third molars Absence of one or more third molars is common anomaly in human dental development. Third molar absence has also been associated with a reduction in the number of other teeth and structural variations (Garn et al. 1962). Attempts were made to explain this deviation through evolutionary and anatomical models, such as Butler s field theory, odontogenic polarity, or Sofaer s model of compensatory tooth size interactions (Vastardis, 2000). i) Butler s theory (1939): this theory attempts to explain why certain teeth fail to form. According to this hypothesis, the human dentition can be divided into three morphologic fields: incisors; canines; and premolars/molars. Within each of those fields, one key tooth is presumed to be stable; the other teeth within this field become progressively less stable. Considering each quadrant separately, for example the key tooth in the molar/premolar field would be, the first molar. This positions the second and third molars at the distal end and the first and second premolars on its mesial end of the field. Based on Butler s theory, the third molar is predicted to be most variable in size and shape. ii) Evolutionary theory: in human evolution there has been a tendancy towards reduction in jaw length and prognathism, mandibular canine size and first molar cusp number, and increased third molar agenesis (Kraus, 1964). The reduction in tooth number is concomitant with the reduction in the size of the jaws in human evolution and is believed to be a continuing evolutionary trend. These changes in dentition correlate to functional adaptations; however it has been difficult to determine what advantage to survival has been confirmed by a reduction in dental structures. Kraus (1964) suggested that reductions of dental structures are controlled by genes that produce other structural changes that are advantageous to survival. Page 29

49 Human molecular genetics: Identification of the underlying cause of a condition starts with the localization of the defective gene in the human genome. Familial tooth agenesis can be the result of a single dominant gene defect. This is a clearly recognizable well defined and relatively common dental anomaly. However, a large family and an accurate assessment of the phenotype are essential to perform genetic linkage studies. The objective of these studies is to determine whether two genetic traits are segregating independently (Vastardis, 2000). The two genetic traits are a genetic marker (DNA polymorphism of known chromosomal location) and the condition of interest (e.g familial tooth agenesis). Genes located close to each other are passed together from parent to child (Ott, 1992). Therefore, cosegregation of a phenotype, such as tooth agenesis and a particular known marker, would suggest that these genetic traits lie close to each other, on the same region of a chromosome, providing at the same time the locus for the defective dental gene. Once the condition locus is identified in one family the above step is designed to determine whether the same chromosomal location is responsible for tooth agenesis in other families (Ott, 1992; Vastardis, 2000). Vastardis (2000) applied the same strategy to an American family presenting with autosomal dominant agenesis of the second premolar and third molars. They were able to find out in which chromosome the abnormal dental gene was located and concluded that a location on chromosome 4p is responsible for tooth agenesis in that family Histology of the dental tissues All teeth consist of acellular enamel that forms the outermost layer. This hard tissue is supported by resilient dentin. The roots of the teeth are covered by cementum. All the dental hard tissues have a distinctive microscopic structure which can be analysed histologically. The following is a brief description of the histology of enamel, dentin and cementum. Page 30

50 Enamel Enamel is an epithelially derived protective covering (of variable thickness) over the entire surface of the crown. During the eruptive phase the ameloblasts that form the enamel are lost. This means that thereafter if the enamel is damaged (e.g. due to caries, attrition, erosion) thereafter, it cannot be reformed. As a compensatory mechanism, enamel has a complex structure with a high mineral content, which makes it the hardest tissue in the human body. i) Physical properties: enamel is composed of 96% inorganic mineral in the form of hydroxyapatite and 4% organic material and water. The hydroxyapatite is a crystalline calcium phosphate that is also found in bone, dentin and cementum. Enamel is white to grayish-white in colour, but appears slightly yellow because it is translucent. Enamel ranges in thickness from a knifelike edge at its cervical margin to about 2.0mm to 2.5 mm maximum thickness over the occlusal (or incisal) surfaces (Avery, 1992). ii) Structure of enamel: enamel is composed of rods that extend from their site of origin to the outer surface of enamel. Each rod is formed by ameloblasts; four ameloblasts form a part of each rod. In cross-section a rod is keyhole-shaped with a head and a tail as shown in Figure The head of the rod is the broadest part; the tail is the elongated thinner part. Each rod is filled with crystals. Rods are formed perpendicular to the dentino-enamel junction and curve slightly towards the cusp tip (Orban, 1976). Page 31

51 Head Crystals Tail Figure 2.10: Diagramatic representation of rods in enamel (from Orban, 1976) Dentin Dentin is the mineralized tissue that forms the bulk of the tooth. As a living tissue it consists of specialized cells called odontoblasts and an intercellular substance. Dentin is structurally unique due to the presence of closely packed dentinal tubules. These tubules traverse the entire thickness of dentin and have a wavy course (Nanci, 2008). Page 32

52 Figure 2.11: Composite diagram of a human tooth in cross-section illustrating the different types of dentin (from Nanci, 2008). i) Physical properties: dentin is yellowish in colour and is composed of 70% inorganic hydroxyapetite crystals, 20% organic collagen fibres (along with small amounts of proteins) and 10% water. It is softer than enamel but slightly harder than bone or cementum. Radiographically, therefore, it is more radiolucent than enamel. Dentin is slightly elastic which allows the impact of mastication to occur without fracturing the enamel. This resilience is due to the presence of dentinal tubules throughout its matrix (Avery, 1992). ii) Types of dentin: dentin is composed of primary, secondary and tertiary dentin as shown in Figure Most dentin external to the pulp chamber is primary (also known as circumpulpal) dentin. The outermost layer of primary dentin is mantle dentin, secondary dentin develops after root formation has been completed. Tertiary (or reparative) dentin is formed in response to disease or trauma, which is formed by odontoblasts that are directly affected by the stimuli (Nanci, 2008). Page 33

53 Dental pulp Dental pulp is the soft connective tissue that supports the dentin. Each tooth has coronal (crown) and radicular (root) pulp. It contains connective tissue, blood vessels, nerves, and cells (such as odontoblasts and fibroblasts). The pulp has several functions, including initiative, formative, protective, nutritive and reparative (Nanci, 2008). i) Histology of dental pulp: the centre of the pulp contains large veins and arteries surrounded by fibroblasts and collagen fibres embedded in a intercellular matrix. More peripherally, along the dentin in both the coronal and radicular pulp, are the formative cells of the dentin termed odontoblasts. Histologically three distinct zones can be identified in the dental pulp as shown in Figure 2.12: A. Odontogenic zone (Odontoblasts); B. Cell free zone; C. Cell rich zone. Figure 2.12: Coronal section of a molar showing the dental pulp zones (from Orban, 1976). Page 34