HOMO - Journal of Comparative Human Biology 65 (2014) 201–213

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Hypodontia: Prevalence and pattern amongst the living Druze population – A Near Eastern genetic isolate Firas Alsoleihat ∗, Ameen Khraisat Department of Conservative Dentistry and Fixed Prosthodontics, Faculty of Dentistry, University of Jordan, Amman 11942, Jordan

a r t i c l e

i n f o

Article history: Received 20 September 2013 Accepted 5 March 2014

a b s t r a c t The Druze population is a small-sized Near Eastern genetic isolate with high rates of consanguineous marriages. The present study aimed at investigating prevalence, pattern and distribution of hypodontia in the permanent dentition, excluding wisdom teeth, amongst this population. Panoramic radiographs, dental casts, and anamnestic records of 85 un-admixed Druze schoolchildren (45 males, 40 females; age range = 14–18 years, mean age = 16 years, SD = 0.5 years) were examined for evidence of hypodontia. Those with any type of facial clefts or craniofacial syndromes were excluded. Hypodontia prevalence was 11.8% (11.1% for males, 12.5% for females) with no statistically significant difference between sexes. The average number of missing teeth per child was 1.4 (1.2 for males, 1.6 for females). The majority (90%) of affected individuals exhibited minor hypodontia (one or two teeth missing). The most commonly missing teeth were upper lateral incisors and canines, followed by lower and upper second premolars. Asymmetrical hypodontia was more prevalent. Maxillary, left side, and anterior segment predominance were observed. The distinct features of hypodontia among the Druze population are the relatively high prevalence of minor hypodontia, and the upper lateral incisors and canines being the most commonly missing teeth. © 2014 Elsevier GmbH. All rights reserved.

∗ Corresponding author. Tel.: +962 777 946 631; fax: +962 6 5355 522. E-mail address: fi[email protected] (F. Alsoleihat). http://dx.doi.org/10.1016/j.jchb.2014.03.003 0018-442X/© 2014 Elsevier GmbH. All rights reserved.

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Introduction Familial (non-syndromic) hypodontia, defined as congenital absence of varying numbers of deciduous and/or permanent teeth as an isolated trait, is one of the most common developmental abnormalities in humans, and the most common dental anomaly across diverse ethnic groups (Bergstrom, 1977; Brook, 1974; Buenviaje and Rapp, 1984; Castaldi et al., 1966; Helm, 1968; Ingervall et al., 1972; Johannsdottir et al., 1997; Locht, 1980; Mckibben and Brearley, 1971; Salem, 1989; Thilander and Myrberg, 1973). The frequency and pattern of hypodontia are well documented in the literature. The previously reported global scale for the prevalence rate of hypodontia, excluding third molars, ranges from as low as 2.2% in a Saudi Arabian sample (Salem, 1989) to as high as 15.68% in a Hungarian sample (Gabris et al., 2001) except for the extraordinarily high frequency (36.5%) observed among the Dariusleut Hutterite Berthren of Western Canada, which is a Europeanderived genetic isolate (Mahaney et al., 1990), and for the extremely low frequency of 0.3% observed in a Jewish sample (Rosenzweig and Garbarski, 1965). However, it is noteworthy that another study of an Israeli Jewish male sample reported a higher frequency of hypodontia (5.3%) (Goren et al., 2005). This wide range could possibly be attributed to variation in many factors including methods of sampling and investigation, as well as the distribution of age, sex, and ancestral origin of individuals (Aasheim and Øgaard, 1993; Bäckman and Wahlin, 2001; Davis, 1987; Eidelman et al., 1973; Silverman and Ackerman, 1979; Rosenzweig and Garbarski, 1965; Wisth et al., 1974). It appears that the prevalence of hypodontia observed in samples that belong to the Sub-Saharan African ancestral category is generally relatively low with a narrow range from 2.75% to 6.30% (Diagne et al., 2001; Ducka-Karska, 1983; Garner and Yu, 1978; Harris and Clark, 2008; Kayembe and Ntumba, 1987; Ng’ang’a and Ng’ang’a, 2001; Onyeaso and Oneyeaso, 2006). Amongst Sino-American (Sinodont) samples, such as: Chinese, Japanese and Korean, the reported frequencies are relatively high with a narrow range from 8.5% to 11.3% (Chung et al., 2008; Davis, 1987; Endo et al., 2004, 2006; Fujita et al., 2009; Goya et al., 2008; Kim, 2011; Lai and Seow, 1989; Niswander and Sujaku, 1963; Sharma, 1983; Tsai and King, 1998; Wu and Feng, 2005; Yanagida and Mori, 1990; Zhu et al., 2007). Western Eurasian samples showed the widest range of hypodontia prevalence from 2.2% to 15.7%, even when excluding the extremely low or high outlying frequencies mentioned earlier (Mahaney et al., 1990; Rosenzweig and Garbarski, 1965). Although limited number of studies observed the prevalence of hypodontia in Sunda-Pacific (Austronesian or Sundadont) samples, it appears that the prevalence range is also wide, extending from 2.8% to 8.6% (Dechkunakorn et al., 1990; Intaraprasong et al., 1983; Nik-Hussein, 1989; Stecker et al., 2007). Unfortunately, no accessible data were available regarding the prevalence of hypodontia amongst the Sahul-Pacific (Australo-Melanesian or Proto-Sundadont) category. With the exception of a few studies (Grahnen, 1956; Maklin et al., 1979; Lai and Seow, 1989; Salem, 1989; Ng’ang’a and Ng’ang’a, 2001), most of the previous reports demonstrated that hypodontia is more prevalent in females than males (about 3:2 female-to-male ratio). Most of the previous investigations conducted on Western Eurasian, Sub-Saharan African and Sunda-Pacific samples have shown that the most common congenitally absent teeth are either lower second premolars (LP2s) or upper lateral incisors (UI2s), however, the lower lateral incisor (LI2) or LP2 agenesis was found to be the most frequent type of dental agenesis amongst East Asian (Sinodont) populations (Chung et al., 2008; Davis, 1987; Endo et al., 2004, 2006; Fujita et al., 2009; Goya et al., 2008; Kim, 2011; Lai and Seow, 1989; Niswander and Sujaku, 1963; Sharma, 1983; Tsai and King, 1998; Wu and Feng, 2005; Yanagida and Mori, 1990; Zhu et al., 2007). The living Druze population is a small Near Eastern minority of about one million individuals residing primarily in Syria (40–50%) and Lebanon (30–40%), and to a lesser extent in Israel (6–7%) and Jordan (1–2%), in addition to about 100,000 individuals in the Druze Diaspora (Nissim, 2003; Shlush et al., 2008). The Druze population is famous for its endogamy (Falah, 2000; Jobling, 2004; Nissim, 2003) which is also corroborated by some genetic studies such as Shlush et al. (2008). This social practice of endogamy and consanguineous marriage, in addition to the fact that the Druze doctrines firmly close their religion to new converts, prohibited admixture with other populations, and thus led to genetic isolation of the Druze minority from the larger population majorities of the countries where they co-reside (Jobling, 2004; Shlush et al., 2008).

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The Druze population is generally thought to be Arabs who separated from Islam in the 11th century (Firro, 1992). Recent genetic clustering analyses have shown close ancestral relationships between the Druze people and Turkish Cypriots and identified similarities of the Druze people to the general Syrian and Lebanese populations, as well as to a number of Jewish groups such as Ashkenazi (German Jews), Sephardi (Spanish Jews), Iraqi, and Moroccan Jews (Behar et al., 2010). Moreover, a dental morphological study, based on 19 non-metric dental traits, has shown a relatively closer affinity for the living Druze people in Israel to all the Jewish groups included in the study (Eastern European Jews, Kurdish Jews, Moroccan Jews and Mount Zion Skeletal Jews), excluding the Habbanites, as well as to Bedouin Arabs and Circassians than to the Habbanite Jews, contemporary Nubians and Samaritans (Sofaer et al., 1986). In addition, a recent dental morphologic analysis has shown that the living Druze population is very closely related biologically to living Jordanians, who belong to the Near Eastern Arabs (Alsoleihat and Khraisat, 2013). Therefore, it appears that the Druze population is most likely ancestrally closest to the Near Eastern Arabs and Jews. The Druze population is almost completely an endogamous group whose individuals do not generally intermarry with their Arab neighbors (Firro, 1992). It has been hypothesized and demonstrated, in a number of previous studies, that high rates of consanguineous marriages are associated with increased frequencies of dental anomalies and agenesis (Eswar et al., 2003; Maatouk et al., 2008; Mahaney et al., 1990; Nelsen et al., 2001; Vona et al., 1993). The present study aims at investigating the prevalence of dental agenesis, excluding third molars, among the living Druze population and comparing the findings of the study with the results available in the literature and obtained from diverse ethnic groups. This would shed light on the influence of high rates of endogamy and consanguineous marriages on the prevalence of dental agenesis. In addition, this would also help assessing the degree of orthodontic needs for this population.

Materials and methods A representative sample was obtained in 2011 by selecting 85 un-admixed Druze school children (45 males and 40 females), from the main two schools in Al-Azraq (Al-Azraq ash-Shamali school for males, Al-Azraq ash-Shamali school for females), which is a city located in the east of Jordan. It is noteworthy to mention that the majority (about 60%) of Druze in Jordan (also known as Bani Ma’roof) live in this one town called Al-Azraq and the vast majority (more than 95%) of the schoolchildren of this town go to either of these two schools, as confirmed by the director of these two schools. Only less than 5% of the Druze schoolchildren in Al-Azraq go to Al-Azraq al-Janubi school for males and Al-Azraq al-Janubi school for females; these were not included in the present study. It is also noteworthy to mention that present study involved most of the Druze school children in Al-Azraq in the age range of 14–18 years, since the Druze who live in Al-Azraq are a small population (i.e. about 8000 individuals) according to various sources. Therefore, the authors believe that the sample of the study is largely representative of the majority of the living Druze population in Jordan. Ethical approval for conducting this type of study was obtained from the authorized body, the Ethical Committee of the Faculty of Dentistry/University of Jordan, prior to commencement of the study. Informed consents were obtained from the parents of all children who chose to participate in the study before the children were subjected to dental examination, radiographic imaging, or impression taking. The average age of the selected individuals was 16 years (SD = 0.5), being between the ages of 14 and 18 years. In this age range, the children were old enough to have a reliable confirmation of congenital absence of any of the permanent teeth; due to the fact that it has been previously demonstrated that no tooth has been found to initiate crown mineralization beyond the age of 12 years across diverse ethnic groups (Aasheim and Øgaard, 1993; Harris and McKee, 1990; Rantanen, 1967). Moreover, the children, in this age range, are young enough to have minimum tooth loss, and dental records which are reliable enough to document extractions and avulsions. They were selected according to the following criteria: all individuals were apparently healthy with no history of serious childhood illnesses, and with no developmental anomalies such as ectodermal dysplasia, facial clefts and craniofacial syndromes, and have no history of previous orthodontic treatment, and no history of tooth extraction or accident related tooth loss.

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Unadmixture status of individuals in this study was determined by self-report because there is no objective criterion to distinguish admixed individuals; however, this was further confirmed by the directors of the schools who are local Druzes of Al-Azraq. Also they helped to rule out any possible adoption of Druze self-identification by schoolchildren with different (e.g. Chechen or Palestinian) ethnic background. Panoramic radiographs, dental casts, and anamnestic records were the only sources of information used in this study to diagnose hypodontia. Several previous studies have demonstrated the reliability of panoramic radiographs for the diagnosis of hypodontia (Aasheim and Øgaard, 1993; Bäckman and Wahlin, 2001; Nik-Hussein, 1989; Nordgarden et al., 2002; Wisth et al., 1974). A tooth was diagnosed to be congenitally absent when no evidence of crown calcification could be identified at the tooth site on the panoramic radiograph and no evidence of previous extraction of the tooth could be found. Dental casts, clinical dental examination, as well as medical and dental records were used as supplements to exclude those with teeth missing due to extraction or avulsion, and to ensure accurate diagnosis of hypodontia. The panoramic radiographs and dental casts of the individuals of the present study were reexamined by another observer three months after the first observation, and 100% reproducibility was achieved in the diagnosis of hypodontia. The prevalence of hypodontia was calculated by tooth type and sex. Statistical differences between frequencies or means of groups were tested by independent samples t-test between percentages or means, respectively (Statistics Calculator, 1997–2010 StatPac, Inc.). Statistical significance was set at the 0.05 probability level. The prevalence of hypodontia in a particular group was calculated by dividing the number of individuals with hypodontia by the total number of individuals in that group. The average number of congenitally missing teeth per hypodontic individual for a particular group was calculated by dividing the total number of missing teeth by the total number of individuals with hypodontia in that group. The relative frequency of agenesis for a particular tooth type amongst the hypodontic individuals of the present sample was calculated by dividing the number of missing teeth corresponding to the tooth type considered by the total number of missing teeth. The prevalence of symmetrical hypodontia was calculated as the number of symmetrical cases of dental agenesis divided by the total cases of agenesis whether symmetrical or asymmetrical. The dental casts and the panoramic radiographs of the individuals of the present study are currently housed at the museum of the Faculty of Dentistry of the University of Jordan. Results A total of 10 (5males and 5females) schoolchildren out of 95 (45males and 40 females) were found to exhibit hypodontia in the permanent dentition. Therefore, the prevalence of hypodontia among this sample is 11.8% (11.1% among males and 12.5% among females). However, this difference between males and females is not statistically significant (p = 0.842). The total number of congenitally missing permanent teeth, among this sample was 14 (6 among males and 8 among females), with an average of 1.4 per hypodontic child (1.2 for males and 1.6 for females). This difference is also statistically insignificant (p = 0.395). The number of missing teeth per child in the present sample ranged from 1 to 2 for males and from 1 to 3 for females. The percentage of children having 1 missing tooth among those with hypodontia was 70% (80% (4 out of 5 individuals) for males, 60% (3 out of 5 individuals) for females; p = 0.5097); those with 2 missing teeth constituted 20% (20% (1 out of 5 individuals) for males, 20% (1 out of 5 individuals) for females), and those with three missing teeth represented 10% (0% (0 out of 5 individuals) for males, 20% (1 out of 5 individuals) for females; p = 0.3226) Out of those exhibiting hypodontia, the prevalence of oligodontia, defined as having five or more missing teeth, was 0% in both males and females. The male-female difference was statistically insignificant in the percentages of children in each group classified by the number of missing teeth. The distribution of missing teeth by sex is shown in Tables 1 and 2. The results vary with respect to the most commonly missing teeth between males and females. Among males with hypodontia, the most frequently missing teeth were the upper lateral incisors (UI2s; 4 cases), followed by upper canines (UCs; 1 case) and second premolars (UP2s; 1 case) which showed equal rates of agenesis. None

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Table 1 Raw data of the distribution of missing teeth by sex and side amongst the Druze sample. Sex/side

Male/right side Male/left side Female/right side Female/left side

Tooth type (n) UI2

UC

LP2

UP2

1 3 0 1

1 0 1 2

0 0 1 2

1 0 1 0

n – number of missing teeth, UI2 – upper lateral incisor, UC – upper canine, LP2 – lower second premolar, UP2 – upper second premolar.

Table 2 Distribution of hypodontia frequencies by sex amongst the Druze sample. Percent Males Hypodontia (all types) Maxillary hypodontia By tooth site By tooth type Mandibular hypodontia By tooth site By tooth type Right side hypodontia Left side hypodontia Anterior segment hypodontia By tooth site By tooth type Posterior segment hypodontia By tooth site By tooth type Symmetrical hypodontia (by tooth type) Asymmetrical hypodontia (by tooth type) UI2 agenesis Frequency by tooth site per total individuals Frequency by tooth type per total individuals Frequency by tooth site UC agenesis Frequency by tooth site per total individuals Frequency by tooth type per total individuals Frequency by tooth site LP2 agenesis Frequency by tooth site per total individuals Frequency by tooth type per total individuals Frequency by tooth site UP2 agenesis Frequency by tooth site per total individuals Frequency by tooth type per total individuals Frequency by tooth site

Percent Females

p-Value

Percent Sexes pooled

12.5% (5/40)

0.84

11.8% (10/85)

100.0% (6/6) 100.0% (5/5)

62.5% (5/8) 71.4% (5/7)

0.21 0.47

78.6% (11/14) 83.3% (10/12)

0.0% (0/6) 0.0% (0/5) 50.0% (3/6) 50.0% (3/6)

37.5% (3/8) 28.6% (2/7) 37.5% (3/8) 62.5% (5/8)

0.21 0.47 0.64 0.64

21.4% (3/14) 16.7% (2/12) 42.9% (6/14) 57.1% (8/14)

83.3% (5/6) 80.0% (4/5)

50.0% (4/8) 57.1% (4/7)

0.30 0.58

64.3% (9/14) 66.7% (8/12)

16.7% (1/6) 20.0% (1/5) 20.0% (1/5) 80.0% (4/5)

50.0% (4/8) 42.9% (3/7) 14.3% (1/7) 85.7% (6/7)

0.30 0.58 0.79 0.79

35.7% (5/14) 33.3% (4/12) 16.7% (2/12) 83.3% (10/12)

11.1% (5/45)

8.9% (4/45)

2.5% (1/40)

0.36

5.9% (5/85)

6.7% (3/45) 66.7% (4/6) 60.0% (3/5)

2.5% (1/40) 12.5% (1/8) 14.3% (1/7)

0.62 0.09 0.22

4.7% (4/85) 35.7% (5/14) 33.3% (4/12)

2.2% (1/45)

7.5% (3/40)

0.34

4.7% (4/85)

2.2% (1/45) 16.7% (1/6) 20.0% (1/5)

7.5% (3/40) 37.5% (3/8) 42.9% (3/7)

0.34 0.58 0.58

4.7% (4/85) 28.6% (4/14) 33.3% (4/12)

0.0% (0/45)

7.5% (3/40)

0.0% (0/45) 0.0% (0/6) 0.0% (0/5)

5.0% (2/40) 37.5% (3/8) 28.6% (2/7)

0.22 0.21 0.47

2.4% (2/85) 21.4% (3/14) 16.7% (2/12)

2.2% (1/45)

2.5% (1/40)

0.93

2.4% (2/85)

2.2% (1/45) 16.7% (1/6) 20.0% (1/5)

2.5% (1/40) 12.5% (1/8) 14.3% (1/7)

0.93 0.83 0.79

2.4% (2/85) 14.3% (2/14) 16.7% (2/12)

0.1

UI2 – upper lateral incisor, UC – upper canine, LP2 – lower second premolar, UP2 – upper second premolar.

3.5% (3/85)

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of lower second premolars (LP2s) were found to be congenitally missing among the male group of the present study. On the other hand, the order of absence frequency among females with hypodontia was as follows: UCs (3 cases) and LP2s (3 cases) followed by UI2s (1 case) and UP2s (1 case). When both sexes were combined, the order of agenesis rates among children with hypodontia was as follows: UI2s (5 cases) UCs (4 cases), LP2s (3 cases), and UP2s (2 cases). There are apparent differences in the pattern of hypodontia between males and females especially regarding the agenesis frequencies of UI2s, UCs and LP2s; however statistical significance was only found in UI2 agenesis. The prevalence of symmetrical hypodontia was 16.7% (2 out of 12 cases) among the Druze children exhibiting any degree of hypodontia (20.0% (1 out of 5 cases) among males and 14.3% (1 out of 7 cases) among females; p = 0.7993), with the most common symmetrical hypodontia among this sample (sexes pooled) being LP2 agenesis (50% of the time; 1 out of 2 cases of LP2 agenesis), followed by UI2 agenesis (25% of the time; 1 out of 4 cases of UI2 agenesis). However, the difference between these two types of symmetrical agenesis was statistically insignificant (p = 0.5734). The congenital absence of UCs and UP2s was found asymmetrical in all hypodontic cases among this sample. The ratios of missing teeth in the upper and lower jaws, on the right and left sides, and in the anterior and posterior segments were calculated and compared. All missing teeth among male children with hypodontia were maxillary teeth, equally distributed on the right and left sides, and with the majority (83.3%; 5 out of 6 cases) being in the anterior segment. Among females 62.5% (5 out of 8 cases) of missing teeth with hypodontia were maxillary, 62.5% (5 out of 8 cases) being on the left side (60% (3 out of 5 cases) with respect to maxillary teeth and 66.7% (2 out of 3 cases) with respect to mandibular ones); and equally distributed in the anterior and posterior segments (80% (4 out of 5 cases) in the anterior segment with respect to maxillary teeth and 0% (0 out of 3 cases) with respect to mandibular ones). When sexes were pooled, 78.6% (11 out of 14 cases) of missing teeth were maxillary, 57.1% (8 out of 14 cases) being on the left side (54.5% (6 out of 11 cases) with respect to maxillary teeth and 66.7% (2 out of 3 cases) with respect to mandibular ones); 64.3% (9 out of 14 cases) being in the anterior segment (81.8% (9 out of 11 cases) with respect to maxillary teeth and 0% (0 out of 3 cases) with respect to mandibular ones). Discussion The prevalence of hypodontia, among the Druze sample was 11.1% for males, 12.5% for females, and 11.8% for both sexes pooled. These rates are at the high end of the wide range previously reported for Western Eurasia, which extends from 2.2% to 15.68%. However, the closest reported frequencies among Western Eurasia to the rates of hypodontia in the present study are those reported by Hunstadbraten (1973) who studied a sample of Norwegians, Behr et al. (2011) and Hickel and Hickel (1989) who studied German samples, O’Dowling and McNamara who studied an Irish sample (1990), Ajami et al. (2010) and Vahid-Dastjerdi et al. (2010) who studied Persian samples, and Maatouk et al. (2008) who studied a Tunisian genetic isolate. Interestingly, the hypodontia prevalence in the Druze sample is very close to the reported frequencies of hypodontia in a number of studies previously conducted on Eastern Asian (Sinodont) samples (Chung et al., 2008; Goya et al., 2008; Kim, 2011). This is difficult to reconcile since the Druze population do not share a recent common ancestry with Sinodont populations. Surprisingly, on the other hand, hypodontia prevalence among the Druze sample is distinctly higher than the reported frequencies of hypodontia in a number of Near Eastern Arabian samples (Abu Alhaija et al., 2005; Albashaireh and Khader, 2006; al-Emran, 1990; Salem, 1989; Salama and Abdel-Megid, 1994), which is inconsistent with their close biological ancestry. This distinctly higher prevalence of hypodontia among the Druze population is possibly related to isolation and the high rates of consanguineous marriages. It is known that the social organization of the Druze encourages inter-marriages between close relatives and discourages marriage with anyone outside the Druze population. This is consistent with the results of some previous studies conducted on genetic isolates that are characterized by high rates of endogamy (Eswar et al., 2003; Maatouk et al., 2008; Mahaney et al., 1990; Nelsen et al., 2001), which observed ostensibly higher rates of hypodontia among these groups. Alternatively, this dissimilarity might be attributed to the close family relations of the studied individuals in Al-Azraq as they are all descendants of a small number of families which settled down in Jordan in early 20th century.

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This study also revealed that the prevalence rate of hypodontia is slightly higher in females than in males among the Druze sample, but this difference is statistically insignificant. This statistically insignificant female predominance in the frequency of hypodontia is consistent with the findings of the majority of previous studies, with the exception of the following studies (Grahnen, 1956; Lai and Seow, 1989; Maklin et al., 1979; Ng’ang’a and Ng’ang’a, 2001; Salem, 1989). In this regard, only a few previous studies demonstrated statistically significant male-female differences (Bergstrom, 1977; Brook, 1974; Nordgarden et al., 2002; Ringqvist and Thilander, 1969; Rosenzweig and Garbarski, 1965; Topkara and Sari, 2011). These findings might suggest a more tendency for the X chromosome toward permitting dental numerical reduction versus a balancing effect of the Y chromosome in conserving the same number of teeth. The number of congenitally absent teeth per child in both sexes combined among the Druze sample was 1.4. This figure is slightly lower than the previously reported range of 1.5–4.8, and is closest to that reported by Davis (1987) who studied a sample of Chinese school children. This relatively high prevalence of hypodontia and small number of missing teeth per individual among the living Druze might be characteristic for this population. However, further comparative data are required from other groups that belong to the Middle Eastern Arab regional group to confirm this postulation. This study also demonstrated that the number of missing teeth per individual among the Druze sample is slightly greater in females than males, but the difference is again not statistically significant. It is noteworthy that many previous studies have shown inconsistent findings as to which sex is predominant in this regard (Aasheim and Øgaard, 1993; Bäckman and Wahlin, 2001; Bergstrom, 1977; Byrd, 1943; Castaldi et al., 1966; Grahnen, 1956; Haavikko, 1971; Horowitz, 1966; Hunstadbraten, 1973; Magnusson, 1977; Muller et al., 1970; Ng’ang’a and Ng’ang’a, 2001; Nik-Hussein, 1989; Ringqvist and Thilander, 1969; Rolling, 1980; Rose, 1966; Rosenzweig and Garbarski, 1965; Thompson and Popovich, 1974; Volk, 1963; Wisth et al., 1974; Zimmerman, 1967). The frequency of 90% in children with one or two missing teeth among the children exhibiting any degree of hypodontia in the present study is within, but occupying the high end of the previously reported worldwide range of 75% in the Norwegian population (Hunstadbraten, 1973) to 97.4% in the Chinese population (Davis, 1987), except for the very low prevalence rate of 49.0% reported by Lai and Seow (1989). In contrast, the prevalence of oligodontia in the present study was found to be 0.0% occupying the lowest end of the previously reported world range of 0.0% in the Eurocanadian (Castaldi et al., 1966) and Chinese populations (Davis, 1987) to 11.3% in a Swedish sample (Grahnen, 1956). The finding of the present study, that the upper lateral incisor has the highest rate of agenesis in children with minor (one or two teeth missing) or moderate hypodontia (three or four teeth missing), followed by the upper canines, the upper second premolars and the lower second premolars in a decreasing order, is partly in agreement with those of a number of studies conducted on Latin American samples (Gomes et al., 2010; Medina, 2012; Silva Meza, 2003) and on Sunda-Pacific samples (Intaraprasong et al., 1983; Nik-Hussein, 1989), and with those observed by Shafi et al. (2008) in an ˙ English sample; Szepesi et al. (2006) and Gabris et al. (2001, 2006) in Hungarian samples; Struzak´ et al. (1990) in a Polish sample; Topkara and Sari (2011), Celikoglu et al. (2010) and Wysokinska Sisman et al. (2007) in Turkish samples; Vahid-Dastjerdi et al. (2010) in a Persian sample; Rosenzweig and Garbarski (1965) in a Near Eastern Jewish sample; Werther and Rothenberg (1939), Byrd (1943), Zimmerman (1967) and Muller et al. (1970) in Euroamerican samples; and Ruprecht et al. (1986) in a Saudi Arabian sample. These studies showed that the upper lateral incisors are the most frequently missing teeth, followed, in most of these studies, by the lower second premolars; or, in some of them, by the upper second premolars (Intaraprasong et al., 1983; Vahid-Dastjerdi et al., 2010), except for one study where the UI2 was followed by the LI2 (Nik-Hussein, 1989). On the other hand, the findings of the present study, regarding the order of frequency of congenitally missing teeth, completely disagree with those observed in Sinodont samples, which showed that the lower second premolars (Endo et al., 2006; Goya et al., 2008; Kim, 2011; Zhu et al., 2007) or the lower lateral incisors (Chung et al., 2008; Davis, 1987; Niswander and Sujaku, 1963) are the most commonly missing teeth in those samples, and also disagree with those observed in Sub-Saharan African samples (Diagne et al., 2001; Ng’ang’a and Ng’ang’a, 2001) and the majority of Western Eurasian samples (Aasheim and Øgaard, 1993; Ajami et al., 2010; Bäckman and Wahlin, 2001; Behr et al., 2011; Bergstrom, 1977; Brown, 1957; Buenviaje and Rapp, 1984; Byrd, 1943; Castaldi et al., 1966; Dolder, 1937; Gimnes, 1964; Glenn, 1961, 1964;

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Gonzalez-Allo et al., 2012; Goren et al., 2005; Grahnen, 1956; Gulzow and Peters, 1977; Haavikko, 1971; Horowitz, 1966; Hunstadbraten, 1973; Ingervall et al., 1972; Johannsdottir et al., 1997; Lai and Seow, 1989; Lynham, 1990; Maatouk et al., 2008; Magnusson, 1977; Maklin et al., 1979; Manrique Mora et al., 1991; Mckibben and Brearley, 1971; Nordgarden et al., 2002; Ringqvist and Thilander, 1969; Rolling, 1980; Rose, 1966; Sabes and Bartholdi, 1962; Silverman and Ackerman, 1979; Thilander and Myrberg, 1973; Thompson and Popovich, 1974; Volk, 1963; Wisth et al., 1974) including Near Eastern Arab samples (Abu Alhaija et al., 2005; Albashaireh and Khader, 2006; al-Emran, 1990; Salama and Abdel-Megid, 1994; Salem, 1989). These studies showed that the lower second premolars are the most common type of dental agenesis among these population groups, followed by either the upper second premolars or the upper lateral incisors. The partial or complete dissimilarity of the findings of the present study regarding the distribution of hypodontia among the Druze population to the findings of the above-mentioned studies conducted on Sinodont, Sundadont and Sub-Saharan African populations is consistent with the different ethnic origin of the Druze population from these three groups. While the dissimilarity to the findings in the above-mentioned Western Eurasian samples especially Near Eastern Arabs and Jews is more difficult to reconcile since they belong to the same ancestral category (i.e. they share a recent common ancestor). A possible explanation for this might be that the Druze population has undergone a major genetic drift from the Western-Eurasian category leading to distinct dental characteristics from other groups belonging to Western-Eurasia, with which it shares a recent common ancestor over a relatively short time span. Similar dental microdifferentiation among groups having recent common ancestral relationships has been well documented in the Near Eastern Jews (Sofaer et al., 1986), the Southwest American Indians (Scott and Dahlberg, 1982; Scott et al., 1983), the Yanomama Indians of Venezuela (Brewer-Carias et al., 1976), and Melanesians (Harris, 1977). Such genetic drifts leading to local differentiation in dental characteristics among biologically related groups over a relatively short period are generally viewed as a consequence of colonization events, population structure such as high rates of endogamy, and small population size (Scott and Turner, 1988). It is also noteworthy that the finding of the present study that the upper canines were the most commonly missing teeth in females (3 out of 8 cases) and the second most commonly missing teeth (following upper lateral incisors) in males (1 out of 6 cases) and in both sexes combined (4 out of 14 cases), which is dissimilar to all previous investigations across diverse ethnic groups. These previous studies showed extreme rarity of upper canine agenesis. It appears that the relatively high rate of upper canine agenesis amongst hypodontic individuals is characteristic for the Druze population. This finding is difficult to reconcile and inconsistent with the general consideration, according to Butler’s evolutionary morphogenic field model (Butler, 1963) and Osborn’s clone model (Osborn, 1978), which state that the upper canines are one of the most stable teeth (similar to lower canines and the mesial members of various dental classes, such as: central incisors, first premolars, and first molars) and least affected by morphometric reduction or agenesis. However, this finding is consistent with the Svinhufvud’s et al. (1988) anatomical model which suggests that teeth adjacent to the junction between the premaxilla and maxilla proper (upper lateral incisors and canines) could exhibit some degree of instability. However, this model failed to account for the considerable stability of upper canines across diverse ethnic groups. Therefore, the relatively high rate of upper canine agenesis amongst the Druze population seems to be more likely due to a mutation or mutations in a gene or a number of genes that act specifically on the odontogenesis of upper canines rather than genes with a more general action on the development of the whole dentition. Such mutations are more likely to accumulate in isolated genetic pools due to high rates of endogamy and small population size-which is the situation for the Druze population. The present study revealed that symmetrical hypodontia is much less prevalent (16.7%; 2 out of 12 cases) than asymmetrical dental agenesis among the Druze sample. This is in disagreement with many previous reports which reported that symmetrical hypodontia is more prevalent (Bäckman and Wahlin, 2001; Bergstrom, 1977; Byrd, 1943; Castaldi et al., 1966; Endo et al., 2006; Glenn, 1964; Goya et al., 2008; Grahnen, 1956; Horowitz, 1966; Hunstadbraten, 1973; Lai and Seow, 1989; Maatouk et al., 2008; Magnusson, 1977; Maklin et al., 1979; Muller et al., 1970; Ng’ang’a and Ng’ang’a, 2001; Silva Meza, 2003; Silverman and Ackerman, 1979; Volk, 1963; Zimmerman, 1967). However, few previous studies (Davis, 1987; Glenn, 1961; Grahnen, 1956; Haavikko, 1971) have shown the opposite,

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thus similar to the finding of the present study. The finding of the present study, that the most common symmetrical hypodontia among the Druze sample is LP2 agenesis, followed by UI2 agenesis, is in agreement with most previous studies which demonstrated that the most common symmetrical hypodontia is LP2 agenesis, followed by either UI2 or UP2 agenesis (Castaldi et al., 1966; Endo et al., 2006; Gimnes, 1964; Grahnen, 1956; Mckibben and Brearley, 1971; Silverman and Ackerman, 1979; Szepesi et al., 2006; Volk, 1963). The present study demonstrated that more teeth were missing from the maxilla than from the mandible among the Druze children exhibiting any degree of hypodontia which ranged in the affected individuals of the present study from one to three missing teeth per individual (i.e. minor to moderate hypodontia). Many previous studies have shown inconsistent findings regarding which dental arch is predominant in terms of how many teeth per arch are missing (Aasheim and Øgaard, 1993; Albashaireh and Khader, 2006; Bäckman and Wahlin, 2001; Bergstrom, 1977; Brown, 1957; Byrd, 1943; Davis, 1987; Dolder, 1937; Endo et al., 2006; Glenn, 1961, 1964; Grahnen, 1956; Haavikko, 1971; Horowitz, 1966; Hunstadbraten, 1973; Legovic´ et al., 1990; Magnusson, 1977; Maklin et al., 1979; Muller et al., 1970; Ng’ang’a and Ng’ang’a, 2001; Nordgarden et al., 2002; Ringqvist and Thilander, 1969; Rolling, 1980; Rose, 1966; Rosenzweig and Garbarski, 1965; Ruprecht et al., 1986; Sabes and Bartholdi, 1962; Salama and Abdel-Megid, 1994; Silva Meza, 2003; Silverman and Ackerman, 1979; Thompson and Popovich, 1974; Topkara and Sari, 2011; Werther and Rothenberg, 1939; Wisth et al., 1974; Zhu et al., 2007; Zimmerman, 1967). However, some previous investigations observed maxillary predominance in this regard, which is consistent with the finding of the present ˙ ´ nska et al., 1990; Vahid-Dastjerdi et al., study (al-Emran, 1990; Sisman et al., 2007; Struzak-Wysoki 2010). The present study demonstrated that, in the present sample, the left side is predominant, especially in the mandible, in terms of the number of missing teeth per side. This finding disagrees with many previous investigations which showed that there is similar distribution of missing teeth between the right and left sides (Aasheim and Øgaard, 1993; Albashaireh and Khader, 2006; Bäckman and Wahlin, 2001; Bergstrom, 1977; Brown, 1957; Byrd, 1943; Dolder, 1937; Endo et al., 2006; Grahnen, 1956; Magnusson, 1977; Maklin et al., 1979; Muller et al., 1970; Ringqvist and Thilander, 1969; Rolling, 1980; Rose, 1966; Rosenzweig and Garbarski, 1965; Silva Meza, 2003; Sisman et al., 2007; Thilander and Myrberg, 1973; Thompson and Popovich, 1974; Wisth et al., 1974; Zhu et al., 2007; Zimmerman, 1967). The present study found that in the Druze children with one or two teeth absent, the majority of missing teeth are in the anterior segment when only maxillary teeth or when all teeth are considered, while when only mandibular teeth are considered, the congenital absence occurs exclusively in the posterior segment. When all children with any degree of hypodontia are considered and when the whole dentition is included in the comparison, the overall anterior predominance considerably decreases. This finding agrees with the findings of some previous reports which pointed out that anterior tooth agenesis is predominant in individuals with one or two teeth absent, but the reverse is true of individuals with three or more missing teeth (Aasheim and Øgaard, 1993; Endo et al., 2004, 2006; Muller et al., 1970; Øgaard and Krogstad, 1995). The findings of the present study provide useful information to dental practitioners on where to focus in dental examination when hypodontia is alleged in patients from the Druze population. In addition, the findings suggest great orthodontic need for this population due to the relatively high prevalence of hypodontia (11.8%) and the observation that dental agenesis occurs predominantly (64.3% (9 out of 14 cases)) in the upper anterior segment.

Conclusions The distinct features of hypodontia among the Druze population, in comparison with other populations, are the relatively high prevalence rate of minor (one or two teeth missing) hypodontia, the extreme rarity of oligodontia (five or more teeth missing), and the upper lateral incisor and upper canine agenesis being the most common type of congenital dental absence among males and females, respectively.

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Acknowledgments This research was conducted at the Faculty of Dentistry of the University of Jordan, and supported by a grant from the Deanship of Academic Research of the University of Jordan (109/2009-2010). We would like to thank Dr. Mahmoud Alnsour, Dr. Khadijah Al-zoubi and Dr. Reham Al-Najjar for helping us taking the impressions for the dental arches, to thank Mr. Ziad Nazik for pouring the impressions and preparing the dental casts, to thank Mr. Ramzi Bani Marof for arranging the school visits, and to thank the directors of schools involved. References Aasheim, B., Øgaard, B., 1993. Hypodontia in 9-year-old Norwegians related to need of orthodontic treatment. Scand. J. Dent. Res. 101, 256–260. Abu Alhaija, E.S., Al-Khateeb, S.N., Al-Nimri, K.S., 2005. Prevalence of malocclusion in 13–15 year-old North Jordanian school children. Community Dent. Health 22, 266–271. Ajami, B.A., Shabzendedar, M., Mehrjerdian, M., 2010. 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