PEDIATRIC DENTAL JOURNAL 19(2): 228–233, 2009

Received on October 16, 2008

Accepted on March 9, 2009

Agenesis of maxillary permanent first molars: Literature review and two case reports

Robert P. Anthonappa, Cynthia K.Y. Yiu and Nigel M. King

Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, CHINA

Abstract Congenital absence of teeth is the most commonly known devel-opmental dental anomaly in humans. Agenesis of the permanent first molars has the least frequency of all the tooth types and when present, usually occurs in association with oligodontia or anodontia. Generally, it is easy to diagnose agenesis of the permanent first molars retrospectively, based on the clinical morphology and x-ray photographic features of the permanent second molars; while it is often debatable when made prospectively. Several hypotheses have been promulgated to explain the etiology of hypodontia with evolutionary and anatomic models. Nevertheless, clinical epidemiology does not completely support these hypotheses and therefore clinicians should be cautious; while speculating the missing tooth type based on these hypotheses. We encompass a comprehensive review of the literature on missing permanent first molars and illustrate two cases of missing maxillary permanent first molars to emphasize the complexity involved in their diagnosis.

Conversely, among the southern Chinese, the reported prevalence of tooth agenesis for both genders combined is 7.3% and after the third molars, the next most commonly missing tooth type is the mandibular incisor5,6).

Several hypotheses have been promulgated to explain the etiology of hypodontia with evolutionary and anatomic models such as, Butler’s field theory7), odontogenic polarity, Sofaer’s model of compensa-tory tooth size interactions8), Svinhufvud’s anatomic model (areas of embryonic fusion)9) and Kjaer’s neuro-osteological developmental fields in the jaws (incisor field, canine/premolar, and molar field)10). Nevertheless, clinical epidemiology does not com-pletely support these hypotheses and therefore clinicians should be cautious while speculating the missing tooth type based on these hypotheses.

For example according to Butler’s field theory7), the mammalian dentition can be divided into three morphologic fields corresponding to incisors, canines/ premolars, and molars. Within each field, one “key” tooth is presumed to be stable and the neighboring teeth within this field become progressively less stable. Considering each quadrant separately, the

Introduction

Hypodontia is the term used to describe the developmental absence of one or more primary or permanent teeth, excluding the third molars. The term “oligodontia” is used for six or more missing teeth and “anodontia” for complete absence of teeth. Hypodontia is the most commonly occurring developmental dental anomaly in man and is found more often in the permanent dentition1). It occurs either as an isolated abnormality or in association with several syndromes or conditions2).

In the general population the reported incidence of permanent tooth agenesis ranges from 1.6 to 9.6%, this is excluding third molars; while in the primary dentition the range is from 0.5 to 0.9%3). Females are more frequently affected than males, in the ratio 3 : 2 respectively2). In the Caucasian population, mandibular second premolars are the most frequently missing teeth, followed by maxillary lateral incisors and maxillary second premolars4).

Key wordsHypodontia,Missing teeth,Permanent first molars

Case Report

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key tooth in the incisor field would be the central incisor. This diagram positions the lateral incisors at the distal end of the field. Therefore, the lateral incisors would be predicted to be most frequently missing. Conversely, Kjaer10), explained the location of tooth agenesis by neural developmental fields in the jaws (incisor field, canine/premolar, and molar field). She proposed that the teeth farthest from the main nerve stem are those most often affected by agenesis. Considering each quadrant separately, for the incisor field in the mandible, the central incisors are more frequently missing as they are farther away from the main nerve stem in this field. Therefore, the concept of a polygenic multi-factorial model of etiology provides a good explanation for hypodontia. Recent developments have indicated the importance of muscle-specific homeobox genes (MSX1)3), and their role in controlling the complex epithelial/ mesenchymal interactions that occur during dental development2). Inactivation11) or silent12) point muta-tions in the paired box domain of human PAX9 gene has also been implicated in tooth agenesis.

Several hypotheses7–10) have been proposed to explain the missing tooth types and the consensus opinion is that the permanent first molars are always present and never missing, except in those patients, whose complete molar tooth series are absent, as seen in severe oligodontia or anodontia. Nevertheless, agenesis of the permanent first molars has been reported in the literature, in isolation from oligo-dontia and anodontia; which refutes the proposed hypotheses. Consequently, it is the purpose of this article to review the literature on missing permanent first molars and illustrate two cases of missing maxillary permanent first molars to emphasize the complexity involved in the diagnosis of the same.

Literature review

The permanent first molars are considered the most important teeth in the dentition; this importance is mainly as a consequence of its position in the dental arch. They perform the major portion of work in mastication and communition of food and are the largest and strongest teeth, both because of their bulk and anchorage. The classification of malocclu-sion is also traditionally based on the relationship of the permanent first molars when in occlusion. Consequently, their absence can adversely affect normal growth and development, disrupt function and phonetics, alter occlusion and cause a serious orthodontic problem13).

The origin and development of the permanent first molars differs from the other permanent teeth in that they do not have preceding primary teeth and are the only permanent teeth whose enamel organ arises directly from the dental lamina in the same way as those of the primary teeth14). The data on the onset of calcification of the permanent first molars show little consistency varying from the sixth fetal month to sixth postnatal month15). They emerge into the oral cavity distal to the primary second molars at an average age of 5.5 to 7 years16) and the postnatal growth changes in the permanent first molars conform to the vertical growth of the jaws.

Agenesis of maxillary and mandibular perma-nent first molars has the least frequency of all the tooth types and it is impossible to determine the true prevalence of missing first molars from many published surveys in the English literature, since they are frequently grouped with other tooth types. Some authors17–22) have reported a prevalence range of 0.2 to 1.5% for missing permanent first molars (Table 1), and a recent meta-analysis4) of dental

Table 1 Reported prevalence of permanent first molar agenesis

Author Year Sample size Prevalence Combined maxilla mandible (maxilla and mandible)

Werther and Rothenberg17) 1939 1,000 1.5% 1.5% —

Grahnén18) 1956 1,006 — — 0.2%

Clayton19) 1956 3,557 — — 1.57%

Rose20) 1966 6,000 — — 0.2%

Horowitz21) 1966 1,000 1.14% 1.14% —

Bergström22) 1977 2,736 2.9% 1.1% —

Polder et al.4) 2004 48,274 0.02–0.05% 0–0.02% —

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agenesis of permanent teeth derived from 48,274 persons from 10 different studies, established the prevalence ranges of 0.02 to 0.05% and 0 to 0.02% for the maxillary and mandibular missing permanent first molars respectively. Furthermore, the dental literature contains several case reports23–33) of isolated agenesis of permanent first molars (Table 2).

Case 1

A 15-year-old Chinese girl presented for a routine dental examination. Her medical history was unre-markable and the mother reported a normal preg-nancy. Extra-orally she exhibited a skeletal class III pattern and competent lips. Intra-orally she was in

her late mixed dentition stage with fair oral hygiene and exhibited a class III incisor relationship. In the maxillary arch (Fig. 1), 13 was transposed with 14 and 15, tooth 12 was microdontic and 22 was missing. The morphology of maxillary permanent molars present distal to the second premolars was not similar to that of the permanent first molars. While in the mandibular arch all the permanent teeth from 37 to 47 were clinically evident. A panoramic x-ray photograph (Fig. 2) confirmed these clinical findings and revealed the agenesis of the maxillary permanent first molars (16 and 26) as the permanent molars present distal to the second premolars had conical roots. In addition, the third molar tooth

Table 2 Reports of non-syndromic permanent first molars agenesis in association with agenesis of other permanent teeth

Author Year Gender Age Ethnicity Location Missing teeth maxilla/mandible

McCall and Samuels23) 1943 — 8 yrs — maxilla 16, 12; 22, 26

mandible 36, 31; 41, 45, 46

— 10 yrs — maxilla 16; 26

mandible 36; 46

— 11 yrs — maxilla 16; 26

mandible 36, 31; 41, 46

— 14 yrs — maxilla 16; 25, 26, 28

mandible 36, 35; 45, 46

Turner and Turner24) 1977 F 8 yrs Caucasian maxilla 16, 12; 22, 26

Neal and Lambdin25) 1980 M — — maxilla 26, 27, 28

Järvinen and Väätäjä26) 1981 F 8 yrs — maxilla 16; 26

mandible 35; 45

Järvinen27) 1984 F 9 yrs — maxilla 16, 15; 25, 26

mandible 36, 35 31; 41, 45, 46

Ranta28) 1985 F 39 yrs Caucasian maxilla 18, 17, 16, 15; 25, 26, 27, 28

M 12 yrs Caucasian maxilla 17, 16, 15, 55; 65, 25, 26, 27

M 9 yrs Caucasian maxilla 17, 16, 15, 55; 65, 25, 26, 27

Muir29) 1985 M 7 yrs — maxilla 16

Lapeer30) 1990 F 8 yrs Caucasian maxilla 17, 16, 12; 22, 24, 25, 26, 27

mandible 37, 35, 31; 41, 47

Zarrinnia and Bassiouny31) 2003 F 8 yrs — maxilla 16, 12; 22, 26

Kau et al.32) 2003 F 38 yrs Caucasian maxilla 18, 17, 16, 15; 26, 27, 28

mandible 38, 36; 46, 48

F 11 yrs Caucasian maxilla 18, 17, 16, 15; 26, 27, 28

mandible 38, 37, 36, 35; 46, 48

F 9 yrs Caucasian maxilla 18, 17, 16, 15; 26, 27, 28

mandible 38, 37, 36; 46, 47, 48

Suprabha and Pai33) 2006 F 14 yrs Asian mandible 36

Anthonappa, R.P., Yiu, C.K.Y. and King, N.M.

231AGENESIS OF MAXILLARY PERMANENT FIRST MOLARS: LITERATURE REVIEW AND TWO CASE REPORTS

germs were not evident. The parents were informed of the missing teeth and a multidisciplinary treat-ment plan was formulated. The patient has declined the proposed surgical-orthodontic treatment approach and is under review in our clinic.

Case 2

A 6-year-old Chinese girl presented for her first routine dental examination. Her medical history was unremarkable and the mother reported a normal pregnancy. Her parents were dentate and reported no missing teeth or associated syndromes among their other family members. She was the eldest of the two sisters in the family and her extra-oral examination showed a normal facial appearance. Intra-orally she was in her early mixed dentition stage with fair oral hygiene. The mandibular perma-

nent first molars had erupted and the incisal edges of her mandibular permanent central incisors were clinically evident. A panoramic x-ray photograph (Fig. 3) confirmed these clinical findings and revealed the agenesis of the maxillary permanent first molars (16 and 26), two mandibular and one maxillary right second premolars (15, 35 and 45) and a mandibular left second molar (37). The patient’s 3-year-old sister was also examined clinically and x-ray pho-tographically. She was found to have a full comple-ment of permanent teeth consistent with her age. The parents were informed of the missing teeth and the patient was placed under review to monitor the eruption of her teeth. Subsequent two-year follow-up panoramic x-ray photograph revealed mesial movement of the maxillary permanent second molars (17 and 27) (Fig. 4), when compared to the previous

Fig. 1 Intra-oral clinical picture of the 15-year-old girl (Case 1) demonstrating transposition of 13 with 14, microdontic 12, retained 53, 62 and 63

Mesio-palatal rotation of 17 and 27 occupying the position of 16 and 26.

Fig. 2 Panoramic x-ray photograph of the 15-year-old girl (Case 1) showing agenesis of 16, 26, 22 and all the third molar tooth germs

In addition, 13 is transposed with 14 and 12 is microdontic.

Fig. 3 Panoramic x-ray photograph of the 6-year-old girl (Case 2) showing agenesis of the 16, 26, 15, 35 and 45

Fig. 4 Panoramic x-ray photograph (Case 2) two years later (8- year-old) showing mesial movement of the 17 and 27

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x-ray photograph. Clinically, no carious lesions were evident and the maxillary central incisors had erupted.

Discussion

Most frequently a comprehensive clinical and x-ray photographic examination will aid in the diagnosis of hypodontia. It is often easy to diagnose agenesis of the permanent first molars retrospectively based on the clinical morphology and x-ray photographic features of the permanent second molars as described in Case 1. This approach indicates that a diagnosis for missing permanent first molars can only be made when the second molars erupt into the oral cavity and complete their root formation i.e. approximately 14 to 16 years16).

A diagnosis of missing permanent first molars is often debatable, especially when made prospectively as in Case 2. In the presented case, the maxillary permanent first molars are likely to be congenitally missing because the dental age34) of the tooth germs in the maxillary permanent molar region (Nolla stage 6; Fig. 4) does not coincide with that of the mandibular permanent first molars (Nolla stage 9; Fig. 4). In addition, their position along with the dental age (similar to the right mandibular permanent second molar, Nolla stage 5; Fig. 4) suggest that they are permanent second molars rather than delayed eruption of permanent first molars associated with immature tooth formation. Furthermore, the dental ages of the affected teeth are much closer to the chronological ages, when assuming the affected teeth to be maxillary permanent second molars rather than maxillary permanent first molars.

Ectopic eruption of the permanent first molars has been more frequently reported than any other tooth13). This may occur as a consequence of an abnormal path of eruption, presumably because of an unusual orientation of the tooth germ35). A physical barrier such as supernumerary teeth, odontoma, or odontogenic tumors in the path of eruption; distur-bances in the dental follicle that result in failure of resorption of the overlying bone as in cleidocranial dysplasia36), inadequate arch length and ankylosis have been reported to impede the eruption of the permanent molars14,30). In the absence of the above mentioned physical barriers, either clinically or x-ray photographically, the concept of delayed eruption of the permanent first molars can thus be ruled out in the Case 2.

The hypothesis that the affected teeth might be maxillary permanent second molars associated with congenitally missing permanent first molars in Case 2 may be criticized, if the permanent first molars are defined as the most mesially erupting of the permanent molars. According to this definition, the congenital absence of permanent first molars can be recognized only when all three permanent molars in the same quadrant are congenitally absent. However, in most of the case reports on congenitally missing permanent first molars, this definition has not been applied. Furthermore, the eruption time of permanent first molars into the oral cavity is considered to be the least variable among all the permanent teeth37). Taken together, these findings support the hypothesis that the maxillary molars in Case 2 are almost certainly the permanent second molars associated with congenital absence of the maxillary permanent first molars.

Early diagnosis of hypodontia is essential, as it allows the practitioner to format a multi-disciplinary treatment plan involving all the possible treatment options, thereby reducing the complexity of subse-quent orthodontic treatment. In the absence of the permanent first molars, the maxillary permanent second molars may freely migrate anteriorly into a more favorable position17) rather than tipping, due to the lower resistance of the maxillary trabecular alveolar bone in comparison to that of the mandible. The conical shaped root of the maxillary permanent second molars causes a mesio-lingual rotation as the tooth move in an anterior direction. Therefore, monitoring the migration during the active eruption stage is imperative21) if this rotation is to be controlled.

In conclusion, we propose that the lack of a confirmatory family history supports the hypothesis of mutation as the possible explanation for the dental agenesis in the presented cases. Consequently, we opine that these cases represent good examples of congenital or developmental absence of the maxillary permanent first molars to discuss the problems associated with the diagnosis.

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AGENESIS OF MAXILLARY PERMANENT FIRST MOLARS: LITERATURE REVIEW AND TWO CASE REPORTS