1

Anomalies of Number

1.1 Introduction

The human dentition may consist of fewer or more than the normal number of 20 deciduous or 32 permanent teeth. Fusion of two teeth may also give the impression of hypodontia (Section 1.2).

1.2 Hypodontia

Hypodontia is the term used for dentitions with fewer than the regular number of teeth due to agenesis, i.e. either absence of a tooth germ or failure of a tooth germ to develop. Anodontia is the congenital absence of all the teeth while the absence of many teeth is known as oligodontia or partial anodontia. Isolated hypodontia is the congenital absence of one or a few teeth. Incomplete dentitions not classified as having hypodontia by definition are those where teeth are absent due to failure of eruption, extraction due to caries or orthodontic treatment,²⁹⁵ or where teeth have been lost due to trauma and other reasons.

Hypodontia is the most frequent of all congenital aberrations in humans, and also occurs in animals such as dogs.²⁴⁵ The incidence of agenetic teeth in Caucasian populations seems to have increased during the twentieth century, but the available data are too limited to suggest a trend.³⁸⁴

The aetiology of hypodontia is not entirely clear, but genetic factors are most certainly involved. Because mutated genes associated with agenesis have develop­mental regulatory functions elsewhere in the embryo, associated defects in other tissues and organs are also possible.⁵⁹¹

The skin develops from the ectoderm, one of the three primary germ layers, which is involved in the formation of the teeth. Patients with isolated dental agenesis may show unusual dermatoglyphic patterns of the palms of the hands and soles of the feet, suggesting a shared origin.²⁸ The combination of freckles, thin eyebrows and hypo­dontia⁶⁰⁹ suggests the same. Therefore, isolated dental agenesis might be a minor manifestation of a systemic disorder.

1.2.1 Isolated Dental Agenesis

As mentioned above, isolated dental agenesis is the result of the absence of one or a few tooth germ(s). Missing teeth can lead to diastema (interdental spaces) in the dental arches. Displacement and tilting of the neighbouring teeth may close these spaces.

Epidemiology

Primary Dentition

Less than 1% of children exhibit hypodontia in the deciduous dentition. The teeth most often involved are the maxillary incisors, followed by the mandibular central or lateral incisors,^{112 134 220 372 394 445 469} and also the first molars.¹¹² When the deciduous canines are agenetic, ^{205 608} a syndrome (see Chapter 11) such as cleft lip is usually present.⁴⁶⁴ Around 50% of children with hypodontia are missing one tooth, usually the maxillary lateral incisor; in the rest usually two or more teeth are missing.¹³⁴ Japanese children show agenesis more frequently (5%), which may represent an ethnic trait.^{112 158}

Permanent Dentition

Surveys, often retrospective, of some 160 000 children and adolescents from different countries and populations indicate that 2% to 10% of the permanent dentitions show isolated dental agenesis, third molars excepted.^{1 10 31 47 55 89 94 112 129 131 159 169 170 208 210 219 231 268 279 294 352 362 371 374 388 410 414 421 423 441 469 485 488 508 541 594 595 629 638 648} One study found that 13% of orthodontic patients had hypodontia,⁴⁷⁶ however, this sample is not representative of the population and the finding may be considered as an outlier.

The main body of data has been collected from European and American Caucasian populations, although some studies have included different ethnic and mixed populations. European and Australian children in general lack more teeth than North American Caucasians.⁴⁴⁶ On average two teeth, frequently homologous teeth,³¹ per individual are agenetic.¹⁶⁰

Girls are significantly more susceptible to agenesis than boys,^{1 31 47 169 200 219 279 476 485 488 452 566 664} but not all surveys have found a difference between the sexes. ^{159 374 497 501 574 665} Isolated dental agenesis may, however, occur about 1.4 times more often in girls than boys,^{384 446} and agenesis of several teeth is also more common in girls.^{210 585 595} In one study, the prevalence of hypodontia in Jewish children did not differ by sex, but girls lacked the maxillary lateral incisors more frequently and the boys the mandibular incisors.¹⁷⁰

The overall prevalence of agenesis in the maxilla is comparable with that in the mandible, but there is a marked difference in the pattern of absence of tooth type between the jaws.⁴⁴⁶ The five teeth most prone to agenesis in order of most to least prevalent, are: third molars > mandibular second premolars > maxillary lateral incisors > maxillary second premolars > mandibular lateral incisors.

However, other rank orders of agenesis of teeth, including teeth other than the ones mentioned above, have also been reported. Specific populations and inclusion of oligodont patients may account for the differences.⁴⁴⁶

Third Molars 

Wisdom teeth are most often implicated in isolated dental agenesis, but the prevalence data vary. In 10–35% of adolescent and young adult dentitions, one to four third molars are absent.^{5 33 34 73 129 200 231 234 250 260 255 276 308 412 441 455 511 535 554 594 596} In studies of third molar agenesis, subjects may not be very young since the teeth develop quite late in some individuals,^{61 474} and older people may not recollect whether the teeth were extracted.

Third molar agenesis seems race-related.^{33 132 260 524 554} For instance, 27% of the white population in the USA versus 2% of East Africans have missing wisdom teeth,³³ and more Chinese lack all four third molars than Caucasians.¹³² Women lack wisdom teeth less often than men,⁴⁴⁴ but some researchers did not find a sex difference.^{346 354}

More so than other teeth, the third molars tends to show bilateral absence.^{129 354 554} Almost 10% of subjects also lack other teeth when one or more third molars are agenetic.⁷³

Mandibular Second Premolar 

Of the four frequently agenetic teeth (excluding third molars), 45% or more^{47 466} are second mandibular premolars; they are bilaterally missing in almost half of the population missing these teeth.⁵⁷⁴ The reported percentages for lower second premolar agenesis vary considerably, but may be as low as 3.5%. In some studies, the maxillary lateral incisors were the most frequently agenetic,^{55 170 410 501 580 638} and in one study it was the mandibular lateral incisors.¹³¹ Such variations in findings might be due to ethnic differences.^{444 524} Studies in Caucasians are more likely to show absence of the mandibular second premolar (and maxillary lateral incisor), and Asian studies of the mandibular incisors. A difference between the sexes has not been established.⁵⁷⁴

Maxillary Lateral Incisor 

A quarter of the four most frequently agenetic teeth (excluding third molars) are the maxillary lateral incisors. In some studies these teeth are reported to be missing even more often than the second mandibular premolars. A meta-analysis showed almost equal rates of agenesis of the maxillary lateral incisors and the maxillary second premolars.³⁸⁴ In about 2.2% of Caucasians and Israelis the maxillary lateral incisor is absent.^{170 580} Bilateral agenesis is common,⁴⁴⁶ and the anomaly may be more common in women.⁵⁶⁶

Figure 1.1 shows a congenitally missing right maxillary lateral incisor and an underdeveloped contralateral incisor, which is rarer than bilateral agenesia.⁵⁶⁶ The reduced, often conical, morphology may represent an incomplete expression of agenesis.⁴⁴⁴

Figure 1.1 Agenetic right permanent maxillary lateral incisor; the contralateral tooth is underdeveloped.

Maxillary Second Premolar 

This tooth accounts for some 20% of the four frequently agenetic teeth, excluding the third molars. The variations in figures may be ascribed to small sample sizes, but the large differences in reported prevalences is substantial and remains unexplained. Bilateral agenesis occurs thrice as often as unilateral agenesis.⁴⁴⁴

Mandibular Central Incisor 

In order of frequency of agenesis in the permanent teeth, the mandibular central incisors usually come last, but not in Chinese children in Hong Kong.¹³¹ Occasionally, the lateral incisor has been found to be missing more often than the central. Figures 1.2 and 1.3 show two and four retained deciduous mandibular incisors, respectively, due to absence of their successors.

Figure 1.2 Agenetic permanent central incisors – the deciduous teeth are retained.

Figure 1.3 Four retained deciduous incisors in the mandible; the permanent successors are agenetic.

Other Teeth 

Any other tooth may be agenetic, and this is termed “aplasia of atypical elements”.⁵²⁴ For instance, absence of the maxillary first premolar,¹⁵⁹ second molar^{47 166 279 410} and the mandibular first premolar has been reported.^{47 210} When the first permanent molar does not develop, other teeth tend to be missing too (oligodontia).^{230 289 344} The maxillary central incisors are almost always present,⁵⁴¹ the canines being more often agenetic.⁵²⁴ The condition in which there is presence of just one central incisor is described in Section 1.4.

Aetiology

Agenesis of teeth has been attributed to infectious diseases such as rubella, birth trauma, endocrine disorders, evolution and heredity.^{268 572} For instance, one mother and her three daughters all had congenitally missing mandibular incisors, albeit different ones.⁴¹⁵

Evolution

The relationship between agenesis and evolutionary processes is not clear.

Human teeth are diphyodont (two generation), except the permanent molars, which are monophyodont.¹⁴⁰ The last element of each tooth class (the third molars, second premolars and lateral incisors) is often agenetic or reduced in size. Bolk (1866–1930) therefore proposed the “terminal reduction theory”: that is, during evolution, the distal element in each tooth class tends to disappear. However, of the four archetypal premolars per quadrant, the third and fourth premolars are present in the dentition of modern humans. The third molars would also have disappeared, but the primary fourth molar became the permanent first molar, accounting for the presence of three molars in modern humans.^{524 628} Bolk’s theory was eventually rejected but is summarised in Table 1.1, which also illustrates the current point of view.

Bolk also postulated that the teeth lost through evolution occasionally re-emerge in Homo sapiens. For instance, the lost lateral incisor reappears as an “additional tooth” between the central incisors (see Section 1.3).

Table 1.1 Teeth per quadrant of the archetypal permanent dentition, the teeth remaining in modern humans according to Bolk, and present point of view⁶²⁸

Heredity

Hypodontia is usually a manifestation of an inherited trait (Figure 1.4) although it can occur sporadically, when it represents an acquired anomaly.⁶¹⁹ It is conceivable that certain mutated genes cause hypodontia.⁵⁹¹ Hypodontia follows an autosomal dominant mode of inheritance, but incomplete penetrance suggests interference of suppressor genes with the phenotypic expression.⁹² Research has focused in particular on the maxillary lateral incisors, and, to a lesser degree, the premolars.

• Agenesis of maxillary lateral incisors is an autosomal dominant trait,^{19 219 376} but other modes of inheritance have also been reported.^{19 284 413}
• Agenesis of premolars is an autosomal dominant trait, with a complete penetrance, but varying expression.⁵⁷⁷
• Agenesis in particular affects the second premolars together with the lateral maxillary incisors, called the “incisor-premolar trait”, with a penetrance of 86%.²⁸⁴

Figure 1.4 Mother (A) and daughter (B) with agenetic maxillary permanent lateral incisors.

One study found that mutations in the genes for growth factors, which have a regulatory role during odontogenesis, were not responsible for dental agenesis.²⁵ Mutations in more than one gene and possibly multiple alleles are needed to explain the variations in dental agenesis.^{37 480 488} One proposition is that to become manifest, agenesis must cross a “biological threshold”¹¹⁰ and penetrance would require altered expression of more than one gene. A statistical analysis⁵⁷⁵ in 171 families¹⁸⁵ concluded that hypodontia must be polygenetic.

• The homeobox genes regulate migration of the neural crest cells and tooth morphology. Mutations of the muscle-specific homeobox gene, MSX1, are linked to autosomal dominant agenesis of specific teeth.³⁵⁷ Inactivation of MSX1 has a highly selective effect on the dentition, but other genes must be involved for hypodontia to occur.⁵¹⁶ MSX1 encodes transcription factors expressed in several tissues including the dental mesenchyme. MSX1 mutations might be related to agenesis of premolars and molars.^{357 626} MSX1 is located on chromosome 4; the locus of Notch2 for third molar agenesis has been mapped to chromosome 3 in mice.⁴²² Multiple genes appear to contribute to interfamilial clinical variations in tooth agenesis.⁶¹⁹ Data on hypodontia fitted a polygenic model better than a single major gene model.⁵⁴⁶ Several independent, defective genes acting alone or in combination, and eventually becoming antagonistic, may lead to a specific pattern of phenotypic agenesis.⁶²⁵
• Environmental factors are also implicated. Exposure in childhood to dioxin after an accident in a chemical factory (Seveso, Italy) resulted in increased incidence of hypodontia.⁹ The association between different cleft types and hypodontia in twins was found to have a weak genetic component,³³⁹ possibly because of the small sample size and the presence of environmental factors.
• Confounding factors also exist. Monozygotic twins, born from one fertilised egg, are in principle, genetically identical, but showed differential expression of hypodontia,^{222 315 403} as mirrored agenesis of mandibular second premolars.³⁴⁵ Regardless of tooth group concordance, mono- and dizygotic twins (born from two fertilised eggs) have similar prevalence rates of bilateral agenetic teeth.⁶⁴

Dizygotic twins are no more similar than siblings. In the past, matched tooth anomalies were used as determinants of di- and monozygosity,⁶⁰⁴ but cleavage of the egg can take place in the two-cell stage or later.⁵²¹ Monozygotic twins may have an abnormal number of (parts of) chromosomes.⁵⁶⁰ Differing chromosomal compositions may exist through gene mutation or post-zygotic (partial) loss of a chromosome in one twin DNA (copy errors), while the other maintains the karyotype of the zygote. It has been established that monozygotic twins show mirror-image tooth anomalies but discordant hypodontia^{222 244 312 339 345 403 604} and other dental features.^{41 52 99 603 604} Moreover, lyonisation is a possibility in monozygotic female twins.

When multiple teeth are agenetic, associated deviant skeleto-dental patterns, such as a retruded maxilla,⁴⁵ have been hypothesised to be due to environmental effects. Horizontal and vertical pressure on the dental lamina during growth supposedly suppresses or distorts the tooth buds, likely affecting the last tooth in each tooth class.³⁹⁷ One study concluded that in orthodontic patients, the frequency of maxillary and mandibular third molar agenesis is related to a decreased anterior–posterior dimension of solely the maxilla.²⁹⁹

Relationships Between Teeth

In isolated dental agenesis, the teeth that are present show smaller crowns, a “tendency” to agenesis.^{308 524} A peg maxillary lateral incisor is also associated with non-eruption and a palatally erupted canine.^{439 440}

• Agenesis of the maxillary lateral incisors increases the probability of other teeth being agenetic.³⁴⁹ The same applies to an increasing number of agenetic wisdom teeth.^{5 201 439 511 554 574} Such relationships are not always present,³¹⁹ and the reverse has also been reported.^{491 532}
• When the wisdom teeth are agenetic, the anterior teeth in particular may be relatively small, but such findings are not consistent.^{38 199 497 648}
• Agenesis of the maxillary lateral incisor seems to be compensated by a larger adjacent central incisor. When the lateral incisor is undersized, this compensation is not seen; in fact the adjacent central incisor also tends to be relatively small.⁵⁵⁰
• Agenetic wisdom teeth (in the Inuit) were not found to be associated with a reduced occlusal pattern of the other molars.²⁴⁹
• In at least 50% of patients with a missing deciduous lateral incisor, the successor is agenetic,^{72 220 290 372 469} and if bilaterally absent, more permanent teeth may be agenetic, such as the rarely involved first and second molars.¹³³

A division of mandibular tooth agenesis into three groups has been suggested, based on the radiological evidence of the mandibular canal. In the anterior part of maxillary jaw, a pronounced lack of teeth was found to be associated with absence or marked reduction in the size of the incisive foramen and nasopalatine canal.^{320 532}

Tooth Germ-Related Causes

For dental agenesis, a tooth germ must be affected during its earliest developmental stages. Likely causes are failure of mesenchyme condensation during the initiation of the tooth bud stage, absence of induction of the subsequent ectodermal reactions and an inability of the ameloblasts to produce enamel following reciprocal induction by the odontoblasts. On combining these tissues in vitro, there was development of tooth-like structures.⁵⁴³ In experimental studies, the formation of the dental papilla was induced in the mandibular epithelial lamina through its contact with non-odontogenic mesenchyme.⁴⁰¹

Odontogenesis is initiated in the epithelium and is guided by interactions with the neural crest cells.^{529 593} Regulation of normal tooth development requires proteins produced by a number of genes for the series of reciprocal interactions between the dental epithelium and mesenchymal condensations, which are accumulations of proliferating cells originating from the neural tube. Some of these cells are pluripotent;⁷⁷ their absence is related to non-initiation of the tooth germ.^{390 542}

The specific morphology of the teeth is also influenced by the cell condenstations.³⁹⁰ Butler’s “field theory” states that all tooth primordia are initially equivalent and that the morphology of the teeth is determined by morphogenes in the antero–posterior axis.⁹³ Molecular investigations have identified single genes (such as the homeobox genes) with site-specific antero–posterior activity. A defective specific gene has been found to be associated with agenesis of specific posterior teeth, but not the anterior teeth. The developing maxillary dentition is not continuous: the upper incisors develop in the medial nasal processes of the first arch,¹¹⁴ in the premaxilla.

The hypothesis that tooth agenesis is associated with prenatal brainstem anomalies has not been confirmed: the frequency of agenesis in such patients and the population was similar.³⁵⁸

Early disruption of the developmental processes is most probably a result of lack of (reciprocal) signals at the right time,⁶¹⁸ due to mutations in genes or an inability of the cells to respond appropriately.

Other Causes

Dental development may be interrupted by diseases such as leprosy or the presence of congenital anomalies involving atrophy or disordered development of the anterior part of the maxilla. Figure 1.5 shows absence of a mandibular second premolar and arrested development of the contralateral tooth.

Figure 1.5 (A) Agenetic lower second premolar with retained deciduous second molar. (B) The development of the contralateral premolar is arrested.

Ionising radiation (radiotherapy) can cause agenesis (and morphologic changes)¹⁶¹ in humans^{240 217 387 505} and animals^{84 85} Just one treatment with 15 Gy leads to a temporary interruption in odontogenesis.^{240 565} After chemotherapy, children showed marked tooth agenesis.¹⁶

Segmental odontomaxillary dysplasia is another con­dition with agenesis of one or both premolars in the affected jaw segment. The disorder consists of a unilateral maxillary enlargement from the canine region to the tuberosity, accompanied by gingival hyperplasia. Superiorly the enlargement occurs at the cost of the maxillary sinus. The spaced deciduous molars may be malformed, with splayed roots and pulp stones in enlarged pulp chambers. If the skin on the affected side contains more sweat glands than normal, the anomaly is called hemi-maxillofacial dysplasia. The cause of the disorder is not known.^{42 123 162 433 459 622 637}

Cutaneous abnormalities, such as “hairy nevus” of the skin and hypopigmentation of the lip border, are less frequently observed.^{459 637} The anomalies originate in utero or in early childhood. Less common features are enlarged crowns and roots.^{42 433} The condition seems to remain stable without significant progression.¹²³

Consequences

Consequences of isolated dental agenesis depend on the permanent tooth that is missing.

• When a permanent maxillary lateral incisor is agenetic, the predecessor will exfoliate, because the broad crown of the erupting permanent central incisor initiates and maintains the resorption of the roots of both the deciduous central and lateral incisors (Figure 1.4). The permanent canine frequently erupts partly mesially, and occasionally at the site of the lateral incisor, while the deciduous canines remain in situ.¹³⁸
• If the mandibular central incisors are absent, the predecessors persist and may be functional for a long time.
• When the mandibular premolars are missing, the deciduous second molars are usually retained (Figure 1.5)⁴⁸⁸ as root resorption is not initiated. In the maxilla, the deciduous second molar may exfoliate (due to resorption influenced by the first molar), but not as a rule.

A retained deciduous molar may function for many years, but undergoes wear and physiological changes, such as a reduction in the size of the pulp chamber and hypercementosis (Chapter 8).⁵⁰⁶ Excessive wear or caries may lead to extraction. Another complication is infra-occlusion (Chapter 4), in which the occlusal surface of the retained tooth stays below the level of the occlusal plane, as the adjacent teeth continue to erupt. In 19–20-year-olds, more than half of the retained deciduous second molars showed 0.5–4.5 mm infra-occlusion.⁵³ The mesio-distal width of the retained tooth exceeds that of the missing mandibular second premolar, which causes a slight malocclusion.⁵³ After late extraction, there is tilting and migration of the neighbouring teeth and overeruption of the antagonist tooth, but this is minimal if the occlusion was stable to begin with.

An early diagnosis of premolar agenesis may be incorrect because these teeth may have delayed development. ^{12 117 393 648} In 6-year-olds, it is possible not to detect any trace of second mandibular premolar development¹²⁰ because initiation of calcification of mandibular second premolars may not start before the age of 9 years.¹²⁷

Agenesis of wisdom teeth is often welcome, because a lack of space within the jaws may inhibit their eruption, causing secondary pathology (Chapter 4). A normal molar relationship (Class I) is observed in these cases.¹⁵¹ Permanent teeth may erupt prematurely when the deciduous predecessors are agenetic.⁴⁶⁴ Isolated dental agenesis may lead to malocclusion, but dentofacial deviations are mostly minor.⁶⁵⁶ The alveolar process may be underdeveloped locally. In one study, patients lacking four or more teeth were found to have a smaller cranial base, a shorter maxillary length, a slightly prognathic mandible with anticlockwise rotation, and retroclination of the maxillary incisors.¹⁷²

Prevention and Treatment

Isolated agenesis in the deciduous dentition does not require treatment, but it is prudent to consider taking radiographs of the patient and their siblings, for counselling and timely treatment of agenetic permanent teeth.

Deciduous molars without successors must be extracted²¹⁴ at a young age,³⁹⁵ taking into account the relationships between the maxillary and mandibular teeth. Early extraction allows the permanent first molar to move mesially spontaneously without excessive tilting, filling most of the extraction site. Spontaneous space closure occurs if the deciduous mandibular molar is extracted before the root of the mandibular first molar is completed and before the second molar erupts.³⁵⁹ Deciduous second molar extraction at 10–13 years in dentitions with a normal occlusion was found to result in a diastema, which half closed within 1 year by the tilting and migration of the adjacent premolars and canines, and, in particular, mesial tilting of the first mandibular molars. The closure was enhanced by the displacement of the maxillary first molar due to growth in a downward direction.^{375 431} Two years later, 10% of maxillary and 20% of mandibular diastemata remained. In contrast with the mandible, a maxillary unilateral extraction did not result in a shift of midline.³⁷⁵ The local width of the maxillary and mandibular alveolar arches decreases considerably after extraction.⁴³¹

An unstable situation requires orthodontic closure of a diastema. Care must be taken that the anterior teeth do not tip lingually.⁶³¹ If the contralateral premolar is present, its extraction must be considered. An alternative approach is space maintenance, followed by the placement of an implant or a bridge. Long-term implant survival is around 90%, independent of its location.¹⁶⁷ Timing of implant placement depends on vertical alveolar growth, which continues beyond puberty: an implant-supported prosthesis inserted in adolescence will eventually become infra-occluded. Sometimes it is possible to transplant a tooth (Chapter 7) that must be extracted for orthodontic reasons into the site of an agenetic tooth.³⁴⁰

Extraction of retained deciduous molars in adulthood necessitates insertion of implants or bridges for function and aesthetics. However, deciduous second molars that are lacking successors and are retained into adulthood remain functional for a long time.⁵⁴⁵ In one study, only a few retained second deciduous mandibular molars were lost because of caries or periodontal breakdown at the age of 48 years;⁵⁴⁵ in a minority of cases root resorption of the deciduous molar will be a problem.^{364 666}

• A patient with agenesis of the permanent maxillary lateral incisors requires orthodontic space closure. In contrast with replacement with partial dentures, moving the permanent canines into contact with the central incisors (Figures 1.6 and 1.7) resulted in a healthier periodontium 7–10 years later⁴²⁴ and greater patient satisfaction.⁴⁸² Restorative reshaping of such canines with composite creates a cosmetically acceptable emergent incisor profile on the canine.⁴⁹⁰ If not treated orthodontically, a rather wide diastema will remain. Space maintenance is required to close the diastema with an adequate implant⁵⁴⁶ or bridge after cessation of the (rapid) alveolar growth. The use of osseointegrated implants in children is problematic for reasons of jaw growth, but is presently under consideration.⁶⁴³
• When the permanent central mandibular incisors are absent, extraction of the predecessors is not indicated. An acceptable prosthetic solution after natural loss of the long-persisting deciduous teeth will require maintenance of the diastema.¹⁸⁰

Figure 1.6 Patient with missing maxillary lateral incisors.

Figure 1.7 The canines of the patient in Figure 1.6 have been moved orthodontically into the locations of the agenetic maxillary lateral incisors. The morphology of the canines may be improved by grinding the cusps and building the lateral margins with composite so that the teeth resemble lateral incisors. Note the presence of only three lower incisors.

1.2.2 Oligodontia

The term oligodontia has been defined by various authors as the absence of either four or six and more or eight and more teeth, excluding agenesis of third molars.^{263 468 648 664} Oligodontia has diverse presentations. Some patients lack many posterior teeth and others lack anterior teeth.⁴⁶⁸ The pattern of missing teeth is often bilaterally sym­metrical. The frequency of oligodontia is low: about 0.1–0.5% people have seven to eight missing teeth.^{1 47 219 371 485 541 594 664}

Oligodontia occurs either sporadically in just one individual^{26 559} or it may be inherited without other anomalies.⁵⁸³ It commonly occurs in association with Down’s syndrome (trisomy 21¹¹⁶) and syndromes in which the epithelium or its derivatives are involved, such as ectodermal dysplasia with abnormal hair, nails and sweat glands (Chapter 11). When a larger number of teeth are missing or more “stable” elements, especially the maxillary central incisors,⁴⁶⁸ are absent, the presence of a syndrome should be suspected. Conversely, three or more (inherited) skin conditions require investigating for dental agenesis,^{517 518} but absence of “stable” teeth is not inevitably a sign of a general disorder.⁵⁷²

In cherubism, a hereditary (autosomal recessive?) painless disorder with oligodontia, the middle and lower part of the face become progressively more and more rounded.³³⁵ The cherubic appearance may be enhanced by skin stretching and downward pulling of the lower eyelids, causing an upward directed look.^{153 467 616}

In microsomia I, agenesis and delayed maturation of teeth is related to the severity of the disorder. It affects the face unilaterally, with underdevelopment of the mandibular ramus and adjacent soft tissues, possibly caused by a vascular lesion of the first and second branchial arches.^{113 175}

Hall (1983) mentions 34 syndromes and a number of clefts with isolated dental agenesis or oligodontia,²³⁵ but the number is at least 120.⁶⁶⁰

In one study, some 60% of oligodontia patients had ectodermal dysplasia; the other 40% had intermediate severity eczema and asthma, and some experienced hyposalivation.⁴²³ “Isolated” oligodontia patients had fewer sweat glands than controls.²¹⁸ Oligodontia (Figure 1.8) is, moreover, characterised by reduced tooth size⁴²³ and morphology (conical crowns, short roots), ^{11 317} and underdevelopment of the alveolar processes.^{80 222 317 480} The face tends to resemble that of an edentulous person, with protrusion of the mandible and pouting lower lip, and often a decrease in vertical dimension.

Figure 1.8 Oligodont dentition with hypoplastic and conical teeth.

(Courtesy of Department of Oral Surgery, University of Groningen.)

Heredity

Analysis of family trees suggests an autosomal dominant inheritance pattern in more than half of oligodontia patients.^{187 213 517 572} Different mutations of the PAX9 gene encoding the PAX9 protein appear to be associated with specific oligodontia patterns, but other genes are most probably involved in specific combinations of multiple missing teeth. PAX9 is expressed in the neural-crest-derived mesenchyme of the mandibular and maxillary arches. MSX1 has been found to be associated with agenetic second premolars and third molars, but PAX9, on chromosome 14,¹⁸⁶ was associated with missing first and second molars.³²⁷ A mutation in MSX1 was found to be unlikely in family members who, to various degrees, had missing first, second and third molars, premolars and mandibular incisors.²¹³

A PAX9 nonsense mutation changes a chain-termination codon from one that specifies an amino acid into one that does not,⁶³³ which results in incomplete protein synthesis and oligodontia.⁴¹⁷ A de novo deletion of the proximal long arm at chromosome 14 resulted in developmental defects, but oligodontia was not mentioned in the report.⁵²² A deletion involving the PAX9 locus was observed in a proband with absence of most deciduous and permanent teeth. A frame-shift mutation in PAX9¹²⁶ encodes an abnormal protein or disrupts the synthesis of the protein completely.⁶³³ Identical twins have been found to have a frame-shift mutation and a premature stop codon.¹²⁵ A missense mutation in the PAX9 gene was found in a family with a distinct oligodontia type.³⁴² The mutation changes a codon specific for one amino acid into a codon />

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