Disorders of development of teeth and craniofacial anomalies

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Introduction

The development of teeth and of the face is regulated by genes, but the genetic programme is very sensitive to disturbances in the environment such as exposure to infection or toxic chemicals, including drugs. The specific genetic abnormalities underlying some developmental disorders are now known, and for several others a strong genetic association has been established, even though the genes have yet to be identified. However, there remain many where the causes appear complex and multifactorial, involving the interaction of genetic and environmental factors.

Disorders of development of teeth

Disorders of development of teeth may be prenatal or postnatal in origin and may be inherited or acquired. Their recognition and evaluation require a thorough knowledge of the normal chronology of the human dentition and of the normal development and structure of the teeth.

Disorders of development of teeth may be due to abnormalities in the differentiation of the dental lamina and the tooth germs, causing anomalies in the number, size, and form of teeth (abnormalities of morphodifferentiation) or to abnormalities in the formation of the dental hard tissues resulting in disturbances in tooth structure (abnormalities of histodifferentiation). Abnormalities of histodifferentiation occur at a later stage in development than abnormalities of morphodifferentiation; in some disorders both stages of differentiation are abnormal.

Disturbances in number of teeth

Hypodontia, anodontia, and associated syndromes

The congenital absence of teeth may be referred to as hypodontia, when one or several teeth are missing, or anodontia when there is a complete absence of one or both dentitions.

Hypodontia is more common in the permanent dentition, occurring in about 2-10 per cent in different populations (excluding absent third molars) compared to the primary dentition where the prevalence is less than 1 per cent. It is more common in females and there are also racial differences. For example, the prevalence of missing mandibular permanent central incisors is much more common in Japanese and Swedish populations than in other groups studied. Hypodontia may be symmetrical when particular teeth or groups of teeth are involved, or haphazard when no pattern is discernible. Although it is very unusual for deciduous teeth to be congenitally absent, it is likely that in such cases the permanent successional tooth will also fail to form. Third molars, permanent maxillary lateral incisors, and mandibular second premolars are the teeth most frequently involved in symmetrical forms and a hereditary trait can sometimes be shown with missing maxillary lateral incisors.

Although the genetic basis of hypodontia is not yet understood, several regulatory genes involved in tooth development have been identified and it is likely that mutations in these result in tooth agenesis .These control or regulatory genes are not unique to tooth development but are the same genes that control the development of the face and of many other tissues and organs in the embryo. Thus, hypodontia may be associated with other craniofacial anomalies and developmental syndromes .

Hypohidrotic ectodermal dysplasia

Severe hypodontia and anodontia are rare and in most cases are associated with other defects, the most frequent being hereditary hypohidrotic ectodermal dysplasia which is characterized by the congenital absence of ectodermal structures. The disorder is rare and is usually inherited as an X-linked recessive trait, although a very rare autosomal recessive form has been described and sporadic cases have been reported. Affected patients have smooth, dry skin with fine, scanty hairs and partial or total absence of sweat glands which leads to hyperthermia. Some have a few teeth present, but these are often retarded in eruption, deformed, and frequently have conical crowns. Female carriers usually show only mild manifestations that may be restricted to minimal hypodontia, such as absent maxillary lateral incisors, but carriers may be detected by a reduced sweat pore count.

Supernumerary teeth (hyperdontia)

These are teeth additional to those of the normal series. They may develop in any tooth-bearing area but occur most frequently in the anterior and molar regions of the maxilla followed by the premolar region of the mandible. Occasionally, they are associated with other defects, such as cleft palate or cleidocranial dysplasia. They may prevent the eruption, or cause malposition or resorption of adjacent teeth, and may develop dentigerous cysts if unerupted. Supernumerary teeth are more common in females, are usually single and occur in about 1-3 per cent of the population in the permanent dentition. They are unusual in the deciduous dentition.


Key points - Hypodontia and Hyperdontia
Hypodontia
· more common in permanent than primary dentition
· may be associated with mutations in developmental control genes
· absence of primary teeth associated with absence of permanent successors
· may be associated with other developmental abnormalities
Severe Hypodontia/Anodontia
· rare
· associated most frequently with hypohidrotic ectodermal dysplasia (HED)
· HED usually X-linked recessive
Hyperdontia (supernumerary teeth)
· more common in maxilla than mandible
· occasionally associated with other developmental defects
· more common in females than males


Supernumerary teeth occurring at certain sites may be referred to by special terms. A mesiodens is a supernumerary tooth developing between the maxillary central incisors and is the most common of all supernumerary teeth. The majority have conical crowns and short roots. A paramolar arises alongside the maxillary molars and is usually buccally placed, and a distomolar develops distal to a third molar. Supernumerary teeth which morphologically resemble those of the normal series are called supplemental teeth but most are reduced in size.

Disturbances in size of teeth

Macrodontia and microdontia

The size of both the teeth and the jaws is influenced by genetic and environmental factors and considerable variation occurs. Studies of twins have shown that for the teeth, at least, genetic factors account for a large part of this variation. The terms 'macrodontia' and 'microdontia' are used to describe teeth which are larger or smaller than normal, respectively, but the limits of normal variation have never been adequately defined. Both macrodontia and microdontia may involve the entire dentition, or only one or two teeth symmetrically distributed in the jaws. Microdontia of the whole dentition may be associated with other defects, for example Down syndrome and congenital heart disease.

Whilst it is convenient to consider abnormalities in the number and the size of teeth separately the anomalies often occur together. For example, hypodontia and microdontia may occur together in several of the conditions listed in. More common examples are seen in patients with one missing permanent maxillary lateral incisor, in which case the contralateral tooth is frequently peg-shaped.

Disturbances in form of teeth

Introduction

Disturbances in tooth form may involve the crown, the root, or both. The most frequent variations of the crowns of teeth affect maxillary permanent lateral incisors, which may be peg-shaped or show an accentuated cingulum - either variation sometimes being associated with an invagination. Premolars or molars with an increased or decreased number of cusps are also frequently seen. Variations in the number, course, form, and size of roots are particularly common.

Dilaceration

The term dilaceration is used to describe a deformity in which the crown of the tooth is displaced from its normal alignment with the root, so that the tooth is severely bent along its long axis. Dilaceration is usually the result of acute mechanical trauma and most frequently involves the maxillary incisors.

Taurodontism

A taurodont tooth (bull-like tooth) is one in which the pulp chamber has a greater apico-occlusal height than in normal teeth, with no constriction at the level of the amelocemental junction. The result is that the chamber extends apically, well beyond the neck of the tooth. The anomaly affects multirooted teeth and is thought to be caused by the failure of Hertwig's sheath to invaginate at the proper horizontal level. It is rare in the primary dentition. Taurodont teeth may occur as incidental findings or be associated with other rare craniofacial or dental anomalies. There is also an association with abnormalities in the number of the sex chromosomes such as in Klinefelter and poly-X syndromes.

Double teeth

Double teeth is a descriptive term used to describe a developmental anomaly where two teeth appear joined together. The degree of union is variable and may involve the crown, the roots, or both. It is very unusual for teeth to be united by enamel only, joining of the dentine and also the pulp chamber being much more frequent.

A variety of other terms have been applied to this anomaly based on its supposed aetiology, such as fusion and gemination. These have been defined as:

Fusion - the union between dentine and/or enamel of two or more separate developing teeth.

Gemination - the partial development of two teeth from a single tooth bud following incomplete division.

However, the aetiology remains unclear although a genetic basis has been suggested. For this reason the general term 'double teeth', which describes the appearance with no implication regarding aetiology, is preferred. The developmental anomaly of double teeth must be distinguished from concrescence which is an acquired condition where teeth are joined by cementum only (see below).

Double teeth are more common in the primary than in the permanent dentition, the prevalence in different series ranging from 0.5-2.5 per cent for the primary and 0.1-0.2 per cent for the permanent dentitions. The incisors (and also the canines in the primary dentition) are most frequently affected and the condition may be bilaterally symmetrical. In the primary dentition the majority of cases involve the anterior mandibular teeth.

Concrescence

Concrescence is an acquired disorder in which the roots of one or more teeth are united by cementum alone after formation of the crowns. This is most frequently seen in the permanent dentition where the roots of teeth develop close together (for example, between maxillary second and third molars) or following hypercementosis associated with chronic inflammation.


Key points - Double teeth
· developmental anomaly
· teeth usually united by dentine (with or without pulp)
· more common in primary than permanent dentition
· anterior teeth mainly involved
Concrescence
· acquired anomaly
· union by cementum alone following hypercementosis

Disturbances in structure of teeth

Disturbances in structure of enamel

Enamel normally develops in two stages. In the first, or secretory, stage, the ameloblasts perform the dual function of matrix production and initial mineralization. Matrix production involves the synthesis and secretion of the matrix proteins, amelogenin, enamelin, ameloblastin, and tuftelin, of which amelogenin accounts for about 90 per cent. Initial mineralization occurs immediately after secretion. In the second stage, the maturation stage, there is withdrawal of protein and water from the enamel accompanied by increase in mineral content before the tooth erupts.

Most classifications of disturbances in enamel formation distinguish between those that affect the secretory stage, resulting in deficient matrix production and thin hypoplastic enamel, and those that affect the maturation stage, resulting in deficient mineral deposition and soft hypomineralized enamel. Although this distinction may at times be hard to sustain as some disturbances affect both matrix formation and mineralization, it remains a useful clinical division.

In enamel hypoplasia the ameloblasts fail to produce a normal volume of matrix but any matrix which is produced generally becomes as fully mineralized as normal enamel. Enamel hypoplasia presents clinically as pits or grooves in the enamel surface, or as a general reduction in the thickness of the whole enamel. The defective enamel has fewer prisms than normal and they may run in abnormal directions. In some cases no prism structure can be seen.

Hypomineralized enamel results from a failure of the ameloblasts to fully calcify the previously formed matrix, and generally such enamel appears clinically as white and opaque. After eruption it may become pigmented buff, orange, or brown and be quickly chipped and worn away. Much of the organic matrix of hypomineralized enamel remains acid-insoluble and is often preserved in sections of decalcified specimens.

Hypoplastic and hypomineralized enamel may result from disturbances affecting a single tooth, a group of teeth, or all of the teeth, and the structure of the enamel formed depends on the severity and duration of the disturbance as well as its nature. Most disturbances of ameloblast function can produce both hypoplasia and hypomineralization, but clinically one type usually predominates in a particular patient.

The classification given in is based on the aetiology of hypoplastic and hypomineralized enamel. It is not exhaustive but includes the common causes.


Key points - Defective amelogenesis
· defective matrix production - enamel hypoplasia
· defective maturation/mineralization - hypomineralized enamel


Localized causes

Local infection or trauma
Enamel hypoplasia or hypomineralization involving a single tooth is most commonly seen in permanent maxillary incisors, or maxillary or mandibular premolars. The usual cause of these abnormalities is infection or trauma related to the deciduous predecessor resulting in damage to the ameloblasts of the permanent successor. Such teeth are often called Turner teeth. Clinically, the defects range from yellowish or brownish pigmentation of the enamel to extensive pitting and irregularity of the surface, the crowns often being smaller than normal. The yellowish colour is sometimes due to the deposition of cementum on the enamel surface.

Enamel opacities
These are white, opaque spots seen in smooth-surface enamel, some of which become brown-stained after eruption . The opacities are common and are seen in as many as one in three children aged 12-14 years. They have a random distribution, and teeth in both the deciduous and the permanent dentition are affected. The maxillary permanent central incisor is most frequently involved. The cause is not known but the opacities are thought to be due to local rather than systemic factors. The prevalence is less in areas with one part per million of fluoride in the drinking water. Histological examination of the enamel shows the opaque spots to be hypomineralized.


Key points - Developmental abnormalities of enamel - aetiology
· local causes
· generalized causes
- environmental/systemic disturbances (chronological hypoplasia)
- genetically determined

Generalized causes

Chronological hypoplasias
Any serious nutritional deficiency or systemic disease occurring during the time of formation of the teeth can lead to enamel hypoplasia or hypomineralization, because ameloblasts are amongst the most sensitive cells in the body in terms of metabolic requirements. Such time-related disturbances are called chronological hypoplasia and most enamel hypoplasias due to environmental causes are of this type. A pitting type of hypoplasia usually results , although ridging and grooving may also be seen, and the disturbance produces a horizontal band of hypoplasia, the distribution of which is related to that enamel which was forming at the time of the disturbance . Thus a disturbance occurring at or soon after birth may affect the incisal edges of the permanent central incisors and the occlusal surfaces of the first permanent molars in addition to the deciduous teeth .

Congenital syphilis
This disease produces characteristic hypoplastic changes in the enamel of permanent incisors and first molars due to infection of the tooth germ by spirochaetes. The mesial and distal surfaces of the incisors taper towards the incisal edges rather than toward the cervical margin, giving a 'screw-driver' appearance, and the incisal edges usually have a central notch (Hutchinson's incisors).These changes are most obvious in the maxillary central incisors. The occlusal surfaces and occlusal thirds of the crowns of the first molars are covered by small globular masses of enamel (Moon's molars or mulberry molars).

Fluoride ions
Ingestion of excess fluoride during the period of tooth formation may result in dental fluorosis, producing hypomineralized or hypoplastic enamel. The development of fluorosis is dependent on the total amount of fluoride ingested from all sources and the duration and timing of exposure. The early maturation phase of enamel formation appears particularly sensitive whereas the secretory phase is the least sensitive. Clinically, dental fluorosis is characterized by faint white flecking of the enamel, white patches or striations, or in more severe cases by yellow or brownish-black discoloration, particularly in teeth most exposed to light. The term 'mottling' is often used to describe the appearances of dental fluorosis . Varying degrees of hypoplasia of the enamel may also seen. The severity of the lesions varies from tooth to tooth and between different areas of an individual tooth, reflecting variations in exposure and in the susceptibility of different phases of enamel formation with time. They are found mainly in the permanent dentition but the deciduous teeth may be involved in severe cases and in areas of endemic fluorosis.


Key points - Chronological hypoplasias; dental fluorosis
Chronological hypoplasias
· time related
· horizontal bands of pitting of enamel
· distribution of bands and tooth involvement reflect the chronology of tooth development
Dental fluorosis
· effects dependent on dose, duration, and timing of exposure
· mottled appearance of the teeth, usually widespread throughout the dentition
· hypomineralization of subsurface enamel; hypoplastic pitting in severe cases


Amelogenesis imperfecta
Amelogenesis imperfecta is a group of hereditary conditions affecting enamel formation. It is usually classified into two major and clinically distinct types depending on whether the abnormality is related to defective matrix production (hypoplastic type) or defective mineralization (hypomineralized/hypomaturation type). Within this broad division many subtypes have been described based on the modes of inheritance and clinical manifestations. It can be inherited as autosomal dominant, autosomal recessive or X-linked forms. Most cases of amelogenesis imperfecta are inherited as autosomal dominant or, less frequently, X-linked traits.

The patterns of inheritance are not related to particular variations in the clinical manifestations (phenotype). However, as the molecular basis for enamel formation is becoming better understood the corresponding genes are being identified. Mutations in these genes have been associated with amelogenesis imperfecta and in the future it may be possible to correlate specific genotypes with particular phenotypes. Amelogenin, the most abundant of the enamel matrix proteins, has been most extensively studied and is coded for by genes on both the X and the Y chromosomes. However, the gene on the X chromosome (AMELX gene) accounts for the great majority of the protein synthesized. The genes for ameloblastin, enamelin, and tuftelin have also been localized and mutations in these are linked to autosomal patterns of inheritance of amelogenesis imperfect.All types of amelogenesis imperfecta affect the deciduous and permanent dentitions and most of the enamel on all of the teeth is involved.

The hypomineralized/hypomaturation type is the most common form. Newly erupted teeth appear normal in size and shape and have enamel of normal thickness. However, the enamel is of a soft chalky consistency and exhibits variable white, opaque to mottled brownish-yellow appearances. Because of the deficient mineralization the enamel has a similar density to dentine on radiographs. It is rapidly lost by abrasion and attrition exposing the dentine. Gross attrition may result in the teeth being worn down to gum level.

In the hypoplastic types of amelogenesis imperfecta the enamel does not reach normal thickness and there is considerable variation in clinical appearances. In some cases, localized areas of hypoplasia are randomly distributed over the surface of the enamel producing generalized roughness and pitting or irregular vertical grooving and wrinkling. In the smooth form, the enamel over the whole of the crown is affected and the teeth have sharp, needle-like cusps. The enamel is very thin but hard and glassy. It lacks a normal prismatic structure and may be laid down in incremental bands parallel to the surface.

In the X-linked form, heterozygous females are less severely affected than males or homozygous females and tend to show alternating irregular vertical bands of normal and defective enamel, reflecting the random inactivation of one or other of the X chromosomes in different groups of ameloblasts (Lyonization effect).


Key points - Amelogenesis imperfecta
· hypomineralized/hypomaturation and hypoplastic types
· autosomal dominant pattern of inheritance the most common for both types
· X-linked forms associated with mutations in the amelogenin (AMELX) gene
· mutations in genes coding for other enamel proteins linked to autosomal patterns of inheritance
Hypomineralized/hypomaturation type
· normal tooth morphology when first erupt
· soft chalky enamel easily lost, exposing dentine
· teeth prone to attrition, sometimes severe
Hypoplastic type
· enamel of normal hardness but of variable thickness
· considerable variation in clinical appearances
· variable pitting/vertical grooving/generalized thinning
· teeth may appear small/show abnormal cuspal morphology

Disturbances in structure of dentine

Dentine is the first-formed dental hard tissue, the cells of the internal enamel epithelium inducing the adjacent mesenchymal cells of the dental papilla to differentiate into odontoblasts. Both the odontoblasts and subodontoblastic cells influence the development of the first-formed or mantle dentine, the subodontoblastic cells forming part of the collagenous matrix which is embedded in a ground substance rich in glycosaminoglycans. As more matrix is formed the odontoblasts migrate centripetally and their processes remain in the matrix which begins to mineralize when it is about 5 um thick. Calcification is initiated by small crystallites (which at first are probably budded from the odontoblasts) and completed by subsequent growth and fusion of discrete globules called calcospherites. Where fusion of calcospherites does not occur, hypomineralized areas of interglobular dentine remain. A layer of uncalcified matrix (predentine) is normally present at the pulpal surface. Peritubular dentine is formed along the internal surfaces of the dentinal tubules throughout life, the tubule diameter being progressively reduced or even obliterated.

Most of the clinically significant disturbances of dentinogenesis have a genetic aetiology, but some environmental or systemic disturbances affecting calcium metabolism or calcification may also produce abnormal dentine. The developmental abnormalities of dentine are listed in .Many are rare.

Dentinogenesis imperfecta (hereditary opalescent dentine)
Three types of dentinogenesis imperfecta are recognized: type 1, which is associated with osteogenesis imperfecta; type II, where only the teeth are affected; and type III, which only occurs in a rare racial isolate in the USA. Type II is the commonest type.

Dentinogenesis imperfecta type II is an autosomal dominant disorder with variable expressivity and is the most common dental genetic disease, involving approximately 1 in 6000 to 1 in 8000 of the population. It has been mapped to the long arm of chromosome 4 (4 q), as have some of the enamel matrix proteins, but the gene involved has not yet been isolated. Both the deciduous and permanent dentitions are affected. On eruption the teeth have a normal contour but an opalescent amber-like appearance . Subsequently, they may have an almost normal colour, following which they become translucent, and finally grey or brownish with bluish reflections from the enamel. Although in most cases the enamel is structurally normal, it is rapidly lost and the teeth then show marked attrition .

Radiological examination shows short, blunt roots with partial or even total obliteration of the pulp chambers and root canals by dentine Histological examination shows that apart from a thin layer of normal tubular mantle dentine (i.e. the dentine immediately adjacent to the enamel or cementum), the dentine contains a reduced number of tubules, many of which are wide and irregular, and areas of atubular dentine may be present. This abnormal dentine partly or totally obliterates the pulp chamber and root canal. Vascular inclusions are often found in the dentine, representing remnants of odontoblasts and pulp tissue.

Analysis of the dentine shows an increased water content and a decreased mineral content when compared with normal dentine. The microhardness of the dentine is low, explaining the rapid attrition of the teeth which occurs following loss of enamel. The latter may be due to the abnormal physical properties of the dentine which render it less able to withstand distortional forces. Caries is unusual in affected teeth, presumably due to the reduced number of invasion pathways in the dentine, with the caries being confined to the superficial layers which are quickly worn away. The pulp cavities in deciduous teeth may not be obliterated, the dentine may remain thin and the pulps may become exposed by attrition (see 'shell-teeth' below).

Dentinogenesis type I is associated with osteogenesis imperfecta and although the two conditions are closely related they are genetically distinct. In many patients with osteogenesis imperfecta the appearances of the teeth in the primary dentition are indistinguishable from those seen in dentinogenesis type II. However, the involvement of the permanent dentition in type I (associated with osteogenesis imperfecta) is very variable and tooth discoloration and attrition do not occur to the same extent.

A similar appearance is seen in dentinogenesis type III which occurs in a particular racial isolate group in southern Maryland, USA (the Brandywine isolate). Genetic studies have shown that type III overlaps with the same region on chromosome 4 as type I, but it is not yet known whether they are genetically distinct or represent variable expression of disease in different groups of patients.


Key points - Dentinogenesis imperfecta
Type I
· associated with osteogenesis imperfecta
· genetically distinct from dentinogenesis type II
· primary dentition more severely affected than permanent
· appearances in the primary dentition as for type II
Type II
· autosomal dominant, affecting teeth only
· primary and permanent dentition affected
· discoloration of teeth, opalescent amber-like appearance
· marked attrition following loss of enamel
· pulp obliteration and stunted roots
· abnormal dentine structure and composition.
Type III
· rare racial isolate in USA
· closely related to type II


Dentinal dysplasia
Two forms of this rare autosomal dominant disease are described. In type I (rootless teeth) the permanent teeth have normal crowns associated with roots composed of dysplastic dentine containing numerous calcified, spherical bodies. The pulp chamber and root canals are largely obliterated and the roots are usually very stunted. The abnormality is due to a defect in Hertwig's root sheath which fragments and is incorporated into the dental papilla where it induces formation of fused globular masses of abnormal dentine. The first sign may be premature exfoliation either spontaneously or with minor trauma.

Type II dentinal dysplasia (coronal dentine dysplasia) may not be a distinct entity. It has been linked to the same area on chromosome 4 as dentinogenesis imperfecta type II. The appearances in the primary dentition are the same but the permanent teeth are of normal colour and root length.

Metabolic disturbances affecting dentinogenesis
In the active phase of rickets the width of the predentine is increased and the recently formed dentine is incompletely calcified. Subsequently, bands and areas of interglobular dentine corresponding to the period of illness are seen in the dentine. Pronounced interglobular dentine is also a feature of vitamin D-resistant rickets (hypophosphataemia), but large pulp chambers and long pulp horns which may extend as clefts to the amelodentinal junction are also seen. The overlying enamel may also be cracked or defective allowing direct access of bacteria to the pulp, resulting in pulpitis and periapical sequelae without carious attack. Increased amounts of interglobular dentine and widening of the predentine may be seen in other environmental disorders affecting mineralization such as hypophosphatasia (see also hypocementosis) and nutritional deficiencies. The effects of drugs vary with the nature of the drug and period of administration. Cytotoxic agents often produce increased prominence of incremental lines coinciding with drug administration.

The teeth in juvenile hypoparathyroidism may be small with hypoplastic enamel. The roots may be stunted and there may be structural abnormalities in the radicular dentine.

Regional odontodysplasia (ghost teeth)
This is an uncommon developmental disorder of unknown aetiology associated with abnormalities of enamel, dentine, pulp, and the dental follicle. Both deciduous and permanent dentitions are affected and the number of teeth and number of quadrants involved varies. The defect occurs most frequently in the anterior part of the maxilla and is usually unilateral.

The teeth are delayed in eruption and generally have a very irregular shape with hypoplastic and irregularly mineralized enamel. The dentine is thinner than normal, hypomineralized, and contains large areas of interglobular dentine. Radiological examination shows reduced radiopacity of the teeth with loss of distinction between the enamel and dentine, described as a 'ghostly' appearance.

Disturbances in structure of cementum

The coronal third of the root is normally covered only by a narrow layer of acellular (primary) cementum, whereas the apical two-thirds and furcation areas are covered by an additional thicker layer of cellular (secondary) cementum. Cellular cementum continues to be formed throughout the life of the tooth and typically shows incremental lines of growth. The thickness of the cementum varies considerably between individuals, but generally increases with age and to compensate for occlusal wear.

Hypercementosis
Hypercementosis may be idiopathic or the result of local or general disorders. It may affect one or several teeth and may be associated with root ankylosis, when cementum is directly continuous with the alveolar bone, or with concrescence . Some causes are given below.

PERIAPICAL INFLAMMATION
Although resorption of cementum may occur close to the centre of the inflammatory focus, apposition of cementum may be stimulated a little further away. This produces a generalized thickening of the cementum or a localized knob-like enlargement.

MECHANICAL STIMULATION
Excessive forces applied to a tooth may produce resorption, but mechanical stimulation below a certain threshold may stimulate apposition of cementum.

FUNCTIONLESS AND UNERUPTED TEETH
Such teeth may show areas of cementum resorption, but excessive apposition of cementum may also occur. In unerupted teeth the cementum may even extend over the surface of the enamel if the reduced enamel epithelium is lost.

PAGET'S DISEASE OF BONE
Hypercementosis is often seen in teeth of patients with Paget's disease, the thickened cementum showing a mosaic appearance analogous to that seen in the bone. The cementum forms irregular masses and ankylosis is common.

Hypocementosis
Hypoplasia and aplasia of cementum are uncommon. In cleidocranial dysplasia there is a lack of cellular cementum following the deposition of acellular cementum. Aplasia of cementum is seen in hypophosphatasia :a recessive autosomal disease, characterized by a reduced serum alkaline phosphatase level and skeletal abnormalities. Premature loss of some or all deciduous and permanent teeth is seen. Dentine formation may also be abnormal.

Craniofacial anomalies

A bewildering array of craniofacial anomalies and associated syndromes have been, and continue to be, identified. The majority have a genetic basis and with the recent advances in molecular genetics the mechanisms underlying some of these conditions are being discovered. Many appear to be associated with abnormalities in the developmental genes, their signalling molecules, receptors, and transcription factors, as discussed previously in relation to hypodontia. Examples include cleidocranial dysplasia in which there is mutation of a master control gene of osteoblast function, and Crouzon syndrome in which there is mutation of a fibroblast growth factor receptor gene. However, others, particularly orofacial clefts (clefts of the lip and/or palate), have a multifactorial aetiology involving the interplay of genetic and environmental factors.

Orofacial clefts are amongst the commonest of all congenital structural birth defects and may occur alone or in combination with over 300 syndromes, although about 70 per cent are non-syndromic in type. The prevalence varies in different parts of the world but ranges from about 1 in 500 to about 1 in 1000 births in most cases. There is a higher incidence of clefts of the lip and palate compared with clefts of the palate alone and there is a family history in about 30% of the former and 15% of the latter.

About 20 possible (or candidate) genes have been suggested from different studies and although some occur more frequently than others, those associated with increased risk have still to be confirmed. In addition, clefts have also been associated with environmental factors, for example smoking, alcohol, and folic acid deficiency, but study of the interplay of these with genetic factors is at an early stage.