4.5 Genetic and Developmental Dental Anomalies
Key Takeaways
- Amelogenesis imperfecta is genetic, generalised and affects both dentitions with normal dentine, while molar incisor hypomineralisation is acquired and affects specific first permanent molars and incisors with sharply demarcated opacities.
- Dentinogenesis imperfecta produces opalescent bulbous crowns with obliterated pulp chambers and short blunted roots, and type I is associated with osteogenesis imperfecta.
- Cleidocranial dysplasia results from RUNX2 mutation and produces multiple unerupted supernumerary teeth with absent or hypoplastic clavicles.
- Gardner syndrome combines osteomas and supernumerary teeth with colonic polyposis of very high malignant potential.
- Turner's tooth is localised enamel hypoplasia of a permanent tooth caused by infection of or trauma to its primary predecessor.
A Named Blueprint Topic
"Genetics/embryology relevant to orofacial disease" is a Paper A blueprint topic. Embryology is covered earlier in this chapter; this section covers the genetic and developmental anomalies that result when odontogenesis is disturbed.
Anomalies of Number
| Anomaly | Definition | Associations |
|---|---|---|
| Hypodontia | Developmental absence of one to five teeth excluding third molars | Commonest missing teeth are third molars, then mandibular second premolars and maxillary lateral incisors; familial pattern common |
| Oligodontia | Absence of six or more teeth excluding third molars | Often syndromic |
| Anodontia | Complete absence | Rare; strongly associated with hypohidrotic ectodermal dysplasia |
| Supernumerary teeth | Additional teeth beyond the normal complement | Mesiodens is the commonest; conical, tuberculate, supplemental and odontome types |
Hypohidrotic (anhidrotic) ectodermal dysplasia is X-linked in its classic form and produces sparse fine hair, absent or reduced sweating with heat intolerance, and severe hypodontia or anodontia with conical teeth. Cleidocranial dysplasia, caused by RUNX2 mutation, produces multiple unerupted supernumerary teeth, delayed eruption, absent or hypoplastic clavicles and a patent anterior fontanelle. Gardner syndrome combines multiple osteomas and supernumerary teeth with colonic polyposis that has a very high malignant potential, so the dental findings carry major systemic significance.
Anomalies of Structure
| Condition | Inheritance | Key features |
|---|---|---|
| Amelogenesis imperfecta | Autosomal dominant, recessive or X-linked | Affects both dentitions; hypoplastic, hypomaturation or hypocalcified types; enamel thin, pitted or soft but normal dentine, so radiographic contrast between enamel and dentine is reduced or lost |
| Dentinogenesis imperfecta | Autosomal dominant | Opalescent brown-blue crowns, bulbous crowns with cervical constriction, obliterated pulp chambers, short blunted roots, enamel shears off leaving rapid attrition; type I associated with osteogenesis imperfecta |
| Dentine dysplasia | Autosomal dominant | Type I has normal crowns with rootless teeth and crescent-shaped pulpal remnants; type II resembles dentinogenesis imperfecta in the primary dentition only |
| Molar incisor hypomineralisation (MIH) | Not inherited; multifactorial systemic insult in early childhood | Demarcated opacities on one to four first permanent molars, often with incisors; post-eruptive breakdown, sensitivity and difficulty achieving anaesthesia |
The distinction between amelogenesis imperfecta and molar incisor hypomineralisation is a standard examination discriminator: amelogenesis imperfecta is genetic, generalised and affects both dentitions, whereas MIH is acquired, affects specific first permanent molars and incisors, and shows sharply demarcated rather than diffuse boundaries.
Fluorosis is distinguished from both by its diffuse, symmetrical, chronologically distributed white flecking or mottling corresponding to the period of excess fluoride exposure during amelogenesis.
Turner's tooth is localised enamel hypoplasia of a single permanent tooth caused by periapical infection of, or trauma to, its primary predecessor — the reason an avulsed primary incisor is never replanted.
Anomalies of Shape and Size
- Dens invaginatus (dens in dente) — infolding of the enamel organ, most often in the maxillary lateral incisor, creating an enamel-lined channel that permits early pulpal necrosis in an otherwise caries-free tooth.
- Dens evaginatus — a tubercle containing a pulp horn, classically on premolars, which fractures on eruption and causes necrosis.
- Talon cusp — an accessory cusp on the palatal surface of an incisor.
- Taurodontism — an enlarged apically displaced pulp chamber with short roots, associated with amelogenesis imperfecta and with Klinefelter syndrome.
- Fusion, gemination and concrescence — fusion joins two tooth germs including dentine; gemination is incomplete division of a single germ; concrescence is union by cementum only.
- Macrodontia and microdontia — the peg-shaped maxillary lateral incisor is the commonest example of localised microdontia and is often familial, occurring in the same families as hypodontia of that tooth.
Anomalies of Eruption
Primary failure of eruption is a genetic condition affecting posterior teeth that do not respond to orthodontic traction. Ankylosis of a primary molar produces an infraoccluded tooth. Ectopic eruption of the maxillary canine affects roughly 1% to 2% of the population and may resorb the lateral incisor root, which is why palpation of the canine bulge from about age 9 to 10 is a standard part of the paediatric examination.
Exam link. A stem describing opalescent brown teeth in both dentitions with bulbous crowns and obliterated pulp chambers in a child with a history of multiple fractures is dentinogenesis imperfecta type I associated with osteogenesis imperfecta.
Inheritance Patterns Worth Memorising
Paper A expects candidates to recognise the mode of inheritance of the common dental genetic conditions because it drives the family history questions in an SBA stem. Amelogenesis imperfecta is genetically heterogeneous, with autosomal dominant, autosomal recessive and X-linked forms; the X-linked form, caused by mutations in AMELX, produces vertical banding in heterozygous females through lyonisation. Dentinogenesis imperfecta types II and III and dentine dysplasia arise from DSPP mutations and are autosomal dominant with high penetrance, so an affected parent is usually evident. Osteogenesis imperfecta with associated dentinogenesis imperfecta (historically type I dentinogenesis imperfecta) reflects type I collagen mutations and is accompanied by blue sclerae, bone fragility and sometimes deafness. Hypohidrotic ectodermal dysplasia is classically X-linked recessive, so males are severely affected with marked hypodontia or anodontia, conical teeth, sparse hair and reduced sweating, while female carriers show milder hypodontia.
Distinguishing the Structural Anomalies Radiographically
Radiographic appearance separates the conditions when the clinical picture overlaps. In dentinogenesis imperfecta the crowns are bulbous with marked cervical constriction, the roots are short and slender, and the pulp chambers and canals obliterate progressively — the tooth eventually shows no visible pulp space. In dentine dysplasia type I the roots are extremely short or absent with crescent-shaped pulpal remnants and frequent periapical radiolucencies in non-carious teeth. In regional odontodysplasia ("ghost teeth") the affected teeth in one quadrant show such thin enamel and dentine that they appear faint and poorly defined against the surrounding bone. In hypophosphatasia, premature exfoliation of primary incisors with the root intact, caused by defective cementum, is the alerting dental sign of a systemic metabolic disease.
Management Priorities in Practice
The examinable management themes are consistent: early diagnosis and referral for shared paediatric and restorative care, aggressive prevention because these teeth are at high caries and wear risk, full-coverage protection of posterior teeth to preserve vertical dimension, and genetic counselling for the family. Extraction of severely affected teeth in a growing child is avoided where possible because alveolar bone volume depends on retaining teeth, and definitive prosthodontic treatment is deferred until growth is complete.
A 9-year-old presents with sharply demarcated creamy-brown opacities on three first permanent molars and two upper central incisors. The primary dentition was entirely normal. The molars are sensitive and difficult to anaesthetise, and one shows post-eruptive enamel breakdown. What is the most likely diagnosis?