VARIATION IN TOOTH MORPHOLOGY

I. Introduction.

Variation of teeth has been an enduring interest to the clinical practitioner and the laboratory scientist. No two teeth are alike. The day-to-day variation of teeth that we see is the norm. It is the odd, peculiar, and strange group of teeth to which we focus our attention here. They are called anomalies.

Some texts describe them as developmental disturbances, recognizing that they are best understood from a developmental viewpoint. They are the more extreme variations from the norm.

Reproduced below is the life cycle of the tooth, taken from the classic text by Schour and Massler used by a generation of dental students.

These few facts about tooth development will assist us in understanding tooth variation:

(1) Teeth form from the cooperative interaction of two germ layers--ectoderm and ectomesenchyme (a neural crest derivative).

(2) Enamel is formed from the ectodermal components.

(3) Dentin, pulp, cementum, the supporting PDL, and bone are all derived from ectomesenchyme.

Some variations, such as the shovel-shaped incisor or the Carabelli trait are curiosities in clinical practice, but are very useful to the dental anthropologist in tracing population movements. This is recognized in the phrase "where armies go, genes flow". Shoveling of the incisor is common amongst the Chinese, Japanese, Mongolians, and Eskimo. The Carabelli trait is most common amongst European peoples.

Other variations, such as agenesis are useful for analysis at the biochemical and molecular level in understanding the fundamental chemistry involved in tooth formation.

Very few heritable dental traits are truly Mendelian (although some 49 traits, many restricted to single families, are recognized). Most dental variations are polygenic, that is controlled by several genes. With these, we often speak of the degree of expression or penetrance.

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II. Disturbances in Size.

-Microdontia means teeth that are smaller than normal. Generalized microdontia is a rare condition associated with uncommon conditions such as pituitary dwarfism. More often, microdontia is limited to just one or two teeth. "Peg-shaped" upper lateral incisors and small third molars are seen regularly by the practicing dentist. Peg-shaped incisors tend to be familial. Occasionally, an upper lateral incisor is missing on one side, peg-shaped on the other side.

-Macrodontia refers to teeth that are larger than normal. Generalized macrodontia is rare, seen infrequently in conditions such as pituitary gigantism. It can be localized involving just one or a few teeth. Macrodontia is much less common than microdontia. You may occasionally see the term 'megadontia' used for this condition.

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III. Disturbances in Number.

-Anodontia is the congenital absense of all teeth. Read that sentence again, so you can contrast anodontia with edentulous, the clinical absence of teeth. (Edentulous means the clinical absence of teeth. Anodontia means the failure of teeth to form at all.) In true (or complete) anodontia, all teeth fail to develop.

-True anodontia is an extremely rare occurrence. When this does occur, it is usually part of a more generalized disorder ectodermal dysplasia, an inherited defect of all ectodermally derived structures.

-Partial anodontia is the failure of one or more teeth to develop. It is much more common than complete anodontia. Third molars, lower second premolars, and upper lateral incisors (in that order) are the most common congenitally absent teeth. (Note this very well: congenital absence of a deciduous tooth is not common. When it does occur, it is most often the upper lateral incisor that is absent. When a deciduous tooth is absent, its permanent successor is usually missing also.)

This is a note on terminology: Terms such as oligodontia or hypodontia are occasionally used in British journals to describe the condition of partial anodontia. These terms don't appear in American dictionaries; therefore, the terms are not always consistent in usage if they appear in journals from our country. You are safe just to know that they mean 'a few teeth are missing.'

-Supernumerary teeth are 'extra' teeth. Most (90%) occur in the maxilla. Their presence in the deciduous dentition is quite rare. Two terms occasionally used to describe this condition of having extra teeth are polydontia or hyperdontia. The most common of all supernumerary teeth is the mesiodens which is a supernumerary that forms in the midline between and lingual to the roots of the maxillary central incisors. Most supernumerary teeth do not erupt; they often are unknown until detected on X-ray films. Fourth molars, themselves a rare occurrence are infrequently called 'paramolars' or 'distomolars.' Supernumerary teeth occur less often than do missing teeth.

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IV. Disturbances in Eruption.

-Premature eruption of all of the teeth, either deciduous or permanent, may suggest an underlying endocrine dysfunction such as hyperthyroidism. A natal tooth seen at birth may be an aberrant 'tooth' nodule, or it may actually be a deciduous incisor that has erupted early. The etiology is unknown.

-Delayed eruption can be due to local or systemic factors. For one or a few teeth, surgical exposure and--if needed--orthodontic guidance can bring the tooth into its proper place. X-ray films are useful in evaluating early or late eruption of teeth.

-Impacted teeth are those that have failed to erupt and remain buried in the alveolar bone. Usually, some barrier to eruption will be seen on the X-ray film. A seldom used term for unerupted teeth is embedded teeth. The third molars and maxillary canine are the most frequently impacted teeth, followed by premolars and supernumerary teeth.

-Ectopic eruption is another type of eruption disturbance; this occurs when a tooth cannot complete its eruption because it is blocked by an adjacent tooth or by a misplaced orthodontic band--usually on a first molar tooth.

-This is an appropriate time to introduce you to another important term. An iatrogenic disease or condition is one that is 'doctor-caused'. If a second molar is inadvertently impacted by a poorly placed molar band as shown in the diagram, it is an iatrogenic condition. These things happen to the best of us. We need to make these occurrences as few as possible, and to be on the lookout for them when they do occur.

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V. Disturbances in Shape (The Odd and the Bizarre)

-Dilaceration is a severe bend in the long axis of the tooth. The bend it located at the junction between the crown and the root. The bend can be as much as 90 degrees. Dilaceration usually results from trauma to the unfinished tooth when development is in progress. Such a tooth in the old literature is sometimes called a 'hawk billed tooth'.

-Flexion is a deviation or bend restricted just to the root portion of the tooth. Usually the bend is less than 90 degrees. It may be a result of trauma to the developing tooth. Carefully contrast and compare the definitions for flexion and dilacertaion.

-Taurodontism literally means 'bull-like teeth'. Taurodont teeth are usually molars. They have an abnormally long pulpal chamber and shortened roots. The pulp chamber has no constriction near the CEJ as do normal teeth. Clinically these teeth appear normal. Taurodontism was relatively common amongst the European Neandertals. It is also found on occasion in people living today. The implication of its appearance in modern people is unknown. No treatment of the condition is necessary apart from due prudence when doing root canal therapy.

-Dens in dente literally means 'tooth within a tooth'. It is an uncommon developmental abnormality that primarily affects maxillary lateral incisors. It is in fact a deeply invaginated lingual pit with an important clinical implication: these are very susceptible to caries and they ought to be restored promptly. Left unattended, these teeth tend to decay very rapidly. Modern literature will often refer this condition as dens invaginatus. This is a term you should know.

-Supernumerary cusps, or as they are often called, extra cusps are occasionally found on teeth. We review here the common types.

(1) The most common one is the Carabelli Cusp which is found on the mesiolingual aspect (on the mesiolingual cusp) of maxillary first molars AND maxillary second deciduous molars.

-This occlusal landmark can appear with varying degrees of intensity either as a cusp or pit. These conditions are known collectively as the Carabelli trait. Clinically it is worth checking out during a dental exam: it is frequently a site of early dental caries. It is a heritable trait most commonly seen in European populations. The Carabelli trait is best understood as a variation, not an anomaly.

(2) A talon cusp is an extra cusp that resembles an eagle's talon (a talon is the claw of a bird of prey). A talon cusp appears as a projection from the cingulum of incisor teeth. These can interfere with occlusion; however, grinding them down is a hazardous endeavor. Talon cusps often contain a prominent pulp horn which is very susceptible to exposure in the younger patient.

(3) Very rarely, a similar projection of enamel can occur on the occlusal surface of the premolar teeth. It forms a tubercle called a 'dens evaginatus'. (Do not confused this with the early condition, dens invaginitus mentioned above.) Dens evaginitus can also contain a pulp horn as does the talon cusp mentioned earlier. Fortunately, these conditions are infrequent.

(4) One other variation of the upper first premolar is the 'Uto-Aztecan' upper premolar. It is a bulge on the buccal cusp that is only found in Native American Indians, with highest frequencies of occurence in Arizona. The name is not a dental term; it comes from a regional linguistic division of Native American Indian language groups.

(5) Peg-sbaped lateral incisors have already been discussed above.

(6) No discussion of 'disturbances in shape' is complete without mention of root variation. Certain teeth exhibit a variation in the number of roots.

Incidentally, most root variation is in the distal third of the root. Sometimes roots can be blunted or shortened. It tends to occur without apparent cause. On occasion, root resorption and root blunting can be unfortunate consequence of orthodontic treatment with fixed appliances. A few patients are very susceptible to this condition, and the practitioner's heart sinks when it is found--without warning--on the post treatment X-ray films. Below is a brief discussion of extra or fused roots.

(a.) Upper first premolars usually have two roots; however, one root is occasionally found in these teeth. More rarely there are three roots.

(b.) Lower permanent canines on occasion have a bifurcation near the apex resulting in two short roots. A question about this condition has appeared frequently on Part I National Boards for dental students.

(c.) Sometimes lower first permanent molars have three roots. It is the mesial molar root that is bifurcated. Clinical articles often consider it an anomaly. It is just a variation. Three rooted lower first molars are found in greater frequency amongst North American Indians with origins in Asia.

(d.) In upper third molars, and on occasion with fused roots, second molars, the roots can be fused together. This is a variation, not a pathological condition.

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Shovel shaped incisors display enhanced marginal ridges and present with a distinctive shovel-shaped appearance on the lingual aspect. They appear frequently in many persons of Asian origin, including many Native American Indians. They are especially prominent in Eskimo/Inuit who are descendants of Siberians about 4,000 years ago.

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This next section might best be called 'joined teeth.'

-Fusion is the union of two teeth by dentin and enamel. The pulp chambers are often shared; however, they can be separate. This condition can be differentiated from gemination (discussed below) by counting the teeth. The diagnosis can be confirmed if there is a reduction in the number of teeth that are present in the dental arch. Can you stand another term? Some authors refer to fusion as 'syndontism'.

-Gemination is the development of two crowns from a single tooth germ. It is an incomplete twinning of the gooth germ. In gemination, there is a shared pulp and root. Unlike fusion, the number of teeth is correct; however, the 'gemination' gemination tooth is unusually wide. Some writers refer to gemination as 'schizodontism'. (Are all of these terms giving you schizophrenia? You might want to take a break before going on.)

-Concrescence is the joining of tooth roots by cementum. It occurs after tooth formation is complete. Usually this condition is 'silent' without clinical significance. When extracting teeth, however, the prudent clinician will carefully examine X-ray films of the patient for this condition. A missed diagnosis can be serious business during an attempted extraction.

-A related occurrence is hypercementosis. It is a consequence excessive cementum deposition. It occurs in older people often when there are greatly increased or decreased occlusal forces.

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VI. Disturbances in Formation

-Enamel pearls (enamelomas) are small nodules of enamel found on the root of the tooth close to or at the cemento-enamel junction. They are found most often at the bifurcation or trifurcation of molars. They do appear radiographically. While they can be a problem in periodontal disease, they should usually be left alone. Their attempted removal can do more harm than good.

-Hutchinson's incisors or mulberry molars are developmental anomalies that result from congenital syphilis. Clinically, the incisor teeth are, on occasion called 'screwdriver teeth' due to their distinctive shape.

-Winged incisors are not a disturbance of development, but rather a special case of rotation of the upper central incisors. It is usually described as a distinctive bilateral rotation to the mesial which is particularly common amongst Native American Indians.

-Enamel hypoplasia can be localized or generalized. Localized enamel hypoplasia is a visible defect that is important in archaeology as an indicator of severe sickness or nutritional deprivation. When the insult during tooth formation is brief in duration, it can be detected histologically in the enamel as a Wilson line. Further information on these features and their significance in archaeology is available at www.uic.edu/classes/osci/osci590

CJ '98