Translation from the original in Italian that you can consult here

DENTAL ANOMALIES 

Odontogenesis 

Odontogenesis, i.e. the formation of dental elements, begins very early in embryonic development. In fact, the precursor of the oral cavity, the stomodeum, is sketched in the fourth week of intrauterine life and the formation of the dental lamina begins immediately afterwards. Thus, for us higher invertebrates, it is a fundamental event as it accompanies us throughout our growth within the maternal uterus and then again, with tooth formation, after birth. There are two dental laminae, the one that will give rise to the deciduous teeth and the one that will give rise to the permanent teeth, which have a temporally displaced development. When we speak in an archetypal way, in order to understand how teeth form, we usually refer to the timing of the development of deciduous teeth and then to the first dental lamina that forms in correspondence to what will be the two drafts of the two arches, of the two maxillae (upper and lower, if we use the anatomical terminology). The formation of the dental plate is simply a thickening of the horseshoe-shaped epithelium that, with the passage of time, tends to invaginate within the ecto-mesenchyma that will constitute our alveolar processes, the bones of the jaws, forming a button. The latter goes on to elongate as a true gubernaculum dentis until it forms a cap and then a bell, as is shown in the histological representations you see in this image (it is necessary to talk about it to describe the abnormalities we will see in a moment). The epithelium is important because it is the initiator of this process (there are many embryological studies on this subject, for example those of Irma Thesleff, not asked in the exam; it is useful to know these bibliographical references in case, in the future, you are interested in delving into the subject that engages entire generations of researchers and that, in the course of the second year, is barely mentioned). This induction of the epithelium is fundamental because it is the growth factors of the epithelium, which deepens within the ectomesenchyma, that drive a recruitment and a transformation of the ectomesenchymal cells, so that they go to form what will be the dental papilla. As we follow the development of the epithelium, it becomes a bud, then a cap and finally a bell and simultaneously, with an induction that becomes reciprocal, the underlying mesenchyme, of neuroectodermal origin, also has its evolution and its thickening.

Enamel organ  development

From the epithelium derives the enamel organ, which is the other name for the structure that will later become the bell proper. The enamel organ is not present in the developed individual or, with the eruption of the tooth, is completely obliterated (erased). The enamel organ is fundamental because it directs the formation of the dental crown, perfectly guiding its morphology and, of course, the quality of the enamel. Moreover, it is recalled (in a morphological and, in part, functional sense) histologically by a whole series of tumors, including adamoblastoma of which we will speak at length (it is not sterile to have in mind the developmental processes). The enamel organ consists of an inner adamantine epithelium, which is what will be in contact with the coronal dentin and which then, within the organ, interfaces with an intermediate layer of (probably) stem cells, with the stellate epithelium and which instead, on the outside of the enamel organ, is called the outer adamantine epithelium. So there are at least four types of cells, within the enamel organ, that have a defined role. We'll talk more about that when we look at amelogenesis imperfecta (this is a slight preview). The inner and outer adamantine epithelium, at the time the dental crown is finished, come together and give rise to Hertwig's lamina, which is the one that drives root formation. The two epithelia without the interposition of the stellate epithelium are therefore at the origin of the root formation and therefore of the dental eruption.

Epithelial rests of Malassez

What have always been described as Malassez remnants or Malassez residues, which can be found along the course of the tooth root, are acquiring, in recent years, more and more interest from those researchers studying tooth formation as a key to trying to replicate, in a future with biotechnology, tooth formation. Our knowledge about the growth factors contained and expressed in the epithelium and mesenchyme is due to the interest we have as a scientific community, not only to study the process in a bookish way, but especially to discover the keys to be able to replicate the process and be able to apply biotechnology, in a future, and reconstruct the tooth, in part and/or in block.

Growth factors and genes

That is why growth factors that are expressed by the epithelium and ecto-mesenchyme during all stages of dental germ formation acquire fundamental importance. And here the citation of tark and sharp is for your own interest. Obviously, of all the growth factors and genes that are turned on when teeth form, some are more important than others (there are hundreds). The ones that contribute the most are no more than five and in particular the "pathways" that are most activated are those of bone morphogenetic protein (BMP), fibroblast growth factor (FGF), sonic hedgehog (SHH) and WMT wingless related integration site and finally ectodysplasin A (Eda-Edar receptor) (important in explaining some types of agenesis, missing teeth). The most obvious thing that could happen is that, when that epithelial horseshoe should form, it does not form in the correct length or has interrupted sections. The direct consequence is a lack of teeth (agenesis) or, conversely, if it forms too exuberantly you will have supernumerary teeth (more teeth). (Quick recap) The inner epithelium generates the enamel prisms, the dental papilla generates the pulp and dentin, the dental sac generates the periodontal tissues. Knowing the extracellular matrix and the cellular component of pulp and dentin is fundamental for a dentist because he intervenes, constantly, on these tissues (e.g. filling or devitalization of a tooth). REMEMBER THE KEY POINTS OF THIS TABLE extracted from a study by Michelle Goldberg, namely the components of dentin and pulp.

Dentin/pulp and collagen proteins

Dentin, from a structural point of view, closely resembles bone (mineralized tissue) but differs because it is richer in certain non-collagenous protein components such as so-called "siblings". They are so named because they are very similar proteins structurally and functionally and have a common ancestral precursor, derived by successive duplications along vertebrate phylogenetic development. One of the most important is DSPP. Returning, however, to the specifics of dentin, which is very similar structurally to bone but differs fundamentally in non-collagenous proteins, dentin has another major difference from bone; it is never reworked. It is a post-term mineralized tissue, it is not reworked and it is not continuously absorbed and redeposited. When we talk about odontoblasts, i.e. the cells that formed dentin during embryonic development and also later during dental organogenesis, we must remember that, once they have formed dentin, they no longer form it and therefore the term odontoblast can be misleading, from a lexical point of view, because it refers only to the phase in which it develops dentin. The osteoblast is an always active cell in all of us. The odontoblast becomes a postmitotic cell in the already formed individual and, probably, contributes to some functions of structural maintenance and nociception, even if, indirectly, the perception of pain (nociception) is entrusted to fibers of type a Delta and C that innervate, copiously, the pulp. There is, however, a hydrostatic theory that explains how odontoblasts within the dentin, as a result of structural changes and mechanical stress (decompression), can contribute to the activation of pain perception (nociception). The tooth does nothing but perceive pain, nothing else. When we talk about secondary dentin, we mean dentin that is slowly deposited (very little) by odontoblasts. Tertiary dentin (which you will see well in endodontics), however, is something different because it is deposited by stem clones that are recruited in a damaged area of the tooth and, for this reason, no longer has the true structure of dentin, so clear, and composed of dentinal tubules and reference structures; it looks very much like a crushed bone.

Numerary alterations in dental development 

• Hypodontia/Oligodontia. 

Talking about alterations in the development of teeth affecting the number, we must introduce the concept of hypodontia, also known as hypodontia. The lack of development of one or more dental elements is called hypodontia. We speak of oligodontia (few teeth) when six or more dental elements are missing in the dental formula we want to consider.

• Anodontia/edentulism. 

Anodontia, a very rare pathology, indicates the complete absence of all teeth. Anodontia is undoubtedly due to the lack of development of the dental lamina and therefore there is a serious impairment of the epithelium which does not thicken in the lamina, does not deepen in the germs and does not give rise to the processes we mentioned earlier. This is very difficult for us to see. Anodontia should always be distinguished from the edentulous individual, i.e. an individual who has had teeth but then they are lost. Patients who are no longer at a very young age may be partially or totally edentulous. Some of us want to use the term edentulism only as an absolute, i.e. 'total' edentulism (i.e. no teeth in the mouth) or rehabilitation with classic full dentures, while others understand it in a more partial sense as they speak of 'partial' edentulism.

• Agenesis. 

Agenesis of single teeth is much more frequent (in your working life it will happen several times) and is related, as I mentioned before, to some of the hundreds of genes that regulate our development; a very interesting one is the beta(?) gene that encodes for ectodysplasin A. (Other well studied ones are AXIN2, MSX1, PAX9 and WNT10A; refer to the two tables above. (Other well-studied ones are AXIN2, MSX1, PAX9 and WNT10A; refer to the two tables above). There are many syndromes related to hypodontia. Hypodontia and agenesis are referred to clinically as syndromic or non-syndromic. They are syndromic when they are correlated with other pathological alterations, thus with other more or less serious signs, and they are non-syndromic when they are apparently only as such; that is, nothing else abnormal is correlated. The fact that a dental anomaly is syndromic or non-syndromic is very important because a dental agenesis can be an indicator of even more serious pathologies.Prevalence figures are very variable and, therefore, it does not make much sense for you to know that it varies between 1.6 and 9.6 %. This that I have given you is taken from a fundamental textbook in the United States, which serves to give you an idea, but it is of no interest that the data is so specific, that it changes over time, depending on where you are and what literature you consult. This is because we are all homo sapiens, but we have considerable differences according to the various ethnic groups (Caucasian, black or Mongolian); we have to be very careful. Statistics on jaw divergence vary a lot by ethnicity in orthodontics; to illustrate this, there are some of us more prone to certain anomalies than others; for example, yellows tend to have claw cusps, we whites may be more prone to agenesis than blacks. There is a slightly higher female predominance than male. A very important paradigmatic syndromic case that you can see is the one in which, not only do we have agenesis of many elements and therefore also an overt oligodontia, but also anhydrotic ectodermal dysplasia. We will be faced with individuals who have few teeth in their mouths, usually not of very good quality as far as enamel is concerned, and, typically, a white, diaphanous, very thin and delicate skin with no (or very few) sweat glands. They will also have very sparse, brittle and unmistakable hair; this is the syndrome of anhydrotic ectodermal dysplasia. 

Studying tooth agenesis (which are not serious pathologies when they are not syndromic) from a genetic point of view is important, not only because it is much easier thanks to the enormous progress that has been made over the years in knowing everything we want from a genetic point of view (it is very easy to have access to a person's genome and sequence it at the points that interest me), but also because, when they are syndromic, they can become an indicator for less obvious but certainly more harmful pathological conditions. So, studying all this and understanding what the mutations that lead to agenesis might be, individually, is of particular interest. A (relatively recent) study published in the Journal of Dental Research, our leading journal with the greatest impact in the dental scientific community (it has been publishing valuable articles for over a hundred years), linked the prevalence of msx1 and pax9 agenesis with the location of the missing tooth. Depending on whether the tooth is missing in the anterior or posterior region, one or the other gene may be more involved. But, in fact, some time ago there was a lot of talk about the fact that there might be, in women, particular types of breast cancer connected with dental agenesis. There were a few articles that dealt with this strong correlation, and it caused a lot of concern among young women with agenesis, who were afraid of having breast cancer; it seemed that the pax gene was involved. This was, over time, much mitigated and much disputed so that now it is no longer talked about. It is a sensationalist piece of news that emerges in the scientific community and is more or less quickly debunked.

• Hyperdontia. 

Hyperdontia is when one has more teeth than there should be. So, in the mouth, there are supernumerary teeth. Again, the genes involved are the same as those involved in the failure to form teeth, but they will have mutations that make them hyper-functional instead of hypo-functional. The prevalence of hyperdontia, whatever statistics you look at, is certainly lower than that of hypodontia; it is easier to see people who are genetically missing some teeth than people who have more teeth than the normal dental formula, which I remind you is 32 permanent teeth and 20 deciduous teeth. Again, hyperdontia is much more likely to show up as a single extra tooth than a large number of teeth. Let's look at some cases. In the slide below, at the bottom left, you can see a mesiodens, not in the shape of an incisor but in the shape of a cone between the two upper central incisors. In contrast to hypodontias, hyperdontias are more frequent in males than in females. Depending on the area, supernumeraries are called mesiodens (that tooth that is born on the sagittal-median line between the two central incisors, usually the upper ones) (very rarely), histodens (a tooth or a conoid tooth that is born even more distal than the wisdom tooth, third molar or eighth tooth; it is called histo-molar or even ninth tooth, but it is preferred to speak of ninth tooth when it is a real molar that is added to the wisdom tooth).

If, on the other hand, we are talking about teeth that are next to each other but not in the arch of the dentition, then we will have paradens or paramolars. We can also have paradens near the premolars or even near the canines. Very rare are those close to the incisors. Depending on the shape we speak of conoid or tubercle teeth. Conoid are, for example, those present in the pictures, or the mesiodens we saw earlier. Tuberculated teeth are those that appear to be tubercular in shape and are therefore more cylindrical than true conoids. Molariformes are those that present the shape of a molar, more or less sketched. Of course, the oriform teeth we mentioned last time in the presence of congenital lue or congenital syphilis are not molariform.

Anomalies in tooth shape

• Microdontia / Macrodontia

There are abnormalities in the shape of the teeth, of course. One speaks of microdontia when the teeth are smaller than normal (it is very difficult to establish what is normal, because if you take any morpho-metric treatise it will tell you how much the teeth differ in shape and size between individuals). We speak of microdontia in syndromic cases, for example, when it is also associated with Down's syndrome or pituitary dwarfism. It is easier, in these cases, to speak of microdontia because it is more supportable. There are abnormalities in the shape of the teeth, of course. One speaks of microdontia when the teeth are smaller than normal (it is very difficult to establish what is normal, because if you take any morpho-metric treatise it will tell you how much the teeth differ in shape and size between individuals). We speak of microdontia in syndromic cases, for example, when it is also associated with Down's syndrome or pituitary dwarfism. It is easier, in these cases, to speak of microdontia because it is more supportable. The threshold that is set to define whether a tooth is small or, on the other hand in macrodontia, whether it is larger than the norm is linked to the sensitivity of the individual.The shape and size of teeth are genetically established very clearly. Therefore, always take this as a beacon or norm; it is easier to see microdontia that are syndromic than non-syndromic.

Gemination / Fusion

Gemination 

occurs when a single enlarged tooth is found that, if counted as one, falls within the normal dental formula. In other words, there are teeth that appear to be split, looking like Siamese twins. For example, those two upper central teeth, which appear as one tooth separated into two, are clearly the result of an attempt to generate supernumeraries during dental formation. This attempt was unsuccessful. The germ was twinned, but not completely; just like Siamese twins. The incisors and canines are the most affected and finding twinned elements is more common in the upper jaw. 

Conversely, fusion 

occurs when a single enlarged tooth is found which, when counted as one, reveals the absence of an element in the normal tooth formula. In this case, not one germ was being separated but two germs that were supposed to be individual were fused. It seems that in Asians fusion is more common than in us Caucasians(?). Again, the canines and incisors are more affected but the mandibular is more common than the maxillary.

Concrescence

We have other interesting anomalies such as concrescence. We talk about concrescence when two elements are fused through the root cementum. That is, they have two distinct root dentines but an unbroken, seamless layer of root cement (as a source of two elements for the root part). This is a fairly rare phenomenon which, however, can occur in posterior teeth, especially in the maxilla. Iconographically depicted is a case of a wisdom tooth which, not erupted, had then concreted with the roots of the seventh. This seventh carious tooth (on the slide) is not at all easy to extract because it is completely attached to the other tooth, completely in bone inclusion. Knowing what concrescence is, for us, has a very bookish value; like all notions, they are never necessarily sterile because it could happen, albeit very rarely, that this phenomenon occurs and therefore, having to extract the tooth, you could find yourself in difficulty. Obviously, in these cases you have to proceed with odontectomy; that is, you have to cut out the root, extracting piece by piece until you get rid of everything you have to remove. Of course, it is always wrong to take the approach of someone who pulls harder and harder with pliers and does not ask the question whether there is something wrong when he feels that the force to be applied is too high.

Accessory cusps

Accessory cusps are perhaps the dental anomaly we see most frequently. In particular, the carabelli cusp, which is very easy to see, is an accessory cusp located on the mesio-palatine surface of the permanent upper molar. When the accessory cusp is mesiobuccal on the lower molars, on the other hand, we speak of a protostylid tooth. However, it is very rare. The claw cusp is, on the other hand, an accessory of the lower molars; this too is more frequent in yellows than in whites. It should be taken into account for one fact; there are various types of possible extroversions of this cusp. Accessory cusps are perhaps the dental anomaly we see most frequently. In particular, the carabelli cusp, which is very easy to see, is an accessory cusp located on the mesio-palatine surface of the permanent upper molar. When the accessory cusp is mesiobuccal on the lower molars, on the other hand, we speak of a protostylid tooth. However, it is very rare. The claw cusp is, on the other hand, an accessory of the lower molars; this too is more frequent in yellows than in whites. It should be taken into account for one fact; there are various types of possible extroversions of this cusp.

Dens evaginatus

Tooth eversions known as dens evaginatus can occur. This, in fact, depends on that fine orchestration of the development of the enamel organ (with the enamel nodes) that establishes the precise conformation of the dental crowns. We will, of course, deal with it next time when we talk about amylogenesis imperfecta, but it is important to know how they form and what abnormalities they may include. It is nothing more than an excessive proliferation of certain portions of the layer that makes up the internal adamantine epithelium; hence, true monstrosities are generated as there is a hyperplasia of this component of the enamel organ. Dens evaginatus, as I said in the case of premolars (as in the photo on the right), is different from the tooth you see here; these are chisel incisors. The dens evaginatus is nothing more than the result of an aberrant hyper-proliferation of the inner adamantine layer of the enamel organ. It can be seen clinically by the presence of accessory tubercles, i.e. conformations that should not occur in the normal anatomy of the tooth; therefore on the premolar there can be an extra cusp in the centre made in the shape of a pyramid, instead of having the classic fully articulated premolar. Or you may have, in the incisors more rarely, wider bands of enamel that make up, for example, the chisel tooth. This means that on the mesial, distal, lingual or palatine margins of the incisors you will have an exuberant presence of enamel (as you can see in the picture above). Evaginatus because it refers to an evagination of the tooth; the presence of an extroversion in the enamel structure of the tooth is not necessarily only enamel because, when the tubercle is very pronounced as in the claw tooth, there can be the underlying presence of dentine and pulp.

Dens invaginatus

The dens can also be invaginatus if, instead of having an eversion, there is an introflexion of the surface.  It has been divided into three types by the disease descriptor. A type 1 dens is when the introflexion is rather limited and usually does not exceed half of the height of the dental crown (it is like a little well you find inside). It is type 2 when it exceeds a certain line, i.e. it is higher than the dental crown itself and sinks until it touches the root. It is type 3 when it creates a communication between the deep periodontium and the mouth. Now you will understand that, in all these cases, dens invaginatus is a serious problem because, even in type 1, a well conformation certainly facilitates the onset of caries; self-detection is impossible and voluntary cleansing of the surface is very difficult. Type 2 is even more serious. In the third type there are problems with the support of the tooth because the deep periodontium is involved and periodontal disease is more likely to develop. In the case of the third type, the dens evaginatus subverts the conformation of the tooth and gives rise to a rather monstrous tooth (which is often extracted). In the first two types, however, it is not certain that they cannot be remedied by conservative restoration.

Dens in teeth

You have a real monster when it comes to dens in a tooth, i.e. an invagination so large that, radiographically, it looks like you can see one tooth inside another. Here, too, there has been serious damage during the formation of the dental germ; there has been a fairly radical disruption of the proliferation of the layers of the enamel organ or a trauma to the enamel that may have caused this. Dens invaginatus and dens in tooth are simply due to clinical description but have never had any relevant genetic studies. If dens invaginatus was bipolar, one can obviously have as an explanation the abnormal proliferation, not only of the enamel organ, but also of the dental lamina.  

Ectopic enamel

Ectopic enamel is enamel that forms where it should not be, i.e. in an ectopic rather than orthotopic series. It refers to any kind of presence (it is an anomaly but not a pathology) of enamel that should not be there, typical of the enamel pearl. Spheriform concretions can occur especially in the molars, often near the root furcation of the lower molars. Nothing is done except when it becomes symptomatic or subject to caries or because, at the patient's request, it has to be removed. Other times it is not a true pearl, that is, a kind of extroversion (like a sphere), but it is, simply, enamel that has been laid down more, too much, and which can therefore lead to an alteration in the correct formation of the seal of the marginal periodontium. When the tooth erupts, when the enamel organ obliterates and the tooth is ready to perforate the gum, a junctional epithelium remains (which, in the body, is a more unique than rare structure as our gum, at the end of the sulcus, is very adherent to the tooth thanks to a whole series of transmembrane molecules that recognise the tooth surface).

The junctional epithelium is most admirable (in the Latin sense, i.e. extraordinary) because on the other side it has a normal interface as if it were an epithelium lying on the lamina propria. It is the only case in which the epithelium has two interfaces and therefore acts as a glue, a seal. If you have an irregular enamel conformation, it is more likely that this epithelium is interrupted, thinner or can be interrupted; hence the reason why some authors have said that the formation of ectopic enamel, even in simple root formation, generates a laxity at the level of this periodontal seal, therefore a greater predisposition to the development and onset of periodontal problems.

Taurodontism

It is an enlargement of the pulp chamber of the multi-rooted tooth. We simply have a lowering, more serious according to the different types of taurodontism, of the furcation of the multi-rooted tooth. We have shorter roots and a larger pulp chamber. It looks more like bull teeth and for this reason it has been called "taurodontism". We, as a species, tended to be defined by some anthropologists as cynodonts (with teeth more similar to those of dogs than to those of ruminant mammals). Here too, the consideration of prevalence leaves something to be desired because it depends on ethnic groups and sub-populations.

Hypercementosis

It is, quite simply, the excessive deposition of root cement. You know how the periodontal ligament is made up and that the root cementum is the mineralised tissue that covers the root dentin. When there is an abnormal, excessive proliferation of cementoblasts for various reasons, over time, layer upon layer of cement is added. This enlarges the roots, which take on an irregular, excessive, abnormal shape that does not require any intervention if there are no symptoms (there usually are not). However, one must be aware that these teeth present hypercementosis and that they must be distinguished by a differential diagnosis with other pathologies which can be more aggressive as well as, sometimes, benign tumours (which can affect the tooth element). A very important note is that hypercementosis is strongly associated with Paget's disease which is a bone pathology that we will see in the future. Keep this in mind: hypercementosis, in some cases, can be the telltale sign that a dentist has, by looking at a normal dental overview, to intuit if the patient may have a quite relevant bone pathology such as Paget's disease ("if he were an adult, when asked if he has Paget's, he will answer in the affirmative and there will be no untimely diagnosis by the dentist").

Dilaceration

Dilaceration is nothing more than the presence of a root which is not in axis with the dental crown, so it is a displaced, bent tooth. This is true for many types of causes but they are usually compressive or traumatic ones that have generated a variation in the longest axis of the tooth during development. Thus, cysts or tumours that compressed during the eruptive phase of the permanent tooth could create a dilaceration.