In dentistry, hypodontia is the condition at which the patient has missing teeth as a result of the failure of those teeth to develop (also called tooth agenesis). Hypodontia describes a situation where the patient is missing up to five permanent teeth, excluding the 3rd molars. Missing third molars occur in 9–30% of studied populations. In primary dentition the maxilla is more affected, with the condition usually involving the maxillary lateral incisor.

The condition of missing over five (six or more) permanent teeth, excluding 3rd molars or wisdom teeth, has been called oligodontia. The condition for missing all teeth, either primary and/or permanent), is called anodontia. A similar condition is hyperdontia, in which there are more than the usual number of teeth, more commonly called supernumerary teeth.

Many other terms to describe a reduction in number of teeth appear in the literature: aplasia of teeth, congenitally missing teeth, absence of teeth, agenesis of teeth and lack of teeth.

Clinical appearance is variable with presentation ranging from gray to yellowish brown, but the characteristic features is the translucent or opalescent hue to the teeth.

In Type I, primary teeth are more severely affected compared to the permanent dentition which has more varied features, commonly involving lower incisors & canines. Primary teeth have a more obvious appearance as it has a thinner layer of enamel overlying dentine, hence the color of dentine is more noticeable.

In Type II, both the dentitions are equally affected.

Enamel is usually lost early because it is further inclined to attrition due to loss of scalloping at the dentoenamel junction (DEJ). It was suggested that the scalloping is beneficial for the mechanical properties of teeth as it reinforces the anchor between enamel and dentine. However, the teeth are not more susceptible to dental caries than normal ones.

However, certain patients with dentinogenesis imperfecta will suffer from multiple periapical abscesses apparently resulting from pulpal strangulation secondary to pulpal obliteration or from pulp exposure due to extensive coronal wear. They may need apical surgery to save the involved teeth.

These features are also present in dentine dysplasia and hence, the condition may initially be misdiagnosed.

Supernumerary teeth can be classified by shape and by position. The shapes include the following:

- Supplemental (where the tooth has a normal shape for the teeth in that series);

- Tuberculate (also called "barrel shaped");

- Conical (also called "peg shaped");

- Compound odontoma (multiple small tooth-like forms);

- Complex odontoma (a disorganized mass of dental tissue)

When classified by position, a supernumerary tooth may be referred to as a "mesiodens", a "paramolar", or a "distomolar".

The most common supernumerary tooth is a mesiodens, which is a malformed, peg-like tooth that occurs between the maxillary central incisors.

Fourth and fifth molars that form behind the third molars are another kind of supernumerary teeth.

A single tooth is larger than the rest. This is unusual and could be the result of fusion and germination that cause enlarged crowns.

All teeth, although the same size, grow in larger than normal. This is seen in cases of growth hormone excess called pituitary gigantism. It is the rarest of the types.

Type I and II have similar radiographic features

- Total obliteration of the pulp chamber and root canals due to deposition of dentine

- Bulbous crowns with apparent cervical constriction

- Reduced root length with rounded apices

Type III shows thin dentin and extremely enormous pulp chamber. These teeth are usually known as "shell teeth".

Periapical radiolucency may be seen on radiographs but may occur without any apparent clinical pathology.

The presence of a supernumerary tooth, particularly when seen in young children, is associated with a disturbance of the maxillary incisor region. This commonly results in the impaction of the incisors during the mixed dentition stage. The study debating this also considered many other factors such as: the patient’s age, number, morphology, growth orientation and position of the supernumerary tooth. Therefore, the presence of a supernumerary tooth when found must be appropriately approached with the correct treatment plan incorporating the likelihood of incisal crowding.

The lesions that appear in teeth affected with MIH can present as opacities that vary from white to yellow-brown. They are usually asymmetrical in appearance, with a sharp demarcation that distinguishes between normal and affected enamel. The lesions usually do not involve the cervical third of affected teeth.

Taurodontism is a condition found in the molar teeth of humans whereby the body of the tooth and pulp chamber is enlarged vertically at the expense of the roots. As a result, the floor of the pulp and the furcation of the tooth is moved apically down the root. The underlying mechanism

of taurodontism is the failure or late invagination of Hertwig's epithelial root sheath, which is responsible for root formation and shaping causing an apical shift of the root furcation.

The constriction at the amelocemental junction is usually reduced or absent. Taurodontism is most commonly found in permanent dentition although the term is traditionally applied to molar teeth.

In some cases taurodontism seems to follow an autosomal dominant type of inheritance.

Taurodontism is found in association with amelogenesis imperfecta, ectodermal dysplasia and tricho-dento-osseous syndrome.

The term means "bull like" teeth derived from similarity of these teeth to those of ungulate or cud-chewing animals.

According to Shaw these can be classified as hypotaurodont, hypertaurodont and mesotaurodont.

According to Mangion taurodontism may be:

- A (mentally retarded) character

- A primitive pattern

- Mendelian recessive character

- Atavistic feature

- A mutation

It has also been reported in Klinefelter's syndrome, XXYY and Down's syndrome .

The teeth involved are invariably molars, sometimes single and at the other times multiple teeth may be involved. The teeth themselves may look normal and do not have any particular anatomical character on clinical examination.

On a dental radiograph, the involved tooth looks rectangular in shape without apical taper. The pulp chamber is extremely large and the furcations may be only a few millimeters long at times.

Talon Cusp will show physical signs of the irregular dental formation of the teeth and cause other symptoms of the disease that could possibly lead to dental problems in the future depending on severity of the deformity. Most commonly, the extra cusp is located on the lingual surface, giving a three-pronged appearance which has been described as an eagle talon. Rarely however the extra cusp may be situated on the facial surface, or there may be extra cusps on both lingual and facial surfaces. There may be a deep groove between the talon cusp and the rest of the tooth. The extra cusp typically contains pulp tissue. When viewing talon cusp from the occlusal, the projection will appear "x-shaped" as well as appears conical and mimicking the shape of an "eagle's talon".

Symptoms of talon cusp include:

- Interference with occlusion or bite

- Irritation of soft tissues and tongue

- Accidental cusp fracture

- Susceptible to dental caries

The phenomenon of gemination arises when two teeth develop from one tooth bud and, as a result, the patient has an extra tooth, in contrast to fusion, where the patient would appear to be missing one tooth.

Fused teeth arise through union of two normally separated tooth germs, and depending upon the stage of development of the teeth at the time of union, it may be either complete or incomplete. On some occasions, two independent pulp chambers and root canals can be seen. However, fusion can also be the union of a normal tooth bud to a supernumerary tooth germ. In these cases, the number of teeth is also normal and differentiation from gemination may be very difficult, if not impossible. In geminated teeth, division is usually incomplete and results in a large tooth crown that has a single root and a single canal. It is an asymptomatic condition.

The prevalence of germination or fusion is 2.5% in primary dentition, and 0.1 - 0.2% in permanent dentition. It is more frequently observed in primary than permanent dentition; anterior than posterior teeth; unilaterally than bilaterally. It commonly occurs in the primary upper incisors.

The distribution of disease in those affected with MIH can vary greatly. It can be common for the enamel of one molar to be affected while the enamel of the contralateral molar is clinically unaffected, or with minor defects only.

In persons of European ancestry, the most common missing teeth are the wisdom teeth (25–35%), the permanent upper lateral incisors (2%), the lower second premolars (3%), or the upper second premolar, with a higher prevalence in females than in males. The prevalence of missing primary teeth is found at 0.1–0.9%, with a 1:1 male to female ratio. Excluding the third molars, missing permanent dentition accounts for 3.5–6.5%. Similar trends of missing teeth can be seen in approximately 3–10% of orthodontic patients.

30-50% of people with missing primary teeth will have missing permanent teeth, as well.

Talon Cusp is a rare dental anomaly. Generally a person with this develops "cusp-like" projections located on the inside surface of the affected tooth. Talon cusp is an extra cusp on an anterior tooth. Other names for talon cusp is eagle's talon, dens evaginatus, interstitial cusp, tuberculated premolar, evaginated odontoma and supernumerary cusp. Although talon cusp may not appear serious (and in some people may be completely benign), it can cause clinical, diagnostic, functional problems and alters the aesthetic appeal.

The term refers to the same condition as dens evaginatus, but the talon cusp is the manifestation of dens evaginatus on anterior teeth. Talon cusp can simply be defined as hyperplasia of the cingulum of an anterior tooth.Talon cusp was first described by W.H. Mitchell in 1982 and named by J. Kimball Mellor B.S., D.D.S. and Louis W. Ripa, D.D.S., M.S. due to its similar appearance to an eagle's talon. Some sources define a talon cusp as an extra cusp which extends at least half the distance between the cementoenamel junction and the incisal edge of the tooth. Other sources classify all enlarged cingula as talon cusps and classify them according to the degree of enlargement.

The incidence has been found to range from 1% to 6% of the population. Talon cusp tends to occur on permanent teeth only. They are vary rare in (deciduous) baby teeth. In most cases the involved teeth are the permanent maxillary lateral incisors (55%), followed by maxillary central incisors (33%), mandibular incisors (6%), and maxillary canines (4%).

Amelogenesis imperfecta (AI) is a congenital disorder that presents with a rare abnormal formation of the enamel or external layer of the crown of teeth, unrelated to any systemic or generalized conditions. Enamel is composed mostly of mineral, that is formed and regulated by the proteins in it. Amelogenesis imperfecta is due to the malfunction of the proteins in the enamel (ameloblastin, enamelin, tuftelin and amelogenin) as a result of abnormal enamel formation via amelogenesis.

People afflicted with amelogenesis imperfecta have teeth with abnormal color: yellow, brown or grey; this disorder can afflict any number of teeth of both dentitions. The teeth have a lower risk for dental cavities and are hypersensitive to temperature changes as well as rapid attrition, excessive calculus deposition, and gingival hyperplasia.

Tooth gemination is a dental phenomenon that appears to be two teeth developed from one. There is one main crown with a cleft in it that, within the incisal third of the crown, looks like two teeth, though it is not two teeth. The number of the teeth in the arch will be normal.

AI can be classified according to their clinical appearances:

- Type 1 - Hypoplastic

Enamel of abnormal thickness due to malfunction in enamel matrix formation. Enamel is very thin but hard & translucent, and may have random pits & grooves. Condition is of autosomal dominant, autosomal recessive, or x-linked pattern. Enamel differs in appearance from dentine radiographically as normal functional enamel.

- Type 2 - Hypomaturation

Enamel has sound thickness, with a pitted appearance. It is less hard compared to normal enamel, and are prone to rapid wear, although not as intense as Type 3 AI. Condition is of autosomal dominant, autosomal recessive, or x-linked pattern. Enamel appears to be comparable to dentine in its radiodensity on radiograpshs.

- Type 3 - Hypocalcified

Enamel defect due to malfunction of enamel calcification, therefore enamel is of normal thickness but is extremely brittle, with an opaque/chalky presentation. Teeth are prone to staining and rapid wear, exposing dentine. Condition is of autosomal dominant and autosomal recessive pattern. Enamel appears less radioopaque compared to dentine on radiographs.

- Type 4: Hypomature hypoplastic enamel with taurodontism

Enamel has a variation in appearance, with mixed features from Type 1 and Type 2 AI. All Type 4 AI has taurodontism in common. Condition is of autosomal dominant pattern.

Other common features may include an anterior open bite, taurodontism, sensitivity of teeth.

Differential diagnosis would include dental fluorosis, molar-incisor hypomineralization, chronological disorders of tooth development.

An anterior crossbite in a child with baby teeth or mixed dentition may happen due to either dental misalignment or skeletal misalignment. Dental causes may be due to displacement of one or two teeth, where skeletal causes involve either mandibular hyperplasia, maxillary hypoplasia or combination of both.

Dental open bite occurs in patients where the anterior teeth fail to touch. However, this is not accompanied by the skeletal tendency of having an open bite. Thus this type of open bite may happen in patients who have horizontal or hypodivergent growth pattern. These patients have normal jaw growth and do not have the long face syndrome. The anterior open bite in these patients may be caused by Macroglossia, Tongue thrusting habit or digit sucking habits. Some of the characteristics of a dental open bite include:

- Normal lower anterior facial height

- Horizontal/Hypodivergent growth pattern

- Occlusal plane diverges after the premolar contact

- Under-eruption of the anterior incisors

- Over-eruption of the posterior incisors

- Proclined upper and lower incisors

- No vertical maxillary excess or gummy smile

- Presence of habits such as thumb sucking, tongue thrusting

- Spacing between anterior incisors due to their proclination

The phenomenon of tooth fusion arises through union of two normally separated tooth germs, and depending upon the stage of development of the teeth at the time of union, it may be either complete or incomplete. On some occasions, two independent pulp chambers and root canals can be seen. However, fusion can also be the union of a normal tooth bud to a supernumerary tooth germ. In these cases, the number of teeth is fewer if the anomalous tooth is counted as one tooth. In geminated teeth, division is usually incomplete and results in a large tooth crown that has a single root and a single canal. Both gemination and fusion are prevalent in primary dentition, with incisors being more affected.

Tooth gemination, in contrast to fusion, arises when two teeth develop from one tooth bud. When the anomalous tooth appears to be two separate teeth, it appears that the patient has an extra tooth, although they have a normal number of tooth roots.

Unilateral crossbite involves one side of the arch. The most common cause of unilateral crossbite is a narrow maxillary dental arch. This can happen due to habits such as digit sucking, prolonged use of pacifier or upper airway obstruction. Due to the discrepancy between the maxillary and mandibular arch, neuromuscular guidance of the mandible causes mandible to shift towards the side of the crossbite. This is also known as Functional mandibular shift. This shift can become structural if left untreated for a long time during growth, leading to skeletal asymmetries. Unilateral crossbites can present with following features in a child

- Lower midline deviation to the crossbite side

- Class 2 Subdivision relationships

- Temporomandibular disorders

In dentistry, anodontia, also called anodontia vera, is a rare genetic disorder characterized by the congenital absence of all primary or permanent teeth. It is associated with the group of skin and nerve syndromes called the ectodermal dysplasias. Anodontia is usually part of a syndrome and seldom occurs as an isolated entity.

Congenital absence of permanent teeth can present as hypodontia, usually missing 1 or 2 permanent teeth, or oligodontia that is the congenital absence of 6 or more teeth. Congenital absence of all wisdom teeth, or third molars, is relatively common. Anodontia is the congenital absence of teeth and can occur in some or all teeth (partial anodontia or hypodontia), involve two dentitions or only teeth of the permanent dentition (Dorland's 1998). Approximately 1% of the population suffers from oligodontia. Many denominations are attributed to this anomaly: partial anodontia, hypodontia, oligodontia, the congenital absence, anodontia, bilateral aplasia. Anodontia being the term used in controlled vocabulary Medical Subject Headings (MeSH) from MEDLINE which was developed by the United States National Library of Medicine. The congenital absence of at least one permanent tooth is the most common dental anomaly and may contribute to masticator dysfunction, speech impairment, aesthetic problems, and malocclusion (Shapiro and Farrington 1983). Absence of lateral incisors represents a major stereotype. Individuals with this condition are perceived as socially most aggressive compared with people without anodontia (Shaw 1981).

The reported symptoms are very variable, and frequently have been present for many months before the condition is diagnosed. Reported symptoms may include some of the following:

- Sharp pain when biting on a certain tooth, which may get worse if the applied biting force is increased. Sometimes the pain on biting occurs when the food being chewed is soft with harder elements, e.g. seeded bread.

- "Rebound pain" i.e. sharp, fleeting pain occurring when the biting force is released from the tooth, which may occur when eating fibrous foods.

- Pain when grinding the teeth backward and forward and side to side.

- Sharp pain when drinking cold beverages or eating cold foods, lack of pain with heat stimuli.

- Pain when eating or drinking sugary substances.

- Sometimes the pain is well localized, and the individual is able to determine the exact tooth from which the symptoms are originating, but not always.

If the crack propagates into the pulp, irreversible pulpitis, pulpal necrosis and periapical periodontitis may develop, with the respective associated symptoms.

Patient with skeletal open bites that accompany dental open bites may have Adenoid faces or Long face syndrome. They are said to have what is known as "Hyperdivergent Growth Pattern" which includes characteristics such as:

- Increased Lower Anterior Facial Height

- Occlusal plane diverges after the 1st molar contact

- May accompany dental open bite

- Narrow nostrils with upturned nose

- Dolicofacial or Leptoprosopic face pattern

- Constricted maxillary arch

- Bilateral Posterior Crossbite

- High and narrow palatal vault

- Presence of crowding in teeth

- Mentalis muscle strain upon forcibly closing of lips

- Possible gummy smile with increased interlabial gap

Cephalometric analysis features of skeletal open bite may include:

- Increased Frankfurt-Mandibular Plane angle

- Steep Occlusal Plane Angle

- Increased SN-MP Angle

- Short Mandibular ramus

- Increased mandibular body length

- Downward and backward position of mandible

- Increased gonial angle

- Proclined upper incisors, retroclined or upright lower incisors

- Posterior part of maxilla is tipped downwards

- Posterior facial height equals 1/2 of anterior facial height

- Increased hard tissue Lower Anterior Facial Height

- Increased total anterior facial height

- Short mandibular ramus

Viken Sassouni developed Sassouni analysis which indicates that patient's with long face syndrome have 4 of their bony planes (mandibular plane, occlusal plane, palatal plane, SN plane) steep to each other.

Cracked tooth syndrome could be considered a type of dental trauma and also one of the possible causes of dental pain. One definition of cracked tooth syndrome is "a fracture plane of unknown depth and direction passing through tooth structure that, if not already involving, may progress to communicate with the pulp and/or periodontal ligament."