A. ABNORMAL CROWN MORPHOLOGY

Crown malformations may be seen clinically upon visual inspection of the oral cavity.

1. Third Molar Malformations

Maxillary third molars have the most variable crown shape of all permanent teeth followed by mandibular thirds. These anomalies can range in shape from a small peg-shaped crown to a multicusped, malformed version of either the first or second molar.

2. Peg-Shaped Lateral Incisors

The most common anomaly in tooth shape in the anterior region of the permanent dentition is the peg-shaped (or cone-shaped) lateral incisor (Fig. 11-9) [which occur in 1% to 2% of the population].⁴ The tooth is somewhat conical in shape and broadest cervically and tapers toward the incisal to a blunt point. Several studies of identical twins seem to indicate that peg-shaped maxillary lateral incisor teeth may be a varied expression of the same genetic trait as missing maxillary lateral incisors.^14,15 A most unusual occurrence is that of peg-shaped maxillary central incisors (Fig. 11-10). Recall that peg-shaped teeth develop from one lobe, instead of the four lobes, which would be normal for anterior teeth.

3. Gemination or Twinning

Gemination or twinning results from the splitting (or twinning) of a single forming tooth. Since the tooth division is incomplete, the twinned crown appears doubled in width compared to a single tooth (Fig. 11-11) and notched. The single root is not split and has a common pulp canal. If the doubled tooth is counted as two teeth, the dental arch containing the geminated tooth will have an extra tooth beyond the normal number of teeth. Gemination occurs more frequently in the primary dentition than in the permanent dentition and most commonly in the region of the maxillary incisors and canines.³ [Gemination occurs in <1% of the population.] Note in Figure 11-12 that the wide, notched crowns of the anterior maxillary incisors of this Native American resemble teeth that have geminated.

4. Fusion

Fusion is the union of two adjacent tooth germs, always involving the dentin. Upon clinical examination, this condition appears similar to gemination since the two fused teeth have one crown that appears doubled in width. However, unlike gemination, they usually reveal two separate but fused roots (seen in Fig. 11-13) with separate pulp chambers as seen on radiographs. Another way to differentiate fusion from gemination is to count the teeth in the arch. If the fused teeth are counted as two, the total number of teeth will reflect the normal number of teeth in that arch (Fig. 11-14A and B). Like geminated teeth, fused teeth occur more commonly in the anterior portion of the mouth [but in <1% of the population] and more often in the primary dentition than in the permanent dentition. The mandibular incisor area is affected more often than the maxilla.^2,3 Look at the primary dentition in Figure 11-14C and determine what condition you suspect.

It is thought that fusion is caused by pressure or force during development of adjacent roots. Many of the reports of fusion involve a supernumerary tooth joining with an adjacent tooth, such as the fusion of a mandibular third and fourth molar seen in Figure 11-15A–C or the fusion of a maxillary lateral incisor and anterior supernumerary tooth.^16–18

5. Hutchinson Incisors and Mulberry Molars

When an infected mother passes syphilis on to her unborn baby, the child’s teeth in both dentitions may develop with unique shapes. Maxillary and mandibular incisors may be screwdriver shaped, broad cervically, and narrowing incisally, with a notched incisal edge. These teeth are referred to as Hutchinson incisors. Note in Figure 11-16A that the crowns of Hutchinson incisors resemble somewhat the notched crowns of fused incisors seen in Figure 11-14A and B. Also, first molars in these dentitions may have occlusal anatomy made up of multiple tiny tubercles with poorly developed indistinguishable cusps. Because of the berry-like shape on the occlusal surfaces, these molars are called mulberry molars (Fig. 11-16B). Other manifestations of congenital syphilis may include scarring of the skin around the mouth, bone pain, and swelling of the joints.

6. Accessory Cusps, Tubercles, or Ridges

Accessory enamel projections may result from developmental localized hyperplasia (increase in volume of tissue caused by growth of new cells), or crowded conditions prior to eruption may result in fusion of a supernumerary tooth, which can appear similar to an extra cusp (Fig. 11-17A–C). A third lingual cusp may develop on mandibular molars on the lingual surface and is called a tuberculum [too BER ku lum] intermedium (Fig. 11-18). If this extra cusp were located on the distal marginal ridge, it would be called a tuberculum sextum.

A talon cusp (like a “claw of bird of prey”) is a small projection on the lingual surface of maxillary or mandibular anterior permanent teeth (Fig. 11-19A). Frequently, the cusp has a pulp horn so that on a radiograph, it may be mistaken for a supernumerary tooth superimposed over an anterior tooth or a dens in dente (described later in this chapter). Removal of this cusp is often necessary because of its interference in jaw closure in the maximum intercuspal position. Since the pulp horn is present, endodontic treatment is usually required when this cusp is removed.^2,19 The malformed marginal ridge that extends over much of the lingual surface on the anterior tooth in Figure 11-19B resembles a talon cusp.

Recall that mandibular second premolars most often have three cusps (one buccal and two lingual). However, the number of lingual cusps can range from one to three, so occlusal morphology can vary greatly in terms of groove and fossa patterns established by the number of lingual cusps.²⁰

Teeth may also exhibit extra small enamel projections called tubercles (Fig. 11-20), or extra accessory cusps. Finally, an unusual prominent ridge is seen on the facial surface of a maxillary central incisor in Figure 11-21.

7. Variations in Tooth Size

Microdontia (very small, but normally shaped teeth) and macrodontia (very large, but normally shaped teeth) may occur as a single tooth, several teeth, or all teeth in a dentition.²¹ Macrodontia most frequently involves incisors and canines, whereas microdontia affects maxillary lateral incisors and third molars.^11,22,23 Some examples of variation in size of teeth are shown in Figure 11-22A and B. One report described a maxillary canine 39 mm long and a maxillary first molar 31 mm long (compared to average lengths of 26.3 and 20.1 mm, respectively), both removed from a pituitary giant.²¹

8. Shovel-Shaped Maxillary Incisors

Possibly not a true anomaly, shovel-shaped incisors are a frequently occurring trait that reflect biologic differences between races.⁴ The lingual anatomy includes a pronounced cingulum and marginal ridges that resemble the shape of a “shovel” (Fig. 11-23A). These teeth occur most frequently in Asian, Mongoloid, Arctic, and Native American populations. Double shoveling refers to the pronounced lingual marginal ridges as well as prominent ridges on the mesial and distal portions of the labial surface as seen in Figure 11-23B.

B. ABNORMAL ROOT MORPHOLOGY

Root malformations are normally only obvious on radiographs, although close examination of extracted teeth reveals much variation.

1. Enamel Pearls

Enamel pearls are small, round nodules of enamel with a tiny core of dentin. Since they are covered with enamel, they prevent the normal connective tissue attachment, may be felt with a probe, and, consequently, may lead to periodontal problems in this region.

They are found most frequently on the distal of third molars and near the buccal root furcation of molars²⁴ (Fig. 11-24). On a radiograph, enamel pearls appear as small round radiopacities (i.e., areas appearing light or white on the exposed film).

2. Taurodontia

In taurodontia, or so-called bull or prism teeth, the pulp chamber is very long, without a constriction near the cementoenamel junction (Fig. 11-25). This occurs only in permanent teeth [with a frequency of less than 1 in 1000 among American Indians and some Arctic populations].²⁵ Taurodontia is caused by a disorganization of the calcified tissues and possibly occurs in dentitions subjected to heavy use.

3. Dilaceration

Dilaceration [die lass er A shun] is a severe bend or angular distortion of a tooth root (Fig. 11-26).²⁶ This unusual occurrence may be the result of a traumatic injury or of insufficient space for development, as is often the case with third molars (Fig. 11-27). Dilaceration is often observed in teeth with accessory roots. Historically, flexion is another term that has been used to describe a sharp curvature or bend of a tooth root. This condition makes it challenging to extract the tooth without breaking the root.

4. Dens in Dente

Dens in dente [denz in DEN tee] (literally “tooth within a tooth”) is a developmental anomaly resulting from the invagination of the epithelium of the enamel organ before the formation of hard tissue (seen in Fig. 11-28A). Clinically, it appears most often as a deep crevice near the cingulum region of incisors, and on a radiograph, it appears like a tooth forming within a tooth (Fig. 11-28B). Although most commonly found in maxillary lateral incisors, this condition has also been noted in maxillary central incisors and in mandibular incisors. Usually, it appears in the coronal third of the tooth but may extend apically into the root. Often peg-shaped lateral incisors, with failure of mesial and distal lobes to develop, are found to have dens in dente upon radiographic examination. [Their occurrence is from 1% to 5% of the population.²]

5. Concrescence

Concrescence [kon KRES ens] involves the superficial fusion or growing together of only the cementum of two adjacent tooth roots (Fig. 11-29). Unlike fusion, these teeth usually become joined after eruption into the oral cavity due to the close proximity of the roots and excessive cementum deposition.⁶ This anomaly occurs most frequently in the maxillary molar region.

6. Dwarfed Roots

Maxillary teeth often exhibit normal-sized crowns with abnormally dwarfed (short) roots (seen earlier in Fig. 11-22B). The incisal edges of maxillary teeth with dwarfed roots are often displaced lingually (as also occurs on mandibular incisors). This condition is often hereditary; however, isolated or generalized dwarfing of roots may also result from orthodontic movement of the teeth (with braces) when the movement has occurred too rapidly.

7. Hypercementosis

Hypercementosis is the formation of excess cementum around the root of a tooth after the tooth has erupted (Fig. 11-30). It may be caused by trauma, metabolic dysfunction, or periapical inflammation. Excess cementum may actually form a thin layer that connects adjacent roots, similar to the thin tissue that connects the “toes” on the webbed foot of a duck.

8. Extra (Accessory) Roots

Usually occurring in teeth whose roots form after birth, accessory roots are probably caused by trauma, metabolic dysfunction, or pressure. Third molars are the multirooted teeth most likely to exhibit accessory roots (Fig. 11-31A).² Other molars may also develop extra roots, as seen on a mandibular molars in Figure 11-31B and C. The single-rooted teeth most likely to have an extra root are the mandibular canines followed by mandibular premolars. Two roots on a mandibular canine (one facial and one lingual) are found rarely enough to be interesting but frequently enough not to be amazing (Fig. 11-32A). Mandibular first premolars may also exhibit a bifurcated root, one buccal, and one lingual (Fig. 11-32B), a condition less common for these teeth than for mandibular canines. A rare occurrence of two roots on mandibular premolars, one mesial and one distal like on mandibular molars, is evident in the radiographs in Figure 11-32C and on an extracted mandibular first premolar in Figure 11-32D. [A Japanese study of 500 mandibular first premolars found that this type of bifurcation occurred in 1.6% of Japanese teeth. These researchers also found one very rare specimen with three roots, two buccal and one lingual.²⁷]

A very unusual maxillary first premolar with three roots (two buccal and one lingual) similar to the roots of a maxillary molar is seen in Figure 11-33. There have also been a number of reports of bifurcated roots on primary maxillary canines: five discovered from routine radiographic examination and the sixth on a routine dental recall examination (as seen in Fig. 11-34).^21,28–33

C. ANOMALIES IN TOOTH POSITION

1. Unerupted (Impacted) Teeth

Unerupted teeth are embedded teeth that fail to erupt into the oral cavity because of a lack of eruptive force. Impacted teeth, on the other hand, fail to erupt due to mechanical obstruction, often related to the evolutionary decreasing size of modern man’s jaw. The most common teeth to be impacted are maxillary and mandibular third molars (Fig. 11-35A and B) and maxillary canines.^2,4,34 [At least 10% of the population have impacted teeth.]

2. Misplaced Teeth (Ectopic Eruption or Transposition)

Occasionally, the cells that form a tooth (tooth buds) seem to get out of place, causing teeth to emerge in unusual locations. The most common tooth involved is the maxillary canine seen in Figure 11-36A [20 of 25 cases reported],³⁵ followed by the mandibular canine (Fig. 11-36B). Maxillary canines can even be transposed to the central incisor region.^36,37 Other abnormalities in the alignment of teeth within an arch were mentioned in Chapter 9: labioversion is when a tooth is located too far to the labial like tooth #24 in Figure 11-37A, linguoversion is when a tooth is too lingual like teeth #7 and #10 in Figure 11-37A, supraeruption when a tooth is erupted beyond the occlusal plane like tooth #1 in Figure 11-37B, and infraversion when a tooth is shorter than the occlusal plane like retained deciduous tooth K in Figure 11-37C).

3. Tooth Rotation

Slight rotation of a tooth is called torsiversion like tooth # 8 in Figure 11-38A. Complete rotation, where a tooth is rotated on its axis by 180°, is a rare anomaly, most common for the maxillary second premolar (Fig. 11-38B), sometimes the maxillary incisor, first premolar, or mandibular second premolar.³⁸

4. Ankylosis

Ankylosis [ang ki LO sis] may be initiated by an infection or trauma to the periodontal ligament, resulting in the loss of its periodontal ligament space so the tooth root is truly fused to the alveolar process or bone. These teeth erupt into the oral cavity but, after ankylosis, fail to reach occlusion with the opposing arch and appear shorter than adjacent teeth in its arch. Many times, ankylosis of a primary tooth occurs when the permanent successor is missing. Primary mandibular second molars most often fail to continue erupting as the jaw grows. Consequently, the ankylosed tooth will be 2 to 4 mm short of occluding with an opposing tooth (as in Fig. 11-37C).

D. ADDITIONAL TOOTH DEVELOPMENTAL MALFORMATIONS (AND DISCOLORATION)

Other tooth malformations may be related to heredity or injury during formation and therefore may affect many teeth rather than just one or two specific teeth. These conditions are not anomalies, but dental professionals should be able to distinguish them from other anomalies.

There are several terms you need to be familiar with in order to understand this section. First, the suffix “-plasia” refers to formation or development. Dysplasia is a generic term that indicates abnormal development. Dysplasia can result from too little mineral content being incorporated (hypomineralization) or too little calcium (hypocalcification) incorporated into enamel or dentin. Hypoplasia is a form of dysplasia that refers to an incomplete formation of a tissue. Dysplasia of the enamel or dentin can result from a number of factors during tooth formation, such as exposure to excessive amounts of fluoride, exposure to tetracycline antibiotics, congenital syphilis, or injury to the tooth.

1. Enamel Dysplasia

Enamel dysplasia is a term used to describe a disturbance in the enamel-forming cells (ameloblasts) during early enamel formation. Enamel dysplasia may be hereditary (as with amelogenesis imperfecta) or could result from systemic causes during early tooth formation (such as exposure to a high fever, nutritional deficiencies, or an excess amount of fluoride) or local disturbances (such as trauma or periapical infection of adjacent primary teeth). Generally, variations in color (from white to yellow and brown) or variations in morphology (such as pitting or roughened enamel) can result. Several examples of enamel disturbances are presented here.

a. Amelogenesis Imperfecta

Amelogenesis imperfecta [ah mel o JEN e sis im per FEC ta] is a hereditary disorder that affects the enamel formation of both dentitions (Fig. 11-39). (“Amelo-” refers to the ameloblasts or enamel-forming cells, and “genesis” refers to the beginning formation of these cells. The word “imperfecta” means imperfect.) The partial or complete lack of enamel results in rough yellow to brownish crowns that are susceptible to decay. This condition is rare [with an incidence in the United States of 1 in 15,000].²

b. Fluorosis

Fluorosis [floor O sis] is a condition caused during enamel formation by the ingestion of a high concentration of ingested fluoride compounds in drinking water that greatly exceeds the concentration recommended for controlling decay. The amount of fluoride compounds in some naturally occurring mineral water that causes this condition is many times greater than the one part per million that is added to drinking water to effectively reduce the prevalence of decay. These teeth can exhibit a color change from white to yellow/brown spots called mottled enamel, and if severe, the tooth enamel can undergo a morphologic change resulting in the formation of pits within the enamel (pitted enamel) (seen on erupting secondary central incisors in Fig. 11-40). Clinically, all permanent teeth may be involved depending on the length of time that the person was ingesting high levels of fluoride. These teeth are most often resistant to decay.

c. Enamel Damage Due to High Fever

Pitted enamel on permanent teeth may result from a very high fever during early childhood due to diseases such as measles.⁴ Usually, the tooth crowns that are developing at the time of the fever are affected. For example, a high fever at about age 2 years and 3 months can damage the enamel forming at that time on mandibular second premolars and second molars (Fig. 11-41).

d. Focal Hypoplasia (or Hypomaturation)

Focal hypoplasia is an incomplete development of enamel seen as a localized discolored spot or deformed area on a tooth. During enamel formation, this condition may result from trauma, a local infection of an adjacent abscessed primary tooth, or some other interference in enamel matrix maturation, most likely to occur in succedaneous teeth (called a Turner hypoplasia) seen in Figure 11-42. Unlike decalcification (early decay), which can usually be seen in the cervical thirds of teeth or on occlusal surfaces of posterior teeth, this form of hypomaturation generally appears in the middle third of the smooth crown surfaces (facial and lingual surfaces). The underlying enamel may be soft making the area susceptible to decay.

2. Dentin Dysplasia

Dysplasias of dentin occur twice as often as those in enamel [1 in 8000].³⁹ Abnormal development of the dentin includes hereditary and systemic conditions as follows.

a. Dentinogenesis Imperfecta

Dentinogenesis [den ti no JEN e sis] imperfecta is a hereditary disorder that affects the dentin formation of both dentitions. Clinically, all teeth have an unesthetic light blue-gray to yellow, somewhat opalescent appearance (Fig. 11-43A), hence the term hereditary opalescent dentin. On a radiograph, there may be a partial or total absence of pulp chambers and root canals since the pulp chambers and root canals may calcify (Fig. 11-43B). These teeth may be weak because of a lack of support in the dentin, so they may be susceptible to severe attrition. [This condition occurs in only about one in every 8000 persons.]

b. Tetracycline Stain

When the antibiotic tetracycline is taken by a pregnant woman, an infant, or a child during the time of tooth formation and calcification, it can affect developing dentin. The result is a change in tooth color depending on the dose of the drug, to a yellow or gray-brown (Fig. 11-44). The resultant staining may be generalized in the primary dentition but may also affect some permanent teeth, depending on the age at which tetracycline was administered. Since only the teeth that are calcifying during the tetracycline therapy are stained, it is possible to confirm this condition by noting the age when tetracycline was given and comparing this to the teeth that were calcifying at that age. Some persons have erroneously blamed the staining from tetracycline antibiotic therapy during tooth formation on fluoridated community drinking water, which is beneficial for both teeth and general health.

E. CHANGES IN TOOTH SHAPE DUE TO INJURY AFTER TOOTH ERUPTION

Reactions to injury are not really anomalies but are unique changes in tooth morphology associated with a specific cause. It is important to recognize these conditions so that their etiology (causes) can be identified and modified, when possible, to avoid the causative factor(s) that could worsen the condition.

1. Attrition

Attrition is the wearing away of enamel (and eventually dentin) due to the movement of mandibular teeth against maxillary teeth during normal function and is made worse by excessive grinding together of teeth known as bruxism. Two examples of severe attrition are shown in Figure 11-45A and B. Stress greatly increases bruxism. Attrition should be distinguished from other forms of tooth damage such as abrasion and erosion since the cause of each condition, and therefore the therapy to prevent further damage, is quite different. (Recall from the discussion on bruxism in Chapter 9 that normal tooth-to-tooth contacts per day in a healthy person without occlusal problems may be as little as 7 to 8 minutes per day during mastication of food with a force that is normally less than 33 lb. Imagine, on the other hand, the potential damage to teeth (as well as muscles and the TMJ) if a bruxer bites together for 5 hours per night at pressures exceeding 190 lb!)

2. Abrasion

The wearing away of tooth structure by mechanical means is called abrasion. A common example of abrasion (sometimes called toothbrush abrasion) results in the loss of enamel near the cementoenamel junction of the facial surfaces of crowns, especially on premolars and canines, due to improper tooth brushing techniques (Fig. 11-46). It can be caused by using a hard bristle toothbrush, a horizontal brushing stroke, and/or a gritty dentifrice. Another contributing factor to the loss of tooth structure near the cementoenamel junction is known as abfraction [ab FRAC shun], which is the bending (flexure) of the tooth caused by heavy occlusal forces. This condition is thought to result in loss of tooth structure due to separation of enamel rods near the CEJ.

Occlusal abrasion results from chewing or biting hard foods or objects or from chewing tobacco and results in flattened cusps on all posterior teeth and worn incisal edges (appearing similar to attrition). An unusual type of abrasion, caused by the use for many years of a toothpick between the maxillary central incisors, has been reported by Melfi (Dr. Rudy Melfi, personal communication circa 1983). The same type of proximal abrasion has been reported from the use of a straight pin for the same purpose over many years.

3. Erosion

Erosion is the loss of tooth structure from chemical (not mechanical) means and affects smooth and occlusal surfaces. Erosion can be the result of excessive intake or use of citric acid (like in lemons) and carbonated beverages or the result of regurgitated stomach acids (seen in bulimic individuals who habitually induce vomiting, as in the “binge and purge” syndrome).² Erosion can also occur from an unknown cause (idiopathic). Severe erosion of the lingual enamel of all maxillary anterior teeth is evident in Figure 11-47A. Careful inspection of the tooth damage evident in the figure reveals that at least one pulp horn has been exposed on the maxillary left lateral incisor. Erosion caused by sucking on lemons between the buccal mucosa and adjacent posterior teeth is seen in Figure 11-47B.

F. UNUSUAL DENTITIONS

Careful examination of the casts of the dentition of a 23-year-old man reveals that the mandibular left first molar (#19) closely resembles a maxillary first molar, complete with what appears to be an oblique ridge and a cusp of Carabelli (Fig. 11-48). On closer examination, the first and second mandibular premolars and first, second, and third mandibular molars on both sides also appear remarkably similar morphologically to maxillary posterior teeth. The mandibular six anterior teeth unquestionably belonged to the mandibular dentition. The occlusion of the young man’s teeth was remarkably good considering the fact that maxillary posterior teeth were occluding against practically identical maxillary teeth on both sides!

Another most unusual dentition of a foreign exchange student from Africa is seen in Figure 11-49. This maxillary dentition has a total of 24 erupted or partially erupted teeth. There appear to be 4 incisors, 1 canine, 6 premolars, and 13 molars (5 of which somewhat resemble mandibular molars).

REVIEW Questions

Circle the correct answer(s).

1. What condition may result when a forming succedaneous tooth is located next to an abscess on an adjacent primary tooth?
   1. Turner hypoplasia
   2. Fluorosis
   3. Tetracycline staining
   4. Dentinogenesis imperfecta
   5. Amelogenesis imperfecta
2. An adult has only three maxillary incisor crowns, but one of the crowns is doubled in width and notched. What do you suspect?
   1. Fusion
   2. Twinning
   3. Gemination
   4. Concrescence
   5. Cementosis
3. Which condition may be caused by habitually sucking on lemons (which are quite acidic)?
   1. Attrition
   2. Erosion
   3. Abrasion
   4. Amelogenesis imperfecta
   5. Hypoplasia
4. Which three of the following locations are most likely to have supernumerary teeth form?
   1. Mandibular premolar area
   2. Maxillary premolar area
   3. Maxillary incisor area
   4. Mandibular incisor area
   5. Third molar area
5. Which one of the following teeth that are normally single rooted are most likely to have a bifurcated root?
   1. Maxillary central incisors
   2. Maxillary lateral incisors
   3. Mandibular canines
   4. Mandibular first premolars
   5. Mandibular second premolars
6. Which two of the following are most likely to exhibit unusually formed crown morphology?
   1. Maxillary central incisors
   2. Maxillary lateral incisors
   3. Mandibular canines
   4. Maxillary third molars
   5. Maxillary first molars

ANSWERS: 1–a; 2–a; 3–b; 4–a, c, e; 5–c; 6–b, d

  CRITICAL Thinking

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