Oral Anatomy

Abstract

The oral cavity consists of an outer vestibule and an inner oral cavity proper. It is bound anteriorly by the lips, laterally by the cheek mucosa, inferiorly by the floor of the mouth, posteriorly by the oropharynx, and superiorly by the hard and soft palate. In addition to being a site for mastication and propulsion of bolus, enzyme release and salivation concurrently occur in order to initiate the digestive process. Additional roles of the oral cavity include tissue mobility and lubrication for speech and swallowing, vital chemosensory functioning, and an alternative respiratory passage.

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The oral cavity consists of an outer vestibule and an inner oral cavity proper. It is bound anteriorly by the lips, laterally by the cheek mucosa, inferiorly by the floor of the mouth, posteriorly by the oropharynx, and superiorly by the hard and soft palate. In addition to being a site for mastication and propulsion of bolus, enzyme release and salivation concurrently occur in order to initiate the digestive process. Additional roles of the oral cavity include tissue mobility and lubrication for speech and swallowing, vital chemosensory functioning, and an alternative respiratory passage.

The oral cavity is lined by a mucous membrane consisting of a stratified squamous epithelium, which may or may not be keratinized. In areas of functional, chemical, and mechanical challenge, the mucous membrane is keratinized and tightly attached to the underlying bone. Conversely, in areas requiring mobility of tissue and elasticity, the mucosa is nonkeratinized and loosely attached to the underlying periosteum.

The mucous membrane is composed of two layers: the surface epithelium and an underlying connective tissue layer, the lamina propria. The submucosa underlying the lamina propria is highly variable. As a result of the variability in the type of epithelium present, as well as the characteristics of the connective tissue, several regions can be distinguished: lining mucosa, masticatory mucosa (mucoperiosteum), specialized mucosa, and a transitional zone (vermillion zone).

The true mucous membrane of the lips and cheeks is characterized by a relatively thin, nonkeratinized epithelium with a thin lamina propria. In these areas, where the lining mucosa covers muscles (lips, cheeks, and ventral surface of the tongue), the submucosa is fixed to the underlying fascia of the muscles. The mucosa here is smooth and very elastic and acts as a safeguard in functional mastication. If it were not elastic it would fold and produce creases that could be traumatized when chewing if it protruded between the teeth.

The mucosa of the soft palate is a transition zone between the mucosa of the lips and cheeks and that of the vestibule and floor of the mouth. The vestibular mucosa and that of the floor of the mouth are loosely attached and allow for the movement of labial and lingual frenum attachments, which are critical for speech, mastication, and swallowing. The mucosa of the vestibule (alveolar mucosa) is quite pink and red in color and very flexible when compared to the attached mucosa over the gingiva.

The attached mucosa is specialized depending on its position within the oral cavity. It is subjected to the forces and friction of mastication (hence the term masticatory mucosa) and thus is thick and keratinized with a dense, firm lamina propria. It is tightly adherent to the teeth and bone due to its dense bands of fibrous connecting tissue that join the mucous membrane directly to the underlying periosteum and bone. The submucous space in the mucosa of the hard palate is subdivided into irregular and intercommunicating compartments of various sizes that are filled with fat in the anterior palate and glands in the posterior palate. The submucous space of the gingiva does not exist, and instead the lamina propria continues into the depth of the tissues and fuses directly with the periosteum of the alveolar process or the cervical region of the teeth.

Dentition

In humans, the teeth are composed of a core, the dental pulp, which is rich in nerves and blood vessels. The bulk of the tooth is formed from dentin, with that portion exposed to the oral cavity called the crown, which is covered with an ectodermal derivative (enamel) (Fig. 3.1). The portion of the tooth embedded in the bony socket is covered by a mesodermal derivative (cementum) that is by its nature almost exactly like bone (Fig. 3.2). The junction of the root and the crown is the cementoenamel junction.

Fig. 3.1

Cross section of a canine tooth

Fig. 3.2

Close-up of dental attachment to the gingiva

Once the tooth is developed and it erupts through the gingival surface, a gingival margin and cuff forms at the junction of the dentoenamel junction. The gingival surface is not attached to the crown of the tooth but rather is attached below the cementoenamel junction by collagenous nonelastic fibers called Sharpey’s fibers of the alveolar crest fibers of the periodontal membrane. It is these Sharpey’s fibers that attach the bone to the cementum, acting as a “shock absorber” and thereby protecting the tooth and the adjacent bone from the constant trauma of chewing. These fibers make up the bulk of the periodontal membrane. The periodontal membrane or periodontium is attached from the junction of the crown and the root and extends all the way down around the apex of the fully developed root as well as around the gingiva at the cementoenamel junction. It is divided into three segments:

• 1. The gingival segment where the free gingival fibers attach to the cementum near the neck of the tooth.

• 2. The transeptal or interdental ligament. These fibers in a strict sense are not contained in the periodontal space but run across the interdental space from one tooth to the next and serve to unite all the teeth of one arch into a functional unit.

• 3. The alveolar ligament, which is horizontal and oblique fibers running from the alveolar bone to the cementum of the root.

If there is any breakdown of the periodontium from inflammation, there will be a destruction or degeneration of the periodontal membrane. Associated with this is the inflammatory destruction of the adjacent alveolar bone. This results in progressive weakening of the tooth in the socket because of its lack of bony support. A relationship between periodontal disease and systemic disease (such as diabetes, preterm, low birth weight, cardiovascular) has been demonstrated.

In a multirooted tooth, the same structures are present and the attachment of the tooth itself is exactly the same as a single-rooted tooth, except that there are more roots, meaning that there are more areas of attachment of that tooth. Also in the periodontal membrane are remnants of the epithelial root sheath of the developing tooth (as mentioned earlier in this chapter) called the epithelial rests of Malassez. They are elliptical segments that can appear as round islands of epithelial cells near the root of a tooth especially at the apex. They are of no consequence except they may proliferate in association with inflammation and have been implicated as forming the epithelial lining of a radicular cyst at the apex of the tooth.

The pulp canal in the roots and the main pulp chamber in the crown, with its arterial, venous, lymphatic, and neural structures, are seen as they pass from the pulp chamber through the apex of the fully formed root (Fig. 3.3). Nerves and vessels are plentiful as they exit through the apex of each tooth and have a wide communication with the blood supply coming from the adjacent bone through the periodontal membrane and the adjacent mucosa. If we consider the presence of a wide plexus, we can easily understand how a tooth’s pulp can be removed and yet the viability of the tooth structures can be maintained. Fig. 3.4 shows the mean lengths of teeth. Cleft lip/palate has been associated with significantly different mean lengths.

Fig. 3.3

Neurovascular supply to a molar tooth

Fig. 3.4

Palatal surface of the maxilla demonstrating 16 teeth

Sensibility and Viability

The term viable tooth tissue refers to a tooth with an intact blood supply either from the pulp or the adjacent tooth tissue. The term sensate refers to the ability of that tooth to emit the sensation of pain when the tooth’s pulp is stimulated with an electric or cold stimulus. The two terms, sensate and viable, are not mutually exclusive. A tooth without a pulp (pulp chamber being removed by endodontics) still will be viable because of the blood supply coming in from the adjacent periodontal membrane, but that same tooth, however, will not emit a painful stimulus from a pulp tester and is therefore viable but nonsensate. The same principal occurs when we cut the maxilla in a Le Fort I osteotomy. All the teeth have lost their nerve supply because of the osteotomy and the transection of the posterior, middle, and anterior superior alveolar nerves. Yet if the blood supply is maintained via the palatal vessels, the teeth will remain viable and be maintained in the alveolar process of the maxilla. If, however, the blood supply is cut and the segment becomes avascular and the viability is destroyed, then the tooth and the adjacent bone will be sloughed from the arch. If the labial or palatal blood supply is maintained, segmental bony structures can be mobilized and moved into various positions.

Description and Nomenclature of the Dentition

If a line is drawn between the central incisors down the medial raphe of the maxilla, it divides the arch into right and left components (Fig. 3.5). Terms such as mesial and distal are used to denote teeth position in the arch and surfaces on the tooth itself. For example, the central incisor is mesial to the lateral incisor, which is mesial to the cuspid. The first bicuspid is distal to the cuspid and the second molar is distal to the bicuspids and first molar. Similarly the central incisor has a mesial surface (towards the midline) and a distal surface (away from the midline). The lateral incisor and all the other teeth also have a mesial and distal surface depending on the surface and its relationship to the midline.

Fig. 3.5

Lengths of the deciduous and permanent teeth

The lingual surface of all the teeth is that surface next to the tongue. The outer surface, however, is different. The anterior six teeth (central through cuspid on each side), because they are next to the lips, have a labial surface. The anterior teeth (bicuspids and molars), because they are next to the cheek, have a buccal surface.

The anterior six teeth have a sharp edge called the incisal surface, whereas the bicuspids and molars, because their surface is flatter and are used to grind food, have an occlusal surface. Thus, the surfaces of molars are mesial, distal, lingual, buccal, and occlusal. The surfaces of the anterior incisors are mesial, distal, labial, lingual, and incisal. The mandibular teeth surfaces are named exactly the same.

The anterior teeth have one more peculiar anatomical segment. Each of these teeth has a cingulum. The cingulum is a small area and is adjacent to the lingual surface of the anterior teeth. The bicuspids erupt in the adult dentition where the deciduous molars are present in the primary dentition. Each of these teeth has two cusps, a buccal and a lingual cusp. These teeth are chewing and grinding teeth and can also tear food, but not as effectively as the incisors and canines. The molars are multicusped teeth (four or more) that also have multiple roots (Fig. 3.6). The shape of the dental arches varies considerably. However, in the average individual, the arch has two curves in space. In the horizontal plane, the arch form is somewhat U-shaped with the labial surfaces of the six anterior teeth. As we progress to the bicuspids, this plane is continued. It is not until we reach the distal cusps of the first molar that the curve begins to bend towards the lingual. This curve of the arch is important because it prevents the teeth from biting the cheek and gives clearance for the coronoid process of the mandible to move within the zygomatic arch.

Fig. 3.6

Occlusal surface of a molar tooth

Deciduous Dentition

The deciduous dentition closely resembles their permanent successor; however, they have smaller crowns and shorter root lengths (Fig. 3.7). The anterior incisors have more curvature on their labial surface when compared to the adult version. The height of contour of the molars is closer to the gingiva than the permanent molars, thus making it much harder to get wires below the height of contour so that they will hold tight to the teeth when applying arch bars. As a result, acrylic splints, circumandibular and transmaxillary wires, drop wires from the piriform apertures, or zygomatic buttresses are frequently used to hold the arch bars in young patients.

Fig. 3.7

Dental eruption schedule

The permanent dentition, in contrast, has contact points at the upper third of the clinical crown above the interproximal papillae. Fixation of arch bars in the permanent dentition can be easily secured with interdental wires if the dentition is intact. The deciduous teeth in the anterior are replaced in kind by the permanent teeth. However, in the case of the deciduous molars they are replaced by permanent bicuspids. Because there are no bicuspids in the deciduous dentition, there are only 20 teeth in the arch. In comparison, the three permanent molars erupt de novo and as a result, there are 32 permanent teeth in the adult arch.

Dental Formula

There are many formulas one can use to indicate the position of a permanent or deciduous tooth in the arch. In this text, two basic methods are discussed. One method frequently used is where a cross was made and the vertical line would separate the right side from the left, while the horizontal line would separate the upper from the lower jaws. Looking at that cross, it would be as though the patient were looking at you.

The deciduous dentition is marked in Roman numerals or letters (A through E), while Arabic numbers are used to represent any erupted permanent teeth (Fig. 3.8). Another method to denote the dentition in the arch is to use a formula where the teeth are numbered starting in the right upper quadrant and going forward all the way around the arch (1–18). The lower left third molar is now 19 and then around to the full complement of teeth in the lower arch to the right third molar, which is 32. Here, for example, the lower left central would be 24 and the lower right cuspid would be 27.

Fig. 3.8

Pediatric dental nomenclature

It must be mentioned that the eruption schedule is only a guide because eruption patterns of human teeth vary considerably. It is only those eruption times that are significantly outside the ranges that are considered as pathologic. The eruption of the deciduous teeth generally follows front to back with the upper centrals and laterals erupting first and the cuspids next and on back to the molars erupting at age 3 years. The sequence in the permanent teeth is quite different. Here, the upper and lower first molars are the first teeth to erupt into the arch. They position themselves just behind the deciduous second molars and are usually fully erupted by age 6 or 7. This is an important concept, for these teeth act as pillars, so that if the loss occurs before the dentin is completely erupted, a disturbance in the occlusion is likely to occur. As stated by Angle, “all teeth are essential yet in function and influence some are of greater importance than others, the most important of all being the first permanent molars.” These teeth act to maintain the dynamic balance of the occlusion between the forces of the anterior arch with the forces of the molars on the posterior portion of the arch. Early loss of these teeth results in tilting of adjacent teeth, overeruption of the opposing teeth, and migration of the adjacent teeth into the first mandibular space, creating marked dental disharmony.

• Maxillary Central Incisor: This tooth has a broad crown and a shovel shape. When the tooth first erupts, it (as well as the lateral incisors in both the maxillary and mandibular arches) has three rounded cusps called mamelons. These are quickly worn away with occlusal abrasion. It is important to note that if in an adult you still see mamelons present on the centrals and laterals, you can assume that very little contact has been made between these teeth, as is frequently seen in class III malocclusions or open bite deformities. The root of the fully formed central incisor is relatively short (about 12 mm) and cone shaped and as a result can be pulled easily inferiorly with a wire or elastic traction. As a result, interproximal wires should not be placed around this tooth and attached to an arch bar. This could easily result in partial extrusion of the tooth below the upper incisal plane. The same is true of the maxillary laterals and the mandibular central and lateral incisors.

• Maxillary Lateral Incisor: This tooth is similar to the central but has a slightly narrower crown width mesially to distal. The root frequently has a slight distal bend at its apex. The root length is also about 12 mm. This tooth has a higher rate of being congenitally missing or malformed.

• Maxillary Cuspid: Here, the incisal edge has a sharp point. It has a short blunted lingual cusp or cingulum. The root is longest and strongest in the dental arch (16 mm) with an overall tooth length of 27 mm. The apex of the root curves distally. This root structure is why this tooth should be included when placing arch bars. However, because of the low lingual crown anatomy, it is necessary to loop the arch wire around the arch bar to hold the wire as close to the neck of the tooth as possible.

• Maxillary First Bicuspid: These teeth are the first to have a true occlusal surface. The buccal cusp is similar to the cuspid but here the cingulum is now a fully developed lingual cusp. The root of the maxillary first bicuspid can have either one or two roots. Sicher states that over 50 % of these teeth had two roots with two root canals.

• Maxillary Second Bicuspid: This tooth is somewhat smaller than the first bicuspid. It has a lingual and buccal cusp of equal height. The root is rarely divided. It frequently has a deep groove and its root canals may be totally separate or may fuse at variable distances from the apex of the roof.

• Maxillary First Molar: Here the occlusal surface is rhomboid shaped. It frequently has an extra cusp on the mesial lingual surface called the cusp of Carabelli. Sicher states it is present in 10–15 % of upper first molars as a well-developed cusp, but a remnant such as a pit or groove may be present in the mesial lingual cusp in up to 40 % of upper first molars. The roots are three, with the lingual (or palatal) and a distal and mesial root.

• Maxillary Second Molar: The overall shape in this tooth is similar to the upper first molar except there is no cusp of Carabelli. The roots are also similar, except they do not diverge as much and are more frequently fused into one mass but still maintain the three separate root canals.

• Mandibular Central Incisor: The crown of this tooth is narrow with a sharp mucosal edge. As with the upper central incisors, there are mammelons present in the newly erupted lower central incisors that quickly are worn away with incisal function. The root is straight and the overall tooth size is quite small, and therefore it too should not be attached to an arch bar because it could easily be avulsed from its socket.

• Mandibular Lateral Incisor: This tooth is very similar to the central incisor, only it is a little longer.

• Mandibular Cuspid: This tooth is smaller and shorter than the maxillary cuspid (overall length 25 mm vs. 27–28 mm for the upper cuspid). Because of its similar shape to the upper cuspid, it must also be wired to the arch bar with a loop wire. The root is shorter than the maxillary cuspid and not as strong, but it should be used to help retain the lower arch bar because it does have good retentive strength.

• Mandibular First Bicuspid: Here, there are two cusps, a buccal and lingual, but when compared with the upper first bicuspid, the lingual cusp is smaller and shorter. The root here is singular with one root canal.

• Mandibular Second Bicuspid: The crown of the mandibular second bicuspid is larger than the first with a larger, more developed, lingual cusp. Its root is also singular with a root canal but is longer and stronger than the lower first bicuspid.

• Mandibular First Molar: This tooth has five cusps, three on the buccal and two on the lingual. It is rectangular shaped. The lingual cusps are slightly higher than the buccal ones because they fit into the central fossa of the upper first molar. There are two roots, a distal and mesial root with two canals, and these roots frequently deviate distally in the arch

• Mandibular Second Molar: The crown has two lingual and two buccal cusps. Both are of equal height. The roots also are two, a mesial and distal root with two root canals.

Eruption of a Tooth

When there is pressure exerted in living bone, it responds either by osteoblastic proliferation (bone formation) or osteoclastic activity (bone resorption). As the tooth develops in its dental sac, there is a resorptive surface at the periphery of the dental sac that permits the growth and development of the crown of the tooth. This is called the preeruptive phase of eruption. With the development of the root, the active phase of eruption begins. As the root grows down into the alveolar bone, the resultant force is upward in the mandible and downward in the maxilla. This process of eruption proceeds as the tooth emerges through the mucosa and continues until the erupting tooth comes in contact with the tooth of the opposite arch. Eruption also occurs in a rotational axis or in a transverse labiolingual or buccolingual axis (tilting or tipping of teeth). Finally, a tooth may move parallel to its long axis or drift, as can occur when an adjacent tooth is absent and the tooth migrates into the adjacent tooth space. Once the tooth is fully developed, it still can erupt if, for example, it has no opposing tooth. It is believed that the negative force created by the unopposed tooth results in bone apposition at the apex of the tooth and facilitates the eruption until opposite contact is made.

In the mandible of a child, the larger size of the developing teeth results in crowding. The follicles of these permanent teeth may be rotated or staggered in the arch and frequently are just above the inferior border of the mandible or just below the orbital floor in the maxilla. The permanent bicuspid crowns are smaller than the overlying deciduous molars and develop immediately below the bifurcation of these molar roots.

The position and growth of the permanent molar teeth is dependent on the forward growth of the mandible. When the permanent molars begin to develop, they lie in the base of the ascending ramus of the mandible. The developing upper molars lie in the expanded maxillary tuberosity. If horizontal growth of the mandible fails to occur or the maxilla is hypoplastic, these molar teeth may be unable to erupt because they become “blocked out” by the erupted adjacent teeth and remain in the bone in an oblique or horizontal position.

The position of the developing and erupting teeth also demonstrates how difficult reduction of fractures of the maxilla and mandible are in the young child (Fig. 3.9). There is little or no room to place plates and screws without risking injury to these teeth. An osteotomy of the maxilla would also be extremely difficult because of the absence of the maxillary sinus and the high position of the developing cuspid and bicuspids in relation to the orbital floor.

Fig. 3.9

Location of the permanent dentition within the pediatric maxilla

Maxillary Sinus

The maxillary sinus at the time of birth is very small - about the size of a pea (Fig. 3.10). As the face grows and the teeth erupt, the sinus enlarges in an inferior direction. This may explain why the maxillary sinus ostium is at the upper end of the adult maxillary sinus cavity. In most individuals, Sicher states the maxillary sinus continues to expand throughout life. This expansion penetrates deeper into the alveolar process and thus we frequently see the apices of the molar teeth sticking up into the sinus floor. The maxillary sinus may even invade into the body of the zygomatic bone. The enlargement of the maxillary sinus explains in part why maxillary fractures are less often seen in children than in adult patients. Also, the intimate relationship of the sinus to the roots of the bicuspid and molar teeth explains why with maxillary sinusitis one can have a feeling of pressure or pain referred to these teeth.

Fig. 3.10

Growth and aeration of the maxillary sinus with age

Curve of Spee

When looking at the maxillomandibular occlusion from the buccal surface, an anterior position curve is noted. This curve is called the curve of Spee. It extends from the tip of the mandibular cuspid and follows along the buccal cusps of the mandibular posterior teeth (Fig. 3.11). This curve is usually slight and only in a few patients does it have a pronounced arc. It is believed to be an adaptation of the teeth to a balanced occlusion and it is caused by the tendency of a single tooth to assume a position in the jaw where its long axis is best suited to the forces of mastication. The long axis of the upper and lower arches form reciprocal curves, which are convex in the maxilla concave in the mandible. This position gives each tooth the optimal resistance under maximal force from the muscles of mastication. The obliquity of the resultant force produces an angular position of the teeth in the arch and is affected by the arc of rotation and angulation of the condyle in the glenoid fossa and by the forces of the muscles of mastication. This curve permits the maximum utilization of tooth contacts during function that would not be possible if there was a flat occlusal plane.

Fig. 3.11

Curve of Spee

The teeth are not in continual contact in normal individuals. Instead there is a position the mandible assumes when the patient is erect and the muscles of mastication are relaxed. This is called the mandibular rest position. Here the teeth are not in contact but have a space of 2–3 mm between them, the freeway space. When the patient is in rest position, even though the muscles are relaxed, there is still some muscle activity present, the muscle tonus, which is present throughout the body unless the patient is paralyzed under general anesthesia. This freeway space is important and must be maintained. If the bite is artificially “opened” and exceeds the freeway space distance, the muscles of mastication, especially the external pterygoid, become chronically stretched with an increase in tone. This can result in fatigue and increased muscle irritation, with muscle spasm and pain.

Centric Relation and Centric Occlusion

Centric relation refers to the most retruded unstrained position of the mandibular condyle in the glenoid fossa. This is also called the terminal hinge position. Centric relation does not in itself indicate occlusion of the teeth. It is simply a condylar glenoid fossa relationship. Centric occlusion, on the other hand, refers to the maximal contact of the incline planes of the opposing cusps of the mandibular and maxillary arches. In this relationship of maximal contact, there must be bilateral symmetrical contact and a balanced and unstrained relationship of the condyle in the glenoid fossa. With premature contact of the cusps (due to loss of teeth and overeruption of the opposing teeth, restorations with too high an occlusal contact, malpositioned teeth, and/or skeletal discrepancy), the centric occlusion can be compromised. Orthodontics and orthognathic surgery serve to reestablish a normal jaw relationship so that when there is centric relation of the condyle in the glenoid fossa, there is also centric occlusion of the dentition.

One other term must also be discussed, and that is habitual occlusal relationship. This is the occlusion a patient goes into when they occlude their teeth several hundred times a day while swallowing, talking, and so forth. It is imperative for dental and temporomandibular joint health that the habitual occlusion and centric occlusion position be the same so that there is harmony between the occluding teeth and the condyle in the glenoid fossa.

Overbite, Overjet, and Crossbite

The normal interarch relationships of the anterior teeth show two different types of overlap: a vertical and a horizontal overlap. The horizontal distance from the labial incisal edge of the lower central incisor to the labial incisal edge of the upper central incisor when the jaws are in centric occlusion is called overjet (Fig. 3.12). In a class II malocclusion, there is frequently an increased amount of overjet (This may not be the case in class II, division 2, and cases with deep bites). In the vertical plane, the vertical overlap is called overbite and is represented by the distance from the incisal edge of the upper central incisor to the incisal edge of the lower central incisor while the teeth are in centric occlusion. A class II, division 2 case with a deep bite will have an increased overbite whereas an end-to-end bite (class III) will have no overbite whatsoever. Some patients may even have a distinct gap between the incisal edges of the central incisors when the molars are in centric occlusion. This is referred to as an anterior open bite deformity and is more likely to be seen in patients with vertical maxillary excess and/or steep mandibular plane angles.

Fig. 3.12

Representation of “overjet” and “overbite”

The occlusion of the posterior teeth can also demonstrate overjet, overbite, open bite, and crossbite. In the normal buccolingual arch relationship of the mandibular and maxillary molar teeth, the mandibular buccal cusps occlude in the central fossae of the maxillary molars and the maxillary lingual cusps articulate in the central fossae of the mandibular teeth. This is explained because in the normal individual the total length of the maxillary arch is 128 mm, whereas the mandibular arch is only 126 mm. This produces some degree of overjet and overbite in this molar region. If there were a gap between the maxillary and mandibular occlusion, then a lateral posterior open bite could exist as well. The overlap of the maxillary molars prevents the cheek from being entrapped when the molar teeth occlude.

When the position of the maxillary teeth versus the mandibular teeth is excessive in the buccal dimension, a “buccal crossbite” or “Brodie bite” can result. When this situation is reversed, then a posterior crossbite can exist. It may be bilateral or unilateral. In this type of malocclusion, the mandibular lingual cusp articulates in the central fossa of the maxillary teeth and the maxillary buccal cusp articulates on the central fossa of the mandibular teeth.

Classification of Malocclusion

The differences in the mesiodistal diameters of the upper and lower incisors cause a distal position of the upper cuspids, bicuspids, and molars to their counterpart in the lower arch. Thus, the upper cuspid occludes distal to the lower cuspid, between it and the lower first bicuspid. In the molar region, this distal position of the upper dentition results in the mesial buccal cusp of the upper first molar to articulate with the buccal groove of the lower first molar, which is the basis for Angle’s classification of occlusion. He divided malocclusions into three broad classes dependent on the relationship of the maxillary and mandibular first molars: class I neutroclusion, class II distoclusion, and class III mesioclusion.

Malocclusion can be divided into dental dysplasia, skeletal dysplasias, and skeletal soft tissue dysplasias. Angle’s classification serves as a tool to describe the anterior-posterior relationships of the maxillary and mandibular dental arches. Using it makes it possible to scientifically categorize malocclusions and gives us “common ground” to communicate information to our colleagues. It has more recently been expanded to include skeletal and soft tissue relationships of the upper and lower jaws.

There are a variety of environmental effects on malocclusion. These include respiratory patterns (mouth breathing causing a high-arched palate and transverse constriction of the palate and maxillary teeth) and habits such as thumb sucking and tongue thrusting. Possible equilibrium influences, whether from masticatory forces, periodontal fibers, and swallowing posture, can have a role in tooth alignment and are dependent on magnitude and duration of force against teeth.

The patient’s basic hereditary or ethnic pattern, which can result in discrepancy in the size and number of the teeth in relation to the size and shape of the skeletal support of the jaws, also has a role on occlusion. In a large study of 21,328 children, 6–18 years of age, by Dr. Woeffel in the USA, 71.7 % had dental malocclusions, whereas 28.3 % had an acceptable occlusion. When classified according to Angle’s classification, 28.3 % had class I occlusions, 22 % had class II occlusions, and 5.7 % had class III occlusions.

Class I (Neutroclusion)

With class I occlusion, the anterior-posterior relationship of the maxillary and mandibular molars is in neutral position. The mesiobuccal cusp of the maxillary first molar articulates with the buccal groove of the mandibular first molar (Fig. 3.13). The anterior occlusion may display dental abnormality, such as flaring, crowding, or excessive lingual tilt. It is also possible to see a class I molar and cuspid relationship but have the entire upper and lower dentition positioned forward on their alveolar bases (called bimaxillary protrusion). Another possibility is having the posterior molar relationship class I, but the anterior teeth and bicuspids out of contact, producing an anterior open bite. It is important to remember that an open bite may also occur in class II and III malocclusions as well. The effects of these malpositioned teeth on the soft tissues may produce excessive eversion of the vermilion, protruding lips in the bimaxillary protrusive case, or the lip incompetence that is frequently seen in anterior open bite or severe overjet cases.

Fig. 3.13

Class I (“neutral”) occlusion

Class II (Distoclusion)

With class II malocclusion, the mandibular arch is in a distal or posterior position in relation to the maxillary arch. The mesial buccal cusp of the maxillary first molar articulates with the distal portion of the mandibular second bicuspid and the mesial cusp of the first molar (Fig. 3.14). The remaining teeth reflect this distal position of the mandible. Angle subdivided this class II into division 1 and division 2, depending on the relationship of the anterior teeth to each other.

Fig. 3.14

(a, b) Class II malocclusion, including divisions 1 and 2

In division 1, the molar relationship is as described above but the anterior teeth are proclined, producing a significant overjet. This results in muscle imbalance with loss of upper and lower lip competence. Here, the lower lip frequently becomes everted because of its contact with the proclined maxillary central and lateral incisors. The arch form is frequently V shaped, rather than U shaped, due to narrowing in the cuspid and bicuspid regions. This narrowed maxillary arch tends to constrict the tongue, which then puts pressure on the anterior teeth with swallowing and thereby accentuates the labial proclination and excessive protrusion of the anterior maxillary teeth.

In division 2, the mandibular molar relationship is class II or in a distal position when compared to the maxillary dentition. The anterior dental relationship, however, is quite different than in division 1. The maxillary dental arch is not narrow but instead may be quite wide. There is an excessive curve of Spee with super eruption and lingual retroclination of the mandibular anterior incisors. This produces a deep bite that can result in trauma to the maxillary palatal gingiva in excessive cases. The maxillary lateral incisors are either flared or in lingual version also. Because this is a deep bite, the forced lingual retroclination of the lower anterior incisors may cause their apices to protrude through the labial plate of bone. The mandibular incisors may occlude on the anterior portion of the hard palate, including the incisive papilla, and cause severe tissue damage and, in certain cases, damage to the roots of the maxillary incisors. This tight locked bite can also force the mandible and its condyle into a retruded position in the glenoid fossa, which may lead to symptoms of temporomandibular joint pain, frequently associated with deep bite patients. These malocclusions can present with strong labial musculature.

Class III (Mesioclusion)

With class IIII malocclusion, the mandibular dentition is mesially positioned when compared to the maxillary dentition. The mandibular first molar articulates with the maxillary first and second bicuspid teeth (Fig. 3.15). In the anterior teeth, there is either an end-to-end bite or a crossbite relationship. In cases of true prognathism, the mandibular incisors are usually lingually displaced because of the increased mandibular length and forward mandibular position. This results in an increased lingual pull by the muscles of the lips on these anterior mandibular incisors, resulting in their lingual retroclination. At the same time, there is a decreased force by the tongue labially because it lies easily in the floor of the mouth, which is larger because of the increased mandibular length. The lingual retroclination of the lower incisors and the labial proclination of the maxillary incisors seen in this class of malocclusion are called dental compensations. These dental compensations must be overcome orthodontically and be placed correctly over basal bone before surgery is performed to correct this malocclusion. A class III relationship can be due to midface hypoplasia, mandibular prognathism, or a combination of both. In addition to the anterior-posterior relationship, the vertical growth pattern and transverse relationship of the maxilla and mandible have significant effects on the treatment plan. A proper diagnosis of must be completed prior to orthodontic and surgical correction in order to ensure an ideal result.

Fig. 3.15

Class III malocclusion

Gingiva

The mucous membrane surrounding the teeth, the gingiva, is subjected to forces of friction and pressure in the process of mastication. The gingiva is sharply limited on its outer surface of both jaws by a scalloped line (mucogingival junction), which separates it from the alveolar mucosa. The gingiva is normally pink, sometimes with a grayish tinge, a variable partly caused by differences in the thickness of the stratum corneum. The alveolar mucosa, on the other hand, is red, showing numerous small vessels close to the surface. A similar line of demarcation is found on the inner surface of the lower jaw between the gingiva and the mucosa on the floor of the mouth. In the palate, there is no sharp dividing line because of the dense structure and firm attachment of the entire palatal mucosa.

Normally the epithelium of the gingiva is cornifed on its surface and contains a granular layer. In the absence of cornification, there is no granular layer and the flat surface cells contain nuclei that are frequently pyknotic. The epithelium that covers the margin of the gingiva also continues into the epithelial lining of the gingival sulcus. The cells of the basal layer here may contain pigment granules (melanin). While pigmentation is a normal occurrence in blacks, it may also be found in Caucasians, especially those with a dark complexion. When found, it is most abundant in the basal areas of the interdental papillae. If, however, there is increased pigmentation of the interdental papillae, especially when associated with increase in pigmentation of the skin, Addison’s disease can be suspected.

The lamina propria of the gingiva consists of dense connective tissues that are not highly vascular. The papillae of the attached gingiva are characteristically long, slender, and numerous. The presence of these numerous papillae permits a sharp demarcation of the gingiva with that of the alveolar mucosa, which usually has fewer and flatter papillae. The tissues of the lamina propria contain only a few elastic fibers that are, for the most part, confined to the walls of the blood vessels. The gingival fibers of the periodontal membrane enter into the lamina propria, attaching the gingiva firmly to the cementum. The gingiva is immovable and firmly attached to the periosteum of the alveolar bone because of the presence of large, coarse collagen bundles that extend from the lamina propria into the bone.

The gingiva may be divided into the free gingiva and attached gingiva (Fig. 3.16). The dividing line between these two parts of the gingiva is the free gingival groove, which runs parallel to the margin of the gingiva at a distance of 0.5–1.5 mm. The free gingival groove is, on histologic section, a shallow, V-shaped groove corresponding to the heavy epithelial ridge that divides the free and attached gingiva. The free gingival groove develops at the level of, or somewhat apical to, the bottom of the gingival sulcus.

Fig. 3.16

Schematic diagram of gingiva

The attached gingiva is characterized by high connective tissue papillae elevating the epithelium the surface of which appears striped. The stippling is most probably an expression of functional adaptation to mechanical impacts. The degree of stippling varies with different individuals. The disappearance of stippling is an indication of edema, secondary to chronic inflammation such as gingivitis. It is important to distinguish between attached gingiva and alveolar mucosa. This is because teeth will not erupt through alveolar mucosa but will most assuredly erupt through attached gingiva. This is especially important in patients in which bone grafting is being performed in the region of the alveolar clefts. If the erupting cuspid is covered by alveolar mucosa, it will not properly erupt into the oral cavity, whereas if attached gingiva is brought over the area of the bone graft, the erupting cuspid can easily erupt into the oral cavity in its normal dental position. It is also important not to bring alveolar mucosa down around the necks of erupted teeth because the alveolar mucosa will not attach to the adjacent tooth structure with a normal architectural pattern. Periodontal pockets can easily develop, the mucosa appears red and beefy, and it will not withstand the forces and friction of mastication as attached gingiva will.

Attached gingiva appears slightly depressed between each tooth, corresponding to the depression of the alveolar bone process between eminences of the sockets. In these depressions, the attached gingiva often forms slight vertical folds. The interdental papilla is that part of the gingiva that fills the space between two adjoining teeth and is limited at its base by a line connecting the margin of the gingiva at the center of one tooth to the center of the other. The interdental papilla is composed of free gingiva and attached gingiva in various relations, depending largely upon the relationship of the neighboring teeth.