Dental Topics for Plastic Surgeons, Part One: Normal Anatomy, Growth and Development

Abstract

Normal and abnormal conditions affecting the dentition and the periodontal structures receive sparse coverage in the plastic surgery literature, textbooks, and training programs. Nevertheless, a working knowledge of this subject matter is important in certain areas of clinical practice, and a fundamental understanding is often part of plastic surgery competency tested for in the In-service and written board examinations. This four-part series is written to provide plastic surgeons with a working knowledge of relevant dental topics. Part 1 covers fundamental aspects of normal dental embryology, growth and anatomy.

A basic dental knowledge is fundamental for several key areas of plastic surgery practice including congenital cleft and craniofacial anomalies, orthognathic abnormalities, and maxillofacial trauma. Surgeons who practice in these clinical areas are often the first to see associated dental problems and must be able to recognize and understand the implications of what they see. Moreover, they must be able to intelligently communicate and collaborate with their dental colleagues. This knowledge is deemed important enough that questions focused on dental anatomy, development, and pathology are frequently tested on the In-service and the written board examinations for plastic surgeons. The plastic surgery literature, unfortunately, provides little guidance about dental principles.

This manuscript is the first of a four-part series written to provide an overview of dental topics and concepts that are directly or indirectly encountered in the field of plastic and reconstructive surgery. Part 1 covers normal dental anatomy, growth, and development. Subsequent sections in this series will cover congenital dental anomalies, infection, traumatic injury, and oncologic processes.

Dental Anatomy

Each tooth may be grossly divided into two components: the crown and the root (Fig. 1), with the junction between the two called the cervical margin. The crown represents the portion of the tooth that projects into the oral cavity. It is protected by an acellular outer layer of highly mineralized enamel, which is composed of hydroxyapatite crystals and is the most highly mineralized tissue in the body. Enamel is supported by a layer of hard connective tissue called dentine, which consists of hydroxyapatite crystals and collagen, resulting in it having a chemical composition similar to bone. Supporting the dentine is a central cavity containing the dental pulp, a soft connective tissue that encompasses the neurovascular components of the tooth, as well as immune cells such as T lymphocytes and macrophages. Along the peripheral boundary of the pulp are odontoblasts, which can be stimulated to produce more dentin throughout one's lifetime.¹

FIGURE 1: Anatomy and structure of the tooth.

Teeth are attached to the bones of the jaw by a supporting apparatus, which includes the cementum, periodontal ligament, and alveolar bone (Fig. 1). Cementum is an avascular mineralized connective tissue that covers the roots of the teeth, and anchors the periodontal ligament to the teeth. It is generally thicker toward the apex of the root. The periodontal ligament connects the tooth to the alveolus, provides support to withstand the forces of mastication, and contains sensory fibers for proprioception. The tooth root is embedded within the alveolus, which is composed of both cortical and cancellous bone. Along the lip or cheek (labial or buccal) aspect of the alveolus, the bony plate is generally thinner compared to the aspect that faces the tongue (lingual or palatal).²

The oral mucosa covering the upper part of the alveolar ridge and immediately surrounding an erupted tooth is referred to as the gingiva and it forms a tight protective cuff around the cemento-enamel junction (neck) of the tooth. The potential space between the gingival cuff and the enamel of the crown is called the gingival crevice. The conglomeration of tissues that surround and serve to support the tooth is collectively known as the periodontium.²

Dental Nomenclature

There are four classes of teeth: incisors, canines, premolars, and molars. The primary (deciduous) dentition consists of 20 teeth, including two incisors, one canine, and two molars in each quadrant (Fig. 2). The permanent (adult) dentition is comprised of two incisors, one canine, two premolars, and three molars in each quadrant (Fig. 3).

FIGURE 2: Dental nomenclature in primary (deciduous) dentition.

FIGURE 3: Dental Nomenclature in adult (permanent) dentition.

The classes of teeth may be further sub-classified based on position and location. The general position of a tooth includes the laterality of the tooth, as well as its presence in the maxilla or mandible. Location further describes a specific tooth based on its position. Examples of location classifications include: central and lateral incisors, first and second premolars, and first, second, and third molars.³ Several taxonomic classification schemes have been developed to identify a specific tooth. The most commonly used dental notation system in the United States is the Universal Numbering System, which uses a unique letter or number for each tooth. Letters “A” through “T” are used to describe primary dentition, and numbers 1 through 32 are used for permanent dentition. The lettering system begins with “A” at the right maxillary second molar in the primary dentition, and continues to “J” at the left maxillary second molar. The counting then continues with “K” from the left mandibular second molar to finish with “T” at the right mandibular second molar (Fig. 4). The numbering system for adult dentition begins with “1” at the right maxillary third molar and continues to “16” at the left maxillary third molar. The counting then proceeds with “17” from the left mandibular third molar to finish with “32” at the right mandibular third molar (Fig. 5).³

FIGURE 4: Universal numbering system for deciduous (primary) dentition.

FIGURE 5: Universal numbering system for permanent (adult) dentition.

Descriptive Anatomic Terminology

Each surface of the tooth is described using specific terms. The part of tooth facing the lip or cheek may be referred to as the facial surface, which may then be further subdivided into labial (incisors and canines) or buccal (premolars and molars) surfaces. The palatal surface describes the surface of the maxillary teeth facing the palate, while the lingual surface describes the surface of the mandibular teeth facing the tongue. The relative position of a tooth is termed “mesial” as the position moves closer to the dental midline; the term “distal” refers to teeth that reside further away from the dental midline. Lastly, the chewing surface of the posterior teeth is referred to as the occlusal surface, while the biting surface of the anterior teeth is the incisal surface (Fig. 6).

FIGURE 6: Anatomic dental orientation and nomenclature.

Occlusal Relationships

The relative position of the occlusal surfaces is described by specific relationships between the maxillary and mandibular dentition. An overbite refers to a vertical overlap between the incisal surfaces of the central maxillary and mandibular incisors (Fig. 7). An overjet refers to a horizontal discrepancy in the position of the maxillary and mandibular central incisal surfaces (Fig. 7). Cross-bite describes either a negative overjet anteriorly, or an abnormal buccal–lingual occlusal relationship of the posterior teeth (Figs. 8 and 9). An open-bite is a vertical space between either the maxillary and mandibular incisal surfaces (anterior open-bite) (Fig. 10) or occlusal surfaces (posterior open-bite).

FIGURE 7: A: Overbite: Vertical overlap between the maxillary central incisors over the mandibular central incisors. B: Overjet: Horizontal discrepancy between the maxillary and central incisors.

FIGURE 8: Lateral view of anterior cross-bite of the central and lateral maxillary incisors.

FIGURE 9: Frontal view of posterior cross-bite.

FIGURE 10: Patient with an anterior open-bite and consequentially mamelons on the upper and lower incisors.

The Angle Classification system describes the relative positions between the mesial buccal cusp of the maxillary first molar and the buccal groove of the mandibular first molar. In an Angle Class I molar relationship, the mesiobuccal cusp is in line with the buccal groove (Fig. 11A and B). In an Angle Class II molar relationship, the maxillary mesiobuccal cusp is anterior to the mandibular buccal groove (Fig. 12A and B). Class II is subdivided into two divisions. In Class II, division 1, patients have minimal crowding of the maxillary teeth and proclined (cusp tilted outward, away from the mouth) upper central incisors, and therefore a significantly increased overjet (Fig. 13A). In Class II division 2, the central incisors are retroclined (cusp tilted inward, toward the mouth), leading to a deep bite or diminished effect on the degree of overjet noted (Fig. 13B). In the Angle Class III molar relationship, the maxillary mesiobuccal cusp lies posterior to the mandibular buccal groove (Fig. 14A and B).⁴

FIGURE 11: A and B: Angle class I relationship. The maxillary first molar mesiobuccal cusp (in red) is in in line with the mandibular first molar buccal groove (in blue).

FIGURE 12: A and B: Angle class II relationship. The maxillary first molar mesiobuccal cusp (in red) is anterior to the mandibular first molar buccal groove (in blue).

FIGURE 13: Angle Class II, division 1; proclined central incisors (A). Angle Class II, division 2 ; retroclined central incisors (B).

FIGURE 14: A and B: Angle class III relationship. The maxillary first molar mesiobuccal cusp (in red) is posterior to the mandibular first molar buccal groove (in blue).

Specific Tooth Features

While the basic arrangement of the composite dental tissues is similar among all teeth, the shape of the crown and root is variable for each type of tooth. Canines and pre-molars (except maxillary first pre-molars) have one root, while maxillary first pre-molars and mandibular molars have two roots, and maxillary molars have three roots. The maxillary canine has the longest root, with an average length of 26.5 mm, placing it at highest risk for damage during a LeFort I osteotomy.^5,6 In describing the crown, the elevated points of the occlusal surfaces found on posterior teeth and canines are known as cusps. Similar to roots, the number of cusps varies from one cusp on canines, to up to four or five cusps on molars.³

The anterior teeth are formed from four centers of development, or lobes. Three of the lobes are present on the facial side of the tooth, and one lobe is located on the lingual side. The cingulum is formed from the sole lingual lobe, and is identified as a v-shaped convexity found on the lingual surface of the anterior teeth. The remnants of the remaining three lobes on the facial surface are known as mamelons. These are typically found to be three small ridges on the incisal edges of the anterior teeth, which typically wear away due to attrition from occlusal contact. Mamelons can be useful landmarks in the management of patients with facial fractures since their presence in adults may indicate premorbid malocclusion from lack of occlusal contact (Fig. 9).³

Dental Growth and Development

Odontogenesis describes the process by which the tooth develops from embryologic cells to a mature tooth that erupts through the alveolus. The tooth germ (dental organ) is derived from ectoderm of the first branchial arch and ectomesenchyme of the neural crest.^2,7 This initial group of cells is composed of the enamel organ, dental papillae, and dental follicle. The enamel organ consists of multiple cell types, which develop into ameloblasts (enamel-forming cells). The dental papillae give rise to odontoblasts (dentin-forming cells), and the formation of the tooth pulp is derived from mesenchymal cells within the papillae. The dental follicle generates cementoblasts, fibroblasts, and osteoblasts. Those three cell types are integral in the formation of the cementum, periodontal ligaments, and alveolar bone, respectively.⁸

In the sixth week of embryologic development, the initiation of tooth development is characterized by the establishment of an identifiable differentiation between the vestibular lamina and dental lamina. Tooth development is then typically described in four stages: bud, cap, bell, and maturation. The bud stage begins with the proliferation of epithelial cells into the ectomesenchyme of the alveolus² and continues as the cellular density surrounding the epithelial outpouchings increases in a process called ectomesenchyme condensation. A total of 10 structures will develop in the distal aspect of the dental lamina of each arch, corresponding to each of the deciduous teeth.⁹ The cap stage follows as the cells begin to assume a more organized arrangement. A collection of ectomesenchymal cells aggregate to form the dental papilla, around which the bud proliferates to become the enamel organ. This structure produces the appearance of a cap covering the dental papillae (Fig. 15). Ectomesenchymal cells proceed to condense between the enamel organ and the papilla to become the dental follicle.²

FIGURE 15: Histologic appearance of the tooth during the cap stage of development where (A) is the enamel organ, (B) is the dental papilla, and (C) is the dental follicle.

The bell stage is characterized by the processes of histodifferentiation and morphodifferentiation (Fig. 16) during which a mass of similar cells transforms into phenotypically and functionally distinct entities, and the tooth crown assumes its final shape, respectively. In the early bell stage, the cells on the edges of the enamel organ distinguish themselves into three separate layers: the cuboidal cells or outer enamel epithelium (OEE), the columnar cells next to the papilla or the inner enamel epithelium (IEE), and the stratum intermedium, which lies between the IEE and the stellate reticulum. The junction of the IEE and OEE is called the cervical loop, and plays a role in formation of the apical part of the tooth bud including the root.² The IEE also influences the ultimate shape of the tooth crown during the bell stage.

FIGURE 16: Histologic appearance of the tooth during the early bell stage where (A) is the outer enamel epithelium, (B) is the inner enamel epithelium, (C) is the dental papilla, and (E) is the dental follicle.

The maturation (crown) stage is characterized by development of the dental hard tissues (Fig. 17), and determination of the cuspal pattern of the tooth. It is at this point that histo-differentiation occurs, leading the development of ameloblasts, odontoblasts, cementoblasts, and fibroblasts.² Dentin and enamel form during this stage under the influence of a process referred to as “reciprocal induction,” in which the formation of dentin always precedes the formation of enamel.¹

FIGURE 17: Histologic appearance of the tooth during the maturation stage showing (A) ameloblasts, (B) enamel, (C) dentin, (D) ondontoblasts.

Specific Tissue Formation

The formation of dentin (dentinogenesis) represents the first event in the maturation (crown) stage of development. Odontoblasts migrate from the outside surface to the inside of the tooth, generating an extension referred to as the odontoblast process, which secretes hydroxyapatite crystals and leads to matrix mineralization.² Enamel formation (amelogenesis) occurs in two phases referred to as the inductive and secretory stage, in which proteins and an organic matrix combine, and the maturation stage during which the mineralization process finalizes. Amelogenesis occurs only once, producing completed enamel, as opposed to dentinogenesis, which occurs throughout a lifetime. Occasionally, a green-gray residue called the Nasmyth membrane forms on newly erupted teeth.¹

Cementogenesis is carried out by cementoblasts, which differentiate from follicular cells. Acellular cementum is first formed as cementoblasts deposit collagen fibrils along the root surface where they will ultimately attach the periodontal ligament fibers to the tooth. Formation of cellular cementum is one of the final processes to take place in tooth development, as it occurs once the tooth meets its opposing tooth in occlusion. Although cellular cementum has a minimal role in attaching periodontal ligament fibers to teeth, it aids in the repair of periodontal tissues, and has an adaptive role in response to tooth movement and erosion.²

The periodontal ligament is derived from ligament fibroblasts of the dental follicle cells and serves as the anchor of the tooth by connecting the cementum and alveolar bone. Osteoblasts forming the alveolar bone respond to tension, while osteoclasts exhibit increased activity under compressive forces, enabling tooth movement with orthodontia. The gingiva is the remaining component of the periodontium and is responsible for the primary epithelial attachment of the tooth through hemidesmosome formation.²

Neurovascular development is an asynchronous process in the tooth. Nerves begin to form during the bud-to-cap stage transition, during which pioneer nerve fibers divide and proliferate within the dental follicle to form an abundant plexus surrounding the tooth germ. Upon dentinogenesis, the nerves penetrate the dental papillae, and thus provide sensory innervation to the developing periodontal ligament and pulp. The nerve fibers never extend into the enamel organ. This pattern of innervation accounts for tooth sensitivity with exposure of dentin.

Vascular development begins in the dental follicle and continues into the dental papillae during the cap stage. Ultimately, the vascular network forms part of the pulp of the tooth. With increasing age, the amount of pulpal tissue, and therefore the blood supply to the tooth, decreases.⁸ The enamel organ is also devoid of blood vessels as it is of epithelial origin, and its mineralized components do not require vascularization.²

Tooth Eruption Process

Teeth will continue to erupt until meeting a point of contact (ie, opposing dentition, tongue, foreign object). The exact mechanism of tooth eruption is yet to be clearly defined, but it is generally accepted that the periodontal ligament is pivotal in initiating the process of eruption through collagen fiber cross-linking and fibroblast contraction (Fig. 18).² Tooth eruption begins for the primary dentition with the eruption of the mandibular central incisors, generally around the age of 6 months. This is followed by the lateral incisor, first molar, canine, and second molar in chronologic order.³ The typical pattern is for mandibular teeth to erupt before maxillary counterparts, and for females to erupt earlier than males. Eruption of the primary dentition should conclude around the age of 30 months (Supplemental Digital Content, Table 1, https://links.lww.com/SCS/B281).^2,10

FIGURE 18: Histologic appearance of tooth eruption depicting tooth (A), gingiva (B), alveolar bone (C), and periodontal ligaments (D).

Excluding the molars, the permanent (adult) teeth are succedaneous, meaning they directly replace the corresponding primary teeth. Since there are no primary premolars, the teeth that precede the permanent premolars are the deciduous molars. Distal (posterior to the dental midline) to these will develop three permanent molars that do not have primary precursors.⁴ The first permanent tooth to erupt is the mandibular first molar, beginning the stage of mixed dentition, which typically occurs at the age of 6 years. Following the first molar, the next teeth to erupt are the central and lateral incisors, respectively, at which point the order begins to differ between the maxillary and mandibular arches. In the mandible, the next tooth to erupt is typically the canine followed by the first premolar, second premolar, second molar, and third molar. In contrast, the next tooth to erupt in the maxilla is generally the first premolar, second premolar, followed by the canine, and second molar (Supplemental Digital Content, Table 1, https://links.lww.com/SCS/B281).⁴

CONCLUSION

Dental development is a complex, but relatively well-defined process that is necessary for the practicing plastic surgeon to understand in order to gain full appreciation of tooth anatomy and occlusal relationships. This overview describing dental anatomy and development provides a foundation for discussion of clinical conditions affecting the dentition that may arise from congenital anomalies, traumatic injury, or oncologic processes relevant to plastic and reconstructive surgery.