Clinical Oral Structures, Dental Anatomy, and Root Morphology
The practice of dental hygiene is based on oral anatomy, a fundamental dental science. A thorough knowledge of oral anatomy provides the basis for assessing, diagnosing, planning, implementing, and evaluating clients during the dental hygiene process of care. Oral structures reflect local and systemic health. Oral anatomy also provides the basis for client education, fluoride and pit-and-fissure sealant therapy, periodontal and tooth assessment, instrumentation, and nonsurgical and periodontal maintenance care, all of which require imagery and tactile perception. The dental hygienist also uses oral anatomy to assess the relationship of teeth, both within and between the arches. These factors influence care plans, evidence-based decision making, professional recommendations, and referral to other health care practitioners.
Oral tissues are indicators of a client’s oral and general health. Abnormal conditions can be recognized if the appearance of normal oral structures is known (Figures 5-1 to 5-8 and Table 5-1). Oral structures are identified according to their specific locations and functions. Generally, oral structures appear in shades of pink and may be pigmented in dark-complexioned individuals. In the oral cavity, the presence of melanin pigmentation is random, scattered, and unpredictable.
FIGURE 5-2 Buccal mucosa.
FIGURE 5-3 Dorsum of the tongue.
FIGURE 5-4 Lateral surface of the tongue.
FIGURE 5-5 Ventral surface of the tongue.
FIGURE 5-7 Hard palate.
FIGURE 5-8 Soft palate and oropharynx.
6. Occlusal—surface of a posterior tooth that is toward the opposite arch; the chewing surface; this surface has elevations and depressions; the expression of these anatomic landmarks varies with the population from which they are derived (Figure 5-9)
FIGURE 5-9 Occlusal anatomy.
(1) Fissure—structural effect of enamel formation manifested as developmental lines or grooves on the external surface of a tooth; the area where the centers of calcification coalesce during tooth development
f. Pit—a sharp, pointed depression generally located at the junction of developmental grooves (fissures) or at their termination; the opening of a pit may be narrow or wide but is smaller than a toothbrush bristle; pits may be shallow or deep, and their apical descent may be steep or gradual; pits in primary teeth are not as deep as those in permanent teeth
D Junction of surfaces—a tooth has curved surfaces; therefore, no “corner,” where one surface begins and another ends, is present; the transition area is called the line angle area and is named for the surfaces that are involved (e.g., MB, mesiobuccal, DL, distolingual, MO, mesio-occlusal)
E Embrasure—the interproximal space between teeth that begins at the contact area and widens in facial, lingual, occlusal–incisal, and cervical directions; functions as a spillway and escapement area by deflecting food and reducing the forces placed on the periodontium during chewing; also provides a self-cleaning area; the interproximal gingiva fills the cervical embrasure
Knowing the length of the crown of a tooth is helpful when assessing the length of its root and the amount of attachment. Maxillary central and lateral incisor crowns are the longest in the dentition, being approximately 9 to inches in length. Anterior crowns are 2 to 3 mm longer than posterior crowns. Roots range between 12 and 17 mm in length; incisor roots are the shortest, and canines are the longest. Proportionally, when comparing the length of roots with their crowns, molars have the “longest” roots overall because of their short crowns; maxillary incisors have the “shortest” roots
a. Eight incisors, two in each quadrant, named central and lateral incisors, respectively. The four quadrants in a dentition are: maxillary right, maxillary left, mandibular right, and mandibular left
3. The Universal Numbering System (UNS) uses Arabic numerals 1 to 32 to specify permanent teeth, beginning with the maxillary right third molar and ending with the mandibular right third molar; the International Standards Organization (ISO) TC 106 designation system (also referred to as the International Numbering System) uses a two-digit code; the first digit—1 to 4—designates the quadrant in the dentition, clockwise from the upper-right quadrant. The second digit—1 to 8—designates the tooth, from the central incisor to the third molar. For example, tooth 11 is the permanent maxillary right central incisor
(4) As a general rule, the mesial crest is more incisal–occlusal than the distal crest, and mesial cusp ridges are shorter than distal cusp ridges; mesial outlines are straighter than distal outlines (Figure 5-11)
c. All facial surfaces of the crown are convex, and the crest of the curvature is located in the cervical third; the lingual surfaces of posterior teeth are convex, and the crest of curvature is located in the middle third; the lingual surfaces of anterior teeth are concave in the middle third; these contours deflect food away from the gingiva and facilitate the function of teeth (Figure 5-13)
d. The CEJ on the proximal surface curves toward the incisal–occlusal surface and is more prominent on anterior teeth than on posterior teeth; the CEJ curves more on the mesial surface than on the distal surface (see Figure 5-13)
e. From a proximal view, the long axes of the crown and the root are in line except for the posterior mandibular teeth, which have the long axis of the crown tilting lingually to the long axis of the root; this lingual inclination enables the intercusping relationship of posterior teeth and the distribution of forces along their long axes (Figure 5-14)
f. Proximal surfaces converge toward the lingual; this is most prominent on maxillary incisors and canines (the two exceptions are the mandibular second premolar and the maxillary first molar) (see Table 5-2 and Figure 5-15)
f. A cervical cross-section of a tooth with one root shows three basic shapes (Figure 5-16; see Table 5-2), which may be slightly altered by the presence of root concavities
(2) The proximal surfaces of maxillary incisors are more accessible from the lingual approach because of the convergence of the proximal surfaces; the proximal surfaces of mandibular incisor roots are difficult to approach because of limited interproximal space and root concavities
(3) As people live longer and keep their teeth, repeated instrumentation on the roots of mandibular incisors places the crowns of these teeth in jeopardy; the very narrow facial and lingual root surfaces are increasingly subject to loss of structure, resulting in unsupported cervical enamel