Anesthesia for dental procedures

Dental development

Initial calcification of the primary tooth buds may be seen in the fourth month of prenatal life. In general, by the end of the sixth prenatal month, all of the primary teeth have begun to develop. The newborn infant is edentulous, with the rare exception of a mandibular central incisor. This natal or neonatal tooth tends to be quite mobile, and in the past it was thought to require immediate extraction. Recent data suggest that by the end of the neonatal period, this mobile tooth becomes quite stable and capable of normal masticatory function. This is indeed fortunate for the infant, because these neonatal teeth are frequently the only primary teeth that develop in that position ( ; ).

The sequence of eruption of human teeth may critically affect infant feeding, behavior, and mastication. Major changes in the appearance of the dentition in the oral cavity may alter important aspects of neurobehavioral development ( ). As an example of eruption-sequence alterations, premature infants and neonates requiring prolonged orotracheal intubation have significant defects in both oral and dental structures that may persist past 5 years of age, even after removal of the orotracheal tube ( ).

The order of appearance of the teeth in the oral cavity tends to follow generalized patterns ( Table 42.2 ). Usually the teeth erupt in pairs; a mandibular right central incisor erupts at approximately the same time as the mandibular left central incisor, at approximately 6 to 7 months of age. The mandibular teeth usually precede their maxillary counterparts; the maxillary incisors erupt approximately 1 month later than the mandibular incisors. The eruption sequence continues and is usually complete by age 2½ years. The last tooth to erupt is the deciduous second molar, called the 2-year molar because of its appearance at 2 years of age.

When completed, the primary dentition totals 20 teeth ( ). As the toddler’s growth continues, the mandible and maxilla enlarge, causing separations, or diastemata, between the primary teeth ( ). The separations also permit sufficient room for the proper alignment of the permanent dentition.

The maintenance of the health and hygiene of the primary teeth is essential to avoid premature tooth loss. When primary teeth are prematurely lost as a result of decay or trauma, the space needed for the permanent tooth eruption may also be lost because the natural tendency of the tooth is to tip mesially (toward the midline) in the oral cavity. Subsequently, dental malocclusions tend to occur. The primary teeth may also function as the permanent teeth if the permanent analogous tooth fails to develop ( ).

The transition period between exfoliation of the primary teeth and eruption of the permanent teeth is called the mixed-dentition phase. This phase continues until the last primary tooth is normally exfoliated or extracted.

Development of the secondary, or permanent, dentition begins at birth with calcification of the buds of the permanent first molars. The permanent dentition begins its eruption pattern with the permanent first molar, usually at approximately 6 years of age. Like their primary counterparts, the mandibular teeth usually precede the maxillary teeth, with the permanent mandibular incisors beginning to appear at approximately 6 to 7 years of age. Unlike the primary dentition, where there is usually a variability of several months in the timing of eruption, the permanent teeth may vary as much as 1 to 2 years in eruption sequence. See Table 42.2 for approximate ages and sequence of eruption. At the completion of the eruption sequence, the permanent dentition consists of 32 teeth ( ).

The third molars, commonly known as wisdom teeth, have the least predictable eruption sequence of any of the human dentition. They may erupt as early as 13 years of age, as late as 25 years of age, or not at all. Quite commonly, the third molars fail to erupt because of dental germinal pattern alterations or impactions in the soft or hard tissues. Impactions usually occur because of insufficient bony growth of the maxilla or mandible in proportion to the space needs for the unerupted third molar teeth.

In addition to the frequently absent third molars, the most commonly congenitally missing teeth are the mandibular premolars and the maxillary lateral incisors, either singly or in symmetric pairs ( ). Occasionally, a tooth that is thought to be congenitally absent is actually impacted in the soft tissues or alveolar bone.

Just as there are congenitally absent teeth, there are supernumerary or accessory teeth. The most common supernumerary tooth is the mesiodens, a conically shaped tooth located in the midline between the maxillary central incisors. Other common supernumerary teeth are the third premolars and fourth maxillary molars ( ). Supernumerary teeth and congenitally missing teeth are often associated with syndromes.

Identification of mobile primary versus transitional, or mixed, dentition can be of importance to anesthesia personnel. It is often recommended that mobile primary teeth be removed prior to airway instrumentation, as they may be at risk for dislodgment and airway obstruction. Likewise, it is important to note that newly erupted permanent teeth are often slightly mobile, with incomplete root formation. For this reason, extra care should be taken during laryngoscopy, oral airway placement, or other jaw manipulation, as immature permanent teeth may be predisposed to luxation or avulsion trauma if too great a force is placed on them.

Dental identification

There are two principal dental identification systems. In both systems, the primary teeth are designated by letters, and the permanent teeth are designated by numbers. These systems differ in the way that the dental arches (mandible and maxilla) are divided. The first system uses a sequential means for identification, with the primary maxillary right second molar designated as tooth A and followed sequentially around the contralateral side of the maxilla to the left second molar, which is tooth J. The primary mandibular left second molar is tooth K, and the sequence is completed on reaching the mandibular right second molar, tooth T. Similarly, the numbering system for the permanent dentition starts with the maxillary right third molar as tooth 1 and continues to the maxillary left third molar, tooth 16. The sequence continues with the mandibular left third molar, tooth 17, and is completed with the mandibular right third molar, tooth 32 ( ). Both pediatric and general dentists commonly use this system of tooth identification.

The second designation system divides the dental arch into quadrants. All primary central incisors are tooth A and follow distally or posteriorly, so that all primary second molars are tooth E. To make the designation more specific, the quadrant is also named. For example, the primary maxillary right lateral incisor is designated maxillary right B. Similarly, the permanent dentition is divided into quadrants. All central incisors are tooth 1 and continue posteriorly, so that all third molars are tooth 8. This system is most commonly used by orthodontists ( Fig. 42.1 ).

Dental anatomy and physiology

The tooth is composed of a crown, which is usually visible for clinical examination, and a root, which is not seen during routine clinical examination. They are separated by the cementoenamel junction or cervical region of the tooth ( Fig. 42.2 ). The cementoenamel junctions are seen more commonly in adult dentition if gingival (“gum”) recession occurs. The crown is responsible for the slicing, ripping, and grinding of foodstuffs (incisors, canines, and molars, respectively). The root structure imparts stability to the tooth in its surrounding tissues. The anterior teeth, the incisors and the canines, are single rooted with a conical shape. The posterior teeth, the premolars and molars, are multirooted and impart most of their stability by both the number of roots and the subtly divergent directions in which the roots may grow.

Surrounding the root structure of the tooth is the periodontium, which is composed of three structures as follows:

• The most external portion is a combination of the gingival and alveolar mucosa, which constitutes the soft tissue covering for the remainder of the periodontal structures.

• The periodontal ligament attaches the external surface of the root to the alveolar bone, acting as a shock absorber and anchor during masticatory function.

• The bony component is called the alveolar bone or tooth socket. Beneath the alveolar bone rests the supporting basal or skeletal bone. Basal bone, the part seen in edentulous patients, forms the skeletal support for full or partial dentures. When the tooth structure is lost, alveolar bone is also lost and is not naturally regenerated.

The individual teeth are composed of enamel, dentin, dental pulp, and cementum ( ) (see Fig. 42.2 ). The enamel covers the external surface of the dental crown. It is the hardest substance in the human body and, unlike bone, has no living cells. When intact, enamel functions as a thermal insulator and an impervious barrier to chemicals and microorganisms.

On the internal surface of the enamel lies the dentin. It is composed of microtubules and has living cells in the dentinal structure. When tooth decay is advanced, noxious stimuli are readily transmitted via the dentinal tubules to the underlying dental pulp, where the neurovascular supply of the individual teeth is contained. Pain is easily elicited by many different stimuli—thermal, tactile, or liquid. The pain is transmitted from the dental pulp through the root apices to the alveolar bone and subsequently to the body’s pain receptors.

The final portion of the tooth structure is the dental cementum, which covers the external surface of the roots. Because it is not nearly as hard and impervious to the surroundings as is enamel, noxious stimuli are perceived when the cementum is exposed. The cementum is similar to the dentin of the tooth. Patients who enjoy good dental health usually do not have exposed cementum. However, with gingival and alveolar recession, root structure and its investing cementum may be exposed to the external environment.

Some morphologic differences exist between deciduous and succedaneous teeth. The most obvious difference exists in the size of the teeth, with deciduous teeth being significantly smaller than their permanent counterparts. With respect to the molars, the buccal–lingual dimensions are proportionately narrower. In contrast, the mesial–distal dimensions are proportionately larger.

Another difference between the sets of dentition rests in the color of the enamel. The primary teeth are milky white, or opalescent; hence the name milk teeth. The permanent teeth are significantly less “milky” because pigment absorption has occurred during their development or has been acquired during the intraoral lifetime of the tooth ( ). Two examples are tetracycline staining (developmental) and caffeine staining (acquired).

The pulp chambers of the primary teeth are larger than the permanent teeth because of the relative thinness of the deciduous enamel and dentin ( ). Less than meticulous dental restorations or large carious lesions may predispose the primary teeth to pulpal or endodontic therapy earlier than their permanent counterparts.

The root structures of the primary molars are more saber shaped and extend laterally beyond the crown width. This unique root structure allows adequate room for the permanent tooth bud to develop and mature until the exfoliative process is completed for the primary tooth ( ).