General Dentistry The anatomy of local anesthesia Robert S. Roda* / Patricia L. Blanton** The trigeminal nerve is the great sensory nerve of the facial portion of the head, andan inti- mate knowledge of this nerve and its surrounding structures is a prerequisite to the applica- tion of profound local anesthesia without complication. In this review anide, the classic description of the relevant anatomy is described and is updated with recently published research into anatomic variations that have an impact on the induction of clinical local anesthesia. Causes of both failure and complications of traditional loeal anesthetic techniques are e-\plained, and recommendations for avoiding these pitfalls are made. (Qumtessence itit 1994:25:27-38.) Introduction However, previous descriptions of the "normal" an- atomy of the area are generalized and incomplete; they Local anesthesia is perhaps the most commonly deliv- do not accurately reflect the anatomic variability of the ered service in dentistry today and occasionally can be various sensory nerves, blood vessels, and other struc- one of the most frustrating, ITie key to minimizing tures. This could be, for example, one reason why the anesthetic failure or postinjection complications in the rate of failure to achieve adequate pulpal anesthesia head and neck region is a thorough knowledge of the via inferior alveolar block injection has been so high.^'^ anatomy of the trigeminal nerve (cranial nerve V) in all Techniques that allow integration of individual an- of its variations. atomic variations are available based on increased The trigeminal nerve is the largest of the cranial knowledge of the anatomic distribution and relation- nerves and is the great sensory nerve of the facial por- ships of the trigeminal nerve. Intimate famiharity with tion of the head tncluding the maxilla and mandible, these relationships and the variation exhibited by all of teeth, and oral mucosa. It has both sensory and motor these structures is esserttial to providing adequate an- roots, the sensory much larger. The classic descriptions esthesia to patients. of the anatomy of innervation of the dentition used by There are two types of local anesthesia: supraperi- dental practitioners to achieve local anesthesia, the ac- osteal, or infiltration, anesthesia and block anesthesia. tual techniques themselves, and the target areas of the The anatomic basis of infiltration anesthesia is simple injections are described elsewhere in the iiterature,'"^ enough, but it has limited usefulness in that it is pri- marily effective for only small areas of oral mucosa and teeth and is totally precluded in many areas of the max- illa and mandible by the nature of the bony anatomy. Block, or trunk, anesthesia involves larger areas of the maxilla or mandible and more teeth, and, to be success- ful and safely applied, requires a more thorough •" Private Practice, 7054 East Cochise. Suiie B-115, Scottsdale, knowledge of the anatomy of the maxilla and mandible Arizona 85253. and related oral structures extending into the depths of »^ Adjunct Professor, Department of Anatomy, Baylor College of the head. Dentistry, Dallas, Texas 75246, Quintessence International Volume 25, Number 1/1994 27 General Dentistry The purpose of this paper is to review the anatomy of Labial infiltration: Central incisors the trigeminal nerve and its surrounding structures, to provide an update on its peripheral distribution and In the region of the maxiiiary centriii incisors, the anatomic variations based on current research, and to prominence of the anterior nasal spine and a promi- demonstrate how an enhanced understanding affects nent floor of the piriform aperture may preclude ap- induction of anesthesia and limits postinjection compli- proximating the apices of these teeth. The anterior cations. This knowledge is essential to the safe and ef- superior alveolar nerves arise in the infraorbital canal fective application of local anesthesia. about 6 to 10 mm posterior to the infraorbital foramen. To obtain complete anesthesia of these nerves, the in- jection may have to be made into the infraorbital ca- nal.^ Successful infraorbital anesthesia will anesthetize Anatomic considerations in achieving complete the incisors and the canine, as weli as the facial gingiva anesthesia and the skin of the upper lip, lower eyelid, and the alae of the nose. Often the first premolar is anesthetiœd. Maxilla Frequently, the anesthetic zone does not reach the mid- Maxillary division block line and so, for complete anesthesia of the ipsilateral central incisor, the nasopalatine nerve must also be With "only one inferior alveolar nerve", the man- blocked. It is interesting to note that the anterior dibular teeth may be anesthetized with only one block superior alveolar nerves overlap the midline for the injection; in the maxilla, which has three superior alve- skin, but do not cross the midline ofthe maxiila,'' olar nerves, multiple injections are required, unle.ss the entire second division is being blocked. For extensive operations in the maxilla, a second division block might be desirable,^ To accomplish this, the injection has to be Palatal infiltration made into the ptery go pal atine fossa, which is accessi- The mucosa ofthe hard palate and the palatal gingiva ble by two routes: through the pterygomaxillary fissure are supplied by the greater palatine and nasopalatine posterior artd superior to the maxilla'^"* or via the nerves. The boundary between the areas innervated by greater palatine canal,'^•""'•' the two nerves corresponds roughly to a line drawn be- tween the maxillary canines; however, the two areas are not so sharply delineated. Recently, Langford'** Buccal infiltration: Molars artd premoiars showed, by severing the nasopalatine nerve, that the The outer cortical plate ofthe maxillary alveolus is al- greater palatine nerve may play a larger role in the in- most always thin and porous enough in the adult to nervation of the anterior palate than had previously make infiltration anesthesia effective. Several possible been thought. The greater palatine nerve enters the anatomic variations may lead the clinietan to modify oral cavity through the greater palatine foramen, which this principle. First, the region of the first molar and se- is infrequently palpable and has been localized on the cond premolar often includes the lower extreme of the hard palate further posteriorly than depicted by most zygomaticoalveolar crest, a borty prominence that texts, Otie study'"" showed this foramen to be opposite precludes approximatirtg the apices of these teeth or slightly distal to the third molar or its extraction site when infiltrating around the middle stiperior alveolar (57%). it has been found to lie 1,9 mm in front ofthe nerve,'"* The middle superior alveolar nerve is also re- posterior border ofthe hard palate and 15 mm from the ported to be absent in many persons.'^'^ In these eases, palatai midhne. Trunk anesthesia immediately in the most if not all of the fibers constituting this nerve are vicinity of the foramen is recommended. incorporated into the posterior superior alveolar Fibers of the superior alveolar plexus occasionally nerve,^* Further, in the pédiatrie patient, the outer cor- join the nasopalatine nerve just below the nasal floor tical plate may be too dense to allow adequate infil- and travel with the nasopalatine nerve to reach the re- tration,^-" These potential problems may be solved spective central incisor. Anesthesia ofthe nasopalatine by successful anesthesia of the posterior superior nerve may be necessary to completely anesthetize the alveolar nerve high on the maxillary tuberosity.* "' This centra! incisors and is best accomplished immediately will render the buccal gingiva in the region, as well as lateral to the incisive papilla with the needle directed the three molars and on occasion the premolars, upward, backward, and slightly medially. insensitive. The greater palatine injection may also add to the ef- 28 Quintessence International VolLittie25, Number 1/1994 General Dentistry ficiency of a huccal or superior alveolar injection, if the molar teeth in 75% of the mandibles, and even with this latter does not render totally effective anesthesia. The level in 22,5 % ,^' Nicholson"' concluded that because of greater palatine injection influences the nerves that the variability in the position of the foramen, his find- enter the palatal root of the molars and/or the lingual ings would offer little help to the clinician wishing to aspect of the premolars,'''" These nerves are displaced anesthetize the inferior alveolar nerve, branches of the buccally located superior alveolar nerves, which enter the teeth from above. To avoid complications such as postoperative pain, Intramandibular course of the inferior alveolar nerve ulcération, or necrosis after palatine injection, trunk Carter and Keen'^ dissected and examined eight hu- anesthesia imtnediately in the vicinity of the foramen is man mandibles and described in great detail many of recommended. Use of strictly local anesthesia in the the anatomic variations of the inferior alveolar nerve. first molar and premolar regions should be avoided be- They compensated for the small sample by dissecting cause the densely textured palatal tissue renders the in- the eight mandibles in exquisite detail. The pattern of jection very traumatic. distribution of the inferior alveolar nerve was classified into three types. Type 1 (n = 6) was a single nerve run- ning in a bony canal close to the root apiees of the man- Mandible dibular teeth. Type 2 (n = 1) was positioned more infe- By far the most common approach to inferior alveolar riorly with longer branches to the dentition that were anesthesia is the traditional Halstead method.^''-" in oriented obliquely. Type 3 (n = 1) split posteriorly into which the inferior alveolar nerve is approached in the a superiorly placed alveolar branch while the main con- p te ryg o mandibular space via an intraoral route just be- tinuation of the nerve occupied a more inferior posi- fore the nerve enters the mandibular foramen"' (Figs 1 tion and continued anteriorly to the mental and incisive and 2), Entrance into this space is through the buccina- branches.