Chapter 11Applied Anatomy

1 Anatomy of Local Anesthesia

This section is not intended as a comprehensive description of local anesthetic techniques, but it is instead intended to provide the student with an anatomical basis for a sub-sequent anesthetic course referenced by appropriate texts and handbooks about local anesthesia.

Pain control is a very important part of dental practice. Operative procedures require cutting through sensitive tooth structures, producing extreme discomfort and pain. Surgical procedures, such as tooth extraction, periodontal surgery, biopsies, and so on, also require a form of pain control.

General anesthetics affect the central nervous system and render the patient unconscious and incapable of feeling pain. General anesthesia, however, is potentially hazardous to the patient, and it is usually administered in a hospital environment.

Local anesthesia is the introduction, by injection, of an anesthetic fluid to a sensory peripheral nerve. The fluid diffuses through the nerve bundles to reach the individual nerve fibers and blocks the transmission of pain-perceived stimuli to the brain. It thus renders the territory supplied by the nerve void of sensation, or numb. This procedure is performed regularly and routinely in the dental office and is relatively safe because the patient remains awake.

A topical anesthetic is applied to the injection site to numb the subsequent penetration of the needle through the mucosa. In addition, before the release of the anesthetic, it is necessary to aspirate, or pull back on the plunger. If blood is not aspirated back into the syringe, this ensures that the tip of the needle has not entered a blood vessel and precludes inadvertent injection of the fluid into the bloodstream and the possibility of unwanted systemic reactions.

TYPES OF LOCAL ANESTHESIA

The two basic types of local anesthetic injections are local infiltration and nerve block.

Small areas of soft tissue or bone may be anesthetized by injecting a small amount of anesthetic fluid directly into the area (Figure 11-1). The fluid diffuses through a small, localized area and blocks the terminal nerve fibers there. Most maxillary teeth may be anesthetized in this manner. Anesthetic fluid is injected deep to the vestibular fold and deposited on the periosteum of the alveolar bone overlying the root apex (supraperiosteal injection). The fluid diffuses through the bone to reach and block the terminal nerve fibers at the apical foramen of the tooth. In general, a localized tooth block may be attempted on any tooth that has a vestibular alveolar plate of bone thin enough to permit diffusion of the solution to the apical nerves.

Figure 11-1 Local infiltration of individual teeth. The needle is inserted through vestibular fold, and anesthetic fluid is deposited over root apex. Fluid percolates through the thin alveolar bone to block terminal sensory nerves passing from root apex.

Nerve Block

Larger areas and several teeth may be anesthetized by blocking a main nerve; that is, the anesthetic solution is deposited adjacent to a main peripheral nerve. This will obviously produce a greater effect because the farther proximally a nerve is blocked, the greater the area anesthetized. In the oral cavity, there are several areas where large peripheral nerves are accessible for nerve blocks.

LOCAL ANESTHESIA OF MAXILLARY STRUCTURES

Local Infiltration

Teeth

Each of the maxillary teeth may be individually anesthetized using local infiltration. The one tooth that may be a problem is the maxillary first molar, because the roots are covered by the zygomatic process of the maxilla. The anesthetic fluid does not always sufficiently diffuse through the bone, and if this is the case, a nerve block must be considered.

Injection Sites

The upper lip or the cheek is retracted to expose the vestibular fold and the root apex or root apices of the tooth involved, and this is noted as the penetration site (Figure 11-2). The needle is introduced through the vestibular mucosa with the bevel of the needle pointing toward the mucosa. The syringe is aspirated. The bevel of the needle should come to rest facing and on the periosteal surface of the vestibular alveolar bone directly overlying the root apex. The proper amount of fluid is slowly injected at the site, and the fluid percolates through the alveolar bone to reach and block the terminal branches of the maxillary nerve entering the apical foramen.

Figure 11-2 Placement of needle over the apex of maxillary right central incisor (A), maxillary right canine (B), and maxillary premolars (C) for local infiltration.

Block Anesthesia

Anterior and Middle Superior Alveolar Blocks (Infraorbital Block)

The maxillary nerve enters the infraorbital groove in the floor of the orbit and changes its name to the infraorbital nerve. The maxillary nerve passes forward, giving off the middle superior and anterior superior alveolar nerves before exiting through the infraorbital foramen as facial branches (see Figure 7-58).

Nerves Blocked

The following nerves may be blocked with a single injection: (1) the terminal facial branches of the infraorbital nerve (superior labial, nasal, and inferior palpebral), (2) the anterior superior alveolar nerve, and (3) the middle superior alveolar nerve (usually).

Injection Site

The site of injection is the mouth of the infraorbital canal (Figure 11-4). At this point, the middle superior and anterior superior alveolar nerves have branched from the main infraorbital nerve and are descending to their respective destinations. At the mouth of the canal, the terminal branches issue onto the face.

Figure 11-4 Placement of the needle in infraorbital foramen for anterior and middle superior alveolar (infraorbital) nerve blocks.

The infraorbital foramen lies on the facial aspect of the maxilla, approximately 0.5 to 1.0 cm below the midpoint of the inferior orbital margin. On a skull, the supraorbital, infraorbital, and mental foramina lie along a vertical line. In the living person, an imaginary line drawn through the palpable supraorbital foramen, pupil of the eye, and the crown of the maxillary second premolar tooth will pass through the infraorbital foramen. Immediately superior to the foramen is the origin of the levator labii superioris muscle, and below the foramen is the origin of the levator anguli oris muscle.

Complications

1. Vascular penetration. The terminal branches of the infraorbital artery and vein pass from the infraorbital foramen. Careful aspiration of the syringe during injection prevents inadvertent release of anesthetic agent into a blood vessel.

2. Intraorbital injection. Directly above the infraorbital foramen are the orbit and its contents. Inadvertent injection of anesthetic fluid into the orbit could result in temporary but unpleasant complications. The motor supply to the extraocular muscles may be blocked, causing double vision, or diplopia, or the optic nerve may be involved and result in temporary blindness.

Teeth Anesthetized

The pulps and periodontal ligaments of the maxillary central incisor, lateral incisor, canine, first premolar, and second premolar are anesthetized.

Other Tissues Anesthetized

In addition to the teeth just mentioned, the adjacent plate of labial alveolar bone, adjacent vestibular gingivae and alveolar mucosa, the skin and mucous membrane of the upper lip, the lateral aspect of the external nose, the skin and conjunctiva of the lower eyelid, and the anterior aspect of the maxillary sinus are anesthetized.

Posterior Superior Alveolar Block

The posterior superior alveolar nerve arises from the maxillary nerve just before it enters the infraorbital canal (see Figure 7-58). The nerve passes downward, along with corresponding branches of the maxillary artery, to the posterior or infratemporal surface of the maxilla.

Injection Site

The site of injection is the point where the posterior superior alveolar nerve enters the posterior superior alveolar foramen on the infratemporal surface of the maxilla (Figure 11-5). On the skull, this may be seen as several tiny foramina on the gently rounded, convex, posterior surface of the maxilla. In the living person, the area may be located by palpating the prominent zygomatic process of the maxilla in the vestibular fold. Posterior to this, the posterior surface of the maxilla may be palpated. If the mouth is opened too widely, the palpating finger is displaced by the coronoid process of the mandible and the attached temporalis tendon.

Figure 11-5 Placement of the needle for a posterior superior alveolar (PSA) nerve block.

Complications

Vascular penetration: The needle could penetrate an accompanying posterior superior alveolar artery or vein. In addition, the pterygoid plexus of veins, a rich anastomosing network situated about the lateral pterygoid muscle, is slightly posterior to the injection site and relatively easy to penetrate with the needle. Aside from inadvertent injection of anesthetic fluid into the bloodstream, the peculiar nature of this injection site makes it particularly vulnerable to widespread intertissue hemorrhage, or hematoma. The area lateral to the site is filled with loose areolar tissue, and puncture of a blood vessel fills the area with blood and produces a reddened swelling on the side of the face. This is traditionally attributed to a penetration of the pterygoid plexus of veins, but, judging by the speed with which the swelling takes place, it is more likely due to an arterial puncture. The hematoma goes through the same colorful stages as a healing black eye and is resolved in a few weeks.

Teeth Anesthetized

The pulps and periodontal ligaments of the maxillary first molar, second molar, and third molar are anesthetized. (Note: The mesiobuccal aspect of the first molar may be innervated by the middle superior alveolar nerve, and a separate injection is usually performed over the apical root of the second premolar to achieve complete anesthesia of the maxillary first molar.)

Other Tissues Anesthetized

Other tissues affected by this block are the buccal gingivae overlying the maxillary molars, the buccal plate of alveolar bone overlying the maxillary molars, and the bone and mucosa of the posterior portion of the maxillary sinus.

Nasopalatine Block

The nasopalatine nerve branches from the pterygopalatine ganglion and enters the nasal cavity through the sphenopalatine foramen (see Figures 7-60 and 7-82). It passes anteriorly and inferiorly along the nasal septum and exits onto the hard palate through the incisive foramen.

Nerves Blocked

The nasopalatine nerve branches from the pterygopalatine ganglion of the maxillary nerve. It passes medially through the sphenopalatine foramen to enter the nasal cavity, deflects downward and forward along the nasal septum and at the anterior aspect of the floor of the nose, then passes through the incisive canal to emerge on the oral aspect of the hard palate.

Injection Site

The injection site is the mouth of the incisive canal immediately posterior to the central incisor teeth (Figure 11-6). From the canal opening, branches of the nasopalatine nerve pass laterally and posteriorly. The right and left incisive canals merge on the palate as one incisive foramen. The opening on the dried skull is approximately 1.5 cm posterior to the alveolar crest between the central incisors. From this point, it funnels anteriorly behind the incisors. In the mouth, it lies in the midline immediately posterior to the incisive papilla. Topical anesthetic is used before injection. A very quick initial injection is performed just to one side of the incisive papilla. This eliminates the discomfort when the needle is subsequently placed into the incisive foramen through the incisive papilla.

Figure 11-6 Placement of the needle in incisive foramen for a nasopalatine nerve block.

Complications

1. Vascular penetration: Branches of the nasopalatine artery and vein accompany the nerve and should be avoided. Any injection in the hard palate is painful.

2. Pain on injection: The mucosa is tightly bound to the underlying bony palate, and, therefore, injections into the area should be administered slowly (see previous description concerning palatal infiltration).

Tissues Anesthetized

Because both right and left nerves emerge through a common midline opening, both nerves are anesthetized with the same injection. The tissues supplied are the palatal mucosa and lingual gingivae of the six anterior maxillary teeth and the lingual plate of alveolar bone and hard palate associated with the six anterior maxillary teeth.

Greater Palatine Block

Nerves Blocked

The greater palatine nerve arises from the pterygopalatine ganglion of the maxillary nerve. It drops through the greater palatine canal and descends to emerge on the posterior aspect of the hard palate (see Figure 7-60).

Injection Site

The injection site is the mouth of the greater palatine canal (Figure 11-7). At this point, the greater palatine nerve emerges on the hard palate between the second and third molars, about 1 cm superior to the margin of the palatal gingiva. From here the nerve passes anteriorly in a groove to supply structures of the palate. The groove and foramen may be palpated under the palatal mucosa.

Figure 11-7 Placement of the needle in greater palatine foramen for a greater palatine nerve block.

Complications

Vascular penetration: Branches of the greater palatine artery and vein accompany the nerve, and these should be avoided. Again, this is a palatal injection, and it should be administered slowly to prevent stripping of the mucosa from the underlying bone.

Tissues Anesthetized

All the palatal mucosa of the side injected and lingual gingivae posterior to the maxillary canines and the underlying bone of the hard palate would be anesthetized.

Maxillary Block

Rarely, circumstances require a block of the entire maxillary nerve for extensive surgery or when traditional local anesthesia is contraindicated because of the presence of infection.

Injection Site

The only accessible point along the route of the maxillary nerve is where the nerve passes across the roof of the pterygopalatine fossa. This location may be approached several ways. The first is a high posterior, superior alveolar block. The technique previously described is used for this block, except that the needle is carried farther superiorly to the pterygopalatine fossa. The second is a greater palatine approach. The greater palatine foramen is entered with a needle and then followed superiorly to where the canal enters the pterygopalatine fossa. The third is an extraoral approach. The needle is passed through the skin of the face, through the mandibular notch of the mandible to the pterygopalatine fossa.

Tissues Anesthetized

A maxillary block results in anesthesia of half the maxillary dental arch and supporting structures, a major portion of the nasal cavity, half the palate, and the maxillary air sinus.

Complications

Vascular penetration: These procedures are performed rarely, and for good reason. Not only are branches of the maxillary blood vessels easily encountered, but the main vessels are vulnerable as well.

LOCAL ANESTHESIA OF MANDIBULAR STRUCTURES

Local Infiltration

In the pediatric patient, all deciduous teeth may be anesthetized using local infiltration. The mandibular first molars, however, are not easily anesthetized with this method. In the adult patient, infiltration may be attempted on the mandibular incisors but is not always successful. The labial and buccal plates of bone of the mandible are relatively thick and preclude successful local infiltration. In general, nerve blocks are used to anesthetize the mandibular dentition.

Block Anesthesia

Standard Inferior Alveolar Block

The inferior alveolar nerve arises from the posterior division of the mandibular nerve and passes inferiorly laterally, and slightly anteriorly toward the midpoint of the internal aspect of the ramus of the mandible (see Figure 7-53). Here it enters the inferior alveolar canal, or mandibular canal, and travels anteriorly and medially in an arc below the roots of the mandibular teeth. It ends at the midline, as does its counterpart of the opposite side. At the level of the second premolar, the inferior alveolar nerve gives off the mental nerve, which passes through the mental foramen to emerge on the face.

Injection Site

The mouth of the mandibular canal is the site accessible for injection (Figure 11-8). It is situated on the medial aspect of the ramus of the mandible. On the dried skull, if one were to ignore the coronoid and condylar processes, the ramus is roughly rectangular, and two imaginary diagonal lines intersect approximately at the mandibular foramen. The opening is large and funnels down into the canal within the bone. The anterior border of the foramen is guarded by a variable-sized, tongue-shaped projection of bone called the lingula. An imaginary line drawn posteriorly from the molar occlusal plane would lie approximately 0.5 cm above the foramen.