Chapter 11Applied Anatomy
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.
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.
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.
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.
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.
The injection sites for palatal local infiltration are at a point midway between the gingival crest of the tooth and the midline palatal raphé (Figure 11-3). This ensures that the needle will enter a relatively thick portion of the palatal mucosa to minimize pain.
Palatal injections can be painful because the mucosa of the hard palate is closely applied to bone. More pressure must be applied to force the solution under the tissue. To offset the pain of palatal injections, a topical anesthetic is absolutely necessary. In addition, pressure can be applied to the injection site with a cotton applicator before and after the injection to minimize the discomfort.
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).
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).
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.
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.
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.
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.
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.
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.
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.)
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
In the mouth, location is rather difficult. Of all the routine injections attempted in the dental office, the inferior alveolar block is perhaps the most difficult to perform effectively. There is a higher degree of failure, and this is usually attributable to not following accepted techniques. There are, of course, individual variations in the locations of the mandibular foramen and its contents. One time-honored method of locating the area is to successively palpate a number of intraoral structures. The external oblique line is palpated in the buccal vestibule and followed posteriorly to where if ascends as the sharp anterior border of the ramus. This concavity is called by clinicians the coronoid notch. The finger is moved medially to engage the temporal crest (internal oblique ridge), and it remains in this position. The finger is now in the retromolar fossa with the fingernail pointing backward. A line is sighted from between the two premolar occlusal surfaces of the opposite side to the midpoint of the fingernail. The line continued posteriorly ends just above the mandibular foramen, at the point where the inferior alveolar nerve enters the canal.
Inferior and Medial. A number of structures run laterally and inferiorly from the base of the skull to the ramus of the mandible. The inferior alveolar nerve and vessels pass from between the medial and lateral pterygoid muscles to run downward and laterally along the medial pterygoid muscle to the mandibular foramen. The needle tip lies just above and lateral to these structures as they enter the foramen. The medial pterygoid muscle continues down to the internal aspect of the angle. The sphenomandibular ligament extends from the spine of the sphenoid laterally and inferiorly to attach to the lingula. The needle tip should rest just above this attachment.
Anteriorly. The lingual nerve, at the level of the injection site, lies about 1 cm anteriorly and approximately 0.5 cm deeper than the inferior alveolar nerve. Use is made of this relationship for simultaneous injections for inferior alveolar and lingual nerve blocks.
Posteriorly. The encapsulated parotid gland lies posteriorly. The fibrous capsule of the deep portion of the gland attaches to the styloid process medially and the posterior border of the ramus of the mandible laterally. The capsule and gland balloon forward into the infratemporal region.
If the needle is directed too far posteriorly, it could penetrate the parotid gland capsule. Anesthetic fluid injected within the capsule quickly diffuses through the glandular tissue and anesthetizes the five main branches of the facial nerve contained within the gland. This results in a facial paralysis of the affected side, but the effects, fortunately, are transient.
The mental branch of the inferior alveolar nerve, however, does supply soft tissues: the skin and mucous membrane of the lower lip (from the mental foramen anteriorly to the midline), labial alveolar mucosa and gingivae, and skin of the chin.
The injection site for the Gow-Gates mandibular nerve block is considerably higher than that of the standard inferior alveolar. The needle tip in this method is placed in the infratemporal region below the insertion of the lateral pterygoid muscle at the anterior aspect of the condylar neck (Figure 11-10).
The injection parallels an external line extending from the intertragal (incisural) notch of the ear to the angle of the mouth. The barrel of the syringe rests over the contralateral canine or the contralateral premolars if the mandibular rami are widely divergent. The needle is inserted through the mucosa distal to the maxillary second molar at the height of its mesiolingual cusp and with the opposite finger lined between the intertragal notch and angle of the mouth as a visual guide, the needle is advanced until the condylar neck is reached (a depth of about 2.5 cm). The needle is retracted slightly and aspirated to prevent an intravascular injection.
Unlike the standard technique described a moment ago, the tip of the needle is remote from the main branches of the mandibular nerve. The Gow-Gates or high mandibular technique relies on a relatively large volume of anesthetic fluid to work. It is released into the infratemporal or pterygomandibular space and the patient is required to keep the mouth open for at least 30 seconds. This enables the fluid to permeate the area and flow around the nerves. The technique is purportedly more reliable and more comfortable than the standard inferior alveolar nerve block.
It is extremely important that careful aspiration be performed before release of the fluid. The needle ends up in the middle of the pterygoid plexus territory. In addition, the maxillary artery is close and may be punctured. In some cases, the maxillary artery lies superior to the lateral pterygoid muscle, making the artery even more vulnerable.
Unlike the standard inferior alveolar block, the buccal and lingual nerves are concurrently anesthetized, obviating separate nerve blocks if anesthesia of these nerves is required as well. The auriculotemporal nerve is also affected but has no bearing on regular dental procedures. The patient should be warned, however, of numbness on the side of the head, because this technique may be new to some patients who have only experienced the standard inferior alveolar block.
Soft tissues anesthetized include (1) the chin, lower lip, and mandibular labial gingiva supplied by the inferior alveolar nerve via the mental branch; (2) the ipsilateral side of the tongue, floor of mouth, and mandibular gingiva via the lingual nerve; and (3) the ipsilateral cheek and mandibular buccal mucosa via the long buccal nerve.
The injection site for this technique lies within the infratemporal or pterygomandibular space but between the site for the standard inferior alveolar block and the Gow-Gates mandibular block (Figure 11-11). It is a preferred technique when opening the mouth is either painful or limited.
The patient is asked to maintain the teeth in gentle occlusion, and the cheek is retracted and the injection site is swabbed with topical anesthetic. The needle is advanced posteriorly through the maxillary vestibule, keeping the barrel of the syringe parallel to the maxillary occlusal plane and at the height of the mucogingival junction of the maxillary buccal segment. The needle is further advanced below the zygomatic process of the maxilla to pierce the mucosa of the retromolar fossa and penetrate to a depth of about 2.5 cm. After aspiration, the appropriate volume of fluid is deposited.