Anatomic Considerations for Local Anesthesia Administration

From an anterior view, each frontal process of the maxilla articulates with the frontal bone. The zygomatic process of the maxilla forms the medial part of the infraorbital (IO) rim with the maxillary process of the zygomatic bone providing the lateral part on its anterior surface (Figures 10-2 and 10-3). Clinicians can palpate a “notch” or depression in the midpoint of the IO rim created by the more vertical zygomaticomaxillary suture located between the two bones, the maxilla and the zygomatic bone. Each maxilla’s orbital surface is separated from the sphenoid bone by the inferior orbital fissure (Table 10-1; Figure 10-1). The inferior orbital fissure carries the IO nerve, zygomatic nerve, IO artery, and inferior ophthalmic vein (see later discussion). The groove in the floor of the orbital surface is the IO sulcus.

The IO sulcus becomes the IO canal and then terminates on the facial surface of each maxilla as the IO foramen (see Figures 10-3 and 10-19 and Table 10-1). The IO foramen is located approximately 10 mm inferior to the midpoint of the IO rim, and is in a linear relationship on the ipsilateral (or same) side of the face with the more superior supraorbital notch of supraorbital rim as well as the pupil of the eye and corner of the mouth (see Figure 12-9).

The IO foramen transmits the IO nerve and blood vessels (discussed later). Palpation of the IO foramen will cause transient soreness to the area in a patient due to the presence of the nerve (see Figure 12-8). The opening of the IO foramen is a landmark for the administration of the IO block (see Chapter 12). Inferior to the IO foramen on each maxilla is an elongated depression, the canine fossa. The canine fossa is just posterosuperior to each of the roots of the maxillary canines.

Also present is a facial and palatal cortical plate, which is part of the alveolar process of the maxilla. The root of each tooth of the maxillary arch is covered by a prominent facial ridge of bone, another part of the alveolar process of the maxilla (Table 10-2; Figures 10-3 and 10-19). The facial ridge over each of the roots of the maxillary canines, the canine eminence is especially prominent, which is a landmark for the administration of the anterior superior alveolar (ASA) block (see Chapter 12).

The alveolar process of the maxilla usually contains the roots of the maxillary teeth within the alveoli (or tooth sockets) (see Figures 10-2 and 10-4 and Table 10-2). The apices of these roots between both the facial and palatal cortical plates of the alveolar process of the maxilla are landmarks for the administration for the majority of the maxillary injections (see Chapter 12). The alveolar process of the maxilla can become resorbed in a patient who is completely edentulous in the maxillary arch; resorption occurs to a lesser extent in partially edentulous cases. However, the more superiorly placed body of the maxilla is not resorbed with tooth loss, but its walls may become thinner in this case.

In general, the alveolar process of the maxillary teeth is less dense and more porous than the alveolar process of similar mandibular teeth as demonstrated on a panoramic radiograph (see Figure 10-2). These differences in bone density allow a greater incidence of clinically effective local anesthesia for the maxillary arch when the local anesthetic agent is administered as a supraperiosteal injection or local infiltration than would occur with similar teeth on the mandibular arch (see Chapters 12 and 13).

From the lateral view, each zygomatic process of the maxilla articulates with the zygomatic bone and its maxillary process laterally, completing the medial part of the IO rim as was discussed earlier (Figures 10-2 and 10-3 and Table 10-2). On the posterior part of the body of the maxilla is a rounded, roughened elevation, the maxillary tuberosity, just posterior to the most distal maxillary molar (Figures 10-2, 10-3, and 10-4).

Posterosuperior on the maxillary tuberosity are the posterior superior alveolar (PSA) foramina that perforate the infratemporal surface of the maxilla multiple times. These foramina are where the PSA nerve branches and blood vessel branches enter the bone from the posterior and then open up onto the infratemporal surface of the maxilla. Both the maxillary tuberosity and the openings of the PSA foramina are landmarks for the administration of the PSA block (see Chapter 12). The maxillary tuberosity also serves as a landmark for the administration of the Vazirani-Akinosi mandibular (V-A) block (see Chapter 13).

From an inferior view, each palatine process of the maxilla articulates with the other to form the anterior hard palate (Figure 10-4; see Figure 10-2 and Table 10-2). A suture is noted between these two palatine processes of the maxilla is the anterior part of the median palatine suture (see Table 10-3). In the patient, this suture is covered by the median palatine raphe, a midline tendinous band of tissue that serves as a landmark for the administration for both the greater palatine (GP) block and anterior middle superior alveolar (AMSA) block (see Chapter 12). In addition, on the inferior surface there are a number of small pores in the maxilla of the anterior hard palate that will allow for the diffusion of local anesthetic agent during the administration of the AMSA block.

In the anterior midline between the two articulating palatine processes of the maxillae, just palatal to the maxillary central incisors, is the incisive foramen (see Table 10-1 and Figure 10-20). This foramen carries branches of both the right and left nasopalatine nerves as well as branches of the sphenopalatine artery from the nasal cavity to the anterior hard palate (see later discussion). The incisive papilla is the soft tissue that bulges over the opening of the incisive foramen. Both the incisive foramen and its incisive papilla are landmarks for the administration of the nasopalatine (NP) block (see Chapter 12).

The palatine bones serve as a link between the maxillae and the sphenoid bone with which they articulate, in addition to articulating with each other. The two horizontal plates articulate with each other at the posterior part of the median palatine suture underlying the median palatine raphe as discussed earlier, and more anteriorly with the palatine processes of the maxillae at the transverse palatine suture (see Figure 10-4 and Table 10-3).

There are two main foramina in each of the palatine bones, the GP and lesser palatine (LP) foramina (Figure 10-6; see Figure 10-20 and Table 10-1 [discussed later]). The larger GP foramen is located in the posterolateral region of each horizontal plate of the palatine bones, usually superior to the apices of the maxillary second (in children) or third molars (in adults). The GP foramen is approximately 10 mm medial and directly superior to the palatal gingival margin. The GP foramen is also midway between the median palatine raphe overlying the median palatine suture and the palatal gingival margin of the maxillary molar. The GP foramen transmits the GP nerve and blood vessels, and its opening is a landmark for the administration of the GP block (see Chapter 12).

The smaller opening nearby, the LP foramen, transmits the LP nerve and blood vessels to the soft palate and tonsils. Both foramina are openings of the pterygopalatine canal that carries the descending palatine nerves and blood vessels from the pterygopalatine fossa to the palate; the pterygopalatine fossa is inferior to the infratemporal fossa.

From an anterior view of the mandible, there are prominent landmarks to note (Figures 10-9 and 10-11). The mental protuberance, the bony prominence of the chin, is located inferior to the roots of the mandibular incisors. In the midline on the anterior surface of the mandible is a faint ridge, an indication of the mandibular symphysis, where the mandibular bone was formed by the fusion of right and left processes.

Farther posteriorly on the lateral surface of the mandible is the opening of the mental foramen, which is usually inferior to the apices of the mandibular premolars (see Figures 10-2, 10-9, 10-10, and 10-25, and Table 10-1). As mandibular growth proceeds in children, the mental foramen alters in direction from anterior to posterosuperior. The mental foramen allows the entry of the mental nerve and blood vessels into the mandibular canal to merge with the incisive nerve and blood vessels (discussed later).

The mental foramen’s posterosuperior opening in adults signifies the changed direction of the emerging mental nerve. This opening of the mental foramen onto the bony lateral surface of the mandible is an important landmark to note intraorally and on a radiograph before administration of both the mental and incisive blocks (see Chapter 13 and see Table 13-6, Figure L). Studies show that the mental foramen can be as far posterior as the apex of the mandibular first molar or as far anterior as the apex of the mandibular canine. Palpation of the mental foramen before an administration of a local anesthetic agent will cause transient soreness to the area in a patient due to the presence of the nerve.

The heavy horizontal part of the lower jaw inferior to the mental foramen is the body of the mandible (see Figures 10-9 and 10-10). Superior to this, the part of the lower jaw that usually contains the roots of the mandibular teeth within the alveoli is the alveolar process of the mandible (see Table 10-2). Also present is a facial and lingual cortical plate, which is also part of the alveolar process of the mandible. During growth of the body, both the body of the mandible and alveolar process of the mandible elongate posterior to the mental foramen, providing space for the additional permanent teeth.

The alveolar process of the mandible can become resorbed if a patient becomes completely edentulous in the mandibular arch; resorption occurs to a lesser extent in partially edentulous cases. This resorption can occur to such an extent that the mental foramen is virtually on the superior border of the mandible, instead of opening on the lateral surface, changing its relative position. However, the more inferiorly placed body of the mandible is not affected and remains thick and rounded. This resorption can possibly alter the landmarks when administering both the mental or incisive blocks as well as the inferior alveolar (IA) block (see Chapter 13).

In general, the alveolar process of the mandibular anterior teeth is also less dense and more porous than the alveolar process of the mandibular posterior teeth as demonstrated with a panoramic radiograph (see Figure 10-8). These differences in bone density allow a supraperiosteal injection or local infiltration of the mandibular anterior teeth by a local anesthetic agent to have greater incidence of clinically effective local anesthesia than the mandibular posterior teeth but always with less clinically effective overall anesthesia than the maxillary teeth (see Chapters 12 and 13).

On the lateral surface of the mandible, the stout flat plate of the mandibular ramus extends superiorly and posteriorly from the body of the mandible on each side (see Figure 10-10). Each mandibular ramus, which serves as the primary area for the attachment of the muscles of mastication; it grows even more superiorly and posteriorly, displacing the mental protuberance of the chin inferiorly and anteriorly when nearing adulthood.

The anterior border of the mandibular ramus is a thin sharp margin that terminates in the coronoid process (see Figures 10-11 and 10-12 and Table 10-2). The main part of the anterior border of the mandibular ramus forms a concave anterior curve, the coronoid notch. This notch is the greatest depression on the anterior border of the mandibular ramus. The anterior border of the mandibular ramus is a landmark for the administration of the buccal block and the coronoid notch is a landmark for the administration of the IA block (see Chapter 13).

Inferior to the coronoid notch, the anterior border of the mandibular ramus becomes the external oblique line. The external oblique line or ridge is a crest where the mandibular ramus joins the body of the mandible. The external oblique line is noted as a radiopaque line on a radiograph superior to the mylohyoid line (discussed next); clinicians may also palpate this line intraorally to help locate the coronoid notch (see Figure 10-8).

The posterior border of the mandibular ramus is thickened and extends from the angle of the mandible to a projection, the mandibular condyle with its neck (see Figures 10-7 to 10-11 and Figure 10-12). The anteromedial border of the neck of the mandibular condylar is a landmark for the administration of the Gow-Gates mandibular (G-G) block (see Chapter 13). The articulating surface of the condyle is an oval head involved in the TMJ. Between the coronoid process and the condyle is a deep concavity, the mandibular notch.

Visible on the medial surface of the mandible are the body of the mandible, alveolar process of the mandible, and the mandibular ramus (see Table 10-2 and Figure 10-12). The medial surface of the mandibular ramus is a landmark for the administration of the IA block as well as the V-A block (see Chapter 13).

At the lateral edge of each alveolar process of the mandible is a rounded, roughened area, the retromolar triangle, just posterior to the most distal mandibular molar and that, when covered with soft tissue, is the retromolar pad. The retromolar triangle is a bony landmark for the administration of the buccal block (see Chapter 13).

Along each medial surface of the body of the mandible is the mylohyoid line or internal oblique ridge that extends posteriorly and superiorly, becoming more prominent as it moves superiorly on the body of the mandible. The mylohyoid line is the point of attachment of the mylohyoid muscle that forms the floor of the mouth. The posterior border of the mylohyoid line also provides for attachment of the pterygomandibular raphe. The roots of the mandibular posterior teeth often extend internally inferior to the mylohyoid line. The mylohyoid line can be noted on a radiograph as the radiopaque line inferior to the external oblique line (see Figure 10-8).

The pterygomandibular raphe is noted in the oral cavity as the pterygomandibular fold, which is a tendinous band located posterior to the most distal mandibular molar as it spans the area between the mandible and the point at which the hard and soft palates meet. The pterygomandibular fold is a landmark for the administration of the IA block (see Chapter 13).

On the medial surface of the ramus is the opening of the mandibular canal, the mandibular foramen (see Figure 10-12 and Table 10-1). Most skulls show the mandibular foramen position as approximately two-thirds to three-fourths the distance from the coronoid notch to the posterior border of the mandibular ramus; this variance can be noted in reference textbooks depending on their currency. The opening of the mandibular foramen is a landmark for the administration of the IA block as well as the V-A block (see Chapter 13).

The IA nerve and blood vessels exit the mandible through the mandibular foramen after traveling in the mandibular canal after the merging of the mental and incisive nerves and blood vessels (see discussion later). The incisive nerve was originally located within the mandibular incisive canal, which is an anterior continuation of the mandibular canal bilaterally between the two mental foramina.

With age and tooth loss, the alveolar process of the mandible is absorbed as discussed earlier so that the mandibular canal is nearer the superior border. Sometimes with excessive alveolar process absorption, the mandibular canal disappears entirely and leaves the inferior alveolar nerve without its bony protection, although it is still covered by soft tissue. The height of the mandibular foramen can also seem to be more superior without the presence of teeth in the mandibular posterior sextant.

Rarely, a patient may have a bifid inferior alveolar nerve, in which case a second mandibular foramen more inferiorly placed exists and can be detected by noting a doubled mandibular canal on a radiograph. Keeping this anatomic variant of the mandibular foramen in mind is important when administering an IA block as well as any changes due to bony resorption (see Chapter 13).

Overhanging the mandibular foramen is a bony spine, the lingula, which serves as an attachment for the sphenomandibular ligament associated with the TMJ. The lingula is also a landmark for the IA block. A small groove, the mylohyoid groove, passes anterior to and inferior from the mandibular foramen. The mylohyoid nerve and blood vessels travel in the mylohyoid groove (discussed later), which in some cases can have an unusual pathway that may cause difficulty with the administration of an IA block (see Chapter 13).

The maxillary nerve is a nerve trunk formed within the pterygopalatine fossa by the convergence of many nerves; the largest contributor is the IO nerve. Tributaries of the IO nerve or maxillary nerve trunk include the zygomatic, anterior, middle and posterior superior alveolar, greater and lesser palatine, and nasopalatine nerves.

After all these branches come together within the pterygopalatine fossa to form the maxillary nerve, the nerve enters the skull through the foramen rotundum of the sphenoid bone. Small afferent meningeal branches from parts of the dura mater join the maxillary nerve as it enters the trigeminal ganglion.

A few small branches that arise from the V₃ trunk or mandibular nerve before its separation into anterior and posterior trunks and include the meningeal branches, which are afferent nerves (see Chapter 2) for parts of the dura mater (see Figure 10-26). In addition, there are muscular branches from the undivided mandibular nerve that serve as efferent nerves.

The anterior trunk of the mandibular nerve is formed by the merger of the (long) buccal nerve and additional muscular nerve branches (Figure 10-24). The posterior trunk of the mandibular nerve is formed by the merger of the auriculotemporal, lingual, and IA nerves (see Figure 10-14).

The pterygoid plexus of veins drains the veins from the deep parts of the face and then drains into the maxillary vein (discussed next) (see Figure 10-32). The middle meningeal vein also drains the blood from the dura mater of the meninges of the brain into the pterygoid plexus of veins.

The pterygoid plexus of veins also drains the PSA vein, which is formed by the merging of its dental and alveolar branches. The dental branches of the PSA vein drain the pulp of the maxillary teeth by way of each tooth’s apical foramen. The alveolar branches of the PSA vein drain the associated periodontium and gingiva of the maxillary teeth.

The IA vein forms from the merging of its dental branches, alveolar branches, and mental branches in the mandible, where they also drain into the pterygoid plexus of veins. The dental branches of the IA vein drain the pulp of the teeth by way of each tooth’s apical foramen. The alveolar branches of the IA vein drain the associated periodontium and gingiva of the mandibular teeth.

Some parts of the pterygoid plexus of veins are near the maxillary tuberosity, reflecting the drainage of dental tissue into the vascular plexus. Due to its location there is a possibility of piercing the pterygoid plexus of veins or nearby maxillary artery when a PSA block is administered incorrectly with the needle being overinserted (see Chapter 12). When the pterygoid plexus of veins or even the nearby maxillary artery is pierced as in this situation, a small amount of the blood escapes and enters the tissue, causing tissue tenderness, swelling, and the discoloration of a hematoma or bruise (see Chapter 15 and Figure 15-2).

A spread of infection along the needle tract deep into the tissue can also occur when the PSA block is incorrectly administered with a contaminated needle. This may involve a serious spread of infection into the pterygoid plexus of veins and then on to the cavernous sinus, a venous sinus deep within the skull (see Figure 10-32). Each cavernous sinus is located on the lateral surfaces of the body of the sphenoid bone. This may result in cavernous venous thrombosis (see Chapter 15).