Maxillary Nerve Block:
The Pterygopalitine Canal Approach

Over the past few years, the dental industry has introduced a number of new local anesthetic delivery systems. Despite the advantages of many of these newer techniques, conventional needle and syringe procedures remain the cornerstone of local anesthesia for most dental practitioners. Most of these "tried and true" approaches are both popular and comfortable for the dentist since they are widely taught in undergraduate dental curriculum. There is one technique, however, that although relatively safe, effective and reliable, is not usually included in undergraduate dental programs. This technique, the maxillary nerve block via the pterygopalitine canal, provides the patient with a profound block of nerve V2 and allows the dentist the ability to perform procedures anywhere in the maxillary quadrant that has been anesthetized.

Regional block anesthesia is very common in dentistry -- primarily in the mandible. The mandibular block is an essential procedure to anesthetize the lower dentition due to the thick cortical plate of bone. It is this bony plate that, in most cases, inhibits the ability of infiltrated local anesthetics to diffuse to the root apicies. Although true for mandibular molars, the anatomy of the maxilla is different in that a thinner cortical plate will allow infiltration anesthesia in most situations. Despite this fact, the ability to block the maxillary nerve has many advantages.

The literature describes three techniques to block the maxillary nerve; one extraoral and two intraoral approaches. The extraoral approach, which is described in detail by Nish et al, will not be discussed here because it is not a common or practical procedure in most dental offices. ^1,2,3

Intraorally, the two techniques to block the maxillary nerve are the high tuberosity approach (similar to the posterior superior alveolar nerve block) and the greater palatine canal approach. Although less predictable and prone to more complications, the high tuberosity approach may be the more comfortable procedure for the dentist because it is essentially a high buccal infiltration. The goal of this technique is to direct the needle superiorly, medially and posteriorly along the zygomatic and infratemporal surfaces of the maxilla to enter the pterygopalatine fossa. The depth of needle insertion is judged by measuring the distance from the gingival crest of the maxillary premolar region to the infraorbital ridge approximating the foramen on the face. This depth can be anywhere from 24 to 44 mm.⁴

Complications associated with this technique include the lack of profound anesthesia either due to inadequate volume of local anesthetic (it may take more than one cartridge of solution to obtain complete anesthesia) or improper positioning and inadequate depth of penetration. The pterygoid plexus of veins is in close proximity to the area approached with this technique and there is a relatively high risk of hematoma.

In view of the disadvantages associated with the extraoral and high tuberosity maxillary nerve blocks, this paper will focus on the greater palatine canal approach as the most predictable, reliable and safest means of obtaining profound maxillary anesthesia.

The maxillary nerve block via the greater palatine canal was first described in 1917 by Mendel.⁵ Despite this technique's longevity however, it is not used frequently. This is perhaps due to the fact that it is not taught as a routine approach in a significant number of dental facilities and because the need to block the hemimaxilla does not arise as frequently as the need to block the mandible.

Advantages

As with any local anesthetic technique, there are advantages and disadvantages. The advantages of the maxillary nerve block include the ability to do quadrant dentistry without the need for multiple injections. The presence of oral infections may not allow a practitioner to infiltrate either because the infection is in the area to be injected or because of the difficulty anesthetizing a tooth with an apical abscess.6 In these cases, a maxillary block via the palatine canal may bypass the area of infection and thus not disseminate an infection by injecting in or around the involved areas.⁶ This approach is also useful for sinus procedures, and for the diagnosis of chronic maxillofacial pain syndromes. As a final advantage, one must consider that this technique has a greater than 95 percent success rate, with minimal risk.⁷

Disadvantages

The disadvantages and contraindications with this technique are few, but important. As with any block, hemostasis will not occur in the proximal areas of operation. One must infiltrate in order to obtain hemostasis. If there is infection in the area of the palatine canal, this technique cannot be used. There are also some anatomical limitations. Difficulty negotiating the canal with the needle tip may occur due to bony exostosis that can preside on the anterior pterygoid plate which compromises the posterior border of the pterygopalatine canal. Approximately 5 percent of the population have been shown to have tortuous canals that impede the needle tip.⁸ Also, in a study done by Westmoreland and Blanton, 300 skulls were examined and of those, 53 (18 percent) were found to have a relatively horizontal course with respect to the palatal anatomy.⁹ This horizontal opening would disallow the passage of the needle in a relatively superior orientation.

Anatomical Considerations

Figure 1. Anatomy of the maxillary nerve.

Any technique of local anesthesia must be based on a thorough and sound knowledge of the anatomy of the area to be injected and anesthetized (Figure 1).¹⁰

The second division of the trigeminal nerve arises from the gasserian ganglion in the medial cranial fossa and exits the skull via the foramen rotundum. The nerve then traverses the superior aspect of the pterygopalatine fossa, where it divides into three major branches: the pterygopalatine nerve, the infraorbital nerve, and the zygomatic nerve.

The Pterygopalatine Nerve

The main portion of the pterygopalatine nerve passes down the ptergyopalatine canal and exits toward the oral cavity at the greater palatine foramen as the anterior palatine nerve. It is the greater palatine foramen and the pterygopalatine canal that are traversed to achieve the second division nerve block.

The pterygopalatine or sphenopalatine ganglion is attached to the medial side of this nerve. However, none of the fibers of the pterygopalatine nerve synapse there. The nasal branches are emitted and the medial branches traverse the septum, ending as the nasopalatine nerve which exits at the incisive foramen.

The Infraorbital Nerve

The main trunk of the maxillary nerve continues through the pterygopalatine fossa and gives rise to the posterior superior alveolar nerve, which divides into an external gingival branch and the internal dental branch. The main nerve trunk then enters the inferior orbital fissure and travels through the floor of the orbit in the infraorbital canal. While in this canal, it emits the middle superior and anterior superior alveolar nerves, which sometimes exist as a nerve plexus. These nerves travel along the lateral sinus wall to the premolar and anterior teeth, respectively, and send sensory branches to the maxillary sinus mucosa. In approximately 30 percent of the population, the middle superior alveolar nerve does not occur and, if this is the case, its areas of innervation are assumed by the posterior and anterior alveolar nerves.¹¹ The infraorbital nerve exits the maxilla at the infraorbital foramen and divides into three terminal sensory branches -- the superior labial nerve to the upper lip, the lateral nasal nerve to the side of the nose, and the inferior palpebral nerve to the lower eyelid.

The Zygomatic Nerve

This branch emerges from the infraorbital nerve and has two divisions of its own. One communicates with the postganglionic parasympathetic fibers from the pterygopalatine ganglion to the lacrimal gland. The other exits the body of the zygoma and the zygomatic orbital foramen and supplies sensation to the malar eminence.

Anatomically, once the greater palatine foramen is located, a number of structures will be approximated by the needle tip. As the needle advances, it will transcend the greater palatine fossa and the associated neurovascular bundles. Although damage to these structures can occur theoretically, the incidence of such damage appears to be low. The incidence of positive aspiration has been reported to be only 8 percent and traumatic injury to the nerve less than 1 percent.¹² The reasons for these low percentages are theorized to be due to two factors. One, the practitioner is advancing the needle virtually parallel to these neurovascular structures and therefore puncturing them is unlikely. Two, the tough perineurial fibrous sheath may offer some protection from the needle tip.1 If a dentist does enter a blood vessel, it is unlikely that it will be an artery. If the needle tip does in fact approximate an artery, the patient feels pain and moves their head away as a protective response, the artery goes into spasm, and the smooth muscle wall of the vessel constricts. This is evidenced by blanching on the ipsilateral side of the patient's face. Therefore, in most cases, a practitioner must be rather aggressive to actually enter an artery. If, however, one punctures a vein, it is possible for a hematoma to occur. This extravasation will be limited to the pterygopalatine fossa and not be clinically evident.

Hard and Soft Tissue Landmarks

Figure 2. Anatomy of the hard palate.

In most cases, the challenge of this technique centers around locating the greater palatine canal and being familiar with the anatomy of the palate (Figure 2). With practice, finding the canal entrance and orienting the needle correctly with respect to the midsagittal plane becomes easier. To maximize one's ability to locate the greater palatine canal, there are five landmarking steps. These are as follows:

1. Using the dentition, providing the patient has a full complement of teeth in that arch, the greater palatine canal is located between the middle portion of the third molar and the middle portion of the second molar 84 percent of the time. It is located mesial to the mid portion of the second molar 10 percent of the time and distal to the mid portion of the third molar in 6 percent of the population.⁹ If there is not a complete dentition in the arch on the ipsilateral side of the injection, these landmarks should be adjusted.

2. The greater palatine foramen is located anterior to the junction of the hard and soft palate. This junction is seen as a color change such that the tissue overlying the soft palate is darker pink than the tissue overlying the hard palate. This border can also be palpated using the dental mirror. The foramen can occur anywhere from 1.8 to 12mm anterior to this border with an average distance of 7mm.^{9, 13}

Figure 3. Depth of the palatal vault.

3. The greater palatine canal is usually located at the junction of the horizontal and vertical bony plates of the hard palate. In patients who have a very deep palatal vault, the canal opening will appear closer to the dentition. Conversely, in patients with a more shallow palatal vault, the foramen will appear closer to the sagittal midline (Figure 3).

Figure 4. Location of greater palatine canal.

4. The greater palatine canal is located along an imaginary line from the ipsilateral hamular process to the ipsilateral cingulum of the lateral incisor (unpublished data) (Figure 4).

5. The soft tissue depression which covers the greater palatine canal is a blanched or whitish area overlying serous and mucus glands. This lighter colored tissue can also be used to locate the canal.

The lessor palatine canal is located posterior to the greater palatine canal on the posterior aspect of the hard palate. This canal is of no clinical significance to this technique since it is not possible to advance the needle into this foramen due to its extremely small size and tortuous path (see Figure 2).

Armamentarium and Technique Modifications

There are modifications which the authors believe will assist in the success of this block. For patient comfort, it is absolutely essential to do a preliminary infiltration in the region of the greater palatine foramen. This, in essence, now becomes a two-injection technique.

The first syringe is used for the preliminary injection and can be done with a 30 gauge short needle with a solution containing 1:100,000 epinephrine. The rationale for this is twofold. First, the 30 gauge needle tip will, in fact, having lined up all the landmarks, be very close to the greater palatine foramen, and, with its small needle gauge, disallow significant backflow from the small puncture. Second, as vascularity is encountered during the preparatory injection, the 1:100,000 epinephrine solution may minimize extravasation submucosally or onto the surface of the hard palate. The suggested volume is 0.5 cc.

The orientation of the pterygopalatine canal is superior and posterior. The angle that the canal creates with the horizontal hard palate can range from 20 to 70 degrees. However, 75 percent of the population have been found to have this angle occurring between 37 to 57 degrees.¹³

Figure 5. Bent needle.

Therefore, the second syringe can be prepared with a bend of 45 degree at the hub, utilizing a 25 gauge or 27 gauge needle with the bevel facing posteriorly, towards the anterior border of the pterygoid plate (Figure 5). In this way, the needle bevel can 'ski' off the above-mentioned surface and advance upwards through the pterygo-maxillary fissure.

Figure 6. "Pen" or "Dart" grip.

Another technique modification which the operator may wish to attempt is the "dart or pen grip". This involves holding the syringe as a pen (Figure 6). The advantage of this positioning is that it allows the dentist an extra degree of motion such that one is able to roll the syringe barrel between the thumb and forefinger. This may help to maneuver the needle tip inside the palatine canal, especially if it touches bone before the final depth of insertion. The dentist must never push in order to advance the needle if this occurs. If one encounters bone before the final depth of insertion, one should pull back 1 mm, re-angulate and try to gently redirect the needle tip. No force should be required for the advancement. With too much force, it is possible to penetrate bone and deposit local anesthetic into the nasopharynx. If this occurs, the patient will complain of a bitter taste in their throat. As well, the practitioner will be aware that they have entered the nasopharynx by aspirating and observing only air being withdrawn into the cartridge.

Clinical Procedure
The patient must be asked to lift their chin off their chest and to open wide. If this does not occur, it will be extremely difficult to visualize the injection site and the mandibular dentition will impede the barrel of the syringe.

Figure 7. Hard and soft palate junction.

The practitioner will use the five landmarks previously described in order to pinpoint an area of injection. Assuming there is no gag reflex, place the mouth mirror at the junction of the hard and soft palate and observe that the greater palatine canal will occur an average of 7 mm anterior to this (Figure 7). If there is a full dental arch present, the foramen will be located

Figure 8. Location of canal.

between the distal marginal ridge of the second molar and the middle aspect of the third molar approximately 85 percent of the time. Next, line up the ipsilateral hamular process and lateral incisor and observe the junction of the vertical and horizontal bony plates of the palate. Finally, look for a whiter depression of soft tissue beneath which lies the opening of the greater palatine canal (Figure 8).

Now that a target zone has been established, the practitioner can administer the preparatory injection directly into this area. A local anesthetic with a vasoconstrictor concentration of 1:100,000 should be used for this injection. This is absolutely essential in order to administer a painless maxillary nerve block. To help the patient experience less discomfort from the preparatory injection, a topical anesthetic, which must be isolated from the tongue, should be used. Counterpressure from the mirror would be beneficial as well. One must ensure that this injection is given 3-5 mm submucosally so that the local anesthetic solution, in an approximate dose of 0.5 cc, does not leak back into the oral cavity. Allow two to three minutes before proceeding with the next injection during which time the second needle and syringe can be prepared.

Figure 9. Angular direction of canal.

With a 25 or 27 gauge needle bent at 45 degrees and the bevel facing posteriorly, the procedure can begin. The length of the needle should be parallel to the mid-sagittal plane and using the needle tip as a probe, the foramen is located (Figure 9). Gently rotate the needle until it falls up into the canal without any resistance. If bony resistance is encountered, pull back 1 mm and gently try to tease the needle tip around this. Advance in a superior and posterior direction to the final depth as measured from the gingival crest in the maxillary bicuspid region to the infraorbital foramen. This is almost the same as the depth measurement for the high tuberosity approach previously mentioned. It can be anywhere from 24-44 mm with most adults falling into the 25-30 mm range (Figure 10 & 11.) If a patient is found to have a distance of 30 mm from the gingival crest in the bicuspid region to the infraorbital foramen we must visualize 5 mm of needle remaining outside of tissue once we have reached our final depth.

Figure 10. Depth of injection.

Figure 11. Depth of injection.

Long needles are generally 35 mm in length. If the dentist encounters an obstruction that is not negotiable or if the patient has a very long face (a depth of 44 mm in the extreme), then the final depth will not be obtainable. This may not matter since it has been suggested that if the practitioner is within 15 mm of the pterygopalatine fossa and a full cartridge of solution is injected, success should occur.¹²

When the final depth has been achieved, aspirate and slowly inject a full cartridge of either a plain local anesthetic solution or one with a vasoconstrictor concentration of 1:200,000 or 1:100,000 epinephrine. It should take 30-45 seconds to inject the full cartridge and the patient should be instructed that they will feel slight pressure in the middle of their face.
The time of onset for the block to occur is 3-10 minutes and, with a 1:200,000 solution, the duration of anesthesia is 1.5 - 2 hours for hard tissue and 2.5 hours for soft tissue.

Complications

The complications of positive aspiration, hematoma and neural damage have been discussed, as has the inability to introduce the needle the entire depth into the canal.

Diplopia of the ipsilateral eye may occur 35.6% of the time.12 This results from the local anesthetic diffusing superiorly and medially to anesthetize the orbital nerves. The patient must be assured that this phenomenon is transient. There are no known reports in the literature of permanent diplopia.¹⁴

Depositing anesthetic solution into the nasopharynx can occur if too much pressure is exerted on advancement of the needle. In this case, the posterior wall of the canal can be perforated. If a dentist enters the tissue of the palate too far distally with the initial needle puncture it is possible to miss the hard palate completely and advance up into the soft palate. The needle advances very easily in this area, and one may have the false sense that they are in the pterygopalatine canal. When solution is deposited in this case, it again is deposited into the nasopharynx and the patient will complain of a bitter taste and may cough. No block will occur in this situation.

Conclusion

The maxillary nerve block in the greater palatine canal is a safe and effective method for achieving anesthesia of the hemimaxilla providing that there is strict adherence to the landmarks and techniques described. This technique carries with it a high success rate and with specific applications can allow a practitioner to provide painless dental treatment.

Authors
J. Mel Hawkins, DDS, is an Assistant Professor at the University of Toronto. He also maintains a private practice specializing in dental anesthesiology. David Isen, DDS, is President Elect of the Ontario Dental Society of Anesthesiology. He maintains a private practice specializing in dental anesthesiology.

References
1. Nish, IA, Pynn, BR, Holmes, HI, and Young, ER, Maxillary nerve block: a case report and report of the intraoral technique. J Can Dent Assoc 61(4):305-310, 1995.
2. Poore, TE, Carney, FMT, Maxillary nerve block: a useful technique. J Oral Surg 31:749-755, 1973.
3. Jastak, JT, Yagiela, JA and Donaldson, D, Local Anesthesia of the Oral Cavity. WB Saunders Co. Toronto, 1995.
4. Canter, SR, Slavkin, HC and Canter, MR, Anatomical study of the pterygopalatine fossa and canal. J Oral Surg 22:36-41, 1964.
5. Mendel N, Puterbaugh, PG, Conduction, Infiltration and General Anesthesia in Dentistry. Dental Items of Interest Publishing Co, 4th ed:140, 1938.
6. Isen, DA, Hawkins, JM, The pharmacology of local anaesthetics. Ont Dent 72(6):18-22, 1995.
7. Malamed, SF, Handbook of Local Anesthesia 4th Ed. Mosby, Toronto, 1997.
8. Mercuri, LG, Intraoral second division nerve block. Oral Surg 47:9-13, 1979.
9. Westmoreland, EE and Blanton, PL, An analysis of the variations in position of the greater palatine foramen in the adult human skull. The Anatomical Record 204:383-388, 1982.
10. Evers, H and Haegerstam, G, Introduction to Dental Local Anaesthesia. ASTRA/Mediglobe SA, Fribourg, Switzerland, 1990.
11. Loetscher CA and Walton, RE, Patterns of innervation of the maxillary first molar: a dissection study. Oral Surg 65:86-90, 1988.
12. Sved, AM, Wong JD, and Dunker, P, Complications associated with maxillary nerve block via great palatine canal. Aust Dent J 37:340-345, 1992.
13. Malamed, SF and Trieger, N, Intraoral maxillary nerve block: an anatomical and clinical study. Anesth Prog 30: 44-48, 1983.
14. Literature Search: Canadian Dental Association Library, Ottawa, Canada.

To request a printed copy of this article, please contact
J. Mel Hawkins at Anaesthesia Associates in Restorative Dentistry, 4800 Leslie Street, Suite 111, Toronto, Ontario M2J 289, Canada.