Overview of Dental Local Anesthesia
Local anesthetic solution is injected to block the production of nerve impulses that relay sensory information. This is accomplished by decreasing the permeability of the ion channels to sodium ions. The decrease in the sodium conductance causes a failure to reach the threshold level of depolarization for the production and propagation of an action potential.
Local anesthetic is delivered in two types of injections. In both types, the local anesthetic prevents the nerve impulse from traveling centrally from the site of anesthetic deposition.
Local infiltration (supraperiosteal injection) is the injection of local anesthetic into a relatively small area with the aim of anesthetizing the terminal nerve branches in the area of the planned procedure. This is most effective for procedures of limited scope.
A nerve block requires the deposition of anesthetic in proximity to the trunk of a nerve usually at a site somewhat distant from the area of the procedure. This usually results in a larger area of anesthesia and is useful for multiple or more extensive procedures.
Basic instrumentation in both types of injection includes a syringe, disposable needle, and anesthetic cartridge.
Commonly used syringe is a metallic, breech loading cartridge type. These can be of an aspirating or self-aspirating type. Alternatives include various disposable plastic syringes that accommodate the cartridge and computer-controlled local anesthetic delivery systems.
The disposable needle that delivers the solution into the tissues from the cartridge is usually made of stainless steel.
Needles are beveled to form the point. The gauge or diameter of the lumen used is 30, 27, or 25. Thirty gauge is the smallest diameter of these and 25 gauge the largest.
Needles come in short and long lengths, with average lengths being 20 mm for the short and 32 mm for the long.
The dental cartridge is a glass cylinder containing 1.8 mL of the selected anesthetic solution.
The smaller end of the cartridge has an aluminum cap that holds a diaphragm of semipermeable latex rubber membrane for the blunt end of the needle to penetrate.
The open end of the cartridge has a stopper, slightly inset, to allow the harpoon of an aspirating syringe to embed for applying back pressure on the plunger.
Mandibular Local Anesthesia
Sensory innervation to the mandibular arch, including teeth, supporting structures, and soft tissue, is derived from the mandibular division of the trigeminal nerve (V3). The specific branches of V3 involved are the inferior alveolar, the lingual, and the buccal. The inferior alveolar further divides into the mental and incisive branches. Block injections are usually chosen due to the anatomy and thickness of the bone of the mandible. The specific dental procedure planned determines which block injection is used.
Inferior Alveolar Nerve Block
The most commonly used block for restorative and surgical procedures in the mandibular arch, this injection is also the most challenging in oral anesthesia.
Nerves targeted: the inferior alveolar before it enters the mandible at the mandibular foramen, which will include the mental and incisive branches of the inferior alveolar, and the lingual, which is just anterior and medial to the inferior alveolar when it enters the foramen (a-1).
Anesthetizes: all the teeth and supporting structures to the midline; buccal soft tissues anterior to the mandibular first molar, the lower lip, and chin to midline; and lingual soft tissues, floor of oral cavity, and the anterior two-thirds of the tongue (a-2).
Anatomic landmarks: coronoid notch, the greatest concavity on the anterior border of the ramus of the mandible; pterygomandibular raphe, a mucosal fold spanning from posterior of the mandibular dental arch toward the maxilla; and the occlusal plane of the mandibular dental arch.
The operator’s thumb in the coronoid notch stabilizes the tissue and indicates the vertical position of the injection because the deepest part of the notch is at the level of the mandibular foramen.
The syringe should approach the injection site parallel to the occlusal plane of the mandibular teeth from the opposite premolar area (a-1).
The needle should penetrate the tissue lateral to the pterygomandibular raphe at a level that will be above the mandibular foramen (a-3).
The needle is advanced about 20 to 25 mm until it is stopped by contact with bone. At this point, it is withdrawn slightly, about 1 mm (a-4).
Aspiration is important to assure against intravascular administration of the anesthetic.
If the aspiration is negative, inject the cartridge of anesthetic slowly over 60 seconds.
Slowly withdraw the needle and allow time for the anesthetic to bathe and penetrate the nerve.
Successful block is indicated by numbness and tingling in the lower lip, which indicates the mental nerve, the terminal branch of the inferior alveolar, has been anesthetized.
Penetration of the needle is inadequate, the lingual nerve may be anesthetized but not the inferior alveolar.
The injection is too low, anesthetic will be below the foramen where the nerve enters the bone and will not be effective.
The needle is advanced beyond the posterior border of the mandible, it enters the parotid gland space, and injection will affect the facial nerve (VII) to cause paralysis of the muscles of facial expression (Bell’s palsy).
If a bony stop is encountered too early, the needle has contacted the mandible too far anterior and must be partially withdrawn and repositioned to reach the proper depth for the injection.
If no bony stop is encountered, the angle of the injection should be increased so the syringe is positioned over the contralateral first molar and advanced until a stop is met.
Inferior alveolar or lingual nerves may be contacted by the needle during the injection, resulting in a “shock” sensation to the patient. This usually results in rapid and profound anesthesia. This seldom results in nerve damage.
Anatomic variations that should be considered effect the position of the mandibular foramen. In children, it is relatively farther posterior on the ramus of the mandible. In adults with a protrusive mandible, it is relatively higher on the ramus.
Fig. 27.1 (a-1) Syringe position for the inferior alveolar nerve block, left lateral view. (a-2) Areas anesthetized by inferior alveolar nerve block, superior view. (a-3) Intraoral injection site for inferior alveolar nerve block. (a-4) Transverse section just above the occlusal plane of the mandibular teeth, superior view.
Gow-Gates Block
Considered to be a true mandibular nerve block, this injection is a variation of the inferior alveolar nerve block in which the anesthetic deposition is very high in the pterygomandibular space and should affect all branches of the mandibular nerve (V3).
Nerves targeted: the inferior alveolar with its mental and incisive branches, the lingual, mylohyoid, auriculotemporal, and buccal (long buccal).
Anesthetizes: all the teeth and supporting structures to the midline; buccal soft tissues of the same side of the mandibular arch; lingual soft tissues, floor of oral cavity, and the anterior two-thirds of the tongue; skin of the cheek and anterior temporal region (b-1).
Anatomic landmarks: lower border of the tragus of the ear and the corner of the mouth extraorally and the maxillary second molar intraorally.
The patient must open their mouth as wide as possible and maintain that position.
The operator should retract the cheek with the thumb in the coronoid notch of the mandible.
The syringe barrel should be positioned toward the injection site from the corner of the mouth on the opposite side. The syringe must be parallel to a line from the angle of the mouth to the lower edge of the tragus of the ear on the side of the injection.
The needle should penetrate the tissue just posterior and lateral to the maxillary second molar at the level of the mesiolingual cusp (b-2).
The needle is advanced to about 25 mm and should make contact with the bone of the neck of the mandible (b-3). At this point, it is withdrawn slightly, about 1 mm.
Aspiration is important to assure against intravascular administration of the anesthetic.
If the aspiration is negative, inject the cartridge of anesthetic slowly over 60 to 90 seconds.
Slowly withdraw the needle. The patient will need to remain in the maximally open position for another 1 to 2 minutes.
Successful block is indicated by numbness and tingling in the lower lip, which indicates the mental nerve, the terminal branch of the inferior alveolar, has been anesthetized.
Failure of this block is rare after the operator has become accustomed to the procedure.
The larger diameter of the nerve at this level may require that a larger volume of anesthetic be used. If anesthesia is incomplete, inject additional solution.
Akinosi Block
This closed-mouth inferior alveolar block is a variation of the inferior alveolar nerve block. It can be used when the patient has a restricted ability to open the mouth.
Nerves targeted: the inferior alveolar with its mental and incisive branches before it enters the mandible at the mandibular foramen and the lingual just anterior and medial to the inferior alveolar.
Anesthetizes: all the teeth and supporting structures to the midline; buccal soft tissues anterior to the mandibular first molar, the lower lip, and chin to midline; lingual soft tissues, floor of oral cavity, and the anterior two-thirds of the tongue (c-1).
Anatomic landmarks: junction of the gingiva and mucosa superior to the maxillary third (or second) molar, the maxillary tuberosity, and the coronoid notch on the anterior border of the ramus of the mandible.
The patient should have their teeth gently touching and the muscles of mastication relaxed.
The operator should retract the cheek as much as possible with their thumb or index finger resting in the coronoid notch.
The syringe is held parallel to the occlusal plane of the maxillary teeth at the level of the mucogingival junction of the third molar (second molar if the third molar is not present).
The needle should penetrate the buccal mucosa and be oriented slightly laterally to a depth of about 25 mm from the maxillary tuberosity (c-2). No bony contact should be expected. At this point, the tip of the needle should be in the middle of the pterygomandibular space.
Aspiration is important to assure against intravascular administration of the anesthetic.
If the aspiration is negative, inject the cartridge of anesthetic slowly over 60 seconds.
Successful block is indicated by numbness and tingling in the lower lip, which indicates the mental nerve, the terminal branch of the inferior alveolar, has been affected.
The needle is directed too far medially, medial to the sphenomandibular ligament, if the lateral flare of the mandible is underestimated. The path of the needle should parallel the ramus of the mandible.
The needle inserted too low will not produce anesthesia. Retry at a higher insertion level.
Inserting the needle too far or not far enough. There is no bony contact to help judge the depth of penetration. The 25-mm recommendation if for average patients. It has to be adjusted for larger or smaller individuals.
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