General Principles of Local Anesthesia
Physiology of Peripheral Nerve Depolarization
When a nerve is inactive (not generating action potentials), there is a potential difference across its membrane which is known as the resting membrane potential (RMP) and is measured in millivolts (mV). Nerve cells have an RMP of −70 mV that is established by differences in potassium and sodium ion concentrations across the resting cell membrane—high potassium concentration intercellularly and high sodium ion concentration extracellularly. At rest, the nerve cell is relatively resistant to ion passage, but on excitation, voltage-gated sodium channels open and there is a slow influx of sodium ions into the cell. When a threshold potential is reached, depolarization occurs, and there is a fast influx of sodium ions into the cell, causing the potential across the membrane to become positive (+ 40 mV). The sodium channels quickly close again, preventing further sodium influx. At the same time, potassium channels open and there is potassium efflux from the cell. This causes repolarization of the cell membrane back to the RMP.
The depolarization of a nerve cell initiates a sequential series of depolarizations along the nerve fiber, thus propagating the impulse (action potential) along the fiber. In myelinated nerve fibers, the depolarizations “jump” from one node of Ranvier to the next (saltatory conduction). In unmyelinated fibers (which do not have nodes of Ranvier), the depolarization spreads to adjacent cells.
Mechanism of Action of Local Anesthetics
Local anesthetics block the inner (cytoplasmic) gate of sodium channels in nerve cells, preventing sodium influx and action potential initiation and propagation. Termination of action of the anesthetic at the site of injection is by diffusion of the active drug into the systemic circulation, followed by metabolism and elimination.
Duration of Anesthesia
Duration of dental local anesthesia can be defined in terms of duration of pulpal anesthesia versus the duration of soft-tissue anesthesia. Usually, dentists try to maximize the duration of pulpal anesthesia and minimize the undesirable persistence of soft-tissue anesthesia. The duration of pulpal anesthesia and the duration of soft-tissue anesthesia of some of the main local anesthetic agents following maxillary infiltration and an inferior alveolar nerve block are listed in ▶ Table 4.1.
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Lidocaine 2% with 1:100,000 epinephrinea |
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Articaine 4% with 1:100,000 epinephrinea |
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Bupivacaine 0.5% with 1:200,000 epinephrinea |
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aThe duration of action is prolonged when combined with epinephrine, a vasoconstrictor. A “plain” solution contains no vasoconstrictive agent. |
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General Injection Technique
The fingers of the supporting hand retract the soft tissue around the injection site, enabling the dentist to visualize the target area. These fingers may also be used to provide stability for the syringe and can act as reference points for some injections.
When ready to inject, the needle is inserted gently and directly, in one continuous movement, into the target area. The dentist should then aspirate to ensure that the tip of the needle does not lie within a blood vessel. Most dental syringes are “self-aspirating,” meaning that if the plunger of the syringe is slightly deployed, it bounces back, aspirating (sucking) as it does so. If the syringe is not self-aspirating, aspiration is performed by drawing back slightly on the plunger. The absence of blood in the local anesthetic cartridge suggests that no vessel has been breached. If there is blood in the cartridge, the needle tip should be repositioned slightly and aspiration repeated. Following a negative aspiration result, the local anesthetic is injected slowly, exerting as little pressure as possible. Injection into the hard palate and interdental papillae are exceptions because the mucosa is tightly adherent to the supporting periosteum in these areas, necessitating that some pressure is used.
Classification of Injection Techniques
Overview of Nerves Anesthetized
Injections given within the oral cavity anesthetize a branch or branches of either the maxillary or mandibular division of the trigeminal nerve (CN V2 or CN V3, respectively) on the same side as the injection ( ▶ Fig. 4.1 and ▶ Fig. 4.2).
Failure of Anesthesia
Patient Variation
A typical dose of local anesthetic profoundly anesthetizes some patients but may not sufficiently anesthetize others. The dentist must try to ascertain whether failure is due to patient variation or improper injection technique. If due to the former, then more local anesthetic may be given to achieve adequate anesthesia. Likewise, differences occur between patients in the duration of action of anesthetics. The best way to mitigate these differences is to begin treatment as soon as anesthesia is achieved, i.e., ~ 2 minutes after injection for an infiltration and ~ 5 minutes after injection for a block.
Acute Pulpitis or Apical Abscess
Acute pulpitis (pulpal inflammation) results in a hyperemic tooth (a tooth in which the pulpal blood vessels dilate causing a painful increase in pressure) that is difficult to adequately anesthetize. The pus of an apical abscess can prevent proper diffusion of the local anesthetic solution to the periapical nerves and vessels.
Intravascular Injection
If all or part of an injection of local anesthetic is deposited intravascularly then there may be little or no anesthesia achieved.
Injection into Muscles or Their Fascia
If the local anesthetic is deposited in a muscle or its fascia instead of the bone near the teeth apices, then the distance the anesthetic has to diffuse to reach the apical nerves and vessels is increased, resulting in reduced anesthesia. Injection into a muscle can also produce trismus (restricted mouth opening).
Complications and Their Treatment
Fainting
This is the most common systemic complication and is likely attributed to anxiety over the procedure. It can be minimized by administering local anesthetic with the patient in supine position. If fainting does occur, place the patient in a supine position, and recovery will occur rapidly.
Allergic Reactions
Allergy to local anesthetic is uncommon but possible. It may be due to allergy to the drug, allergy to the additives for compounding the drug, or latex allergy to the rubber bung located at one end of a local anesthetic cartridge. Allergy manifests as facial flushing, swelling, rash, itching, and wheezing. The patient should be sent for allergy testing to determine the precise cause. For minor allergic reactions, provide reassurance and antihistamines if necessary. For severe (anaphylactic) reactions, urgently call for an ambulance, place the patient in a supine position, and give emergency medication as needed (e.g., intramuscular epinephrine, intravenous hydrocortisone, and oxygen by mask).
Cardiovascular Collapse
Cardiovascular collapse may be precipitated by, or exacerbated by stress, excessive amounts of local anesthetic, and improper aspiration, leading to deposition of local anesthetic in a blood vessel. Epinephrine in the local anesthetic can act directly on the heart, which, if previously diseased, can cause arrhythmias.
If this occurs, urgently call for an ambulance, place the patient in a supine position, and maintain airway and circulation.
Hematomas
Small hematomas are of little consequence, but larger hematomas can compromise the airway. No treatment is needed for small hematomas; whereas large hematomas due to arterial bleeding, if they are not self-limiting, may require ligation of the vessel.
Trismus
Trismus is the inability to open the mouth normally. It usually occurs after an inferior alveolar nerve block that is given too low, resulting in hematoma formation in the medial pterygoid. This may be accompanied by infection. Treatment includes reassurance, antibiotics, and encouragement to progressively try to open the mouth.
Facial Paralysis
Facial paralysis (or palsy) may occur following an improperly placed inferior alveolar nerve block. If the needle is directed too far posteriorly, the tip may enter the superficial layer of deep cervical fascia that surrounds the parotid gland. Thus the local anesthetic is able to penetrate the gland and anesthetize the five branches of the facial nerve that are embedded within it. This is manifested by the patient’s inability to frown or blink and drooping of the mouth on the affected side. The facial paralysis is transient, normally lasting for ~ 1 hour.
Treatment includes reassurance and a protective eye covering until the blink reflex returns.
Maxillary Anesthesia
Maxillary Incisors and Canines
Anatomy
The incisors and canines and their associated periodontal ligaments, buccal gingiva, mucosa, and supporting bone are innervated by the anterior superior alveolar nerves which branch off the infraorbital nerve just before it emerges from the infraorbital foramen (▶ Table 4.2). These nerves anastomose over the midline. The palatal gingiva, mucosa, and supporting bone are innervated by the nasopalatine nerve, which emerges through the incisive foramen.
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Areas anesthetizeda (see ▶ Fig. 4.3c and ▶ Fig. 4.3d) Nerve (▶ Fig. 4.3b) |
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Maxillary central, lateral incisora and canineb and their associated periodontal ligaments, buccal gingiva, mucosa, and supporting bone |
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aThis applies when the injection is placed superior to the maxillary lateral incisor. bThe root of the canine is longer and the apical part of the root is often distally oriented; therefore, the maxillary right canine may not be sufficiently anesthetized for cavity preparation from this injection alone. |
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The medial spread of local anesthetic may be hindered by the labial frenulum that anchors the lip to the attached gingiva in the midline.
The maxillary bone has a thin, porous lamina (layer) easily penetrated by an infiltration of local anesthetic solution.
Injection Technique
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Insert the needle in the mucobuccal fold immediately superior to the crown of the tooth being anesthetized and pass it axially toward the apex of the tooth (▶ Fig. 4.3a, b). The needle should be in close proximity to the bone to ensure that the local anesthetic solution has minimal diffusion distance before it bathes the periapical nerves and vessels.
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Following a negative aspiration result, slowly inject 1.0 to 1.8 mL of local anesthetic.
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For central incisors, the injection is best placed distally due to the close proximity of the anterior nasal spine.
Fig. 4.3 Infiltration of the maxillary lateral incisor. (a) Injection technique. (b) Nerves anesthetized, anterior view. (c) Areas anesthetized, right lateral view. (d) Areas anesthetized, inferior view. Note the lips roughly extend to the first premolar region on each side; the cheeks are colored gray. (From Baker EW. Anatomy for Dental Medicine. Illustrations by Voll M and Wesker K. Second Edition. New York: Thieme Medical Publishers; 2015).
Infraorbital Nerve Block
Anatomy
The infraorbital nerve is a continuation of the maxillary nerve as it enters the infraorbital canal. The anterior superior alveolar nerve and middle superior alveolar nerve (when present) branch from this nerve just before it exits the infraorbital canal and are, therefore, also anesthetized by diffusion of local anesthetic from the injection site (▶ Table 4.3).
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Nerve (▶ Fig. 4.4b) |
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Incisors and canine and their associated periodontal ligament, buccal gingiva, mucosa, and supporting bone |
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Premolars and possibly the mesiobuccal cusp of the first molar and their associated periodontal ligament, buccal gingiva, mucosa, and supporting bone |
Middle superior alveolar nerve or fibers from the superior dental plexus |
Injection Technique
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Palpate the center of the inferior margin of the orbit with the index finger of the supporting hand. At a point ~ 1 cm below the orbital margin, the infraorbital foramen can be palpated. Hold the index finger at that point, while retracting the upper lip with the thumb of the supporting hand. Insert the needle at the mucobuccal fold immediately superior to the first maxillary premolar, parallel to the long axis of the tooth, toward the tip of the index finger (▶ Fig. 4.4a, b).
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Following a negative aspiration result, slowly inject ~ 1 mL of local anesthetic.
Fig. 4.4 Infraorbital nerve block. (a) Injection technique. (b) Nerves anesthetized, right lateral view. (c) Areas anesthetized, right lateral view. (d) Areas anesthetized, inferior view. (From Baker EW. Anatomy for Dental Medicine. Illustrations by Voll M and Wesker K. Second Edition. New York: Thieme Medical Publishers; 2015).
Clinical Considerations
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To avoid having to give more than one injection, an infraorbital block may be used to anesthetize multiple teeth for cavity preparation or pulpal procedures. It may also be used when infiltration has failed to achieve pulpal anesthesia or is contraindicated (e.g., infiltration would require an injection into an infected area).
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Extractions of any of the teeth anesthetized by an infraorbital block will require supplementary anesthesia of the palatal gingiva by a nasopalatine or greater palatine block or by local infiltration of the palate.
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Hematoma is rare with this injection, but there is potential for iatrogenic (accidental, clinician-induced) damage to the patient’s eye.
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To obtain complete anesthesia of the central incisor on the same side as the injection, it may be necessary to block anastomosing fibers from the anterior superior alveolar nerve on the contralateral side of the midline. This is achieved by placing a supplemental 0.5 mL of local anesthetic in the contralateral buccal fold just distal to the central incisor.
Maxillary Premolars
Anatomy
The premolar area is innervated by the superior dental plexus, which is formed by convergent branches from the posterior superior alveolar nerve and the anterior superior alveolar nerve. Sometimes there is a middle superior alveolar nerve that, when present, innervates the premolars, their periodontal ligaments, buccal gingiva, and supporting bone, and often the mesiobuccal root of the first molar (▶ Table 4.4). The palatal gingiva, mucosa, and supporting bone adjacent to the premolars is mainly innervated by the greater palatine nerve, but the area of the first premolar may also be innervated by fibers of the nasopalatine nerve.
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Nerve (▶ Fig. 4.5b) |
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Both maxillary premolarsa and their associated periodontal ligaments, buccal gingiva, mucosa, and supporting bone |
Middle superior alveolar nerve or branches of the superior dental plexus |
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Canine and the mesiobuccal cusp of the first molarb and their associated periodontal ligaments, buccal gingiva, mucosa, and supporting bone |
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aThis applies when the injection is placed between the premolars. bThese teeth, soft tissues, and bone may also be anesthetized to a lesser extent. |
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Diffusion of local anesthetic deposited in the mucobuccal fold is especially good in this area because the bone lamina is thin and the apices of the premolars lie very close to the lamina. Consequently, small volumes of local anesthetic are required, and the palatal roots of the premolars are almost always anesthetized by this one injection.
Injection Technique
The same infiltration technique is used in this case as for the incisors and canines. Deposit 1.0 to 1.5 mL of local anesthetic solution around the apex of the premolars (▶ Fig. 4.5a, b).
Fig. 4.5 Infiltration of the maxillary premolars. (a) Injection technique. (b) Nerves anesthetized, right lateral view. (c) Areas anesthetized, right lateral view. (d) Areas anesthetized, inferior view. ([a]: From Daubländer M in van Aken H, Wulf H: Lokalanästhesie, Regionalanästhesie, Regionale Schmerztherapie. Third Edition. Stuttgart: Thieme; 2010; [b-d]: From Baker EW. Anatomy for Dental Medicine. Illustrations by Voll M and Wesker K. Second Edition. New York: Thieme Medical Publishers; 2015).
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