Administering Dental Nerve Blocks
Dental nerve blocks provide excellent pain management for many oral procedures. They are easy and inexpensive to perform, provide a rapid onset of action, and work in concert with anesthetic and other analgesic modalities. This article explains how to administer various dental nerve blocks and discusses the indications and precautions associated with their use.
Introduction
Pain management in veterinary patients has become common only within the last 10 years or so. Contemporary research indicates that when pain is adequately treated in veterinary patients, they recover faster and return more quickly to normal behaviors (e.g., grooming, eating) than patients that do not receive adequate pain management, and less morbidity and mortality are observed.1
Preemptive analgesia further improves the quality of pain management. Preemptive analgesia provides desensitization before the onset of painful stimuli in order to prevent the central nervous system (CNS) responding to the painful event itself. Preventing “wind-up,” or the opening of the pain gates, results in the need for lower doses of analgesics at a reduced frequency over the course of the painful event.2,3,4 It is important to remember that the drugs used for the induction and maintenance of anesthesia do not provide preemptive analgesia.3
Local anesthesia can be defined as a temporary loss of sensation in a specific, limited area of the body without loss of consciousness. Regional anesthesia causes a loss of sensation in a particular region of the body by interrupting the sensory nerves conducting impulses from that region; the term is often used to describe field blocks or a specific nerve block. The differences between local and regional anesthesia are somewhat arbitrary, but in general, smaller volumes of anesthetics are needed to produce a local nerve block than to produce a regional field block.2 A nerve block is performed by injecting a small volume (usually <1–2 mL) of a local anesthetic close to the nerve whose conductivity is to be interrupted.2,5 One advantage of using many of the nerve blocks described in this article is that precise placement of the agent is not crucial because the effects occur primarily by diffusion of the agent around the nerve.
Local anesthetic agents are classified as either amides or esters on the basis of their chemical structure. Lidocaine and bupivicaine are examples of amides, whereas procaine is classified as an ester. At the amounts commonly used, many local anesthetics produce vasodilation, which can increase the speed of uptake of the drug into the bloodstream. Vasodilation decreases the efficacy at the site of injection and can increase the potential for toxic side effects. For this reason, many local anesthetics are combined with a vasoconstrictor (such as epinephrine) to help maintain their concentration at the site of administration and increase the duration of action.
Local anesthetics exhibit their effects by stabilizing the membranes of excitable tissues, thereby preventing the transmission of the nerve impulses. In addition, local anesthetics selectively bind to sodium channels in the nerve membrane at one or more sites,5 an action that prevents a large increase in membrane permeability to sodium ions, which can occur as a result of a noxious stimulation. Finally, local anesthetics stop the transfer of the noxious information to the CNS, thereby blocking pain perception.
Nerve blocks are considered to be a “perfect” means of analgesia because they are the only type of analgesic capable of inhibiting 100% of the perception of pain, albeit temporarily. In general, the nerve blocks described here are easy to perform, and the supplies needed are inexpensive and readily available. The blocks provide a rapid onset of analgesia and act in concert with other analgesic modalities, making them an essential part of a balanced analgesic and anesthetic protocol.6,7
Balanced anesthesia and analgesia involve the use of multiple drug classes to provide effective pain management and minimize the amount of anesthetic needed. Most pain is evident in the first 24–72 hours after dental surgery, necessitating continued pain management after the patient leaves the hospital.3 Fortunately many drug options—such as the often used combination of nonsteroidal anti-inflammatory drugs and opioids—are available to round out a postoperative pain management plan.2,8
Benefits and Indications
Use of dental nerve blocks offers several benefits. In addition to providing preemptive analgesia and lowering analgesic dose requirements, nerve blocks allow patients to be maintained on reduced concentrations of the inhalant anesthetics, thereby minimizing complications from hypotension and hypoventilation. Patients maintained on lower anesthetic concentrations recover more quickly and with fewer complications than those receiving higher concentrations. 3,9 Using dental nerve blocks also provides a smoother anesthetic episode because the inhalant will not need to be increased during particularly painful parts of the procedure, e.g., extractions. Reducing the amount of anesthesia used is important because many patients undergoing anesthesia are elderly and may have preexisting conditions (such as heart murmurs or endocrine dysfunction) that increase the risk of adverse effects associated with anesthesia. Use of dental nerve blocks provides benefits outside of surgery as well. Clients have come to expect pain management from veterinary facilities and may prefer a hospital that offers advanced pain management.
In addition to tooth extractions, procedures for which dental nerve blocks may be indicated include bone and soft tissue reconstruction following trauma, mass excisions and biopsies, root canal therapy, oronasal fistula repair, cleft palate repair, periodontal flap surgery, and maxillectomy and mandibulectomy. Dental nerve blocks are valuable in any procedure for which local pain control is beneficial.9
Performing the Blocks
There are five basic dental nerve block regions: the major palatine, infraorbital foramina, maxillary, middle mental, and mandibular (also referred to as the inferior alveolar). Some authors recommend performing dental nerve blocks using an aseptic technique. With all dental nerve blocks, it is essential to aspirate before injecting the agent to avoid accidental intravenous injection of the local anesthetic. Always inject the agent slowly.
Major Palatine Block
The major palatine foramen is located on the palate, halfway between the palatine midline and the dental arcade at the level of the mesial root of the maxillary first molar in dogs [Figure 1A] and the palatal root of the maxillary fourth premolar in cats [Figure 1B]. The major palatine nerve provides sensory innervation to the oral side of the hard and soft palates. The foramen is not palpable, and the needle usually cannot be placed into the foramen. Given that the injection site can be difficult to identify and the palatine nerve is also a branch of the maxillary nerve, use of this block does not provide a significant advantage over other blocks. Therefore, this block is not further discussed.
Infraorbital Nerve Block, Cranial and Caudal
The infraorbital nerve has multiple sensory branches that innervate the nasal and buccal soft tissues as well as the incisor, canine, and premolar teeth. In dogs, the foramen is located on the maxilla, dorsal to the caudal (distal) root of the third maxillary premolar [Figure 2]. The caudal extent of the infraorbital canal is located at the level of the medial canthus of the eye, which can serve as a guide for maximum needle advancement. Caution must be exercised when performing this technique on cats and brachycephalic dogs. Because the infraorbital canal is short, compared with that of normocephalic and dolichocephalic dogs, take care to avoid inserting the needle beyond the medial canthus.6 The infraorbital block can be performed using either a cranial or caudal technique.
The cranial infraorbital nerve block desensitizes the incisor, canine, and premolar teeth rostral to the fourth premolar as well as the associated ipsilateral soft tissues. This block is performed at the opening of the infraorbital foramen, which can be accessed either intraorally or extraorally, using a percutaneous approach. The needle should not be inserted deeply into the infraorbital canal, and injection of only a small amount of local anesthetic (usually 0.25–0.5mL) is necessary.a [Figures 3A, 3B] After the local anesthetic has been injected, apply pressure over the injection site for 60 seconds while elevating the head to further encourage caudal diffusion of the local anesthetic agent.10
The caudal infraorbital nerve block desensitizes the same areas desensitized by the cranial infraorbital block, plus all of the bone, soft tissue, and dentition rostral to the maxillary first molar. The area affected by this block depends on the degree of caudal diffusion of the anesthetic agent. Unless the agent diffuses beyond the caudal borders of the infraorbital canal, adequate analgesia cannot be provided to the maxillary fourth premolars and molars. Therefore, if anesthesia to the caudal cheek teeth is required, it is recommended that the maxillary nerve block (described subsequently) be performed.9 The caudal infraorbital nerve block is administered similarly to the cranial infraorbital nerve block except that the head is kept elevated and digital pressure is applied over the foramen and needle during, rather than after, injection of the local anesthetic agent. Pressure over the foramen and needle helps force the local anesthetic caudally; maintaining the head elevation while placing direct digital pressure over the infraorbital canal postinjection for 60 seconds further encourages caudal flow of the agent deep into the infraorbital canal and regional tissues. 10 An additional 50% of the calculated volume of local anesthetic can be administered for the caudal infraorbital nerve block to encourage adequate diffusion to the middle superior alveolar nerves that lie within the infraorbital canal.9
In cats, the infraorbital foramen is located above the maxillary second premolar and is within the junction of the zygomatic arch and maxillary bone [Figure 4]. The infraorbital canal is only about 4-mm long in a mature cat and ends at the level of the medial canthus. A 27-gauge needle attached to a tuberculin syringe containing the calculated local anesthetic dose is inserted into the foramen and advanced into the infraorbital canal for a maximum of 3–4 mm.3 It is important to maintain pressure over the site for up to 60 seconds postinjection.
Maxillary Nerve Block
The following techniques for performing the maxillary nerve block in dogs and cats have not yet been reported in the literature. These easy-to-perform techniques were pioneered by Benjamin Colmery, DVM, AVDC, during his tenure at the Animal Medical Center in New York and reflect careful study of the cranial anatomy of dogs and cats.10 The maxillary nerve block affects all of bone of the maxilla, the soft and hard palates, the soft tissue of the nose and upper lip, and dentition rostral to the maxillary second molar (i.e., the entire hemimaxilla). Additional drug quantities (up to 2 mL in very large breeds) can be used to flood this nerve for desensitizing the orbit, eye, conjunctiva, eye-lid, and forehead skin.10
In dogs, the maxillary nerve block is performed intraorally by inserting the needle just caudal and center to the last maxillary molar [Figure 5A]. Advance the needle dorsally to a level just beyond the root tips of the last molar, and then aspirate and slowly inject the agent [Figure 5B]. The maxillary nerve block is preferred over the infraorbital nerve block for providing analgesia to the maxillary molars.10
In cats, the maxillary nerve block is performed intraorally at the base of the ‘V’ notch or divot near the soft palate juncture, palpable just medial to the caudal root tips of the maxillary fourth premolar [Figures 6A, 6B.] Aspirate and inject the agent slowly.10
Middle Mental Block
The middle mental nerve block disrupts sensation to all of the ipsilateral canine and incisor teeth as well as bone and soft tissue rostral to the second premolar of the mandible. If the agent is deposited outside of the foramen, only the ipsilateral buccal soft tissue from the canine teeth forward will be desensitized.4 This block can be performed either intraorally or percutaneously.
The location of the middle mental foramen varies with the breed, size, and species of the patient. Easily palpable in larger dogs, the foramen can be hard to palpate in smaller dogs, which makes the middle mental block more difficult to perform in smaller patients. Dental radiology may be helpful in locating this foramen in cats and small breeds of dogs.
In dogs, the middle mental foramen is located ventral to the rostral (mesial) root of the second premolars [Figure 7]. If the area cannot be palpated, place the needle in the submucosa at the ventral third of the mandible at the level of the diastema between the first and second mandibular premolars. Place a finger over the needle during the injection to help direct the anesthetic into the foramen. The agent should be injected very slowly. Otherwise, as the agent is injected directly into the canal, pain may induce a rise in the heart rate. Digital pressure should be maintained over the injection site for 60 seconds postinjection to ensure maximum caudal diffusion of the agent into the foramen.9
The middle mental foramen is very small and difficult to locate in cats, making placement of this block problematic. The foramina of the middle mental nerves are located at (or immediately caudal to) the apex of the mandibular canine tooth, and the targeted foramen is located most rostrally.3 In cats, this block is performed as described for the dog, but use of a 27-gauge needle may allow easier access into smaller feline foramina. Because the middle mental nerve block provides limited analgesia to a circumscribed part of the mandible, the mandibular nerve block (described subsequently) may constitute a more practical alternative for use in cats.10
Mandibular or Inferior Alveolar Nerve Block
The mandibular nerve block desensitizes the lower dental arcade (including teeth and bone on the ipsilateral side) as well as the associated soft tissues (possibly the tongue) that are innervated by the mandibular nerve (i.e., the entire hemimandible).4 This block can be performed either intraorally or percutaneously.
The foramen is located on the medial side of the ramus of the mandible in dogs and cats [Figures 8A, 8B]. The mandibular nerve fossa can be easily palpated intraorally in most dogs and is located approximately two-thirds of the distance caudally from an imaginary line drawn from the bone beginning after the last mandibular molar to the caudal prominence of the angular process of the mandible. To determine the injection site extraorally for percutaneous drug administration, palpate the vascular notch located on the ventral aspect of the mandible, and place the needle into the medial aspect of the mandible. Whether the agent is administered intraorally or percutaneously, intraoral palpation of the needle during injection of the local anesthetic can help ensure proper needle placement.
Because this foramen cannot be directly entered in the dog or cat, the nerve is blocked before it enters the mandible. Analgesia is accomplished secondary to regional diffusion of the local anesthetic. When performing the mandibular nerve block, ensure that the bevel of the needle is facing the foramen. Then aspirate and slowly inject the agent. Caution must be used when administering this block because inadvertent inclusion of the lingual nerve can cause self-mutilation of the tongue and lip-chewing upon recovery, requiring sedation of the patient until the agent disperses.3,6
Splash Block
Another technique that may be used for local pain control in oral surgery is the splash block, in which a local anesthetic is placed directly into a surgical site or injected into a closed space where the agent may diffuse.6
Supplies, Drugs, and Dosages
Supplies necessary to perform dental nerve blocks include tuberculin or 3-cc syringes; 30- to 22-gauge, 3/4- to 1 3/8-inch needles; and a regional anesthetic agent. The most commonly used agents include lidocaine, mepivicaine, and bupivicaine [Table 1.] The addition of epinephrine reduces the rate of absorption of the local anesthetic by about 30% and can increase the effect and duration by up to 50%.3 Epinephrine also aids in hemostasis. The most commonly used dental block agent is 0.5% bupivicaine combined with epinephrine at 1:200,000.b This preparation is also available in 1.8-mL individual dose cartridges that can be used with dental aspirating syringes.
In general, the volume of agent administered per site should range from 0.1 to 0.5 mL for dogs (depending on body size) and 0.1 to 0.3 mL for cats [Table 2]. When using bupivicaine (with or without epinephrine), the maximum total body dose for dogs is 2 mg/kg, and the maximum total body dose for cats is 1 mg/kg. When blocking multiple locations in smaller patients, use extreme care to ensure that maximum doses are not exceeded. If necessary, local anesthetics may be diluted with 0.9% sodium chloride (not sterile water) to provide additional volume and avoid toxic doses.2
Toxicity and Complications of Local Anesthetics
Systemic toxicity from local anesthetics, although rare, can result in CNS signs that include muscle twitching, seizure, depression, unconsciousness, coma, and respiratory arrest. It is important to keep in mind that these signs can be masked in the anesthetized patient. Treatment of CNS toxicity is supportive.5
Because bupivicaine dissociates slowly from sodium ion channels, it is more cardiotoxic than lidocaine or mepivicaine. Bupivicaine-induced cardiovascular dysfunction causes profound depression of the cardiac-conducting tissue, resulting in decreased cardiac output and systemic hypotension leading to cardiovascular collapse and/or arrest. Cardiac toxicity is greater in the presence of hypoxia, acidosis, and hyperkalemia.5 Cardiovascular toxicity resulting from bupivicaine is difficult to treat unless addressed promptly; treatment consists of supportive care that can include intravenous fluids, vasopressors and positive inotropes.2 If epinephrine is combined with bupivicaine, tachycardia, bronchospasm, or dysrhythmia may result.3,6
Metabolites or preservatives of local anesthetics also can cause rare allergic-type reactions.5 The most common indicators of anaphylaxis are cutaneous and respiratory reactions, which often are secondary to accidental intravenous injection or excessive doses of local anesthetics. Methemoglobinemia is also possible.7
According to the literature, complications resulting from oral nerve blocks performed on humans have included hematomas, iatrogenic tissue trauma, and broken small-gauge needles.3 All patients should be closely monitored in the hours following the administration of dental blocks. Although it occurs infrequently following dental blocks, some animals self-mutilate by chewing on their desensitized facial features (e.g., lips and cheeks).8 Excessive pawing of the face may also be observed. If the patient appears uneasy following the loss of sensation to the face, tranquilization may be indicated.8
Although extremely rare, permanent damage to the inferior alveolar nerve, lingual nerve, or both can occur as a result of mandibular nerve blocks. Neurologic complications can include facial nerve palsy, transient blindness, Horner’s syndrome, transient nerve paralysis, and unilateral deafness. Given that use of local anesthetics is avoided in human patients prone to malignant hyperthermia, it may be prudent to avoid their use in veterinary patients with this condition.9
Contraindications for epinephrine use include uncontrolled hyperthyroidism, cardiac dysrhythmias, and possibly asthma. Additionally, halogenated agents (e.g., halothane) can sensitize the heart to catecholamines. Mepivicaine or bupivicaine alone can be substituted if bupivicaine with epinephrine is contraindicated.3,6
Conclusion
Dental nerve blocks are inexpensive to perform and easy to master and can significantly benefit patient comfort. When combined with other analgesic modalities, dental nerve blocks are a valuable part of a balanced anesthetic and analgesic protocol.
Although some experts describe techniques that involve insertion of the needle deep into the foramen, the techniques described in this article involve “nerve-friendly” methods to decrease the possibility of nerve damage during performance of the blocks.
Abbott Laboratories, North Chicago, IL 60064.
Dentition Formula Review
\(\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \begin{eqnarray*}&&\begin{array}{lllllllllc}Canine-&I&6&C&2&P&8&M&4&\\&I&6&C&2&P&8&M&6&=\ 42\ teeth\end{array}\\&&\begin{array}{lllllllllc}Feline-&I&6&C&2&P&6&M&2&\\&I&6&C&2&P&4&M&2&=\ 30\ teeth\end{array}\end{eqnarray*} \end{document}\)Glossary
Buccal—pertaining to or directed toward the cheek
Diastema—space
Distal—away from the center line of the dental arch
Foramen—natural opening into or through a bone (pl. Foramina)
Frenulum—where lip attaches to bone
Ipsilateral—affecting the same side
Labial—pertaining to a lip
Lingual—pertaining to or toward the tongue
Mandible—bone of the lower jaw
Maxilla—upper jaw bone; includes floor of the orbit, side and lower walls of the nasal cavity, palate, and teeth
Mesial—toward the center line of the dental arch
Palatine—pertaining to the palate
Ramus—vertical portion of the caudal mandible
Rostral—toward the nose
Acknowledgments
A special thank you to Benjamin Colmery III, DVM, Diplomate, AVDC, for his eternal wisdom, expertise, and contributions to this article, as well as to Iris Cloyd, LVT, for her assistance in editing this article, and to Nichole Danova, DVM, for her assistance in adapting my photographs into the correct format for this article. All photographs were supplied courtesy of the author.
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Citation: Journal of the American Animal Hospital Association 43, 5; 10.5326/0430298
Location of the major palatine foramen in the dog (A) and cat (B).
Location of the canine infraorbital foramen.
A correct needle placement for performance of the canine infraorbital nerve block (A). During administration of the canine infraorbital nerve block, the nose of the patient should remain elevated to encourage caudal flow of the local anesthetic (B).
The feline infraorbital foramen is located within the junction of the zygomatic arch and the maxillary bone.
Landmark for performance of the canine maxillary nerve block (A). During performance of the canine maxillary nerve block, advance the needle dorsally to just beyond the root tips of the last maxillary molar (B).
Landmark for performance of the feline maxillary nerve block (A). Correct needle placement for performance of the feline maxillary nerve block (B).
Correct needle placement for performance of the middle mental nerve block in the dog.
Location of the mandibular nerve fossa in the dog (A) and cat (B).
