Local Anesthetic for the Dental Hygienist

OVERVIEW

History

For centuries, the Peruvian people have appreciated the pharmacologic actions of the leaves of the Erythroxylon coca, a shrub growing high in the Andes mountains. Chewing or sucking these leaves produces a sense of well-being. The plant ash releases an alkaloid in a form that can be absorbed across mucous membranes. This alkaloid is more commonly known today as cocaine.

The pure alkaloid of cocaine was isolated in 1880, and its pharmacodynamic properties were further investigated. In 1884, cocaine was used clinically as a local anesthetic in ophthalmology, dentistry, and surgery. In 1905, the first synthetic local anesthetic, procaine, was developed and became the prototype for these agents. There are currently 16 chemical agents used in local anesthesia; lidocaine, bupivacaine, and tetracaine are the most commonly used in clinical practice.

Uses

Local anesthetics are used to decrease pain, temperature, touch proprioception, and skeletal muscle tone. The degree of these actions is dependent on dose and concentrations of the drug, degree of hydrophobicity, and integrity of the application site.

Local anesthetics are used in a variety of clinical situations, from topical application to the skin or mucosa membranes to injectable agents used for peripheral, central, or spinal nerve block. Some agents are used for specific indications in preparations intended for anorectal or ophthalmic use.

Action

All local anesthetics reversibly block nerve conduction by decreasing nerve membrane permeability to sodium. This decreases the rate of membrane depolarization, thereby increasing the threshold for electrical excitability. All nerve fibers are affected, albeit in a predictable sequence: autonomic, sensory, motor. These effects diminish in reverse order.

Since these drugs can also block sodium channels in myocardial tissues, an antiarrhythmic action is created. Lidocaine and procainamide are routinely used as antiarrhythmics. Mexiletine and tocainide, oral congeners of lidocaine, possess a mechanism of action similar to lidocaine and are also used as antiarrhythmics. Currently, mexiletine and tocainide are not available in topical or injectable preparations for local anesthetic purposes. Finally, lidocaine has also been used as an alternative agent in the treatment of status epilepticus. While the mechanism of this effect is unclear, it is likely that blockade of neuronal sodium channels may be involved.

Features

Since the mechanism of action is similar, local anesthetics are selected based on their pharmacodynamic and pharmacokinetic features. Ideally, the agent should possess low systemic toxicity, have a rapid onset of action, have a duration of action long enough to complete the intended procedure, and not be irritating to the tissue.

Local anesthetics are listed according to their basic chemical class. Typically, local anesthetics are grouped as either "amides" or "esters". The "esters" include the prototype procaine, and also benzocaine, butamben picrate, chloroprocaine, cocaine, proparacaine, and tetracaine. These agents are derivatives of paraaminobenzoic acid and are hydrolyzed by plasma esterases. The "amides" include the prototype lidocaine, and also bupivacaine, dibucaine, etidocaine, mepivacaine, prilocaine, and ropivacaine.

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The amides are derivatives of aniline and are metabolized primarily in the liver, and the metabolites are then excreted renally. Lidocaine, mepivacaine, and tetracaine also may have some biliary excretion. Dyclonine and pramoxine do not fit into either chemical classification and might be suitable alternatives in patients with allergies to amides or esters.

Bupivacaine is commonly used for epidural anesthesia in obstetrics because of its motor-sparing properties and long duration although its systemic toxicity is greater than less potent agents such as lidocaine or mepivacaine.

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Selection of a local anesthetic is based largely on its pharmacokinetics. The site and route of administration can influence the characteristics of an agent. Most local anesthetics have a rapid onset when administered parenterally for infiltrative anesthesia, the fastest being lidocaine (0.5-1 minute) followed by prilocaine (1-2 minutes). The average onset of action for the remaining agents is between 3-5 minutes.

The exception is tetracaine, which can take up to 15 minutes. The onset of action is longer with epidural administration, averaging approximately 5-15 minutes for most local anesthetics. Procaine (15-25 minutes), tetracaine (20-30 minutes), and bupivacaine (10-20 minutes) all have slightly slower onsets of action when administered epidurally than the other agents.

Duration

The duration of action of the injectable anesthetic agents is important in their selection for various procedures. Procaine and chloroprocaine are the shortest-acting agents (0.25-0.5 hours), followed by lidocaine, mepivacaine, and prilocaine, which have slightly longer durations of action (0.5-1.5 hours). The longer-acting agents include tetracaine (2-3 hours), bupivacaine (2-4 hours), etidocaine (2-3 hours), and ropivacaine. Ropivacaine exhibits a duration of 8-13 hours when used for peripheral nerve block but the effective analgesia when administered epidurally is only 2.5-6 hours.

Epinephrine prolongs the duration of action of most local anesthetics, however, this is not always a consistent finding. Spinally administered local anesthetics tend to have a slightly shorter duration of action compared with other routes of administration.

Applied topically, local anesthetics reach peak effect at different times when applied to mucous membranes. Benzocaine is the fastest (1 minute), followed by lidocaine = cocaine < pramoxine < tetracaine < dyclonine and < dibucaine. All of the topical products have a duration of action ranging from about 30 minutes to an hour. Cocaine's effects can last up to 2 hours after topical application, and dibucaine has the longest duration of action at 3-4 hours.

The topical local anesthetics are used for skin and mucous membrane anesthesia. Most agents are used for both indications, except dibucaine, butamben picrate, and pramoxine, which are not used on mucous membranes. Pramoxine was found to be too irritating to the eyes and nose. Dyclonine and cocaine are not used in skin disorders, perhaps because they are well absorbed through the skin.

Tetracaine and proparacaine are available as ophthalmic preparations. Benzocaine and pramoxine are also included in a variety of anorectal preparations. Benzocaine candy or gum is used as a diet aid in combination with dieting. It works by decreasing the ability to detect degrees of sweetness by altering taste perception.

Adverse Reactions

Systemic absorption of local anesthetics can produce increased serum concentrations, resulting in toxicity in the CNS or the cardiovascular system. The longer the duration of action, the higher the potential for toxicity, which can include neurolysis with sloughing and necrosis of surrounding tissue. Parenteral local anesthetics are often coadministered epinephrine or other local vasoconstrictor to delay systemic absorption, prolong duration of action, and promote local hemostasis.

Local hypersensitivity reactions are more likely to occur with the "ester" type of local anesthetics. The "amide" type of local anesthetics has not demonstrated cross-sensitivity with the esters. Some of the topical preparations contain tartrazine and sulfites and should be used with caution in patients with these hypersensitivities.

LOCAL ANESTHETIC

Local Anesthesia

Local anesthesia should be a standard procedure performed before nonsurgical periodontal therapies. It is important that the clinician let the patient know of the standard at the initial examination appointment as well as when scheduling future appointments. The objective is to discuss the procedure before the actual administration of local anesthesia.

This allows the patient to say at an early juncture that they do not want local anesthesia, at which point the clinician should explore the patient’s concerns. The clinician can inform the patient that most patients need it for comfort and to allow for the best treatment. Keep in mind that the short amount of time for local anesthesia allows much more time for the actual treatment on a patient.

Administration from the Dental Hygienist

Dental hygienists who have completed required continuing education course work in local anesthesia administration have provided safe, effective injections. They have reported this service to be of value to the practice, and have had very few complications.

Injection

The patient is supine during the injection procedure, with the clinician seated, to promote comfort.4 This also avoids problems with patient syncope, which is the most common dental emergency and most common side effect of local anesthesia. Finally, having the patient supine helps the clinician easily view the landmarks necessary for safe and effective injections.

The tissue in the area of the injection is gently dried before the needle enters to reduce contamination from the oral cavity into deeper tissues. There should also be consideration for the use of an antimicrobial preprocedural oral rinse to sanitize the oral cavity as recommended by the latest Centers for Disease Control Guidelines for Infection Control in Dental Health-Care Settings (2003).

Oral Rinse

Antimicrobial oral rinses (for example, chlorhexidine gluconate, essential oils, or povidone-iodine) used by patients before a dental procedure are intended “to reduce the number of microorganisms the patient might release in the form of aerosols or spatter that subsequently can contaminate dental health-care personnel and equipment operatory surfaces.”

In addition, “preprocedural rinsing can decrease the number of microorganisms introduced in the patient’s bloodstream during invasive dental procedures such as nonsurgical periodontal therapies.” It is important to never inject in areas in the oral cavity with infection so as to prevent the spread of infection to deeper tissues (by way of needletrack infections)

Topical

When using injected local anesthetics, topical anesthetic is placed for at least one minute, preferably for two to three minutes (as per the package insert for all presently marketed topical anesthetics), on the soft tissue to increase comfort.

Topical anesthetics actually do penetrate mucous membranes of the oral cavity two to three millimeters below the surface to reach free nerve endings. Achieving comfort during the puncture of the tissue by the needle is especially important for maxillary facial injections. The clinician should inform the patient: “I am using this surface anesthetic to make the rest of the procedure more comfortable for you.” The injection will need to be performed within one to two minutes after removal of the topical anesthetic for maximum effectiveness.

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The clinician should not use an excessive amount of topical anesthetic which may cause an increased risk of toxicity and an increase of uncomfortable soft tissue anesthesia. In order to control these side effects, in preparation of tray set-up, the clinician should place a small amount of topical anesthetic in the inner rim from the bottom of the bottle. Roll the cotton tip applicator in the inner rim of the mouth of the bottle to imbed topical. Then place the cotton tip applicator with the topical anesthetic lightly on the tissue and only on the area of puncture; surface anesthesia is by contact and not by pressure!

Topical anesthetic sprays can make it difficult to control the amount of anesthetic and location of application and, thus, are not usually recommended for prepuncture soft tissue anesthesia in the oral cavity.

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The overlying palatal tissues are very dense and adhere firmly to the underlying palatal bone, the use of pressure anesthesia to the injection site before and during the injection to blanch the tissues will reduce patient discomfort by producing a dull ache to block pain impulses arising from needle penetration. Mirror handles are NOT recommended for pressure.

During the injection of the facial alveolar mucosa, the soft tissue should be kept taut by strong and steady retraction with two fingers and not a mirror. This will allow the needle to easily and comfortably penetrate the tissue, and the tissue will not have a chance to possibly reflect over the needle tip at the last minute. The index finger is used on the dominant side intraorally and the thumb extraorally. On the nondominant side, the thumb is used intraorally and the index finger extraorally. With this level of retraction, a safe zone of 10 to 15 millimeters is created around the needle tip to prevent needlestick injuries.

In cases where slippage of tissue is anticipated, a small gauze square is effectively used to help retract the tissue during the injection (see figure 6). This occurs mainly when administering the anterior superior alveolar nerve block or any maxillary anterior infiltration as the clinician retracts the upper lip. It is important, though, to not allow the gauze to interfere with the injection. Using the same two-finger retraction with the inferior alveolar, incisive, and buccal (long) blocks is also helpful in maintaining control during the injection.

Administration Techniques

The bevel of the needle is always placed toward the bone, deep within the tissue. To easily check the bevel position, the drip of the solution is noted off the bevel surface of the needle. If the bevel is not toward bone and bone is touched, there may be tearing of the periosteum covering the bone and resultant discomfort.

Newer needles are available having a colored graphic on the hub of the needle located on the same side as the bevel that can easily help the clinician with the determination of the bevel position. This is especially helpful when performing the inferior alveolar block due to its deep tissue penetration (average depth of the needle is 25 millimeters) .

The clinician should not move the needle horizontally deep within the tissue, because it can tear the tissue and also can cause needle deflection. With deflection, the clinician does not know the exact position of the needle tip or bevel hidden in the tissue. If the position of the needle is not correct with initial placement, the needle should be removed from the tissue completely or most of the way by the clinician before re-entering the tissue. The discomfort may not be felt at the time of the injection, but the area will feel sore to the patient later due to tissue tears.

An increase in the occurrence of movement of the needle in the tissues by some clinicians when she has viewed the administration of the posterior superior alveolar nerve block can happen. The uncontrolled movement of the needle in this case can lead to the complication of an extraoral hematoma if the maxillary artery or pterygoid plexus of veins is inadvertently punctured. In addition, bending the needle is also to be avoided with this maxillary block for the same reason. At other times, the author has noted horizontal needle movement when viewing clinicians administering the inferior alveolar nerve block, which is problematic due to its deep tissue penetration as noted earlier

Extra needle deflection can also be avoided by the use of a proper gauge for the needle depending on the injection given. The use of a smaller gauge needle for patient comfort is unwarranted since studies show that patients cannot feel the difference between needle 13 A 25-gauge long needle (inferior alveolar block and long buccal block by default) and a 27-gauge short needle (all other injections) are recommended due to their ability to decrease deflection, provide accuracy, and allow for safety when aspirating.

Studies using human tissue and radiography have demonstrated that a 30-gauge needle inserted to a depth of 25 millimeters deflects four millimeters, a 27-gauge needle deflects two millimeters, and a 25-gauge needle deflects one millimeter.

Needles

This use of a larger gauge of needle is especially important for the inferior alveolar block injection with its considerable depth of penetration when accuracy and aspiration ability are key to success. With this injection, the needle tip must be within one millimeter of the landmark of the mandibular foramen for accurate deposition. This block also has the highest risk for positive aspiration.

The differences in deflection between needles with varying needle gauges as the needle enters tissue can be noted using glycerin soap. The differences with aspiration between the different needle gauges can also be noted using colored water in a cup. Aspiration is important; if the local anesthetic agent is deposited into a blood vessel, no anesthesia is obtained and serious complications can occur.

Pain Control

The main factor for pain control during an injection, though, is the slow dispensing of the local anesthetic agents. Each full cartridge should be dispensed over one minute. The needle entering the tissue is not the major cause of discomfort, contrary to popular thought; it is rather the pressure and the volume of the fluids being injected into the tissues that causes the most distress to the patient.

Even when the patient has received an initial dose of agent, he or she may not immediately feel the pressure of the added agent if given too quickly, but will later feel tissue soreness. The clinician should use this time dispensing to talk softly, assuring the patient of comfortable treatment. The clinician can communicate with the patient: “I am depositing solution slowly so it will be more comfortable, but you are not receiving any more than usual.

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The clinician may want to check the usual rate of deposition of the agent outside the patient sphere by using a cup. First, the clinician can observe to make sure that the rate does not produce a stream but a constant drip when dispensing the agent; later the clinician should cover the tip and just watch the rubber stopper move at a constant rate and have a partner check the actual rate of deposition of the agent, thus simulating the actual situation where the agent enters blindly into the tissue of the patient.

The newer computer-controlled anesthetic devices automatically incorporate this pain reduction strategy since they deliver the local anesthetic agent more slowly than any clinician can. Data from clinical studies indicate that patients generally find the computer-controlled local anesthetic injections to be less painful than those delivered by traditional syringe, even in the palatal area.

Current Concepts

One common reason for anesthetic failure is due to an inaccurate injection; the local anesthetic is not delivered near enough to the relevant nerve or bony area and thus it is either totally ineffective or only partial anesthesia is attained. Injection into muscle and not close to bone will usually prevent the local anesthetic from working and may lead to excessive muscle soreness afterwards.

Another common reason for anesthetic failure is that too little local anesthetic agent is used. The patient may have large teeth with unusually long roots or the bone surrounding the roots may be excessively thick.

The final common reason for anesthetic failure in patients is that treatment is started before the local anesthetic can take effect; usually three to five minutes is required for most local anesthetic agents to take effect in average situations. This time period allows the nerve to be fully bathed in the local anesthetic agent.

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This particular problem is especially noted with injections for the procedures involving the mandibular arch due to its density and anatomical variations. Placing the patient in an upright position for 20 seconds after the injection series has been shown to promote deeper anesthesia in most mandibular arch cases. 

It is important to note that all these common reasons for failure just discussed are based on anatomical considerations. Management of pain through local anesthesia requires a thorough knowledge of the anatomy of the skull, trigeminal nerve, and related tissues, and how those are presented in each individual patient. A review of a textbook on local anesthetics by the clinician may be considered or a continuing education course in the subject may be necessary to reinforce the needed basic anatomical information

Vasoconstrictor

Dental hygiene procedures usually involve varying levels of bleeding during instrumentation without utilization of local anesthesia with vasoconstrictor, hampering effective treatment by the clinician. This is especially true in the areas of periodontal furcations or deep periodontal pockets. Obtaining hemorrhage control using vasoconstrictor with the local anesthetic agent is one of the key ways to increase the effectiveness of basic local anesthetic procedures.

Additionally, the vasoconstrictor makes the injection safer: there is slower absorption into general circulation and over a longer period of time, so there is a lower chance of overdose from the local anesthetic agent. As an added bonus, the patient experiences a longer duration of anesthesia, possibly until a postoperative analgesic can take effect after instrumentation.

Most contraindications to the use of vasoconstrictor during elective dental procedures are relative and not absolute. An absolute contraindication indicates that under no circumstance should this drug be administered to this patient because the possibility of potentially toxic or lethal reactions is increased.

With a relative contraindication, the drug in question may be administered to the patient after carefully weighing the risk of using the drug against its potential benefit, and if an alternative drug is not available. Most of these relative contraindications due to medical history complications recommend a reduction of the vasoconstrictor but not a complete lack of it.

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This overall reduction of vasoconstrictor can be accomplished by the use of more than one type of local anesthetic agent, giving a series of injections (“cocktail approach”).

Half of the amount of agent with vasoconstrictor is administered initially in the specific areas according to the patient’s needs and staying within the maximum recommended dosage. Then later, the other half of the amount needed is administered in the same region using a plain agent.

Thus, the overall amount of agent is not reduced; the patient responds with an adequate depth and length of anesthesia, and hemorrhage control is assured, while using less vasoconstrictor.

All types of agents can be used together without any anticipated problems. A series of injections with different agents to reduce vasoconstrictor level has been used for many years in dentistry with success. However, there is no formula for the correct degree of reduction and it is important to use the smallest dose of vasoconstrictor that still will allow dental treatment.26 Another method to reduce the amount of the vasoconstrictor would be to reduce the procedures to be accomplished during the appointment (sextant instead of quadrant, quadrant instead of half-mouth).

The American Dental Association (ADA) recommends the use of a reduced amount of vasoconstrictor in controlled cases of cardiovascular disease (CVD) to allow these patients much needed profound anesthesia. This use of smaller amounts of vasoconstrictor with the agent also minimizes the risk of direct vascular injection, which could be dangerous in these patients.

Block vs. Infiltration

It is recommended that pain control in dentistry be performed using regional block anesthesia instead of infiltration methods. Stanley F. Malamed, DDS, professor of anesthesia and medicine at the University of Southern California in Los Angeles, states, “Fewer penetrations of mucous membrane and smaller volume of local anesthesia is administered (with regional blocks) to provide adequate anesthesia.”

This use of regional block anesthesia works well with the standard dental hygiene protocol of performing quadrant or half-mouth procedures during nonsurgical periodontal therapy (see figure 10).29 There is a move to the use of sextants for nonsurgical periodontal therapy, especially in medically compromised cases, to improve quality of procedures performed, reduce systemic complications in the patients, and increase local healing. In most cases, it is important to perform regional block anesthesia on the mandibular arch, since infiltration is not as effective in the mandibular arch due to the increased density of the bone, especially in the posterior region of the arch.

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Assurance of pulpal anesthesia for nonsurgical periodontal therapy is important, and it is more easily obtained for a longer length of time with regional block anesthesia, in addition to soft tissue anesthesia. Pulpal anesthesia allows more comfortable instrumentation on possibly sensitive dentin-covered root surfaces.

The clinician should inform the patient that more anesthetic is available if needed, and always add more local anesthetic agent as needed, if requested. However, the clinician should always keep below the recommended maximum dosage for that individual patient.

Patients need an additional amount over the suggested amounts in local anesthesia textbooks if they have extreme root length and density of bone or increased levels of inflammation that promotes acidity in the tissues. The clinician should let the patient know that when agent is added, it is usually not even felt if given properly, since the surface tissues still have a significant amount of anesthesia.

Dental Hygiene Procedures

When using regional block anesthesia, it is important to inject from posterior to anterior based on increased levels of sensitivity of the tissues as one moves from posterior to anterior in the oral cavity.

The clinician should consider periodontal probing of each quadrant with anesthesia after the regions are instrumented. Probing patients during an initial examination with inflammation and without anesthesia can be very uncomfortable to the patient, and associated bleeding and deposits may reduce accuracy.

Both the ADA and the American Academy of Periodontology recommend that the Periodontal Screening and Recording (PSR) System® be used during the initial examination to screen for periodontal disease instead of a full-mouth periodontal probing. This system takes only a few minutes and tells the clinician immediately the level of periodontal involvement and treatment options needed by the patient. The PSR also summarizes necessary information with minimum documentation for submission to third party payers. However, the PSR is not intended to replace a comprehensive periodontal examination when indicated.

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If the clinician is individualizing the use of local anesthetic agents, as should be the case, each tray will have to be prepared after the patient is seated; the universal use of one type of local anesthetic agent is not in the best interest of the patient since medical and dental histories can vary per patient. It is important that the dental practice keep supplies for local anesthesia nearby where it is used and not in another treatment room or lowest drawer, so that the clinician does not have to get up to assemble the local anesthetic tray.

As an example of individualizing the approach to the use of local anesthetic agents, only a few of the agents are more highly recommended for pregnant patients than others

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Most local anesthetics have not been shown to be teratogenic in humans and are considered relatively safe for use during pregnancy.35 The recommendation for caution (Category C) relates primarily to limited data collected in animal studies. Other agents are not at full caution; instead adverse drug effects to humans cannot be ruled out for all these agents (Category B). Because all local anesthetics can cross the placenta and cause fetal depression, limiting the dose to the minimum required for effective pain control is obviously advisable. Epinephrine, a naturally occurring hormone, is generally considered to have no teratogenic effects when administered with dental anesthetics.

There are also a few drug interactions to keep in mind when using local anesthetics and vasoconstrictors. A review of a textbook on local anesthetics by the clinician may be considered or a continuing education course in the subject may be necessary to have the updated information when considering the use of various agents. No local anesthetic agent should be given to a pregnant patient without the consideration of her physician’s desires and her wishes.

What to do

The clinician should make sure to change the needle after three to four injections or when the patient is sensitive, so that they are always sharp and can easily enter the tissue. Dental practices should be wary of low cost or old local anesthesia supplies. Hard rubber stoppers in the cartridges that do not easily allow engagement by the harpoon - forcing the clinician to slam them on a countertop to engage the needle - do not promote a very friendly way to introduce the patient to a gentle local anesthetic technique.

Needles that are hard to open and remove from the syringes (one has to use a rheostat) may increase needlestick risk for the clinician. Harpoons on syringes that are bent, dull, or even rusty do not allow for effective use of local anesthesia. Traditional syringes do not have an unlimited lifetime and should regularly be checked with colored water in a cup to make sure they can aspirate.

The clinician needs to keep current in pain control. A new topical anesthetic for the oral cavity has been developed containing low levels of lidocaine, prilocaine, and a thermosetting agent. This new formulation is similar to the eutectic mixture of local anesthetics (EMLA) that is commonly used on the skin prior to needle puncture, since the present formulation is only recommended for dermatological use by the Food and Drug Administration.

Pain control should be an integral part of dental hygiene nonsurgical periodontal therapies. Clinicians involved in these therapies should consider use of the Velvet Touch when utilizing local anesthesia during dental hygiene treatment. This is true whether the dental hygienist can directly administer local anesthesia or the supervising dentist performs this procedure.

Desirable Properties

A local anesthetic agent should have many desirable properties, although no one drug is bound to have all of them. The following is a list of such properties:

1. The local anesthetic should not be irritating to the tissue when applied.2. The anesthetic action of the agent should be completely reversible. In

other words, it should not alter permanently nerve structure.3. The time of onset of anesthesia should be as short as possible.4. Anesthesia produced should last long enough to allow the dentist to

complete the procedure, yet not so long that the patient takes hours to recover from its effect after the procedure is completed. Anesthetic drugs used today produce clinical duration of action, ranging from 30 minutes to over 90 minutes depending upon the drug selected and the particular preparation of the drug used. The dentist should select a local anesthetic agent depending upon the length of anesthesia desired.

5. Systemic toxicity of the drugs is a significant factor to consider in its selection for use as a local anesthetic.

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The anesthetic drug must be effective regardless of whether it is injected into the tissue or applied locally to mucous membranes. For example, procaine and mepivacaine are effective injectable local anesthetics but are relatively weak agents when applied topically to mucous membranes. If used in higher concentrations, these drugs do produce effective topical anesthesia; however, at these concentrations they irritate the tissues and increase the risk of systemic toxicity. Lidocaine and tetracaine are two agents that meet the criterion of producing effective local anesthesia regardless of how they are applied, whether injected into the tissue or used topically in clinically acceptable concentrations.

There are some additional desirable properties of an ideal local anesthetic agent: The drug should be sufficiently potent to give complete anesthesia in clinically

acceptable concentrations. There should be no allergic reaction to the drug. It should be stable in solution and readily undergo biotransformation in the body. It should either be sterile or be capable of being sterilized by heat without

deterioration.

Classification

EstersEsters of benzoic acid: Butacaine CocaineEthyl aminobenzoate (benzocaine)Hexylcaine PiperocaineTetracaineEsters of para-amino- benzoic acid:ChloroprocaineProcainePropoxycaineAmides Articaine BupivacaineDibucaineEtidocaineLidocaineMepivacainePrilocaineQuinolineCentbucridine

TYPES

Selection

Selection of a local anesthetic should take into account three factors:

a. duration of the procedure or the length of time for which pain control is desired;

b. potential for pain after treatment; long duration agents should be employed when postoperative pain is thought to be a factor;

c. contraindications for a particular anesthetic

Lidocaine

Lidocaine is an amide local anesthetic. Compared to procaine, it possesses more rapid onset of action, produces more profound anesthesia, and has a longer duration of action and a greater potency. Lidocaine is the most widely used local anesthetic in dentistry.

Lidocaine is available in three formulations _ 2% without a vasoconstrictor, 2% with 1:50,000 epinephrine, and 2% with 1:100,000 epinephrine. Lidocaine without a vasoconstrictor is rarely used in a typical dental practice, because the vasodilation effect limits pulpal anesthesia to about 5-10 minutes, leads to high blood level of the drug, which may lead to overdose reaction and the possibility of excessive bleeding into the region of anesthetic administration.

2% Lidocaine with 1:50,000 epinephrine resolves these problems. It produces approximately 60 minutes of pupal and 3-5 hours of soft tissue anesthesia.

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For duration and depth of pain control in a typical dental patient, 2% Lidocaine with 1:100,000 epinephrine is recommended over 2% Lidocaine with 1:50,000 epinephrine. The lesser amount of epinephrine in the former solution makes it more acceptable to those individuals who are sensitive to vasoconstrictors. 2% Lidocaine with 1:100,000 epinephrine provides excellent hemostatic action by decreasing tissue perfusion in the region of injection.

Lidocaine produces topical anesthetic action in clinically acceptable concentrations.

Mepivacaine

Mepivacaine, an amide category of anesthetic agent, produces only slight vasodilation. The duration of pulpal anesthesia with mepivacaine without a vasoconstrictor is 20 to 40 minutes and 2 to 3 hours of soft tissue anesthesia. Mepivacaine is available in 2 formulations: 3% without a vasoconstrictor, and 2% with a vasoconstrictor.

3% Mepivacaine without a vasoconstrictor is recommended for patients in whom a vasoconstrictor is not indicated and for dental procedures not requiring lengthy pulpal anesthesia. Mepivacaine without a vasoconstrictor is the most often administered local anesthetic in pediatric dentistry and is just as often used in geriatric patients.

2% Mepivacaine with a vasoconstrictor gives pulpal anesthesia of approximately 60 minutes and soft tissue anesthesia of 3 to 5 hours, similar to those obtained with Lidocaine-epinephrine solutions.

Mepivacaine does not produce topical anesthetic action in clinically acceptable concentrations.

Prilocaine

Prilocaine, an amide category of agent, is characterized by the clinical action delivered depending upon the type of anesthetic technique employed. There is a significant variation in the duration of anesthesia produced depending upon whether the agent is delivered supraperiosteal or nerve block.

Prilocaine plain through infiltration (supraperiosteal) produces shorter duration of pulpal (5 to 10 minutes) and soft tissue (l 1/2 to 2 hours) anesthesia; regional block (e.g., inferior alveolar nerve) provides pulpal anesthesia for up to 60 minutes and soft tissue anesthesia for 2 to 4 hours. Thus prilocaine plain is frequently able to provide anesthesia that is equal in duration to that noted with lidocaine and mepivacaine with vasoconstrictor.

Prilocaine is used with vasoconstrictor epinephrine in the formulation of 1:200,000 which provides lengthy pulpal anesthesia of 60 to 90 minutes and soft tissue anesthesia of 3 to 8 hours. This formulation has the advantage that it offers the least concentrated epinephrine solution currently available.

Bupivacaine

Bupivacaine, an amide category anesthetic agent, is used as a 0.5% solution with 1:200,000 epinephrine. It is generally used under circumstances with lengthy dental procedures requiring pulpal anesthesia in excess of 90 minutes. (e.g., full mouth reconstruction and extensive periodontal operations), and procedures in which postoperative discomfort is anticipated (e.g., endodontic, periodontal, and oral surgery).

One consideration in the selection of Bupivacaine as an anesthetic agent is the possibility of post operative soft tissue injury produced by self-mutilation because of the long lasting anesthesia produced.

COMPLICATIONS

Safety

The local anesthetics used in dentistry are considered very safe and have a low incidence of adverse reactions associated with their administration. These adverse events may be classified as either systemic or localized. Systemic complications can occur as a result of psychogenic reactions induced by anxiety, toxicity secondary to high levels of the drug in blood, or allergy.

Localized complications may manifest in a number of ways, and the focus of this article is to briefly review them. It is important to note that complications are not always preventable, and their occurrence does not necessarily imply poor technique by the dentist. The reality is that with an estimated 300 million injections being administered yearly in the United States alone, even those events that may be considered rarer will be experienced by many patients.

Paresthesia

Complete anesthesia or an altered sensation in the lip or tongue may persist beyond the expected duration of action of a local anesthetic. Commonly referred to as a paresthesia, these neuropathies may manifest as a total loss of sensation (anesthesia), a burning or tingling, pain to non-noxious stimuli (dysesthesia), or increased pain to noxious stimuli (hyperesthesia). Prolonged anesthesia or paresthesia in the tongue or lip is known to occur following surgical procedures such as extractions, and it is assumed that the cause is direct trauma to either the lingual or inferior alveolar nerve. However, persistent anesthesia or paresthesia can also occur following nonsurgical dentistry. Most these are transient and resolve within eight weeks, but they may become irreversible. Whereas the former are an annoyance for the patient, the latter are much more distressing.

There are several putative causes of post injection paresthesia. Hemorrhage into the nerve sheath may lead to an intraneural hematoma, which then causes pressure on nerve fibers, impairing normal conduction. The hematoma and associated edema usually resorb within several weeks, and symptoms subsequently resolve. If scar formation results, there may be permanent loss of sensation. Direct trauma by the needle may also lead to similar damage. In addition, administration of local anesthetic from a cartridge contaminated by alcohol or sterilizing solution may induce paresthesia. Finally, neurotoxicity may be a factor, since a review of the literature suggests that local anesthetics have this potential.

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How often do paresthesias occur in nonsurgical dentistry? A recent study led to an estimated incidence of 1 irreversible paresthesia out of every 785,000 injections. It has been stated in a legal case in Canada that this low frequency would not warrant advising every dental patient of this risk prior to each injection. This same study did note that specific drugs were more likely to be associated with paresthesia.

It can be seen that two drugs, articaine (which is available in Canada and parts of Europe under the trade name Ultracaine, among others) and prilocaine (Citanest), were more likely to be associated with paresthesia compared with the other anesthetics, and this was statistically significant when compared to the distribution of use. These same two drugs were again found to be significantly more likely to be associated with paresthesia in 1994.

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The reasons for this relationship to the type of anesthetic are speculative only. Differences in metabolism of these drugs would not be relevant since it occurs in organs away from the site of the neuropathy. Their only common feature is that they are the only injectable local anesthetics in dentistry that have a concentration of 4 percent, whereas the others are lower. It may be conjectured that toxicity may manifest simply because of the higher concentration of these drugs, as opposed to any unique characteristic. Needle trauma to the nerve combined with deposition of a large quantity of drug may be more likely to induce residual nerve damage.

Supporting a role of drug concentration are reports of neurologic deficits in animal studies using 4 percent lidocaine and in human studies of spinal anesthetics with 5 percent lidocaine. This should be contrasted with the rare reports of neuropathy with 2 percent lidocaine (Xylocaine, among others), which is used in dentistry. 

There is no guaranteed method to prevent paresthesia or prolonged anesthesia. The inferior alveolar nerve block requires the practitioner to advance the needle near the inferior alveolar and lingual nerves.

Practitioners attempt to place the needle near these nerves without intentionally striking them, yet this can occur and may be perceived as an "electric shock" sensation by the patient. Interestingly, this sensation does not imply that paresthesia will result. Directly contacting these nerves is not an indication of improper technique; it is simply a risk of carrying out intraoral injections.

Prevention

• If the patient feels "electric shock," move needle away from this site prior to injecting.

• Do not store cartridges of local anesthetic in disinfecting solutions.

Most paresthesias are transient and resolve within eight weeks. This is fortunate as there is no definitive means of improving the patient's symptoms. The dentist must show concern and reassure the patient that these events can occur and usually resolve over time.

The dentist should note the signs and symptoms and maintain contact with the patient. A change in the character of the symptoms can be an encouraging sign that there may be resolution of the neuropathy. It may indicate that there is healing of the nerve, and with time the patient may regain normal sensation.

Management

The patient who has had no change in symptoms over a prolonged period, such as several months, is less likely to have a satisfactory outcome. Restoring sensation by microsurgery may be considered by those with training in this area. It has been stated that microsurgery is most likely to be successful if the patient is evaluated within the first month or the first three months. There is no guaranteed method of treating prolonged anesthesia or paresthesia.

Reassure the patient that the condition is usually temporary although, rarely, it can remain indefinitely.

• Note signs and symptoms and follow up within one month. • If symptoms persist for more than two months, refer to an oral and

maxillofacial surgeon with experience in this field.

Trismus

Limited jaw opening, or trismus, is a relatively common complication following local anesthetic administration. It can be caused by spasm of the trismus muscles of mastication, which in turn may be a result of needle insertion into or through one of them. The most common muscle to be the source of trismus is the medial pterygoid, which can be penetrated during an inferior alveolar nerve block using any of the three main techniques: the conventional approach, the Vazirani-Akinosi (closed mouth) technique, or the Gow-Gates. Rarely, the temporalis may be penetrated before it attaches onto the coronoid process if the needle is inserted too far laterally.

Even more rarely, the lateral pterygoid muscle may be penetrated if a block is administered too far superiorly. Bleeding into the muscle following injection may also cause muscle spasm and trismus. Furthermore, injection of local anesthetic directly into muscle may cause a mild myotoxic response that can lead to necrosis. In the rare situation of an infection from the injection, trismus may also develop.

The main symptom of trismus is the limitation of movement of the mandible, which is often associated with pain. Symptoms will arise from one to six days following an injection. The duration of symptoms and their severity are both variable. Improvement should be noted within two to three days. If there is no improvement within this time, the dentist should consider other possible causes, such as infection, and treat accordingly.

Prevention of trismus: • Follow basic principles of a traumatic injection technique.

Management of trismus: • Apply hot, moist towels to the site for approximately 20 minutes every

hour. • Use analgesics as required. • The patient should gradually open and close mouth as a means of

physio- therapy.

Hematoma

A hematoma is a localized mass of extravasated blood that may become clinically noticeable following an injection. In this context, it can occur following the inadvertent nicking of a blood vessel during the penetration or withdrawal of the needle. When carrying out intraoral injections, practitioners often pierce blood vessels; but only when there is sufficient blood leaking out can hematoma be seen. The vessels most commonly associated with hematomas are the pterygoid plexus of veins, the posterior superior alveolar vessels, the inferior alveolar vessels, and the mental vessels.

The patient will notice development of swelling and the discoloration of a bruise lasting seven to 14 days. It is important to note that a hematoma will form independently of aspiration results. A negative aspiration does not guarantee an absence of a hematoma, as the needle may nick a blood vessel either on the way in or upon withdrawal. Aspiration results merely report the contents at the needle tip at the time of aspirating. Similarly, a positive aspiration does not imply that a hematoma will result, since the needle may simply have entered a vein at the time of aspiration, and the amount of blood leaking out from this vessel penetration may be clinically unnoticeable.

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Prevention of hematoma: Follow basic principles of a traumatic injection technique. Minimize the number of needle penetrations into tissue. Use a short needle for the posterior superior alveolar nerve block.

Management of hematoma: If visible immediately following the injection, apply direct pressure, if

possible; Once bleeding has stopped, discharge the patient with instructions to: Apply ice intermittently to the site for the first six hours. Do not apply heat for at least six hours. Use analgesics as required. Expect discoloration. If difficulty in opening occurs, treat as with trismus, described above.

Pain

Occasionally, injection of local anesthetic can be accompanied by pain or a burning sensation. Passing the needle through a sensitive structure such as muscle or tendon may cause pain. It may occur during injection if the solution is administered too quickly and therefore distends the tissue rapidly.

Local anesthetic solutions that are too cold or too warm may also cause discomfort. Solutions that are more acidic, namely those with vasoconstrictor, may cause a short-lasting burning sensation. Cartridges stored in a disinfecting solution such as alcohol may have residual amounts of solution on the end of the cartridge that can then be administered inadvertently during injection.

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Prevention of pain:Inject slowly: Take at least one minute to

administer one cartridge.Store cartridges at room temperature.

Do not store cartridges of local anesthetic in disinfecting solutions.

Management of pain:Pain or burning on injection is usually self-limiting

because it is treated by the onset of anesthesia.

Needle Breakage

This event is very rare. Sudden, unexpected movement of the patient is the primary cause. It is believed that smaller-diameter needles, i.e., 30 gauge, are more likely to break than larger-diameter, i.e., 25 gauge. Needle breakage usually occurs at the hub, which is the reason for never inserting a needle completely into tissue.

Although bending a needle may be considered for injection techniques such as the Vazirani-Akinosi or the maxillary nerve block, some advise against this practice. If it is done, it is important to do so only once because repeated bending will weaken the connection at the hub and predispose the needle to breakage.

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Prevention of needle breakage:Do not insert a needle into tissues up to its hub; always

leave a portion exposed.Use long needles if a depth of more than 18 mm is required.Use larger-diameter needles (25 gauge is ideal) for the

deeper blocks, such as the three mandibular block techniques (conventional, Gow-Gates, and Vazirani-Akinosi) and the maxillary nerve block.

Do not apply excessive force on the needle once it is inserted in tissue.

If redirecting a needle is required, withdraw it almost completely before doing so.

Do not bend a needle more than once.

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Management of needle breakage: Remain calm. Ask the patient to remain still; keep their mouth open by not

removing your hand.

If a portion of the needle is visible, remove it with a hemostat or similar instrument.

• If the needle is not visible: - Inform the patient. - Record the events in the chart. - Refer the patient to an oral and maxillofacial surgeon. - Surgical removal should only be attempted by someone

experienced with surgery of the involved region and after radiographs have been taken to help localize the needle.

Soft Tissue Injury

With the loss of sensation that accompanies a successful block, a patient can easily bite into his or her lip or tongue. Swelling and pain will result following the offset of anesthesia. This event is most common in children or patients who are mentally challenged or demented, such as those with Alzheimer's disease.

The child's parent or guardian, or the caregiver with the mentally challenged patient or those with dementia, should be advised to carefully observe the patient for the expected duration of anesthesia. Nevertheless, soft tissue injury may also be a concern for mentally normal patients who are at risk of an exaggerated response to trauma. Therefore, patients with bleeding abnormalities should also be warned of the risks of lip and tongue biting.

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Prevention of soft tissue injury: • For pediatric, mentally challenged, or demented patients, use a local

anesthetic of appropriate duration. • Warn the parent, guardian, or caregiver to watch the patient carefully

for the duration of soft-tissue anesthesia to prevent biting of tissue. • In children, consider placing a cotton roll between the mucobuccal

fold for the duration of anesthesia. • Explain the risks of soft tissue injury to patients with bleeding

abnormalities.

Management of soft tissue injury: • Use analgesics as required. • Uses rinses or applications with lukewarm dilute solutions of salt or

baking soda. • Consider applying petroleum jelly over lip lesion.

Facial Nerve Paralysis

Anesthesia of the facial nerve may occur if the needle has penetrated the parotid gland capsule and local anesthetic is then administered within. This nerve, the seventh cranial nerve, is contained within the parotid gland and provides motor function through its five branches–the temporalis, zygomatic, buccal, mandibular and cervical. Needle placement into the parotid may occur if there is over insertion during an inferior alveolar nerve block or the Vazirani-Akinosi block. The result of anesthesia of these branches of this nerve includes a transient unilateral paralysis of the muscles of the chin, lower lip, upper lip, cheek, and eye. There will be a loss of tone in the muscles of facial expression. In the past, the term Bell's palsy was commonly used to refer to all paralyses of the facial nerve, but it is now restricted to those induced virally.

Facial nerve paralysis secondary to local anesthetic injection is temporary and will last the expected duration of anesthesia of soft tissue for the particular anesthetic administered. There are risks if there is a loss of the protective reflex to close the eyelid.

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Unwanted anesthesia of other nerves may also occur. Ocular complications following temporary paralysis of cranial nerves III, IV, or VI, as well as the optic nerve, have been described.

The proposed mechanism for these events is intravenous transport of local anesthetic to the cavernous sinus. Careful aspiration to avoid intravenous injection should prevent this complication.

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Prevention of facial nerve paralysis:• Follow basic principles of a traumatic injection technique.

• Avoid over insertion of the needle. • For the conventional inferior alveolar nerve block, do not inject unless bone has been contacted at the appropriate depth.

Management of facial nerve paralysis:• Reassure the patient of the transient nature of the event. 

• Advise the patient to use an eye patch until motor function returns. • If contact lenses are worn, they should be removed.

• Record details in the patient's chart

Infection

With the introduction many years ago of sterile disposable needles, infection is now an extremely rare complication of local anesthetic administration.It may occur if the needle has been contaminated prior to insertion. The normal flora of the oral cavity is not a concern since they do not lead to infection in patients who are not significantly immunocompromised. In fact, bacteria enter the tissues with every needle insertion, yet the body's normal defense prevents a clinical infection. In patients who are severely immunocompromised, a topical antiseptic or an antiseptic rinse such as chlorhexidine could be considered prior to needle insertion.

If an infection does occur, it will likely manifest initially as pain and trismus one day post injection. If these symptoms persist for three days and continue to worsen, the possibility of infection should be considered. At this stage, this patient should be examined for other signs of infection, such as swelling, Iymphadenopathy, and fever.

When there is an active site of infection, such as an abscess, needles should not be inserted. This is not only because the low pH will prevent the onset of local anesthetic action, but also because there is the potential for spreading the infection.

Continued

Prevention of infection:• Use sterile disposable needles.• Do not contaminate the needle by contacting

nonsterile surfaces outside the mouth.• In severely immunocompromised patients, consider a

topical antiseptic prior to injection.

Management of infection:• Prescribe antibiotics, such as penicillin, in an

appropriate dose and duration. • Record details in the patient's chart and follow up to

determine progress. 

Mucosal Lesions

Occasionally, the intraoral mucosa may show signs of sloughing or ulceration. The epithelial layer may desquamate from prolonged application of topical anesthetic. It is possible, but not common, that necrosis of tissues may result from high concentrations of vasoconstrictor, such as 1:50,000.

Sites of ulceration consistent with a diagnosis of aphthous stomatitis may also result following local anesthetic administration. For each of these, the lesions will be present for one to two weeks and resolve irrespective of treatment. Drug therapy is seldom warranted. Simple measures such as saline or sodium bicarbonate rinses may assist healing by keeping the sites relatively clean.

Prevention of mucosal lesions:• Do not leave topical anesthetic on mucosa

for prolonged periods.

Management of mucosal lesions:• Reassure the patient; advise him or her of

the expected duration of one to two weeks.• Use rinses with lukewarm dilute solutions

of salt or baking soda, until symptoms resolve.