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ESSAYLocal Anesthesia in DentistryRitu BahlSchool of Dental Medicine, University of Connecticut Health Center, Farmington, Connecticut

This paper was th e First Place winning paper of the ADSA Student Essay AwardContest for 2003.

Pain and dentistry ar e often synonymous in th eminds of patients, especially those with poor den-tition due to multiple extractions, periodontal disease re-quiring surgery, or symptomatic teeth requiring end-odontic therapy. Members of t he public perceive a gooddentist as a practitioner who causes little or no discom-fort. In turn, dental practitioners identify a good anes-thetic as one that allows them to focus solely on oper-ative procedures without distractions from pain-inducedpatient movements. The everyday practice of dentistryis therefore based upon achieving adequate local anes-thesia.Research has shown that th e fear of pain associatedwith dentistry is closely associated with th e most com-mon method for blocking pain during dental proce-dures-intraoral administration of local anesthetics. Thisis considered aversive due to the pain associated withthe injection and th e perceived threat of needle punc-ture prior to th e injection.1 Another survey finding wasthat those individuals who reported themselves as highlyfearful of dentistry were worried about receiving oral in-jections and demonstrated an association between highdental anxieties and missed or delayed appointments.2Pain is a result of stimulation of nociceptors that arereceptors preferentially sensitive to a noxious stimulusor a stimulus that will become noxious if prolonged.When nociception reaches th e cerebral cortex, it maybe perceived as pain. Pain may be abolished by inter-rupting th e pathways that carry th e information of th estimulus from th e periphery of th e body to th e centralnervous system, by blocking the central nervous system,or by removing th e stimulus. Local anesthetics blocksensory neuronal conduction of noxious stimuli fromreaching th e central nervous system.

Received June 20 , 2003; accepted for publication J ul y 1 2, 2004.Address correspondence to R itu Bahl, School of Dental Medicine,University of Connecticut Health Center, 263 Farmington Avenue,Farmington, CT 06030; [email protected] Prog 51:138-142 2004) 2004 by the American Dental Society of Anesthesiology

NEUROANATOMYThe sensory supply to th e teeth, jaws, and oral mucosais derived from th e maxillary and mandibular division ofthe trigeminal (fifth cranial) nerve, whose cell bodies ar efound in th e Gasserian ganglion. The maxillary nervecarries purely sensory fibers, exits th e skull through theforamen rotundum, and enters th e pterygo-palatine fos-sa. At this point the maxillary nerve gives branches tothe sphenopalatine ganglion. Among the nerves thatpass through th e sphenopalatine ganglion is th e naso-palatine nerve (also called th e long sphenopalatinenerve) that passes along the nasal septum and emergesat the incisive foramen on th e anterior hard palate. Itsupplies sensation to th e gingival soft tissues of th e an -terior hard palate. The greater and lesser palatinenerves also pass through th e sphenopalatine ganglionand course through th e greater and lesser foramina, re-spectively. The greater palatine innervates th e palatalmucoperiosteum and the gingiva from th e molars to th earea near the cuspid region that abuts tissue supplied byth e nasopalatine nerve. The lesser palatine nerve sup-plies the tissues of th e soft palate and uvula. The max-illary nerve also gives rise to th e posterior superior al -veolar nerve, which supplies sensation to th e buccal gin-giva and periodontium adjacent to the maxillary molarteeth and th e pulps of all molar teeth except th e mesio-buccal pulp of the upper first molar. That mesio-buccalpulp is supplied by another branch of th e maxillarynerve, th e middle superior alveolar nerve, which alsoinnervates the pulps, buccal gingiva, and peridontium ofth e maxillary premolars. The final branch of the maxil-lary nerve, the anterior superior alveolar nerve, suppliesthe pulps of the upper incisors and cuspid along withth e associated buccal gingiva and periodontium.3,4Unlike th e maxillary division, th e mandibular divisionof the trigeminal nerve is a mixed motor and sensorynerve. The mandibular nerve exits th e skull through th eforamen ovale to enter th e infratemporal fossa. It then

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Chemical Classification and Duration of Action of Local Anesthetic AgentsLocal Anesthetic Classification Duration

Lidocaine Amide IntermediatePrilocaine Amide IntermediateMepivacaine Amide IntermediateBupivacaine Amide Long-actingEtidocaine Amide Long-actingArticaine Amide with an ester side chain IntermediateProcaine Ester Short-acting

divides into anterior and posterior divisions. The ante-rior division has some sensory branches: the long buccalnerve that supplies the buccal mucosa and the gingivaadjacent to th e lower molar and second premolar teeth.Other fibers supply sensation to th e skin of the cheek.The posterior division is primarily sensory. It branchesto give th e auriculotemporal, lingual, and inferior alve-olar dental nerves. The lingual nerve innervates th e lin-gual gingiva, floor of t he mouth, and anterior two thirdsof th e tongue. The inferior alveolar nerve supplies sen-sation to th e pulp and periodontium of all the molar andpremolar teeth on 1 side of th e mouth. Near th e mentalforamen, th e inferior alveolar nerve branches into th eincisive and mental nerves. The mental nerve innervatesth e buccal gingiva and th e mucosa from th e mental fo-ramen forward to th e midline, including th e skin of th elower li p and chin. The incisive nerve supplies the pulpsof the first premolar, canine, and incisor teeth.3,4LOCAL ANESTHESIALocal anesthesia is defined as a loss of sensation in acircumscribed area of the body by a depression of ex-citation in nerve endings or an inhibition of t he conduc-tion process in th e peripheral nerves. In clinical practicea localized loss of p ai n sensation is desired. Althoughth e terms dental anesthesia and dental analgesia areused synonymously in dentistry, local analgesia is moreaccurate. Local anesthesia can be achieved by a numberof mechanisms including mechanical trauma, anoxia,and use of neurolytic agents in addition to traditionallocal anesthetic drugs. However, clinically only reversiblelocal anesthetic agents and other reversible techniquessuch as temperature reduction or electronic stimulationare useful to prevent pain.The use of reversible local anesthetic chemical agentsis the most common method to achieve pain control indental practice.5 Some ideal properties of local anes-thetics are as follows:* Specific action* Reversible action

* Rapid onset of action* Suitable duration of action* Active whether applied topically or injected* Nonirritant* Causes no permanent damage* No systemic toxicity* High therapeutic ratio* Chemically stable and a long shelf life* Ability to combine w it h o th er agents without loss ofproperties* Sterilizable without loss of properties* Nonallergenic* NonaddictiveIn spite of the major advances made in the field of an -esthesia, th e ideal local anesthetic agent does not exist.Local anesthetic agents can be classified in severalways (as shown in th e Table):* Chemical structure: local anesthetics are classifiedusually as either esters or amides.* Duration of action: local anesthetics maybe classifiedas short acting, intermediate-acting, or long-acting.The injectable local anesthetics used in dentistry have acommon core structure consisting of5,6* Hydrophilic amino terminal* Intermediate chain* Lipophilic aromatic terminalThe combination of hydrophilic and lipophilic prop-erties in 1 molecule is essential for an injectable localanesthetic to be effective. The hydrophilic portion of th emolecule consists of a substituted secondary or tertiaryamine. Solubility in water is essential for 2 reasons-toallow for th e dissolution in a solvent to permit injection,and to allow penetration through interstitial fluid follow-ing administration.5,6The intermediate chain consists of either an amide orester linkage. This allows spatial separation of th e hy-drophilic and lipophilic components of th e molecule.The older agents, procaine and cocaine, are ester-based

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drugs but are no longer widely used as dental anestheticsdue to their unwanted side effects, such as toxic or al -lergic reactions.The lipophilic part of th e local anesthetic agent is anaromatic residue that is essential for it s ability to pene-trate fatty tissue such as th e lipid sheath of nerves inorder to gain access to the nerve cell membrane toreach it s site of action.Different drugs have different proportions of hydro-philic and lipophilic components. These differencesmodify th e characteristics and/or the properties of th eanesthetic agents in th e following ways:* Intrinsic anesthetic potency: the minimum concentra-tion of local anesthetic required to reduce th e nerveamplitude by half it s amplitude within 5 minutes. It is

a measure of pharmacologic action of th e agent.* Onset of anesthesia: th e onset of anesthesia is de -pendent on th e speed at which th e agent passesthrough th e tissue, th e proximity of site of injection

to th e nerve to be anesthetized, and th e diameter ofthe nerve fibers. Thin fibers are anesthetized morerapidly as compared with thick fibers, possibly be -cause th e nodes of Ranvier are closer together.

* Duration of action: duration of action of anesthesia isdependent on th e rate of diffusion along a concentra-tion gradient away from it s site of action-the ionchannels in the nerves.* Effects on other tissues including toxicity: th e func-tions of lipid-containing organs and tissues such as

th e brain and heart may be affected by high levels oflocal anesthetics.* Rate of degradation, both systemically and locally:most amide local anesthetic agents are broken downby hepatic dealkylation and hydrolysis and are sub-sequentially conjugated with glucuronic acid and ex-creted in th e urine. Esters are metabolized by ester-

ases that are widely distributed in the body.The general constituents of a dental cartridge of anes-thetic solution are:* Local anesthetic agent* Vasoconstrictor: this is sometimes included to delayth e removal of th e anesthetic from th e tissues by de -

creasing the blood flow through adjacent blood ves-sels. A vasoconstrictor produces the following advan-tages: (a) longer duration of local anesthetic action,(b ) reduced bleeding of a surgical site, and (c) reducedsystemic effects. The most commonly used vasocon-strictors are epinephrine (adrenaline) and octapressin(felypressin). Only epinephrine is available in th e Unit-ed States.

* Reducing agent: this prevents oxidation of the vaso-

constrictor and acts by competing with th e vasocon-strictor for oxygen available in th e solution. The mostcommonly used reducing agent is sodium metabisul-fite.* Preservative: a bacteriostatic preservative prolongsth e shelf life of th e solution, but since preservativescan provoke allergic reactions, they ar e no longercontained in dental local anesthetic cartridges in th eUnited States. The typical shelf life of an anestheticwithout preservative is approximately 18 months to2 years.* Fungicide: Thymol is used occasionally as a fungicide.* Carrier solution: an acidic aqueous solution dissolvesth e local anesthetic salt and maintains it at an ac-ceptable pH.

MOLECULAR BASIS OF LOCAL ANESTHESIAAll local anesthetic agents used in dentistry work by ob -structing th e exchange in Na+ permeability, which is es-sential for th e initial phases of a neuronal action poten-tial. This mechanism prevents th e development andpropagation of the action potential by preventing th ewave of depolarization.FAILURE OF ANESTHESIAFailure of local anesthetics to achieve profound anal-gesia may be related to:* inaccurate anatomic placement of local anesthetic so-lution* placing too little solution* allowing insufficient time for it to diffuse and take ef-fect* injecting into inflamed or infected tissues* using an outdated or improperly stored anesthetic so-lution.It is recommended that a local anesthetic not be injectedin infected tissue because of th e risk of spreading th einfection and th e increased probability of achieving lessthan effective anesthetic results owing to th e low pHwithin th e infected tissue maintaining th e ionized, non-lipid-soluble state to th e anesthetic.COMPLICATIONS OF LOCAL ANESTHETICSComplications of local anesthetic administration includeboth local effects and systemic effects.7 Local compli-cations include:

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* Spread of infection: occasionally infection may bespread into the tissues by the needle passing througha contaminated tissue or by the needle being contam-inated before use.* Hematoma: damage of a bloo d vessel by th e tip of aneedle may lead to bleeding into the tissues, resultingin the formation of a hematoma. Significant bleedingmay produce swelling, act as an irritant to th e tissues,and c au se p ai n and trismus. Theoretically, th e local-ized collection of blood becomes an ideal culture me-dium for bacteria, although infection of a hematomais unusual.* Nerve damage: rarely, during an injection the needlemay pierce a n er ve b un dl e during placement, pro-ducing an immediate electric shock sensation to thepatient. It is usually followed by a partial sensory def-icit, bu t subsequently a complete return to normalsensation usually follows.* Blockade of th e facial nerve: if the injection is givenin close proximity to the facial nerve, a motor block-ade causing temporary paralysis of th e muscles of fa -cial expression may occur. The effect may last for 1-2 hours. In such circumstances, the desired branch ofthe trigeminal nerve will no t be anesthetized, and asubsequent injection will be required at the correctanatomic location to achieve the desired effect.

Systemic complications include:* Regional or systemic infection: th e spread of infection

within th e perioral tissues can be potentially spreadthrough planes of th e head and neck by passage of aneedle through an infected area.* Endocarditis risk: injections such as the intraligamen-tary injection can force bacteria into the systemic cir-culation and cause bacterial endocarditis.* Cardiovascular disease: patients with ischemic heartdisease (angina pectoris, previous myocardial infarc-tion) or who have had previous cardiac s urg ery orcirculatory dysfunction such as cardiac failure, showhigher plasma levels of lidocaine when comparedwith healthy subjects given th e same dose. Thereforeit is recommended that the maximum safe dose behalved in such patients.8 Low plasma potassium levelsand acidosis also potentiate adverse effects of localanesthetics on the myocardium.7* Liver disease: patients with reduced hepatic functionmay exhibit an abnormally decreased rate of metab-olism of amide local anesthetics, resulting in poten-tially toxic blood levels. Dosage levels must thereforebe reduced for these patients.* Pseudocholinesterase deficiency: local anesthetics ofth e ester type (eg, procaine) should be avoided in pa-tients who have this rare familial enzyme defect as

metabolism of these drugs is impaired. Ester-type lo -cal anesthetics ar e no longer routinely used for dentalprocedures.* Methemoglobinemia: this is a rare complicationcaused by a metabolite of prilocaine that oxidizes th eferrous component of heme in re d blood cells to th eferric state. This reduces their oxygen-delivering ca-pacity and results in tissue hypoxia.

USE OF LOCAL ANESTHETICS DURINGPREGNANCYThe adverse drug reactions during pregnancy may affecteither the mother or the fetus. Hypersensitivity, allergy,or toxicity reactions in th e mother may compromise herhealth and limit her ability to support a pregnancy. For-tunately, doses of local anesthetics in dentistry are usu-ally relatively small and are generally unlikely to causecomplications during pregnancy. All local anestheticscross th e placenta to some degree.9-11 Highest concen-trations in the fetal circulation follow injection of prilo-caine, and the lowest follow bupivacaine, with lidocainein between.12."3Felypressin, which is a derivative of vasopressin andis related to oxytocin, has th e potential to cause uterinecontractions. Although this is a highly unlikely effect atth e low dose of felypressin used in local anesthetics, itis best avoided during pregnancy. Lidocaine with epi-nephrine is commonly used for pregnant dental pa -tients.The performance of common dental treatments for apregnant patient is highly variable. In a telephone surveyusing a standardized questionnaire, 78 resident dentistsin Germany, Switzerland, and Austria were interviewedwith respect to several aspects of th e dental treatmentof pregnant women. Only 58% of the interviewees de-cided clearly in favor of using local anesthetics, 59%supported th e use of analgesics, 70% supported a pos-sible antibiotic therapy, and 33% would agree with aradiological examination during pregnancy.9"13 In addi-tion, according to references in th e specialist literature,guidelines for the dental treatment, drug therapy, andradiological diagnosis of pregnant women ar e present-ed.12 The local anesthetics should have a high plasmaprotein bonding capability (Articaine, bupivacaine, eti-docaine) and minimum epinephrine concentrations.Acetaminophen is th e usual analgesic of choice forpregnant dental patients. If an antibiotic treatment is re-quired, penicillin, cephalosporin, and erythromycin arerecommended. In particular, during th e first 3-monthperiod, radiological examinations should be restricted tothe absolute minimum and performed only if no rea-

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sonable alternative is available, although th e radiologicalburden on the fetus falls 500,000 times short of th elimit value of 50 mgray (5 rad) in th e c as e of a micro-radiogram, and 50,000 times short of th e limit value inthe case of an orthopantomogram.2LOCAL ANESTHETICS IN CHILDRENFixed pediatric dosage recommendations for a given agerange are no longer endorsed for local anesthetic andsedative agents. Available data suggest that adverse re-actions in pediatric patients are commonly caused byinadequate dosage reduction.14 Maximum recommend-ed doses of local anesthetics is based upon th e weightof th e child, usually expressed as milligrams pe r kilo-gram of body weight. For very obese children, th e max-imum dose should be calculated on the basis of lean-body weight or ideal weight, no t the true body weight.The specific number of milligrams pe r kilogram used forcalculating th e maximum recommended dose differsamong th e various local anesthetics.MEDICAL-LEGAL CONSIDERATIONSIt is incumbent upon every dental practitioner to treathis or her patients in an appropriate way, taking intoconsideration both their dental needs and any specialprecautions related to their past medical history. To pre-vent any implication of negligence, a practitioner mustadminister appropriate treatment. Furthermore the pa-tient should receive adequate information about the pro-posed dental treatment and must submit themselves will-ingly to a local anesthetic as a part of the proposeddental treatment after th e benefits and risks ar e ex-plained. Medico-legal complaints arising from adminis-tration of local anesthesia are few in number. There are,however, some particular complications arising directlyfrom the local anesthetic drugs or their delivery thatmerit consideration.Persisting anesthesia or paresthesia due to damage tovarious branches of th e trigeminal nerve is a commoncomplication in dental surgical procedures, especiallyassociated with lower third molar removal. Cases relat-ing to sensory loss of lingual nerve and inferior alveolarnerve following inferior dental block injections for re-storative procedures have occasionally been presentedas a legal complaint.In a study of over 12,000 inferior dental block injec-tions, all given for restorative treatment, 18 patients

(0.15%) were found to have some lingual sensory dis-turbance following treatment. Of these 18 patients, 17patients totally regained normal sensation within 6months, and 1 patient still had a loss of sensation after1 year (0.008%). Of the 12,000 patients, 856 (7%) ex -perienced an "electric shock" type feeling in th e tongueat th e time of injection, suggesting that th e tip of theanesthetic needle had touched the lingual nerve.3Although th e medico-legal issues tend to frighten th edental practitioner, statistical data demonstrate that ifth e current standards of practice are observed, th e den-tist is unlikely to run into these types of problems.REFERENCES

1. Rosenberg ES . A computer-controlled anesthetic deliv-ery system in a periodontal practice: patient satisfaction andacceptance. J Esthet Restor Dent. 2002;14:39-46.2. Robinson PD, Pitt Ford TR, McDonald F. Local anes-thesia in dentistry. London: Reed Educational and ProfessionalPublishing; 2000.3. Kraft TC, Hickel R. Clinical investigation into th e inci-dence of direct damage to the lingual nerve caused by localanesthesia. J Craniomaxillofac Surg. 1994;22:294-296.4. Dionne RA, Phero JC, Becker DE. Management ofpain and anxiety in the dental office. Philadelphia: WB Saun-ders; 2002.5. Haas DA. An update on local anesthetics in dentistry.J Can Dent Assoc. 2002;68:546-551.6. Meechan JG, Robb ND, Seymour RA. Pain and anxietycontrol for the conscious dental patient. London: Oxford Uni-versity Press; 1998.7. Chen AH. Toxicity and allergy t o local anesthesia. JCalif Dent Assoc. 1998;26:683-692.8. Preshaw PM, Rowson JE. The use of lignocaine localanesthetic. Br Dent J. 1996;181:240.9. Pertl C, Heinemann A, Pertl B, et al. The pregnantpatient in dental care. Survey results and therapeutic guide-lines. Schweiz Monatsschr Zahnmed. 2000; 110:37-46.10. Lawrenz DR, Whitley BD, Helfrick JF. Considerationsin the management of maxillofacial infections in th e pregnantpatient. J Oral Maxillofac Surg. 1996;54:474-485.11. Avraamides EJ, Craen RA, Gelb AW. Anesthetic man-agement of a pregnant, post liver transplant patient for dentalsurgery. Anesth Intens Care. 1997;25:68-70.12. Johnson CG. Local anesthetics and pregnancy. J AmDent Assoc. 1985;110:302.13. Watson AK. Local anesthetics in pregnancy. Br DentJ. 1989;166:36.14 . Hunter ML, Hood CA, Hunter B, Kingdon A. Oralhealth advice: reported experience of mothers of children aged5 years and under referred for extraction of teeth under gen-eral anesthesia. Int J Pediatr Dent. 1998;8:13-27.

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