An Update on Local Anesthetics in Dentistry

1 Journal of the Canadian Dental Association546 October 2002, Vol. 68, No. 9 CLINICALPRACTICEI ntraoperative pain control by means of Local anesthesiais an intrinsic part of clinical practice in dentist in Canada injects approximately 1,800cartridges of Local anesthetic yearly,1and it has been esti-mated that more than 300 million cartridges are adminis-tered by dentists in the United States every ,all dentists should have expertise in Local anesthesia. Thisarticle provides a brief overview of Local Anesthetics to rein-force dentists knowledge of these follows here is a brief synopsis of the pharmacol-ogy of Local Anesthetics .

2 Dentists should be familiar withsources that provide more detailed information on anesthesia is induced when propagation of actionpotentials is prevented, such that sensation cannot be trans-mitted from the source of stimulation, such as a tooth orthe periodontium, to the brain. Local Anesthetics work byblocking the entry of sodium ions into their channels,thereby preventing the transient increase in permeability ofthe nerve membrane to sodium that is required for anaction potential to , Local Anesthetics have specific fundamentalfeatures in common. These include a lipophilic group,joined by an amide or ester linkage to a carbon chainwhich, in turn, is joined to a hydrophilic group.

3 Local anes-thetics are classified by these amide or ester linkages. Alllocal Anesthetics available in dental cartridges in Canadatoday, namely articaine, bupivacaine, lidocaine, mepiva-caine and prilocaine, belong to the amide class. The proto-type for the ester group is procaine (Novocain, Abbot),which is no longer available in dental cartridge form. Thetopical anesthetic benzocaine is an onset and duration of action of Local Anesthetics areinfluenced by several factors, as summarized in Table most important factors affecting onset are pH of thetissue and pKa of the drug.

4 The pH may drop in sites ofinfection, which causes onset to be delayed or evenprevented. Clinically, there are no significant differences inpKa among the amides, with the exception of bupivacaine,which has a slightly higher pKa and hence a slower onset ofaction. Proximity of the deposition of Local anesthetic to thenerve can also be a factor, which is why infiltration is asso-ciated with rapid onset whereas the Gow-Gates block isrelatively slow. Nerve morphology is a factor, in that therelatively thin pain fibres are usually anesthetized limits, higher concentration and greater lipid solu-bility improve onset to a small duration of action depends on the length of timethat the drug can stay in the nerve to block the sodiumchannels.

5 Local Anesthetics cause vasodilatation, whichleads to rapid diffusion away from the site of action andresults in a very short duration of action intraorally whenthese drugs are administered alone. This diffusion can bereduced by the addition of a vasoconstrictor, usuallyepinephrine. Bupivacaine is unique in that it provides long-duration anesthesia for soft tissue in both the arches andpulp of mandibular teeth. The duration of action of localanesthetics is summarized in Table ,4,6In general, blockslast longer than infiltrations, and soft-tissue anesthesia lastslonger than pulpal of amides occurs primarily in the is also metabolized in the plasma and kidney, andone of its metabolites may lead to methemoglobinemia, asAn Update on Local Anesthetics in Dentistry Daniel A.

6 Haas,BSc, DDS, BScD, PhD, FRCD(C) AbstractLocal Anesthetics are the most commonly used drugs in Dentistry . This article provides a brief Update on thepharmacology, adverse effects and clinical applications of these drugs, as well as the role of Key Words:anesthesia, dental; Anesthetics , Local /adverse effects; pharmacology; vasoconstrictor agents J Can Dent Assoc 2002; 68(9):546-51 This article has been peer 2002, Vol. 68, No. 9547 Journal of the Canadian Dental AssociationAn Update on Local Anesthetics in Dentistrydiscussed below. Esters are biotransformed by plasmacholinesterase, also known as pseudocholinesterase.

7 Patientswith the genetic disorder pseudocholinesterase deficiency canbe expected to metabolize procaine at a much slower , little clinical effect would be expected unless thedose was very a practical viewpoint, the dentist should beconcerned about alterations in biotransformation only inpatients with severe liver dysfunction. Reduced hepaticfunction predisposes the patient to toxic effects but, unlikethe situation for systemically administered drugs, shouldnot significantly increase the duration of action of locallyadministered Anesthetics . In this context, it must be remem-bered that hepatic function does not affect the duration ofaction of Local anesthesia, which is determined by redistrib-ution and not biotransformation.

8 Therefore, a patient withliver disease needs the standard amount of Local anestheticat each site. However, the total dose is a concern. Therefore,when treating a patient with significant liver disease, it isprudent to treat one quadrant at a time, thereby minimiz-ing total dose. Use of an ester may not offer any advantage,because pseudocholinesterase is also synthesized in the regard to efficacy, no studies have shown anysignificant differences among the agents. Therefore it isappropriate to assume that each of the 5 amides is ReactionsLocal Anesthetics should be considered relatively safe,but with the high number of injections given yearly, adversereactions are seen (Table 3).

9 Psychogenic ReactionsAnxiety-induced events are by far the most commonadverse reaction associated with Local Anesthetics indentistry. These may manifest in numerous ways, the mostcommon of which is syncope. In addition, they may presentwith a wide variety of symptoms, including hyperventila-tion, nausea, vomiting and alterations in heart rate or bloodpressure. Psychogenic reactions are often misdiagnosed asallergic reactions and may also mimic them, with signs suchas urticaria, edema and ReactionsPatient reports of allergic reactions to Local anestheticsare fairly common, but investigation shows that most ofthese are of psychogenic ,12 True allergy to an amideis exceedingly rare, whereas the ester procaine is somewhatmore allergenic.

10 An allergy to one ester rules out use ofanother ester, as the allergenic component is the breakdownproduct para-aminobenzoic acid, and metabolism of allesters yields this compound. In contrast, an allergy to oneamide does not rule out use of another amide. Allergy toepinephrine is patient may be allergic to other compounds in theanesthetic cartridge. For example, methylparabens arepreservatives necessary for multidose vials and were presentin dental cartridges in the past. They are no longer included as dental cartridges are single-use items.