Local Anesthesia in Dentistry: LOCAL ANESTHESIA

In: Anesthesia

24 Sep 2009

Local anesthesia is defined as a loss of sensation in a circumscribed area of the body by a depression of excitation in nerve endings or an inhibition of the conduction process in the peripheral nerves. In clinical practice a localized loss of pain sensation is desired. Although the terms dental anesthesia and dental analgesia are used synonymously in dentistry, local analgesia is more accurate. Local anesthesia can be achieved by a number of mechanisms including mechanical trauma, anoxia, and use of neurolytic agents in addition to traditional local anesthetic drugs. However, clinically only reversible local anesthetic agents and other reversible techniques such as temperature reduction or electronic stimulation are useful to prevent pain.

The use of reversible local anesthetic chemical agents is the most common method to achieve pain control in dental practice. Some ideal properties of local anesthetics 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 with other agents without loss of properties
  • Sterilizable without loss of properties
  • Nonallergenic
  • Nonaddictive

In spite of the major advances made in the field of anesthesia, the ideal local anesthetic agent does not exist.
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Local anesthetic agents can be classified in several ways (as shown in the Table):

  • Chemical structure: local anesthetics are classified usually as either esters or amides.
  • Duration of action: local anesthetics maybe classified as short acting, intermediate-acting, or long-acting.

The injectable local anesthetics used in dentistry have a common core structure consisting of

  • Hydrophilic amino terminal
  • Intermediate chain
  • Lipophilic aromatic terminal

The combination of hydrophilic and lipophilic prop­erties in 1 molecule is essential for an injectable local anesthetic to be effective. The hydrophilic portion of the molecule consists of a substituted secondary or tertiary amine. Solubility in water is essential for 2 reasons—to allow for the dissolution in a solvent to permit injection, and to allow penetration through interstitial fluid following administration. buy tadalafil online

The intermediate chain consists of either an amide or ester linkage. This allows spatial separation of the hydrophilic and lipophilic components of the molecule. The older agents, procaine and cocaine, are ester-based drugs but are no longer widely used as dental anesthetics due to their unwanted side effects, such as toxic or allergic reactions.

The lipophilic part of the local anesthetic agent is an aromatic residue that is essential for its ability to penetrate fatty tissue such as the lipid sheath of nerves in order to gain access to the nerve cell membrane to reach its site of action.

Different drugs have different proportions of hydro-philic and lipophilic components. These differences modify the characteristics and/or the properties of the anesthetic agents in the following ways:

  • Intrinsic anesthetic potency: the minimum concentration of local anesthetic required to reduce the nerve amplitude by half its amplitude within 5 minutes. It is a measure of pharmacologic action of the agent.
  • Onset of anesthesia: the onset of anesthesia is dependent on the speed at which the agent passes through the tissue, the proximity of site of injection to the nerve to be anesthetized, and the diameter of the nerve fibers. Thin fibers are anesthetized more rapidly as compared with thick fibers, possibly because the nodes of Ranvier are closer together.
  • Duration of action: duration of action of anesthesia is dependent on the rate of diffusion along a concentration gradient away from its site of action—the ion channels in the nerves.
  • Effects on other tissues including toxicity: the functions of lipid-containing organs and tissues such as the brain and heart may be affected by high levels of local anesthetics.
  • Rate of degradation, both systemically and locally: most amide local anesthetic agents are broken down by hepatic dealkylation and hydrolysis and are sub-sequentially conjugated with glucuronic acid and excreted in the urine. Esters are metabolized by esterases that are widely distributed in the body. viagra online pharmacy

The general constituents of a dental cartridge of anesthetic solution are:

  • Local anesthetic agent
  • Vasoconstrictor: this is sometimes included to delay the removal of the anesthetic from the tissues by decreasing the blood flow through adjacent blood vessels. A vasoconstrictor produces the following advantages: (a) longer duration of local anesthetic action, (b) reduced bleeding of a surgical site, and (c) reduced systemic effects. The most commonly used vasoconstrictors are epinephrine (adrenaline) and octapressin (felypressin). Only epinephrine is available in the United States.
  • Reducing agent: this prevents oxidation of the vaso constrictor and acts by competing with the vasoconstrictor for oxygen available in the solution. The most commonly used reducing agent is sodium metabisul-fite.
  • Preservative: a bacteriostatic preservative prolongs the shelf life of the solution, but since preservatives can provoke allergic reactions, they are no longer contained in dental local anesthetic cartridges in the United States. The typical shelf life of an anesthetic without preservative is approximately 18 months to 2 years.
  • Fungicide: Thymol is used occasionally as a fungicide.
  • Carrier solution: an acidic aqueous solution dissolves the local anesthetic salt and maintains it at an acceptable pH.
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MOLECULAR BASIS OF LOCAL ANESTHESIA
All local anesthetic agents used in dentistry work by obstructing the exchange in Na+ permeability, which is essential for the initial phases of a neuronal action potential. This mechanism prevents the development and propagation of the action potential by preventing the wave of depolarization.


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Blog invites submissions of review articles, reports on clinical techniques, case reports, conference summaries, and articles of opinion pertinent to the control of pain and anxiety in dentistry.

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