Local Anesthetic Agents in Dentistry

In dentistry, local anesthetic agents have excellent efficacy and safety in dental practices, but they also have common indications, usage, and infrequent adverse effects.

What Is the Molecular Structure of a Local Anesthetic?

Local anesthesia is mainly or majorly definable as a loss of sensation in a circumscribed body area caused by a depression of excitation in nerve endings or an inhibition of the conduction process in peripheral nerves. The nerve conduction occurs by the inhibition of the influx of sodium ions. The local anesthetic agent primarily relies upon its molecular structure to interrupt temporarily for a specific period.

The local anesthetic molecule consists again further of three primary and pivotal components:

1. Lipophilic Aromatic Component - It improves or enhances the lipid solubility of the compound for diffusion through the nerve sheath.
2. Intermediate Ester or Amide Chain - It determines a pattern of molecular biotransformation.
3. Terminal Amine - Its property is to make the molecules more water-soluble.

Each of these three components makes up the molecular structure of an anesthetic molecule by contributing distinct properties to the nature and mechanism of action.

The main compositions of local anesthesia used in dentistry are:

• Lignocaine (anesthetic).

• Adrenaline (vasoconstrictor).

• Methylparaben (agent).

• Sodium metabisulphite (fungicide).

• Water.

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Local anesthetic can be available and used as an injectable and in the forms of gel, spray, topically applicable ointments, creams, patches, and liquids. For simple procedures like dental scaling and cavity preparation in hypersensitive teeth or simple pediatric procedures, lidocaine gels and sprays are commonly used in dentistry.

For surgical procedures like simple to complicated tooth extraction, gum surgeries, placement of dental implants, and root canal procedures, the local anesthetic is injected through specific techniques based on the tooth involved and the site of the injection decided by the dental surgeon. The injection techniques can range from simple intraligamentary and infiltration block to nerve blocks like the maxillary posterior superior alveolar nerve block and the mandibular inferior alveolar nerve block.

What Are the Desirable Properties of Local Anesthetic Agents?

• The agent should not irritate the local tissues to which it is applied.

• It should not cause any permanent alteration or variation in the nerve structure.

• The systemic toxicity of the LA agent should be low.

• The time of onset of anesthesia should be preferably as short as possible.

• The duration of action must be long enough to permit completion of the surgical procedure but not exceed the length that may require an extended recovery period for the patient.

• The LA agent should not produce allergic reactions.

• It should be stable in solution and readily undergo biotransformation in our body.

How Are Local Anesthetics Classified?

Dental anesthetics are mainly grouped into esters and amides. Though usually for injectable anesthetic drugs for dental surgical procedures, esters are no longer used; some practitioners use Benzocaine as an effective topical medication especially for pain relief and to produce adequate numbness at the affected or inflamed site of the patient's oral cavity. In injectable anesthetic, the most commonly used remains Lidocaine 2% solution. It can also be used as a topical anesthetic for the applicatory purpose at the site of painful oral lesions, sores, and affected areas to relieve pain efficaciously.

The broad general classification of local anesthetics used in dentistry are given as below:

Based upon the molecular structure, local anesthetics are classified into:

1. Esters:

a. Esters of benzoic acid -

• Butacaine.
• Cocaine.
• Ethyl aminobenzoate (Benzocaine).
• Hexylcaine.
• Piperocaine.

b. Tetracaine ester of Para Aminobenzoic acid-

• Chloroprocaine.
• Procaine.
• Propoxycaine.

2.Amides:

• Articaine.
• Bupivacaine.
• Dibucaine.
• Etidocaine.
• Lidocaine.
• Mepivacaine.
• Prilocaine.
• Ropivacaine.

3. Quinoline:

• Centbucridine.

What Is the Composition of Local Anesthetics?

The main components that are present in a local anesthetic bottle are,

1. Local Anesthetic Agent - Lignocaine Hydrochloride 2% (20 mg/mL)

The properties of a local anesthetic molecule have been elaborated as mentioned above, i. e. mainly its function is to effectively inhibit nerve conduction for a specific period. For all dental surgical procedures, 2% Lidocaine or Lignocaine hydrochloride is used commonly and widely preferred with other injectable anesthetics like Mepivacaine and Prilocaine (that are also preferred by some dental surgeons in terms of enhanced anesthetic performance over lignocaine.) In rare cases of patients with Lidocaine allergy, research establishes the use of Articaine anesthetic.

2. Vasoconstrictor - Adrenaline 1:80,000 (0.012 mg)

Vasoconstrictor function in the anesthetic solution is to decrease the flow of blood at the injected site to increase the local uptake of the anesthetic agent in and around that area and prevent toxicity. One more advantage is that bleeding is controlled at the administration site of the anesthetic. The duration of action is crucial to an anesthetic agent's success during the surgical period; hence the vasoconstrictor plays a significant role by fulfilling multiple functions.

3. Reducing Agent - Sodium Metabisulphite (0. 5 mg)

They act mainly as preservatives for the vasoconstricting agents. The vasoconstrictor solution can become unstable by undergoing oxidation on exposure to sunlight. Sodium metabisulphite is a commonly used reducing agent that competes for the available oxygen and maintains the stability of the solution.

4. Isotonic Solution - Sodium Chloride or Ringer’s Solution (6 mg)

It is also known as the "vehicle" of the anesthetic solution. The discomfort at the injection site is reduced considerably because of the isotonicity of the vehicle solution.

5. Preservative - Methylparaben 0. 1% (1 mg)

Xylotox and methylparaben are the constituents of the preservative agent that help maintain the stability of the anesthetic solution.

6. Nitrogen Bubble

The nitrogen bubble aims to prevent the contamination by oxygen from being trapped within the cartridge. If oxygen is trapped within the cartridge, then the action of the vasopressor is potentially threatened. The bubble is around 1 to 2 mm in diameter

7. Other Agents

To adjust the pH, sodium hydroxide is used. Thymol is the commonly used fungicide agent, and distilled water is used as a diluting agent.

Conclusion:

Local anesthesia is a very safe and commonly employed method before almost all dental surgical procedures. The dental surgeon must understand the mechanism and implications of local anesthetics for delivering standardized and quality patient care, especially for performing the procedure painlessly or with as little discomfort and pain as possible.