Local Anesthesia in Dentistry: Techniques and Complications

Basic Definitions As It Concerns Anesthesia

• Anesthesia: Loss of all forms of sensation: Pain, temperature, touch, pressure
• Local: Within a given area, nearby, within, etc.
• General: Whole
• Analgesia: Loss of pain sensation only
• Pain: An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.

Why Do We Require Local Anesthesia in Dentistry?

• With the exception of a few non-invasive procedures, virtually all procedures in Dentistry will result in pain.
• Pain must be eliminated to carry out these procedures effectively.
• Oral and perioral structures are richly supplied with nerve endings (nociceptors) that serve pain, which are classified into:
  • Finely myelinated A-delta fibers
  • Unmyelinated C fibers

Why Use Local Anesthesia? (Cont’d)

• Nociceptors are activated by intense or noxious stimuli:
  • Some nociceptors are unimodal and respond only to thermal or mechanical stimuli.
  • Others are polymodal and respond to mechanical, thermal, and chemical stimuli.
• Nociceptors encode the intensity, duration, and quality of a noxious stimulus.
• Stimulation of these nerve endings results in pain, irrespective of the agent (mechanical, chemical, etc.).

Overview of Pain Pathway

• The fifth cranial nerve (Trigeminal nerve) is the sensory nerve of the oral region, conveying impulses along relevant branches (mandibular or maxillary) to the central nervous system.
• Remember: sensory impulses pass from the periphery to the CNS, while motor impulses travel from the CNS to the periphery.

Overview of Pain Pathway (Cont’d)

• Information associated with pain is carried in the divisions of CN V to the trigeminal (Gasserian) ganglion.
• The central processes of neurons enter the pons, descending in the brainstem as the spinal trigeminal tract.
• Fibers from the spinal trigeminal tract synapse in the adjacent spinal nucleus of CN V at the medulla but extend to C2 of the spinal cord, merging with the dorsal gray matter.
• The nucleus caudalis is the principal site in the brainstem for nociceptive information.

Overview of Pain Pathway (Cont’d)

• Axons from the spinal nucleus of CN V cross to the opposite side and ascend to the ventral posteromedial (VPM) nucleus of the thalamus.
• At this point, there are projections to the reticular formation and the medial and intralaminar thalamic nuclei.
• From the thalamus, neurons course and end at the somatosensory cortex in the post-central gyrus.

Advantages of Local Anesthesia

• Safe, effective, and convenient means of obtaining anesthesia for dental treatment.
• Simple to carry out; the armamentarium is cheap, not bulky, and easy to transport compared to machines for general anesthesia.
• Premedication is not routinely given.
• In most cases, patients can return to their workplace immediately.

Disadvantages/Contraindications

• Presence of infection at the site of injection can spread the infection, and the local anesthesia may be ineffective.
• Hemorrhagic disorders (e.g., hemophilia) require treatment in a hospital setting.
• Pregnancy and cardiovascular disease may require caution with certain vasoconstrictors in local anesthetics.

Mechanism of Action

• At rest, the resting membrane potential within a peripheral nerve is about -50 to -70 mV, i.e., negative relative to the outside.
• When stimulated, there is an initial slow phase of depolarization with the internal membrane potential becoming less negative.
• At threshold potential, the interior becomes positively charged (may reach 40 mV).

Mechanism of Action (Cont’d)

• Repolarization restores the resting membrane potential.
• The negative charge is due largely to potassium ions. During stimulation, membrane permeability increases causing an influx of sodium ions (Na+) responsible for depolarization.
• At maximum depolarization, sodium passage is arrested, and potassium exits the cell, thus repolarizing the membrane.

Mechanism of Action (Cont’d)

• Na+ and K+ movements during depolarization are passive (dependent on concentration gradient).
• Post-repolarization, an ionic imbalance occurs, with too many intracellular Na+ and extracellular K+.
• The Na+ is extruded by the Na+ pump, using energy from oxidative metabolism to restore ATP.

Mechanism of Action (Cont’d)

• Other mechanisms for restoring membrane potential may include active transport of K+ or transport of K+ along electrostatic gradients between the resting cell and its environment.

Propagation of Nerve Impulses

• The change in electric potential results in impulse propagation along the nerve.
• For myelinated nerves, depolarization occurs at the nodes of Ranvier, while in unmyelinated nerves, each segment activated the adjacent one.

Pharmacology of Local Anesthetics

• Local anesthetics provide reversible blockage of nerve conduction. They can be classified as:
  • Amino-esters
  • Amino-amides

Amino-Esters

• Clinically useful amino-ester local anesthetics are esters of PABA (Para-amino-benzoic acid).
• Prototype: Procaine (Novocaine); benzocaine is mainly used as a topical anesthetic.

Amino-Amides

• Prototype agents: Lignocaine, used since 1944, due to its superior pharmacological properties it has replaced procaine.
• Properties: Lignocaine has topical action, while other amide links include carbocaine and prilocaine.

Local Anesthetic Agents

Lignocaine

• Uses: Available as 2% gel, 5% ointment, or 10% spray.
• Presentation:
  • Strength: 2%
  • Plain: without adrenaline (do not exceed 200 mg)
  • With adrenaline: do not exceed 350 mg

Lignocaine (Cont’d)

• Toxicity:
  • CNS depression (drowsiness, sedation)
  • Possible tremors and convulsions.

Mepivacaine

• Uses: For infiltration and regional anesthesia; lacks topical action.

Presentation of Mepivacaine

• 2% or 3% solution (the former usually has 1:80,000 adrenaline; the latter is plain).
• Maximum dose: do not exceed 5mg/kg (e.g., 70 kg person – max of 350 mg).

Toxicity

• Effects are those of CNS stimulation.

Contraindications of Mepivacaine

• Allergy to amide-type L.A.
• Liver disease.

Prilocaine

• Uses: For infiltration and regional anesthesia; less toxic than lidocaine or mepivacaine.
• Properties: Lacks topical action; maximum dose should not exceed 400 mg.

Contraindications of Prilocaine

• Infancy, hypoxia, heart failure, liver disease.

Articaine

• Available in Europe since 1976, it comprises 35.6% of the local anesthetic market in the U.S.
• It's an amide with ester characteristics, 1.5 times more potent than lidocaine.

Contraindications of Articaine

• Known hypersensitivity to amide-type local anesthetics or known bisulfite allergy.

Mechanism of Action of Local Anesthetics

• Decrease the rate of rise of depolarizing phase of the action potential by reducing Na+ influx.
• The membrane resting potential isn’t influenced, preventing action potential firing.

Mechanism of Action (Cont’d)

• Local anesthetic agents affect the Na+ channels on the internal surface of the axon membrane.

Vasoconstrictors

• Adrenaline (epinephrine) in concentrations of 1:50,000 – 1:100,000; 1:80,000 most common.
• Felypressin (octapressin) – 0.03mg/ml (1:200,000).

Advantages of Vasoconstrictors with Local Anesthetics

• Increase the depth and duration of anesthesia.
• Reduce systemic toxicity by slowing absorption.

Precautions

• Use adrenaline cautiously in hypertensive patients.
• Felypressin: do not exceed 8.8ml in patients with ischemic heart disease.

Armamentarium for Local Anesthesia

1. Dental Syringe: Various types include:
   • All-metal reusable, sterilizable
   • Plastic (disposable/single use)
   • Breech loading, side loading, non-aspirating, and aspirating
2. Cartridges: Made of glass or plastic, single-use.
3. Dental Needle: Designed to pierce the cartridge and soft tissues; available in long and short sizes.

Notes on Use of Needle

• The bevel of the needle should face bone.
• Avoid unnecessary movement within tissues to reduce the risk of fracture.

Techniques of Local Anesthesia

1. Preparation of the Patient:

   • Reassure the patient; provide full disclosure.
   • Address anxiety with premedication if needed, like diazepam or midazolam.

2. Patient Position for Local Anesthesia:

   • Maintain a proper position for visibility and accessibility.
   • Ensure comfort by loosening tight clothing.

3. Preparing the Mucosa:

   • Cleanse with chlorhexidine or iodine.
   • Consider topical anesthesia before injection for pain reduction.

4. Preparing the Syringe:

   • Open the needle and attach it to the syringe.
   • Load the cartridge securely and check for patency.

Speed of Injection

• Injection should be slow, as too rapid an injection may cause pain.

Testing for Anesthesia

• Subjective: Ask patients how their mouth feels.
• Objective: Test various sites based on administered blocks.

Failure to Obtain Anesthesia

• Causes include faulty technique, inadequate knowledge, and injection into muscle or infection.

Complications Following Local Anesthesia

• While rare, they can arise; dentists should be prepared for prevention, diagnosis, and management.

Local Complications

• Failure to obtain anesthesia, pain during or after injection, hematoma, trismus, etc.

General Complications

• Syncope, drug interaction, sensitivity reactions, etc.