Local Anesthesia: Principles & Mechanisms

• Analgesia is the loss of pain sensation without the loss of other sensations like pressure.
• Anaesthesia is the loss of all sensations including pain, touch, temperature, and pressure.
• Both analgesia and anaesthesia can be local or general.
• The aim of dental treatment is to contain active disease and prevent its recurrence.
• Oral tissues are well-supplied with nerves, which deliver sensations to the brain.
• Some dental treatments are considered painful.
• The same stimulus can produce different reactions in different people and even within the same patient at different times.
• Differences in how pain is perceived is explained by the Gate Control Theory.
• Pain is abolished by interrupting neural pathways, which can be permanent or temporary.
• Reversible pain control at a local level is required for dental treatment since removing the stimulus is not possible.
• Local anaesthetics block painful impulses.
• Local analgesia is enhanced by the environment, hypnosis, and distraction due to the gate control theory.
• Local anaesthesia can be achieved by applying cold, pressure, or drugs.

How Local Anaesthetic Agents Work

• Local anaesthetic agents work by blocking the transmission of impulses in sensory nerves.
• A nerve cell produces an electrical impulse known as an action potential.
• This impulse travels in one direction.
• The frequency or number of pulses of the impulse can vary.
• Ion channels in the axon are voltage-gated during impulse transmission.
• Depolarization in one axon segment opens ion channels in the next segment.
• An action potential spreads like a wave of depolarization along the axon.
• A resting potential includes high extracellular Na+.
• A resting potential includes low intracellular K+.
• The nerve cell membrane is semi-permeable for resting potential.
• The resting potential is -70mV.
• With a stimulus, there is increased permeability of the nerve cell membrane to Na+ ions causing them to rush into the cell.
• Membrane potential becomes positive (+40mV) during stimulus.
• A sodium-potassium pump actively moves Na+ and K+ ions across the nerve cell membrane to restore balance during repolarization.
• The membrane potential returns to approximately -70mV during repolarization.
• Local anaesthetic agents set up a chemical roadblock between the source of the impulse and the brain.
• LA agent acts in two ways:
  • Binds to specific receptors in the sodium channels in the nerve cell membrane, blocking the inward movement of Na+ ions during impulse conduction.
  • Non-specific expansion of the nerve cell membrane, causing physical obstruction of the Na+ channels.
• An action potential not generated, so no impulse travels to the brain.

Local Anaesthetic Agents

• Local aesthetic agents are drugs that bring about a temporary loss of pain sensation in a limited area of the body.
• The drug acts by making nerve fibres incapable of transmitting an impulse on stimulation.
• The ideal LA agent should be:
  • Effective and reversible on nerves and nerve endings
  • Able to diffuse through the lipid membrane of the nerve fibre
  • Non-irritating or injurious to the tissues
  • Have a rapid onset of numbness
  • Provide adequate working time
  • Possess a high safety margin
  • Compatible for combination with a vasoconstrictor
  • Isotonic
  • Have an adequate shelf life
  • Capable of sterilization
• A dental LA cartridge includes:
  • Analgesic agent for blockade of nerve conduction (e.g., Lidocaine, Prilocaine, Articaine, Mepivacaine)
  • Vasoconstrictor to increase the depth and duration of analgesia (e.g., Epinephrine, Felypressin)
  • Reducing agent to prevent oxidation of the vasoconstrictor (e.g., Sodium metabisulphite)
  • Vehicle to carry the above agents and ensure the solution is isotonic (e.g., Sterile saline)
• LA agents are categorized as either esters or amides.
• Esters include benzocaine, cocaine, procaine, and amethocaine.
• Amides include lidocaine, prilocaine, mepivacaine, articaine, bupivacaine, ropivacaine, and levobupivacaine.

General Features of LA Agents

• Organic weak bases are insoluble in water.
• LA agents are converted into soluble salts (hydrochlorides) for clinical use.
• There are two chemical forms: un-ionized and ionized (weak base).
• The un-ionized form is lipid soluble and able to cross the fatty sheath around the nerve to gain access to nerve fibres.
• The ionized form actually blocks nerve conduction once the solution is inside the nerve fibres.

Amides

• Lidocaine 2% with epinephrine 1:80,000:
  • The gold standard for most dental procedures.
  • Effective with a rapid onset.
  • Provides a useful duration of pulpal and soft tissue analgesia.
  • Has low toxicity.
  • Provides a good topical effect.
• Lidocaine is available in topical preparations:
  • As a spray: 10mg Lidocaine per spray.
  • As a gel: Lidocaine 5%, cetrimide 0.15%, with an onset of action in 2-5 min.
• Prilocaine:
  • Is as potent as lidocaine.
  • Has a shorter duration of action.
  • Has less vasodilation than lidocaine can be delivered without a vasoconstrictor.
  • Is sometimes combined with felypressin but causes less vasoconstriction than epinephrine.
  • Is rapidly dispersed and cleared.
  • Has a very low incidence of side effects.
  • Available as Citanest 3% + Octapressin (Felypressin) (0.03iu/ml) and Citanest 4% plain (No vasoconstrictor)
• Mepivacaine:
  • Has similar properties to prilocaine.
  • Has shorter analgesia if used as infiltration (15-30 minutes)
  • Available as Scandonest 3% Plain and Scandonest 2% with epinephrine 1:100,000.
• Articaine:
  • A powerful analgesic agent with a rapid onset of action.
  • Provides a longer duration of soft tissue analgesia.
  • Is rapidly metabolized in plasma and liver.
  • Cannot be given as an inferior dental block.
  • Available as Septanest 1:100,000 (4% articaine hydrochloride with epinephrine 1:100,000) and Septanest 1:200,000.
• Bupivacaine:
  • Is very long lasting (6-8 hours).
  • Depresses cardiac activity and causes dysrhythmias.
  • Useful for post-surgery.
  • Not available in dental use cartridges, only given with standard medical type syringe as Marcaine + epinephrine.

Vasoconstrictors

• Increase the depth and duration of anaesthesia.
• Epinephrine:
  • Found naturally in the body in concentrations of 1:80,000, 1:100,000 or 1:200,000
  • Provides more profound analgesia
  • Controls haemorrhages
• Felypressin (0.03 IU/mL) is a synthetic octapeptide, but not such a profound vasoconstrictor.

Duration of Anaesthesia

• Lidocaine 2% + epinephrine 1:80,000 provides 45 minutes of pulpal anaesthesia and 3+ hours of soft tissue anaesthesia.
• Prilocaine 3% + felypressin 0.03iu/ml provides 30-45 minutes of pulpal anaesthesia and 2 hours of soft tissue anaesthesia.
• Articaine 4% + epinephrine 1:100,000 provides 75 minutes of pulpal anaesthesia and 3-4 + hours of soft tissue anaesthesia.
• Mepivacaine 3% plain provides 20 minutes of pulpal anaesthesia and 1 hour of soft tissue anaesthesia.
• Bupivacaine provides 4 hours of pulpal anaesthesia and 6-8 hours of soft tissue anaesthesia.
• Maximum safe dosages of local anaesthetics:
  • Lidocaine 2% maximum dose is 4.4 mg/kg, allowing for 6.8 cartridges (2.2ml) in adults (70kg) and 2.0 cartridges in a 5 year old child (20kg).
  • Prilocaine 3% maximum dose is 5.0 mg/kg, allowing for 6.0 cartridges (2.2ml) in adults (70kg) and 1.8 cartridges in a 5 year old child (20kg).
  • Mepivacaine 3% maximum dose is 4.4 mg/kg, allowing for 4.5 cartridges (2.2ml) in adults (70kg) and 1.3 cartridges in a 5 year old child (20kg).
  • Articaine 4% maximum dose is 7.0 mg/kg, allowing for 5.0 cartridges (2.2ml) in adults (70kg) and 1.5 cartridges in a 5 year old child (20kg).
• Effectiveness of L.A. depends on:
  • Analgesic potency of the agent
  • Concentration of the agent
  • Solubility of the agent in water and lipid
  • How long it stays at the injection site (concentration of agent and vasoconstrictor)
  • The rate at which the agent is metabolized at the injection site
  • Accuracy of technique
  • Amount of spread and diffusion of agent
  • Presence of infammation and infection

General Contraindications and Cautions

• Latex allergy: all LA cartridges at Eastman have latex-free bungs, but it's necessary to check in other dental practices.