12.1 Local Anaesthesia

Mechanism of Action

Local anaesthetics block voltage-gated sodium channels on nerve axons. By binding to the open sodium channel from the intracellular side, they prevent sodium influx, abolish depolarisation, and halt action potential propagation. Recovery is spontaneous as the drug diffuses away.

Nerve Fibre Sensitivity

Different nerve fibres are blocked in a predictable order. Small, myelinated fibres are most sensitive because the local anaesthetic has a shorter distance to reach the nodes of Ranvier.

This explains the clinical sequence: patients report loss of sharp pain before loss of touch or motor function.

Agents Used in UK Dental Practice

All routinely used dental injectable anaesthetics in the UK are amides. Ester agents survive mainly as topical preparations.

Dose Calculation in Practice

A 70 kg adult receiving lidocaine 2% with 1:80,000 epinephrine:

• Weight-based limit: 7 mg/kg × 70 kg = 490 mg (below the 500 mg absolute ceiling)
• Cartridges to reach weight limit: 490 ÷ 44 mg ≈ 11 cartridges (lidocaine limit)
• Epinephrine is the practical limiting factor: a healthy-adult epinephrine ceiling of approximately 200 micrograms per appointment ÷ 27.5 µg per cartridge ≈ 7–8 cartridges. For patients with cardiac disease, the cautious epinephrine ceiling is approximately 40 micrograms (~1–2 cartridges).

Always use the most conservative of the weight-based and epinephrine-based limits.

Vasoconstrictors

Epinephrine (adrenaline) is added to local anaesthetics to cause local vasoconstriction, which:

1. Reduces systemic absorption, lowering toxicity risk and extending duration
2. Decreases bleeding at the operative site
3. Increases the effective maximum safe dose

Concentrations are expressed as ratios: 1:80,000 (lidocaine) and 1:100,000 (articaine). Felypressin (octapressin), a synthetic vasopressin analogue, is paired with prilocaine as an alternative for patients in whom epinephrine must be minimised, although it has weaker vasoconstrictor action.

Epinephrine Cautions

Epinephrine is not absolutely contraindicated in most cardiac disease, but caution is required. Safe practice:

• Aspirate before injecting to avoid intravascular delivery
• Use the lowest effective concentration and volume
• Avoid in uncontrolled hyperthyroidism and unstable cardiac disease
• Limit to approximately 40 micrograms epinephrine (about 1–2 cartridges of 1:80,000) for patients with significant cardiovascular disease
• True absolute contraindications to epinephrine are rare; the risk of epinephrine is generally lower than the risk of giving plain anaesthetic that fails to achieve adequate analgesia

Allergy to Local Anaesthetics

True allergy is uncommon but clinically important.

• Amide agents (lidocaine, articaine, mepivacaine, bupivacaine, prilocaine): allergy is rare. Reactions reported are more often vasovagal, toxicity, or epinephrine side effects.
• Ester agents (benzocaine, procaine, amethocaine): higher allergy risk because they are metabolised to para-aminobenzoate (PABA), a known allergen. Cross-reactivity exists across ester agents but not with amides.

If a patient reports a genuine amide allergy, refer to allergy testing — do not assume all amides are contraindicated without confirmation.

Topical Anaesthesia

Topical agents anaesthetise mucosa before injection or for minor procedures:

• Lidocaine 5% ointment or 4% gel — widely used; amide, low allergy risk
• Benzocaine 20% gel — ester; risk of methaemoglobinaemia, especially in children, infants, and patients with G6PD deficiency

Topical should be applied to dried mucosa for 1–2 minutes and used sparingly.

Injection Techniques

Articaine's lipid solubility has expanded the role of buccal infiltration in the mandible, reducing reliance on the ID block for some restorative procedures, though the ID block remains standard for profound pulpal anaesthesia of mandibular molars.