3.1 Local Anaesthetic Agents, Vasoconstrictors & Injection Techniques

3.1 Local Anaesthetic Agents, Vasoconstrictors & Injection Techniques

Local anaesthesia forms the cornerstone of pain control in clinical dentistry. In Australian dental practice, practitioners must possess a thorough understanding of local anaesthetic (LA) pharmacology, vasoconstrictor interactions, dosage calculations based on Australian standard 2.2 mL cartridges, anatomical injection techniques, and the immediate management of adverse reactions.

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Chemical Classification & Pharmacokinetics

Local anaesthetic agents are amphiphilic molecules comprising a lipophilic aromatic ring connected via an intermediate ester or amide chain to a hydrophilic tertiary amine group.

• Ester Local Anaesthetics (e.g., Procaine, Benzocaine, Tetracaine): Hydrolysed rapidly in plasma by pseudo-cholinesterase enzymes. Their metabolic product, para-aminobenzoic acid (PABA), carries a relatively high risk of IgE-mediated allergic reactions. Ester agents are rarely used for parenteral injection in contemporary Australian dental practice, though Benzocaine remains in topical formulations.
• Amide Local Anaesthetics (e.g., Lignocaine/Lidocaine, Articaine, Prilocaine, Mepivacaine, Bupivacaine): Biotransformed primarily by hepatic cytochrome P450 enzymes. True allergic reactions to amides are extremely rare; hypersensitivity manifestations are usually attributable to the preservative sodium metabisulfite added to adrenaline-containing formulations.

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|                                 LOCAL ANAESTHETIC MOLECULAR STRUCTURE                             |
|                                                                                                   |
|   Lipophilic Aromatic Ring  =====>  Intermediate Chain (Ester/Amide)  =====>  Hydrophilic Amine    |
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Mechanism of Action

Local anaesthetics reversibly block voltage-gated sodium channels ($Na_v1.5$ and related subtypes) along the internal membrane surface of nerve axons. By binding to specific receptors within the sodium channel pore, LAs prevent influx of sodium ions, thereby inhibiting membrane depolarisation and blocking action potential conduction along unmyelinated $C$ fibres (dull pain, temperature) and small myelinated $A\delta$ fibres (sharp, acute pain).

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Pharmacological Profiles of Dental Local Anaesthetics

In Australia, local anaesthetics are supplied in 2.2 mL cartridges. The specific concentration and vasoconstrictor ratios dictate maximum dosage and clinical application.

Key Agent Characteristics

1. Lignocaine (Lidocaine) 2% with 1:80,000 Adrenaline: Considered the gold standard reference agent in Australian dentistry. Provides reliable pulpal anaesthesia for 60 minutes and soft tissue anaesthesia for 3–5 hours.
2. Articaine 4% with Adrenaline: Contains a thiophene ring enhancing lipid solubility and tissue diffusion. Uniquely metabolised predominantly in blood plasma by esterases (90-95%), yielding an exceptionally short half-life (~20 minutes) and reduced systemic toxicity cumulative risk. Excellent for maxillary and mandibular infiltration. However, due to reports of persistent paresthesia (predominantly involving the lingual nerve), many Australian guidelines advise caution when selecting 4% formulations for deep nerve blocks (such as the Inferior Alveolar Nerve Block).
3. Prilocaine 3% with Felypressin: Felypressin is a synthetic analogue of vasopressin that acts on vascular smooth muscle $V_1$ receptors without significant cardiac $\beta_1$ stimulation. Recommended when adrenaline is strictly contraindicated. Metabolites include o-toluidine, which can induce oxidation of haemoglobin to methaemoglobin.
4. Mepivacaine 3% Plain: Low vasodilatory property, making it suitable for short procedures where a vasoconstrictor is undesirable.
5. Bupivacaine 0.5%: High protein binding (95%) and lipid solubility confer long-acting pulpal ( up to 1.5–2 hours) and soft-tissue anaesthesia (up to 8–12 hours), ideal for post-operative pain control in extensive surgical procedures.

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Maximum Recommended Dosage (MRD) Calculations

Dosage calculations must be performed on a milligram-per-kilogram (mg/kg) basis, taking into account the patient's lean body mass and maximum ceiling limits defined by Australian Therapeutic Guidelines.

Standard Australian Cartridge Contents (2.2 mL Volume)

• 2% Solution = 20 mg/mL $\times$ 2.2 mL = 44 mg agent per cartridge.
• 3% Solution = 30 mg/mL $\times$ 2.2 mL = 66 mg agent per cartridge.
• 4% Solution = 40 mg/mL $\times$ 2.2 mL = 88 mg agent per cartridge.
• 0.5% Solution = 5 mg/mL $\times$ 2.2 mL = 11 mg agent per cartridge.

CALCULATION FORMULA:
1. Patient Weight (kg) x Agent MRD (mg/kg) = Maximum Allowable Dose (mg)
2. Compare with Absolute Maximum Ceiling Limit (choose lower value)
3. Maximum Allowable Dose (mg) / mg per 2.2 mL Cartridge = Maximum Cartridges

Agent Dosage Thresholds (Australian Therapeutic Guidelines)

• Lignocaine (with Adrenaline): 4.4 mg/kg; absolute maximum cap of 300 mg (or up to 400-500 mg under strict medical guidelines; standard Australian dental cap is 300 mg).
  • Example calculation for a 70 kg adult:
    • $70\text{ kg} \times 4.4\text{ mg/kg} = 308\text{ mg}$.
    • Apply 300 mg maximum cap limit.
    • $300\text{ mg} \div 44\text{ mg/cartridge} = 6.81\text{ cartridges}$. (Safely rounded down to 6.5 cartridges).
• Articaine (with Adrenaline): 7.0 mg/kg; absolute maximum cap of 500 mg.
  • Example for a 70 kg adult:
    • $70\text{ kg} \times 7.0\text{ mg/kg} = 490\text{ mg}$.
    • $490\text{ mg} \div 88\text{ mg/cartridge} = 5.56\text{ cartridges}$ (5.5 cartridges).
• Prilocaine (with Felypressin): 6.0 mg/kg; absolute maximum cap of 400 mg.
  • Example for a 70 kg adult:
    • $70\text{ kg} \times 6.0\text{ mg/kg} = 420\text{ mg} \rightarrow$ Capped at 400 mg.
    • $400\text{ mg} \div 66\text{ mg/cartridge} = 6.06\text{ cartridges}$ (6.0 cartridges).

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Vasoconstrictors in Dental Local Anaesthesia

Vasoconstrictors are included in local anaesthetics to offset intrinsic vasodilation, prolong duration of action, reduce systemic absorption/toxicity, and enhance local haemostasis.

Adrenaline (Epinephrine)

Acts on $\alpha_1$ receptors (vasoconstriction of skin and mucosal blood vessels), $\beta_1$ receptors (increased heart rate and myocardial contractility), and $\beta_2$ receptors (bronchodilation and peripheral vasodilation in skeletal muscle).

• Healthy Adult Maximum Dose: 0.2 mg of adrenaline per appointment.
  • In 1:80,000 adrenaline (12.5 µg/mL), 2.2 mL contains 27.5 µg (0.0275 mg). Max cartridges based on adrenaline = $0.2 / 0.0275 = 7.27\text{ cartridges}$.
• Cardiovascular Compromised Maximum Dose (ASA III/IV): 0.04 mg (40 µg) of adrenaline per appointment.
  • Restricts 1:80,000 adrenaline to a maximum of 1.4 cartridges (approx 1.5 cartridges) or 1:100,000 to 1.8 cartridges.

Absolute & Relative Contraindications to Vasoconstrictors

• Uncontrolled Hyperthyroidism / Thyrotoxicosis: Risk of severe thyroid storm and malignant arrhythmias (Absolute Contraindication for adrenaline).
• Recent Myocardial Infarction (<6 months) or Unstable Angina: High risk of re-infarction or fatal arrhythmia (Absolute Contraindication for adrenaline).
• Non-Selective Beta-Blockers (e.g., Propranolol): Unopposed $\alpha_1$ stimulation leads to severe hypertension accompanied by compensatory reflex bradycardia.
• Tricyclic Antidepressants (TCAs) (e.g., Amitriptyline): Inhibit reuptake of noradrenaline, potentiating adrenaline effect 2- to 4-fold.
• Sulfite Hypersensitivity: Sodium metabisulfite is used as an antioxidant in adrenaline formulations. Patients with true bisulfite allergies must receive plain LAs (e.g., Mepivacaine 3% plain).

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Injection Techniques & Clinical Efficacy

1. Inferior Alveolar Nerve Block (IANB): Targets the inferior alveolar nerve prior to entry into the mandibular foramen. Landmarks: coronoid notch, pterygomandibular raphe, mandibular occlusal plane (10 mm above). Aspiration in 2 planes is imperative (high intravascular injection incidence ~10-15%).
2. Gow-Gates Technique: True mandibular nerve block targeting the anteromedial border of the condylar neck, below the insertion of the lateral pterygoid muscle. Anaesthetises V3 branches (IAN, lingual, mylohyoid, buccal, auriculotemporal). Higher success rate and lower intravascular aspiration rate (~2%) compared to traditional IANB.
3. Vazirani-Akinosi (Closed-Mouth) Technique: Indicated when patient presentation includes severe trismus. Target is the pterygomandibular space at the level of the mucogingival junction of the maxillary molars.
4. Mental / Incisive Nerve Block: Targets mental foramen adjacent to mandibular premolars. Does not anaesthetise lingual soft tissues.
5. Articaine Buccal Infiltration in Mandibular Molars: Due to high lipid solubility, Articaine 4% buccal infiltration provides pulpal anaesthesia in mandibular first molars comparable to IANB in adult patients, serving as an effective primary or supplemental technique.

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Complications & Toxicity Management

Local Anaesthetic Systemic Toxicity (LAST)

LAST occurs secondary to accidental intravascular injection or severe overdose resulting in plasma levels exceeding toxic thresholds.

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|                                 LAST CLINICAL PROGRESSION STAGES                                  |
|                                                                                                   |
|  Initial CNS Excitation ====>   CNS Depression   =====>  Cardiovascular Collapse                  |
|  - Metallic taste              - Slurred speech          - Severe hypotension                     |
|  - Perioral numbness           - Loss of consciousness   - Ventricular arrhythmias                |
|  - Tinnitus & tremors          - Generalized seizures    - Cardiac arrest / Asystole              |
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Management of LAST Protocol

1. Stop Administration immediately and call for emergency assistance (000 in Australia).
2. Maintain Airway: Administer 100% oxygen, prevent hypoxia and hypercapnia (which worsen toxicity).
3. Control Seizures: Administer intravenous Benzodiazepines (e.g., Midazolam 2.5–5 mg IV/IM).
4. Lipid Emulsion Therapy: Administer 20% Intralipid: Bolus dose $1.5\text{ mL/kg}$ IV over 1 minute, followed by continuous infusion of $0.25\text{ mL/kg/min}$ to scavenge lipophilic local anaesthetic from circulation ("lipid sink" mechanism).

Methaemoglobinaemia

Associated primarily with Prilocaine (and high-dose Benzocaine). Metabolic conversion of haemoglobin to methaemoglobin ($Fe^{3+}$ state) impairs oxygen-carrying capacity. Cyanosis unresponsive to supplemental oxygen occurs when methaemoglobin exceeds 15-20%. Treatment: Intravenous Methylene Blue 1–2 mg/kg over 5 minutes.