Topical anaesthetic reduces the slight discomfort associated with insertion of the needle. It is affective to a depth of 2-3mm. Although its application is beneficial for reducing patient discomfort during the initial phase of local anaesthetic administration, it may be a disadvantage in children if the taste is disagreeable to the patient. Also excessive length of application time may increase apprehension of the approaching procedure.
It is available in gel, liquid, ointment, patch and pressurized spray forms. The most common topical anaesthetics used in dentistry are those containing benzocaine or lidocaine.
Benzocaine (ethyl aminobenzoate) is an ester local anaesthetic. It is available in up to 20% concentrations. It is not known to produce systemic toxicity but can produce local allergic reactions especially after prolonged or repeated use. It exhibits poor solubility in water and poor absorption into the cardiovascular system, thus it remains at the site of application longer, providing a prolonged duration of action. Systemic toxic (overdose) reactions are virtually unknown. Benzocaine is reported to inhibit the antibacterial action of sulfonamides.
Lidocaine is available as a solution or ointment up to 5% concentration and as a spray up to 10% concentration. It has a low incidence of allergic reactions but is absorbed systemically and application of excessive amounts of topical lidocaine may absorb rapidly into the cardiovascular system leading to higher local anaesthetic blood levels with an increased risk, especially in the pediatric patient, of overdose reaction. Thus a minimal amount of topical gel should be applied to the tissue and a metered spray is suggested if an aerosol preparation is selected.