Oral and maxillofacial surgery/Local anesthesia/Topical anesthetics

Topical anesthetics are local anesthetics which are applied topically to induce reversible analgesia in the superficial tissue layers. This learning resource focusses on the use of topical anesthetics in the mouth prior to intra-oral injections of local anesthetic. The skin is a more significant barrier with its thick keratin layer compared to mucous membranes. Hence, topical anesthetics are less effective on skin, especially adult skin.

Oral and maxillofacial surgery

School of Medicine
School of Dentistry

Intra-oral:

Lidocaine
Benzocaine 20% gel
Ethyl chloride ("freezes" tissue)

Skin:

EMLA (eutectic mixture of local anesthesia, lidocaine, prilocaine)
Ametop (tetracaine 4%/methocaine)

General points:

Some clinicians do not use topical at all as they see it as time wasted on something which is ineffective
For maximum benefit, topical should be used with liberal suggestion, in much the same way the effect of nitrous oxide sedation is said to be largely non-pharmacologic
Topical can be useful in children
Some adult patients will think more highly of their clinician if they always get the "numbing gel", as they interpret it as extra attention to detail and genuine care for their welfare.
Generally, topical is not indicated prior to blocks, only infiltrations. Topical should be avoided particularly for IANB's where the topical just runs down the throat triggering coughing. Having said this, some patients might be so attached to the idea of topical from their previous dentist that it is quicker and easier to just use it rather than waste time trying to convince them otherwise
Topical sprays are available. These are difficult to direct, usually taste bad. They are generally inferior except for tasks such as anesthetizing the oropharynx prior to manage an oversensitive gag reflex prior to a procedure.
Generally, topical anesthetics tend to be esters rather than amides.

Technique

Describe the topical in positive terms. Pain is subjective and has a strong emotional component. Unsurprisingly, persons who are given topical and told confidently "this will make numbing the mouth much more comfortable" actually experience significantly less injection pain compared to persons who are given topical with no explanation, or a negative comment such as "this doesn't work that well". The power of suggestion is very important.
Usually it is best to have the patient sitting upright to avoid the topical running down the back of the throat. They induce salivation and may numb the soft palate, tongue and oropharynx if it runs down the throat or if the patient swallows the saliva mixed with topical. Anesthetized oropharynx is sometimes desirable (e.g. over-sensitive gag reflex preventing impression taking), but generally it is very uncomfortable. It may lead to coughing which continually interrupts the procedure as the patient loses the ability to stop saliva running into the larynx. Applying the topical towards the back of the mouth, especially when the patient is supine, may cause this.
Apply a small amount of topical to a cotton applicator or cotton wool roll. It is a common mistake to use too much topical. The excess will be unhelpful and just causes salivation, mixes with the saliva and washes around the mouth and throat causing unnecessary problems. Instead, aim to be accurate with only a small amount.
Identify the exact spot where the needle will penetrate. This requires the tissues to be retracted and stretched otherwise the area that has had the topical will not be in the same place when you administer the injection.
Dry the mucosa gently with the 3 in 1 syringe. Saliva acts as a barrier between the topical and the mucosa.
A full 2 mins with the topical in place are required for any real difference in the pain caused by an injection. Some clinicians use less time than this, and are consequently relying on the placebo effect to have any change in the injection pain.
The mucosa may wrinkle, which is sometimes attributed to muscle relaxation in the underlying tissue.