Local anesthetics work by blocking nerve impulses. Nerve impulses are electric signals that carry both stimulus to a muscle to have it function and sensation, including pain, from tissues to the brain. At a cellular level this occurs by blocking sodium channels in the nerve membranes. When sodium is blocked in this way, the nerve cannot conduct an impulse and therefore no sensation can be transmitted. Different local anesthetic drugs differ in their side effects, dosages and duration of action.
In a dental context there are two types of anaesthetic injections. In Canada we call such an injection, ‘freezing’, in the USA it is called a shot. Not sure if that is a geographical or political distinction. In a lower anaesthetic usually the a half of the mandible is anaesthetized. This involves a block of the entire inferior alveolar nerve. This nerve blocks sensation to the lower teeth, lower lip, chin and tongue on the half of the lower jaw that is injected. To achieve the anesthesia the dentist injects into the area behind the last lower molar. A ‘landmark’ is used to properly inject the anesthesia into the site where the nerve exit’s the inside of the jaw. But, in some cases the block may not occur as the anatomy may vary between patients. In such cases the dentist will place another carpule of anesthesia into the area. A branch of the ‘mandibular’ nerve is the mental nerve. It exit’s the jaw area near the lower bicuspids. It supplies innervation to half of the lower lip and the chin. This is why the dentist will ask if the lip is “frozen”. If a patient’s lower lip and chin are numb it means we can proceed with treatment. In some cases a dentist may place some anesthesia near the tooth or teeth being treated, this helps numb supplementary innervation.
The upper teeth are usually anesthetized with anesthesia placed directly beside the tooth or teeth in question. This is called an infiltration procedure in that the anaesthesia will penetrate the thin bone surrounding the tooth. In most cases where a filling is being provided this will allow painless treatment. In other cases such as a dental extraction, the anesthesia may be introduced to several areas around the upper teeth / tooth. This can include an anesthesia to the palate, which can be sensitive.
In most applications of local anesthesia a gel containing some anesthesia is placed on the injection site. It is in fact more of a psychological aid because it only anesthetizes the surface. Once the needle penetrates the soft tissues the effect of the topical gel disappears. But if a dentist slowly injects a few drops as he / she proceeds the discomfort is minimalized. Anesthesia usually lasts about thirty minutes. But in some cases where a longer duration is desired, an anaesthesia with adrenalin (Epinephrine) in the concentration of one in 100,000 is used to constrict the blood vessels near the nerve, this reduces the time it takes for the anesthesia to be removed from the site. After it is circulated it is reduced by the liver to an inert material.
In rare instances a lower mandibular block can result in a numbing which can last for several weeks. This is due to an unavoidable situation where the needle may cause some injury to the nerve. A lip or tongue can remain slightly numb for several weeks.
It is also important that the patient and the dentist be aware of any medical problems which should be addressed before injecting an anesthesia. High blood pressure, heart problems, diabetes are a couple of situations the dental team should be appraised of.
Dr Michael Pilon DDS