A Primer on Dental Local Anesthetics
I use local anesthetics every single day at work without fail. Even most of my ardent holistic patients who shun "chemicals" don't object to its use. The reasons are clear. Their use is generally very safe, and most people don't want to risk feeling dental pain, which can be intense. This post will cover what local anesthetics are, what they do, the side effects, and the risks of their use.
What is a local anesthetic?
Local anesthetics interrupt our ability to feel pain where they are administered. They do so by inhibiting sodium ions influx into nerves. When this happens, the nerve cannot fire properly, and we feel no pain.
The history of local anesthesia
Cocaine is a naturally occurring local anesthetic that the Coca plant produces. Researchers believe humans have been using it for 5000 years. The earliest known cultivation is over 2000 years old. The Spanish knew about its use, and in 1653, Bernabé Cobo mentioned in his works that it relieves toothaches. In 1860, Friedrich Wöhler isolated the active chemical, which he named cocaine. Scientists were able to elicit the chemical formula in 1898, and it was first synthesized in 1923 by Merk. However, its use as an anesthetic was noted by Basil Von Anrep in 1880. Due to his findings, he recommended its use as a surgical anesthetic. Dr. Koller used it for eye surgery in 1884 and published the results. Unfortunately, cocaine is addicting and has other adverse reactions. This led to the advent of Novocaine in 1905. Novocaine was a weak anesthesia, but it took until the mid-forties for researchers to develop lidocaine, the standard to this day. Even though most of us refer to local anesthetics as Novocaine, it is not in wide use today. 1
Many dental procedures don't require anesthesia.
I frequently perform procedures on teeth without numbing the patient. Here are the reasons. First, if a tooth has a root canal, the nerve has been removed and cannot sense pain. Second, as we age, the nerve recedes, causing much less, if any, pain sensitivity. Third, sometimes the painful part of placing a filling, the "drilling," only takes a few seconds, so some patients opt to bear with the discomfort for a few seconds to avoid several hours of numbness. Lastly, many treatments are superficial, and there is little risk of pain. Conversely, sometimes teeth near where I am working are so sensitive to air and water that they must be numbed in addition to the tooth I am working on. However, I can often cover them with a thin layer of impression material instead of numbing them.
Ways your dentist can make the injections more tolerable.
First, a topical numbing agent should be applied before the injection. Second, the solution should be warmed to body temperature. The inner temperature of our body is a constant 98-plus degrees when healthy. Lowering our inner temperature is uncomfortable and accounts for much of the discomfort from injections. Third, thinner gauge needles should be used as they are easier to tolerate. Fourth, the injection should be placed slowly to allow the solution to dissipate. This will result in lower pressure from the solution. Other techniques, like distracting the senses through tugging on the lips, are often employed.
The most common problems with local anesthetics.
The biggest one is the numbness after the procedure is finished. Sometimes the numbness can last for up to 5 hours. The feeling can be uncomfortable. It can result in speech impediments and a droopy look to the face. The other big issue is accidentally biting the lip, tongue, or cheek. For this reason, it is important not to eat solid foods while numb. A reversing agent is available on the market, but it is of little clinical value.
What nerves does it work best on?
Sometimes, on lower block injections, the facial nerve can be affected, which can cause the motor nerves to cease functioning properly. The usual area affected is the eye, which looks like Bell's palsy. Luckily, the motor nerves return quickly, so there are usually no long-term issues. The eye can become dry, so taping it shut can help.
Local anesthetics work best on small, rapidly firing nerves (neurons). Dentists take advantage of this because the sensory nerve of the head, called the trigeminal nerve, is comprised of only small, rapidly firing neurons. They are most effective on autonomic fibers, followed by sensory fibers, and, lastly, motor fibers (involved in movement). For instance, as patients recover from spinal anesthesia, they first regain voluntary muscle movement, then sensation returns, and finally, they can urinate, which is an autonomic function. Additionally, patients numbed during extraction can still sense the pressure because those nerve fibers are slow firing.
Local anesthetics don't always work; why?
Due to the acidic nature of the mixture, the molecules exist in a quaternary, water-soluble state at the injection time and cannot penetrate the neuron. Once they are in the neutral PH of the body, they convert to a form that can enter the nerve cell. In cases where patients are inflamed, the environment's low PH (acidity) can block the local anesthetic's action. This is one reason I prefer patients to seek treatment early, as the infection can make numbing difficult.
Another common reason is that injections in the lower jaw, called mandibular blocks, require that the solution is deposited next to the nerve. The nerve is covered by the cheek muscle, however. Sometimes the solution is deposited on the wrong side of the muscle, and it never gets to the nerve.
Local anesthetics and obesity.
Local anesthetics are fat-soluble, which causes them to become absorbed into fatty tissues. Articaine, also called Septocaine, is the most fat-soluble one. For this reason, when I am working on obese patients, it sometimes wears off more quickly than expected. This is hardly an issue because most dental procedures don't last long enough for the anesthetic to wear off. Typically, the patient notices a slight pain sensation, and more anesthetic is delivered.
Why do local anesthetics make some people jittery?
To keep local anesthetics in the injection site longer, epinephrine is added to the solution. Epinephrine (epi) is also called adrenalin, which explains the uneasy feeling that can happen after a local anesthetic is administered. The heart rate increases by around 8–10 beats per minute, and blood pressure changes by around 5 points on average. Peak influences of epinephrine are generally observed rapidly within 5–10 minutes following injection decline. The half-life of epi is only 1-3 minutes. You may notice that it is over quickly when you are startled and get an adrenalin rush. The effects of epi, which are not always seen, disappear in about ten minutes. Contrary to popular belief, epinephrine is not as dangerous to cardiac patients as popularly believed. It should still be used cautiously when treating patients with high blood pressure or other heart and circulatory conditions.
Epinephrine constricts the blood vessels locally. When the blood flow is diminished, the anesthetic is not carried away from the site. As a result, the numbness lasts longer. Additionally, the blood concentration of the local anesthetic is kept lower, allowing gradual metabolism and diminishing the systemic effects of the local.
What are the side effects of local anesthesia?
I have spoken about the jitters due to epi; what about the local anesthesia’s side effects? Local anesthetics are considered central nervous system depressants. They are used to suppress cardiac arrhythmias and seizures and can suppress respiration. In higher concentrations, local anesthetics can cause seizures. Their ability to cause seizures is greater when there are higher CO2 blood concentrations with poor respiration that can occur with sedation.
Paresthesia is a condition where one is left with partial feeling. It is usually associated with lower mandibular blocks because the needle tip can touch the large nerve that supplies the jaw. Otherwise, infiltrations given adjacent to teeth do not go near large nerves, so the risk of paresthesia is much lower. This is a rare side effect of injections with local anesthesia for two reasons. First, a nerve can be touched and injured by the tip of the needle. The other cause is the irritating nature of local anesthetics. Slow injections of smaller volumes help avoid this damage. In 1993, only 14 cases were reported in Canada. In every case, the solution was either articaine or prilocaine because these solutions often come in higher concentrations. Most often, the paresthesia is temporary.
Caution must be used when patients are on sedatives and narcotics. Luckily, dentists normally use doses too low to induce systemic effects. Unfortunately, problems can occur when children are sedated for dental procedures and receive high doses of local anesthetic. Bupivacaine has greater potential for direct cardiac toxicity than other agents. Articaine (Septocaine) is inactivated in the blood; for this reason, fewer systemic effects are expected with its use. Having said all of this, the concentrations used, especially for routine procedures like fillings, crowns, and root canals, are low, and systemic effects are rare.
Do allergies to local anesthetics occur?
It is not unusual for patients to incorrectly claim they are allergic to local anesthetics. This is due to the palpitations or increased heart rates from the epinephrine. Allergies to the local anesthetics themselves are rare. Allergic reactions following local anesthetic injections are more likely attributable to preservatives (methylparaben) or antioxidants (sulfites) contained in the solution. Sulfites are used when the anesthetics contain epi. For this reason, if sulfite allergies are present, using anesthetics without epi is important. If a patient describes a reaction that is at least clinically consistent with allergy, the dentist should avoid using the offending agent until an allergist evaluates the patient.
Which local is the best?
There is no good answer to this question. Lidocaine 2% with epinephrine 1:100,000 is the most common local anesthetic. Its use is practically universal. For longer procedures, bupivacaine, also called Marcaine, is used. Carbocaine, also called mepivacaine, 3% plain, is usually the choice for people requiring no epi. It may not result in profound anesthesia and does not last very long because it has no epi to potentiate its effects. I prefer articaine because it is highly fat soluble and acts quickly. It is also metabolized systemically in the blood. It performs better for lower mandibular blocks than lidocaine. One study disproves what I just told you; however, having given hundreds of thousands of injections, certain patients' lower nerves are covered by thicker than normal cheek muscles, and in their cases, articaine is more reliable. I am aware it can cause paresthesia about 7% more than lidocaine. However, the rate is so low that the 7% increase has little clinical relevance. I have only caused it twice in over 30 years, which was transient. The risk of causing paresthesia is mitigated by superior technique; as I mentioned earlier, slow injections are the key to more tolerable injections and decreasing the odds of paresthesia. I hope this post is useful for you should you be confronted with choosing to have a local anesthetic or not when visiting the dentist.