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  • Dr. David Alfaro

Ask Dr. Dave: Why do my teeth not "freeze" well?

It has been a long time since someone has asked me to write about something! A good friend from back in my Cactus Club days is wondering why dental anaesthesia doesn't seem to work well for him. This is a difficult topic that is still very mysterious.

Let us talk about dental "freezing" (getting ya numb, for my US friends).

What is pain anyways?

We have all been in physical pain at some point, but how well can we describe it? It is a hard thing to get a grasp of because pain is not just a sensation, it is an experience. Pain perception is affected by local factors at the site of the injury, there are areas in the spinal cord where modification of the pain signal occurs, and then centrally in the brain there are further interactions that can alter the feeling that happens in response to a noxious stimulus. The extent of pain that felt is therefore very different between people, and within people, depending on a whole set of parameters that can even include emotional state, awareness, and previous experiences.

Pain's complexity makes it very difficult to study. It is not as straight forward as studying a drug, where we can administer a specific dose, measure the amount in the blood stream, evaluate outcomes numerically (such as blood sugar levels), and then come up with confident conclusions about the effects of the treatment. A pin prick to one person could be no big deal, but for someone else could be a horrible, traumatizing experience.

How do dental local anaesthetics work?

Getting people out of pain, and preventing pain, are some of the major stressors that I experience every day as a dentist. It is emotionally draining knowing that every single person that comes in to my office has some level of anxiety and is not looking forward to the pain or the numbness of freezing, and in the case of surgery, the days of healing that are required. It hard for us dentists too; I do not like inflicting pain, but it is an often unavoidable part of my job.

If if wasn't for gloves and local anaesthetic, there is no way that I would have become a dentist.

I will skip the history and pharmacokinetics of local anaesthetics, but basically they work by shutting down the nerves that transmit the pain signal, along with the sense of touch. Nerves transmit impulses by allowing ions to cross their membranes, causing a change in voltage. Local anesthetics block the ion channels leaving the nerves unable to transmit the electrical signal.

There are different anaesthetics that vary in how strong they are and how long they last, based on various things such as how easily they enter the nerve, where and how quickly they get broken down, and whether or not they also have epinephrine (adrenalin).

Why would someone have a hard time getting "frozen"?

There a many reasons why there could be difficulties in acheiving profound dental anaesthesia.

1. Injection technique and anatomic variability

There are various ways to apply local anaesthesia: we can target nerves directly, we can infiltrate the anaesthetic to the bone that surrounds the tooth, and we can even inject the solution into the periodontal ligament space or into the pulp itself, if it is already exposed.

All of these techniques have different indications, success rates, complications, and discomfort levels.

When most people think of getting a "shot" at the dentist, they are thinking of the inferior alveolar nerve block. There is a pretty big nerve that enters the mandible on the inside of the cheek, very far back. Like most big nerves, it runs with blood vessels in a bundle. This nerve supplies sensation to most of the lower jaw and teeth, one on each side.

To successfully "block" this nerve a dentist has to deposit the anaesthetic solution right beside the nerve. Even with proper training, it is possible to "miss the block". Depending on the study, and type of anaesthetic used, research indicates that it could be around 25% of injections that fail.

2. Local Factors

When there is an active infection or inflammation, the nerve does not respond well to anaesthetic. It is well documented that people with teeth that are "symptomatic", have a harder time getting frozen. There are many possible reasons for this, including local changes in pH, high concentrations of inflammatory mediators, increased regional blood flow, and potentially even changes to the ion

channels themselves.

3. Systemic and Central Factors

Some people just gobble up anaesthetic. They get nice and frozen but then it wears off quickly. This again could be for various reasons. As most modern dental anaesthetics are metabolised in the liver, anything that ramps up liver enzyme function can theoretically speed up the rate that these anaesthetics are broken down.

There are individual differences in pain tolerance as well, and there are various theories, ranging from very biological explanations such as variations in receptors and ion channels, to more psychobiological explanations, where experiences and emotions cause the upregulation or downregulation of enzymes and receptors involved in the interpretation of the pain experience. Some people tolerate pain well, while others flinch at every light touch.

Are there risks to local anaesthesia?

Of course. Everything has its pros and cons.

Whenever any medication is administered, there is always the risk of allergy or sensitivity to the drug or its byproducts. Fortunately, most commonly used local anaesthetics and their preservatives are very well tolerated.

Depending on which anaesthetic is used, there can be serious adverse effects such as cardiac arryhthmias or seizures if administered in high doses.

Many of the reported adverse outcomes could be related to the addition of epinephrine to common local anaesthetic solutions. It is added in order to constrict the blood flow in the region where it is applied, helping to keep the anaesthetic in place, instead of allowing it to diffuse systemically. It also helps provide hemostasis. If too much epinephrine is used, or if the injection is deposited within a blood vessel instead of within the tissues, patients may experience increases in heart rate and blood pressure.

Finally, there is always the risk of nerve damage and needle breakage when an inferior alveolar block is performed. The risk of the anaesthetic itself damaging the nerve is not the concern here, but it is the needle itself. When administering a "block", the dentist must blindly place the needle close to the nerve. We obviously cannot see the nerve, but use anatomical landmarks to estimate where the nerve should be. Then the goal is to aim for right beside the nerve; we do not want to hit the nerve or blood vessels, but to get it close by. This is tough to do and contributes to the low success rates of the inferior alveolar nerve block.

Sometimes we are too accurate and hit the nerve straight on with the needle. Patients will often report a sharp electric feeling when this happens, and really there is nothing that a dentist can or cannot do to avoid this. Most of the time this is just a zinger that goes away quickly, but some patients can experience lingering altered sensation that could even last for months. Fortunately, the nerve damage is rarely permanent.

What can a dentist to do provide better anaesthesia?

Technique and anaesthetic choices aside, the provision of efficient dental anaesthsia is truly an art of patience.

When I was a kid, I was pretty good at the dentist, but was still not too keen on getting shots or drilling done. At the office I went to, and at many other offices, the dentist was always rushing around treating a bunch of patients at once. To me, this was a source of stress. I was totally fine chatting with the assistant and hygienist, but I knew once the dentist walked in, that it was pain time. Furthermore, he didn't really say anything to me. He would ask the assistant, "is he frozen?" and would then just do some stuff then walk away. If I wasn't frozen, he would just load me up with more, and then would walk off to do something else.

After having had that experience growing up, I decided that I would not practice dentistry that way. The administration of anaesthesia and anxiety management is something that I take my time with, and I am fortunate that I can do that in my practice. I do not have multiple appointments going on and my patients get my undivided attention. It is how I manage dental anxiety and how I ensure that my patients are comfortable before beginning any procedure.

What about sedation?

Vancouver is completely saturated with offices promoting sedation dentistry. I have even seen "Dentistry while you sleep!" ads on the bus. Yes. I take the bus and skytrain to work.

Don't get me wrong, sedation has a very important role in the provision of complex dental care. It is a great tool when dealing with a combative patient, or when completing larger, invasive, dental surgery.....but....people are using sedation as a primary anxiety management technique, and even worse, as an income generator. Sedation is safe, but when things go wrong, they go very wrong, and the bottom line is, the general population does not need sedation for their basic dentistry.

Sedation adds a whole layer of complexity to the provision of dental care. It adds a new element of risk to every procedure. Instead of worrying about the challenges of the dental treatment that is being provided, the risks of the sedation become the greatest concern, and procedures need to be completed as quickly as possible. I do not understand how anyone would be willing to add so much risk for dental procedures that can be completed with basic local anaesthesia.

I can say this comfortably because I do complex prosthetic and surgical dentistry everyday, without sedation. Extractions, dental implant surgery, bone grafting, multiple crown preparations. Hard stuff. Yes, I have had patients who regularly take anxiolytics ask if they can take one before an appointment, but I do not even have a stock in the office, nor do I need to. If someone absolutely insists on sedation, I refer them to the appropriate specialist.

In my practice, I manage dental anxiety without the use of drugs. I book extra time, I let patients rest, I make sure that I give them time to ask all the questions they need, and I sit by their side until they are calm and comfortable before starting any procedure, even for something as simple as a scaling and root planing. It is takes time to provide dental care in this manner, so yes, people pay specialty fees, but they get specialty care. And for the most works.

This kind of treatment just does not work out in an office like I went to as a child and teenager, where the dentist was running from room to room doing fillings and hygiene checks. He did not have time to even talk to me directly, let alone sit with me while the freezing took effect. In a busy, volume based, general dental practice, anxiety cannot be managed with time the way I do in my little specialty clinic. Adding sedation to manage dental anxiety to a busy routine like that doesn't work either, because sedation requires one on one attention. This trend of the incorporation of complex dental treatments (such as sedation) to the busy routine of restorative dentistry oriented general dental practices is a whole different topic, and a controversial one at I wont go there......

Thanks for reading!

Dr. Dave

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