As a dentist in private practice, I hear stories ALL the time about patients who “couldn’t get numb at the dentist.” Not surprisingly, many of the comments that readers post here on this site also deal with this very issue.
In response to this, I decided to compile a Top 5 list of reasons for why this happens. So here goes:
1. You have an infection
Sometimes, a patient comes in with an active infection and it can be difficult to get them completely numb. This is obviously very frustrating for both the dentist and the patient. Why does this happen?
An active dental infection will usually result in the presence of pus. In most cases, the pus is acidic. Conversely, dental anesthetics (lidocaine, novcaine, etc.) function best in slightly basic environments. The end result is that the unique chemistry of the infection “deactivates” the local anesthetic, making it so that more anesthetic is needed. And in cases of severe infections, sometimes you simply cannot get the patient 100% numb.
2. You moved during the injection
Let’s face it – some injections – but not all – hurt! And it just so happens that the one that hurts the most is the one that requires the most patient cooperation. I speak from experience having done this well over ten thousand times.
For lower back teeth, we nearly always need to do a nerve block. This is where we have to go deep through muscle and other tissue and deposit the local anesthesia near the nerve. We can’t actually see the nerve – we have to use various anatomical landmarks to guide us to the area.
If you move suddenly, the needle will also move. Most often, it will have moved away from the nerve! So we then end up depositing the anesthetic farther away from the nerve than we would like. What happens next? You don’t get numb. Fortunately, if we have to administer a second injection, it will rarely hurt, and then we can place it spot on.
On rare occasions – and this has never happened to me nor most dentists – you can move so much that the needle can actually break!
3. I am not using epinephrine
I’ve blogged about this before. Epinephrine is added to dental local anesthetics because it enhances the numbness. How does it do this? Epinephrine acts as a vasoconstrictor and reduces blood flow in the area of the injection. The end result is that the local anesthetic stays around much longer and gives a more profound feeling of numbness.
In certain circumstances, we use a local anesthetic that does not contain epinephrine. Why? Patients with certain cardiac conditions or who take certain medications are best served with one that does not contain it. Others have experienced a mild adverse reaction (some mistakenly think they are allergic to epinephrine) and prefer we don’t use it. A small fraction of patients are allergic to the sulfite preservative so we can’t use it in those cases either.
If we can’t use epinephrine, there is a chance you won’t feel numb enough. Or we’ll have to re-inject multiple times.
4. You’re wired differently
The human body is incredibly variable. People are double jointed. Remember the kid in grade school who could move his ears? Why is Usain Bolt faster then any other human? You get the picture.
Do you think your nerves look like the drawing below?
If you answered yes, then you’re probably wrong!
Most people have what I might call “standard anatomy.” This means that the nerves going to your teeth are where you might expect them to be located. But just like Usain Bolt and the kid from fifth grade who could move his ears, some patients have extreme variability with the nerves going to their teeth. We see this most frequently with lower molars.
Some people may have up to 4 nerves going to their lower molar teeth. This can mean 4 different injections to get them numb! This doesn’t mean your dentist is incompetent – it means you’re wired differently. So if that happens to you, just think about Usain Bolt and the kid from fifth grade who could move his ears.
5. You have red hair
But to summarize, people with red hair have a built in resistance to local anesthetics. This means that more local anesthetic is required to achieve profound numbness in those people with red hair. The reason behind this is complex, but the genetics for red hair also confers resistance to local anesthetic.
I can say definitively that from personal experience, redheads nearly always require more local anesthetic. All of my red haired patients are aware of this – and we joke about it each and every visit!
Want to see reasons 6 through 10? Here they are!