Why do dental injections hurt? -
Needle size, injection location, type of tissue? | Which kinds of dental shots are the most painful? | Techniques dentists use to minimize injection pain. | What to expect during yours.
A patient receiving a dental injection.
One question that dental patients always have is: Will my injection hurt?
Unfortunately, that question can't always be answered in advance. But there are some basic factors and rules of thumb that can give you a pretty good idea of what to expect. That's what this page outlines and explains.
Why do some shots hurt and others not so much?
Location, location, location.
Anyone who's had very much dental work done knows that the level of pain they'll experience with any single injection can vary by quite a bit.
And even though most patients have their mind set on the needle as being the main culprit, the more important factor is the location (type of tissue) in which the shot is given. It's this factor that explains why some injections end up hurting more than others.
The dental injection process.
Every shot is composed of 3 basic parts:
- The needle insertion. - This is the act of initially piercing the patient's skin.
This step literally is just a pin prick. And even when it's not totally painless, it only takes a split second to accomplish.
- Needle placement. - Once penetration through the skin has been made, the next step is for the dentist to advance the needle to the position where the anesthetic needs to be deposited.
There can be some discomfort associated with this process. But, as you'll read below, the dentist has a simple way of minimizing what's felt.
- Depositing (squirting out) the anesthetic solution. - This is the step that's typically responsible for whatever amount of pain is felt. As a process, it involves the act of depositing the needed quantity of anesthetic solution into soft tissues.
The ease with which this is accomplished depends on the nature of the tissue itself. And this factor explains why the location where a shot is given is the biggest determinant of how much it's going to hurt. (See below for more details.)
The steps of the injection process - How a dentist gives a shot.
Here's what a dentist does, or at least tries to accomplish, when they give an injection. Note that some of these steps are taken specifically to help to minimize the amount of pain you feel.
- Placing topical anesthetic. - This step precedes the actual injection. It involves placing a numbing agent on the surface of your skin where the shot will be given, in hopes that you won't feel the prick of the needle as it's inserted.
Unfortunately, this step really isn't as effective as you'd expect. But that's not to say that it doesn't improve your experience. We explain what we mean in greater detail below.
- Needle insertion. - For this step, your dentist will stretch your skin taut (so it's easily pierced) and advance the needle through it about 1/4th of an inch.
They'll then express a few drops of anesthetic and pause for about 5 to 10 seconds as it takes effect.
- Needle placement. - Of course, the needle needs to be positioned properly before the bulk of the anesthetic can be deposited.
To get to this location, the dentist will advance it slowly, in small steps, while simultaneously expressing additional drops of anesthetic. This way the needle's movement is always into tissue that's already numb.
A dentist may deposit as much as 1/4th of the anesthetic in the syringe during the needle's travels.
- Depositing the anesthetic. - Once the tip of the needle has reached its destination, the anesthetic solution can be placed. And as we explain why below, this is the process during which the greatest amount of pain is typically felt, if any is felt at all.
In short, the slower the rate of injection of the solution the less pain you'll feel. A rate of about 1 ml per minute (or less) is usually considered optimal for minimizing discomfort.
Since the standard dosing held by a dental syringe is around 1.8 ml. This part of your dental injection will ideally (in terms of minimizing pain) last close to 2 minutes.
- The needle is withdrawn. - After the needle has been taken out, the anesthetic's complete effect (including numbing your tooth) should take place within 3 to 5 minutes.
Why does the rate of injection correlate with the amount of pain felt?
It's the physical act of depositing a quantity of liquid (the anesthetic) into tissues that has the greatest potential to cause injection discomfort.
That's because the liquid itself has to find a location to occupy. And the more rapidly it's expressed out of the dentist's syringe, the larger the amount of disruption within the tissues it causes while doing this.
The type of tissue involved matters too.
Actually, the rate of injection may not make much of a difference when an injection is made into "loose" tissues. But when "tight" ones are involved, the pinch of the solution as it forces its way into them may hurt quite a bit. And this fact alone is why shots in different locations have such varying potential to be painful.
a) "Loose" tissues.
The type of tissue in some locations is comparatively "loose" (freely movable), thus making it easy for the injected anesthetic to quickly find space to filter into and occupy.
b) Dense, firmly attached tissues.
In other areas, the nature of the tissue will be dense and tight. And as the anesthetic solution is deposited, it must forcibly make its own space.
It's the pressure that builds up during this process that causes the pinching sensation you feel, and likely confuse with being caused by the needle itself.
Research about injection pressures.
A number of studies have documented that slow, low-pressure injection technique is the key to performing painless dental shots (Primosch 2002, Nagasawa 2003, Kudo 2005) [page references]. That's because it allows a maximum amount of time for the anesthetic to diffuse into the neighboring tissues.
In situations where the injection rate is too rapid, or the tissues involved are very dense, pressure is created as the anesthetic must force its way into the surrounding tissues. And it's this tissue stretching and trauma that causes pain.
- Pashley (1981) calculated that the pressure of anesthetic exiting the needle during an injection could be as high as 330 to 660 psi in dense tissues (your car's tires are inflated to around 32 psi).
In looser tissues, this value dropped on the order of 50 fold.
- Another study (Kudo 2005) studied injection pressures (.6 to 9.5 psi) when shots were placed in loose oral tissues.
It determined that a pressure of just 6 psi (or less) was optimal for minimizing both pain and patient anxiety.
One obvious problem with all of this discussion about injection pressure is how does the dentist know how much they are creating?
Well of course they don't precisely. They just know to inject slowly, and if the patient seems to feel very much, to go even slower.
A more precise technique involves the use of a "computer-controlled local anesthetic delivery system." Some of the brand names associated with these devices are: The Wand, Comfort Control Syringe and Quicksleeper.
- These units replace the use of a conventional dental syringe (they are syringes in their own right).
- They can be calibrated to deliver the anesthetic at a precise (slow) rate, thus minimizing the amount of solution pressure that's created.
- What you do feel, exactly like when any other type of syringe is used, is the initial prick of the needle.
- What you may not feel as much is the anesthetic being deposited.
Research studies involving controlled-delivery syringes.
When used with dense tissues (possibly the best application for these units), Nusstein (2004) determined that fewer patients felt pain (conventional dental syringe 43% vs. 25% when a Wand unit was used).
But not all research confirms that this high degree of control over flow rate is needed. Studies by Asarch (1999), Ram (2003), Grace (2003) and Shah (2012) each found no significant difference in the patient's experience/satisfaction when conventional or computer assisted injections were administered.
In an attempt to lessen their patients' injection pain, some dentists pre-warm dental anesthetic cartridges before they're used.
Schwartz (2015) suggests that the temperature of injected anesthetic solution should lie somewhere between room and body temperature. And in fact states that using cartridges that are too warm tends to increase injection discomfort. (Warming the solution helps it to penetrate into tissues more readily. Liu 2009)
Employing this technique may be of some benefit but Davoudi (2016) states that the number of clinical trials documenting its efficacy are too few to be conclusive. Even so, if you find that your dentist does go to the trouble of taking this step at least they're making a gesture toward making your injection less painful.
So, will your dental shot hurt?
Now that you know why injections can hurt, how do you know if yours will?
a) You're probably going to feel the prick of the needle.
Your dentist can't give you an injection with out the needle piercing your skin. So you may feel that. But even if you do, the pain should only last a split second or so.
A review of dental literature by Nusstein (2003) concluded that 14 to 22% of people receiving mandibular block injections (see below) considered the initial prick of the needle to have caused moderate to severe pain.
[It would be our conjecture that with many other types of injections (especially some infiltration procedures, see below) that that number is significantly less.]
What about using topical anesthetic?
In our outline above, the first step we mention involves the application of topical anesthetic. This product can be a gel, ointment, liquid, patch or spray.
It's placed on your skin where the injection will be given in hope that you don't feel the prick of the needle so much. After an application time of 1 to 2 minutes, topical anesthetics are typically effective to a depth of 2 to 3 mm (just a bit more than a 1/16th of an inch). (Schwartz 2015)
The compounds most frequently used as topicals are: benzocaine, lidocaine, tetracaine and dyclonine hydrochloride. Flavoring is frequently added so to make them more palatable.
The truth of the matter is that placing a topical anesthetic isn't as effective as most of us would hope.
b) Other than the initial prick, you really may not feel very much discomfort.
We also described in our outline of the injection process ...
- How the movement of the needle can be preceded by placing drops of anesthetic. (So the needle is always advancing into numbed tissue.)
- And how slowly depositing the anesthetic solution can help to control injection pain by minimizing the amount of pressure that's created.
For these reasons, it's quite possible that you really may not feel much, or even any, discomfort. Certainly many patients don't. But it wouldn't be right to suggest that all shots can always be painless, because that's not accurate.
The fact of the matter is that some injections, due to the location in which they are given and the tissues they involve, often, even typically, do cause some level of discomfort. (That's the subject of our next section.)
Which types of dental shots tend to be the most painful?
A study by Kaufman (2005) evaluated the pain response of patients when receiving some of the more frequently used types of dental injections used to numb up teeth.
Here they are, listed from least (i) to generally more painful (vi), and our explanation of when each might be needed.
- local infiltration
- mental nerve block
- periodontal ligament injection
- maxillary incisor infiltration
- inferior alveolar nerve block
- palatal injections
i) Local infiltration.
If you've every had a dental shot that was totally painless, to the point where you didn't even know that anything was going on when you received it, it was probably one of these. The looseness of the tissue involved is why.
- This technique works with any upper tooth (see our comments below about upper incisors).
- On the lower jaw, it only works well with incisors.
ii) Mental nerve block.
This is another type of shot that's typically easy for a patient to receive.
Nerve "block" technique involves placing the anesthetic at a point along a nerve, beyond which its function is affected. The advantage of using a block is that several teeth end up getting numb, not just the one or two in the immediate area of the injection.
- The Mental nerve is found on the lower jaw.
- This method can be used to numb up lower premolars (bicuspids), eyetooth (canine, cuspid), and incisors (on the side of the jaw the injection is given).
iii Periodontal ligament injection.
Periodontal ligament (intraligamentary) injections are interesting shots, in the sense that they're used to numb up precisely one tooth at a time.
The patient tends to feel the pressure of the process being performed, but often no pain. So, whereas with the shots already discussed you may not realize much is going on, with this one it's obvious.
- In theory, this type of shot can be used to numb any tooth. Although it's generally easier to use this technique with single-rooted teeth toward the front of the mouth where access is easier and more straight forward.
- It's frequently used as an aid when other types of injections have not been totally effective.
iv) Maxillary incisor infiltration.
If you've ever had a shot given for routine dental work on an upper front tooth and it brought an unexpected tear to your eye, it was probably one of these.
The way a dentist makes this shot painless is by placing a small amount of anesthetic initially. And then, after it's taken effect, go back and deposit the rest of the needed dose.
- Upper central and lateral incisors are usually numbed up using this technique.
v) Inferior alveolar nerve block.
This injection, also referred to as a "mandibular block," is used to numb lower teeth. If you've had much work done, it's the shot that's given in the very back of your mouth.
Most people take notice of getting one of these. Our source for this section ranked it as the most painful of the common dental injections. (We discuss pain levels associated with this type of shot below.)
It's a type of nerve block (see above), and therefore has the advantage that it numbs several teeth simultaneously.
- Any tooth on the lower jaw (on the side the injection is given) can be numbed using this method.
vi) Palatal injections.
Our reference source for this section didn't include palatal (roof of mouth) injections because it only evaluated those used to numb up teeth. But Friedman (2001) ranks this type of shot as being more painful than any of those listed above.
- This type of injection isn't used to numb up teeth, just gum tissue. And fortunately not that many procedures require it (gum surgery and some tooth extractions likely would).
- In many cases a dentist can accomplish adequate tissue anesthesia in this region using other means. So, you don't have to be unduly worried about ever having to experience one of these shots.
Which type of shot will you get?
It's probably safe to assume that whenever possible your dentist will give you the least painful type of shot possible. After all, why would they want your procedure to hurt more than it has to?
Having said that, there are other considerations they must weigh. The box below explains.
Research: How badly do mandibular blocks hurt?
In our list above, it's the "inferior alveolar nerve block" (mandibular block) that's singled out as the most painful of the routine injections used to numb up teeth.
Van Wijk (2012) performed a study to determine exactly how much pain is felt when one of these injections is given. It evaluated 230 oral surgery patients.
- Roughly 20% of the patients expected their pain would be at a level of 7 to 10 (considered "substantial" pain by this study).
- After receiving their injection, only 3% of these same subjects reported actually feeling that much discomfort. (Expected pain intensity was higher than experienced pain.)
For all 230 subjects as a group:
- 8% experienced discomfort at a level of 7 to 10 ("substantial" pain).
- The mean pain intensity was between 2 and 3 ("mild" pain).
Within the study's ability to measure pain duration:
- On average, the pain lasted for 5.3 seconds (with a range of 1 to 25 seconds).
- 36% of subjects felt pain for less than 2 seconds.
- 15% felt pain for more than 10 seconds.
The paper's statement was: "A mandibular block injection can be considered to be a mildly painful experience lasting only a few seconds for the majority of patients."
(Keep in mind, this type of injection was found by our study above as being the most painful type of shot you're likely to get.)
a) Size doesn't matter.
There's a common misconception among patients that the larger the needle that's used, the more their shot will hurt.
Research doesn't bear this out. There's a long history of studies (Hamburg 1972, Fuller 1979, Brownbill 1987, Carr 2001, Flanagan 2007, Malamed 2010) that have evaluated the issue of the needle size used for an injection (30, 27 and 25 gauge, the common sizes used in dentistry) and the level of pain the dental patient feels. They have determined that the size of the needle makes no difference in what is felt.
Larger needles have advantages.
The size of the needle that your dentist uses is chosen for good reason, primarily dealing with safety (avoiding complications) and comfort. The box below explains.
b) Needle sharpness.
A factor that may matter in regard to pain felt is needle sharpness. Scanning electron microscope evaluation shows that the point of a needle tends to blunt after being used to give several shots. This it true even if bone tissue is never contacted. (Rout 2003)
A study by Meechan (2005) determined that using the same needle when giving a second injection in a second location resulted in a higher level of pain being felt. As a patient, during subsequent injections you may actually be able to notice that it's more difficult for the needle to penetrate your skin.
Of course this factor may not be an important issue if the follow-up injections are given in areas that are already numb.
c) Needle-free syringes / Jet injectors.
Needle-free dental syringes (like the Injex system) do exist. They express a narrow high-pressure jet of anesthetic solution that's able to penetrate through gum tissue "virtually painlessly." (Used for infiltration procedures only.)
Even though this type of syringe has been available for decades, it's not widely used by dental practitioners. Research seems to suggest that these injectors are neither quite as effective or substantially less painful than traditional methods (Davoudi 2016).
Controlling injection fear.
There is no question that a patient's mental state can affect their dental experience, including how much pain they feel.
There are ways for your dentist to help control your dental anxiety, if they know that they're needed. So, be up front with your dentist, as much in advance of your procedure as possible, so plans can be made.
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