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Updated: Apr 7, 2022


One of the keys to a successful injection is needle selection. Putting the correct needle on your syringe will help you hit your target just right.

There are two measurements of the needle to consider, length and gauge.

To hit the right spot, you need to have the length to get there. The length should be appropriate for the depth of injection. The long needle is about 32 mm in length which is perfect for most of the mandibular injections, but not the best option for an injection in which minimal penetration is needed, like a PDL. An extra-short, about 10 mm long, would be ideal for this one.

Don’t forget the important safety rule of not “hubbing the needle”. The hub (where the needle and adapter meet) is the weakest point of the needle. While it is rare, there have been enough cases in which a needle has broken during the injection. When a needle breaks while it is in the tissue, the mucosa can have a rebound effect, and the tissue slides up the needle a few millimeters. If the tissue is at the hub or close to it, the tissue will “swallow” the needle. Then all of a sudden you are dealing with a plethora of other issues…NO THANK YOU! So, for those deeper injections like the mandibular blocks or the IO, grab a long needle.

Let’s be safe!

The other aspect we often forget about and is just as important to an accurate injection, is the gauge. The needle gauge is based on lumen size (diameter of the hole). The different gauges commonly used in dentistry are 25, 27, and 30.

One would think that the lower the number, the smaller the lumen of the needle. However, it is the exact opposite. A 30-gauge needle has a smaller opening while a 25-gauge is larger. Check out the diagram to see the size difference.

While the length of the needle helps us reach the target point, it is the gauge of the needle that will help us get there accurately. The larger the gauge of the needle the less flexible it is. The needle is less likely to bend, which increases the success rate (you get straight to the point). This means that when giving an injection that has a significant depth to the targeted deposition, a 25-gauge needle will not bend or flex as much.

For example, when giving the IA injection, you may need to readjust the syringe during insertion to avoid premature contact with the bone. When you readjust, the needle can bow a little. I know you have probably seen the needle do this outside of the tissue, but it even happens when the needle is deep into the mucosa, and even if you don’t need to readjust. With a smaller gauged needle, like the 30, the needle is more likely to bend, like a rainbow. This means the opening will not be at the targeted deposition site. Alas, one reason why a 30-gauge long needle is a rarity in the dental office.

Also, the larger lumen provides the opportunity for a more accurate aspiration. Due to the larger diameter, it is easier for the fluid to flow into the needle. As the diameter of the lumen decreases fluid resistance increases. This makes the act of aspiration harder (it can even cause the harpoon of the syringe to become dislodged from the rubber stopper, WHOOPS!). Some may think that the use of a 25-gauge needle causes a higher rate of positive aspiration. When what is being observed is a more accurate indication of vascular penetration.


Often clinicians will select a needle size to help reduce the pain of the initial injection. The pain perception for needle size is reported to have no difference between a 25, 27, or a 30-gauge. According to Malamed, a study was conducted assessing the patient’s awareness of needle size based on pain perception. The participants were injected over the maxillary anterior teeth (without any topical) and could not tell any difference in the needle size. Now that is an interesting research study…I wonder how they were able to get the volunteers for that one.

As a side note, to reduce the sensation of the needle insertion the use of topical anesthetic and pulling the tissue so it is tight will allow for an easier time for you and your patient.

So next time you are setting your syringe up make sure to consider the length and gauge of your needle. Making the correct selection will help you hit your best shot.



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Malamed: Handbook of Local Anesthesia 6th ed

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