The following is a review of available research on effective pain reducing tactics during local anesthesia injections.
Some of these are commonly used while others are not. Incorporating many or most of these ideas within our technique will take us one step closer to "zero pain" local anesthesia injections.
First it's important to understand what causes pain during the injection:Needle
- Needle penetration: Initial contact of the tip with the mucosa.
- Needle movement: Movement of the tip to the target site after entering the mucosa.
- Needle contact: Penetration/contact of the needle with muscle tissue or nerve fibers.
- Solution volume: The pressure caused by the tissue volume.
- Solution pH: Irritation/sting caused by the low pH of the solution.
- Solution temperature: Room temperature is much lower than body temperature.
Review of the above list makes it obvious that a pain free injection would require that the technique used compensates for each of the above causes of pain and discomfort. The following is a list of pain reducing measures to take in order to ensure patient comfort:
- Distraction: Using calm comforting words and distracting patients' attention away from the injection needle is of course effective. Tapping or blowing air on a patient's hand, offering them dark glasses, and headphones with music are commonly advocated.
- Needle gauge: Research shows that using 25 gauge for blocks and 27 for infiltration is optimum; going any narrower does not reduce pain perception, in fact it is likely to cause more needle deflection and therefore adversely affect success.
- Topical anesthetic: This will help mitigate the pain of needle penetration through the mucosa. Opinion is divided about its effectiveness. Maxillary anterior sextant is most helped with the topical anesthetic; 20% benzocaine is commonly used. Anecdotal reports say EMLA and other pharmacologic formulations are even more effective.
- Vibration: Pinching, wriggling the lip or using specifically designed products   to cause sufficient stimulation of the tissue being injected helps. This stimulates the larger A-fibers to inhibit the stimuli from smaller C-fibers conveying pain.
- Two-step anesthesia: A technique of injecting a few drops of anesthetic, waiting for few minutes and then returning to inject and complete the injection has been advocated. This method reduces the pain of needle penetration and movement because it is done in two separate steps. Though not practical for routine cases, it might be useful in some selective cases.
- Slow speed: Injecting at a speed of such that injecting one carpule would take about one minute; this puts the least trauma to the tissue and is shown to reduce pain.
- Buffering anesthetic: A Cochrane Systematic review  showed that buffering the anesthetic reduces pain. Proponents of this technique claim that this also speeds the onset of action, which would be an additional benefit.
- Warming the capsule: Systematic review studies  have shown that warming the anesthetic cartridge from a room temp of about 21°C to body temperature of about 37°C reduces the pain perception
- Technique: This is especially critical for inferior alveolar blocks. The classic technique involves insertion of a needle into the pterygomandibular space by piercing the buccinator muscle. If the needle indeed traverses the ideal path it just pierces the mucosa and enters loose areolar tissue. Increased pain and discomfort is caused when the needle strays from the path to penetrate adjacent anatomical structures like the medial pterygoid muscle.
To keep the needle on an ideal path, it is necessary that the site and level of needle penetration and its angulation and depth is appropriate to the underlying anatomy of that patient. Anatomical variations like wide or flaring mandible, long ramus, endentulous mandible, can be best accommodated by using both intra-oral and extra-oral landmarks .
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Vivek Mehta DMD, FAGD, Visiting Faculty, Spear Education. Follow him on Twitter @Mehta_DMD.