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Updated: Feb 26, 2021

Oh, this wonderful injection, why does this easy one have to be so hard? Do you ever feel like you are Austin Powers trying to move a vehicle when you are getting to your site of deposition?

Friends, here is your public service announcement (PSA) about the posterior superior alveolar injection (PSA)....yeah I totally cheesed up the play on acronyms.

This injection is ALL about correct angulation and palpating from the very beginning.

For review the PSA anesthetizes the molars of the maxillary teeth along with the associated facial structures. Of course, just like most things there is an exception to the rule. Sometimes the MB root of the first molar receives innervation from the MSA nerve instead. So, make sure you account for this when you are working with the first molar.


Before placing your topical make sure you palpate the zygomatic process of the maxilla. This broad boney extension can create a roadblock to your success.

While in general the posterior portion of this structure ends near the maxillary first molar, there are times when it can extend a little further back, which will block your needle pathway. If the path is clear, your point of insertion will be over the distal buccal root of the maxillary second molar. Remember you may have to go a little further distal if the zygomatic process is in your way or if there are third molars present.

Now, let’s talk about the angles of this injection. If you were ever wondering why you had to learn geometry…well, this injection truly is geometry in motion. I know...go ahead and get that UGH out.

I can write about the many 45 degree angles you are supposed to be looking for, but I want to start you with the EASY imaginary line which will help you get all the correct angles. If you draw an imaginary line from the inner canthus of the eye down towards the top of the shoulder you are set up for nearly perfect angles. This line is also parallel to the nasolabial groove…you know the smile line that goes from the nose to the lip.

This line will get your barrel to line up about 45 degrees away from the midline and 45 degrees away from the maxillary occlusal plane. To help you facilitate this you really need to have great retraction. The reason why retraction is important is due to the barrel size of the syringe and the restrictions the mouth can have (if only our patients really could remove their cheeks!) There are several options for retraction available, but that’s a story for a different day. For now, just remember the key is to have the buccal tissue as far away from the teeth as possible so you can really open up the vestibule to see your insertion site, AND…you may have to do what I call a “retraction dance”.

I’ll do my best to put this into words…

When you begin the injection, the corner of the mouth can be in the way a little bit. So, when you start to insert into the tissue, the retraction hand lowers towards the chin until the barrel is clear of the obstacle of the lower lip (about 2 mm). Then, as you are inserting the needle, the retraction hand moves back up toward the eye to open the vestibule back up and create space at the corner of the mouth for the barrel to have room.

Man, that is a lot of information. Take a moment to reread that again! Basically, your retraction hand starts in an upward fashion so you can see the insertion site. Then, moves towards the chin to clear the lip from the barrel and then moves back up to the beginning location so there is room to have the 45-degree angle away from the midline.

I highly recommend using a short needle because you only need to insert the needle about 14-16 mm. If you use a long needle that means you are only inserting half the length of the needle. It is important to remember to not over insert on this injection since the pterygoid plexus of veins is near the deposition site. This is another reason why aspiration is highly important (3% positive aspiration rate) for this injection. Because of the complex angles and the vascularity associated with this injection I prefer to aspirate a couple of times prior to and during this injection. Some may think that is "over kill", but due to variations of patients and the tendency to drift out of proper alignment I like the reassurance that I am not in that pterygoid plexus.

Depending on the anesthetic choice and the procedure conducted give anywhere from 1/2 to a full cartridge of anesthetic.


Have you ever had the patient’s mandibular teeth get numb too? There could be a couple of reasons. Like they have different anatomy, but more than likely, the barrel of the syringe was to flat. Meaning it was nearly parallel with the maxillary occlusal plan than 45 degrees away from it. When the barrel is more in alignment with the occlusal plan or the mucogingival margin, the solution will start to flow towards the condyle of the mandible which is near the pathway of the inferior alveolar nerve. To correct that issue, lower your syringe hand towards the patient’s shoulder and keep the barrel towards the corner of the mouth.

If you find that you contact a boney obstacle too early (about 5 mm or less), I encourage you to completely withdraw and check your anatomy again. This will reduce the impact of tissue trauma. I have seen too many times when someone has maneuvered the syringe so many times it looks like a sewing machine. This can cause quite a bit of tissue trauma, therefore increasing the chance for a hematoma and post injection pain.

Should a hematoma occur apply finger pressure to the injection site to help stop the bleeding. Placing an ice pack to the area will help reduce the swelling and bleeding as well.


Don't forget to palpate for the zygomatic arch and align your syringe with the nasolabial groove. Also, check your insertion depth and aspirate.

Friends, enjoy your geometry in motion and I know when you implement these strategies the PSA will be one of your best shots!



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