MSA INJECTION (Middle Superior Alveolar)
Updated: Jul 19, 2022
This maxillary injection is most widely used when pain management of the maxillary premolars is required. While I prefer the Infraorbital (IO) or the Anterior Middle Superior Alveolar (AMSA) injection, the middle superior alveolar (MSA) injection is the obvious next choice. Due to its simplicity, ease of visibility, and stabilization this is usually one of the very first injections an oral health care provider learns. Even though it is simple there are a few items to keep in mind.
One important factor to remember is the MSA nerve may provide sensation to the mesiobuccal root of the maxillary first molar. This is important to consider when completing quadrant anesthesia. Due to anatomical variation, anywhere from 50%-72% of people don't have the MSA nerve. Yep, let me say it again, just a little differently. Only 28%-50% of your patients actually have this nerve. It is rather complicated to identify who does and does not have this nerve. The most successful way is to complete an anatomical dissection. I have yet to find a patient willing to endure that process, so, a good rule of thumb is to ere on the side of its presence. This will guarantee the dental plexus providing innervation to the premolars (and the MB root of the first molar) will be anesthetized.
As I said before, giving this injection is simple. Once your topical has taken effect, insert your short needle at the height of the mucobuccal fold. About 3 millimeters away from the alveolar process between the first and second premolar. Some literature recommends inserting above the second premolar. However, I've found the most success to be in between the premolars. If the patient only has one premolar insert your needle between the premolar and the
molar. Now, if you were to be tested on this information the correct answer for the site of insertion is above the second premolar 😊. But clinically sometimes things are a little different.
The needle penetration is about 3-6 mm. This all depends on the depth of the patient’s vestibule and the length of the roots (another great reason to have your x-rays available). After confirming a negative aspiration deposit the solution. Depending on your procedure and anesthetic type about half a cartridge should be sufficient.
KEYS TO SUCCESS
Here are some keys to a successful injection. Make sure you take time to palpate the mucobuccal fold above the premolars. I say this because of the large variations in size and shape of the zygomatic arch. The zygomatic process of the maxilla (I know big anatomical term) could potentially be a roadblock to the success of this injection. Textbook anatomy would say the zygomatic process should line up around the first molar or the second premolar. However, there are cases in which the process will come as far anterior as the first premolar,. And as broad to extend posteriorly to the molar. Which will block access to the final deposition site between the first and second premolars at the height of the root apex. If you encounter this situation your best bet is to do an ASA injection, with a little more angulation towards the distal.
Another tip for success is to be aware of the angle of the maxilla. Remember the maxillary bone has a tilt toward the back of the head. Making sure you are parallel with the maxillary process assures that you won’t over angulate the needle, which could result in contacting bone (ouch).
The last thing to review is retraction. Excellent retraction can make or break any injection. For the MSA, having proper retraction allows you to see the exact insertion point. Because the retraction hand is near the barrel of the syringe you can rest a knuckle or finger on it. Don’t forget, when retracting the lip away from the alveolar process you will see several small frenum in this area. You as the clinician, need to be aware of these attachments and be careful to avoid them. If you hit one it would cause major discomfort to your patient and limit the success of your injection. Your quick fix is to move the patients lip toward or away from the midline. This will cause the frenum position to change. Plus, when you retract, pull the lip/cheek out so the tissue is firm. This will allow the needle to insert easily. Its such an easy win for you!
Even the simplest injection like the MSA may cause you or your patient anxiety, but when you incorporate these steps into your routine it doesn’t have to be. Apply that topical, review your anatomy, retract the cheek, and breathe. You can do it. Now go and give YOUR BEST SHOT.
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