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From a broad perspective these three injections can seem similar. All three anesthetize maxillary teeth, their insertion sites are nearly identical or relatively close to each other and have a similar approach. Plus, depending on when and how you learned your techniques you may even use these injection names interchangeably.

Let’s look at each injection individually and then compare them.

The IO injection anesthetizes pulpal and facial tissues of the premolars thru the midline on the side injected. This means the Anterior Superior Alveolar (ASA) and Middle Superior Alveolar (MSA) nerves are impacted along with smaller branches of the IO nerve which innervate the skin below the eye, side of the nose, and upper lip. Because this injection impacts are variety of nerves it is considered a nerve block (nerve block -vs-infiltration).

The needle selection can be a long or short needle depending on the patient’s anatomy. Factors such a foramen location and vestibule height will impact this decision. In general, I like to select a long needle since there are occasions when the depth of penetration could be over 20 millimeters.

The insertion site is in alignment with the IO foramen, which is usually near the maxillary first premolar. And the needle pathway is parallel to the midline.

Finally, to achieve a higher success rate, the clinician should massage the anesthetic into the foramen after anesthetic delivery.

The MSA injection anesthetizes the maxillary premolars and the MB root of the maxillary fist molar. Because this injection primarily impacts the MSA nerve and the dental plexus it is considered a field block. While the upper lip region can become numb, it isn’t as extensive as other maxillary injections.

Using a short needle is standard practice for this injection because the insertion depth is only 3-6 millimeters. Also, due to the insertion site over the maxillary premolars this is a relatively simple injection to give. FYI-I like to insert between the fist and second premolars, while the literature suggests over the second.

Keeping the barrel of the syringe parallel to the midline, or with a slight tilt to the distal, is acceptable. After giving the MSA there is no need to do anything further to help with anesthesia success.

The ASA injection anesthetizes the canine through the central incisor of the side injected. Again, some upper lip anesthesia can happen, but it is minimal. This injection is also considered a field block due to the primary anesthesia impact being the dental plexus of the anterior teeth.

Using a short needle is appropriate as well, due to the shallow insertion of 3-6 millimeters. The main consideration to be made, is the site of insertion is mesial of the canine while the deposition site is on the distal. This means there needs to be angulation away from the midline. As with the MSA injection there isn’t anything further the clinician needs to do to enhance the numbing effect.


You may already see the similarities and differences between these three injections. The most obvious is the structures anesthetized. The IO impacts the ASA and the MSA in just one shot. This is great when you want to reduce how many injections you give your patient. Also, the IO is most successful with a little help. When the anesthetic is massaged into the foramen, it moves deeper into the infraorbital canal (this is where the ASA and MSA nerves begin to split away from the IO nerve) allowing the anesthetic to bathe these important nerves. The other injections don’t require this extra step.

Regarding the insertion site, the MSA and IO are nearly identical, while the ASA is a lot more anterior. If you look at the MSA and the IO as they are described here, the IO is practically the same as the MSA accept a lot deeper, which impacts the larger nerve trunk. Now, here’s the thing…the ASA injection can easily turn into an IO injection too. If the clinician understands where the IO foramen is located, then the more anterior approach is doable. But a lot more challenging (I don’t recommend it).

Let’s put a little more confusion into the mix. Some textbooks don’t use the term IO injection, instead it is called the ASA injection, but with the IO injection technique I described here. While other textbooks have a clear difference between the IO and the ASA injections. Yep, feel free to shake your head in frustration. Why, oh why, can’t we all agree on a name and technique!

So, depending on when and how you learned, you may have learned the ASA injection is performed in the manner of how I describe the IO injection.

The advantage of documenting the injections as MSA, ASA, or IO (as described here) accurately depicts which nerves are involved in the numbing process. Leaving no doubt as to where the patient is going to be numb.

Next time your considering which injections to perform for a procedure, I encourage you to think about which structures need to be anesthetized and choose appropriately…and hopefully this helps you make that decision.



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