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Dental Anesthesia

Mesial Injection

12/05/2011

Question:

Why should we give two injection mesially in the upper first molar?

Answer:

Dear NetWellness Reader, 

Your question is not clearly written, but I'll try to answer what I think you might want to know. I presume you are a dental student or dentist because you know the term "mesial".  The type of injection in your question is not specified so I'll cover some possibilities. 

The periodontal ligament injection is typically given into the periodontal ligament between the tooth root and alveolar bone on both the mesial and distal aspects of a molar tooth in order to allow the spread of the anesthetic to surround the tooth. The intraosseous local anesthetic injection can also be placed into a small hole drilled into the buccal bone near the mesial aspect of the tooth. Both of these injections can be used as primary local anesthetic techniques but are also useful as supplemental injections whenever a simple infiltration injection technique or a nerve block might provide pretty good anesthesia but does not provide complete anesthesia. Thus one could infiltrate local anesthetic on the buccal aspect of the molar and if that did not provide complete pulpal anesthesia of the tooth, a second injection via one of the above supplemental techniques might provide profound pulpal anesthesia. 

However, I suspect that what you may be referring to in your question may have to do with the enervation of the upper first molar. You should know that typically the distal root and the palatal root contain nerve fibers from the posterior superior alveolar nerve (PSA). If you were to do a PSA nerve block by placing the needle through the greater palatine canal and injecting 2 to 4 ml of local anesthetic solution, both the distal and palatal nerve fibers to the tooth pulp should be anesthetized, but the fibers from the mesial root portion of the tooth should be unaffected because it receives enervation from the middle superior alveolar nerve. Thus an infiltration of the middle superior alveolar nerve (MSA) on the mesial aspect of the tooth is needed for complete pulpal anesthesia of the tooth. 

Finally, if the planned anesthetic technique did not involve a PSA nerve block but rather a simple buccal infiltration above the upper first molar, the local anesthetic would be expected to diffuse through the periosteum and the buccal bone and into the nerve fibers of both the MSA and PSA nerves that supply the pulp. One would expect that one injection on the buccal of the tooth would be sufficient to do this. However, the zygomatic buttress is in that area, and its extra thickness may prevent the anesthetic from diffusing forward to catch the MSA fibers if the injection is given more distally or prevent the anesthetic from diffusing distally if the anesthetic is injected on the mesial side of the "hump". Thus one my decide to inject on both the mesial and distal aspects to improve the chances of successful pulpal anesthesia. 

I presume you are interested in pulpal anesthesia for doing typical restorative procedures. If you are interested in extracting the upper first molar, not only do you need pulpal anesthesia, but also palatal soft tissue anesthesia because you will be tearing that palatal soft tissue in the tooth extraction process. So even if you were to extract a broken-down endodontically-treated tooth with no pulpal enervation, you still would need to infiltrate both the buccal and palatal soft tissue so avoid causing pain. 

For more information:

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Response by:

Joel M Weaver, II, DDS, PhD
Faculty Emeritus
Dentistry Administration
College of Dentistry
The Ohio State University