What percentage of patients is the mesiobuccal root of the first maxillary molar not innervated by the PSA, and which nerve innervates it in these cases?

• A. 28%; MSA
• B. 50%; ASA
• C. 28%; PSA
• D. 15%; Infraorbital nerve

Answer: (A)

The important idea here is that dental innervation in the upper jaw is not identical in everyone. The posterior superior alveolar (PSA) nerve typically supplies the molars behind the first molar, but in about 28% of people, the mesiobuccal root of the first maxillary molar is actually innervated by the middle superior alveolar (MSA) nerve instead of the PSA. Because of this, a PSA block may not fully anesthetize that MB root in those patients. In those cases, the MSA (or the infraorbital nerve, which gives off the MSA) would need to be targeted to achieve complete anesthesia. This variation is why the MB root sometimes responds differently to the standard PSA block and why supplemental infiltration or a different nerve block is considered.