Which root of the maxillary first molar may not be innervated by the PSA nerve?

• A. MB Root of the Maxillary First Molar
• B. MB Root of the Maxillary Second Molar
• C. Palatal Root of the First Molar
• D. Distobuccal Root of the First Molar

Answer: (A)

Variation in nerve supply to the maxillary molars means not every root follows PSA innervation. The mesiobuccal root of the maxillary first molar can receive its sensory fibers from the middle superior alveolar nerve, which comes off the infraorbital nerve and is variable in presence. When the middle superior alveolar nerve is present and supplies that root, the posterior superior alveolar nerve does not provide its innervation, so a PSA block alone may not fully anesthetize that root. Clinically, this means you can encounter incomplete anesthesia of the mesiobuccal root if you rely solely on a PSA block. The other roots are more consistently supplied by PSA or by other nerves (for example, the palatal root tends to have its own course via the greater palatine nerve), so they don’t carry the same variability.