What is the depth of penetration of the needle when performing a PSA nerve block?

• A. 8 mm
• B. 12 mm
• C. 16 mm
• D. 20 mm

Answer: (C)

The depth of penetration for this block is about 16 millimeters from the mucobuccal fold. This distance brings the needle tip to the area where the posterior superior alveolar nerve travels near the PSA foramina on the infratemporal surface of the maxilla, close to the apex of the second molar. With the needle inserted in the buccal vestibule above the second molar and aimed posteriorly and slightly medially, you typically reach bone around this depth, indicating you’re near the nerve before it enters the maxilla. Stopping there and depositing the anesthetic allows diffusion to the nerve. Going deeper than about 20 mm increases risk of complications or hitting unintended structures.