What is the depth of penetration for a MSA nerve block?

• A. Just above apices of the premolars until you reach bone
• B. Into the pterygopalatine fossa
• C. Into the maxillary sinus
• D. At the apex of canine

Answer: (A)

The key idea is that the MSA nerve block is a shallow, targeted injection. You place the needle in the mucobuccal fold above the second premolar and advance until the tip lightly contacts bone near the apices of the premolars. This positions the needle close to where the middle superior alveolar nerve travels as a branch of the infraorbital nerve, allowing the anesthetic to bathe the nerve before it enters the alveolar bone. Going deeper would risk deposition in spaces like the pterygopalatine fossa or the maxillary sinus, or miss the premolar innervation entirely if you stop too high. So the correct depth is just enough to reach bone near the premolar apices.