How far do you insert the needle on an IANB?

• A. 5-10 mm
• B. 10-15 mm
• C. 20-25 mm or 3/4 the length of the needle
• D. 30-35 mm

Answer: (C)

Depth for an IANB is about 20-25 mm, roughly three-quarters of a typical needle length. This depth places the needle tip in the pterygomandibular space near the mandibular foramen, where the inferior alveolar nerve enters the mandibular canal. Depositing the anesthetic at this location surrounds the nerve trunk, providing pulpal anesthesia to the lower teeth on that side and associated tissues.

If the needle is inserted shallower, the anesthetic may not reach the nerve, leading to inadequate anesthesia. If it’s advanced too deeply, there’s a greater risk of contacting or injuring neighboring structures. In practice, clinicians aim for this 20-25 mm depth and use bone contact as a landmark, withdrawing slightly before depositing after aspirating to confirm no intravascular placement.