Nerve Blocks Explained — Landmarks, Targets & Why They Sometimes Fail

Inferior Alveolar Nerve Block (IANB)

The IANB is the most commonly used block in dentistry, numbing the lower teeth on one side of the mouth, along with the lower lip and chin. The needle targets the inferior alveolar nerve as it runs through the mandibular foramen, on the inner surface of the jawbone.

This is also the block most prone to missing — the mandibular foramen's height varies person to person, and an injection placed too high or too low can numb the wrong structures or fail to numb the target tooth at all. A classic sign of a missed IANB: the lip and chin go numb, but the patient still feels the drill on the tooth itself, because the lingual or buccal nerve branches weren't fully bathed in anesthetic.

Mental Nerve Block

The mental nerve is a terminal branch of the inferior alveolar nerve, exiting through the mental foramen on the front of the mandible, usually near the second premolar. A mental block numbs the lower lip, chin, and facial gum tissue in that area — but notably not the teeth themselves, since it's downstream of the main IANB.

This block is often used for soft tissue procedures on the lip or chin rather than tooth work.

Infraorbital Nerve Block

For upper anterior teeth and premolars, the infraorbital block targets the nerve as it exits the infraorbital foramen below the eye socket. It numbs the upper lip, side of the nose, and lower eyelid on that side, in addition to the teeth.

Because the injection site sits close to the eye, technique and landmark accuracy matter — patients are understandably anxious about a needle anywhere near their face, and clear chairside communication helps.

Lingual Nerve Block

Often anesthetized at the same time as the IANB since the lingual nerve runs nearby, this block numbs the tongue and floor of the mouth on one side. Patients frequently describe this sensation as the tongue "feeling huge" — a normal and expected effect, worth mentioning beforehand so it doesn't alarm them.

Why nerve blocks sometimes fail

A handful of factors explain most failed or partial blocks:

• Anatomical variation — the height and position of the mandibular foramen varies enough between patients that a "standard" injection depth doesn't land correctly every time
• Accessory innervation — some patients have extra nerve branches (like a bifid inferior alveolar nerve) supplying the same teeth, meaning one successful block still leaves a pathway unblocked
• Active infection or inflammation — acidic, inflamed tissue lowers local pH, which reduces how effectively the anesthetic agent works — this is part of why a tooth with an active abscess is often harder to numb
• Injection technique — incorrect needle angle, depth, or injecting too quickly can all reduce how well the anesthetic reaches the target nerve

What the assistant should watch for

During and after a block, a few signs help confirm it's working: the patient reports lip or tongue numbness in the expected distribution, and there's no flinch response when the dentist tests the area. If a patient reports feeling sharp pain once a procedure starts despite a block being given, flag it immediately rather than assuming it will "kick in" — a supplemental injection is often needed.