Mental nerve block

Basic. The foramen is easy to find by feel or ultrasound, and the injection is shallow.

The mental nerve is the end branch of the inferior alveolar nerve, from the mandibular division of the trigeminal nerve (V3).

It leaves the mandible through the mental foramen, which in adults lies below the second premolar (or between the two premolars), about halfway between the gum line and the lower border of the jaw, and roughly in line with the pupil.

The foramen opens upwards and backwards. The nerve divides into branches to the skin of the chin and to the skin and lining of the lower lip.

In children the foramen is lower on the jaw, and in people without teeth it moves up towards the top of the bone.

• Second lower premolar tooth.
• Midway between the gum line and the lower border of the mandible.
• In line with the pupil and the supraorbital and infraorbital foramina.

• Lower lip and chin surgery: lacerations, skin lesions, lip biopsy.
• Chin augmentation and other cosmetic procedures.
• Lower front teeth and gum procedures (incisive branch, intraoral approach).

• Mental nerve (V3)
• Often the incisive nerve (lower canine and incisors)

Surface anatomy, relevant structures and landmarks

Feel the foramen below the second premolar, halfway down the jaw.

Positioning

Supine or sitting.

Block technique

• Intraoral: pull the lower lip down, insert the needle into the fold between lip and gum next to the second premolar and advance 5–10 mm towards the foramen. Aspirate and inject 1–2 mL.
• Extraoral: insert the needle through the skin over the foramen, aiming slightly down and forwards to meet the opening. Do not enter the canal.
• Press on the site for a minute.

Needle choice

25–27 G, 25 mm.

Neurostimulation technique

Not used.

Choice of local anaesthetic (including volume)

• 1–2 mL of 1–2% lidocaine with adrenaline, or 0.25–0.5% bupivacaine or levobupivacaine for longer effect.

Show scan

DAO Depressor anguli oris muscleDLI Depressor labii inferioris muscleMandible MandibleMF Mental foramen

Positioning

Supine with the head turned slightly away.

Scanning technique

• Place a small linear probe along the lower jaw, below the premolar teeth.
• Slide along the bone until the bright line breaks: the mental foramen.
• Colour Doppler shows the mental artery.

Structures to identify

• Mandible (bright line)
• Mental foramen
• Mental nerve and artery
• Depressor anguli oris and depressor labii inferioris

Position / ergonomics

Stand on the side of the block or at the head of the bed.

Probe

Small linear high-frequency (hockey-stick) probe.

Settings

Superficial preset, depth 1.5–2 cm.

Depth

0.3–1 cm.

Needle size

25–27 G, 25 mm.

Needling technique

Out of plane, aiming at the foramen.

Optimal needle-tip position

At the opening of the foramen, outside the canal.

Drug choice

See above.

Volume

1–2 mL.

Optimal current if nerve stimulation is used

Not used.

Muscle responses acceptable

Not applicable.

Testing block success

• Numb lower lip and chin on that side

Not used.

• The intraoral route is less painful and leaves no skin mark.
• For midline lip surgery, block both sides: fibres cross the midline.
• In people without teeth the foramen moves up towards the top of the jaw.

• Allam and colleagues (2018) reviewed the anatomy of the mental foramen and the ultrasound-guided technique, as part of a review of trigeminal branch blocks.
• Evidence for the mental block comes mainly from dental and dermatological practice; there are few trials in anaesthesia.

• Allam AE, Khalil AAF, Eltawab BA, Wu WT, Chang KV. Ultrasound-guided intervention for treatment of trigeminal neuralgia: an updated review of anatomy and techniques. Pain Res Manag 2018;2018:5480728. PubMed