The mental nerve is a branch of the inferior alveolar nerve from the third division of the trigeminal nerve. The inferior alveolar nerve enters the mandibular foramen to travel within the mandibular canal and form the mental nerve. At its exit from the mental foramen, the mental nerve is divided into several branches.¹ The mental nerve emerges from the mental foramen to supply the chin, lower lip, and teeth. The mental nerve can provide innervation to the lower incisors.²

The mental foramen lies about halfway between the upper and lower borders of the mandible, although the position of the foramen relative to the mandible varies with age.^3,4 Elderly patients who are edentulous have decay of the alveolar ridge. This brings the mental foramen closer to the upper border of the mandible. A severely resorbed alveolar ridge can make identification of the mental foramen difficult.

Suggested Technique

Ultrasound imaging can establish foramen location and morphology for optimal approach. With proper needle positioning within the canal, more extensive block can result.

The mental foramen typically has posterior or right-angled inclination.⁵ When the nerve emerges at right angles, it is funnel shaped. The longest diameter of the mental foramen is about 3 mm.⁶ The presence of multiple mental foramina is rare, being observed in about 2% of cases.^3–57

For this block, the operator stands at the head of the bed and faces the feet of the patient. The block needle is advanced well into the mental canal (≈6 mm) for successful block and to achieve anesthesia of the lower incisors. For this, 1- to 2-mL injection of local anesthetic is required. The block needle passes through the platysma and depressor anguli oris muscles on the approach to the canal. Care is taken not to puncture the facial artery, which usually lies on the posterior side of the foramen.

Key Points

Mental Nerve Block	The Essentials
Anatomy	The MTN exits the mental foramen, with nearby facial artery.
	The MF is about 3 mm in diameter.
Positioning	Supine
Operator	Standing at the side of the patient
Display	Across the table
Transducer	High-frequency linear, 23- to 38-mm footprint
Initial depth setting	20 mm
Needle	25 gauge, 38 mm in length
Anatomic location	Halfway between the upper and lower borders of mandible near the corner of the mouth
Approach	Transverse view of MF, in-plane from lateral to medial
	Place the needle tip into the MF.
Sonographic assessment	Injection distribution observed within the MF
Anatomic variation	Multiple foramina (2%)

MF, Mental foramen; MTN, mental nerve.

Clinical Pearls

• Advance the block needle 6 mm into the mental canal for successful block.

• Local anesthetic injected within the mental foramen tracks centrally along the inferior alveolar nerve within the mandibular canal.

• Use a 25-gauge, 38-mm needle for placement in the mental canal.

• Facial hair can make imaging for mental canal block challenging.

• The facial artery and vein lie close to the mental foramen.

FIGURE 57-1 External photograph showing an approach to mental nerve block. An in-plane approach from the posterior aspect of the mandible is shown (A). The corresponding sonogram of the mental foramen is shown (B). Power Doppler verifies the presence of a blood vessel lying over the foramen (C).

FIGURE 57-2 Ultrasound image of the mental foramen (A) contrasted with the smooth bony contour of the mandible (B).

FIGURE 57-3 Image sequence showing mental nerve block in the mental foramen. The mental foramen is first identified and the angle of approach planned (A). The facial artery is identified posterior to the mental foramen (B). The block needle is placed within the mental foramen for injection of local anesthetic (C).