Mental Nerve Block

One method of providing anesthesia of the lower lip and skin innervating the chin is the mental nerve block. This simple nerve block can provide the analgesia needed for laceration repairs, surgical interventions including tumor resections, and even relief from postherpetic neuralgia. The mental nerve is a branch of the inferior alveolar nerve that exits via the mental foramen and divides into 3 branches below the depressor anguli oris muscle. One branch supplies the skin of the chin and the other 2 innervate the skin and mucous membrane of the lower lip. Note that this nerve block does not provide anesthesia to the teeth or mandibular soft tissue. The pain associated with the mental nerve block is significantly less than with other intraoral nerve blocks. ^{[1, 2, 3, 4]}

Mental nerve block is indicated for the following:

• Lacerations of the lower lip, especially if the vermillion border is involved

• Lacerations to the soft tissue of the chin that extends from the lip anteriorly to the alveolar process and caudally to the mid-body of the mandible

• Surgical removal of facial tumors/lesions

• Relief from postherpetic neuralgia

Mental nerve block is contradicted for the following:

• Noncooperative patient

• Overlying infection

• Allergic reaction to local anesthetic 

• Patient refusal

• Distorted anatomy

Some local anesthetic agents that are typically used for dental procedures include:

• Lidocaine 1–2% (Xylocaine); rapid onset with duration of action lasting 30–60 min (longer duration if epinephrine added)

• Mepivacaine 3% (Carbocaine); moderate onset with duration of action lasting 45–90 min

• Bupivacaine 0.25–0.5% (Marcaine, Sensorcaine); slow onset with duration of action 2–4 h ^{[5, 6]}

Topical anesthetic agents are often useful prior to local anesthetic administration but are not required. These include:

• Viscous lidocaine (Xylocaine)

• Tetracaine (Pontocaine)

Equipment for mental nerve block includes the following:

• Local anesthetic agent of choice

• 25–27 gauge needle

• 5 mL syringe

• Topical anesthetic (optional)

• Gauze pads

• Adequate light source

• Chloraprep

• Ultrasound with high-frequency probe if performing an ultrasound-guided nerve block

• Ultrasound gel

Place the patient in a semi-recumbent position, or in a dental/ENT chair, if available.

Apply topical anesthesia if desired.

Draw 2–5 mL of local anesthetic into the syringe. Agents may be combined (eg, lidocaine and bupivacaine) to achieve a longer duration of anesthesia

While the mental nerve block can be performed via an extraoral or intraoral approach, the intraoral approach to the mental nerve block with adjunctive topical anesthesia has been reported as being less painful. ^[7]

To perform the intraoral approach: 

• Retract the cheek laterally

• The mental foramen is palpable between the 2 lower premolar teeth in adults and between the first and second primary molars in children (see image below)

• The position of the mental foramen may differ among ethnic groups ^[8]

• Insert the needle along the lower gum line into the buccal fold between the premolar teeth (in adults) near the foramen (see image below). Do not place the needle directly into the foramen

• Mental nerve block injection site.

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• Aspirate

• Inject 2–3 mL of anesthetic

  • Do not inject directly into the foramen

  • Slow injection is significantly more comfortable than rapid injection (P< 0.001) ^[9]

  • Stop injecting and withdraw slightly if a paresthesia is experienced

The mental nerve block provides anesthesia only to the lower lip and soft tissue of the chin. It does not anesthetize the teeth, which would require an inferior alveolar nerve block. 

If the foramen is not directly palpable, the anesthetic can be injected into the buccal mucosa between the 2 lower premolar teeth. Never inject into the foramen, as this can permanently damage the neurovascular bundle.

Ultrasound can help identify the mental foramen, especially in the novice operator.

A dental aspirating syringe with a dental carpule can be used in place of a regular syringe. Slide the carpule into the syringe and place the needle on the end.

Complications of mental nerve block can include the following:

• Bleeding

• Infection

• Needle fracture

• Intravascular injection

• Neurapraxia (secondary to direct injection into the mental foramen)

Ultrasound-guided technique

Place a linear ultrasound probe in the transverse orientation over the inferior portion of the mandible at the level of the second premolar. Bone will appear as a hyperechoic linear structure on the ultrasound image.

Scan cephalad until a hypoechoic cleft appears; this is the mental foramen (ultrasound image). Mental foramen typically appears below the second premolars.

Aspirate and inject as outlined above.  

Ultrasound-guided blocks have contributed to shortening performance time and reducing number of passes. ^[10]

The image below shows an ultrasound-guided mental nerve block. Part A shows how the transducer should be positioned to obtain an adequate image.

Part B highlights the mental foramen, which appears as a hypoechoic cleft. 

Ultrasound may be used dynamically to watch spread of anesthesic or to guide the area where needle should be guided.