Local anesthesia of the face, mouth, and teeth is required for a variety of procedures. These include dental procedures and laceration repairs, particularly of the lower lip and buccal mucosa, as well as the external lower lip, cheek, and chin. Facial nerve blocks are employed by dentists, dermatologists, plastic surgeons, otolaryngologists, oral surgeons, facial plastic surgeons, and emergency room providers as a means of analgesia that is more comfortable than local infiltration, and that will not cause local tissue distortion. Local blocks can also negate the need for procedural sedation, which is particularly important in patients with multiple medical comorbidities. The mental and incisive nerves supply sensation to the lower teeth and skin of the chin and lower lip as well as the buccal mucosa, making this nerve block useful for multiple procedures.
The third branch of the trigeminal nerve (the fifth cranial nerve), often abbreviated V3, splits into 2 branches, anterior and posterior. The posterior trunk has 3 branches, one of which is the inferior alveolar nerve, which has sensory-only branches that traverse the mandibular canal to exit via the mental foramen. Once the inferior alveolar nerve exits the mental foramen of the mandible, it is known as the mental nerve, which supplies sensation to the skin and buccal mucosa of the lip and the skin of the chin. The incisive nerve continues in the mandibular medullary cavity to the premolars, canines, and incisors.
The mental nerve exits through the mental foramen bilaterally in the mandible. The mental foramen is typically located halfway between the upper (alveolar crest) and the lower edge of the mandible in direct line with the second bicuspid (premolar). Exact mental canal location, however, can be variable - particularly in patients with other mandibular or craniofacial anomalies. In edentulous individuals, there is alveolar bone resorption, which makes the location of the mental foramen closer to the alveolar crest in a majority of patients. The location of the foramen has been found on a horizontal plane mesial, distal, or between the apices of the premolar roots. On a vertical plane, the foramen has been found above, at, or below the apex of the premolars. The variations are influenced by gender, age, race, and technique used for assessment (i.e., palpation, Panorex, CT scan). Given this variability, testing for anesthesia prior to the initiation of any procedure is important. Full anesthesia, however, has been demonstrated in multiple studies with localization of the foramen with palpation and landmarks alone.
Given the reliability and easy localization of the mental foramen, anesthesia of the mental nerve is particularly useful for dermatologic procedures. In the emergency setting, laceration repair of skin, buccal mucosa of the lower lip, or chin can often be performed without local injection, which can distort anatomy. In most studies, all patients report lip numbness following a mental nerve block. The technique can also be applied to dermatologic surgeries such as Mohs micrographic surgery of these same areas and has been used as an alternative to procedural sedation, which can be riskier in elderly patient or patients which multiple comorbidities. Lacerations or other procedures near the mandibular midline may require a bilateral mental nerve block.
Dentists use the mental/incisive nerve block either alone or in addition to an inferior alveolar nerve block. Studies have shown that 47% of patients develop pulp anesthesia in the first molars, 82% to 100% develop pulp anesthesia to the bicuspids (premolars), and 38% of patients develop pulp anesthesia in the lateral incisors. There is a greater chance of success of incisive nerve block if the injection is given inside the foramen for the second bicuspid and the canine. There was no difference in the success of injection inside versus outside of the mental foramen in anesthesia of the first bicuspid. Injection external to the foramen is typically successful for skin and mucosal procedures.
Contraindications to mental/incisive nerve block include:
This procedure can be done as a sole operator or with an assistant to retract the lower lip.
The patient should ideally be seated or laying with their head placed back against a firm surface to stabilize the head and prevent movement during the procedure.
There are two described techniques for performing this nerve block: intraoral and extraoral (percutaneous). In one study, the intraoral approach with the pre-application of topical anesthetic was shown to be less painful than the percutaneous approach. The decision can also be made based on the location of the procedure - intraoral procedures are more successfully anesthetized via an intraoral approach, while skin anesthesia can be achieved with either approach with equal efficacy.
*Slow injection of anesthetic has been proven more comfortable to patients than brisk injection, and anesthesia success is the same for rapid versus slow injection.
Needle entry into the mental foramen is not necessary for successful anesthesia of the mental nerve and may increase the risk of permanent nerve damage or accidental intravascular injection. Dental providers have historically entered the foramen with improved anesthesia of the incisive nerve and anterior maxillary teeth. However, for anesthesia of the mental nerve, extra-foraminal injection is safest and still provides complete anesthesia to the skin and soft tissues anterior to the nerve.
Anesthesia via nerve block of the mental nerve just as it exits the mental foramen provides excellent anesthesia to the skin and soft tissues of the ipsilateral chin and the lower lip. This block is relatively easy to perform with landmarks, is safe, and is easy to teach operators. It can negate the need for procedural sedation or the risk of tissue distortion with local anesthetic in an outpatient emergency, dermatology, or surgical setting.
Nerve block of the mental nerve provides excellent anesthesia to the skin and soft tissues of the ipsilateral chin and the lower lip; as a consequence, all members of the healthcare team should be familiar with the procedure and assisting in performing this block. [Level 5]
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