A nerve block of the mental nerve anesthetizes the ipsilateral lower lip and skin of the chin, as well as the lateral (buccal) gingiva and mucosa anterior to the mental foramen up to the midline.
Indications for Mental Nerve Block
- Laceration of lower lip or chin, or intraoral mucosa
- Surgical excision of skin or lip lesions
A nerve block is used instead of local anesthetic infiltration when accurate approximation of wound edges is important (eg, skin or lip repair), because a nerve block does not distort the tissue as does local infiltration.
Contraindications to Mental Nerve Block
- Allergy to the anesthetic agent or delivery vehicle
- Absence of anatomic landmarks needed to guide needle insertion (eg, due to trauma)
- Infection in the path of needle insertion: Use procedural sedation or other anesthesia.
- Coagulopathy*: When feasible, correct prior to procedure.
- Pregnancy: Avoid first trimester treatment if possible.
*Therapeutic anticoagulation (eg, for pulmonary embolism) increases the risk of bleeding with nerve blocks, but this must be balanced against the increased risk of thrombosis (eg, stroke) if anticoagulation is reversed. Discuss any contemplated reversal with the clinician managing the patient's anticoagulation and then with the patient.
Complications of Mental Nerve Block
- Allergic reaction to the anesthetic
- Toxicity due to anesthetic overdose (eg, seizure, cardiac arrhythmias)
- Intravascular injection of anesthetic/epinephrine
- Spread of infection, by passing the needle through an infected area
- Failure to anesthetize
- Needle breakage (very rare)
Most complications result from inaccurate needle placement.
Equipment for Mental Nerve Block
- Dental chair, straight chair with head support, or stretcher
- Light source for intraoral illumination
- Nonsterile gloves
- Mask and safety glasses, or a face shield
- Gauze pads
- Cotton-tipped applicators
- Dental mirror or tongue blade
Equipment to do local anesthesia:
- Topical anesthetic ointment* (eg, lidocaine 5%, benzocaine 20%)
- Injectable local anesthetic such as lidocaine 2% with or without epinephrine† 1:100,000, or for longer duration anesthesia, bupivacaine 0.5% with or without epinephrine† 1:200,000
- Dental aspirating syringe (with narrow barrel and custom injectable anesthetic cartridges) or other narrow barrel syringe (eg, 3 mL) with locking hub
- 25- or 27-gauge needle: 3-cm long for nerve blocks
* CAUTION: All topical anesthetic preparations are absorbed from mucosal surfaces and toxicity may result when dose limits are exceeded. Ointments are easier to control than less-concentrated topical liquids and gels. Excess benzocaine rarely may cause methemoglobinemia.
† Maximum dose of local anesthetics: Lidocaine without epinephrine, 5 mg/kg; lidocaine with epinephrine, 7 mg/kg; bupivacaine, 1.5 mg/kg. NOTE: A 1% solution (of any substance) represents 10 mg/mL (1 gm/100 mL). Epinephrine causes vasoconstriction, which prolongs the anesthetic effect. Patients with cardiac disease should receive only limited amounts of epinephrine (maximum 3.5 mL of solution containing 1:100,000 epinephrine); alternatively, use local anesthetic without epinephrine.
Additional Considerations for Mental Nerve Block
- Document any preexisting nerve deficit before doing a nerve block.
- An intraoral or extraoral approach to the mental foramen may be used. The intraoral approach, much preferred and discussed here, causes less pain than the extraoral approach.
- Nerve block may fail if the anesthetic is not placed sufficiently close to the nerve.
- Use a new needle with each attempt (the previous needle may have become blocked with tissue or blood, which would obscure an errant intravascular placement).
- Stop the nerve block procedure and find a different method of anesthesia if you are unsure where the needle is or if the patient is uncooperative.
Relevant Anatomy for Mental Nerve Block
- The mental nerve is a terminus of the inferior alveolar nerve, which itself is a branch of the mandibular nerve.
- The mental nerve emerges from the buccal side of the mandible through the mental foramen (just below the apex of the second premolar tooth) to innervate the lower lip and chin, as well as the lateral (buccal) mucosa and gingiva anterior to the mental foramen ipsilaterally, with some extension across the midline. Repair of a midline chin or lower lip laceration will require use of bilateral mental nerve blocks.
Positioning for Mental Nerve Block
- Position the patient upright or slightly reclined, with the occiput supported, and with the neck in neutral position.
- Optimize access to the injection site (lower mucobuccal fold) by positioning the patient's head approximately level with your elbows, and such that the occlusal plane of the mandibular teeth is parallel to the floor when the mouth is opened.
Step-by-Step Description of Procedure
- Wear nonsterile gloves and a mask and safety glasses, or a face shield
- Using your index finger and thumb, grasp and laterally retract the lower lip.
- Palpating the gingiva, identify the mental foramen 1 cm anterior and inferior to the apex of the 2nd lower premolar tooth. The foramen usually lies just medial to the mid-pupillary line.
- Use gauze to thoroughly dry the mucobuccal fold adjacent to the lower 1st and 2nd premolar teeth. Use suction as needed to keep the area dry.
- Apply topical anesthetic with cotton-tipped applicators and wait 2 to 3 minutes for the anesthesia to occur.
Inject the local anesthetic
- Instruct the patient to slightly open the mouth and relax the jaw and cheek muscles.
- Again retract the lower lip laterally, to delineate the mucobuccal fold.
- Hold the anesthetic syringe such that the needle bevel faces the mandible.
- Insert the needle into the mucobuccal fold between the lower 1st and 2nd premolars.
- Advance the needle inferiorly, parallel to the teeth, about 0.5 to 1 cm. Do not contact bone.
- Aspirate, to rule out intravascular placement. If aspiration reveals an intravascular placement, withdraw the needle 2 to 3 mm, then re-aspirate prior to injection.
- Slowly inject about 1 to 2 mL of anesthetic adjacent to, but not into, the mental foramen.
- Massage the area externally for about 10 seconds, to hasten the onset of anesthesia.
Aftercare for Mental Nerve Block
- Have the patient rest, with mouth relaxed, while awaiting onset of anesthesia (5 minutes)
Warnings and Common Errors of Mental Nerve Block
- To minimize the risk of needle breakage, do not bend the needle prior to insertion, do not insert the needle to its full depth (ie, to the hub), and instruct the patient to remain still, with the mouth wide open, and resist grabbing your hand.
Tricks and Tips
- Distraction techniques (eg, talking to the patient or having the patient hold someone else's hand) may help to reduce patient anxiety.
- Inject the local anesthetic solution slowly (30 to 60 seconds) to reduce the pain of injection.
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