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When used in dentistry, local anesthesia involves the injection of an anesthetic solution adjacent to the nerves that provide sensation to a region of the oral cavity where treatment will be delivered. The anesthetic solution temporarily prevents the propagation of nociceptive nerve impulses, thus allowing for the pain-free delivery of dental treatment. This activity reviews the indications, contraindications, anatomical considerations, equipment, techniques, and complications of local anesthesia in dentistry and highlights the dental team's role in the safe and efficient delivery of local anesthesia. Objectives: Identify the indications and contraindications of local anesthesia in dentistry. Review the anatomical course of the nerves supplying the oral cavity. Explain the different local anesthesia techniques utilized in dentistry. Outline the potential complications of using local anesthesia in dentistry. Access free multiple choice questions on this topic.

Pain is an unpleasant sensory and psychological experience resulting from actual or potential tissue damage and is commonly associated with dental treatment.[1] Local anesthesia is a safe and effective way of managing pain. Anesthetic agents work by reversibly binding to sodium channels, preventing the entry of sodium into the cells and thereby inhibiting the propagation of nerve impulses. Consequently, nociceptive impulses associated with painful stimuli do not reach the brain, and the patient does not perceive pain.[2][3] The maxillary and mandibular branches of the trigeminal nerve provide sensory innervation to the soft and hard tissues of the oral cavity.[4][5] The anesthetic solution must be injected adjacent to the nerve supplying the area where dental treatment will be performed. A thorough understanding of these trigeminal nerve branches and associated anatomical landmarks is essential.[2] Local anesthesia in dentistry can be given as either infiltration or block anesthesia. Generally, infiltration anesthesia is commonly used in the maxilla, whereas block anesthesia is frequently used in the mandible.[6] In addition, there are supplemental local anesthesia techniques that can be utilized when infiltration and block methods have failed to achieve profound anesthesia. Supplemental techniques include intraligamentary, intraosseous, intrapulpal, and interseptal anesthesia.[7] This paper will explore the anatomical course of the nerves supplying the oral cavity and the different techniques utilized to anesthetize these nerves prior to delivering dental treatment.  The equipment required for dental local anesthesia, including common anesthetic agents and their applications, will be considered. Furthermore, the prevention and management of complications associated with local anesthesia will be discussed.

Allergic Reactions Allergy to amide local anesthetics is very rare, and an incidence of 0.1 to 1% has been reported. Some patients may have an allergic reaction to the sodium metabisulfite preservative found in many local anesthetics. Allergy to ester anesthetics, such as topical benzocaine, is more common. Allergic reactions can range from aphthous ulceration to anaphylaxis. Clinicians must be able to identify and manage such reactions.[3] Failure to Achieve Anesthesia Causes for anesthetic failure include anatomical variations, inadequate technique, patient anxiety, and local infection or inflammation.[9] Anesthetic failure is more commonly associated with the conventional IANB method. The reported success rate of IANB is 80 to 85%. However, lower success rates ranging from 19 to 70% have been described for mandibular molars with irreversible pulpitis.[11] Accessory innervation is a common cause of anesthetic failure and should be suspected when an IANB provides anesthesia for the lower lip but not the dentition.[11] The mylohyoid nerve can provide accessory innervation to mandibular teeth, and an IANB generally fails to anesthetize the mylohyoid nerve as this nerve branches off superior to the point of injection. Furthermore, the sphenomandibular ligament provides a physical barrier to the diffusion of the anesthetic solution to the mylohyoid nerve, and the mylohyoid nerve sometimes travels within the bone. The Gow-Gates, Vazirani-Akinosi, or supplemental techniques can help address the mylohyoid accessory innervation. These techniques can also help overcome failure resulting from bifid mandibular canals, a variation that has been described in 0.35 to 1% of the population.[9] In addition, the buccal and great auricular nerves can provide accessory innervation to mandibular teeth via the retromolar foramen when present.[11] Injecting a small amount of anesthetic into the retromolar area can help anesthetize any accessory nerves entering the retromolar foramina. Furthermore, contralateral incisive nerve innervation is common, and when profound anesthesia in anterior teeth is not achieved, a contralateral incisive nerve block may be required.[5]

In addition, the buccal and great auricular nerves can provide accessory innervation to mandibular teeth via the retromolar foramen when present.[11] Injecting a small amount of anesthetic into the retromolar area can help anesthetize any accessory nerves entering the retromolar foramina. Furthermore, contralateral incisive nerve innervation is common, and when profound anesthesia in anterior teeth is not achieved, a contralateral incisive nerve block may be required.[5] There is also a link between skeletal pattern and IANB failure, with a greater risk of IANB failure in patients with retrognathic mandibles. One study found a failure rate of 14.5% for patients with retrognathic mandibles compared to 7.3 % for patients with class I skeletal patterns. This can be explained by the higher position of the mandibular foramen and a smaller mouth opening in retrognathic mandibles, both due to short condyles. Injecting the IANB at a higher point than usual, or using alternative techniques such as Gow-Gates and Vazirani-Akinosi methods, have been proposed to improve anesthetic success in retrognathic mandibles.[20] Furthermore, infection lowers the pH of the environment, and this acidity can affect the efficacy of local anesthesia. It is recommended that the anesthetic solution is injected as far as possible from the area of infection.[5] The pH of the anesthetic solution also plays a role. Local anesthesia is generally acidic, with the commonly used lidocaine with adrenaline having a pH of 3.5. Studies have shown that increasing the pH of local anesthetic, for example, by buffering with sodium bicarbonate, can shorten the speed of anesthesia onset and reduce patient discomfort during injection.[10] Hematoma Occasionally, blood can extravasate and accumulate to form a hematoma when the needle punctures a blood vessel. A hematoma can cause soreness, facial swelling, and trismus. If a hematoma is suspected, the clinician should apply pressure to help stop the bleeding and ensure hemostasis has been achieved before the patient’s departure.[3]

Occasionally, blood can extravasate and accumulate to form a hematoma when the needle punctures a blood vessel. A hematoma can cause soreness, facial swelling, and trismus. If a hematoma is suspected, the clinician should apply pressure to help stop the bleeding and ensure hemostasis has been achieved before the patient’s departure.[3] In addition, there are reported cases of patients where a maxillary artery puncture has led to a hematoma in the infratemporal fossa resulting in pressure in the IAN and lingual nerve, with sensory disturbances such as hyperalgesia, allodynia, and facial pain as a consequence [5]. Due to the higher needle insertion point, there is a greater risk of a maxillary artery or pterygoid plexus injury with the Gow-Gates and Vazirani-Akinosi techniques.[10] Intravascular Injection Intravascular injection of local anesthesia can lead to palpations, visual disturbances, headaches, and vertigo. The patient should be monitored and reassured that the symptoms should be transient and self-limiting. An intravascular injection can be prevented by aspirating before injection. The negative pressure of aspiration withdraws blood into the cartridge if the needle tip is inside a blood vessel, warning the operator not to inject the anesthetic solution at this location.[3] The conventional IANB technique has the highest risk of positive aspiration.[2] The inferior alveolar vessels run posterior to the IAN and can be exposed to trauma from the needle during IANB. In an IANB, the needle must be retracted 1 to 2 mm before aspiration as the inferior alveolar vein may be pinned to the bone, resulting in a false negative result of aspiration.[11] Methemoglobinemia Exceeding the maximum dose of ester anesthetics or prilocaine can lead to methemoglobinemia. Methemoglobinemia is when the iron in hemoglobin is changed from a ferrous to a ferric state, creating methemoglobin. The ferric hemoglobin has a high affinity for oxygen, preventing oxygen from dissociating and depriving tissues and organs of oxygen. This can lead to hypoxemia, with symptoms such as shortness of breath and cyanosis. It is a medical emergency, oxygen must be administered, and emergency services contacted. This condition is treated with intravenous methylene blue.[3] Needle Fracture

Exceeding the maximum dose of ester anesthetics or prilocaine can lead to methemoglobinemia. Methemoglobinemia is when the iron in hemoglobin is changed from a ferrous to a ferric state, creating methemoglobin. The ferric hemoglobin has a high affinity for oxygen, preventing oxygen from dissociating and depriving tissues and organs of oxygen. This can lead to hypoxemia, with symptoms such as shortness of breath and cyanosis. It is a medical emergency, oxygen must be administered, and emergency services contacted. This condition is treated with intravenous methylene blue.[3] Needle Fracture Needle fracture is a rare complication of local anesthesia with modern needles. The most common cause of breakage is sudden movement. This usually involves patient movement, but cases of dentist movement, change in needle direction, and sudden swallowing have been reported. Patients may develop symptoms such as pain, trismus, swelling, and dysphagia. A few patients may remain asymptomatic.[21] Needle breakage is most likely with mandibular nerve blocks and if a needle with a small diameter is used. Therefore, a 27 gauge or less needle should be used for mandibular blocks. A long needle must be used so that at least 5 mm of the needle is outside the mucosa to allow retrieval in the case of breakage. The mandible should be held firmly, and the patient should be advised against sudden movements. Furthermore, forceful or repeated pre-bending of the needle should be avoided, and the needle should be changed if repeated injections are required. If a needle breaks, the broken fragment should be removed as soon as possible. Broken needle fragments following a mandibular block most commonly sit within the pterygomandibular region. However, migration to the external auditory canal, superficial post-auricular region, and base of the skull has been reported.[21] Nerve Injury and Altered Sensation Nerve injury may result from direct injury by the needle, damage to intraneural blood supply leading to a hematoma compressing the nerve, or neurotoxicity of the anesthetic solution. Neurotoxicity is believed to be greater for articaine and prilocaine than lidocaine.[22]

If a needle breaks, the broken fragment should be removed as soon as possible. Broken needle fragments following a mandibular block most commonly sit within the pterygomandibular region. However, migration to the external auditory canal, superficial post-auricular region, and base of the skull has been reported.[21] Nerve Injury and Altered Sensation Nerve injury may result from direct injury by the needle, damage to intraneural blood supply leading to a hematoma compressing the nerve, or neurotoxicity of the anesthetic solution. Neurotoxicity is believed to be greater for articaine and prilocaine than lidocaine.[22] Nerve injury can lead to temporary or permanent loss of sensation. Following an IANB, the reported incidence of temporary IAN and lingual nerve impaired sensation ranges from 0.15 to 0.54%, while permanent sensory loss ranges from 0.0001 to 0.01%.[23] The nerve at most significant risk of injury during an IANB is the lingual nerve, as it sits close to the mucosa without the bony protection of the lingula or sulcus colli bone.[9][22] Nerve injury with maxillary infiltration is very rare.[22] When a patient presents with sensory loss following local anesthesia, careful examination of the distribution of sensory loss should be performed. The patient is followed up regularly until resolution, and referral to a specialist should be considered.[22] Ocular Complications Ocular symptoms have been described as complications following intravascular injection into the maxillary artery. Retrograde flow of the anesthetic solution to the lacrimal and optic arteries can lead to anesthesia of the oculomotor, trochlear, and abducens nerves. Symptoms may include diplopia, ptosis, ophthalmoplegia, amaurosis, mydriasis, transient vision loss, and periorbital skin blanching. These are generally self-resolving.[3][5] Psychogenic Reactions Patient anxiety regarding the administration of anesthesia or the dental treatment itself can lead to increased heart rate, changes in blood pressure, dyspnea, pallor, nausea, vomiting, and syncope.[3] Soft Tissue Trauma Patients should be warned that soft tissues such as lips, cheeks, and tongue can still be numb following dental treatment, and care must be taken until the anesthesia fully wears off.  There is a risk of patients biting these soft tissues due to poor sensory feedback, leading to soft tissue trauma.[3] Systemic Toxicity

Soft Tissue Trauma Patients should be warned that soft tissues such as lips, cheeks, and tongue can still be numb following dental treatment, and care must be taken until the anesthesia fully wears off.  There is a risk of patients biting these soft tissues due to poor sensory feedback, leading to soft tissue trauma.[3] Systemic Toxicity Toxicity can occur if the maximum safe dose of local anesthesia is exceeded and can lead to neurological and cardiac adverse effects. Excitatory neurological symptoms manifest initially. These may include visual disturbances, sensory disturbances, and seizures. This is followed by depressive symptoms, such as decreased consciousness which could lead to coma and respiratory arrest. Cardiac manifestations can range from tachyarrhythmia to bradyarrhythmia to cardiac arrest. Dental practitioners must be able to recognize and manage symptoms of toxicity.[3] Toxicity can be avoided by not exceeding the maximum anesthetic dose based on the patient’s weight. Table 2 outlines the maximum dosage for common anesthetic solutions used in dentistry. The percentage of anesthetic solution signifies the grams per 100 ml. For example, a 2% solution has 2 grams in 100 ml, therefore 20 mg/ml. To calculate the amount of anesthetic agent in a cartridge, the mg/ml figure is multiplied by the total solution in the cartridge, usually 1.8 ml or 2.2 ml. Table 3 illustrates an example of the calculation.[3] Toxicity is an emergency and, therefore, emergency medical service must be contacted. Management involves supplemental oxygen and intravenous medication, including Intralipid, midazolam if the patient has seizures, and vasopressor if the patient has hypotension or bradycardia. The patient should be monitored for ventricular tachycardia, ventricular fibrillation, acidosis, hyperkalemia, and hypercarbia, and these should be treated accordingly.[3] Table Table 2: Maximum dosage for anesthetic solutions commonly used in dentistry. Table Table 3: Example of maximum dose calculation. Transient Facial Nerve Palsy Transient facial palsy is a rare complication and can be immediate or delayed. Immediate palsy can occur due to direct anesthesia of the facial nerve after injection into the parotid gland when an IANB is given too posteriorly. Facial weakness occurs within minutes of the injection and resolves within 3 hours.[24]

Transient Facial Nerve Palsy Transient facial palsy is a rare complication and can be immediate or delayed. Immediate palsy can occur due to direct anesthesia of the facial nerve after injection into the parotid gland when an IANB is given too posteriorly. Facial weakness occurs within minutes of the injection and resolves within 3 hours.[24] Delayed palsy occurs several hours or days after the local anesthetic administration, and recovery can take 24 hours to many months. The mechanism is more complicated, and postulated theories include: the anesthetic agent or mechanical injury from the needle leading to a sympathetic vascular reflex and ischemic neuritis; mechanical trauma leading to the activation of a latent herpes simplex or varicella-zoster virus, and these viral infections leading to neural sheath inflammation and facial palsy; nerve damage due to breakdown products of the local anesthetic; prolonged mouth opening stretching the facial nerve; and intravascular injection. It is important to protect the eye with an eye patch when a patient develops transient facial palsy. Artificial tears and sunglasses are also recommended.[24] Trismus Trismus can result from muscle spasticity if the needle pierces through a muscle of mastication or from a hematoma preventing mandible movements. Trismus is usually managed conservatively with a soft diet, analgesia, and physiotherapy.[3]

All members of the dental team have a responsibility to ensure patient safety. The operator and dental assistant should ensure that the anesthetic solution has not expired and the maximum dose is not exceeded. The dental team members must be aware of the risk of toxicity and understand how to prevent, identify, and manage toxicity. All team members must also be able to recognize and address any medical emergencies that may occur during or after the administration of local anesthesia. Dental assistants can also play a key role in patient care, for example, by providing reassurance to anxious patients. All team members must work together to deliver patient-centered care to ensure good patient outcomes.