This review discusses the most prevalent factors causing local anesthesia (LA) failures in dentistry, ranging from operator error to systemic conditions. Various pharmacologic, inflammatory, and psychological factors contributing to LA failure have been discussed. Knowledge about updated evidence on anesthetic agents, block techniques, anatomic variations such as bifid canals and accessory innervation, and the role of patient-specific factors, including systemic conditions and anxiety, is also explored. Strategies for prevention of anesthetic failure have also been discussed.
Key points
-
•
Several factors can contribute to the failure of achieving successful anesthesia, including operator errors, anatomic variations, certain syndromes, infections, and psychological factors, among others.
-
•
A comprehensive understanding of the different injection techniques, oral anatomy, and its variations is necessary to decrease the rate of local anesthesia (LA) failure.
-
•
Inflammation, like in cases of irreversible pulpitis, can reduce the LA’s ability to dissociate, thereby decreasing the availability of LA to anesthetize the tissues.
-
•
Patient anxiety, dental phobia, and genetic variations can influence the pain threshold of the patient. Hence, patient-specific management strategies are necessary to overcome LA failure.
-
•
Advanced technologies like computer-controlled local anesthetic delivery systems used to monitor the rate of flow of anesthetic fluid, can help minimize LA failure.
Abbreviations
| CBCT | cone beam computed tomography |
| EDS | Ehlers-Danlos syndrome |
| GPC | greater palatine canal |
| IANB | inferior alveolar nerve block |
| LA | local anesthesia |
| MC1R | melanocortin-1 receptor |
Introduction
Local anesthesia (LA) is generally defined as the procedure by which a clinician facilitates lack of sensation in a tissue, brought about by the inhibition of excitatory effects of the nerve endings or inhibition of conduction by sensory nerves. Inadequate anesthesia not only undermines a clinician’s credibility and prolongs appointment durations but also can create a stressful atmosphere for both the patient and the clinician. This may have certain legal implications as well. Several factors can contribute to the failure of achieving successful anesthesia, including operator error, anatomic variations, certain syndromes, infections, and psychological factors, among others. This narrative review explores the factors and the mechanisms of LA failures in dentistry, their clinical implications, and strategies to mitigate them.
Classification
Classification Based on Chemistry
Anesthetics are typically classified based on their chemical structure into 2 categories: amides and esters ( Fig. 1 ). Amides are more commonly used in clinical practice, as esters have been associated with allergic reactions due to the metabolic conversion into para -aminobenzoic acid. ,, Fig. 1 ; Figs. 2 and 3 elaborate the different local anesthetics based on their chemistry, routes of administration, and the biological site of action, respectively. ,,
Different local anesthetics based on their chemistry. ,,
Classification of local anesthetics based on routes of administration. ,,
Classification of local anesthetics based on the biological site of action. ,,
Local anesthetics in dentistry
Lidocaine is one of the most used anesthetic agents in dentistry. Structurally, it is an amide. In addition to being used as a local anesthetic for dental procedures, it also has applications in pain management of mucosal lesions. Others include benzocaine, which is an ester. Admixtures, which are combinations of anesthetics include cetacaine R+ (benzocaine, tetracaine, benzalkonium chloride, and butyl aminobenzoate), which can be used to minimize gag reflex. Compounds such as fluori-methane, although not strictly classified as a local anesthetic, can be used for bringing about a transient local anesthetic effect. , Table 1 exhibits salient features of various local anesthetic agents.
Table 1
Salient features of different local anesthetics
| Local Anesthetic | Onset of Action | Duration | Pregnancy Category |
|---|---|---|---|
| Lidocaine | 2–4 min | Moderate | Class B |
| Articaine | 2–4 min | Long | Class B |
| Bupivacaine | 5–8 min | Long | Class C |
| Mepivacaine | 2–4 min | Moderate | Class C |
| Prilocaine | 2–4 min | Moderate | Class B |
| Ropivacaine | 2–4 min | Long | Class B |
| Procaine | 14–18 min | Short | Class B |
Criteria for anesthesia success
One of the criteria for measuring the success of LA is soft tissue anesthesia. , However, in cases where soft tissue numbness is not evident to the, dentists should rely on tests such as a lack of response to cold tests, electric pulp tests, or pain during access cavity preparation. Among the 3 mentioned, pain during access cavity preparation is considered the gold standard to determine pulpal anesthesia success. ,
Reasons for failed local anesthesia
There are several factors responsible for LA failure ( Table 2 ).
Table 2
Common causes of local anesthesia failures
| Category | Cause of Anesthesia Failure |
|---|---|
| Anatomic factors | Central core theory, mylohyoid nerve involvement, accessory nerve supply |
| Neurophysiological changes | Expression of tetrodotoxin-resistant sodium channels, increased nerve excitability |
| Inflammatory factors | Lower pH reducing anesthetic penetration, upregulation of inflammatory cytokines |
| Psychological factors | Increased anxiety lowering pain threshold, higher nerve excitability |
| Technique-related errors | Poor needle placement, incorrect injection technique, inadequate solution volume |
| Pharmacologic errors | Choice of anesthetic type, concentration, and use of vasoconstrictors |
| Injection-related pain | Mechanical trauma from needle, pain from the anesthetic deposition |
| Supplemental techniques | Failure of primary blocks, need for intraligamentary or intraosseous injections |
| Patient-specific variations | Age, gender, and genetic variations in pain and anesthetic response |
Operator Error
The major factor in LA failure is operator error. These include poor injection technique, incorrect needle orientation and positioning, using the wrong type of anesthetic solution, not giving enough anesthetic, failing to aspirate before injecting or not considering the uniqueness of the patient’s local anatomy.
Choice of block
A comprehensive understanding of the anatomy of the oral cavity is essential for both clinicians and dental students. This includes a thorough knowledge of the nerve supply to each tooth and its roots, which is crucial for selecting the most appropriate anesthetic technique ( Fig. 4 ). Specifically, using infiltration anesthesia for treating pulp disease in the mandibular permanent premolars and molars is often an error that can lead to inadequate anesthesia. ,
Maxillary and mandibular LA techniques and the areas they anesthetize, respectively.
( Adapted from Rodella LF, Buffoli B, Labanca M, Rezzani R. A review of the mandibular and maxillary nerve supplies and their clinical relevance. Arch Oral Biol. Apr 2012;57(4):323-34. https://doi.org/10.1016/j.archoralbio.2011.09.007 .)
Choice of solutions
For most dental procedures, lidocaine with epinephrine is the preferred local anesthetic, while epinephrine-free options may be better for some medically compromised patients. Lidocaine without epinephrine may have the disadvantage of wearing off prematurely.
Poor technique
The success rate for achieving anesthesia depends on the operator’s technical skills and clinical knowledge. Failing to aspirate before injecting could result in intravascular administration. Slower injections can improve the chances of successful anesthesia. If injected too forcefully, the anesthetic could be pushed away from the nerve, making the block less effective. Achieving anesthesia with the inferior alveolar nerve block (IANB) has a success rate of more than 80%.
-
•
Needle orientation error: Improper orientation of the needle during IANB can result in inadequate anesthesia due to the anesthetic solution being deposited away from the mandibular foramen. Hence, correct needle placement is paramount in achieving a successful block ( Fig. 5 ). If the needle is positioned more posteriorly, the anesthetic could be deposited in the parotid region, causing facial paralysis.
Fig. 5 Represents the orientation of needle and target site postinsertion for an IANB.
(Illustration by Dr. Kainat Anjum, BDS, MS.)
-
•
Inadequate drug delivery: The volume of anesthetic deposited influences the success of LA. For IANB, if pulp is inflamed, 4 mL of LA can increase the possibility of successful anesthesia. For maxilla, 1.2 mL of 2% lidocaine with 1:100,000 epinephrine was found to provide greater success. ,
-
•
Failure to aspirate: This can lead to intravascular injection of the anesthetic that can not only reduce the effectiveness of LA but also increase systemic toxicity.
-
•
Possible buffering by local bleeding: Bleeding at the site of injection can reduce local pH and impede the anesthetic’s ability to penetrate nerve membranes, while dilution lowers its effective concentration.
Anatomic Factors
Bifid canal
Anatomic variations of the mandibular canal can include bifid or trifid canals and double mandibular foramen. These anatomic differences are significant factors that can impede LA in the mandibular region.
Position
Anatomic landmarks can differ significantly from one patient to another. A limited understanding of the locations of structures such as the greater palatine canal (GPC), mandibular foramen, and infraorbital nerve can impede successful anesthesia delivery. Misjudging the length of the GPC may result in inadequate diffusion of the anesthetic agent, leading to complications like diplopia. Furthermore, the position of the mandibular foramen can be variable, which can be influenced by factors such as the occlusal plane, age, and variations in condylar length.
Accessory innervation
Variations in the number of nerve branches within specific teeth or regions of the oral cavity can hinder the successful administration of anesthesia. In particular, the presence of additional nerve branches from the mylohyoid nerve can complicate anesthesia for the mandibular anterior teeth ( Fig. 6 A, B). ,, Teeth with multiple roots tend to experience a slower onset of pulpal anesthesia compared with those with a single root, primarily due to a greater likelihood of accessory innervation within the pulp canals. Additionally, if sensitivity is noted in the mandibular posterior teeth while the lower lip is anesthetized, this may be attributed to accessory innervation from the great auricular nerve and the buccal nerve.
( A ) Outline of the submental, submandibular, and mandibular region depicting the mandible, hyoid bone, mylohyoid muscles, and terminal branch of the right mylohyoid nerve entering the mandible through the inferior retromolar foramen. The diagram does not represent the spatial orientation of the hyoid. ( B ) Outline of the frontal view of the mandible to show the mylohyoid nerve communicating with the incisive nerve of the opposite side after penetrating the mandible through the inferior retromental foramen at the level of the inferior border.
(Illustration by Dr. Kainat Anjum, BDS, MS.)
The mylohyoid nerve, carrying afferent fibers from mandibular teeth, branches from the inferior alveolar nerve approximately 15 mm above the mandibular foramen, beyond the reach of conventional techniques. Alternative approaches include Gow-Gates or Akinosi blocks, lingual infiltration adjacent to the tooth, or intraligamentary and intraosseous techniques targeting the medullary space around the tooth. While other nerves, such as the lingual, buccal, and transverse cervical nerves ( Fig. 7 ) have been suggested as accessory sources, definitive evidence is lacking. , The most reliable strategy for overcoming accessory innervation involves intraligamentary or intraosseous injections.
Depiction of the accessory innervation of the mandible from the cervical plexus: The transverse cervical nerve entering the posterior mandible via the inferior border of the mandible (++) and the greater auricular nerve from the cervical plexus, supplying the angle of the mandible (∗∗). This is an anatomic variation that is not seen in all patients. ,
(Illustration by Dr. Kainat Anjum, BDS, MS.)
Patient Factors
Local infection and inflammation
Inflammation and infection may compromise the effectiveness of LA. Acidotic tissue fluid near inflamed tissue interferes with the dissociation of the anesthetic agent. Inflammation may induce primary hyperesthesia, hyperexcitability, and hyperalgesia, increasing patient sensitivity to stimuli. Despite the correct technique, anesthetic failure can occur in one-third of cases involving pulpitis. , To mitigate the risk of infection spread, administering the anesthetic at a distance from the infected area is recommended. Furthermore, repeated anesthetic injections in inflamed or infected tissues are discouraged, as they can result in tachyphylaxis. Additionally, vasodilation in inflamed tissues promotes anesthetic absorption and removal. This issue may be addressed by increasing the anesthetic concentration (rather than the volume). The more the prosperity pain, the more the difficulty to achieve optimal anesthesia.
Allergy
Allergic reactions to amide-type local anesthetics are rare. Symptoms resembling allergies may also result from psychogenic responses or reactions to additives such as sodium metabisulfite, a preservative associated with allergic responses. Previously, methylparaben, a bacteriostatic agent, was another common allergen in multidose vials. Ester-based anesthetics such as benzocaine, commonly used in topical applications, are more likely to cause allergic reactions. In these cases, the allergy to local anesthetics or its components may preclude the patient from being anesthetized efficiently. ,
Systemic conditions
Dental professionals should anticipate an increased likelihood of anesthetic failure during endodontic treatments in patients with diabetes. Increased incidence of LA failure has been reported in patients with Ehlers-Danlos syndrome (EDS), particularly during regional anesthesia procedures such as spinal or epidural blocks. Mechanisms proposed for LA resistance in EDS include altered tissue permeability or spread of anesthetic due to abnormal collagen structures and potential genetic mutations of sodium channels. ,,, There may be multiple systemic conditions potentially contributing to LA failure; a complete discussion is beyond the scope of this article.
Psychological factors
Fear of injections and LA is a significant barrier preventing patients from seeking dental care. Stress is a key factor in discussions about pain management, highlighting the need for clinicians and students to be trained in assessing patients’ anxiety levels, fears, and expectations prior to treatment. Additionally, past negative experiences with dental treatment, coupled with anxiety, can considerably influence both the amount of anesthetic needed and the overall success of the procedure.
Stay updated, free dental videos. Join our Telegram channel
VIDEdental - Online dental courses
