Failure to achieve Inferior alveolar nerve block Local Anesthesia is most common problem many Dentists face during Root canal Treatment, it leads to the patient experiencing pain while pulp extirpation and increases difficulty along with consuming more time for the Dental Practitioner or Endodontist. Having facing many such cases even after maintaining proper Injection technique I have listed down some simple tricks which will help you achieve Anesthesia in failure of Inferior alveolar nerve block.

It is of utmost importance to make sure that you follow all the landmarks and a precise Inferior Alveolar Nerve block is given. It is common to see anatomical variations in patients which makes it difficult to achieve proper path of insertion of the needle in IANB. Some Doctors have coined a term for the teeth which fail to attain Anesthesia even after proper anesthesia technique as “Hot Tooth“. It is a commonly used terminology for discussion in Endodontic circles.

When is a tooth termed a Hot Tooth?

Hot does not imply to a tooth being attractive or an increase in temperature in the tooth. Hot Tooth is used to identify a tooth which has irreversible pulpitis accompanied by moderate to severe pain which does not get anesthetized after a nerve block. In conditions where other Objective symptoms such as lip numbness, etc are achieved but pulpal anesthesia is not achieved it is termed as “Hot Tooth“.

According to the “Central Core Theory” outer nerves of the inferior alveolar nerve bundle supply the molar teeth, whereas the nerves for the anterior teeth lie deeper. Anesthetic solution if administered deeper do not anesthetize the nerve bundle to the anterior teeth and vice versa.

Failure of Inferior alveolar nerve block or hot tooth

Causes of lack of anesthesia after Inferior alveolar nerve block:

Local Pathological factors: Failure rates in posterior Mandibular teeth with Irreversible pulpitis are very high – 30-81% which is a result of the activation and sensitization of the nociceptors and stimulation of a greater number of nerve fibers. Local infections in the pterygomandibular space or the mandibular nerve branch creates a acidic environment interfering with the onset of anesthesia.

Improper injection technique: It can be due to improper positioning of the needle, over extension or under extension of the needle from the location of the nerve to be anesthetized. The proper path of insertion of the needle has to be followed for the needle tip to reach the desired position, if the path of insertion is not proper it tends to extend into a muscle or adjacent space.

Insufficient Local anesthetic injected: To achieve Anesthesia certain quantity of LA liquid has to be administered depending on the type of Injection technique – Nerve block, intraligamentary, topical, intraosseous injection, etc. Depending on the type of injection a certain amount of LA liquid has to be injected so that it can diffuse into the nerve fibers and attain anesthesia. So if less amount of LA liquid is injected proper Anesthesia will ne be attained.

Anatomic variations of Nerve supplying the Mandibular Molars

Skeletal anamolies like in case of Retrognathic mandible where the distance from the mandibular foramen to the condylar tip is significantly shorter, which places the mandibular foramen position higher. When the needle is inserted above the occlusal line the solution is infused lower than the foramen. Similarly in Prognathic mandible cases, the foramen is position inferior to where it usually is located

Additional nerve supply to the molars – Mylohyoid nerve (located in the lingual sulcus) this is seen in certain patients where an additional branch of Mylohyoid nerve supplies to the Mandibular molars. In most cases a proper inferior alveolar nerve block cannot anesthetize the Mylohyoid nerve. Although widely believed that IANB + Mylohyoid inj is effective Clark and colleagues when tested this in practice there was not much improvement in mandibular anesthesia.

Increased Buccal Cheek fat can sometimes be a hindrance for following the correct path of insertion of the needle, this can sometimes deflect the needle Lingually away from the path to be followed.

Physiological factors: Certain factors such as Anxiety or Dental Phobia are one of the major factors for failure of Dental anesthesia. Even if the nerve is anesthetized the patient feels pain due to neuro-physiological factors. Certain stimulus like, sound of handpiece, smell from tooth cutting or the pressure applied can make the patient imagine the feeling of pain even with the nerve being anesthetized. The solution is to counsel the patient before the procedure to give him or her confidence.

How to get Anesthesia in case of Hot Tooth:

The exact reason for the Hot tooth is yet to be determined and according to many studies there are multiple reasons for a hot tooth like Anatomic variations, Improper Injection technique, quantity of LA agent, Type of LA agent, etc.

Repeat Inf Alveolar Nerve block:

Repeating the IANB is the first thing to do in case of a failed Nerve block as you can rectify the mistake of placement or injection technique.

Intraligamentary Injection:

The injection is given in the cancellous medullary bone surrounding the tooth into the periodontal ligament using high injection pressure and high gauge needle. This gives the best result in terms of failure of any Nerve block technique. Smith et al. reported a success rate of 93% in intraligamentary injection followed by IANB (failure) which is quite high and the best solutions for failure of IANB.

Buccal Infiltration / Intraosseous Anesthesia / Intrapulpal Anesthesia:

These three types of Anesthesia can be given in case of failed IANB to get anesthetic effect.

Intrapulpal Injection: In case of teeth requiring Endodontic treatment or extraction, access opening can be done and intrapulpal injection can be given to anesthetize the tooth in question.

Other Blocks: Closed Mouth block or Vazirani/Akinoai block and Gow Gates mandibular nerve block.

Modifications of Inferior alveolar nerve block techniques:

  • Modified IANB by Thangavelu et al.
  • Arched needle technique
  • Modified IANB by Palti et al.

Change the local anesthetic agent injected:

According to research – 3% mepivacine plain, 4% prilocaine, 4% prilocaine with 1:200,000 epinephrine, 2% mepivacaine with 1:200,000 levonordefrin and 4% atricaine with 1:100,000 epinephrine to 2% lidocaine with 1:100,000 epinephrine showed not much of a difference in terms of anesthetization of the tissue.