Why won't my patient get numb? (Part 2)

In our last post we reviewed a portion of  the Winter 2009 Colleagues for Excellence Newsletter and its discussion of multiple factors that can lead to the failure of our patients to get adequately numb for restorative or endodontic procedures.  In this edition we will review more material from the Newsletter which  focuses on some ways to address failed attempts at anesthesia and also debunks some common myths.
                       
With specific regard to the inferior alveolar nerve block (IANB), it is helpful to keep in mind that while lip numbness usually occurs rather quickly (5-9 minutes), pulpal anesthesia usually requires significantly longer (15-16 minutes).  According to one source (1), in 19-27% of cases, pulpal anesthesia in mandibular teeth takes longer than 15 minutes to achieve after administration of the IANB.  

In light of this information, one important step toward achieving adequate anesthesia is to simply give the anesthetic solution sufficient time to set in.  Additionally, administering the anesthetic slowly (over a period of 60 seconds) rather than quickly (15 seconds) has been shown to be not only less painful for the patient, but also more effective at producing pulpal anesthesia (2).  So when delivering the IANB, remember to slow your roll.                                              
While a second IANB following achievement of lip numbness has not been shown to be helpful, a local infiltration of articaine 4% with epinephrine 1:100,000 following IANB with lidocaine 2% w/ epinephrine 1:100k has been shown to increase pulpal anesthesia.  A 2008 study (3) showed that this method resulted in pulpal anesthesia in mandibular teeth 88% of the time while the success rate was only 71% when the buccal infiltration was done with lidocaine rather than articaine.


Occasionally when lip numbness has been achieved and pulpal anesthesia still eludes us, we as clinicians are tempted to deliver a second cartridge of anesthetic solution via the IANB.  However, studies have shown that if lip numbness is achieved following the first block, adding more solution via a second IANB does not result in better pulpal anesthesia.  The same is true of using an anesthetic formulation with a higher concentration of epinephrine; it does not improve pulpal anesthesia. The perception that a second block results in better pulpal anesthesia is likely the result of the fact that in the time it takes to stop the procedure and administer the second injection, the anesthetic has finally been given sufficient time to fully set in.  Of course, if lip numbness is not achieved after the first attempt, a second cartridge should be delivered.                        

If the patient continues to experience sensation after successful IANB (positive lip sign achieved) and administration of a local infiltration of articaine, an intraligamentary (PDL) or intraosseous injection may be indicated.  While the PDL injection is less successful than the intraosseous injection (3), it is still an excellent adjunct to conventional techniques (IANB and local infiltration) and may be a better option for clinicians unfamiliar with the intraosseous injection technique.  When all else fails, the intrapulpal injection can be utilized for endodontic procedures.  It is particularly effective when back-pressure can be achieved and requires no special syringes or needles. 

Myths:

The Winter 2009 Colleagues for Excellence Newsletter also debunks several myths regarding local anesthesia.  Some of these are listed below.

1.  Contrary to what many of us were taught in dental school, orientation of the bevel (toward or away from the ramus) actually does not affect the success of an IANB (4).

2. Despite popular sentiment to the contrary, studies have consistently failed to show statistically significant superiority of articaine over lidocaine for nerve blocks.

3.  On the question of whether articaine causes paresthesia when used for IANB, conflicting studies have reported different outcomes but the preponderance of the evidence seems to indicate that if articaine is more prone to cause paresthesia, the incidence is extremely low and the authors deem the relationship between use of articaine for IANB and resultant IAN disturbances as "questionable."

4.  Cross innervation does occur in mandibular anterior teeth but it is not the cause of most IANB failures in these teeth.  Rather, these failures are most often due to the anatomy of the IAN bundle (as discussed in the previous blog post).  Therefore, performing an IANB on the contralateral side to the teeth in question is unlikely to produce the desired result (5).

5. "Sticking" or "poking" the soft tissue adjacent to the tooth to be worked on is not a good test to assess the degree of pulpal anesthesia obtained.  Application of a cold cotton pellet to the tooth is a much more useful test of pulpal anesthesia.

    
  
1. Nusstein J, Reader A, Beck M. Anesthetic efficacy of different volumes of lidocaine with epinephrine for inferior alveolar nerve blocks. Gen Dent 2002;50:372-5

2.  Kanaa MD, Meechan JG, Corbett IP, Whitworth JM. Speed of injection influences efficacy of inferior alveolar nerve blocks: A double-blind randomized controlled trial in volunteers. J Endod 2006;32:919-23

3. Haase A, Reader A, Nusstein J, Beck M, Drum M. Comparing anesthetic efficacy of articaine versus lidocaine as a supplemental buccal infiltration of the mandibular first molar after an inferior alveolar nerve block. J Am Dent Assoc 2008;139:1228-35
4. Steinkruger G, Nusstein J, Reader A, Beck M, Weaver J. The significance of needle bevel orientation in achieving a successful inferior alveolar nerve block. J Am Dent Assoc 2006;137:1685-91.
5.  Yonchak T, Reader A, Beck M, Meyers WJ. Anesthetic efficacy of unilateral and bilateral inferior alveolar nerve blocks to determine cross innervation in anterior teeth. Oral Surg Oral Med Oral Pathol Oral Radiol  Endod 2001;92:132-5.

Why won’t my patient get numb?

Occasional difficulty in achieving profound anesthesia for dental procedures is a common experience and one that can be frustrating for both clinicians and patients. What are the factors that occasionally prevent us from achieving anesthesia and how can we improve the frequency with which we successfully get our patients numb? These issues were addressed in the Winter 2009 edition of the biannual clinical newsletter published by the American Association of Endodontists (AAE) entitled Colleagues for Excellence.

The authors of that publication begin by addressing several misconceptions related to the delivery of anesthetic solution via the inferior alveolar nerve block (IANB). One common misunderstanding is the belief that lower lip numbness assures adequate pulpal anesthesia for restorative or endodontic procedures. This is actually not the case. Even after achieving a positive lip sign, adjunctive measures are often needed to achieve pulp anesthesia. Similarly, poking the gingiva or mucosa with an explorer without response from the patient does not assure pulpal anesthesia.

 So does lack of pulpal anesthesia following an IANB indicate that the injection technique was inadequate or that the block was “missed”? In most cases, especially when positive lip sign has been achieved, the answer is no. Studies using ultrasound and radiographs to assure accurate positioning of the needle next to the neurovascular bundles targeted for anesthesia have shown that even with perfect needle placement, pulpal anesthesia is sometimes not achieved. In other words, don’t beat yourself up if your patient doesn’t get numb with a well placed IANB.

 Several factors may contribute to continued sensitivity even following an accurate IANB. According to the authors, the best explanation may be simple anatomy. The inferior alveolar nerve that supplies innervation to all the mandibular teeth on its respective side of the jaw is arranged in layers. As the nerve travels through the mandible, the layers sequentially branch off to supply the molars, followed by the bicuspids, and finally the anterior teeth. This means that at the site of anesthetic delivery, the nerves that supply anterior teeth are in the center or at the core of the nerve bundle and are insulated to some degree from the anesthetic solution. This explanation is consistent with experimental results that show higher IANB failure rates in anterior teeth. 

Several other factors that affect the successful achievement of profound numbness are related to the unique aspects of inflamed tissues. One theory posits that the increased acidity of inflamed tissues inhibits the formation of the base form of the anesthetic solution which is needed to cross the membrane of the nerve cells. The possible effect of this phenomenon when delivering local infiltration anesthesia is clear, however its relationship to IANB failures is unclear since with the IANB the solution is usually delivered at some distance from the focus of inflammation. Additionally, nerves in inflamed tissues have decreased excitability thresholds which can result in the inability of anesthetic solution to prevent impulse transmission in the nerves.

 Another factor contributing to anesthesia failure in areas of inflammation has to do with the types and quantities of ion channels in the membrane of the neuron. Studies have shown that nerve cells in pulps with irreversible pulpitis show a greater expression of sodium channels than nerves in non-inflamed pulps thereby affecting their excitability. And a specific class of sodium channels called Tetrodotoxin-resistant (TTXr) channels has been shown to resist the action of local anesthetics. Lastly, patients in pain are often apprehensive. Though the mechanism is not completely understood, this apprehensiveness has been shown to actually decrease pain thresholds.

So as the preceding points make clear, there are many factors affecting our ability as clinicians to help our patients achieve profound anesthesia prior to restorative or endodontic procedures. In our next post we will address multiple helpful strategies for overcoming inadequate anesthesia as well as debunk some other common misconceptions.