The Importance of Technique Selection for Administration of Local Anesthetics

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The Importance of Technique Selection for Administration of Local Anesthetics
April 2007
Alan W. Budenz, MS, DDS, MBA
Contemporary Oral Hygiene

Abstract
Many variables can affect the achievement of profound anesthesia produced by injection of local anesthetic agents. For every dental procedure, the hygienist or dentist must first decide whether injection of a local anesthetic agent will improve patient comfort and/or successful completion of the procedure. If an injection is indicated and determined safe for the patient's health status, then a choice must be made between an infiltration injection technique and a block injection technique. Next is the decision of which anesthetic agent to use and whether to use a "plain" anesthetic solution or one containing a vasoconstrictor. These 2 decisions should be based on the duration and depth of anesthesia required, and the potential need to control bleeding. The practitioner must weigh the risks against the benefits for each anesthetic agent, for use of a vasoconstrictor, and for the delivery technique. This thought process maximizes the safety of local anesthetic injections for each patient and each dental procedure. This article also will discuss the safety factors for use of the anesthetic agent articaine and the 3 major techniques for mandibular quadrant anesthesia.

In dentistry, local anesthetics are fundamental to the control and management of patient pain. Although hygienists and dentists receive extensive training and practice in the administration of local anesthetic injections, many variables affect attainment of successful anesthesia in any patient population. Certainly human variables are significant: anatomical variations in the size, morphology, and location of structures in the jaws of different individuals, including the dental sensory nerves, are well recognized.1 There are also significant variances in chemical sensitivity from one person to another: not all patients will experience the same degree of anesthesia from any single anesthetic agent, nor will a given patient be equally sensitive to every anesthetic agent available for dentistry. Part of the reason for this variability is attributable to differences in the numbers, types, and physiologic state of anesthetic binding receptor sites within sensory nerves.2 Table 1 provides guide?lines to achieve success with local anesthetic agents. This article will ex?plore some general principles of anesthesia and offer suggestions for achieving more predictable success with local anesthetic injections for mandibular quadrant anesthesia.

Infiltration vs Block Techniques
After deciding that a local anesthetic injection will benefit the outcome of a patient's dental procedure, a fundamental choice must be made between an infiltration injection technique and a block injection technique. In general, a block injection will give adequate anesthesia for approximately 2 times as long as an infiltration injection. However, this "rule of thumb" does not hold true for all anesthetic agents available in dental injection cartridges. For example, bupivacaine 0.5% HCl with 1:200,000 epinephrine (Mar?caine, Cooke-Waite, Kodak Dental Products, Ro?chester, NY, www.kodak.com/dental), is potentially the longest acting anesthetic available in a dental cartridge. However, if administered via an infiltration technique in the maxilla, it has an average duration of pulpal anesthesia of 40 minutes, which is not as good as the duration of the same injection using lidocaine or mepivacaine (Carbocaine, Cooke-Waite; Polocaine, Dentsply Pharmaceutical, York Pa, www.dentsply.com; Iso?caine, Septodont, Inc, New Castle, Del, www.septodontinc.com).3,4 The long duration of pulpal anesthesia with bupivacaine, up to 4 hours in some patients, is realized only if administered as a block injection.3 Another example is prilocaine 4% HCl (Citanest Plain, Dentsply Pharma?ceuticals). When this anesthetic is administered as an infiltration injection for maxillary teeth, it has a duration of pulpal anesthesia of approximately 10 minutes, as determined by electrical stimulation studies.5 When administered via a block technique, this same anesthetic produces pulpal anesthesia for 40 to 60 minutes.4 If vasoconstrictor is added to this anesthetic, prilocaine 4% HCl with 1:200,000 epinephrine (Citanest Forte, Dentsply Pharmaceuti?cals), duration of pulpal anesthesia is less technique sensitive and can be extended up to 90 minutes as a block injection.4 Exactly how long anesthesia will last for either type of injection with any agent is highly dependent on a number of patient variables, as well as the relative volume of anesthetic injected and the accuracy of administration of the technique used.

A further example of the duration of anesthesia being dependent on the technique selected is illustrated by the 2 anterior middle superior alveolar (AMSA) maxillary injection techniques: the facial approach and the palatal approach. The standard facial approach AMSA injection, a true block injection properly called the infraorbital block injection, is given on the face just below the eye. The lower rim of the orbit is palpated, feeling for the lowest part of the rim, which is usually more toward the base of the nose rather than right in the center. The finger is then dropped down 1 cm below the rim, which places the finger right over the foramen. Leaving that fingertip over the foramen, another fingertip is slipped up into the top of the maxillary vestibule in the area of the bicuspids. The distance between those 2 fingertips will be the depth of penetration of the needle, usually about 10 mm. The needle is inserted into the top of the vestibule paralleling the slope of the maxillary bone until the needle is felt underneath the fingertip that remained over the foramen. After injecting the anesthetic, the bolus of anesthetic is massaged into the foramen. By using the more posterior approach over the bicuspids, this technique is more comfortable for the patient than the approach entering over the lateral to cuspid region.

The palatal-approach AMSA injection administers the anesthetic lower down the pathway of the anterior and middle superior alveolar nerves than does the facial approach. Using the midline palatine raphe and extending a perpendicular line to between the 2 bicuspids, halfway along that line is the injection site at the junction of the alveolar process and the hard palate. Depositing a small amount of anesthetic at this site will anesthetize the anterior and middle superior alveolar nerves.

The primary advantage of this technique is palatal, pulpal, and buccal anesthesia from the second bicuspid to the central without anesthesia of the upper lip. The facial infraorbital injection does not provide palatal anesthesia and the upper lip is unavoidably anesthetized. The disadvantage of the palatal AMSA injection is that it does not have as long a duration of anesthesia as the infraorbital/facial approach because it is not as high of a block along the nerve pathway as the infraorbital injection. For root planing or veneer cases, when it is desirable to keep the smile line and long duration is not needed, the palatal AMSA is an efficient and beneficial injection technique. But if more invasive or time-consuming procedures are anticipated, such as crown preparations from bicuspid to bicuspid, the infraorbital block technique in combination with the nasopalatine block will provide better duration.

Vasoconstrictors
The presence or absence of a vasoconstrictor is an important factor for duration of anesthesia. The presence of a vasoconstrictor in an anesthetic solution has 3 main effects: (1) increased duration of anesthesia by holding the anesthetic at the local injection site longer by constriction of the local vasculature; (2) slowed uptake of the anesthetic agent into the bloodstream, resulting in a lower concentration of anesthetic in the bloodstream over time, which reduces the risk of systemic toxicity; and (3) maintainance of hemostasis during dental procedures, such as root planing or surgical procedures that produce bleeding.6-8 A block injection will not provide adequate hemostasis to a distal site; only local infiltrations at the site of bleeding can effectively control bleeding.4 Unfortunately, infiltration injections provide shorter duration of anesthesia, and possibly less profound anesthesia.

To decide which technique to use, consider the duration and profundity of anesthesia that will be needed and if hemostasis will be needed. For some procedures, such as root planing or surgery, it may be best to use both techniques: a block injection with a vasoconstrictor-containing anesthetic for dura?tion and profundity of anesthesia and infiltration injections with an anesthetic with the same or a higher vasoconstrictor concentration for hemostasis.

The duration of anesthesia is increased by the presence of a vasoconstrictor; however, the specific concentration of the vasoconstrictor does not significantly alter the clinical duration of anesthesia.4,8-10 For example, lidocaine is available in the United States with epinephrine as the vasoconstrictor in both 1:50,000 and 1:100,000 concentrations. The 1:50,000 concentration of epinephrine will provide the best hemostasis when used as an infiltration injection.4,9 However, a block injection with the 1:50,000 concentration will not provide significantly longer duration of anesthesia than a 1:100,000 concentration, but is more likely to produce systemic cardiovascular side effects, such as tachycardia.8,9

It is this author's recommendation to use the 1: 50,000 concentration only for infiltration injections and the lower 1: 100,000 concentration for block injections. In general, the lowest concentration of vasoconstrictor available is preferable for all block injections. Arti?caine hydrochloride 4% (Septo?caine, Septodont, Inc; Zorcaine, Cooke-Waite) is now available in the United States with either a 1:100,000 or a 1:200,000 concentration of epinephrine. Using the 1:200,000 concentration of vasoconstrictor will decrease the risk of cardiovascular side effects without significantly reducing the duration or profundity of anesthesia.10-12

Epinephrine is the primary vasoconstrictor used in dental anesthetics in the United States. Levo?nordefrin (Neo-Cobefrin, Cook-Waite) also is available. It is used only with a 2% mepivacaine formulation. Because levonordefrin has only one sixth the potency of epinephrine, it is used in a 1:20,000 concentration. While levonordefrin does have a reduced tachycardic effect on the heart, which is preferable in particularly epinephrine-sensitive patients, it is contraindicated for use in patients for whom epinephrine is contraindicated, ie, if epinephrine should not be used for a patient because of its possible deleterious effect on the patient's medical condition, levonordefrin should not be substituted.13 If a vasoconstrictor is contraindicated, only a plain anesthetic solution without a vasoconstrictor should be used.

Articaine
Articaine has become a popular dental anesthetic in the United States since its introduction in early 2000. Although many practitioners have made claims of a faster onset, more profound anesthesia, more successful anesthesia with fewer misses, or better success when anesthetizing "hot" teeth or "difficult-to-numb" patients, these claims are largely anecdotal. Only 1 clinical study to date has shown any support for these claims, while many other studies have shown no clinically significant differences compared with other anesthetic agents.14-16 Essentially, the studies show that all of the amide anesthetic agents currently available for dentistry are very effective.

Equally anecdotal are reports that there is a greater risk of nerve injury, specifically paresthesia, with use of articaine. Many anecdotal reports in the dental literature purport that both of the 4% anesthetic solutions, articaine and prilocaine, have higher incidence of nerve injury, particularly when administered for a traditional inferior alveolar nerve block. However, to date, no evidence-based research supports these claims. The general assumption is a reasonable one: that the 4% solutions have a greater potential for neurotoxicity injury to sensory nerves. But in their 2005 review of local anesthetic-associated paresthesias, Missika and Khoury concluded that "a clear causal relationship has not been established in the literature between the anesthetic agent and neurological complications, such as paresthesia."17 Nonetheless, the volume of anecdotal reports does merit careful consideration before choosing to use either of the 4% anesthetic solutions, particularly for the inferior alveolar nerve block injection.

It is the opinion of this author that at least some of the observed injuries with administration of the 4% anesthetic solutions are caused by overdosage of the anesthetic at the local injection site. Two simple precautions when using articaine or prilocaine are: (1) reduce the volume of the anesthetic delivered by roughly half as compared with 2% lidocaine; and (2) deliver that volume slowly. These precautions are in line with the manufacturers' recommendations for all dental anesthetics: use as low a dosage as possible to achieve effective anesthesia and deliver the anesthetic solution as atraumatically as possible. These precautions will not eliminate the risk of nerve injury, but they may well reduce the size of the risk.

Gow-Gates Mandibular Block Injection
Because the vast majority of nerve paresthesias reported for all anesthetic agents occur in the mandible following a traditional inferior alveolar nerve block injection, it is worth examining the safety record of the various mandibular division nerve block techniques. The traditional inferior alve?olar nerve block technique was first described by William Halsted in 1885, and is the most commonly used method for gaining mandibular anesthesia in the dental office.18 This technique uses the deepest point of the concavity of the anterior border of the ramus of the mandible as the origin for a plane, running posteriorly and parallel to the occlusal plane of the mandible. The anterior̉posterior width of the mandible along this plane is then bisected to determine the injection target on the medial surface of the mandible, which will be above the mandibular foramen, where the inferior alveolar nerve enters into the mandible. With the needle penetration point lateral to the pterygomandibular raphe, the needle is advanced to the target point until bone is contacted.

In 1973, George Gow-Gates described an approach much higher along the nerve pathway for anesthetizing the mandibular division of the trigeminal nerve, Cranial Nerve V, using the anteromedial as?pect of the condylar neck of the mandible as the targeted contact area.19 With the mouth opened maximally, the neck of the condyle translates forward to lie immediately lateral to the foramen ovale, the portal of entry of the mandibular division of the trigeminal nerve into the infratemporal fossa (Figures 1A and 1B). The needle penetration site for this technique is in the maxillary vestibule adjacent to the maxillary second molar and angled posterolaterally to contact the anteromedial aspect of the neck of the condyle. A full cartridge of anesthetic is deposited at this site, and the mouth should be kept wide open for a full 90 seconds after the anesthetic is deposited as per Dr. Gow-Gates' recommendation. Use of a mouth prop, such as a bite block, simplifies maintenance of this maximally opened position for the patient.

Another technique of mandi?b?ular division nerve block anesthesia done with the mouth closed was described by Vazirani in 1960 and similarly by Akinosi in 1977.20,21 Similar to the Gow-Gates technique, the needle penetration site is in the maxillary vestibule. However, the syringe is oriented posteriorly, parallel with the mucogingival line of the maxilla, between the maxilla and the mandibular ramus. The needle is inserted to the midpoint of the ramus, approximately 25 mm, before deposition of a full cartridge of the anesthetic. There is no bone contact with this technique; the needle is kept in close proximity, but parallel, to the mandibular ramus. Figure 2 illustrates the site of anesthetic deposition for each technique.

Although reported success rates have varied considerably from one study to another for all 3 techniques, the Gow-Gates technique is most frequently reported to have the highest success rates, ranging from 90% to 100%.22-25 The success rate of the Halsted traditional technique has been reported to be between 65% and 86%,19,22,23 and the success rate of the Vazirani-Akinosi technique has been reported to be between 76% and 93%.21,26,27 The authors of these studies are careful to point out that the success rate of any technique has many variables, not the least of which is the practitioner's knowledge, experience, and comfort with each technique. No significant differences in duration of anesthesia among the 3 techniques have been reported.26

Risks and Side Effects
Because the infratemporal fossa is a highly vascular area with considerable anatomic variability of artery and vein positions, the possibility of intra?vascular injection or vascular injury, such as hema?toma, is of concern to dental practitioners. The occurrence of positive aspirations is highest with the traditional Halsted technique,26,28,29 occurring in 7% to 22% of injections in a series of 14 studies reviewed by Watson and Gow-Gates.30 The Gow-Gates and Vazirani-Akinosi techniques are typically found to produce far fewer positive aspirations than the traditional technique, with similar values of up to 2% reported for both techniques (Gow-Gates 0% - 2%; Vazirani-Akinosi 2%).26,28,29 The majority of studies show that the Gow-Gates technique, unique among oral nerve block procedures, uses a target zone for optimal analgesia efficacy that corresponds with the site least likely to yield a positive aspiration.30 This is in contrast to the traditional Halsted technique. A study by Donkor and colleagues29 showed that although the Halsted technique could produce an exceptionally high success rate of anesthesia (97%), it was accompanied by the highest incidence of positive aspirations (22%) among the 14 studies reviewed by Watson and Gow-Gates.30

The incidence of other undesirable side effects, such as muscle trismus, nerve paresthesia, or anesthesia of structures other than the desired dental nerves (the facial nerve, the auriculotemporal nerve, or maxillary rather than mandibular nerves) is also worth noting. The Vazirani-Akinosi technique has the highest reported incidence of anesthesia of structures other than the intended inferior alveolar nerve block,29 and the traditional Halsted technique has the highest reported incidences of trismus and paresthesia, while the Gow-Gates technique has the lowest reported incidences of all types of unusual side effects.31

Because the Gow-Gates mandibular division nerve block technique has the highest reported success rate, including the greatest likelihood of anesthetizing all major mandibular dental nerve pathways (inferior alveolar, lingual, long buccal, and mylohyoid) as well as any accessory or anomalous pathways with just one injection, coupled with the lowest incidence of positive aspiration, muscle trismus, and paresthesia, the Gow-Gates injection technique possesses clear advantages over the other techniques. Of course, it is not without disadvantages: primarily, a slower rate of anesthetic onset (up to 10 minutes for full anesthesia) and a more difficult learning curve.

Conclusion
The selection of a technique for administering local anesthetic injections is important to the overall goals of local anesthetic use in dental procedures: adequate anesthesia to maintain patient comfort throughout the duration of the specific procedure, maintenance of hemostasis when bleeding is anticipated, and delivery of the anesthetic agent of choice in as safe and atraumatic a manner as possible. For specific dental procedures requiring complete quadrant anesthesia of the mandible, the Gow-Gates technique has been shown as the most efficient and the safest technique for administration of all of the anesthetic agents available in dental cartridges.31

Acknowledgments
Figures 1A and 1B courtesy of the Spencer R. Atkinson Library of Applied Anatomy, University of the Pacific, Arthur A. Dugoni School of Dentistry.

References
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Alan W. Budenz, MS, DDS, MBA