Innovations in Local Anesthesia are Easing the Pain of Dentistry

It took nearly a century from the time cocaine and its local anesthetic properties were identified and isolated in 1860, to its topical application for ophthalmologic surgery, to the introduction of needles and syringes that could permit nerve block and infiltration anesthesia, to the synthesis of procaine and other ester anesthetics, until the discovery of lidocaine, the first amide local anesthetic introduced into dental practice. The glass cartridge system, containing formulations of the highly effective amide anesthetics lidocaine, mepivacaine, prilocaine, articaine, and bupivacaine, has become one of the most sophisticated single-dose sterilized packaging systems ever developed.

Today, the solution of 2% lidocaine with 1:100,000 epinephrine remains the most versatile and popular local anesthetic formulation used in the United States.¹ It is ostensibly the "gold standard" to which all other local anesthetics are compared. The 2% lidocaine 1:50,000 epinephrine formulation provides the therapeutic benefit of enhanced hemostatic properties, which is particularly useful for oral and periodontal surgery. Articaine, introduced in the United States in 2000, has gained popularity among dentists because of its superior onset, duration, potency, and tissue diffusion properties. Mandibular infiltrations have been found to aid in establishing profound anesthesia when inferior alveolar block anesthesia is incomplete.² Bupivacaine, the only long-acting local anesthetic used in dentistry, has found an important niche in the management of postoperative pain. The 6 to 8 hours of potential anesthesia and analgesia seen following bupivacaine injection can minimize postoperative pain after oral surgical procedures such as third-molar extractions and thereby possibly limit the need for prescription opioid analgesics. Mepivacaine and prilocaine are the only local anesthetics in dentistry formulated without a vasoconstrictor and have found a role in treating cardiovascularly impaired patients and other medically compromised patients who may not tolerate a local anesthetic containing a vasoconstrictor.

The amide agents currently available in dentistry are extremely safe and fulfill most of the characteristics of an ideal local anesthetic (Table 1). These local anesthetic agents can be administered with minimal tissue irritation and with little likelihood of inducing severe allergic reactions. The available agents and formulations provide rapid onset and can be tailored to surgical procedures of various durations. The agents offer anesthesia that is completely reversible, and systemic toxicity is rarely reported. Case reports of children experiencing local anesthetic toxicity have been scarce in the past 25 years because of education toward more conservative dosage calculations, improved emergency care, and greater awareness of the potential for local anesthetic toxicity among dental practitioners. Unfortunately, the ideal local anesthetic agent, one that would induce regional "analgesia" by selectively inhibiting only pain pathways without interrupting transmission of other sensory and motor nerve functions, has yet to be discovered.³

Using these safe and effective amide local anesthetics, various innovations have been advanced in dentistry that provide valuable options for managing the needs of unique patient populations and fulfilling specific procedural requirements. With the development of specialized high-pressure syringes, single-tooth anesthesia can be delivered using periodontal ligament (PDL) injection techniques. Such techniques provide a valuable alternative when block anesthesia is inadequate. Similarly, intraosseous injections have become increasingly popular because of improved armamentarium.⁴ Advances in computer technology have also been applied to dental anesthesia to control the rate of administration, thereby limiting injection discomfort.

Additional innovative strategies to advance the field of dental anesthesia have been introduced in the past decade. A small startup company recognized the desire to reverse the numbing sensation in soft tissue following routine local anesthesia in certain patient populations. The pharmacologic strategy for reversal of local anesthesia is based on the use of an alpha adrenergic antagonist, phentolamine, that opposes the vasoconstrictive effects of epinephrine. By administering phentolamine, the vasoconstrictive properties of epinephrine can be reversed and the duration of soft-tissue anesthesia significantly decreased. In essence, the formulation of 2% lidocaine with 1:100,000 epinephrine, where the duration of lip and tongue anesthesia may last 3 to 4 hours, more closely resembles 2% lidocaine plain (with a soft-tissue duration in the 45- to 60-minute range) following the phentolamine injection. This product has been shown to accelerate the return of soft-tissue sensation and function, the loss of which is associated with mandibular and maxillary dental anesthesia, by about 50%.⁵The product has a niche among adult patients who find prolonged anesthesia annoying and dysfunctional. Local anesthesia reversal may prevent children and special-need patients from chewing and mutilating their lips following dental procedures.

A needleless anesthetic for maxillary anesthesia that has been developed and marketed employs technology that uses an intranasal spray delivery system of 0.2 mL per dosing unit. The anesthesia blocks the anterior and middle superior alveolar nerves. The anesthetic solution contains tetracaine (30 mg/mL) and is combined with the vasoconstrictor oxymetazoline (0.5 mg/mL) to improve the duration of anesthesia. With repeated intranasal sprays, pulpal anesthesia of maxillary premolars, canines, and incisors can be achieved without the use of a needle injection.⁶ This approach has found a place in treating patients who are needle phobic and may be advantageous with younger children.

Several companies are developing local delivery preparations that may extend the duration of local anesthesia beyond a few hours to potentially a few days. If postoperative pain can be safely and effectively blocked for an extended period, the need for opioid analgesics may be diminished. Although several strategies are being developed, the product currently on the market uses the long-acting local anesthetic bupivacaine encapsulated in a formulation of liposomes. The proposed mechanism for the prolonged anesthetic is a delayed, sustained breakdown of the liposomes in tissue resulting in prolonged release of anesthetic at the injection site.⁶ Studies are underway to demonstrate the utility of this potentially opioid-sparing strategy.

In medicine, it has been known for decades that combining the two topical local anesthetics lidocaine and prilocaine creates a eutectic mixture (EMLA) that increases the efficacy of topical anesthesia. A product for use in dentistry has been developed that provides an alternative to dental injections to establish the soft-tissue anesthesia required for periodontal procedures such as deep scaling and root planing.⁷ The product used in dentistry is formulated with an additive that has reverse thermoplastic properties. The formulation, packaged in a glass cartridge, is a liquid at room temperature but converts to a gel at body temperature after application into a periodontal pocket. The efficacy of this product appears to be enhanced because the gel remains in the pocket for an extended period of time.

Dental local anesthetic cartridges contain solutions that are formulated at an acidic pH (2.5 to 6.5) to maintain the stability of local anesthetics in an aqueous solution. The higher acidity may cause a stinging sensation when infiltration anesthesia is administered. Various systems that buffer the solution with a small amount of sodium bicarbonate have been developed for dental practice. Many patients report less discomfort upon injection, and the time needed to establish profound anesthesia may be decreased.⁸

Each of these advances is aimed at better achieving the goals of an ideal local anesthetic for dentistry. Some newer treatment strategies to improve patient comfort come from procedures that are less invasive and require little to no anesthesia. For example, the routine use of sealants and the introduction of silver diamine fluoride can help temporarily arrest dental caries and reduce the need for invasive painful operative procedures.

Translating these highly advanced technologies into everyday dental practice can be challenging. Some of these innovations can be expensive and their potential advantages may not warrant the costs. Others may necessitate additional chairtime and, therefore, may be difficult to integrate into routine practice. Additionally, the advantages of these new and innovative local anesthetic strategies may be thwarted by a lack of patient acceptance for nasal spray administration or prolonged facial numbness.

Dentistry has come a long way from when cocaine was discovered and the therapeutic value of local anesthesia was first realized. The safety and effectiveness of dental local anesthesia has now been established. Dental practitioners continue to pursue anesthetic innovations that meet the needs of their practices and provide better pain control and satisfaction to their patients.

Paul A. Moore, DMD, PhD, MPH
Professor, Pharmacology and Dental Anesthesiology, Department of Dental Public Health,
University of Pittsburgh School of Dental Medicine, Pittsburgh, Pennsylvania