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Comparing the pH Change of Local Anesthetic Solutions Using Two Chairside Buffering Techniques
Jason H. Goodchild, DMD; and Mark Donaldson, PharmD
BACKGROUND: Alkalinization, or buffering of local anesthesia, is a well-documented technique to mitigate low pH levels of the preparations with reports indicating clinical benefits such as decreased onset time and injection pain. OBJECTIVE: Two methods for buffering local anesthetics are available to dentists: hand-mixing with 8.4% sodium bicarbonate or using the Onpharma® mixing system. This in vitro study compared the pH buffering ability of these two methods in seven commercially available dental local anesthetic preparations. METHODS: The authors prepared unbuffered and buffered samples of each local anesthetic preparation. The buffered samples were prepared to a 9:1 ratio (local anesthetic to sodium bicarbonate) using the Onpharma mixing system and by hand mixing. Sample pH levels were recorded using a pH meter. The samples were stored and retested after 3 and 7 days; pH was recorded via the same pH meter. RESULTS: Regardless of buffering technique used, the pH measurements of each sample increased. For six of the seven preparations, there was no significant difference between the buffering from the Onpharma mixing system and hand mixing. The 2% lidocaine with epinephrine 1:50,000 groups showed a significant difference in pH levels between the Onpharma mixing system and hand mixing (7.09 versus 6.90 respectively, P = .022). In all cases, pH of the buffered solutions continued to rise by day 3, and in most cases rose just slightly more by day 7. CONCLUSIONS: For all but one local anesthetic preparation, there was no significant difference between the pH buffering ability achieved by the Onpharma mixing system and hand mixing. The pH levels of all buffered samples increased over time. CLINICAL IMPLICATIONS: Dentists can buffer local anesthetic preparations by using the Onpharma mixing system or by hand mixing and achieve similar results. More research is needed to establish the proper ratio for buffering of 4% local anesthetic solutions. It is recommended to use buffered preparations immediately after mixing. The data from this study indicate a further alkalinization of the solution over time, and the impact of using stored solutions clinically is not clear.
Local anesthetic preparations are commonly packaged at an acidic pH measurement to increase shelf life and stability.1 Under normal conditions, local anesthetics exist in two forms: a charged form that is soluble in water and an uncharged, base form that is insoluble in water. The relative proportion of these two forms is dependent on the pH measurement of the solution and can be calculated using the Henderson-Hasselbalch equation.2
For solutions containing a vasoconstrictor, the pH range can be approximately 3 to 5, and it is believed that this low pH levels can contribute to slow onset and injection-site pain.3 To mitigate the effects of low pH-level local anesthetic solutions, the addition of sodium bicarbonate to buffer or alkalinize the pH measurement closer to physiologic range has been widely studied in medicine and dentistry. Because the uncharged, base form of local anesthetics is lipid soluble and will readily cross nerve membranes, alkalinization of solutions to drive the stoichiometric equation toward more uncharged local anesthetic molecules should lead to a faster, more effective, and profound local anesthesia clinically.
In two early in vitro studies, it was shown that buffering local anesthetics with sodium bicarbonate can potentiate their nerve impulse-blocking action on peripheral nerves.4,5 More recently, an in vivo study by Malamed et al6 demonstrated a statistically significant decrease in onset time when patients received buffered lidocaine with epinephrine compared with an unbuffered solution for an inferior alveolar nerve block. In that study, the time it took for 70% of participants to achieve pulpal anesthesia after administration of buffered and unbuffered lidocaine was 1:57 minutes and 8:30 minutes, respectively.
In 2010 the Food and Drug Administration approved the Onpharma® mixing system (Onpharma Inc., www.onpharma.com) for buffering of lidocaine. The mixing system consists of three parts: the Onpharma® mixing pen, the Onpharma® cartridge connector, and the Onset® Sodium Bicarbonate Injection, 8.4% USP Neutralizing Additive Solution. On the mixing pen, the desired amount of sodium bicarbonate can be selected via a numbered volume dial; the manufacturer recommends the addition of 0.18 mL for a 9:1 ratio of lidocaine to sodium bicarbonate.7
The aim of this in vitro study was to compare the pH buffering of seven commercially available local anesthetic preparations using a chairside hand-mixing technique with commercially available 8.4% sodium bicarbonate, versus the commercially available Onpharma mixing device with the Onset Sodium Bicarbonate Injection, 8.4% USP Neutralizing Additive.
Materials and Methods
Seven commercially available local anesthetic solutions were supplied by DENTSPLY Pharmaceutical (www.dentsply.com) for pH testing (Table 1). All samples were prepared and stored in a temperature-controlled room at 23°C ± 2°C for the entire experiment. The study design involved preparation of two separate samples for each local anesthetic solution, both the unbuffered and buffered. Testing of sample pH level was accomplished using an Accumet® Research AR50 Dual Channel pH/Ion/Conductivity Meter (Fisher Scientific, www.fishersci.com) calibrated with reference solutions of pH levels of 4, 7, and 10. After each pH reading, the probe was wiped clean and the meter was recalibrated before switching to a different local anesthetic solution. Before each sample was tested, it was mixed for 10 seconds with a new mixing stick and then immediately tested.