Pain has been defined by the International Association for the Study of Pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” [20]. The purpose of using topical anaesthesia before dental injection is to eliminate or reduce the pain as effectively and non-invasively as possible. Although previous authors have questioned the effect of topical anaesthesia before injection in the oral cavity [7], many studies have shown a good effect [5, 6, 21, 22].
A confounding factor in the study of pain and discomfort is that the sensation and feelings are subjective, leading to difficulties in objectively comparing experiences of pain and discomfort from different individuals [23]. We therefore decided to compare pain from one side to the other in the same patient. Lidocaine 5% gel was used as a control substance to compare with the ice, since its effect has been described earlier [21, 22] and it is commonly used as topical anaesthesia in dentistry [4]. To minimize the discomfort and pain, and to standardise the injection speed, we used a 30-gauge ½ inch needle along with a computerised standardised anaesthetic injection device, The Wand, as described in a previous study [24].
We decided to use pulse change and VAS ratings as indicators of pain and discomfort, as these have been found to be reliable indicators of a patient’s response to pain [23, 25] and have also been used for evaluation of discomfort [24, 26]. The study was designed as a randomized split-mouth crossover study, which reduced the risk of bias since each patient received both treatments. This gives more reliable results, since there is a variability between different patients’ response to pain [27]. To minimize the risk for bias related to the evaluation of the buccal site, topical anaesthesia on the palatal site was not applied until after the completion of injection and evaluation of the buccal side.
In 2009 Aminabadi and Farahani [14] stated that only one previous study [13] had investigated the effects of local mucosal cooling prior to infiltration of local anaesthesia in dentistry. Even if later published studies have shown results indicating that pre-cooling is effective, the pre-cooling can be administered in many different ways and in combination with various other techniques. An article from 2015 [16] stated that even if the use of cryoanesthesia to reduce injection pain had been reported to be promising, only sparse literature reports exist regarding the clinical efficacy of these agents. Our study is an effort to increase the available publications in this field of knowledge.
Many of the published articles dealing with pre-cooling of oral mucosa prior to dental injection have used ice in combination with pharmaceutical topical anesthetics [28,29,30], why the genuine effect of the cooling has not been evaluated in those studies, as it was done in our study.
The study design had two main limitations. Firstly, it was unblinded, which always increases the risk of bias. Unfortunately neither patients nor clinicians could have been blinded, since the temperature difference between lidocaine 5% gel and ice, and the different methods of application, made it obvious which method was used. Secondly, although a control group treated with placebo could have made the results even stronger, we felt that ethical considerations ruled this out as an option, since we did not wish to cause any painful experience that could increase the risk of dental anxiety.
The VAS is a frequently used reproducible method for the measurement of painful procedures [23, 31, 32]. The only difference between the genders was that the girls scored the VAS pain at injection higher than boys, which is in accordance with several other reports [2, 24, 33]. Generally speaking, the VAS pain intensity ratings in our study were low, for both ice and lidocaine gel and at all time points, when compared to medical pain ratings [34]. Several previous studies – using ice as a pre-cooling agent before dental injection - have only evaluated a shorter time for application of the cooling agent; usually 1 min [16, 28,29,30]. In our study the ice was kept in place during 5 min before injection was performed, and evaluation of insertion pain was measured three times during that time. Our results showed that there was no additional reduction of insertion pain after a 1-min application time for either ice or gel, which makes it reasonable to suggest that for both these methods an application time of 1 min is sufficient to reach superficial topical anaesthetic effect before injection using a 30 gauge needle. An earlier study concluded that lidocaine 5% gel might provide a better superficial anaesthesia of the oral mucosa if the application time was extended to 15 min [24], although the recommended application time on the summary of product characteristics is 2–3 min [35]. Our findings suggest that this application time can be reduced to 1 min, but since this is only applicable to the superficial insertion pain, it is necessary to perform the injection gradually and very slowly to avoid pain from tension in the tissue. It is also important to slowly increase the depth of the needle into already anaesthetized tissue before further injection of local anaesthesia can be performed by stages, in order to make the injection as pain-free as possible.
The finding that a palatal injection was more painful than a buccal injection is well known to dental clinicians, and is probably due to the absence of free mucosa on the palatal side. The fact that the mucosa of the hard palate is highly keratinized and very dense and tight [36] makes the injection pain from pressure in the tissue more prominent. Several studies have reported failure to satisfactorily anaesthetize the palatal site [37,38,39,40].
Our finding that a significantly higher proportion of the participants reported a bad or very bad taste following treatment with lidocaine 5% gel on the buccal site, in comparison to when ice was used on the same site, confirms that the patient’s experience of topical anaesthesia can be improved if a different substance is used or if the method for application is improved. Previous studies have also emphasized the bad taste experience associated with anaesthetic gels [41].
The simple method for topical anaesthesia of oral mucosa (using a plastic syringe filled with frozen water for direct application of ice on the oral mucosa) described in our manuscript has not been published previously, and an evaluation of the comparison between this method and Lidocain 5% gel for topical anaesthesia of the oral mucosa has therefore not been presented heretofore. The methods used by several other authors have included ice incapsulated in plastics [14, 29] or in empty cartridges of glass filled with ice [30] or ice-filled cotton buds [28]. All these mentioned methods have the disadvantage of not having direct contact between the ice and the mucosa. Under such circumstances the plastics or glass will isolate the ice from the surrounding. The superficial part of the ice will melt first and melted water will surround the ice (when covered by plastics or glass), diminishing the temperature effect from the ice on the oral mucosa after a short time of application. When an ice-filled cotton roll or cotton bud is used, the ice on the surface will melt first and after a short time there will be no ice in direct contact with the oral mucosa. The temperature on the oral mucosa will therefore be higher for all those other methods than when ice is in direct contact with the oral mucosa. The method described in our study makes it possible to have direct contact between the ice and the oral mucosa during the entire time of application (Fig. 6).
In a study by Lathval et al. [16] a custom made ice cone was used for topical anaesthesia of the injection site, but the size and shape of the ice cone was not described, neither how the ice cone was kept in place [16]. Jayasuriya et al. [28] stated that a limitation of the method when ice is placed directly on the oral mucosa, is that ice will slip from the operators hand due to wetness and will cool the operators fingers [16]. These problems are, anyhow, not present when our described method is used, since the operator’s fingers are only in contact with the plastic syringe and will not touch the ice itself (Figs. 5 and 6).
No previous study has presented the temperature of the ice used for cooling of the oral mucosa. The temperature of the cooling agent is important, since too low temperature or too long application time may cause frostbite to the oral mucosa – especially if other cooling agents than ice are used [16].
The method described for application of ice on oral mucosa is readily available, and according to the dentists in our study is easy to perform. It could therefore be a good alternative to the commercially available lidocaine 5% gel, especially when other types of topical anaesthesia are not available, or if certain pharmacological components should be avoided due to the risk of allergic reactions or intoxication [10, 42].
Regarding heart rate, the negative Pearson correlation between baseline pulse and age confirmed the findings of previous studies that HR decreases with age [43].We chose to evaluate the relative HR change rather than the actual HR change, to avoid bias from diverging HR at baseline. Unfortunately, the HR from 13 patients was not measured immediately before each intervention, and so their values could not be included in the evaluation of relative HR change. The results showed no significant difference in relative HR change between ice and lidocaine 5% gel, meaning that the two methods have a similar effect on HR change. The significantly larger reduction in relative HR when using lidocaine 5% gel after palatal injection, compared to buccal injection, indicates that the gel had a better pain-reducing effect on the palatal mucosa than on the buccal mucosa. When using ice, there was no significant difference in HR change following injection between the buccal and palatal sites, indicating that topical anaesthesia with ice had similar pain-relieving effects on both the buccal and palatal mucosa. A limitation of the HR analysis was that data were only available from 27 of the 40 patients. However, we believe that this is an acceptable number of patients from which to draw conclusions, since previous similar studies have also included around 30 patients [24, 25, 37, 44, 45]. Another limitation is that HR is a very unspecific metric, and that it can e.g. be influenced by memories or emotional reactions, not least related to smell [46].
The individual comments by the patients showed an obvious variety of preferences. While the effectiveness of the topical anaesthesia was important to several patients, comfort was more important to others. This implies that a continuous communication with the patient is important in choosing the best method of topical anaesthesia for each individual.