Our results showed that the operators experienced difficulty in controlling the flow rate of the anesthetic (only 14% of the trials were without pulses), particularly during the first few seconds. They also suggested that men inject faster than women and practitioners faster than students.
This experiment was performed ex vivo with no natural tissue resistance. A previous study showed that intracartridge pressures generated when injecting into air are less than those needed to inject into tissue.12
Furthermore, when tissue resistance increases, the neuromuscular system of the practitioner instinctively reacts by exerting more force on the plunger.13
The present study must thus be regarded as prospective and will have to be extended using conditions more closely resembling those encountered in vivo. Our assessments mainly focused on injection speed and pulses generated during the SI.
Injection speed is indicative of the initial needle entry pressure and the needle exit pressure, which is related to the initial pressure, the needle diameter, and the injection site.13
Although injection times were significantly shorter with the 27-gauge needle, the mean time for emptying a full cartridge generally exceeded 60 seconds for both gauges, which is longer than recommended and reported in the literature.14
Only 17% of the operators emptied more than 25% of a cartridge in 15 seconds using the 30-gauge needle, indicating that there was a certain degree of control of the initial pressure with the 30-gauge that was not observed with the 27-gauge needle, where 46% of the operators emptied more than 25% of a cartridge. When the pressure is too high, the drop-by-drop flow turns into pulses of anesthetic solution associated with the high pressure around the site of injection and possibly with pain.
Pulse frequency and moment of onset may influence the pain felt by the patient. Tissue overpressure–related pain may appear, especially at the beginning of the injection before the anesthesia has taken effect. The early pulses are thus those that may trigger injection pain. In nearly two thirds of the SIs, the first pulse occurred during the first 3 seconds whereas no pulses were observed in the first 15 seconds in less than one third of the SIs, irrespective of the needle gauge. This may be due to the higher force required to mobilize the plunger at the beginning of the injection compared to the force needed to maintain its motion.8
Furthermore, at the beginning of an injection, the plunger is completely out of the body of the syringe, leading to poorer manual control of the pressure. However, the threshold of interstitial overpressure associated with pain is difficult to determine.
A recent study conducted using a computerized anesthesia delivery system (the Wand) suggests that 306 mm Hg of pressure is the threshold for triggering pain, but with major variations depending on the patient's stress level.9
The level of pain may also depend on the injection site. Injections in high-density tissues, such as intraligamentary and palatal infiltrations (palatal approach to the anterior superior alveolar nerve block [P-ASA], palatal approach to the anterior middle superior alveolar nerve block [A-MSA]), potentially generate more pain because of higher pressures required for the injection.4,13,15–,17
The use of computer-assisted injection systems might be a solution for these problems of overpressure,8,11,18
although the potential improvement may be less evident in anxious patients.10,19
These computer-assisted systems administer anesthetic drop by drop during the initial stages of the injection, with a slow increase in the injection rate over time.
In the present study, given the limits imposed by the experimental conditions, the practitioners performed faster SIs than the students, with significantly more pulses during the first few seconds. The lack of experience likely made the students more careful. It cannot be ruled out that some of the students and practitioners understood the hidden nature of the study and tried to change the way they injected, thus introducing bias into the study. On average, men emptied a cartridge faster than women and performed more SIs with pulses.
These results are in agreement with previous findings indicating that men generated higher pressures than women during periodontal ligament injections.17
Other studies have also shown that female students have a different attitude toward dental anesthesia and tend to be more stressed.20,21
In addition, female dentists are more concerned about causing pain to patients22
and are more motivated by human-oriented factors than their male counterparts.23
The present study also revealed gauge-related differences, with higher injection speeds and more frequent pulses during the first 3 seconds with the 27-gauge needle. Given that we performed an ex vivo study, we did not assess the intensity of the pulses and, consequently, we could not verify whether the pressure generated by the pulses would have caused pain. However, the larger gauge would distribute the pressure over a larger area (30-gauge, 0.071 mm2
, compared to 27-gauge, 0.126 mm2
), which would reduce the risk of generating interstitial overpressure. As such, the larger volume injected with the 27-gauge needle might be compensated by the higher pressure generated by the 30-gauge needle. This complexity might partially explain why studies on the relationship between needle gauge and pain and, as a result, recommendations on the appropriate needle gauge, are contradictory.4,9,24,25
Within its methodological limits, the present study showed that it is difficult to control injection pressure with a metal syringe, especially during the first few seconds of injection. The intensity of the pulses and thus the putative intratissular pressure was not evaluated. It is likely that a powerful stream of anesthetic solution is potentially more likely to generate intratissular pain than a steady stream at fairly low pressure. More studies are needed to determine whether all pulses can induce overpressure-related pain. Pulses occurred most frequently during the first 3 seconds, which is when the tissues are unlikely to be anesthetized. These pulses thus appear more likely to cause pain-generating interstitial overpressure. The injection mode and rate varied widely among operators and with needle size. Practitioners, especially men, emptied the cartridges more rapidly, which, in vivo, would increase the risk of generating high interstitial pressure, thus causing pain. These problems might be overcome by injecting the anesthetic solution using computer-assisted systems to ensure a steady flow rate.