This anesthetic injection requires a little amount of the LA solution (0.6 to 0.9 ml) to be deposited near the apices of the premolars, from where it diffuses through the nutrient canals and porous bone to envelop the subneural dental plexus [27]. This results in anesthesia of the area supplied by the PSA, MSA, and ASA nerves [789]. Additionally, the LA solution diffuses beneath the mucoperiosteum of the palate and reaches the branches of the GP and NP nerves. Therefore, it also provides anesthesia to most of the palatal hard and soft tissues [10].

The patient is placed in a supine position with slight hyperextension of the head and neck. After application of a topical anesthetic at the injection site for 1 min, the needle bevel is placed against the palatal tissue and rotated in an axial manner (45° clockwise and 45° counterclockwise) for needle insertion. It is important to inject the anesthetic solution at a uniformly slow rate (0.5 ml/min) [11]. Once an adequate amount of the LA is deposited, the operator should wait for 10 s before removing the needle. This facilitates the dissipation of the LA within the tissue and minimizes dripping during needle withdrawal [7].

Friedman and Hochman reported that each injection required approximately 90 s to administer and the teeth were fully anesthetized within 2 min. The duration of anesthesia lasted between 45 and 90 min [4]. Lee et al. found a gradual onset time ranging from 9 to 26 min, with anesthesia lasting for around 60 min. They evaluated the effect of anesthesia by using an electric pulp tester [7]. In another study by Velasco and Soto on 31 adult patients, the anesthetic induction time of AMSA injections using conventional syringes ranged from 6 to 12 min, and the duration of pulpal anesthesia ranged between 23 and 40 min [12]. However, for periodontal procedures, the duration of anesthesia using AMSA injections has been reported to be in the range of 55 to 65 min [13] and 90 to 120 min [8]. As an exception, Acharya et al. found that in two out of 50 subjects, the effect of anesthesia lasted up to 180 min, especially on the palatal tissues [8]. In general, the duration of periodontal anesthesia appears to be higher than that of pulpal anesthesia.

There is a wide controversy among the publications regarding the extent of anesthesia achieved using AMSA injections. Freidman and Hochman were the first to report that its effect extends from the central incisor to the mesiobuccal root of the first molar [4]. However, most of the studies evaluated pulpal anesthesia and found the effect of AMSA injections to be unpredictable for the anterior teeth and premolars. Velasco and Soto reported that it was completely ineffective in any of the teeth in 26.7% of the patients [12]. Most of the studies reported that whenever it was effective, the extent of pulpal anesthesia was observed from the central incisor to the second premolars, among which the central incisors were found to be least affected [471214]. However, Velasco and Soto found it to be effective on the palatal soft tissue in 100% of patients, with its effect extending from the central incisors to the mesial side of the first molars [12]. Nevertheless, the effect of the injection did not cross the midline in any of the above studies.

Studies evaluating AMSA injections for different periodontal procedures have also reported the varying extent of anesthesia. It appears that the injection is effective for the entire ipsilateral palate, i.e., from the midpalatine raphe to the gingival margins (Fig. 1B) [8151617]. Acharya et al., however, observed that the blanching crossed the midline in some patients, while most of the patients also experienced the effect of anesthesia on the soft palate [8]. Blanching of the soft palate was also reported by Lee et al. [7]. As far as the anesthesia on the buccal periodontium is concerned, the existing literature suggests that the AMSA injection alone is insufficient. Patients often required additional infiltrations and blocks for the anterior teeth [81317] and molars [15], respectively. Tolentino et al., however, reported that the AMSA injection was as effective as local infiltration for anesthesia of the buccal tissues extending from the central incisors through to the premolars [9]. Only one study reported that the AMSA injection was completely effective in the buccal periodontium of the molars [8].

Various advantages of this technique over the conventional nerve blocks and supraperiosteal infiltrations have been reported in the literature. Most important among these is the reduction in the number of penetrations and cumulative volumes of the LA solution and vasoconstrictor [17]. The higher amount of vasoconstrictor may result in a higher risk of local and systemic complications. Necrosis of the palatal soft tissue may occur when a high concentration of vasoconstrictor is used [6]. Tolentino et al. reported that the amount of LA solution administered in local infiltration was 3 times more than that used in AMSA injections to achieve a similar anesthetic effect [9]. Loomer and Perry found AMSA injections to be less painful and less time consuming than the set of conventional injections they replaced [15]. It also spares the upper lip and muscles of facial expression, thereby ensuring the stability of the smile line during aesthetic procedures in the anterior teeth [718]. A randomized controlled clinical trial reported that pain during anesthetic delivery using a conventional syringe was significantly less in AMSA injections than in IO nerve blocks for the extraction of the maxillary anterior teeth [11]. Another split-mouth clinical trial reported no significant difference in pain during AMSA injections and local infiltrations for open-flap debridement. However, the postoperative pain was significantly less in the quadrant operated under anesthesia performed using the AMSA injection [19]. Another advantage of using AMSA injections for periodontal procedures is adequate hemostasis on the palate for harvesting soft-tissue grafts [1317], scaling and root planing (SRP) [16], open-flap debridement, and osseous surgery [17]; in contrast, conventional nerve blocks provide limited hemostasis only in the immediate areas surrounding the injection sites. The disadvantages of AMSA injections include operator fatigue when using a conventional syringe [4] and the frequent need for supplemental anesthesia around the incisors. Rapid injections may also result in ischemia, pain, and ulceration at the injection site [13].

Individual variations in the anatomical nerve pathways in the maxilla are not uncommon, and these affect the successful outcome of different anesthetic injections. In some cases, secondary pulpal innervation may be observed from the GP and NP nerves [20]. This might be a reason for the high success rate of AMSA injections in pulpal anesthesia reported in some studies [141821]. The presence of the MSA nerve is highly variable. It may be duplicated [10] in some individuals, while it may be absent in 28% [22] to 54% [23] of individuals. Other variations in the anatomy that might affect the outcome of AMSA injections are the course of the MSA and ASA nerves, which are the main contributors to the subneural dental plexus. The MSA nerve may originate from any part of the IO canal, while the ASA nerve mostly originates from the midpoint of the canal [6]. The ASA nerve may also arise near the IO foramen [6]. Since the variations in nerve supply cannot be confirmed in clinical settings, these are major limiting factors in the predictability of anesthetic outcomes of AMSA injections.

The rationale of AMSA injections is based on the dissipation of the LA solution through the numerous nutrient canals in the maxilla. Hence, the outcome could be influenced by the variations in the frequency, distribution, and size of these canals. Porosity and thickness of the palatal cortical bone may also play an important role in the effectiveness and extent of anesthesia [2425]. Cetkovic et al. used computed tomography and micro-computed tomography to evaluate these variations in 20 human skulls [25]. They found that despite having more thickness, the palatal cortex was more porous than its buccal counterpart in the region of the AMSA injection. The canals that pass through the whole thickness of the cortical bone are the most important structures providing the pathway to the subneural dental plexus, which lies in the cancellous bone. The number and width of these canals have also been found to be higher in the palatal cortex than in the corresponding buccal cortex. It is important to note that the foramina of the nutrient canals have been found to be wider in female skulls [25], which may ensure better anesthetic outcomes when using AMSA injections in females than in males. However, Tolentino et al. observed that AMSA injections are less effective than supraperiosteal infiltrations in females [9], suggesting the role of other factors as well. Cetkovic et al. have also reported that in female skulls, the majority of these canals were present in the palatal process, while in the male skulls, most of these canals were in the border zone, i.e., the junction of the alveolar and palatal processes [25]. In view of these findings, the recommended sites of AMSA injection may be changed according to the patient's sex to achieve predictable anesthesia.

The perception of pain varies among different individuals because of the difference in their pain threshold. It is affected by several factors like age, sex, stress, anxiety, and previous experience of LA injections [126]. Even cultural factors can affect the pain perception of an individual [27]. Females, in general, are known to have a lower pain threshold than do males [2829]. It is not surprising that even during AMSA injections, females report more pain than do males [9].

Different anesthetic agents have different potential and ability to diffuse through the tissues. Articaine is reported to be 1.5 times more potent than lignocaine, and it has greater ability to diffuse through the tissues owing to its high liposolubility [30]. A meta-analysis also reported that articaine is 3.81 times more likely to achieve anesthetic success than lignocaine, when used for local infiltrations [31].

Saraf et al. compared 2% lignocaine with adrenaline (1:80000) to 4% articaine with adrenaline (1:100000) for AMSA injections as well as IO nerve blocks. They found that articaine was more efficacious than lignocaine for achieving pulpal anesthesia in the maxillary anterior teeth and premolars, irrespective of the injection technique. Articaine also resulted in a faster onset of anesthesia than did lignocaine [30]. Tomić et al. compared the anesthetic efficacy of AMSA injections by using three different anesthetic solutions: 3% mepivacaine plain, 2% lignocaine with adrenaline (1:80000), and 4% articaine with adrenaline (1:100000). They concluded that predictable efficacy of AMSA injections was achieved for the extraction of the permanent premolars, regardless of the LA used [32]. Both of these studies also reported an insignificant difference in pain on injection with different LA solutions and vasoconstrictors [3032]. The conflicting results in these two studies regarding the efficacy of different LA solutions for AMSA injections may be attributed to the intended procedure. Saraf et al. used it for root canal therapy and evaluated pulpal anesthesia, while Tomić et al. used it for the extraction of the premolar teeth, which requires additional effect in the periodontium.

Maximum benefits of AMSA injections are achieved when they are administered using the computer-controlled local anesthetic delivery (CCLAD) system [71516]. A conventional syringe cannot accomplish the simultaneous high pressure and slow speed required for LA delivery [4]. Therefore, palatal injection has been reported to be more painful when using conventional syringes than when using CCLAD systems [1516]. Hochman et al. also reported that injections performed using CCLAD systems are 2 to 3 times less painful than are conventional injections [33]. In another study, 19 out of 20 patients reported that injection using a CCLAD system was less painful than any injections they had received previously [15]. However, few latter studies reported that both the techniques result in similar pain during AMSA injections [303234].

While using a conventional syringe, it is difficult for the operator to apply a constant pressure for a long time while maintaining the recommended speed of LA delivery at 0.5 ml/min. CCLAD systems ensure this unique setting of a constant high pressure along with the simultaneous slow rate of volume flow [4]. Although it takes 4 to 5 min to complete the injection process by using a CCLAD system, it is virtually pain-free [47]. A rapid injection tends to displace the tissues, whereas CCLAD directs the solution through the connective tissue, periosteum, cortical bone, and medullary bone [4].

Many CCLAD systems are currently available in the market. However, the Wand® (Milestone Scientific Inc, Livingston, NJ, USA) was the first device developed for this purpose and is the most commonly reported in the scientific literature [2].