Discussion - References.
A number of studies have assessed the success of mandibular incisor anaesthesia following different techniques. These studies have shown that when using block anaesthesia successful anaesthesia of molar pulps is not an indication of successful incisor anaesthesia. Hinkley et al. (1991) reported that the lateral incisor was anaesthetized in 36% of cases, whereas the molar was successfully anaesthetized in 54% of cases when lidocaine with adrenaline was used for inferior alveolar nerve blocks. In the similar study of Clark et al. (1999), the corresponding figures were 50% and 73%. In this last study, the central incisor was successfully anaesthetized in only 33 % of cases. Using different methods of approach to the nerve may increase successful anaesthesia of the inferior alveolar nerve. ‘High’ approaches such as the Gow-Gates (Gow-Gates1973) and Akinosi-Vazirani techniques (Akinosi1977 ) may help counter accessory supply from nerves such as the mylohyoid (Wilson et al. 1984 ) and auriculotemporal (Heasman & Beynon 1986). Indeed, impressive success rates have been reported with the Gow-Gates method (Malamed 1981). Nevertheless, even complete anaesthesia of an inferior alveolar nerve may not provide satisfactory anaesthesia of the mandibular central incisor due to cross-over from the contralateral inferior alveolar nerve. Similarly, the effectiveness of incisive nerve blocks differs between the teeth. A volunteer study has shown that pulpal anaesthesia is not as reliable following incisive nerve blocks compared to inferior alveolar nerve blocks (Nist et al. 1992). That study showed that incisive nerve block anaesthesia was poor for mandibular anterior teeth but that premolar pulpal anaesthesia of short duration could be obtained. The use of Infiltration anaesthesia in the mandibular incisor region is described in dental local anaesthetic texts (Jastak et al.1995, Robinson et al.2000) and is used in practice. Some recommend the technique as a means of blocking contralateral inferior alveolar nerve supply (Rood 1977, Jastak et al. 1995). Indeed, Rood (1977) reported 100 % success for pulpal anaesthesia of mandibular central incisors following a combination of inferior alveolar nerve blocks and labial Infiltration in the mandibular incisor region. However, there is little information in the literature concerning the efficacy of Infiltration anaesthesia in the adult anterior mandible. A recent study by Yonchak et al. (2001) reported on the use of labial and lingual Infiltrations in isolation to produce anaesthesia of the mandibular anterior teeth in adults. These workers noted no difference in success between the two types of Infiltration but suggested that the low incidence of successful anaesthesia (27=63%) argued against the use of these methods in this region of the mandible.
The incidence of 50% successful anaesthesia of mandibular central incisors following buccal Infiltration recorded in the present study is identical to that reported by Haas et al. (1990) for anaesthesia of mandibular canines following buccal Infiltration when using 4% prilocaine. It is also identical to the success rate for mandibular central incisor pulpal anaesthesia reported by Nist et al. (1992) following the combination of inferior alveolar and incisive nerve block injections with 2% lidocaine with 1:100 000 adrenaline. It is lower than the 63% success recorded by Yonchak et al. (2001) following buccal Infiltration of 1.8 mL 2% lidocaine with 1:100 000 adrenaline for mandibular central incisors. It is better than the 33% incidence of mandibular central incisor pulpal anaesthesia reported by Clark et al. (1999) following inferior alveolar and mylohyoid nerve blocks with 2% lidocaine and 1 :100 000 adrenaline. Lingual Infiltration alone also produced success in 50% of cases in the present investigation. This is similar to the 47% success rate reported by Yonchak et al. (2001) following the lingual Infiltration of lidocainewith1:100 000 adrenaline in the mandibular incisor region.
The results of the present study are similar to those reported by Yonchak et al. (2001) when the labial and lingual Infiltration methods were used in isolation. The important result from the present study is the ￠finding that the combination of labial and lingual Infiltrations was more likely to produce anaesthesia of mandibular central incisor pulps than either of these approaches used in isolation. This improved efficacy is not due to an in crease in local anaesthetic volume or concentration as the same amount was used during the dual technique as was injected with a single labial or lingual Infiltration. A possible explanation for the increased effectiveness of the combination is that the lingual Infiltration may counter any accessory supply to the pulps from the lingual and mylohyoid nerves (Wilson et al.1984).However, the use of lingual Infiltration anaesthesia on its own was no better than a labial Infiltration. It is interesting to note that Clark et al. (1999) found no increase in incidence of mandibular central incisor anaesthesia between an inferior alveolar nerve block administered alone or in combination with a mylohyoid nerve block. However, that combination would not counter contralateral inferior alveolar and mylohyoid nerve innervation; such collateral innervation would be affected in the present study. When anaesthesia was achieved in the present study there was no difference in duration between the three Infiltration techniques. This supports the view that the combination of labial and lingual injections may be more effective than either of these injections alone due to the blocking of transmission in more than one nerve supplying the tooth.
The anaesthetic effect peaked for buccal Infiltration and the combined Infiltration method 8 min after injection; following lingual Infiltration the peak effect was at 10 min. These data are similar to those reported by Yonchak et al. (2001) and reinforce the message that a reasonable time should elapse after the administration of an anaesthetic prior to stimulating the pulp. The data here would suggest that allowing10 m into elapse before insulting the pulp following Infiltration anaesthesia in the mandibular anterior region is wise.
The success of intraligamentary anaesthesia has been reported to differ between teeth. Cowan (1986) found significant differences in the efficacy of intraligamentary anaesthesia between the jaws, recording 55% success in the mandible compared to 83.3%inthemaxilla.White et al. (1988) investigated the efficacy of intraligamentary anaesthesia with different teeth and reported the least success for pulpal anaesthesia occurred with mandibular lateral incisors (18.2% success); mandibular central incisors were not investigated in that study. In the present investigation, intraligamentary injections produced no case of pulpal anaesthesia for mandibular central incisors. There are a number of explanations for this. Firstly, it has been suggested that the definition of anaesthesia used in this trial may reduce the incidence of success when compared to subjective responses to clinical challenges (White et al.1988). One obvious cause of failure is poor technique, however, the operator performing this study was experienced in the method and had used it successfully on many occasions. The recommended dose during intraligamentary anaesthesia is 0.2 mL per root although there is no explanation as to why this dose was chosen. The syringe used in this study delivered increments of 0.06 mL per lever depression and three depressions gave an administered dose of 0.18 mL. It is possible that this dose (10% lower than that usually recommended) is the cause of the poor efficacy. In addition, there are anatomical explanations for failure. Intraligamentary anaesthesia is a form of intraosseous anaesthesia (Smith & Walton1983) and to be successful solution has to enter the cancellous space via the cribriform plate of the alveolus. In the mandibular anterior region, there is a limited zone of cancellous bone in the inter-radicular area. In addition, there are fewer perforations in the cribriform plate in this region compared to more posterior areas of the jaws and this will limit access of the anaesthetic solution into the cancellous bone (Birn 1966,White et al. 1988). Finally, the design of this study would not allow detection of any early transient anaesthetic effect on the pulp following intraligamentary anaesthesia as the first stimulus was performed 2 min after injection.
A number of studies have considered injection discomfort. Nakanishi et al. (1996) demonstrated variations in the pain threshold at different sites in the mandibular alveolar mucosa. These workers reported that the mucosa in the incisor and canine region had a lower pain threshold than the premolar and molar areas. It has been claimed that intraligamentary injections are less painful than inferior alveolar nerve blocks (Grundy 1984) but produce similar discomfort to Infiltration anaesthesia (Kaufman et al.1984). There is a variation between individuals having intraligamentary injections at similar sites and some individuals report the method as painless (D’Souza et al. 1987, Meechan & Thomason 1999). However, it is certainly not a completely painless technique (Kim1986). The results of the present investigation suggest that in the mandibular incisor region intraligamentary anaesthesia is more uncomfortable than labial and lingual Infiltrations; half of the volunteers recorded a visual analogue scale score representing at least moderate discomfort during the intraligamentary injection. Labial and lingual Infiltrations did not differ in the discomfort they produced, only three out of 24 (12.5%) administrations being considered moderately uncomfortable. This finding is similar to that of Nist et al. (1992) who noted that18%of their subjects reported discomfort that was considered moderate to severe following incisive nerve blocks.
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