July 1, 2005
by Gregory A. Barnett DDS, Bruce R. Pynn MSc, DDS, FRCD(C) , Robert E. Wood DDS, MSc., PhD, FRCD(C), Le
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Purpose: To determine if there are significant differences in mandibular anaesthesia anatomical hard tissue landmarks in Canadian Caucasians and Aboriginals.
Methods: A total of 97 panoramic radiographs were retrospectively assessed by taking measurements of known hard tissue landmarks relevant to mandibular anaesthesia. The mandibular foramen and the mental foramen were the focus of the investigation.
Results: The distances between the landmarks were found to have a wide range. The position of the mandibular foramen relative to the occlusal plane was found to be more inferiorly placed in Aboriginal females compared with Caucasians, but with a great degree of overlap. The distance between this foramen and the antegonial notch was shorter in Aboriginal males compared with Caucasians.
Conclusions: There is variability in the position of key anatomical landmarks in both populations, which limits the general applicability of recommendations for achieving mandibular anaesthesia. Nevertheless, the data suggest that a lower insertion point in patients of Aboriginal descent may improve the success rate of the mandibular block.
Consistently achieving effective mandibular anaesthesia is one of the challenges in clinical dentistry. At best, the success rate for the standard inferior alveolar nerve block is reported to be only 85 percent,1 with other studies reporting even lower values.2,3 This technique depends on accurate placement of local anaesthetic in the pterygomandibular space where the inferior alveolar nerve enters the mandibular foramen (MandF) on the medial aspect of the mandibular ramus. Its success depends on placing the needle tip in close proximity to the MandF. As this is an internal structure which cannot be palpated clinically, standard teaching is to use landmarks to estimate its location.1 Success also depends on several factors, one being the specific anatomy of the patient involved.4-9 Another is the dentist’s proper interpretation of the presenting anatomy in the form of landmarks to be used to orient the injection approach. Anatomic variability may be one source of local anaesthetic failure.
Variability in the location of the MandF, as noted in previous studies, may lead to failure of anaesthesia. The average position of the MandF has been found to be roughly in the centre of the ramus anterio-posteriorly, or just posterior to this.10-15 If the mandibular notch and inferior surface of the mandible are used as superior and inferior references, the MandF has been found to again be at approximately the mid-point, and has also been described as being two-thirds of the way down a line connecting the tip of the coronoid process and the gonial angle.13
Racial variances in mandibular dimensions have been studied among several Caucasian, African, Asian, and Arab populations.13-16 What is not known, is whether there are significant variations on these mandibular landmarks in Canadian Aboriginal patients compared with those already reported.
An important landmark in carrying out the mental or incisive nerve block1 is the mental foramen (MentF). The position of this foramen has also been studied, with racial differences again being found.17-23 There are no apparent published reports on its relative location in Canadian Aboriginal populations.
The purpose of this study was to determine if there are significant differences in the average positions of these two foramina in relation to clinically useful anatomical landmarks in two groups, Canadian Caucasians and Aboriginals. This information will add to our body of knowledge. The presence of differences may suggest modification of the traditional approach and may assist in improving the success of mandibular anaesthesia.
A total of 97 panoramic radiographs, therefore 194 total sides, were assessed for measurements involving the MandF. There were 49 films from the Aboriginal population group, comprised of 27 females and 22 males. There were 48 films from the Caucasian population, comprised of 31 females and 17 males.
Of the 97 radiographs used, 86 were found to have at least one side meeting the inclusion criteria for assessment of the MentF, resulting in a total of 160 measurements. For this part of the study, the Aboriginal population contained 32 right and 39 left sides, the Caucasian population contained 44 right and 45 left sides.
The panoramic radiographs were obtained from patients’ charts from a single oral and maxillofacial surgery office in northwestern Ontario. All films were from a retrospective source from patients who already had these radiographs taken, as they had been required as part of routine dental treatment. No person was directly or indirectly exposed to ionizing radiation for the purposes of this study. The radiographs were taken from this retrospective source in a manner that protected the identity of the patient. The radiographs were coded and classified by the oral and maxillofacial surgeon who did not take part in the actual measurements. The films were then provided to the examiner, who did not know whose radiograph was being assessed. The assortment of radiographs were blinded and randomized in order to assure no recorder bias during the measuring process. Patient anonymity was ensured throughout.
Patients were classified as belonging to the Aboriginal population group if they possessed a Band Number. In this study, these patients were primarily those of Ojibway descent. The radiographs from the Caucasian population were collected from the same geographical region as the Aboriginal population. Patients were classified as belonging to the Caucasian population at the determination of the patient’s oral and maxillofacial surgeon.
All radiographs were taken on the same machine, a Siemens Orthopantomograph 5. Patients had to be 15 years of age or older at the time the radiograph was taken. To assess the MandF, at least 3 of the 5 posterior teeth (2 premolars, 3 molars) in the quadrant had to be present. To assess the MentF, both premolars in the relevant quadrant had to be present to evaluate the MentF. Those quadrants containing premolars that showed excessive tilting or drifting were excluded from assessment of the MentF.
The bony landmarks used in this study are illustrated in Fig 1. Most of these are the same as used previously in the study by Afsar et al,12 and are similar to those used by Nicholson,13 with additions used to evaluate the MentF. The distances between the landmarks and the mandibular foramen or mental foramen were measured to the closest half-millimetre. The first 10 radiographs measured were re-measured at the end of the study to assure consistency in measuring technique.
The mean, standard deviation, and range of the measurements were tabulated. The averages for distances and ratios were analysed to determine whether a significant difference existed between any of the racial groups. Linear regression was performed using a Huber/White robust variance estimator to account the repeated measurements (left and right sides) taken on each patient.26,27 Males and females were modelled separately. Because ten distances were compared, a *Sidk adjustment for multiple comparisons was employed, making the test for statistical significance more stringent.28 Differences were judged significant if the *Sidk-adjusted p-value was less than 0.05.
The mean and standard deviations for the measurements of the Aboriginal and Caucasian populations are shown in Table 1.
The average differences between the Aboriginal and Caucasian distances are shown in Table 2. Two results were statistically significant. In females, the MandF-OP distance was 3.8mm shorter in Aboriginals than in Caucasians. In males, the MandF-AnteGo distance was 3.1mm shorter in Aboriginals than in
Caucasians. Male Aboriginals in this sample also had, on average, a shorter MandF- occlusal plane distance (3.4mm), but that result was not statistically significant.
No statistically significant difference was shown for the position of the MentF between the two groups in regard to any landmarks used in this study. The differences found in the ratios were also non-significant.
A substantial amount of variability was noted among study subjects and between left- and right-side measurements. The boxplots in Figure 2 depict the range of variability in the difference between the left- and right-side measurements. Note that the width of the boxplots is substantially larger than the Caucasian-Aboriginal differences presented in Table 2. Figures 3 & 4 put the two statistically significant differences between Aboriginals and Caucasians (MandF-AnteGo in males and MandF-occlusal plane in females) in the context of left-right differences and the variability between patients.
These results clearly demonstrate the variability of the location of both the MandF and MentF. The variability in the position of the MandF confirms previous findings,12 and may explain one of the reasons for failure of the traditional inferior alveolar nerve block. As before, these results support the use of the Gow-Gates or Vazirani-Akinosi mandibular nerve blocks, which access the inferior alveolar nerve superior to its entry into the MandF, and are therefore less dependent on a predictable position.
The variability among individuals would appear to override inter-racial differences in most cases. The 3.8mm average difference the MandF-occlusal plane distance between Caucasians and Aboriginal females is only 13 percent of the range of measurements (-10,19). However, Figure 2 also shows that there are a substantial number of outliers. 90 percent of the MandF- occlusal plane measurements are within the range (-4,11) = 15. Removing the outliers, the 3.8mm average difference amounts to 25 percent of trimmed range. As well, the MandF-AnteGo distance in males has an average difference of 3.1mm, which accounts for 11 percent of the range. However, 90 percent of the MandF-AnteGo measurements lie within the range (33,48.5) = 15.5. In this context, a difference of 3.1mm amounts to 20 percent of the range.
The MandF-occlusal plane distance is normally utilized when performing a standard inferior alveolar nerve block. This result suggests that the technique for the standard inferior alveolar nerve block could be improved by lowering the insertion point of the needle in the patient of Aboriginal descent compared with the Caucasian. Therefore, instead of placing the needle at the greatest depth of the coronoid notch, it may need to be placed approximately 3.5mm inferiorly. The weakness in this recommendation is the high variability in the location of the MandF between and within the two racial groups, as noted in Tables 1 & 2. Therefore, the evidence may not be strong enough to warrant arbitrarily altering the technique for either group, unless there is history of difficulty achieving mandibular anaesthesia. Such a history may merit modification of standard recommendations.
Similarly, the lack of differences in the measurements involving the MentF, would warrant against any recommendation to alter the technique for the mental or incisive nerve blocks.
In conclusion, this study confirmed that the precise MandF location is highly variable, and also showed that there is variability in the MentF position. This study found that this variability exists in both populations. Because of this unpredictability, it is difficult to assume that there will always be clinically significant differences in the mandibular anaesthesia landmarks between Canadian Caucasian and Aboriginal patients, which could form the basis for recommending an overall adjustment to mandibular anaesthesia approaches. Although not entirely conclusive, the data do suggest, however, that a lower insertion point for standard mandibular blocks may be required in an Aboriginal patient. This latter recommendation should be considered if there is a history of prior anaesthetic failure. Further studies would be required to confirm this conclusion.
The authors wish to thank Diane Scott, illustrator, Paleontology Lab, University of Toronto at Mississauga, for preparing Figure 1.
Gregory A. Barnett was a dental Student at time of project. Currently in Orthodontic Residency, University of Alberta.
Bruce R. Pynn is in private practice in oral and maxillofacial surgery, Thunder Bay, ON.
Robert E. Wood is Associate Professor, Princess Margaret Hospital, Faculty of Dentistry, University of Toronto.
Lee Sieswerda is an Epidemiologist, Thunder Bay District Health Unit, Thunder Bay, ON.
Daniel A. Haas is Associate Dean Clinical Sciences, Faculty of Dentistry, University of Toronto; Professor and Head of Anaesthesia, Faculty of Dentistry; and Active Staff, Sunnybrook and Women’s College Health Sciences Centre.
Oral Health welcomes this original article.
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