Dimitrios Apostolakis Jackie E. Brown John Wiley & Sons A/S. symphysis

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1 Dimitrios Apostolakis Jackie E. Brown The anterior loop of the inferior alveolar nerve: prevalence, measurement of its length and a recommendation for interforaminal implant installation based on cone beam CT imaging Authors affiliations: Dimitrios Apostolakis, Maxillofacial Radiology and Diagnosis Center, Chania, Crete, Greece Jackie E. Brown, Department of Dental Radiology, King s College London Institute of Guy s, King s and St. Thomas Hospitals, London, UK Corresponding author: Dr Dimitrios Apostolakis Maxillofacial Radiology and Diagnosis Center Plateia 1866 No 39, Chania Crete, Greece Tel.: Fax: dentalradiology@hotmail.com Key words: anterior loop, cone beam computed tomography, inferior alveolar nerve, symphysis Abstract Objectives: Interforaminal implant surgery requires anatomical knowledge of the area and adequate information on the location of the various landmarks of significance such as the mental foramen, the anterior loop of the inferior alveolar nerve and the mandibular incisive canal. Cone beam computed tomography (CBCT) is a relatively new imaging modality that provides a multi-dimensional view of the facial skeleton with, in most instances, lower radiation dose to the patient compared to medical CT. The present study aims to use CBCT to identify and measure variation in the presence and extent of the anterior loop of the inferior alveolar nerve. This information may be used to provide recommendations to the surgeon without access to a 3D scan of the dento-alveolar region. Material and methods: Ninety-three patients scanned with a Newtom VG device for a variety of clinical indications were included in this retrospective study. Using the multiplanar capabilities of the device s software the prevalence and length of the anterior loop was assessed. Results: The results show that an anterior loop could be identified in 48% of the cases with a mean length (range) of 0.89 mm (0 5.7). Conclusions: In almost half of the surveyed cases an anterior loop was present. Even though in 95% of the study cases the loop was <3 mm, a 100% safety margin in the placement of anterior mandibular implants, in the absence of a CBCT scan, would only be achieved with a distance of 6 mm between the anterior border of the mental foramen and the most distal interforaminal implant fixture. Date: Accepted 30 May 2011 To cite this article: Apostolakis D, Brown JE. The anterior loop of the inferior alveolar nerve: prevalence, measurement of its length and a recommendation for interforaminal implant installation based on cone beam CT imaging. Clin. Oral Impl. Res. 23, 2012, doi: /j x The final part of the inferior alveolar nerve sometimes passes below the lower border and the anterior wall of the mental foramen. After giving off the smaller mandibular incisive branch, the main branch curves back to enter the foramen and emerge to the soft tissues, as the mental nerve. The section of the nerve in front of the mental foramen and just before its ramification to the incisive nerve can be defined as the anterior loop of the inferior alveolar nerve. Selective surgery in the area of the anterior mandible such as implant installation in the interforaminal region or symphysis bone harvesting, may violate the anterior loop resulting in neurosensory disturbances in the area of the lower lip and chin (Misch & Crawford 1990; Wismeijer et al. 1997). To avoid such a sequel a 5-mm safe distance to the most distal fixture from the anterior loop (Magnusson 1992) and a 5-mm distance from the mental foramen for chin bone harvesting have been proposed (Hunt & Jovanovic 1999). Even though these general safety margins do exist, the problem relates to the ability of the surgeon to identify the anterior loop preoperatively or even intra-operatively to safely plan his actions, thus avoiding the risk of altered lip and chin sensation and permitting the placement of the implant fixtures in a more favourable position from a prosthetic standpoint (i.e. close to the mental foramen increasing the available useful interforaminal space) John Wiley & Sons A/S

2 Exposure of the mental foramen during implant surgery in the symphysis area provides a direct view of the mental nerve. Safe implant placement would always be in the space above the level of the foramen. However, in many cases the limited bone available in the anterior mandible above that level and the need for longer implants will force the surgeon to place the distant implant at or more commonly below the level of the foramen risking violation of the anterior loop, if present. Clinical identification of the anterior loop with the use of a probe has been suggested; however, it is recognized that it is not possible to differentiate between an anterior loop and an incisive canal, by probing (Greenstein & Tarnow 2006). Radiography provides the clinician with information not readily available by any other diagnostic method. However, the ability of conventional two dimensional radiological methods (panoramic tomography, periapical radiographs etc.) to reveal the anterior loop is limited and their reliability and accuracy, questioned (Mraiwa et al. 2003; Jacobs et al. 2004; Ngeow et al. 2009). In recent years the use of medical CT with special dental software programs has been recognized as a useful adjunct to implant surgery (White et al. 2001). However, its use is complicated by limited availability of implant software, cost and the potentially high radiation dose to the patient. It might be because of these reasons or because old habits die hard that panoramic radiography is still used by the great majority of surgeons as the only imaging modality when it comes to implant installation in the mandible (Zitzmann et al. 2008). This is especially true for the more experienced of the surgeons, who advocate the placement of implants using only a panoramic radiograph (Vazquez et al. 2008). Even though panoramic tomography has some reliability when it comes to the identification of the mental foramen (Yosue & Brooks 1989), this is not the case with the anterior loop (Mraiwa et al. 2003; Jacobs et al. 2004; Ngeow et al. 2009), raising amongst others, questions of liability in the case of an untoward sequel. Cone beam computed tomography (CBCT) is a relatively new imaging modality which provides multiplanar views of the facial skeleton with a reduced radiation dose, compared to the most commonly used by MDCT, exposure protocols (Tsiklakis et al. 2005; Ludlow & Ivanovic 2008; Suomalainen et al. 2009) whilst the accuracy and reliability of the measurements done with a CBCT device has been proved by a number of in vitro studies (Lascala et al. 2004; Baumgaertel et al. 2009; Liu et al. 2010; Sherrard et al. 2010). Taking into account the inability of the clinical and common radiographical methods to give accurate information about the anterior loop, it is the aim of the present observational study of 3D data to provide evidence and recommendations about the safe distance of the most distal interforaminal implant from the anterior border of the mental foramen. These recommendations are based on details of the prevalence and on the measurements of the length of the anterior loop acquired with the use of a CBCT device in a population of patients. Material and methods From a pool of 320 CBCT consecutive scans taken for various clinical indications such as implant planning, trauma, assessment of impacted teeth, etc. using a Newtom VG CBCT device in a private radiological practice, cases were selected for the measurements of the anterior loop length (ALL). The selection criteria were: (i) the front part of the body of the mandible bilaterally, at least 2 cm distal to the mental foramen and up to the lower cortical border, had to be included in the volume (ii) no pathology that could affect the position of the mandibular canal and mental foramen should be identified by imaging or history and (iii) the images must be of adequate diagnostic quality. A total number of 320 volumes were examined. The first selection criterion (the border of the mandible depicted) was satisfied by 101 volumes. Of these 101 volumes, five were excluded due to pathology affecting the image (one implant in mental foramen, one osteoradionecrosis, one giant cell lesion, one dense bone island, one artefact due to gunshot pellets). Furthermore, three volumes were excluded due to motion artefacts that rendered the images non-diagnostic. Therefore 93 volumes, representing 93 different patients, were available for evaluation. The cases were categorized by side (left or right), gender, age, dental status and mode of scanning. For statistical analysis, the patients were divided into six age groups: (i) 21 30, (ii) 31 40, (iii) 41 50, (iv) 51 60, (v) and (vi) years. The cases were considered dentate when even one tooth was present in front of the mental foramen. The scanning of the patients was accomplished using, for each patient, one of the following imaging protocols available with this CBCT unit: Standard mode with a field of view (FOV) of cm and a 0.3-mm voxel size, Zoom mode (FOV cm, 0.24 mm voxel size), High resolution mode (FOV cm, 0.15 mm voxel size) and each always included the full area of interest. There were five patients scanned with high resolution mode, 16 patients scanned with zoom mode and 72 patients scanned with standard mode. The KVp was 110 kv, the exposure time ranged between 3.6 and 5.4 s, whilst the ma setting was computed by the device based on the anatomy of the patient within a range of 1 20 ma. There were 51 female and 42 male patients. The mean age was 53 years and the range was between 21 and 89 years. Most of our patients (82%) were between 41 and 70 years old. There were three edentulous and 90 dentate patients. Measurements All the reconstructions and measurements were accomplished with the use of the propriety Newtom VG software (NNT 2.19/ NNT 2.21). On each volume the axial slices were reconstructed parallel to the lower border of the mandible and on the appropriate selected axial slice the most anterior part of the mental foramen was marked (Figs 1 and 2). Then using again the axial views, the most anterior part of the inferior alveolar nerve was marked. It was defined as the most mesial area of the mental nerve just before a sudden reduction of the width (constriction) of the nerve was noted as the incisive nerve divided to pass anteriorly in the incisive canal (Fig. 3). It was not clear in every case, however, where the end of the supposed anterior loop was. So the mark was taken as an indication of the border of the anterior loop and the loop was again evaluated and marked on the cross sections and on all other available reconstructed Fig. 1. Lateral scout view to identify and position the region of interest. The axial images were reconstructed parallel to the lower border of the mandible John Wiley & Sons A/S 1023 Clin. Oral Impl. Res. 23, 2012 /

3 Fig. 2. Axial reconstruction; the anterior border of the mental foramen (right side) is identified. Fig. 3. Axial reconstruction; the anterior border of the anterior loop is seen as a constriction of the canal. The narrowest position of the mandibular canal-incisive canal complex is marked as an indication of the anterior border of the anterior loop. In the image shown the mark is absent to demonstrate the difficulty in the accurate positioning of the border of the anterior loop. views. Based on the available literature of the size of the incisive canal, we devised a cut-off point of 3 mm for the maximum diameter of the incisive canal. That is, a canal of more than 3 mm was always considered part of the mandibular canal (anterior loop) and never of the mandibular incisive canal. The length of the loop was measured by counting the number of the consecutive contiguous vertical cross sections performed between the anterior border of the mental foramen and the anterior border of the loop (Fig. 4). This number was multiplied by the thickness of the slices. The initial slice for measurement was considered to be the first slice just after the anterior margin of the mental foramen disappeared. For example in a case where the number of cross sections between the anterior border of the mental foramen and the edges of the anterior loop were 6, the length of the loop would be = 1.8 mm (in standard mode). For the development of our method and for the evaluation of its precision a small pilot study was undertaken. The available literature on a protocol on the measurement of the anterior loop on a CBCT or even on a medical CT is limited. Actually there are only four studies that tried to measure the anterior loop using CT or CBCT (Jacobs et al. 2002; Kaya et al. 2008; Uchida et al. 2009; Watanabe et al. 2010). In the first three studies all the measurements were done on vertical cross sections on printed film (no other slices were used). The method of Uchida et al. (2009) used the capabilities of the software to do the measurements. We decided to employ the method used by Uchida et al. (2009). In their study in seven cadaver hemimandibles they used transverse (not vertical cross sections) reconstructions to measure the length of the anterior loop. The linear distance between the most anterior border of the mental foramen to the most anterior margin of the anterior loop was defined as the ALL. Five cases (10 sides) were evaluated with this method. Fifteen days later we re-examined the cases and we discovered large variations in our measurements. It became obvious that the above method was not precise enough, maybe due to the different capabilities of the software used in our case. After that we decided to use all the available reconstructions, achieved by our software, to mark the nerve and finally perform the measurements on the vertical cross sections. We re-examined the five cases 2 weeks later to discover adequate intra-rater reliability. These five cases that were measured four times were used for the development of the instructions and for training at the same time. All measurements were done by the first of the authors with more than 15 years of experience in oral surgery and in dental implant installation with the use and interpretation of DentaScan CT software and 3 years recent experience in the acquisition, manipulation and interpretation of CBCT images. The second author, a consultant in dental and maxillofacial radiology in a large London teaching hospital, provided advice on the method and the interpretation of the images. Statistics Descriptive The prevalence of the anterior loop was calculated and an ALL distribution chart was produced (Fig.5). The mean values, range, SD of the ALL measurements were calculated and categorized by side, gender, age group, dental status and mode of scanning. From Fig. 5 it can be seen that our data were severely skewed. As a consequence, median and interquartile values were also calculated to give a better description of our data set (Table 1). Comparative To compare the measured values for differences between gender, mode and dental status, multiple group comparisons were made using Kruskal Wallis ANOVA whilst the Mann Whitney U test was used for two group comparisons. These analyses were performed for each side (left or right) separately. Differences between left and right sides were investigated using the Wilcoxon matched pairs, signed-rank test. The results were considered significant where P Spearman correlation was used to estimate the relationship between the length of the anterior loop and age. The difference in age between the presence and absence of the anterior loop was assessed using a Mann Whitney U test. Ten cases (20 sides), representing the 11% of the total cases were re-examined after a month by the same examiner. Intraclass correlation was used to provide an estimate of the reliability of the measurements. Also the range of absolute errors between the two measurements and the average absolute mean error of the two measurement attempts was calculated. Finally, Bland Aldman analysis was undertaken to investigate whether or not there is a relationship between the difference between the two measurements and the mean size of the ALL. Results The ALL was evaluated in all 93 patients (186 sides). An anterior loop was identified in 91 sides (48% of the sides). The mean and the range of the ALL were 0.89, mm, respectively, whilst the median and the inter Clin. Oral Impl. Res. 23, 2012 / John Wiley & Sons A/S

4 Fig. 4. Cross-sectional reconstructions; the anterior loop can be seen on the images No (arrows). The length is measured as = 1.5 mm (standard mode) Table 1. Our findings on the length of the anterior loop of the inferior alveolar nerve Anterior loop length (mm) No of cases quartile range values were 0.0 and mm, respectively. The 95% confidence intervals for the median were mm. Table 1 summarizes our findings on the length of the anterior loop. In Figs 6 8 the images of the longest anterior loop, and in Figs 9a,b and 10 the images of another case of anterior loop, are shown. In 62% of the cases with an anterior loop, the length was up to 1 mm, whilst it was up to 2 mm in 85% of the cases. Up to 3 mm was in 95% of the cases. Two per cent of the sides showed an ALL more than 4 mm (Fig. 5). The longest loop measured was mm Fig. 5. Length of the anterior loop. Group Range Median IQR Mean SD All (n = 186) Right (n = 93) Left (n = 93) Gender Male (n = 84) Female (n = 102) Age (n = 8) (n = 18) (n = 52) (n = 52) (n = 48) (n = 8) Dental status Dentate (n = 180) Edentulous (n = 6) Mode Standard (n = 144) Zoom (n = 32) Hi Res (n = 10) mm with one more loop measuring 5.2 mm, on patients aged 57 and 49 years old, respectively. In 95 sides (95/186, 52%) no loop could be identified. In 84% of these cases (80 sides, 40 patients) the absence of the loop was bilateral and in 16% (14 sides, 14 patients) an anterior loop could not be identified only on one side of the mandible (it was visible on the other side). Analysis of differences revealed no statistically significant differences between ALL and mode of scan, dentate or edentulous patients. A significant difference was revealed with 2011 John Wiley & Sons A/S 1025 Clin. Oral Impl. Res. 23, 2012 /

5 Fig. 6. Cross-sectional reconstructions through the body of the right mandible. The longest anterior loop; = 5.7 mm. The first slice for the measurements is No 205 and the last is No 223. Mental nerve/anterior loop Fig. 7. Cross-sectional slice No 212 (2.4 mm in front of the mental foramen). The anterior loop of the inferior alveolar nerve can be seen as a double canal (arrows). B = buccal and l = lingual. Fig. 8. Para-sagittal section of the same case illustrating measurement parameters where R = posterior and L = anterior. the ALL being longer on the right side of the mandible (P = 0.025). Spearman correlation coefficient revealed no relationship between ALL and age (r = 0.092, P = 0.211). There was also no difference in age between the presence and absence of the anterior loop (P = 0.248). In the reproducibility study the difference between each measurement ranged between 0 and 2 mm with a mean absolute difference of 0.5 mm. The intraobserver agreement, as calculated with intraclass coefficient was excellent, with r = for the left side and for the right side (Table 2). Bland Aldman analysis of the results revealed that a change on the mean length of the loop is not related to a change in the difference (error) between the measurements. The arithmetic mean of the error was 0.24 mm (95% CI 0.58, 0.1) with the apparent bias just expected by the sampling variation (Fig. 11). Discussion There are a number of studies, where the authors using various methods, (anatomical, radiographical and combined), have attempted to measure the length of the anterior loop of the inferior alveolar nerve (Greenstein & Tarnow 2006; Uchida et al. 2007, 2009). The actual existence of the loop has been debated (Rosenquist 1996) and large variations on the mean length and on the range have been noted. To our knowledge this is the first study to use CBCT scans of actual patients to identify the anterior loop. Concerning the methods used, different studies have shown that panoramic radiographs do not accurately identify the incidence or the extent of the anterior loop (Arzouman et al. 1993; Kuzmanovic et al. 2003; Jacobs et al. 2004; Kaya et al. 2008; Ngeow et al. 2009). Considering therefore that the validity of the studies where panoramics were used as a sole method for detection and measurement is questioned, it is our intention to exclude these particular studies from the present discussion. In almost half of our cases (48%) an anterior loop could be identified radiographically. This lies approximately in the middle of the range of existing anatomical studies reporting either no anterior loop or a universal existence of an anterior loop. Considering that CBCT has been used only once for the measurement of the anterior loop and in a limited number of cadaver mandibles (Uchida et al. 2009), the current study is useful in illustrating the value of CBCT in implant assessment. Kieser et al. (2002) in an anatomical study reported that no anterior loop could be found in a large sample of Negro, Maori and Caucasian units (skulls and cadavers). It is striking, however, that in that study the main pattern of emergence for the mental nerve was type 1026 Clin. Oral Impl. Res. 23, 2012 / John Wiley & Sons A/S

6 (a) (b) 4.8 mm Fig. 9. (a) Para-sagittal slice through the anterior loop of the mandible of another case. R = anterior and L = posterior. (b) Measurement of the size of the anterior loop. R = anterior and L = posterior. The estimation of the anterior loop length is not based on this image only. 1, a posterior inclination, which is shown in a diagram in their article. It is obvious that this pattern of emergence creates an anterior loop that would have been accounted for, in our study. The conclusions of this study came in support of earlier studies of Rosenquist (1996) and Bavitz et al. (1993) where a small number of short anterior loops were identified. The conclusions of Rosenquist (1996) were derived from measurements taken during surgical procedures performed on actual patients. Taking into account the various factors that could have affected the surgical field, their conclusion should be viewed with caution. A number of anatomical studies present a prevalence of anterior loops that could be Fig. 10. Cross-sectional slice through the right body of mandible just anterior to the mental foramen. The anterior loop is seen as a double canal (arrows). considered slightly higher or lower than the number of loops identified in our study (Solar et al. 1994; Kuzmanovic et al. 2003; Hu et al. 2007; Uchida et al. 2007, 2009) whilst Arzouman et al. (1993) reported a 100% incidence of anterior loops with Neiva et al. (2004) following with 88%. On the right side of the male patients the loop was statistically longer. This finding suggests clinically insignificant gender and side relationship with the length of the anterior loop. The CBCT resembles medical computerized tomography on the bone windowed images it produces and in the capabilities of the software. Results from CBCT and CT Table 2. The reproducibility study. The values are in mm imaging of the mandible would therefore be expected to be similar. Jacobs et al. (2002) reported an incidence of 7% on the anterior loop on their CT sample whilst Kaya et al. (2008) 34%. In the most recent CT study Watanabe et al. (2010) reported a prevalence of 55%, a percentage that lies very near our finding. The mean length of the anterior loop in our study was 0.89 mm whilst various studies report a mean length ranging from 0.4 to 6 mm. The longest loop in the literature is reported by Neiva et al. (2004) being 11 mm, followed by Uchida et al. (2009) with a length of 9 mm. Our longest anterior loop was 5.7 mm whilst another case had an ALL of 5.2 mm. Taking into account the preference of the surgeons for panoramic radiographs when implants are placed into the anterior region of the mandible along with the limited accuracy of the panoramics concerning the anterior loop, our results may suggest a strategy for the surgeon who exposes the mental foramen to place implants in the interforaminal region and a computed tomography scan is not available. In 52% of the cases in this study no loop was found to exist, whilst in 95% of all the cases where the loop did exist the anterior loop was measured with CBCT, up to 3 mm. Placing the implants 3 mm from the anterior border of the mental foramen can be considered safe even without the use of CBCT for the great majority of the cases (95% of the sites). However, since the longest loop identified in our study was 5.7 mm and in 5% of the cases the anterior loop was more than 3 mm, there is always the possibility that a Patient no Measurement 1 Measurement 2 Absolute difference L R L R L R L R L R L R L R L R L R L R Mean R, right; L, left John Wiley & Sons A/S 1027 Clin. Oral Impl. Res. 23, 2012 /

7 Measurement 1 - measurement SD Mean SD Average of measurement 1 and measurement 2 Fig. 11. Bland Altman plot with the arithmetic mean ( 0.24 mm), the 95% CI of the limits of agreement (mean ± 1.96SD) and the 95% CI of the mean of differences. long anterior loop may be encountered during implant surgery (Figs 12 14). This may explain the findings of Wismeijer et al. (1997) who, in their prospective study of 110 edentulous patients found sensory dysfunction at the area of the lower lip in 7% of the cases, 16 months after the operation, even with a safety margin of 3 mm. The 100% safe distance, without the use of CBCT would be 6 mm, limiting the space available for implant placement to the diameter of approximately three implants. The suggestion that the implants must be placed 5 mm from the loop (Magnusson 1992) limits even more dramatically the interforaminal space. It should also be remembered that in many situations the most distal implant should be located as close as possible to the mental foramen, to extend the distal cantilever as far as it is biomechanically tolerable (Bou Serhal et al. 2002). Therefore when a CBCT scan is not available a 100% safety in the placement Anterior loop Fig. 13. Raycast 3d rendering of the same patient. The mandibular canal has been marked using a panoramic reconstruction of the data and the Newtom VG software. The anterior loop is represented as the part of the canal between the yellow lines. Measurement of the length is not based on this image. of implants may be achieved, but at the expense of the available space; not an advisable strategy from a prosthetic standpoint. Using 93 CBCT volumes, the incidence and length of the anterior loop of the inferior alveolar nerve was examined for this study. A very low number of edentulous cases (on the anterior mandible) were examined. It seems that even though a large number of edentulous patients are restored with implants, panoramic radiographs remain the preference of the dentists for the anterior region. A similar low number of cases scanned with high resolution and zoom imaging protocols were examined. Even though the smaller voxel size of these modes should increase the identification of small structures, any difference between these modes and standard mode could not be statistically proven in our study, due to the low number of the scans undertaken with zoom and high resolution protocols. So the larger mean values of the ALL for high resolution and zoom modes identified in our study (1.06, 1.00 mm, respectively) when compared to the standard mode (0.85 mm) may show a trend but it is not statistically significant. Sources of errors in linear measurements in this study include the voxel size, the partial inclusion of endpoints within the first and last cross-sectional slices, the partial volume averaging effect, small movement artefacts, artefacts introduced by dental materials, limited contrast resolution, mouse sensitivity and radiologist s time. Concerning the last, it became obvious during our study that identification of the various landmarks is not a trivial task and took considerable time to perform. Frustration and time restrictions on a busy radiological practice may influence the accuracy of the measurements. Experience in the identification of the anatomy and in the use of the software was felt to increase the accuracy of the measurements in the real clinical situation; a fact that needs further investigation. These errors, however, are random in nature and no systemic errors have been introduced in our study. The reliability of our measurements was high (r = for the single observer) even though it must be stressed that appropriate selection and manipulation of the reconstructed images was paramount for the reliable identification of the anterior loop. In support of that came the estimation of the absolute error between the two measurements. The mean absolute difference between the two measurements was 0.5 mm, a number that can be considered small. However, the largest absolute difference was 2 mm. Even though we could be considered experienced users of the software, a 2-mm possible difference between the measurements would be clinically significant in borderline cases with limited interforaminal space. Inexperienced users and surgeons with a limited time available may expect to vary Fig. 12. The mental foramen of one of the patients examined. The volume 3d reconstruction is done with the Newtom VG software. No anterior loop can be identified in this view, as it is the case in a real clinical situation. Fig. 14. Another patient. Panoramic reconstruction, thin section. The mandibular canal, the anterior loop and the mandibular incisive canal are depicted. The red line marks the anterior border of the mental foramen. The green line marks the anterior border of the anterior loop. The distance between the 2 lines is measured as 4.2 mm. Placement of the implant 4 mm from the anterior border of the mental foramen may violate the anterior loop in this case. Measurement of the anterior loop length in this study was not based on this image alone Clin. Oral Impl. Res. 23, 2012 / John Wiley & Sons A/S

8 more in their measurement of the anterior loop. The identification of the anterior loop was based on signs such as the existence of two separate canals beyond the anterior border, an oval or elongated shape of the same canal and finally on its size. Concerning the size of the anterior loopincisive canal complex a review of the literature on the diameter of the incisive canal was undertaken to clarify the subject. In the studies of Mardinger et al. (2000) and Bavitz et al. (1993) anatomical measurements were included and the diameter of the incisive canal ranged between 0.5 and 2 mm. However, there are two studies from the same group of researchers that report a maximum incisive canal diameter of 6.6 mm (Uchida et al. 2007, 2009). The size of the mandibular canal has been reported as ranging from 2 to 5 mm (Rajchel et al. 1986; Ikeda et al. 1996; Sato et al. 2005) with the actual mean nerve size being 2.2 with a SD of 0.4 (Ikeda et al. 1996). Since the incisive nerve is a part of the inferior alveolar nerve we hypothesized that its diameter should always be less than the size of the mandibular canal, i.e. <5 mm and we devised a cut-off point of 3 mm for the maximum diameter of the incisive canal. We feel that this cut-off point includes the vast majority of the true maximum diameters of the incisive canal at its origin. Even though it seems possible to encounter incisive canals with larger diameters, the possible overestimation of the ALL resulting by our method is clinically more significant than its probable underestimation. Another possible drawback of this study was the fact that the measurements were performed by the same observer, allowing some consistency but this always includes the possibility for methodological bias. In conclusion, CBCT scans of patients, taken for various reasons, were used to provide information on the prevalence and on the length of the anterior loop of the inferior alveolar nerve. 1 In 48% of the cases an anterior loop was identified. 2 The mean length (range) of the anterior loop was 0.89 mm (0 5.7). 3 No statistically and clinically significant differences between the various groups examined were identified. 4 The intrarater reliability of our measurements was excellent. Finally taking into account the inability of two dimensional imaging modalities to accurately and reliably depict the anterior loop and that in almost half of our cases an anterior loop existed and was measured with a length of up to 5.7 mm, we recommend the use of CBCT for implant planning in the anterior region. If this is not possible, our findings suggest that a safe distance of at least 6 mm between the anterior border of the mental foramen and the most distal interforaminal implant fixture must be observed. Acknowledgement: The authors would like to thank Dr Wilson Ron, Statistical Advisor, King s College London for his help with the statistics of this paper. References Arzouman, M.J., Otis, L., Kipnis, V. & Levine, D. (1993) Observations of the anterior loop of the inferior alveolar canal. International Journal of Oral & Maxillofacial Implants 8: Baumgaertel, S., Palomo, J.M., Palomo, L. & Hans, M.G. (2009) Reliability and accuracy of conebeam computed tomography dental measurements. American Journal of Orthodontics and Dentofacial Orthopedics 136: Bavitz, J.B., Harn, S.D., Hansen, C.A. & Lang, M. (1993) An anatomical study of mental neurovascular bundle-implant relationships. 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