The anterior loop is an extension of the inferior alveolar nerve, anterior to the mental foramen, which loops back to exit the mental foramen. This structure is important in determining a safe interforaminal area for the placement of dental endosseous implants and other surgeries such as open reduction of a mandibular fracture and genioplasty to prevent neurosensory disturbances. Several studies have been performed to detect and measure the anterior loop length using different diagnostic methods on panoramic radiographs; cone beam computed tomography (CBCT) and CT scans of patients and cadaver. ¹ , ² , ³ , ⁴ , ⁵ Studies have shown panoramic radiographs are not reliable because of high rates of false positive or false negative interpretations, so CT scans and CBCT are recommended before the surgeries in interforaminal region. Among them, CBCT is preferred because of lower radiation doses. ³ , ⁴ , ⁶ Because of lack of studies on Iranian population, the present study was conducted to assess the prevalence and length of the anterior loop of mental nerve using CBCT and to compare the differences between age, gender, and side.

A total of 180 CBCT images, taken from the patients for diagnostic purposes in the Department of Maxillofacial Radiology at Isfahan school of dentistry from 2010 to 2015, were analyzed by two maxillofacial radiologists on an LG LED computer viewer (E2042C, Korea) using Sirona Galelio's software. The images were divided between the two maxillofacial radiologists. For inter-rater reliability, one-third of the images which had been checked by one of them were randomly chosen and checked by another one, and then Kappa and interclass correlation coefficient were calculated. All CBCT images were taken by Sirona Orthophos, GALILEOS version 1.7, (Sirona, Germany) with a flat panel detector. The adjusted scan parameters were 85 kVp and 10–42 mA, depending on the size of patients. The exposure time was 14 s, the effective exposure time was 2–6 s and the voxel size was 0.3 mm × 0.3 mm × 0.3 mm. The inclusion criterion consisted of the patients older than 18 years whose skeletal growth was completed (partial or full edentulous and dentate patients). The exclusion criterion comprised of the patients with pathologic lesions in the mandible.

CBCT projections were analyzed in different sectional planes (tangential, cross-sectional, and axial). The inferior alveolar nerve of mandible was determined by the nerve option of GALILEOS software on both sides of each projection. To measure the length of anterior loop in tangential plane, two parallel lines from the anterior point of mental foramen and anterior point of anterior loop were drawn using length measuring option on Galelio software. The distance between these two lines was measured by drawing a perpendicular line on them and was considered as the length of anterior loop Figure 1. Figure 2shows anterior loop, mental foramen, and inferior alveolar nerve. Comparisons were made between genders, sides, and mean ages. Figure 1

The length of anterior loop on tangential plane.

From 180 CBCT images, 84 and 96 images belonged to men and women, respectively. The mean age of the patients was 48.6 years. There were 75% partial edentulous, 17.2% full edentulous, and 7.8% dentate cases in the right side and 75% partial edentulous, 16.7% full edentulous, and 8.3% dentate cases in the left side. Mental foramen was present in both sides in all of the images.

The Presence of anterior loop

A total of 59 (32.8%) images had the anterior loop and 121 (67.2%) images had no anterior loop on both right and left sides. McNemar's test showed no statistically significant differences between the right and left sides in terms of the presence of anterior loop (P = 0.99). Forty two images had bilateral anterior loop.

The frequencies of the presence of anterior loop in both genders in the right and left sides are shown in Table 1. The results of Chi-square test did not show significant differences between the presence of anterior loop and gender (the right side P = 0.64 and the left side P = 0.43).{Table 1}

In the right side, the mean age of samples with anterior loop was 47 (standard deviation [SD] = 13.17) and that of the samples without anterior loop was 49 (SD = 14.23). The results of t-test did not show a statistically significant difference between the presence of anterior loop and age in the right side (P = 0.45). In the left side, the mean age of samples with anterior loop was 46 (SD = 12.44) and that of the samples without anterior loop was 49 (SD = 14.50). The results of t-test did not show a statistically significant difference between the presence of anterior loop and age in the right side (P = 0.27).

The results of Kappa coefficient between two observers were: Kappa = 1.00 in the right side (P< 0.001) and Kappa = 1.00 in the left side (P< 0.001).

Length of anterior loop

The mean length of anterior loop in the right side was 2.69 mm (SD = 1.56), (the minimum length = 0.44 mm and the maximum length = 7.52 mm). In the left side, the mean length of anterior loop was 2.36 mm (SD = 1.16), (the minimum length = 0.68 mm and the maximum length = 6.19 mm). The findings of t-test did not show statistically significant differences between the mean lengths of anterior loop in both sides (P = 0.18).

The mean lengths of anterior loop in both genders in the right and left sides are shown in Table 1. The findings of t-test did not show statistically significant differences between the lengths of anterior loop and genders (the right side P = 0.33, the left side P = 0.62).

There were no statistically significant differences between the mean age of samples and the mean length of anterior loop (right side: R =0.150, P = 0.26 and left side: R = −0.146, P = 0.27).

The results of interclass correlation coefficient (ICC) between two observers were: ICC = 1.00 in the right side (P< 0.001) and ICC = 1.00 in the left side (P< 0.001).

The successful placement of dental implants depends on proper diagnosis and pretreatment planning. The anterior loop is an important structure in the interforaminal area of mandible. Great care is required in surgeries of this area to prevent mental nerve injury. The anterior loop has been evaluated by different radiographic methods, including panoramic radiography, CT, and CBCT. Panoramic radiography, which is a two-dimensional conventional method, has a limited potency to detect and trace the anterior loop. Studies have shown that panoramic radiography is not sufficient for presurgical implant planning in the mental region and may need to be supplemented with other modalities such as CT for better visualization of the area. ² , ¹³ , ¹⁴ The CT and CBCT allow three-dimensional imaging without magnification and distortion. Of these, CBCT has the advantage of providing high-quality images with less radiation dose than CT. ⁴ , ⁷ , ¹⁰

In this study, on CBCT images of Iranian population, anterior loop was present in 32.8% of image samples. The presence of anterior loop shows a high diversity in different studies. In a study on 230 spiral CT scans in Belgium, the anterior loop of mandibular canal was visualized in 7% of the cases. ⁷ A total of 34.4% of panoramic images had anterior loop in Ngeow et al.'s study carried out in Malaysia. ² From several studies shown in Table 2, it can be concluded that the prevalence of anterior loop is varying from 0% to 88% among different populations. These wide ranges of the incidence may be related to interindividual anatomical variability associated with race. It has been found that the incidence of visualization of anterior loop in the studies on cadavers and three-dimensional images is higher than that of panoramic radiographs. However, they are similar in direct visualization on cadaver and CT or CBCT. ¹ , ⁵ {Table 2}

In the present CBCT study, the incidence and length of anterior loop did not change with age increase, while in the study on panoramic radiographs, the incidence of anterior loop was reduced with aging because of the difficulty in visualizing the anterior loop in older patients. ² There was no correlation between the incidence and length of anterior loop and gender and side in this study, which was in accordance with the results of previous studies. ² , ⁹ Some studies showed a relationship between the length of anterior loop and age, side and race. However, they concluded that the influence of race was more than other factors. ⁶ , ⁸ , ²⁰ , ²¹ , ²³

The length of anterior loop has been reported to vary in different populations. The reported mean lengths of anterior loop in different studies are shown in Table 2. A length range of 0.1 mm to 11 mm has been reported in different studies. These differences may be, at least partly, due to the racial influence. A safe distance anterior to the mental foramen is required when performing surgery in the mandibular premolar region. About 6 mm safe distance anterior to the mental foramen is frequently recommended, especially when the anterior loop of the mental nerve cannot be determined definitely or without the use of CBCT or CT. ⁴ , ⁶ , ¹² There are also other suggestions from 1 to 4 mm for the safe distance anterior to the mental foramen. ¹ These diverse recommendations for the safe distance reflect the length variations of anterior loop in each patient; therefore, a fixed distance anterior to the mental foramen is not safe, and the anterior loop length should be determined for each individual to avoid injury to the mental nerve. ²⁴ , ²⁵ , ²⁶ , ²⁷

Great care is required when placing implants in proximity to mental foramen to avoid anterior loop injury. Because of the length variations of anterior loop in each patient, a fixed distance anterior to the mental foramen is not safe, and the length of anterior loop should be determined for each individual. The use of CBCT provides accurate measurements of the length of anterior loop.

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.