Cone-Beam Computed Tomography Analysis of the Prevalence, Length, and Passage of the Anterior Loop of the Mandibular Canal

Recent studies have suggested that implant-retained overdentures will become the new standard of care to restore the function and esthetics of edentulous mandibles.^1,2  The ideal location for this type of implant is in the anterior mandible, but the presence of multiple neurovascular bundles affects the placement of dental implants in this area. A thorough understanding of the anatomy of the anterior mandible will help improve the rates of successful implant placement.

The inferior alveolar nerve (IAN), a terminal division of the mandibular branch of the trigeminal nerve, is confined within the mandibular canal and innervates all the mandibular teeth, from the most distal molar to the central incisor.³  The IAN originates from the mandibular foramen and runs obliquely downward and forward in the ramus and then horizontally within the body of the mandible.⁴  The IAN gives off the mental nerve, which exits the mandible via the mental foramen, supplying sensory branches to the chin, lower lip, mucous membranes, and the gingiva, from the midline to about the second premolar region.⁵  The IAN then continues anteriorly and becomes the mandibular incisive nerve, which innervates the mandibular canines and incisors.⁶  Before exiting the mental foramen and branching into the incisive nerve, the IAN may continue anteriorly and form an anterior loop. This anterior loop is the segment where the IAN navigates mesially and occlusally before reversing direction and coursing distally toward the mental foramen.⁴ 

The prevalence and length of the anterior loop has been evaluated using periapical, panoramic radiograph, cone-beam computerized tomography (CBCT), and intrasurgically. These studies have shown a prevalence between 26% and 71%, and a mean length of 0.57–2.7 mm.^7–11  Two-dimensional (2D) imaging and periapical and panoramic radiographs have inherent limitations, such as superimposition, distortion, and magnification, which compromise diagnostic accuracy.¹²  Although direct intrasurgical measurement may be the most accurate method available, it is neither time effective nor feasible due to the infliction of unnecessary damage to the patient. CBCT is considered to be an important tool in the evaluation of craniofacial anatomy and pathology, because it produces images with high dimensional accuracy in all spatial planes, with much lower costs and absorbed doses than conventional CT.^13,14  In a study to locate the metal foramen and the mandibular canal, Sonick et al.¹⁵  found that the average distortion for periapical, panoramic, and CT images was 14%, 24%, and 1.8%, respectively, compared with caliper measurements on mandibles of cadavers. This study confirmed that 3D imaging is superior to its 2D counterparts in anatomical assessment.

Although CBCT has been used to investigate the prevalence and length of the anterior loop, very few studies examined the passage and the alveolar bone available coronally in the mandible. This information is critical for planning an implant treatment in the anterior mandible. The aim of the current study was to investigate the prevalence, length, and passage of the anterior loop, as well as the alveolar bone available coronally to the anterior loop, and to further analyze how the sex, age, side, and dentition status of the subjects affected these parameters. Our results are expected to provide comprehensive anatomical information on the anterior loop and help dentists make informed decisions on how to place implants in the interforaminal region of the mandible.

Patients with CBCT scans taken at the Imaging Clinic of the institution's dental school between September 2014 and October 2015 were screened for inclusion. The scans were taken for restorative, orthodontic, or other therapeutic purpose. The inclusion criteria were as follows: (1) entire mandible captured; (2) mental foramen and anterior mandible clearly delineated; and (3) no pathology or treatment in the anterior mandible. The exclusion criteria were as follows: (1) inability to identify the mental foramen in the scan; (2) interference of pathology or excessive scatter artifact with the visualization of the mental foramen and/or the anterior mandible; and (3) excessive amount of the inferior border of the mandible not captured in the image. This study was approved by the Institutional Review Board.

All CBCT scans were exposed at 90 kVp, 10 mA, with a medium field of view (90 × 150 mm), and 200-μm voxel size using a Kodak 9500 scanner (Carestream Health Inc, Rochester, NY). Patients were positioned in the scanner with their Frankfort plane parallel to the floor. All the radiation safety procedures were followed according to the institutional guidelines. The acquired CBCT data were stored in the MiPACS system (Medicor Imaging, Charlotte, NC) as a standard DICOM 3.0 format (Digital Imaging and Communication in Medicine). The DICOM files were then retrieved from MiPACS and viewed with the Invivo 5.0 software (Anatomage Inc, San Jose, Calif).

To standardize head placement, each scan was oriented to have the hard palate parallel with the horizontal plane in the sagittal view prior to the measurements. One of the coauthors (NR) did all the measurements and repeated the analysis.

The presence of the mental foramen and the anterior loop was identified using reconstructed panoramic views and confirmed with cross-sectional views (Figures 1a and b and 2a and b, respectively). The formula used to calculate prevalence was as follows: (cases with anterior loop/total cases analyzed) × 100%. Prevalence of the anterior loop was calculated at both patient and hemi-section levels.

On the cross-sectional views, the interval thickness was set at 0.1 mm. The slice numbers corresponding to the most mesial aspect of the mental foramen, and the most mesial aspect of anterior loop, were recorded. The length of the anterior loop was calculated as the difference between the slice numbers multiplied by 0.1 mm.

For dentate and edentulous alveolar ridges, in the cross-sectional views corresponding to the most mesial aspect of the mental foramen and the anterior loop, the distances from the alveolar crest to the most coronal aspect of the mental foramen and anterior loop were measured; these values represented the alveolar bone available coronally to these structures (Figures 1b and 2b, respectively).

In the cross-sectional view corresponding to the most mesial aspect of the anterior loop, the distances from the buccal cortical plate of the mandible to the buccal wall of the anterior loop, from the buccal cortical plate of the mandible to the lingual wall of the anterior loop, and from the buccal to the lingual cortical plate of the mandible, were measured (Figure 2b). The distance from the lingual wall of the anterior loop to the lingual cortical plate of the mandible was calculated as follows: (distance from buccal to lingual cortical plate of mandible) − (distance from buccal cortical plate of mandible to lingual wall of anterior loop). The buccal-lingual width of the alveolar ridge was further divided into buccal, middle, and lingual thirds. Passage of the anterior loop was indicated by which of these segments the loop occupied.

The normal distribution of the data was tested by conducting a Skewness analysis, which revealed that the anterior loop measurements did not follow a normal distribution; instead, they were positively skewed. Therefore, the results for loop lengths were mainly reported as median, range, and interquartile range. The Wilcoxon signed rank test was used to compare loop lengths between different demographic groups due to the non-normal distribution of the data. In addition, a Fisher's exact test was used to reveal the associations between anterior loop parameters and demographic status, because some expected numbers in the frequency tables were less than 5; therefore, the approximation for a χ² distribution was inadequate, and the Fisher's exact test was a better choice. The intraobserver agreement was assessed by the intraclass correlation coefficient (ICC). The statistical analysis was run with the SPSS program (version 24, IBM, Armonk, NY), with a statistical significance set at P < .05. All the statistical analysis methodologies were reviewed and approved by an independent statistician.

This retrospective study included CBCT scans from 124 patients, 68 women and 56 men, with ages between 16 and 83 years. Twenty-six patients were edentulous, whereas the remaining 98 patients were dentate (Table 1). All the results presented below were checked and confirmed by an independent statistician.

Of a total of 124 patients and 248 hemi-sections evaluated, 28 patients presented the anterior loop bilaterally, and 3 patients had the anterior loop only on one side of the arch. The prevalence of the anterior loop at the patient and hemi-section levels was 25% (31/124) and 24% (59/248), respectively. When the data were restratified according to the demographic and dentition status of the patients, it was found that sex, age, and dentition status did not affect prevalence of the anterior loop (P > .05; Table 2).

For the population studied, the overall median, range, and interquartile range of the length of the anterior loop was 1.63, 0.52–3.92, and 1.28–2.07 mm, respectively. Sex, age, dentition status, and side did not appear to affect the length of the anterior loop (Table 3).

The amounts of alveolar bone coronal to the mental foramen and the anterior loop are shown in Table 4. On average, 12.92 mm of alveolar bone (42.55% of the total alveolar ridge height) was found to be coronal to the mental foramen and 17.06 mm (55.83% of the total alveolar ridge height) coronal to the anterior loop. This showed that anterior loop was more apically located relative to mental foramen. The side of the arch did not appear to affect the amount of bone existing coronally to the mental foramen and anterior loop.

In general, the anterior loop was located closer to the buccal cortical plate than the lingual plate, as shown in Table 5, leaving an average of 5.27 ± 2.10 mm of bone between the lingual wall of the anterior loop and the lingual cortical plate of the mandible. Apparently, the side of the arch did not affect the passage of the anterior loop. The percentage distribution of the anterior loop in the buccal, middle, and lingual one-third of the alveolar ridge was 83.1%, 16.9%, and 0%, respectively.

The ICC for intraobserver agreement was 0.99, which demonstrates high reproducibility and reliability with the same evaluator.

This study identified that the prevalence of the anterior loop was 25% at the patient level and 24% at the hemi-section level. Wide variations in prevalence of the anterior loop have been reported in the literature. Sahman et al.¹⁰  and Kaya et al.⁷  observed an anterior loop prevalence of approximately 29% and 34% in their respective populations. The reported prevalence of the anterior loop at the hemi-section level ranged between 26% and 71%.^11,16–19  This variation in the prevalence of the anterior loop is likely due to differences in ethnicities, evaluation methods (surgical assessment, cadaver dissection, imaging studies), and radiographic acquisition/view protocols (CBCT unit, exposure setting, view software). In the current study, the presence of the anterior loop does not seem to be correlated with the age, sex, side, or dentition status of the patients. All these findings are consistent with what has been previously reported.^{8,10,11,16,17} 

Due to the skewed nature of the data distribution, the lengths of the anterior loop were mainly reported as median and range in the present study, 1.63 and 0.52–3.92 mm, respectively. Apostolakis et al.¹⁶  obtained similar findings, showing that the length of the anterior loop did not follow a normal distribution, and reported a length of 0–5.7 mm. The authors recommended a 6-mm safety margin from the mental foramen for an implant placed in the anterior mandible. However, much longer anterior loops, for example, with 9¹¹ and 11 mm,²⁰  have been reported in literature, which emphasizes the importance of case-by-case CBCT evaluation before implant insertion in the anterior mandible. In this study, no statistically significant differences were found in the length of the anterior loop between patients with different ages, sexes, sides, and dentition status. However, there are inconsistencies in the literature regarding how these factors influence the length of the anterior loop.^{8,10,11,16,17}  Presumably, the heterogeneity of the subjects and differences in assessment methods contribute to the observed variations.

The current study identified approximately 13 and 17 mm of alveolar bone coronal to the mental foramina and the anterior loop, respectively, available for implant placement before contacting the neurovascular bundle. Recent studies have shown that short implants with a length of 5.5–7.0 mm and roughed surfaces are able to induce a favorable rehabilitation effect similar to that of standard implants with a length of 10 or 12 mm.^21,22  Considering that the alveolar bone coronal to the mental foramen and the anterior loop was more than 12 mm for many subjects in the current study, many of them may be able to receive implants placed coronally to these structures without neurologic disturbance. For patients who undergo tooth extractions in the anterior mandible, there are large variations in the speed of bone loss between individuals,²³  with the highest rate of bone loss occurring in the first few months, but bone loss in the mandible can continue up to 25 years after extraction.^24–26  Therefore, a case-by-case evaluation of the residual alveolar dimension at the anterior mandible is deemed necessary prior to implant placement.

The pathway of the anterior loop, when present, was consistently found to be lingual to the mental foramen, but within the buccal or middle aspect of the alveolar ridge for most of the patients in the present study. The range of alveolar bone between the lingual wall of the anterior loop to the lingual cortical plate of the mandible was 3.17–7.37 mm. Studies have shown that implants with a diameter of 4.1 or 4.8 mm, and a length of 8 mm or above, are able to generate tolerable crestal stress and strain, resulting in fair osseointegration and satisfactory restorative and esthetic outcomes.^27,28  Based on the current observations, placing an implant lingual to the mental foramina or anterior loop seems to be suitable for quite a few subjects.

Although designed as carefully as possible, this study presents limitations. The anatomy of the subjects is known to differ according to ethnicity.²⁹  Although some of the published studies may have evaluated a more homogenous population, the patients at our institution present a wide variety of ethnicities, including Caucasian, Hispanic, African American, and Asian ethnicities, with many having a mixed ethnical background. This heterogeneity is considered one of the major cofounding factors in the study, and future analyses with large sample sizes and representative ethnic compositions of various geographic locations will help uncover the relationship between anterior loop anatomy and subject ethnicity. In addition, of the total 124 patients, 98 were dentate and 26 were edentulous. There were variations in the periodontal health status of dentate patients, as well as in the length of time from tooth loss in the edentulous patients. There variations contributed to the complexity of data interpretation; therefore, it is considered a limitation of the study. More detailed studies analyzing how the amount of alveolar bone located coronally to the mental foramen or anterior loop is related to the health status of the periodontium and the extent of edentulism will shed light on the anatomic variations of these structures. Also, all the CBCT scans were acquired with a relatively old scanner, Kodak 9500, which had a smallest voxel size of 200 μm. Compared with newer CBCT units with much smaller voxel sizes, such as 80 μm for the Accuitomo scanner,³⁰  the much larger voxel size of the unit used in our study may limit the detectability of the presence and dimension of the anterior loop. Different exposure settings may also affect imaging quality and accuracy of the anatomic identification. More thorough investigations with different CBCT units and various exposure conditions are expected to provide additional information on the anatomic variations of the anterior loop contributing to suitable planning of implant treatments in the anterior mandible.

To restore the esthetic and function of the edentulous anterior mandible, dental implants may be placed mesially, coronally, or lingually to the mental foramina and the anterior loop, depending on the dimensions of the residual alveolar bone in this area. A case-by-case CBCT evaluation of the critical anatomic structures in the anterior mandible is considered necessary before implant placement in the area.

Abbreviations

CBCT:

cone-beam computed tomography

IAN:

inferior alveolar nerve

The authors thank Dr Liang Zhu, a statistician of the University of Texas Health Science Center at Houston School of Nursing, for reviewing and confirming all the statistical analyses and results.

The authors declare no conflicts of interest.