Dental 3D Cone Beam CT Imaging: Part IV
Anterior Extension of IAN from Mental Foramen
Dental 3D CT cone beam imaging has many purposes including accurate dental implant placement and the identification of key anatomic structures. While there is an extensive literature analyzing the many aspects of the anterior loop extending from the mental foramen which compare the frequency and accuracy of this landmark from radiographs, cadaver dissection, and CT imaging (1-5), no study has measured the anterior extension of the inferior alveolar nerve (IAN) relative to distance from the edentulous crest, apices of teeth, and more. In Greenstein and Tarnow’s exhaustive literature review of the anatomy and clinical ramifications of the mental foramen, they conclude that since there is little correlation between viewing the anterior loop on radiographic and its actual clinical location, they recommend “leaving a 2mm zone of safety between an implant and the coronal aspect of the nerve” (6). In their literature review, it is suggested to use CT scans to help identify the anterior loop. They recommend that particular attention to variations in the anatomy in the area of the mental foramen be considered in addition to both functional and esthetic demands in order to avoid complications to the neurovasculature.
This study, using dental cone beam 3D CT scanners, evaluated various normal and abnormal landmarks noted in 500 consecutive patients referred to dental CT radiological labs for a variety of reasons, but most of which were for the insertion of dental implants. Part I of this study analyzed the demographics and reasons for the patients being referred for CT scans; Part II studied anatomic considerations of the insertion of the lingual artery into the mandible; Part III analyzed the frequency and location of bifid canals. In this study, Part IV, the anterior extension of the inferior alveolar nerve is studied on 3D CT images, and its clinical ramifications are discussed.
Data from five hundred (500) consecutive patients sent to i-dontics center from 9 centers located in 3 states for 3D dental CT studies, were evaluated. Scans were taken on either i-CAT (8 centers) or NewTom 3G scanners and uploaded to a central data center. All studies were converted to SimPlant™ (Materialise, Glen Burnie, MD). When not specified, the data was converted to SimPlant™ version 10.
Two hundred and Ninety-six (296) mandibles were studied on 3D CT studies using the measuring tool on the SimPlant Master™ Program. The following were measured: the length of the anterior extension of the IAN from the most mesial aspect of the mental foramen as identified in a 1mm slice; the distance of the anterior extension from the edentulous alveolar crest; the distance of the anterior extension from the apices of teeth; how many anterior extensions of the IAN were connected from right to left at the midline; and how many mental foramina were located on the alveolar crest.
Almost 97% of the 296 mandibles analyzed in this study had at least one measurable extension of the IAN anterior to the most mesial aspect of the mental foramen (Figure 1), as noted on cone beam dental CT studies.
Figure 1. Nearly 97% of all mandibles had an anterior extension; nine patients did not have a measureable anterior extension of the IAN as seen on a 3D cone beam study.
Fourteen patients (4.73%) did not have an extension on the right side; eleven patients (3.72%) did not have an extension on the left side.
The average length of the anterior extension extending from the mesial rim of the mental foramen is 12.0 mm on the right and 11.8 mm on the left (Figure 2).
Figure 2. The average length of the anterior extension from the mental foramen bilaterally is nearly 12.0 mm.
The distance from the superior most portion of the anterior extension of the IAN to the edentulous crest was measured on images from a 3D dental cone beam scanner. In most instances, the bone loss was level and the radiographic extension was parallel to the crest enabling a single measurement either on the right or left side, or both. The average depth of the extension under an edentulous right crest was 10.5mm and 11.0mm under the left side (Figure 3-3A).
Figure 3. Average measurement to edentulous alveolar crest was 10.5mm on right anterior mandible. (Stereolithographic model courtesy of BioMedical Modeling, Boston, MA).
Figure 3A. Average measurement to edentulous alveolar crest was 11.0mm on left anterior mandible. (Stereolithographic model courtesy of BioMedical Modeling, Boston, MA).
The distance from the anterior extension of the canal to the apex of a tooth measured from 3D CT images when the dentition was present was measured from the most anterior extent of the canal vertically to the apex of the adjacent tooth. The preponderance of teeth measured in this situation were situated under the apices of the lateral incisors. In all instances, the measurement was vertical from the tooth apex to the top of the extended canal. Both right and left measurements, seen on 3D CT images, were identical: 17.0mm from the anterior extent of the canal to the nearest root apex.
A continuous loop, defined as an extension of the canal that emanates from both the right and left mental foramina and is seen to connect in the midline was viewed on 77 patients (26.01%). In Figure 4, the anterior extensions of the canal can be seen in different coronal (panoramic) slices of the same individual.
Figure 4. Coronal slices in the same patient demonstrate examples of a continuous canal as observed and noted in this study.
Figures 5 and 5A demonstrate a continuous anterior extension that joins in the midline. Various branches emanate from the canal displaying that may be viewed in CT cone beam images but not 2D dental X-rays.
Figure 5. Example of a continuous anterior extension of the canal with multiple branches extending from the main trunk.
Figure 5A. Some of the many branches extending from the main canal and the anterior extension are highlighted.
The anterior loop of the mental nerve is commonly described as that part of the neurovascular bundle that transverses anterior and inferior to the mental foramen only to loop back to exit the mental foramen (8-11). While it is often difficult to follow its extend on 2D dental X-rays, it is easier to view these anterior loops on 3D CT images.
When mandibles were dissected, the anterior loop was detected in 60% of 37 cadaver mandibles (12). The length of the loop in this 3D CT study (12) ranged from 0.5 to 5mm. In another study, Neiva et al (13) probed the anterior loop in 22 cadavers, noting that it was present in 88% of the patients and that its mean length was 4.13mm ranging from 1.0 - 11.0mm.
Studies have found CT scans more accurate than traditional 2D dental imaging (14-18) and they should be considered for locating inferior alveolar canals or mental foramina not easily viewed on traditional 2D dental images (periapical films or panoramic images) when considering implant placement, removal of impacted teeth, or treating pathologic lesions.
This 3D Ct cone beam study confirmed, along with other studies, that the alveolar ridge resorbs approximately 6.0-6.5mm when teeth are extracted, which is the average distance noted between the ridge to the anterior extension when compared to root apices to the anterior extension of the canal (1-2).
Using CT scans, Rothman found that the length of the anterior loop could be as long as 10.0mm (19) compared to this study, where it was found the average length for the anterior continuation of the nerve 11.8mm on the left side and 12.0 mm on the right. What is apparent in this study is that the average length of the anterior extension of the canal is greater than in other reported studies. It is unclear if this finding is a function of interpretation or is a result of the accuracy of dental cone beam scanners. What is clear is that these radiographic canals exist, but their clinical importance is a matter of speculation and requires further investigation.
For example, are these radiographic canals filled with nerve and vascular tissues or are they empty? Do they innervate gingival tissues? The answers to these questions are clinically relevant for a host of reasons. When these anterior extensions of the canal are identified on 3D CT images, should they be avoided during implant placement? Should increased bleeding be anticipated? Should the patient be informed that there is a chance there will be an altered sensation to the gingiva and adjacent tissues if these canals are penetrated during dental implant surgery? Further investigation will help determine the answers to these questions.
In this 3D dental cone beam CT study, 296 mandibles were studied on dental cone beam scanners. The anterior extension of the IAN was measured relative to length and distance from edentulous ridges or apices of teeth. The clinical ramifications of this anatomic entity were discussed, including the value of 3D imaging when inserting dental implants.
Support for this study was generously given by Nobel Biocare AB Gothenberg, Sweden (Grant 2006-492) and Imaging Sciences Inc., Hatfield, PA.
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