Over the past several decades, endosseous dental implant surgery has become a common daily practice for the edentulous area and performed ubiquitously on daily basis. An important issue that implant surgeons face during the insertion of dental implants in the anterior mandible is the presence of the anterior loop (AL) of the mental nerve as an endosseous extension of the mandibular canal. The mental foramen is usually considered as an essential reference point during treatment planning to determine the position of the most distal implants in the anterior mandible.1,2 The most posterior implant in the anterior mandible is placed as close as possible to the mental foramen to expand the supporting prosthetic polygon.3,4 However, the benefit of trying to achieve an optimal distribution of implants across the anterior mandible may lead to adverse clinical outcome if the AL of the mandibular canal along with the containing inferior alveolar nerve is damaged during implant insertion.
The intraosseous AL is described as an aberrant anatomical variation of the mental canal, which exists as an anatomical continuation of the mandibular canal and runs outward, upward, and backward to open at the mental foramen. Although the length of the anterior component of the mental canal is not clearly described, it is widely acceptable that it is consistently present.5
Prevalence of the AL varies strongly throughout the literature ranging from 27%1 to 100%.6,7 Several studies throughout the literature have further shown wide variations of AL length.1,2,5,7–18 Such a great variety of the prevalence and the length of the AL lead to the lack of a consensus about the precise morphology of the mandibular canal and should therefore be further investigated with available imaging modality such as computer tomography (CT). Using a dental treatment planning software, CT is able to locate the intraosseous anatomical structures in the mandible precisely, giving a higher contrast-to-noise and signal-to-noise ratio than cone-beam CT (CBCT).19,20
With regard to this anatomical variation of the mandibular canal, various clinical recommendations concerning “safety distances” for implant insertion mesial to the mental foramen were proposed in the literature, ranging from 1.0 mm up to 6.0 mm.1,2,8,9,12,21–24 Is any safety distance recommendable? Will a defined safety zone combined with a panoramic radiograph successfully prevent from any serious neurovascular damage? At which point are further imaging methods needed?
The purpose of this study is to use CT scans to measure the extension and prevalence of the AL given the higher accuracy of CT scans in detecting osseous anatomical structures.25,26 Human prosthetically untreated mandibles were used to simulate the clinical situation of edentulous patients wanting to have dental implants. Using these measurements, it might be possible to give a recommendation for safe implant insertion in the anterior mandible with prevention of any neurovascular damage.
Materials and Methods
A set of 48 meticulously preserved and macerated edentulous human mandibles with obvious alveolar bone atrophy were selected for this investigation. Those mandibles were obtained from an anatomical collection from the 19th century of the University Innsbruck, Austria. Our inclusion criteria for the study was preserved bony integrity of the mandibular body, alveolar crest, ramus, as well as preservation of the mental protuberance. Of special interest was the mandibles' state of untreated edentulism. It is likely that only tooth extractions from barbers or general physicians had been performed at that time. Eleven mandibles were excluded from this study, due to the loss of one or both mandibular condyles decreased reproducibility during the CT. Thus, 37 mandibles were included in the study population. The mandibular body was positioned between 2 elastics on a CT headrest (Fig. 1), and the condyles were fixed with adhesive tape. Neither elastics nor adhesive tape would produce artifacts during CT images. The radiographic examinations were performed using CT (Brilliance Ten Multislice, Philips Medical). A radiologist recorded the CT images and assessed the diagnostic quality of the captured scans. Using the Philips workstation (“Extended Brilliance Workspace 3.5.0 Philips Medical”) with the help of dental planning software, cross-sectional views (×2.0 mm) perpendicular to the mandibular body as well as panoramic reconstructions of all 37 mandibles were produced. The panoramic reconstructions are a series of cross-sectional images along curved planes through the lower jaw that are merged together. All images were printed with a Kodak View 5800 Laser Imager and viewed on a standard light box. Each mandible selected for the CT analysis produced 3 images: 2 showing the cross-sectional views and one showing the panoramic reconstructions of the mandible. One investigator was responsible for the selection of images and measurements. To ensure statistical independence of this investigation, both sides of the edentulous mandibles were selected for analysis and were compared accordingly.
Prevalence and Extend of the AL
Prevalence and length of an intraosseous “loop” of the inferior alveolar nerve in the mandibular canal that extended mesially of the mental foramen was investigated using the cross-sectional views and panoramic reconstructions. Reference points for measurement were set at the radiographic mesial boarder of the mental foramen on both imaging methods (cross-sectional views and panoramics; Figs. 2 and 3). Any intraosseous mesial extension of the coursing of the mandibular canal on those images was measured and reported in millimeters (Fig. 4).
Data Collection and Statistical Analysis
All data was entered in Microsoft Excel 2010 on Windows 7 Ultimate. The database was imported in a statistical software program (version 19; IBM SPSS, Armonk, NY). In the first instance, descriptive statistics were used to quantitatively describe the main features of the data collection. Descriptive data was graphically demonstrated using boxplots with their 5-number summaries: the smallest observation (sample minimum), lower quartile, median, upper quartile, and the largest observation (sample maximum).
To perform a separate computing of variables of different groups in the overall data, an exploratory data analysis (case processing summary) was created. Frequencies were calculated and graphically demonstrated using bar charts.
An independent samples t test compared the means of 2 sets of values from one variable. The 2 means are displayed, the SD and SE for the 2 means. The independent sample t test includes a t test for equality of means and a Levene test for equality of variances. The P-value is known as the significance level and counts as the probability of observing data at least as extreme as that observed, given that the null hypothesis is true. A P-value smaller or equal to 0.05 is considered statistically significant.
Descriptive statistics showed that the mean value of the loop (n = 74) was 2.26 mm (SD: 1.72 mm). Minimum was 0.0 mm, maximum 7.0 mm (Fig. 5).
Frequency analysis presented in most cases a loop of 2.0 mm (24.3%), followed by a loop of 4.0 mm (17.6%). The lowest distribution of values was found at the maximum loop of 7.0 mm with 1.4% (Fig. 6).
There was one mandible with no loop on both sides (2.7%). In 9 mandibles (24.32%), intraosseous mesial extension was found only on one side of the mandible. The other 27 mandibles (72.9%) presented with ALs on both investigated sides.
The right side showed a slightly higher mean length with 2.46 mm compared with the left side with 2.07 mm (Fig. 7). The left side of the mandible presented with a maximum loop of 7.0 mm, followed by a maximum AL extend of 6.0 mm on the right side (Fig. 7).
The Levene test explored if the AL would be equal on both sides. This would be with a probability of 28.2%. With the t test for equality of means, the equality of the AL was tested in the overall population. It showed that in 33.1%, the AL was equal in the overall population.
This CT-based study attempts to further characterize the morphology and the prevalence of the AL of the mandibular canal. Only careful presurgical planning and accurate surgery will deliver a satisfying result in implantology for both, the surgeon and the patient. Nowadays, dental implants are successful treatment option for the replacement of missing teeth.27 Planning involves careful patient examination and precise evaluation of the surgical site. To prevent intraoperative complication and optimal positioning of the implant, the localization and the extension of such an AL in the mandible is essential to recognize.
Placement of dental implants in the anterior edentulous mandible is relatively safe given the presence of only few vital anatomical structures in this location.5 Thus, implants are frequently inserted in this interforaminal area using basic presurgical imaging like conventional panoramic radiographs. Nevertheless, complications of dental implant insertion may arise because of variations of the vascular distribution in the anterior mandible, which might causes accidental vascular injuries,28 or inadvertent nerve injury if the inferior alveolar nerve extends beyond the mental foramen as an intraosseous AL.23 If such a loop extends beyond the mental foramen, implant insertion in the interforaminal area might result in sensory disturbances in the distribution of the mental nerve.23
Conventional panoramic radiographs are suggested as the preferred presurgical imaging tool in the original Brånemark protocol.29 Imaging modality with the ability of cross-sectional viewing, like CT or CBCT, should be considered in difficult cases according to the European Association for Osseointegration guidelines.30 The literature shows that various recommended tools for the identification of the AL exist. Some authors focus on the use of panoramic radiographs,1,5,12,14,18,31 other use surgical dissections,1,5,7,11–13,16,32 CT,8,15,18,22,31 or CBCT6,33 (Tables 1–3). Varied results may be attributed to different diagnostic techniques followed to detect and characterize the AL in diverse patient groups.
The prevalence of the AL is reported in the literature to range from 27% (Kuzmanovic et al1) to 100% (Hu et al7 and Chen et al6) and varies significantly. The absence of the AL seems to be very common throughout the literature, although only one mandible with no loop was detected in this study. The results of Rosenquist et al32 showed that in 74% of investigated dissections, no loop was found. They assumed that the AL of the alveolar nerve does not exist in most patients, and if it does exist, it does not seem to extend more than 1.0 mm.
Apostolakis et al8 reported in their CBCT study that in 52% of their investigated cases (n = 95), no AL was identified. Only 16% revealed an AL on one side of the mandible. They concluded that this approximately lies in the middle of range of existing anatomical studies reporting of either no AL or universal existence. The prevalence of the loop in the CBCT study by Filo et al33 showed a frequency of 69.7% (n = 1384 mandibular sides). Our CT-based study revealed a prevalence of 97.3% (n = 36).
As with the discussion over the existence of the AL and its prevalence, there is a wide variety of data regarding the side and the length of the loop throughout the literature.
Two authors, Arzouman et al5 and Oliveira-Santos et al,31 considered a loop over 2.0 mm as surgically significant. Some authors stated that there is neither a significant difference for the intraosseous loop for the right or left side of the mandible,16,17 nor between being dentate or edentulous mandibles.2,8,16,17 Ngeow et al14 reported that most of the ALs were present bilaterally, followed by loops on the right side only. Arzouman et al5 and Oliveira-Santos et al31 stated that the loop on the right side of the mandible is longer. The results of our investigation support this hypothesis of a side-specific dominance of the loop. A more prominent loop was found on the right side of the mandible with a mean extend of 2.46 mm. The left side presented a mean loop of 2.07 mm. Apostolakis et al8 and Ngeow et al14 confirmed with their results that the AL was longer on the right side of the mandible.
Arzouman et al5 advise against the use of panoramic radiographs. According to them, their use will lead to underestimation of the occurrence of an AL and its length. In their study, a significant loop (>2.0 mm) was identified in 92% to 96% using dissections. The 2 conventional radiographic methods identified ALs in 56% and 76% only. The average physically measured length of the loop was 6.95 mm. The radiographic measurements of loops were 3.18 mm and 3.45 mm.
Kuzmanovic et al.1 investigated the AL with panoramic radiographs versus surgical dissections. Only 27% of the ALs could be identified with panoramic radiographs (range, 0.5–3.0 mm). The dissections revealed a measured range of 0.1 to 3.3 mm of the AL (mean 1.2 mm). Their findings confirm the results of Iyengar et al,34 which advocate against the use of panoramic images for identification of the AL during treatment planning.
In their CBCT study, Oliveira-Santos et al31 described an AL extension of 2 mm or less in 67% of their investigated cases. Some authors stated that the presence of an AL longer than 2 mm is an anatomical exception.5,16 They supported their hypothesis by the results of their studies based on CT scans and dissections, in which a short intraosseous loop was identified in most cases. Referring to their results, we defined a long intraosseous loop >2.0 mm. 71.6% of our investigated cases showed an AL extension of up to 3.0 mm. 28.4% of investigated mandibles presented with loops longer than 3.0 mm. Apostolakis et al8 stated that an AL up to 3.0 mm as significant, which was found in 95% of our investigated cases. Five percent of the loops extended more than 3.0 mm. Uchida et al17 described a prevalence of 6% of loops of 5.0 mm or longer in their population. Oliveira-Santos et al31 even reported of loops longer than 6.0 mm (prevalence 1%).
A more detailed overview about the mean values and prevalence of observed ALs is given in Tables 1–3. Mean values of all AL studies are graphically displayed in the Figure 8.
Given that the data regarding the presence and extension of an AL varies strongly, different recommendations exist for the proposed intraoperative safety distances to avoid neurovascular damages during implant insertion. Apostolakis et al,8 Solar et al,2 and Gerlach et al24 state that a safety distance of at least 6.0 mm without further imaging between the mesial border of the mental foramen and the most distal interforaminal implant is recommended.
Juodzbalys et al,21 Kuzmanovic et al,1 and Koh et al22 recommended a safety distance of 4.0 mm mesial to the mental foramen. Mardinger et al12 and Wismeijer et al23 described a safety zone of 3.0 mm mesial to the mental foramen, although Wismeijer et al reported that 7% of their patients reported sensory disturbances at the lower lip area after dental implant insertion following this safety margin. Bavitz et al9 recommended implant insertion 1.0 mm mesial to the anterior aspect of the mental foramen.
The results of our study recommend a safety distance of 5.0 mm mesial to the mental foramen in the anterior mandible. 71.6% of our investigated loops are 3.0 mm or less. 91.9% of all investigated loops extend up to 4.0 mm or less. Clinicians doubting the AL's position on a panoramic radiograph before placing an implant should consider our 5 mm safety distance to avoid any neurovascular damage. If any closer position to the mental foramen is chosen, further imaging methods like CBCT or CT are recommended. The results of Forni et al26 showed that CT provides a much better identification of the AL than conventional panoramic radiographs.
One drawback of this investigation was that the measurements were performed by only one observer, but hence allowing consistency. It has to be taken under consideration that this always includes the possibility of methodological bias.
Another limitation is the lack of information concerning the human mandible specimens that were investigated. There is no additional information about the anatomical specimen available because no recordings were found.
Most of the investigated studies mentioned above lack of certain specifics like age, gender, or race of the human mandibles. Some even failed to mention whether the investigated individuals were dentate or edentulous. This makes a data comparability difficult.
An advantage of this study was the use of CT, giving its higher accuracy of the determining anatomical structures compared with conventional panoramic radiographs.35 Additionally, the use of prosthetically untreated human mandibles presents a model with an authentic resorption pattern of the alveolar crest, which is often found in patients' previous implant insertion.
The AL of the mandibular canal is an aberrant anatomical variation that needs to be kept in mind during implant insertion in the anterior part of the mandible. Prevalence of the loop is high at 97.3% (n = 36), either on one side of the mandible (24.3%, n = 9) or on both sides (72.9%, n = 27). This study shows that the AL on the right side of the mandible was slightly longer (mean, 2.46 mm) compared with the left side (2.07 mm). Regarding our findings, prevalence of the AL is bilateral, but its mesial extension is subject to anatomical variety. Symmetry of the AL cannot be assumed in human mandibles.
A long AL was defined as a loop longer than 2.0 mm and was found in 71.6% of investigated cases. In 91.9% of all examined subjects, ALs were found to be 4.0 mm or less in length. According to those findings, a safety distance of 5.0 mm (AL: 4.0 mm, safety distance: 1.0 mm) mesial to the mental foramen for implant insertion in the anterior mandible is recommended to avoid neurovascular sequelae.
The use of 3-dimensional imaging methods like CT or CBCT is advised during presurgical planning phase if the course of the AL is not clearly identifiable on conventional panoramic radiographs.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
This investigation was started on the basis of the “Recommendations for the Care of Human Remains in Museums and Collections1” and “Guidance for the care of human remains in museums2” upon approval by the Department of Anatomy Innsbruck, Austria, the Department of Anatomy, Salzburg, Austria, and the Paracelsus Medical University, Salzburg, Austria.
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