Cone Beam Computerized Tomography Analysis of the Posterior and Anterior Mandibular Lingual Concavity for Dental Implant Patients

In the modern practice of dentistry, the use of implants has become integral to the goals of restoring patients to complete form and function. This is due in large part to the poor fit and the frequency of unfavorable patient outcomes with the use of removable prostheses. The general psychosocial perception of tooth loss and increased public awareness of this issue has also contributed to the rise in the use of implants, as has the rapidly growing aging population. Implants are an excellent option for replacing missing teeth because of their high success rates, improved retention of bone in edentulous areas, increased ability to maintain a clean tooth structure, and decreased risk of caries in adjacent teeth when compared with fixed partial dentures.¹ 

Several factors must be taken into account before incorporating a dental implant into a treatment plan. In the past, periapical and panoramic films were used for implant treatment planning. Now, 3-dimensional imaging has become widely available and is frequently used in implant dentistry. The cone beam computerized tomography (CBCT) scan produces 3-dimensional images of maxillary and mandibular morphology, providing multiple views with minimal radiation, which enables interactive treatment planning. By allowing a stronger visualization of the anatomy and morphology of the surgical site, this type of presurgical diagnostic imaging allows the practitioner to evaluate the amount of remaining bone in any given site, in addition to gaining a better understanding of the intraoral anatomical structures in any given area.²  According to the American Academy of Oral and Maxillofacial Radiology, CBCT should be considered as the imaging modality of choice for preoperative cross-sectional imaging of potential implant sites.³ 

The lingual anatomy of the mandible presents with several structures that play key roles in treatment planning and implant placement. These structures include the inferior alveolar nerve, the mental nerve, and the submental/sublingual arteries.^4–6  In 2010, Chan et al⁷  performed a cross-sectional analysis of the mandibular lingual concavity and created a classification system based on the cross-sectional mandibular morphology seen in CBCT scans. Their classification system (C/P/U types) took into account the mandibular first molar region. The anatomical locations of each of these structures, as well as the anatomy of the mandibular lingual concavity, must be carefully identified, evaluated, and analyzed in all mandibular implant cases prior to surgical placement.

The aim of this study was to analyze the variations of the lingual concavity in both the posterior and anterior regions of the mandible and to create a classification system for both regions.

The study was approved by the Institutional Review Board of the Lake Erie College of Osteopathic Medicine (LECOM) School of Dental Medicine as protocol 23-106 on January 5, 2016.

CBCT images were acquired from a private practitioner affiliated with the LECOM School of Dental Medicine. All images were taken with a Sirona XG3 CBCT. Imaging parameters for all scans were as follows: area dose 693 mgy × cm², tube current 6 ma, and tube voltage 85 kvp. The field of view was set at 8 cm × 8 cm for each patient. Total radiation received per patient was set to 166 μSv. The images were analyzed with an implant-planning software program, Galaxis/Galileo Implant Viewer by Sirona. Once all data regarding the lingual concavity was gathered, general information on patient age and race was released, but no personal information was released to examiners or used in any way during this study.

A total of 104 images were analyzed for purposes of this study. The specific areas of interest were both the right and left mandibular first molar region and the mandibular central incisor region. All measurements taken from the CBCT scans were completed by 1 examiner (M.P.). A cross-sectional image was selected on both the left and right mandibular first molar region, through the furcation area, as close to the midline as possible. If the first molar was absent, a midpoint was established between the mandibular second premolar and the mandibular second molar. The measurements of the mandibular anterior region were taken at the midpoint between the 2 central incisors.

The angle of the lingual concavity was determined by the following method: a horizontal line (HL) was drawn approximately 1.5 mm above the mandibular canal²  in the posterior region and 1.5 mm above the inferior border of the mandibular anterior region. The most prominent lingual aspect of bone was marked as a reference point (SLP). A vertical line (VL) was drawn from the SLP meeting the HL, effectively forming the legs of a triangle. The angle of the lingual concavity was measured using the HL of the border of the mandible that formed the hypotenuse of the triangle (Figures 1 and 2).

Several distinct types of mandibular morphology were seen in each of these areas, and based on the type of morphology, each image was classified into 1 of 3 types: (1) parallel, (2) concave, or (3) convex.

In measuring the posterior mandibular regions, when the angulation was less than 85°, the area was classified as “concave” (Figure 3). When there was no obvious lingual undercut, or the morphology was between 85° and 95°, the image was classified as “parallel” (Figure 4). If the angle was larger than 95°, the image was classified as “convex” (Figure 5).

Because of the morphological differences in the anterior and posterior mandible, separate measurement criteria were used to classify cross-sectional images in the anterior region. When there was a lingual undercut, less than 60°, the image was classified as “concave” (Figure 6). When angle was between 60° and 70 °, the image was classified as “parallel” (Figure 7). If the angle was larger than 70°, the image was classified as “convex” (Figure 8).

Statistical analysis was performed and results were analyzed to determine if race, age, and gender were significant variables (P ≤ .05). For race, a single-factor analysis of variance test was used to determine variance. For age and gender, a 2-sample t test was performed.

A total of 104 scans were selected for this study. Of these 104 CBCT scans, 2 images were not analyzed because of corrupt DICOM files. The remaining 102 scans included 47 men (M) and 55 women (F) and included an age range of 21 to 89 years, with the average age being 54.8 years.

The scans were analyzed in cross sections, and the following findings were observed: in the posterior mandible, 61 (60%) were concave, 38 (37%) were parallel, and 3 (3%) were convex (Figure 9). In the anterior region, 39 (38%) were concave, 52 (51%) were parallel, and 11 (115) were convex (Figure 10).

The average lingual concavity seen in the anterior regions was 82.88° (F) and 83.65° (M). In the posterior first molar region, the average concavity was 75.87° (F) and 75.45° (M) on the right and 75.39° (F) and 75.19° (M) on the left. The average concavity length was 17.80 (F) and 18.66 (M) in the anterior region. In the posterior right region, the average length was 15.78 (F) and 16.20 (M), and in the posterior left region, the average length was 16.14 (F) and 16.27 (M; Table).

Statistical analysis revealed a significant difference detected with age in which the posterior right concavity vertical length (VL) and posterior left concavity vertical length (VL) were lower for older patients (Figures 11 and 12). There were no significant differences detected for race or gender.

Previous studies have evaluated the mandibular lingual concavity region in which a unique classification system was developed. Watanabe et al⁸  in 2010 evaluated the posterior lingual concavity in a Japanese population. Based on their study, they reported that a lingual concavity was present in approximately 36% to 39% of all patients. Chan et al⁷  also evaluated numerous CBCT scans for the presence of a lingual concavity in the posterior region. Their results revealed that 66% of their subjects presented with a posterior mandibular lingual concavity. These studies are consistent with our results, in which we found that 60% of posterior mandibles exhibited a lingual concavity.

The mandibular lingual concavity in the anterior region has not been extensively researched. A study performed by Kamburoğlu et al⁹  evaluated the prevalence of the submandibular and sublingual region. Their results found that the mean depth and volume of the sublingual region were 1.3 mm and 26.5 mm³. Although they accurately identified and mapped the sublingual region, they did not apply a classification system. To our knowledge, there are few studies that evaluate this region in particular for implant placement or attempt to classify any concavity found.

Perforation of the lingual plate concavity can have serious consequences for the patient. Arising from the external carotid artery, the lingual artery loops around to pass between the genioglossus and hyoglossus muscles.¹⁰  A large lingual concavity in the anterior region of the mandible can pose a risk of perforation in this area.¹¹  Piercing of the vasculature on the ventral side of the tongue can lead to potentially fatal arterial hemorrhage.^12,13 

In the posterior region, a perforation of the lingual plate concavity is not as life threatening. There are minimal vital structures in the submandibular space except for submandibular glands and lymph nodes. However, it does not mean that this area should be ignored for implant placement. The position of the lingual nerve must also be considered for posterior mandibular implant placement. The lingual nerve is a branch of the posterior trunk of the mandibular nerve given off in the infratemporal fossa, coursing close to the lingual aspect of the mandible at the region of the third molar.¹⁴  Because of the varying course of the lingual nerve, perforation of the lingual cortical plate during implant placement can result in nerve dysfunction.

If a perforation of the lingual plate is left undetected and an implant placed, the extruded implant may cause persistent inflammation or infection. This potentially could cause significant issues with the spread of infection. Because of the location of the concavity, an infection in this area could easily spread to the parapharyngeal and retropharyngeal space, leading to more severe complications such as mediastinitis, mycotic aneurysm formation with subsequent rupture of the internal carotid artery, internal jugular vein thrombosis with septic pulmonary embolism, or upper airway obstruction.¹⁵  Although these types of complications do not immediately occur, the severity of these entities should be taken seriously during the treatment planning and placement of implants in the posterior mandible.¹⁶ 

An interesting finding with this study is in regard to the amount of residual bone present. The design of our study included a measurement of both the anterior and posterior mandibular ridge height from the most prominent lingual bone to a fixed point in the mandible (VL). The greatest amount of residual bone height was found in the anterior mandible, which makes it an ideal site for implant placement. In the posterior mandible, a decrease in VL was observed in older patients, with a significant change occurring around age 63. This could be due to mandibular bone resorption of the edentulous area, which is increased with aging.¹⁷  Because of this factor, implant placement can be more difficult because of the decreased height and resorption in edentulous areas, especially in the elderly, in whom bone resorption is common.

Although no significant differences were detected for race or gender, a larger sample size may have helped to find a significant correlation. Further studies on this topic should include a larger sample size and multiple calibrated examiners to rule out variability.

Preoperative imaging is imperative to the success of implant placement. Combined with proper treatment planning and analysis of the surgical site, a CBCT scan is a useful diagnostic tool that can provide important information about anatomical structures and morphological variations in the sites of interest. In this study, vital landmarks such as the inferior alveolar nerve canal and the inferior border of the anterior mandible were used as reference points in data collection. With these points, a system was developed to classify the mandibular undercut in both anterior and posterior regions. In the posterior mandible, 61 (60%) were concave, 38 (37%) were parallel, and 3 (3%) were convex (Figure 9). In the anterior region, 39 (38%) were concave, 52 (51%) were parallel, and 11 (115) were convex. The lingual concavity, if present, should be measured and carefully assessed to determine if the implant placement is a viable option in the posterior first molar region of the mandible. Likewise, in the anterior region, the sublingual and submental arteries should be carefully avoided by assessing the lingual concavity in order to decrease the risk of hemorrhage and airway obstruction because of its close proximity to the mandible. By analyzing these points in reference to the lingual concavity anatomy using CBCT, preoperative diagnostic studies can be performed to ensure proper implant placement to avoid potential life-threatening complications.

Abbreviations

CBCT

cone beam computerized tomography

HL

horizontal line

LECOM

Lake Erie College of Osteopathic Medicine

SLP

the most prominent lingual aspect of bone

VL

vertical line

The authors would like to acknowledge Mr William R. Potrikus for his contribution to the statistical analysis of the data collected. The authors would also like to acknowledge Ms Andona R. Zacks-Jordan of the Institutional Planning, Assessment, Accreditation and Research (IPAAR) for her contribution in editing the manuscript.