A CBCT Assessment of the Incidence and Location of the Lingual Foramen in the Anterior Mandible

The placement of dental implants in the anterior mandible is a relatively safe and common procedure.¹  The incidence of surgical complications in this area is relatively low.¹  This site is commonly used for the placement of the implants to retain mandibular overdentures.^1,2  Several articles have reported incidences of hematoma following and during dental implant surgery in this area.^1–8  If an implant drill cuts through the vascular contents of a lingual foramen, the exsanguination can expand into the sublingual space. A hematoma or bleeding in this space would probably not compromise the airway and thus may not be life threatening. Nonetheless, an expanding hematoma in the sublingual space may force the tongue into the pharynx and potentially close the airway, thus creating a life-threatening condition. Such a complication may require intubation or an emergency tracheotomy.^2–4,7,8  If an expanding hematoma occurs that threatens airway compromise, an emergency tracheotomy may be considered. This may be best placed at the first tracheal ring.⁹  The superior thyroid vein should be palpated and digitally pressed to one side for the entry. The first tracheal ring is chosen because it is superior to the thyroid gland and inferior to the thyroid cartilage, which may contain vocal cord ligaments. Access through the septum of the thyroid cartilage may be proximate to the ligaments of the vocal cords, and these may be damaged on entry.

Cadaver-based studies or dry skull specimen–based studies have reported the incidences of the lingual foramina.¹  The present study was made on collected patient cone beam computerized tomograms (CBCT) from a large database comprising of patients who had attended the Department of Oral Radiology at Saveetha Dental College and Hospitals in Chennai, India. There was assessment of the distance between the mandibular border and the lingual foramen. The presence or absence of an anastomosing loop was also assessed. Any branching of the lingual foramen and its incidence in the midline were also noted. The path of vessels was recorded, and demographic data were recorded.

A total of 70 CBCTs of the anterior mandible were collected from the database and assessed. Of these 70, 5 were deemed unsuitable for assessment due to poor readability or patient positioning errors. Thus, 65 CBCTs were included in the study. All of the CBCTs were obtained for the purpose of other diagnosis, and samples were randomly selected. None of the CBCTs used for assessment in the present study were obtained solely for the purpose of inclusion in this study.

There were 5 groups for age, extending from 21 to 80 years. The midline of the mandible in all samples was identified using the genial tubercles. This area was then examined for lingual foramina. Any lateral lingual foramina found a further distance from the midline were also recorded on the interdental foramen region extending from the left to the right mental foramen. The number of foramina in various sites was noted, and the vasculature within the structure of the anterior mandible was also noted. Vessels that anastomosed through other foramina at the midline were noted. Measurements were made perpendicular to the horizontal from the primary lingual foramen or the superior lingual foramen. The superior lingual foramen was regarded as the more occlusal foramen of 2 foramina if 2 foramina were noted with an anatomical continuity of vasculature between them. The lower foramen that was closer to the base of the mandible was regarded as the inferior lingual foramen.¹⁰  The distance was measured and noted between the superior lingual foramen and the base of the mandible in an attempt to identify whether there was a relevant anatomical consistency in the location of this foramen. This foramen was also chosen because it is closer to the crestal alveolar bone and is therefore at a greater likelihood of encountering from a dental implant osteotomy. Foramina were found outside the midline and noted.

The methodology and results of the present study were reviewed by an independent statistician. Following the completion of the study, the data were statistically analyzed using SPSS Statistics for Windows, version 17.0 (SPSS Inc, Chicago, Ill). The descriptive statistics were mean, standard deviation, frequency, and percentage. Analysis of variance was used for quantitative analysis. Statistical significance was considered to be at P < .05.

A total of 65 mandibles were included in the study. The sample consisted of 42 males and 23 females. All mandibles were assessed with regard to the incidence of the lingual foramen, the number of branches in the lingual vasculature, and the incidence of studied anatomy in the midline. Any incidence of a loop was noted. Various parameters were studied, and the results following statistical analysis are presented. Gender and age did not have any effect on occurrence or morphology (P > .05).

The incidence of the superior lingual foramen was noted to be 100% of the CBCTs. A minority of arteries had branches after penetration through the mandibular cortex. The incidence of arterial branching after entry into the mandible was 9.2%. Of the 9.2%, 7.7% showed 2 branches, and 1.5% showed 3 branches. Wherever branching was found, their path was noted (Table 1).

A total of 96.9% of mandibular CBCTs showed the incidence of a superior lingual foramen at the midline. The midline was noted as being between the genial tubercles on the CBCT axially (Table 2).

When an artery entered the superior lingual foramen, anastomosing occurred in most of these; 87.7% of mandibles had a demonstrable anastomosis that met with an artery from the inferior lingual foramen, and 12.3% of mandibles had no anastomosing, and the artery terminated in the anterior mandible (Table 3).

Gender did not influence the occurrence of the lingual foramen (P > .05). However, gender did influence the location of the lingual foramen (P = .006). The lingual foramen was located farther away from the base of the mandible in males than in females (Figure 1, Table 4).

The mandibles were classified into 5 age-groups. No significant age influence variation was found (P = .947) (Figure 2, Table 5).

This study is in agreement with a study conducted by McDonnel et al¹¹  using 314 dry mandibles. They found a 99.04% incidence of a midline lingual foramina that was described to be the point of entry of an artery that was an anastomosis of the sublingual branches of the right and left lingual artery.¹¹  However, the study did not consider the incidence of any other accessory lingual foramina.¹¹  The present study found a 100% incidence of the superior lingual foramen in all CBCTs and an 87.7% incidence of what was regarded as the inferior lingual foramen (Table 3).

A superior lingual foramen was found in all CBCTs. This finding is at variance with 2 other studies that assessed the incidence of the superior lingual foramen at the midline. A macroanatomical study based on 50 Caucasian dry human mandibles found a midline lingual foramen in 98% of samples, with only 1 sample in the study lacking a true midline foramen.¹⁰  The same study also identified midline lingual foramina as being either superior or inferior with only a 62% incidence of a superior foramen and a 38% incidence of an inferior foramen.¹⁰  The present study found superior and inferior midline lingual foramina in 87.7% of CBCT samples. All of these had an anastomosis with the other penetrating arteries; 12.3% of mandibles in the present study had only a superior lingual foramen, and no mandibles had a sole inferior lingual foramen (Table 3).

Another study involving dried adult mandibles of Indian populations found a midline foramen in only 72.45% of mandibles. This same study also assessed the presence of foramina in dissected cadavers and found a sole foramen in only 55.5% of samples.¹²  There may be a practical difficulty in finding these small foramina in cadavers as opposed to viewing them on CBCTs. The present study used CBCT as a means of assessment, and it is unclear whether this offered an advantage in terms of assessment of the incidence of the superior lingual foramen.

Two older studies by Sutton¹³  and Shiller and Wiswell¹⁴  reported an incidence of 85% and 88.9%, respectively, in anatomical dissections. Nonetheless, both of these studies took accessory lingual foramina into account and did not specifically report on a midline lingual foramen.

Considerable confusion has been created by the nomenclature of these foramina. Some studies refer to the accessory foramina as either lingual foramina or genial spinal foramina or foramen interspinosum or foramen infraspinosum.^15–17 

An older radiographic study from 1942 reported the incidence of a lingual foramen in only 28% of mandibles. The finding of this low incidence could probably be attributed to the quality of the radiographic system.¹⁸  Oral radiology texts discuss the presence of lingual foramina in the mandible. Lingual foramen may be more easily found in radiologic observation rather than in a clinical or dissection observation.

In a macroanatomical study by Liang et al,¹⁰  22% (11 mandibles) of samples showed at least 2 foramina, while 4% (2 mandibles) showed 3 foramina, while most other mandibles showed a single superior lingual foramen.¹⁰  In the study herein, multiple foramina were observed in only 1 CBCT (1.5%) (Table 1). A single superior lingual foramen at the midline was a consistent finding in the present study. Lateral foramina were not assessed in the present study, as the focus of the study was the analysis of consistent anatomical structures that affected the placement of dental implants in the anterior mandible. Lateral foramina are inconsistent anatomical findings in previous studies. The incidence of additional or accessory foramina between the mandibular midline and the canine region has been assessed in one study.¹⁰  This study found multiple lateral foramina and canals to the left and right of the midline.

A CBCT assessment of the anterior mandible for implant treatment is important to prevent any accidental severance of vital structures. Resorption of the alveolus will change the positional relationship of any foramen and the morphology of the ridge contour and volume.

In the study herein, branching of the vascular bundle that entered through the superior lingual foramen was noted in 9.2% of mandibles (Table 1). The exact contents of this bundle are a point of contention among studies. In 1 cadaver study, a histological assessment of this structure showed an artery with no accompanying vein or nerve.¹¹  In an older study, though, this structure has been previously surmised to be a neurovascular bundle.¹³  However, the diameter of the artery has been deemed to be sufficient to cause a hemorrhage that would be difficult to control. This is due to significant anastomosing with the submental and incisal arteries.¹²  Some studies have found that the content of the vascular bundle in the superior lingual foramen is different from that of the inferior lingual foramen.^{6,11,13,15–17} 

A literature review also suggests contradictory descriptions regarding these macroanatomic foramina and their anatomical relationships. One study stated that the lingual foramen transmits a branch of the incisive artery to anastomose with the sublingual artery.¹⁵  A branch of the sublingual artery has been suggested by others.^6,11,17  Yet another blunt dissection study suggested the passage of branches of the mylohyoid nerve through a foramen inferior to the genial spines.¹⁶  Another study agreed but thought the mylohyoid vessels were present as well.¹³ 

Dissection studies in Indian cadavers suggest that only a vascular bundle entered the superior lingual foramen and that this appears to be a single branch of the sublingual artery with no evidence of a venous or neural component.¹²  The study suggested that a venous-arterial bundle was possible with anastomoses to the branches of the left and right sublingual arteries. There may be a racial genetic explanation for this.

The influence of gender and age was found to be insignificant when correlated with the incidence and location of the lingual foramen.

The present study calculated the distance between the superior lingual foramen and the base of the mandible in an effort to identify a “danger zone” with regard to placement of dental implants in the anterior mandible. A cadaver dissection study on 12 cadavers identified the mean distance between the lingual foramen and the alveolar crest as being approximately 15.6 mm. Alveolar crest resorption would make these measurements inaccurate. The present study measured the distance from the base of the mandible because this landmark may be more reliable.

Another study used 50 dry human mandibles and measured the location of the superior lingual foramen from the base of the mandible using a technique that was very similar to the one used in the present study.¹⁰  However, clinical evaluation is not possible, and radiological CBCT evaluation seems to offer the only effective alternative. The base of the mandible is also a reliable point of reference, as the only alternative point of reference, the crest of the ridge, is unreliable due to resorption. The present study used a digital measurement tool in the Galaxis Implant software to measure the distance between the superior lingual foramen and the base of the mandible as a perpendicular line from the inferior border of the mandible. The aforementioned study, using 50 dry mandibles, found the superior height of the lingual foramen to be 12.6 mm with a standard deviation of 5.6 mm.¹⁰  The present study calculated the distance of the superior lingual foramen from the inferior border of the mandible to be 15.53 ± 1.88 mm for males and 14.18 ± 1.66 mm for females (Table 4). This would indicate a “danger zone” between about 14.2 and 15.6 mm that can serve as a guideline to help prevent an encounter with the superior lingual foramen.

The method of assessment used in the present study is also a limited modality. Disadvantages of CBCT include the inability to accurately represent the internal structure of soft tissues and the presence of various types of artifacts.¹⁹ 

Technical variability also means that standardized CBCT examination protocols are another limitation.²⁰  However, the present study used multiple CBCTS obtained from a single machine with a standardized protocol for full-volume CBCT studies.

A systematic review of the literature would also serve to generate an accepted modality of measurement that would resonate with the implant clinician as well.

The authors would like to draw attention to several limitations in the present study that should guide the readers' conclusions. The present study used only a radiological means of assessment. If this is to be accepted, would a CBCT study of the anterior mandible be justified in every patient having an implant placed in the region? CBCT does involve higher doses than 2-dimensional imaging,¹⁹  and the author suggests that CBCT be considered only when the lingual foramen is not clearly discernible on 2-dimensional imaging or if guided implant surgery is being planned.

The present study is also limited by its sample size, which is limited to a geographical population. It is very possible that racial variations may exist, and it would be useful to see further studies in other population groups. In any case, the numerical findings of the present study in terms of an anatomical average should never be taken at face value. The present study also limits itself to assessment of an offending artery that is found at the midline of the mandible. It does not consider aberrant anatomy that may present itself anywhere in the interforaminal region of the anterior mandible. The reader is encouraged to always assess this region carefully on a CBCT given the high incidence of complications in this region.²¹ 

Notwithstanding its limitations, the study seeks to draw attention to an important anatomical structure with significant clinical consequences.

A hematological complication in the anterior mandible is life threatening and nearly always results in a compromised airway that needs to be managed by intubation and surgical techniques.²¹ 

Over the years, multiple case reports have been appearing in the literature of bleeding complications in the anterior mandible. Several of these bleeding complications involve the sublingual artery at variable anatomical positions.²¹  The present study seeks to therefore draw attention to more careful anatomical assessment of this region and suggest a method to assess the 1 structure in the region that seems consistent in its anatomical presentation. A recent review of the literature suggests that there may be other aberrant anatomy that contributes to hematological complications.²¹  However, it is hoped that objective assessment of the lingual foramen and an understanding of the radiological path of the sublingual artery within the anterior mandible will help reduce such complications.^{5,7,8,22–27}  It is hoped that this article draws more attention to the important anatomical structures in the anterior mandible with some guidance to help assessment.

Several publications in the literature have attempted to measure the positions of the superior and inferior lingual foramina from varying reference points. Many of these studies were confined to specific population groups, and it is obvious that there is significant variation in the findings. Some of the variations involve the incidence of the foramina themselves, while a significant number of studies showed an incidence between 93.33% and 100%. Few studies showed a 100% incidence of the lingual foramen in all patients assessed.^28–30  These studies were based on CBCT assessment of multiple patients.

A significant number of studies used the inferior border of the mandible as a reference point for measurement. But there is a significant variation in the measured position of the lingual foramen, depending on the population group. Variations extended between 7.06 mm³¹ and 17.4 mm.³²  These variations are obviously defined by differences in population groups, and the importance of data from different population groups is obvious. The present study found a 100% incidence in the population groups studied, limited to patients from South India. The present study utilized the base of the mandible as a reference point, and a literature review suggests that this finds acceptance in several studies. Only 2 studies have used the apex of the tooth as a point of reference, but the present study discusses the advantages of using the base of the mandible as a reference point. It is the contention of the authors that the superior lingual foramen is the main anatomical landmark of clinical relevance, and this has been the focus of the present study.

Lingual foramina serve as osseous entry channels for a significant artery, the sublingual artery. In this study, lingual foramina in the anterior mandible were found in 100% of the CBCTs examined. Preoperative CBCTs can identify anatomical structures to be avoided. The astute clinician should be aware of emergency procedures for management of hemorrhage in the floor of the mouth.

Abbreviation

CBCT:

cone beam computerized tomography