Subjects

Thirty-eight Japanese cadavers (76 hemimandibles) were obtained from the Department of Anatomy at Saga Medical School and examined with the approval of the Ethics Committee of Saga Medical School. All of the cadavers were fixed in a 10% neutral formalin solution. Fifteen were male (30 hemimandibles) and 23 were female (46 hemimandibles). The age range at the time of death was 55–96 years (mean 78 ± 10 years). The 38 mandibles (76 hemimandibles) were divided into two groups: an edentulous group (20 mandibles, 40 hemimandibles) and a dentate group with at least one tooth remaining (18 mandibles, 36 hemimandibles).

CT measurement

The 38 mandibles (76 hemimandibles) were scanned with a Multislice CT device (SOMATOM Emotion 16-slice configuration; Siemens AG, Germany). Imaging parameters were the following: tube potential 130 kV, tube current 150 mA, scan time 1 s, gantry tilt 0, slice thickness 0.6 mm, beam pitch 1.0, reconstruction interval 0.6 mm, and reconstruction function H90s. The field of view was 150 mm, and the voxel size was 0.3 mm for the x and y axes and 0.6 mm for the z -axis. The mandible sample was fixed on an acrylic fixture device with the inferior mandibular margin (IMM) plane aligned in parallel with the scanning plane and scanned with the mid-sagittal plane of the sample aligned with the vertical midline of the gantry. The CT data were saved in Digital Imaging and Communications in Medicine (DICOM) format.

The CT data obtained were analyzed with software program Landmark System LAND marker Direct, version 6.11 (iCAT, Osaka, Japan), as described below.

First, among the slices vertical to the dental arch both mesially and distally from the mental foramen (MF), slices with possible SF were selected for measurement; the deepest point of the SF (SFP0) was defined on the lingual surface and the depth of the SF (SFD0) was measured on each slice ( Fig. 1 ). The largest value among SFD0s was defined as the SFD; the corresponding SFP0 was defined as the SFP ( Fig. 2 A) . The vertical distance between the alveolar crest (AC) and the SFP (AC–SFP; Fig. 2 A) and the horizontal distance between the MF and the SFP (MF–SFP; Fig. 2 B) were measured.

Orthoradial views of slices with the sublingual fossa (SF). The SF depth is measured on each view. The most prominent superior and inferior points (red-encircled black dots) of the depression on the lingual side are joined with a straight line (white line). The perpendicular distance from this straight line to the depression on the lingual surface is greatest at the intersection of the perpendicular line and the cortical bone surface of the lingual side (SFP0) (indicated by a black-encircled white dot); the distance is defined as the depth of the SF (SFD0; distance between the black and white arrows). The inferior mandibular margin (IMM) plane is used as the reference plane. Li: lingual side.

Measurement sites related to the sublingual fossa (SF) on the CT images. (A) Orthoradial view of the slice with the deepest point of the SF (SFP; black-encircled white dot). The greatest depth of the SF (SFD; distance between the black and white arrowheads) is the largest value of SFD0. The vertical distance between the alveolar crest (AC, green-encircled black dot) and the SFP (black-encircled white dot) is measured (AC–SFP). (B) Axial view at the mental foramen (MF) level. The red lines represent the slice vertical to the dental arch passing through the most mesial margin of the MF and that passing through the SFP (red-encircled black dot), respectively. The linear distance indicated by the double-headed black arrow is measured as the horizontal distance MF–SFP.

The LLF was defined as a foramen on the lateral lingual surface that was different from those around the mental spine on the mandibular midline. The LLF was identified in each sample; the vertical distance between the AC and the LLF (AC–LLF; Fig. 3 A) , the horizontal distance between the MF and the LLF (MF–LLF; Fig. 3 B and C), the vertical distance between the SFP and the LLF (SFP–LLF-Ver; Fig. 4 A) , and the horizontal distance between the SFP and the LLF (SFP–LLF-Hor; Fig. 4 B and C) were measured.

Measurement sites related to the lateral lingual foramen (LLF) on the CT images. (A) Orthoradial view of a slice with the LLF. The vertical distance between the alveolar crest (AC, green-encircled black dot) and the upper margin of the LLF (AC–LLF; indicated with the double-headed arrow) is measured. (B) and (C) Axial views at the mental foramen (MF) level. Cases in which the LLF is located on the mesial side (B) and distal side (C) of the MF. In (B), the red lines represent the slice vertical to the dental arch passing through the most mesial margin of the MF and that passing through the most distal margin of the LLF. In (C), the red lines represent the slice vertical to the dental arch passing through the most distal margin of the MF and that passing through the most mesial margin of the LLF. The linear distance indicated by the double-headed black arrow is measured as the horizontal distance MF–LLF (B and C). The horizontal diameter of the MF (shown by the red double-headed arrow) is measured (C).

Measurement sites related to the sublingual fossa (SF) and the lateral lingual foramen (LLF) on CT images. (A) Orthoradial view of a slice with the LLF. The vertical distance between the SFP-equivalent level (white arrow) and the top margin of the LLF (SFP–LLF-Ver, indicated by the black double-headed arrow) is measured. (B) and (C) Axial views at the LLF level. (B) A case in which the SFP is located on the mesial side of the LLF is shown; the red lines represent the slice vertical to the dental arch passing through the SFP and that passing through the most mesial margin of the LLF. (C) A case in which the SFP is located on the distal side of the LLF is shown; the red lines represent the slice vertical to the dental arch passing through the SFP and that passing through the most distal margin of the LLF. The linear distance shown by the two black arrowheads is measured as SFP–LLF-Hor (B and C).

A positive value was assigned to MF–LLF when the LLF was located mesially relative to the MF ( Fig. 3 B), while a negative value was assigned when it was located distally relative to the MF ( Fig. 3 C). In the samples with negative MF–LLF values, the horizontal diameter of the MF was measured ( Fig. 3 C). In the samples with multiple LLFs in a hemimandible, the largest LLF was selected.

Physical measurement

Both the SF and the LLF were identified by CT in 24 of the 76 hemimandibles. These samples were cleared of soft tissues to identify the SF and the LLF grossly ( Fig. 5 A and B) . The mandible sample was placed on a flat experimental table and the IMM was defined as the reference plane. Lines (red lines in Fig. 5 A) were drawn vertically to the dental arch on the lingual side of the mandibles in the range in which the SF was present; the deepest point of the SF (SFP0) was defined along each line and the depth of the SF (SFD0) was measured with a mini digital tread depth gauge (MDDG; MonotaRo, Japan). SFP0, corresponding to the longest SFD0 in the SFD0s for the multiple lines (red lines in Fig. 5 A), was defined as the SFP ( Fig. 5 B). Callipers (N10S; Mitutoyo Corporation, Kanagawa, Japan) were used to measure the linear distance between the SFP and the LLF (SFP–LLF, Fig. 5 B).

Physical measurement: the mandibular lingual side cleared of soft tissues. (A) The sublingual fossa (SF) area is encircled with the white line. The red lines are drawn perpendicular to the dental arch on the lingual cortical bone surface with the SF. The red encircled black dots show the most prominent superior and inferior points relative to the lingual side depression, respectively. The black dotted lines connect these two red-encircled black dots. When the perpendicular distance between the black dotted line and the depression on the lingual surface is largest, the intersection of the perpendicular line (red arrow) and the lingual cortical bone surface is defined as SFP0 (white-encircled black dot); the distance is defined as the depth of the SF (SFD0; distance of the red arrow). (B) SFP0 corresponding to the largest SFD0 is the deepest point of the SF, which is indicated as SFP (white-encircled black dot). The lateral lingual foramen (LLF) is observed on the lower side of the SFP. The linear distance between the SFP and the LLF (SFP–LLF) indicated by the double-headed black arrow is measured.

All CT and physical measurement procedures were performed by one expert examiner.

Identification of the vessel penetrating the LLF

Prior to the removal of soft tissues for physical measurement, the origin of vessels penetrating the LLF was identified grossly in 15 mandibles (30 hemimandibles) selected from the 38 mandibles.

Statistical analysis

All statistical analyses were conducted using SPSS statistical software, version 11.0J (SPSS Japan, Tokyo, Japan). P -values of <0.05 were considered statistically significant. The χ ² test was used to compare the frequencies of the presence of SF and LLF in all samples. The range, mean, and standard deviation (SD) of the measurement values related to the location of the SFP and LLF were calculated for all samples and by gender and dental status. The unpaired t -test was used for comparison of the measurement values related to the location of the SFP and LLF, by gender and by dental status.