Relationships Between the Gonial Angle and Mandibular Ramus Morphology in Dentate Subjects: A Panoramic Radiophotometric Study

Dental panoramic radiographs (DPRs) provide extensive information about the condition of the maxillary and mandibular dental arches and jawbone structures. In clinical practice, DPRs are indispensable for the examination of the bone tissues around missing or remaining teeth in the dental arches, the assessment of bone quality and quantity at implant sites,¹  and the correct diagnosis of the anatomic risk areas (maxillary sinus, incisive canal, mental foramen, and mandibular canal).²  DPRs are also morphometrically useful^2–7  to estimate the height of the alveolar ridges in edentulous jaws,⁸  to measure the amount of alveolar bone resorption with loss of teeth,³  and to detect osteopenia or osteoporosis.^4,5,9–11 

In addition, DPR analysis is more useful for assessing the right and left mandibular morphology compared with the analysis performed using cephalograms, which present the image with one side of the ramus of the mandible (mandibular ramus) superimposed on the other.^{3,6,7,12–17} 

Dental panoramic radiography is advantageous over computed tomography with respect to radiation exposure dose, swiftness, and cost.^2,12,18  While there is a tendency to use computed tomography prior to other imaging procedures before implant surgery in Japan, we believe that DPR examinations should be prioritized over computed tomography scans from the point of view of patient protection against possible radiation hazards.

Morphometric studies using DPRs have thus far centered on the relationships between the sizes of the gonial angle (GA) and the mandibular body^6,7  and occlusal forces.^19–21  The relationships between GA and mandibular ramus morphology have received little attention.^17,18 

The purpose of the present study was to use DPRs to clarify the differences in the morphologic parameters of the mandibular ramus (height, width, cortical bone thickness, depth and angle of the posterior ramus notch, and the indices) of young-to-aged dentate subjects between large and small GA groups, as well as between genders, and to investigate the correlations between GA size and variables of mandibular ramus morphometry.

A total of 156 dentate subjects (78 for each gender) with more than 15 teeth on their jaws were enrolled in this study. They were outpatients of a dental office and had no history of systemic diseases, metabolic bone diseases, or parathyroid disorders (for details, see Table 1). The subjects were informed that they would have to undergo dental panoramic radiography, and their DPRs would be used for this study, as well as for devising their teeth and periodontal treatment plans. All subjects provided informed consent. The X rays were taken by an experienced technician, who precisely positioned the heads of the examinees (setting of the Frankfort horizontal plane and the position of the midsagittal line, the posteroanterior relationships, as well as the tilt and rotation of the head).^{3,6,7,12,13,19–22}  The DPRs were obtained using a single panoramic apparatus (at ×1.2–1.25 magnification) (Auto II, Asahi Rentogen Inc, Co Ltd, Kyoto, Japan). Exposed films were developed using an automatic processor (Max-Rhein, Nix, Tokyo, Japan). This study only used those DPRs that presented clear, good contrast, and symmetrical images without distortion in the region of the mandibular ramus. Furthermore, only those films on which the size of the GA in both the right and left sides was less than 120° (low gonial angle [LGA] group) or more than 125° (high gonial angle [HGA] group) were selected.

An experienced dentist overlaid each DPR with a 60-μm thick transparent overhead projector film in a dark room, marked the anatomic reference points, and drew reference lines using a 0.1-mm graph pen on a light box (Figure 1). Within 2 weeks, the same dentist reassessed the reference points and lines and determined the final positions. Linear measurements were obtained using a digital vernier caliper (Mitsutoyo, Tokyo, Japan), while angular measurements were obtained using a semicircular protractor (Staedtler, Germany).

After each parameter was measured, calculated, and assessed, the mean value and standard deviation were computed using Microsoft Excel. The ramus cortical index (RCI) was evaluated by the same dentist twice (Figure 2). If the first and second observations differed, another reassessment was conducted.

The Mann-Whitney U test was used to compare the differences in mean values between the parameters for the LGA and HGA groups and between men and women. The symmetry and distribution of the 3 categories of RCI between the LGA and HGA groups and between men and women were analyzed by using the χ²  test and contingency table test. Pearson correlation coefficients were computed by using Microsoft Excel. The significance of the correlations between GA size and the ramus morphometric variables was assessed by simple regression analysis. Statistical analysis of the data was conducted using StatView software (Abacus Concepts Inc, Berkeley, Calif) and a Power Macintosh personal computer (Apple Inc, Cupertino, Calif).

Table 2 shows the results of comparisons between the parameters of the mandibular ramus morphology in the LGA and HGA groups. The mean values of Ar′-Go, MaF-Go, RW, RCW, and RD were significantly larger in the LGA group than in the HGA group irrespective of gender; however, the mean value for ramus angle (RA) was significantly smaller in the LGA group than in the HGA group.

Table 3 shows the results of the classification of RCI. Symmetry was dominant in both the GA groups. There was no significant difference in the distribution of symmetry between the 2 GA groups. Table 4 shows the distribution of the 3 categories of RCI for both GA groups. Significant differences between the LGA and HGA groups were noted in the RCI distribution.

The means values of Ar′-Go, MaF-Go, RW, and RD were significantly larger in men than in women, whereas those of PRI (LGA group) and RCI (HGA group) were significantly larger in women than in men (Table 2). No significant difference was noted in the symmetrical distributions of RCI classification between men and women (Table 3). Significant gender differences were noted in both GA groups with respect to the distribution of the 3 categories of RCI (Table 4).

Analysis of the correlation between GA size and ramus morphometric variables revealed that the GA size had significant negative moderate correlations with Ar′Go, MaF-Go, and RW (Figure 3). It further revealed significant negative low correlations of GA size with RCW, RD, and RCI (r < −0.4, P < .05, and P < .01) and significant positive low correlations with RA (r < 0.4, P < .01) in both genders.

The results of our study revealed that Ar′-Go, MaF-Go, RW, and RD, which are located in the posterior area of the maxillofacial skeleton, were markedly larger in the LGA group than in the HGA group (Table 2).^20,21,23,24  These findings were consistent with those reported previously using lateral cephalograms.^20,21,23,24  Hence, when the posterior facial portion of the LGA group appeared large,^20,25  the masticatory force was supposed to be strong. Therefore, prosthetic appliances and implant fixtures should be designed and selected with adequate consideration of the strong occlusal force when oral rehabilitation supported by implants is recommended to dentate subjects in the LGA group.

RCW in the posterior area of the mandibular foramen was also thicker in the LGA group than in the HGA group (Table 2), indicating that bone remodeling around the parameter was active in subjects with smaller GA. On the other hand, the increase and decrease in RA were in proportion to those of GA. Therefore, occlusal force can be thought to be stronger in the subjects with thicker RCW and smaller RA.

We presumed that the PRI used in this study corresponded to the PMI proposed by Benson et al.¹⁰  PMI is useful for osteoporosis screening and is affected by the size of GA. However, in this study, PRI proved to be unaffected by the size of GA (Table 2). With regard to the parameter of RCI, we assumed that the LGA group would show smaller values than those of the HGA group, but contrary to our expectation (Table 2).

When the means of each parameter for men and women were compared, significant sexual dimorphism in the 4 parameters of the mandibular ramus (Ar′-Go, MaF-Go, RW, and RD) regardless of the size of GA was noted (Table 2), and the mean values were larger in men than in women. This finding was consistent with those of other investigators who had conducted lateral or posteroanterior cephalometric analyses.^20,21,23,24  In addition, masseter muscles, temporal muscles, and the anterior belly of the digastrics muscles were found to be thicker in men than in women.^24,25  On the basis of these findings, we speculated that prostheses and oral rehabilitation with implant treatment designed for men must be stronger than those for women.

The mean values of the mandibular cortical width in the mandibular body were larger in men than in women, indicating that bone remodeling around the mandibular cortical width is active in men.^3,7,13  Nonetheless, a similar finding was not obtained for the RCW.

The classification of RCI in the dentate subjects revealed that their symmetry was dominant regardless of the size of GA and gender (Table 3). This finding was similar to that of the classification of the mandibular cortical index of the mandibular body in the dentate individuals.²⁶  The classification of the 3 categories of RCI also showed that the smaller the GA became in men, the more varied were the morphologic changes in the ramus cortical bone, and the case was vice versa in women (Table 4). In other words, there was a different tendency in men and women. Further detailed studies will be required to elucidate the reasons for these gender differences by using visualization procedures for the examination of the cortical bone erosion.^4,27,28 

The linear relations of the GA size to the variables of Ar′-Go, MaF-Go, RW, and RD showed that the smaller the GA was, the larger the variables became in both genders (Figure 3).^19–21,24  This finding supported the results of previous studies. It also suggested that the designs for implant prostheses should take into account the negative relation between GA size and occlusal force. Regarding the linear relation between GA size and RA, RA becomes smaller in inverse proportion to occlusal force. A similar finding has been reported by researchers who studied the correlation between GA size and the antegonial notch angle in the mandibular body.²⁶  Hence, the assessment of RA and antegonial notch angle will be needed in implant-supported oral rehabilitation.

Combining the findings of the present and previous studies,^{19–21,24,29}  we arrived at the conclusion that the GA size and gender were associated with the morphology of the mandibular ramus and occlusal force. This study, therefore, suggested that morphometric analysis of the mandibular ramus on DPRs depending on the GA size and gender should be used while devising a strategic treatment plan for implant-supported oral rehabilitation.

Further study will be required to investigate the differences and relationships between GA and mandibular ramus morphometry in edentate subjects.

DPR

dental panoramic radiographs GA: gonial angle

HGA

high gonial angle

LGA

low gonial angle

RA

ramus angle

RCI

ramus cortical index