Computed tomography evaluation of mandibular ... - luca lombardo

ORIGINAL ARTICLE

Computed tomography evaluation of mandibular incisor bony support in untreated patients Antonio Gracco,a Lombardo Luca,b Maria Cristina Bongiorno,c and Giuseppe Sicilianid Ferrara, Italy Introduction: In this study, we aimed to verify, via computed volumetric tomography, a correlation between the morphology of the mandibular symphysis and the various facial types. Methods: From a sample of 148 digital volumetric tomographs, the subjects were classified as either short face (25 subjects), normal face (27 subjects), or long face (28 subjects) according to the average values of their Frankfort-mandibular plane angle. The 80 healthy subjects were between 12 and 40 years of age. Tomography was carried out using NewTom 3G volume scanner (QRsr1, Verona, Italy). The following parameters were measured on the sections corresponding to the 4 mandibular incisors: height, thickness, and area of the entire symphysis; height, thickness, and area of the cancellous bone of the symphysis; distance of the vestibular and lingual cortices from the apices of the 4 incisors; and possible inclination of each mandibular incisor, expressed in degrees. The F test or analysis of variance (ANOVA) and the Tukey HSD Test were subsequently used. Results: The total thickness of the symphysis was greater in the short-face subjects than in the long-face subjects. No statistically significant differences in the total and cancellous areas of the symphysis were found between the 3 facial types. In all 3 groups, the total and cancellous heights and areas were greater at the central incisors than at the lateral incisors. Conclusions: There is a statistically significant relationship between facial type and the total thickness of the mandibular symphysis. (Am J Orthod Dentofacial Orthop 2010;138:179-87)

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he attempt to identify an orthodontically ideal, long-lasting, and equilibrated position of the incisors that will not cause periodontal problems, future articular pathologies, or crowding relapse, and will be esthetically pleasing, has included the possible determination of the anterior-most limit of the teeth.1 The mandibular symphysis is the anatomic factor that limits the movement of those incisors, so awareness of this structure lowers the risk of potential damage to tooth roots and alveolar bone when moving teeth orthodontically.2-4 Thus, the choice of treatment plan should be greatly influenced by the morphology of the symphysis and the position of the mandibular incisors. Several studies have demonstrated a correlation between facial type and the alveolar bone morphology From the Department of Orthodontics, University of Ferrara, Ferrara, Italy a Research assistant, Postgraduate School of Orthodontics, University of Ferrara. b Research assistant, Postgraduate School of Orthodontics, University of Ferrara. c Chairman, Postgraduate School of Orthodontics, University of Ferrara. d Director. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Antonio Gracco, Via E. Scrovegni 2, 35100 Padova, Italy; e-mail, [email protected]. Submitted, March 2008; revised and accepted, September 2008. 0889-5406/$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.09.030

of the mandible.1,5,6 In 1991, Siciliani et al,1 while conducting a teleradiographic study of the correlation between facial biotypes and the morphology of the mandibular symphysis in 150 orthodontically untreated patients, found that the symphysis is thin and elongated in patients with long faces, whereas it is thicker in those with short faces. In 1998, Tsunori et al,5 using computed tomography (CT), found a correlation between facial type, mandibular cortical bone thickness, and the buccolingual inclinations of the first and second molars. However, because bidimensional radiographic representation of the region of the mandibular symphysis is plagued by intrinsic errors such as superimposition of anatomic structures, difficulty in identifying single dental elements, and magnification error of the x-ray because of the divergence of the radiant beam, it is only by using computed axial tomography that we can achieve accurate evaluation of the bony support of the mandibular incisors.7,8 Cone-beam CT yields highdefinition images of the teeth and bone at a far lower dosage of radiation than that in medical imaging and closer to the range of standard dental film series.9-11 Therefore, we used computed volumetric tomography in this study to verify a correlation between the morphology of the mandibular symphysis and the various facial typologies. 179

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Fig 1. Sections perpendicular to the line traced at the halfway point of the root canals.

MATERIAL AND METHODS

Cone-beam CT images of 148 patients between 19 and 43 years of age were analyzed. These tomographs were obtained by using a NewTom 3G volume scanner (QRsr1, Verona, Italy) at the following settings: field of view, 12 in; 110 kV (AP-LL); 2.00 mA (AP); and 1.00 mA (LL); exposure time, 5.4 seconds, and section thickness, 0.50 mm. Subjects with syndromes, craniofacial malformations, or evidence of trauma, and those who had had surgery on the stomatognathic apparatus or previous orthodontic treatment, and those with metal prostheses that could generate scattering phenomena were excluded from the initial sample of 148 digital volumetric tomography scans. The remaining 80 tomographs were from subjects (Class I or Class II malocclusion) from 19 to 37 years of age (mean, 29.5 years). Each computed volumetric tomograph was then subjected to NewTom 3G analysis by an orthodontist who was an expert in the use of this software. This secondary reconstruction permitted the creation of 3-dimensional (3D) maximumintensity projection images, from which it was possible to make the linear and angular measurements. Subsequently, the points required for construction of the diagnostic triangle according to Tweed were identified on each image: porion and orbitale to determine the Frankfort plane, and menton and gonion to determine the mandibular plane. Thus, via Tweed’s cephalometric analysis, the Frankfort-mandibular plane (FMA) angles were calculated for each subject, so that they were classified by facial type: 25 short-face subjects (FMA, 15 -21 ),

Fig 2. Measurements on the sagittal section. Red, Height of the mandibular symphysis; yellow, cancellous bone height; dark blue, thickness of the mandibular symphysis; light blue, cancellous bone thickness; dark green, vestibular part; light green, lingual part.

27 normal-face subjects (FMA, 22 -28 ), and 28 long-face subjects (FMA, 29 -35 ). A further secondary reconstruction was created for each digital volumetric tomography scan using NewTom 3G software to obtain axial sections that allowed identification of the dental canals of the central and lateral incisors. A line passing through the center of each canal of the root was then traced, from the left canine to the right canine, and the software automatically generated sagittal sections 0.5 mm apart of the mandible perpendicular to this line (Fig 1). Constructing the sagittal plane through the center of the radicular canal ensured that the various sagittal scans of each tooth corresponded to the central axis of each root. Moreover, the availability of many sagittal sections can be extremely advantageous in evaluating the structures involved. In this case, the sagittal sections analyzed were those passing through the central axes of the 2 central and 2 lateral incisors. For each of these

American Journal of Orthodontics and Dentofacial Orthopedics Volume 138, Number 2

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Fig 4. Variables calculated with AutoCAD. Fig 3. Points identified on the symphysis and mandibular incisor.

sections, the following measurements were calculated (Fig 2): (1) height of the mandibular symphysis, the segment parallel to the axis of the incisor from prosthion to the external surface of the lingual cortex; (2) cancellous bone height of the mandibular symphysis, the line parallel to the axis of the incisor from the vestibular to the lingual cortex of the symphysis; (3) thickness of the mandibular symphysis, the segment perpendicular to the axis of the incisor that passes through the apex of the root between the external surfaces of the lingual and vestibular cortices; and (4) cancellous bone thickness of the mandibular symphysis, the line perpendicular to the axis of the incisor that passes through the apex of the root coincident with the thickness of the symphysis between the internal surfaces of the lingual and vestibular cortices. We subsequently divided the cancellous bone thickness into vestibular and lingual portions (Fig 2). Twenty-four measurements (6 for each section) were made with the NewTom 3G software for each of the 80 patients, giving 1920 measurements. Each sagittal section was saved in JPEG format and imported into the planning program AutoCAD (16.2,

2005, Autodesk inc, San Rafael, California), which allowed us to calculate the total area and the internal (cancellous bone) area of the symphyses. The mandibular incisors and the osseous variables in the region of the incisors were defined as follows (Figs 3 and 4): (1) axis of the incisor, the line that passes through the apex of the root (point L) and the central incisal margin (point L2); (2) points As and Ps, the most anterosuperior and posterosuperior points of the mandibular alveolar process, respectively; (3) center of rotation (C), the hypothetical center of rotation of the mandibular incisors was considered the median point of the portion of the root embedded in bone, halfway along an imaginary line passing through As and Ps and the root apex; and (4) points A and P, the points on the internal surfaces of the anterior cortex and the posterior cortex, respectively, where the radicular apices would meet if a trajectory of the hypothetical rotation of the roots of the mandibular incisors around their center of rotation was plotted. The other variables, calculated with AutoCAD, were as follows (Fig 4): (1) A-P, the arc between points A and P that corresponds to the cancellous bone thickness of the alveolar process; (2) A-L, the arc between

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Table I.


Means and standard deviations of the values measured at the right lateral incisor, compared between the facial

types

Total bone height (mm) Cancellous bone height (mm) Total bone thickness (mm) Cancellous bone thickness (mm) Labial part (mm) Lingual part (mm) Total area (mm2) Cancellous area (mm2) A-P ( ) A-C-L ( ) L-P ( ) A-L ( ) P-C-L ( )

Short (S) Mean (SD)

Long (L) Mean (SD)

Normal (N) Mean (SD)

ANOVA test F

20.92 (3.17) 11.22 (2.64) 9.32 (2.20) 5.16 (1.89) 3.36 (1.57) 1.80 (0.98) 200.44 (50.81) 78.48 (30.12) 4.59 (2.01) 2.80 (1.36) 1.79 (1.20) 47.85 (17.63) 29.00 (18.86)

22.64 (2.51) 12.03 (1.92) 7.42 (2.20) 3.70 (2.09) 2.33 (1.23) 1.39 (1.19) 191.88 (33.53) 78.54 (23.37) 3.57 (2.11) 2.10 (1.16) 1.47 (1.23) 32.65 (12.39) 21.55 (14.81)

21.79 (2.61) 11.36 (2.78) 8.39 (2.02) 4.42 (2.23) 2.90 (1.58) 1.52 (0.90) 184.05 (34.77) 69.87 (20.57) 4.07 (2.37) 2.54 (1.60) 1.53 (0.92) 43.20 (20.92) 25.40 (13.32)

NS NS 0.026 NS NS NS NS NS NS NS NS 0.023 NS

Tukey HSD test

0.019 S-L

0.020 S-L

NS indicates that the values, even if different, were not significant. When ANOVA resulted in significance at 95%, the Tukey HSD post-hoc test was applied and reported in the last column.

points A and L that identifies the labial portion of the cancellous bone of the symphysis and indicates the distance that the apex of the root could move in a vestibular direction without involving the cortex; (3) L-P, the arc between points L and P identifies the lingual portion of the cancellous bone of the symphysis and indicates how far the apex of the root could move in a lingual direction without involving the cortex; and (4) angle A-C-L, the intersection of lines A-C (the point where the radicular apex meets the internal profile of the vestibular surface of the cortex and center of rotation) and C-L (center of rotation and radicular apex) and indicates how much the crown of the mandibular incisor could be moved lingually and the apex moved vestibularly; and (5) angle P-C-L, the intersection of lines L-C (radicular apex and center of rotation) and C-P (center of rotation and the point where the radicular apex meets the internal profile of the lingual surfacial of the cortex) and indicates how much the crown of the mandibular incisor could be moved vestibularly and the apex moved lingually. AutoCAD was used to make 20 measurements (5 for each section) for each of the 80 patients, giving 1600 measurements, which, with the measurements made with the NewTom 3G software, made a total of 3520 measurements.

Statistical analysis

The F test, or 1-way analysis of variance (ANOVA), was used for the analysis, in which we initially compared the various facial types and then analyzed the 4

incisors of each facial type. The means and standard deviations of each value were then calculated. When ANOVA was significant at 95%, the Tukey HSD post-hoc test was applied to verify where the statistically significant differences were correlated. RESULTS

In Tables I through IV, the means and standard deviations of the measurements for each incisor in the 3 groups (short, normal, and long faces) are detailed; in the last 2 columns, the results of the statistical analysis are reported. At the mandibular right lateral incisor (Table I), because the values for the total thickness and angle A-C-L were significant with ANOVA, the Tukey HSD test was used to compare each facial type with the others. The mean thickness of the symphysis in short-face subjects (9.32 6 2.20 mm) was significantly greater (P 5 0.019) than that in the long-face subjects (7.42 6 2.20 mm). The Tukey test also had a P value of 0.020 for angle A-C-L, showing that the mean is distinctly greater in subjects with short faces (47.85 6 17.63 ) than in those with long faces (32.65 6 17.63 ). At the mandibular right central incisor (Table II), ANOVA showed statistically significant differences in total thicknesses, thickness of the vestibular portion, angle A-C-L, and angle P-C-L. From the Tukey HSD test, we can see, by comparing the facial types, that the mean A-C-L angle in short-face subjects (53.90 6 12.83 ) was significantly greater (P 5 0.001) than that in longface subjects (37.20 6 9.98 ), as was the mean in normal-face subjects (47.35 6 14.47 , P 5 0.035). For

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Table II.

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Means and standard deviations of the values measured at the right central incisor, compared between facial

types


Short (S) Mean (SD)

Long (L) Mean (SD)


ANOVA test F

21.81 (3.33) 13.08 (2.66) 9.61 (2.03) 5.59 (1.87) 3.61 (1.24) 1.98 (1.04) 221.06 (52.98) 96.96 (34.35) 4.85 (1.77) 2.93 (0.99) 1.91 (1.14) 53.90 (12.83) 34.10 (17.68)

23.30 (2.57) 13.72 (2.26) 7.78 (2.23) 4.07 (2.00) 2.57 (1.03) 1.50 (1.18) 206.75 (39.61) 88.76 (25.88) 3.92 (2.07) 2.38 (1.06) 1.54 (1.18) 37.20 (9.98) 23.50 (16.06)

22.31 (3.00) 12.96 (2.81) 9.30 (2.62) 5.07 (2.39) 3.60 (1.69) 1.47 (1.07) 201.26 (35.14) 82.30 (27.10) 4.39 (2.26) 2.99 (1.44) 1.40 (0.98) 47.35 (14.47) 21.95 (13.01)

NS NS 0.034 NS 0.024 NS NS NS NS NS NS 0.001 0.035

Tukey HSD test

0.039 S-L 0.045 S-L 0.047 N-L

0.001 S-L 0.035 N-S 0.045 S-N


Table III.

Means and standard deviations of the values measured at the left central incisor, compared between facial

types


Short (S) Mean (SD)

Long (L) Mean (SD)


ANOVA test F

21.18 (3.24) 13.29 (2.65) 9.62 (2.43) 5.66 (2.18) 3.72 (1.74) 1.94 (1.13) 222.94 (55.06) 98.52 (34.46) 4.85 (2.02) 2.91 (1.31) 1.94 (1.29) 49.40 (17.86) 32.45 (19.96)

22.33 (2.22) 13.19 (2.36) 7.58 (1.99) 3.89 (2.11) 2.52 (1.23) 1.37 (1.14) 206.38 (37.77) 93.30 (29.38) 3.64 (2.03) 2.20 (1.06) 1.39 (1.20) 40.55 (16.22) 23.05 (16.31)

21.46 (2.64) 12.93 (3.35) 9.10 (2.68) 4.96 (2.34) 3.47 (1.50) 1.49 (1.12) 210.80 (36.88) 84.17 (29.83) 4.45 (2.42) 2.90 (1.41) 1.56 (1.31) 49.95 (15.25) 24.10 (17.54)

NS NS 0.024 0.047 0.036 NS NS NS NS NS NS NS NS

Tukey HSD test

0.023 S-L 0.037 S-L 0.038 S-L


angle P-C-L, the mean in short-face subjects (34.10 6 17.68 ) was significantly greater (P 5 0.045) than that in normal-face subjects (21.95 6 13.01 ). The mean total thickness in short-face subjects (9.61 6 2.03 mm) was significantly greater (P 5 0.039) than that in longface subjects (7.78 6 2.23 mm). Relative to the vestibular part, the mean thickness in short-face subjects (3.61 6 1.24 mm) was significantly greater (P 5 0.045) than that in long-face subjects (2.57 6 1.03 mm) and normal-face subjects (3.60 6 1.69 mm, P 5 0.047). For the mandibular left central incisor (Table III), the F-test showed significant differences in total thickness, cancellous bone thickness, and thickness of the vestibular

part. The Tukey HSD test showed that the mean total thickness in short-face subjects (9.62 6 2.43 mm) was significantly greater (P 5 0.023) than in long-face subjects (9.10 6 2.68 mm), the mean cancellous bone thickness in short-face subjects (5.66 6 2.18 mm) was significantly greater (P 5 0.037) than in long-face subjects (3.89 6 2.11 mm), and the mean thickness of the vestibular part in short-face subjects (3.72 61.74 mm) was significantly greater (P 5 0.038) than those measured in long-face subjects (2.52 6 1.23 mm). The mandibular left lateral incisor (Table IV) showed no significant differences between the 3 facial biotypes with ANOVA.

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Table IV.


Means and standard deviations of the values measured at the left lateral incisor, compared between facial

types


Short (S) Mean (SD)

Long (L) Mean (SD)


ANOVA test F

20.49 (3.04) 11.39 (2.77) 9.37 (2.20) 5.28 (2.03) 3.73 (1.56) 1.56 (0.89) 201.58 (57.92) 82.29 (32.31) 4.82 (2.07) 3.16 (1.42) 1.61 (0.99) 46.80 (16.55) 22.85 (11.05)

21.60 (2.05) 12.13 (18.4) 7.62 (2.18) 3.75 (2.67) 2.61 (1.52) 1.14 (1.19) 192.66 (40.26) 80.61 (25.02) 3.43 (2.12) 2.24 (1.29) 1.20 (1.26) 37.35 (14.95) 18.05 (18.05)

20.95 (2.69) 11.16 (2.53) 8.96 (2.76) 4.76 (2.60) 3.29 (1.78) 1.47 (1.17) 191.90 (37.83) 70.53 (23.69) 4.01 (2.20) 2.63 (1.41) 1.38 (1.14) 42.55 (18.20) 19.45 (14.37)

NS NS NS NS NS NS NS NS NS NS NS NS NS

NS indicates that the values, even if different, were not significant.

Table V.

Means and standard deviations of the values measured in short-face subjects, comparing the 4 incisors

Total bone height (mm) Cancellous bone height (mm)

4.2 (A) Mean (SD)

4.1 (B) Mean (SD)

3.1 (C) Mean (SD)

3.2 (D) Mean (SD)

ANOVA F test

Tukey HSD test

20.92 (3.17) 11.22 (2.64)

21.81 (3.34) 13.08 (2.66)

21.18 (3.24) 13.29 (2.65)

20.49 (3.04) 11.39 (2.77)

0.002 0.001

0.05 A vs B 0.001 B vs D 0.001 A vs B-C 0.001 B vs D 0.001 C vs D

Total bone thickness (mm) 9.32 (2.20) 9.61 (2.03) 9.62 (2.43) 9.37 (2.20) Cancellous bone thickness (mm) 5.16 (1.89) 5.59 (1.87) 5.66 (2.18) 5.28 (2.03) Labial part (mm) 3.36 (1.57) 3.61 (1.24) 3.72 (1.74) 3.73 (1.56) Lingual part (mm) 1.80 (0.98) 1.98 (1.04) 1.94 (1.13) 1.56 (0.89) 220.44 (50.81) 221.06 (52.98) 222.94 (55.06) 201.58 (57.92) Total area (mm2)

NS NS NS NS 0.001

Cancellous area (mm2)

78.48 (30.12)

96.96 (34.35)

98.52 (34.46)

82.29 (32.31)

0.001

A-P ( ) A-C-L ( ) L-P ( ) A-L ( ) P-C-L ( )

4.59 (2.01) 2.80 (1.36) 1.79 (1.20) 47.85 (17.63) 29.00 (18.86)

4.85 (1.77) 2.93 (0.99) 1.91 (1.14) 53.90 (12.83) 34.10 (17.68)

4.85 (2.02) 2.91 (1.31) 1.94 (1.29) 49.40 (17.86) 32.45 (19.96)

4.83 (2.07) 3.16 (1.42) 1.61 (0.99) 46.80 (16.55) 22.85 (11.05)

NS NS NS NS 0.031

0.001 A vs B-C 0.001 B vs D 0.001 C vs A-D 0.01 A vs B-C 0.01 B vs D 0.01 C vs D

0.03 B vs C

NS indicates that the values, even if different, were not significant. When ANOVA resulted in significance at 95%, the Tukey HSD post-hoc test was applied and reported in the last column. 4.2, Mandibular right lateral incisor; 4.1, mandibular right central incisor; 3.1, mandibular left central incisor; 3.2, mandibular left lateral incisor.

In Tables V through VII, the means and standard deviations of the measurements for each group are reported, with, in the last 2 columns, the results of ANOVA and the Tukey HSD tests. In the short-face group (Table V), significant differences in total height, cancellous bone height, total area, and angle P-C-L were shown by ANOVA. Subsequent application of the Tukey HSD test showed that cancellous bone height, total area, and cancellous bone area of the symphysis were all greater at the central incisors than at either the left or right lateral incisors. The Tukey test was also applied to the values obtained for angle

P-C-L at all 4 incisors; it highlighted that the mean at the left lateral incisor (22.85 6 11.05 ) was significantly lower (P 5 0.030) than the mean at the right central incisor (34.10 6 17.68 ). In the long-face group (Table VI), significant differences were found in total and cancellous bone heights, total and cancellous bone areas, and angle A-C-L. According to the Tukey test, the total height of the symphysis at the mandibular left lateral incisor was less than at the mandibular central and right lateral incisors, whereas total height of the symphysis was greater at the right central incisor than at the left central incisor.

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Table VI.

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Means and standard deviations of the values measured in long-face subjects, comparing the 4 incisors 4.2 (A) Mean (SD)

4.1 (B) Mean (SD)

3.1 (C) Mean (SD)

3.2 (D) Mean (SD)

ANOVA F test

Tukey HSD test

Total bone height (mm)

22.64 (2.51)

23.30 (2.57)

22.33 (2.22)

21.60 (2.05)

0.0000003

Cancellous bone height (mm)

12.03 (1.92)

13.72 (2.26)

13.19 (2.36)

12.13 (1.84)

0.0000001

0.003 A vs D 0.006 B vs C 0.001 B vs D 0.001 A vs B 0.002 A vs C 0.001 B vs D 0.005 C vs D

Total bone thickness (mm) 7.42 (2.19) 7.78 (2.23) 7.58 (1.99) 7.62 (2.18) Cancellous bone thickness (mm) 3.70 (2.09) 4.07 (2.00) 3.89 (2.11) 3.75 (2.67) Labial part (mm) 2.33 (1.23) 2.57 (1.03) 2.52 (1.23) 2.61 (1.52) Lingual part (mm) 1.39 (1.19) 1.50 (1.18) 1.37 (1.14) 1.14 (1.19) 191.88 (33.53) 206.75 (39.61) 206.38 (37.77) 192.66 (40.26) Total area (mm2)

NS NS NS NS 0.0000003

Cancellous area (mm2)

78.54 (23.37) 88.76 (25.88) 93.30 (29.38) 80.61 (25.02)

0.0000001

A-P ( ) A-C-L ( ) L-P ( ) A-L ( ) P-C-L ( )

3.57 (2.11) 3.92 (2.07) 3.64 (2.03) 3.44 (2.12) 2.10 (1.16) 2.38 (1.06) 2.20 (1.06) 2.24 (1.29) 1.47 (1.23) 1.54 (1.18) 1.39 (1.20) 1.20 (1.26) 32.65 (12.39) 37.20 (9.98) 40.55 (16.22) 37.35 (14.95) 21.55 (14.81) 23.50 (16.06) 23.05 (16.31) 18.05 (18.05)

NS NS NS 0.038346 NS

0.001 A vs B-C 0.001 B vs D 0.001 C vs D 0.001 A vs B-C 0.005 B vs D 0.001 C vs D

0.003 C vs D


Mean cancellous bone height, total area, and cancellous bone area of the symphysis were consistently greater at the central incisors than at the lateral incisors. The mean of angle A-C-L measured at the right lateral incisor (32.65 6 12.39 ) was significantly less (P 5 0.022) than the mean at the left central incisor (40.55 6 16.22 ). In the normal-face group (Table VII), significant differences in total and cancellous bone heights, total and cancellous bone thicknesses, vestibular part, and total and cancellous bone areas were shown. The Tukey test showed that the total height of the symphysis at the right central incisor was greater than at the left central incisor, and cancellous bone height in the region of the central incisors was consistently greater than at the lateral incisors. The total thickness of the symphysis was also greater at the central incisors than at the right lateral incisor, but only cancellous bone thickness in the region of the right central incisor was significantly greater than at its lateral neighbor. For the total and cancellous bone areas of the symphysis, all mean values measured at the central incisors were significantly greater than those at the lateral incisors, except for the total area at the left lateral incisor. DISCUSSION

In orthodontic diagnosis, the position of the mandibular incisors is critical and frequently is a limiting factor

when planning treatment. Decisions regarding orthodontic surgery are greatly influenced by the extent of proclination or retroclination of these teeth that can feasibly be achieved.12 In this context, reduced labiolingual size of the alveolar process in this area indicates that the layer of bone supporting the mandibular incisors is thin and liable to sustain iatrogenic damage. Thus, a more robust layer of bone is preferred. Clinical studies have shown that, especially in patients whose symphysis is thin and elongated, extensive orthodontic movement is a risk factor for progressive bone loss.2 Laterolateral teleradiography of the cranium is widely used to estimate the thickness of the labiolingual bone around the incisors; this technique also permits evaluation of the craniofacial relationship of the incisors on the sagittal plane. The position and inclination of the mandibular incisors and the definition of cephalometric planes have been suggested as focal points in cephalometric diagnosis and treatment planning.13 Lateral teleradiography is commonly used to complement the clinical examination in initial evaluations of form, height, and extension of the symphysis. However, it is almost impossible to examine the inclination or the labiolingual thickness of the alveolar process in the region of the mandibular incisors with this method, because radiographic images of the labial and lingual surfaces of the alveolar process are projected from the more anterior and posterior parts of the bone, and do not correspond precisely to the region

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Table VII.


Means and standard deviations of the values measured in normal-face subjects, comparing the 4 incisors

Total bone height (mm) Cancellous bone height (mm)

4.2 (A) Mean (SD)

4.1 (B) Mean (SD)

3.1 (C) Mean (SD)

3.2 (D) Mean (SD)

ANOVA F test

21.79 (2.61) 11.36 (2.77)

22.31 (3.00) 12.96 (2.81)

21.46 (2.64) 12.93 (3.35)

20.95 (2.69) 11.16 (2.53)

0.002461 0.000004

Total bone thickness (mm) 8.39 (2.02) 9.30 (2.62) 9.10 (2.68) 8.96 (2.76) Cancellous bone thickness (mm) 4.42 (2.23) 5.07 (2.39) 5.00 (2.34) 4.76 (2.60) Labial part (mm) 2.90 (1.58) 3.60 (1.69) 3.47 (1.50) 3.29 (1.78) Lingual part (mm) 1.52 (0.90) 1.47 (1.07) 1.49 (1.12) 1.47 (1.17) 184.05 (34.77) 201.26 (35.14) 210.80 (36.88) 191.90 (37.83) Total area (mm2) 69.87 (20.57) 82.30 (27.10) 84.17 (29.83) 70.53 (23.69) Cancellous area (mm2) 4.07 (2.37) 4.39 (2.26) 4.45 (2.42) 4.01 (2.20) A-P ( ) 2.54 (1.60) 2.99 (1.44) 2.90 (1.41) 2.63 (1.41) A-C-L ( ) 1.53 (0.92) 1.40 (0.98) 1.56 (1.31) 1.38 (1.14) L-P ( ) 43.20 (20.92) 47.35 (14.47) 49.95 (15.25) 42.55 (18.20) A-L ( ) 25.40 (13.32) 12.95 (13.01) 24.10 (17.54) 19.45 (14.37) P-C-L ( )

0.001374 0.026853 0.012603 NS 0.0000001 0.000041 NS NS NS NS NS

Tukey HSD test 0.001 B vs D 0.001 A vs B-C 0.001 B vs D 0.001 C vs D 0.001 A vs B 0.014 A vs C 0.024 A vs B 0.010 A vs B 0.001 A vs B-C 0.001 C vs D 0.004 A vs B


of the incisors. Cephalometric teleradiography also has another intrinsic flaw, since the images of all structures overlap in 3D space, thereby creating a noticeable geometric enlargement error because of the divergence of the x-ray beam. High-definition CT, on the other hand, permits close examination of the labiolingual osseous support of the incisors without the disadvantages of conventional radiography. These images, in addition to being 3D, are not subject to distortion or superimposition, and secondary computerized reconstructions also facilitate quantitative and qualitative evaluation of the bone surfaces, quantitative evaluation of the relationship between teeth and bone,14 and selection of the desired sections.2 Furthermore, the identification of the cortical surface of the labiolingual bone permitted by this methodology allows better adaptation to biomechanical treatment.15 However, despite its usefulness, few studies of the quantitative relationship between the morphology of the mandibular alveolar process and the inclination of the incisor have been carried out by using CT.8 Our study shows that the total thickness of the symphysis in short-face subjects is almost always greater than that in long-face subjects, confirming the results of Siciliani et al1 with laterolateral teleradiography, even though the mean values they reported were greater than those we measured. This discrepancy can, in all probability, be blamed on the less precise images obtained by x-ray than those yielded by CT in our study. Another dissimilarity between our measurements and those of Siciliani et al1 was the difference in total and cancellous bone heights

of the symphysis; they found that to be significant, but we did not. Our results, however, were confirmation by those of Aki et al,16 who associated a characteristic morphology (small height, large thickness, small proportions, and large angle of the symphysis) with mandibular growth in an anterior direction, and the opposite morphology (large height, small thickness, large proportions, and small angle) with growth in a posterior direction. Moreover, Handelman17 confirmed our finding that the distance between the root apices of the central incisors and the internal surfaces of the vestibular cortex is greater in short-face than in long-face subjects. That author also concluded that orthodontic movement of the teeth must be limited in patients with a thinner alveolar process—ie, in long-face patients—because they are more exposed to possible iatrogenic damage.17 Using the measurements of distances A-C-L and P-C-L and angles A-C-L and P-C-L (the limits of the hypothetical rotary motion of the incisors about the center of rotation in the cancellous bone of the symphysis) allowed us to establish how far these teeth can be moved without coming into contact with the cortex. Our data showed that angle A-C-L is always greater than angle P-C-L, and that the radicular apex of each incisor is closest to the internal surface of the lingual cortex of the symphysis. These results, however, contrast starkly with those of Yamada et al,8 although they analyzed a sample of untreated patients with mandibular prognathism, in which the incisors had spontaneously inclined lingually to compensate for their skeletal discrepancy.


Therefore, we maintain that, during treatment, orthodontic movement in the cancellous bone portion (in our case, angle A-C-L—ie, the labial portion of the cancellous bone) is much safer than in the more restricted portion (in our case, angle P-C-L—ie, the lingual portion), where movement could cause damage because the radicular apex is too close to, or even touching, the internal cortex. Upon analysis of the comparison between the results obtained for each incisor, we observed that the total and cancellous bone heights and areas of the symphysis were greater at the central incisors than at the lateral incisors in all 3 groups. This indicates that the symphysis is substantially wider at the central incisors, compared with the surrounding areas. CONCLUSIONS

Analysis of the differences in symphysis morphology between the 3 facial types led us to the following conclusions. 1.

2.

3.

In almost all cases, total thickness of the symphysis is greater in short-face subjects than in their longface counterparts. The vestibular portion of the cancellous bone thickness of the symphysis is greater at the central incisors in short-face subjects compared with long-face subjects, with normal-face patients in the middle. The total and cancellous bone areas of the symphysis showed no statistically significant differences between the 3 face types.

When we compared the measurements obtained for the 4 mandibular incisors in each group, however, we observed that total and cancellous bone heights and areas of the symphysis were greater at the central incisors than at the lateral incisors in all 3 facial types. REFERENCES 1. Siciliani G, Cozza P, Sciarretta MG. Considerazioni sul limite anteriore funzionale della dentatura. Mondo Ortod 1990;15:259-64.

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