Two-dimensional vs 3-dimensional comparison of alveolar bone over maxillary incisors with A-point as a reference

Introduction

Our objectives were to compare, relative to A-point, (1) bone thickness over the most forward maxillary incisor (MFMI) in 2 dimensions vs 3 dimensions, and (2) bone thickness and inclination of each maxillary incisor in 3 dimensions.

Methods

Thirty-four cone-beam computed tomography (CBCT) images were coded, and 2-dimensional (2D) cephalograms were derived from each image using Dolphin software (Dolphin Imaging and Management Solutions, Chatsworth, Calif). A-point and the MFMI crown were located. After reliability tests, alveolar bone buccal to 3 points on the MFMI root, bone to reference line Frankfort horizontal (FH)–A-point, and incisor inclination were measured. This procedure was repeated on the 3-dimensional (3D) CBCT images comparing MFMI with all maxillary incisors. The 2D and 3D measurements were compared using paired t tests, and 3D measurements were compared with analysis of variance. A 5% significance level was used for all tests.

Results

The MFMI’s buccal bone thickness at the root apices and the distance between buccal bone and FH–A-point line at 2 root points were significantly greater in 2 dimensions than in 3 dimensions. In 3 dimensions, bone thickness at MFMI’s root apex and the distance from FH–A-point line at all root points were significantly greater than those of the lateral incisors. Bone buccal to MFMI was significantly smaller than at the lateral incisors 3 mm from the cementoenamel junction.

Conclusions

Evaluation of 2D CBCT derivations can result in overestimation of alveolar bone buccal to the maxillary incisor root apices compared with 3D evaluations. The anterior nasal spine obscures bone measurements over the maxillary incisors in 2 dimensions.

Highlights

  • Alveolar bone thickness over maxillary incisor root apices was measured in 2 and 3 dimensions.

  • Thicknesses were overestimated in 2D measurements compared with 3D measurements.

  • Be cautious when adding labial root torque to maxillary incisors, especially laterals.

A-point was originally called Point A by Downs. On a lateral cephalogram, it is the deepest midline point on the premaxilla between the anterior nasal spine (ANS) and prosthion, and it is used to determine the most forward position of the maxilla. However, there are a number of problems with using A-point as a landmark. For example, A-point can be difficult to identify reliably in 2 dimensions because of overlying structures and positioning of the head. A-point can be affected by the movement of the anterior incisors. It is influenced by the position of the ANS. Van der Linden believed that the position of ANS caused the deepest point on the concavity to be more superior than the alveolar bone of the maxilla. Since ANS is a midline structure, it could mask the amount of bone over the apical portion of the incisor root when viewed in 2 dimensions, thus obscuring the amount of labial bone present for labial root movement.

Another problem in measuring the amount of bone over the incisors is the identification of the actual root of the most forward maxillary incisor (MFMI) using a 2-dimensional (2D) radiograph. Overlying structures such as the roots of the other incisors, the canine, and even other alveolar bone—ie, the canine eminence—can obscure the root outline of the MFMI. Also, the amount of bone over the other incisors cannot be determined using 2D radiographs. Several authors have reported the amount of bone over the maxillary incisors using cone-beam computed tomography (CBCT) images, but there is little information concerning the location of A-point and the amount of bone over the maxillary incisors. Little information comparing 2D with 3-dimensional (3D) alveolar measures exists. Lack of recognizing the amount of bone over the maxillary incisors can compromise treatment when the roots are orthodontically moved through the bone.

The purposes of the study were to compare the amount of bone located over the MFMI in 2 dimensions vs 3 dimensions and to determine the influence of A-point on the measurements by relating the distance of the alveolar bone to A-point. Another purpose was to compare in 3 dimensions the amount of bone and the inclination of each maxillary incisor and the relationship with A-point.

Material and methods

This retrospective study was approved by the Indiana University Institutional Review Board. After a sample size calculation, 35 CBCT images (27 male, 7 female; white ethnicity; age range, 18-37 years) were randomly selected from the pool of initial research records available in the university’s graduate orthodontic clinic. All 3D CBCT images were taken with the same machine (iCAT; Imaging Sciences International, Hatfield, Pa), set for a full 13-cm field of view, 8.9-second scanning time, 120 kV(p), 18 mA, and a resolution of 0.3 mm voxel size. Inclusion criteria included eruption of all permanent teeth anterior to the first molar. All malocclusions (20 Class I, 6 Class II, 8 Class III, based on Angle classifications) were accepted. Exclusion criteria included impacted maxillary anterior teeth, craniofacial abnormalities, severely resorbed roots, previous orthodontic treatment, and noticeable periodontal diseases. Noticeable periodontal disease was based on vertical bone defects or alveolar bone greater than 3 mm from the cementoenamel junction (CEJ), as diagnosed from the constructed panoramic radiograph. During the study, 1 CBCT image was discarded because of poor resolution in the area of measurement, resulting in 34 images. Radiographs were coded and randomized so that the investigator (T.J.K.) was blinded to their identity. The CBCT files (DICOM) were exported into Dolphin Imaging (version 11.8; Dolphin Imaging and Management Solutions, Chatsworth, Calif) for measurements.

A 2D cephalogram was created from the CBCT image in Dolphin. The 2D derivations of all CBCT images were standardized using the same Dolphin Mask Filter 1. A standardized horizontal position (Frankfort Horizontal [FH]) was constructed using the patient’s right porion and right orbitale on the cephalogram. A perpendicular line was drawn from FH through A-point (FH–A-point line) as a vertical reference for tooth and bone measures. The MFMI was located and the root measured from the labial CEJ to the apices. Three points along the root (root apex, half the length of the root, and 3 mm gingival to the CEJ) were determined. The thickness of the alveolar bone buccal to the 3 points on the roots was measured on a line horizontal to the FH. Then the distance along the same lines from the edge of the bone to the FH–A-point line was measured ( Fig 1 ). The inclination of the incisor was measured as an angle between a line from the incisor tip through its apex to the FH.

Fig 1
Two-dimensional image of A-point and the FH–A-point line with the MFMI showing measurements of overlying bone over the 3 points on the root and the measurement of the distance from FH–A-point to the edge of the overlying bone on a line parallel to the FH–A-point line.

On the 3D CBCT image ( Fig 2 ), the FH line was marked, using the right porion and orbitale. Perpendicular lines were made (as follows) to standardize measures and relate A-point to all the incisors. The radiograph was oriented frontally with the FH parallel to the floor, and a vertical orientation (midsagittal plane) was made with the skeletal midline through nasion perpendicular ( Fig 3 ). A-point was identified on the right lateral view of the 3D volumetric image and transferred to the frontal view. From the frontal view, a perpendicular line through A-point was then made to the FH (FH–A-point line). A line horizontal to the FH and perpendicular to the FH–A-point line at A-point was drawn across the frontal view. A-point was marked at each maxillary incisor. The MFMI was identified and the midsagittal line transferred to the center of its crown. Root length was measured from the labial CEJ to the root apex. On a sagittal 0.3-mm section through the midsagittal line of the incisor crown, 3 points were identified (root apex, half of the root length, and 3 mm apical to the CEJ), similar to the 2D cephalogram. A perpendicular line was drawn from the FH through A-point ( Fig 4 ) via the Dolphin software. The FH–A-point line was transferred via software to the midsagittal section of each incisor. The thickness of the alveolar bone buccal to the 3 points on the roots was measured on a line horizontal to the FH ( Fig 5 , A ). Then the distance from the edge of the bone to the FH–A-point line was measured along the same lines ( Fig 5 , B ). The inclination of the incisor was measured as the angle between a line from the incisal tip, extending through the root apex, to the FH ( Fig 6 ). The other maxillary incisors were located using the previously mentioned points, and the same measurements as described for the MFMI were performed. A total of 34 central incisors that were not the most forward incisor and 68 lateral incisors (34 right, 34 left) were measured. The uncertainty of the delineation of the bone edge on 1 MFMI affecting 2 parameters led to a decision to exclude those values, resulting in only 33 measurements for the MFMI at (1) root apex to bone and (2) A-point to bone at root apex. Only the Dolphin magnification function was used to enhance the image of the incisor during measurement.

Fig 2
Three-dimensional image of the axial orientation of a skull along the FH.

Fig 3
Three-dimensional image of the midsagittal orientation of a skull along the skeletal midline and nasion.

Fig 4
Three-dimensional image of a sagittal slice showing the FH–A-point line extended from the facial midline to the middle of a central incisor.

Fig 5
Three-dimensional image of sagittal slices with the FH–A-point line extended from the facial midline to the middle of a central incisor: A, measurement of overlying bone over the 3 points on the root, and B, measurement of the distance from FH–A-point to the edge of the overlying bone on lines parallel to the FH–A-point line.

Fig 6
Three-dimensional image of the inclination of the incisors to the FH.

Before data collection, a reliability test was performed using both 2D and 3D radiographs. One investigator (T.J.K.) measured 10 randomly selected radiographs twice, 2 weeks apart.

Statistical analysis

Intraclass correlation coefficients (ICCs), Bland-Altman plots, and measurement errors were calculated to evaluate the reliability of the 2D and 3D radiograph measurements and to check for patterns of disagreement in the reliability measures. Normality of the data was examined and determined to be acceptable. Comparisons between the 2D and 3D measurements were made using paired t tests. To determine whether sex and occlusion affected the comparisons between 2 and 3 dimensions, analysis of variance (ANOVA) was used, with fixed effects for 2 dimensions/3 dimensions, sex, occlusion, and their interactions. A random subject effect was used to create the 2D-3D pairing in ANOVA. Comparisons between the MFMI and the other maxillary incisor roots were made using ANOVA tests with a fixed effect for maxillary incisor root; a random subject effect was included to allow correlations among the roots within a subject. To control the overall significance level, pair-wise tests between the roots were only conducted when the overall effect was significant in ANOVA. Pearson correlation coefficients were calculated to evaluate the associations of the amount of bone with the incisor inclination. A 5% significance level was used for all tests.

Before the study, sample size calculations showed that with a sample of 35 subjects, with 2D and 3D images evaluated for each subject, the study would have 80% power to detect an effect size of 0.5 for the difference between the 2D and 3D measurements, assuming 2-sided tests, each conducted at a 5% significance level. However, near the end of the study, 1 subject was dropped because of resolution of the 3D image, resulting in 34 subjects.

Results

In 2 dimensions ( Table I ), 5 measurements had excellent ICC values, 0.90 or greater, and 1 was 0.79. Two had moderate-strength ICC values of 0.59 to 0.70. In 3 dimensions, 6 measurements had strong ICC values, greater than 0.70, and 2 had moderate strength. Measurement error was also determined ( Table I ). Measurement error was the standard deviation of the within-image repeats, and 95% confidence intervals (CI) were considered approximately ± 2 measurement errors from the estimated difference between the repeated measurements. Bland-Altman plots showing the 95% CI had no underlying patterns of disagreement in the reliability data ( Supplemental material, Figs 7 and 8 ). We concluded that there were no significant differences between the 2D and 3D ICC values.

Table I
Reliability measures for 2 dimensions vs 3 dimensions
Measurement 2D 3D
ICC (95% CI) ME + ICC (95% CI) ME
Root length 0.99 (0.97-1.00) 0.23 0.71 (0.40-1.00) 0.53
Root apex to bone 0.96 (0.92-1.00) 0.42 0.86 (0.69-1.00) 0.41
One-half root to bone 0.79 (0.54-1.00) 0.27 0.43 (−0.09 to 0.95) 0.28
Root 3 mm from CEJ to bone 0.66 (0.30-1.00) 0.28 0.66 (0.30-1.00) 0.30
A-point to bone at root apex 0.59 (0.17-1.00) 0.20 0.85 (0.67-1.00) 0.28
A-point to bone at 1/2 root 0.90 (0.77-1.00) 0.35 0.88 (0.74-1.00) 0.28
A-point to bone 3 mm from CEJ 0.99 (0.97-1.00) 0.16 0.88 (0.74-1.00) 0.43
Incisor angle to FH 0.98 (0.95-1.00) 0.92 0.99 (0.99-1.00) 0.47
ME , Measurement error (standard deviation of the within-image repeats representing the estimate of the measurement error; the confidence intervals are approximately ± 2 measurement errors from the estimated difference between the repeated measurements).

Summary statistics were developed ( Table II ). The root length of the MFMI was significantly greater in the 2D cephalograms than in the 3D ones ( Table III ). The thickness of bone at the root apices was significantly greater in 2 dimensions than in 3 dimensions. The distance from A-point to the labial aspect of the bone was significantly greater in 2 dimensions vs 3 dimensions at the root apex and at 3 mm from the CEJ. No other 2D vs 3D measures were significantly different.

Table II
Summary statistics of 2D and 3D measures of root length, thickness of alveolar bone labial to root, distance from bone to A-point along maxillary incisors, and incisor angulation
Measurement Location n Mean SD Minimum Maximum
Root length (mm) 2D most forward crown 34 14.9 1.9 11.8 20
3D most forward crown 34 13.7 1.6 10.2 16.2
3D left lateral incisor 34 13.3 1.7 9.8 17.4
3D left central incisor 17 13.9 1.8 9.8 16.8
3D right central incisor 17 13.8 1.8 10.6 17
3D right lateral incisor 34 13.5 1.6 10 17.8
Root apex to bone (mm) 2D most forward crown 34 5.5 1.9 1.3 9.3
3D most forward crown 33 3.2 1.1 1.5 6.9
3D left lateral incisor 34 2.3 0.9 1.1 4.7
3D left central incisor 17 3.3 1.3 1.9 7.1
3D right central incisor 17 3.2 1.1 1.8 5
3D right lateral incisor 34 2.2 0.7 1 4.1
1/2 root to bone (mm) 2D most forward crown 34 2.0 0.8 0.5 4.5
3D most forward crown 34 1.8 0.6 0.8 3.3
3D left lateral incisor 34 1.9 0.7 0.8 4.9
3D left central incisor 17 1.7 0.6 0.9 3.3
3D right central incisor 17 1.7 0.3 1.2 2.3
3D right lateral incisor 34 2.0 0.7 0.8 4.5
Root 3 mm from CEJ to bone (mm) 2D most forward crown 34 1.3 0.4 0.4 2.2
3D most forward crown 34 1.4 0.4 0.6 2.2
3D left lateral incisor 34 2.0 0.7 0.5 4
3D left central incisor 17 1.4 0.4 0.7 1.9
3D right central incisor 17 1.4 0.4 0.8 2
3D right lateral incisor 34 1.9 0.6 1 3
A-point to bone at root apex (mm) 2D most forward crown 34 0.3 0.3 −0.4 1.2
3D most forward crown 33 −1.9 1.1 −4.1 0.2
3D left lateral incisor 34 −5.1 1.4 −7.9 −0.6
3D left central incisor 17 −1.5 1.4 −3.8 0.6
3D right central incisor 17 −1.8 1.4 −4.4 0.6
3D right lateral incisor 34 −5.0 1.3 −7.5 −1.9
A-point to bone at 1/2 root (mm) 2D most forward crown 34 1.7 1.0 −0.2 4.1
3D most forward crown 34 1.3 1.0 −1.5 3.7
3D left lateral incisor 34 −1.1 1.1 −4.2 1.3
3D left central incisor 17 1.7 1.1 −0.3 3.8
3D right central incisor 17 1.0 0.9 −0.4 2.5
3D right lateral incisor 34 −0.9 1.4 −4.3 2.1
A-point to bone at 3 mm from CEJ (mm) 2D most forward crown 34 3.6 1.4 1.1 6.7
3D most forward crown 34 3.1 1.2 1.1 5.9
3D left lateral incisor 34 0.8 1.1 −1.2 3.5
3D left central incisor 17 3.5 1.3 1.1 5.9
3D right central incisor 17 2.7 0.9 1.2 3.9
3D right lateral incisor 34 0.9 1.4 −1.2 4.2
Angle to FH (°) 2D most forward crown 34 116.4 7.8 98.2 133.8
3D most forward crown 34 116.1 7.5 97.5 131.5
3D left lateral incisor 34 116.0 7.1 94.1 127
3D left central incisor 17 115.2 9.2 91.6 127.1
3D right central incisor 17 113.1 6.1 102.8 122.3
3D right lateral incisor 34 115.7 7.4 96.8 127.7

Table III
Comparisons between 2D and 3D most forward crown with paired t tests
Measurement Mean difference SD 95% CI P value
Root length 1.16 1.01 0.81 1.51 <0.0001
Root apex to bone 2.30 1.74 1.70 2.91 <0.0001
1/2 root to bone 0.23 0.77 −0.04 0.50 0.0901
Root 3 mm from CEJ to bone −0.09 0.39 −0.23 0.04 0.1704
A-point to bone at root apex 2.18 1.11 1.79 2.57 0.0001
A-point to bone at 1/2 root 0.36 1.17 −0.05 0.77 0.0837
A-point to bone 3 mm from CEJ 0.51 1.00 0.17 0.86 0.0051
Angle to FH 0.31 3.41 −0.88 1.50 0.6008
The table shows the means, standard deviations, and 95% confidence intervals for the differences between 2 and 3 dimensions, calculated as 2 dimensions minus 3 dimensions, along with the P value from the test, n = 34.

Significant.

Occlusion affected the 2D vs 3D comparison ( Table IV ). The inclination of the MFMI was significantly greater in 2 dimensions than in 3 dimensions for Class III subjects ( P = 0.0029), but not for Class I ( P = 0.15) or Class II ( P = 0.10) subjects. The 2D vs 3D comparison results were not affected by occlusion for any other measurement or by sex for any measurement.

Table IV
ANOVA table for 2D vs 3D comparisons with sex and occlusion factors (n = 34)
Measurement Effect Num df Den df F value P value
Root length 2Dv3D 1 28 31.84 <0.0001
Occlusion 2 28 1.96 0.1602
2Dv3D*Occlusion 2 28 3.59 0.0409
Sex 1 28 7.77 0.0094
2Dv3D*Sex 1 28 2.05 0.1632
Occlusion*Sex 2 28 2.01 0.1534
2Dv3D*Occlusion*Sex 2 28 0.13 0.8829
Root apex to bone 2Dv3D 1 27 36.34 <0.0001
Occlusion 2 27 0.58 0.5673
2Dv3D*Occlusion 2 27 2.28 0.1215
Sex 1 27 4.33 0.0471
2Dv3D*Sex 1 27 1.06 0.3122
Occlusion*Sex 2 27 0.65 0.5278
2Dv3D*Occlusion*Sex 2 27 0.71 0.4990
1/2 root to bone 2Dv3D 1 28 5.68 0.0242
Occlusion 2 28 0.04 0.9653
2Dv3D*Occlusion 2 28 0.30 0.7465
Sex 1 28 0.73 0.3999
2Dv3D*Sex 1 28 2.13 0.1555
Occlusion*Sex 2 28 1.64 0.2127
2Dv3D*Occlusion*Sex 2 28 1.32 0.2826
Root 3 mm from CEJ to bone 2Dv3D 1 28 0.03 0.8662
Occlusion 2 28 0.16 0.8550
2Dv3D*Occlusion 2 28 0.07 0.9348
Sex 1 28 0.06 0.8053
2Dv3D*Sex 1 28 1.94 0.1746
Occlusion*Sex 2 28 1.66 0.2085
2Dv3D*Occlusion*Sex 2 28 0.06 0.9463
A-point to bone at root apex 2Dv3D 1 27 53.49 <0.0001
Occlusion 2 27 1.34 0.2790
2Dv3D*Occlusion 2 27 2.82 0.0775
Sex 1 27 2.12 0.1572
2Dv3D*Sex 1 27 5.46 0.0271
Occlusion*Sex 2 27 2.45 0.1057
2Dv3D*Occlusion*Sex 2 27 2.75 0.0820
A-point to bone at 1/2 Root 2Dv3D 1 28 0.08 0.7758
Occlusion 2 28 0.65 0.5276
2Dv3D*Occlusion 2 28 0.45 0.6428
Sex 1 28 3.63 0.0672
2Dv3D*Sex 1 28 1.16 0.2907
Occlusion*Sex 2 28 1.85 0.1753
2Dv3D*Occlusion*Sex 2 28 1.33 0.2795
A-point to bone from 3 mm from CEJ 2Dv3D 1 28 2.95 0.0971
Occlusion 2 28 4.84 0.0156
2Dv3D*Occlusion 2 28 0.63 0.5426
Sex 1 28 1.01 0.3241
2Dv3D*Sex 1 28 0.01 0.9251
Occlusion*Sex 2 28 3.93 0.0314
2Dv3D*Occlusion*Sex 2 28 1.04 0.3684
Angle to FH 2Dv3D 1 28 7.12 0.0125
Occlusion 2 28 0.61 0.5478
2Dv3D*Occlusion 2 28 7.11 0.0032
Sex 1 28 0.06 0.8121
2Dv3D*Sex 1 28 3.97 0.0560
Occlusion*Sex 2 28 3.08 0.0617
2Dv3D*Occlusion*Sex 2 28 2.14 0.1365
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Dec 12, 2018 | Posted by in Orthodontics | Comments Off on Two-dimensional vs 3-dimensional comparison of alveolar bone over maxillary incisors with A-point as a reference
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