The crown-root morphology of central incisors in different skeletal malocclusions assessed with cone-beam computed tomography

Abstract

Background: To determine the discrepancy of crown-root morphology of central incisors among different types of skeletal malocclusion using cone-beam computed tomography (CBCT) and to provide guidance for proper torque expression of anterior teeth and prevention of alveolar fenestration and dehiscence.

Methods: In this retrospective study, a total of 108 CBCT images were obtained (ranging from 18.0 to 30.0 years, mean age 25.8 years). Patients were grouped according to routine sagittal and vertical skeletal malocclusion classification criteria. The patients in sagittal groups were all average vertical patterns, with Class I comprised 24 patients-14 females and 10 males; Class II comprised 20 patients-13 females and 7 males; and Class III comprised 22 subjects-13 females and 9 males. The patients in vertical groups were all skeletal Class I malocclusions, with low angle comprised 21 patients-12 females and 9 males; average angle comprised 24 patients; and high angle comprised 21 patients-11 females and 10 males. All the CBCT data were imported into Invivo 5.4 software to obtain a middle labio-lingual section of right central incisors. Auto CAD 2007 software was applied to measure the crown-root angulation (Collum angle), and the angle formed by a tangent to the central of the labial surface of the crown and the long axis of the crown (labial surface angle). One-way analysis of variance (ANOVA) and Scheffe's test were used for statistical comparisons at the P < 0.05 level, and the Pearson correlation analysis was applied to investigate the association between the two measurements.

Results: The values of Collum angle and labial surface angle in maxillary incisor of Class II and mandibular incisor of Class III were significantly greater than other types of sagittal skeletal malocclusions (P < 0.05); no significant difference was detected among vertical skeletal malocclusions. Notably, there was also a significant positive correlation between the two measurements.

Conclusions: The maxillary incisor in patients with sagittal skeletal Class II malocclusion and mandibular incisor with Class III malocclusion present remarkable crown-root angulation and correspondingly considerable labial surface curvature. Equivalent deviation during bracket bonding may cause greater torque expression error and increase the risk of alveolar fenestration and dehiscence.

Keywords: Collum angle; Cone-beam CT; Crown-root morphology; Labial surface angle; Skeletal malocclusion.

Fig. 1

a, b The inclination of the root and crown in maxillary and mandibular incisors are inconsistent with each other in the surface view, which indicates the crown-root angulation phenomenon

Fig. 2

Measurements to classify sagittal and vertical skeletal malocclusion. A, A-point, deepest bony point on the contour of the premaxilla below ANS; B, B-point, deepest bony point on the contour of the mandible above pogonion; ANB, angle between point A, B and point N; 1. Wits, perpendicular lines are dropped from points A and B onto the occlusal plane, Wits is measured from Ao to Bo; 2. S, sella, center of sella turcica; N, nasion, the most anterior limit of the frontonasal suture on the frontal bone in the facial midline; SN, connection between S and N, stands for anterior cranium base plane; Go, gonion, the most posterior inferior point of mandible angle; Me, menton, most inferior point of the bony chin; MP, connection between Me and Go, stands for mandibular plane; SN-MP, angle between SN and MP; 3. S-Go, the distance between lines parallel to FH plane passing through S and Go, represents the posterior facial height; N-Me, the distance between lines parallel to FH plane passing through N and Me, represents for the anterior facial height; FHI(S-Go/N-Me), facial height index, the ratio of posterior and anterior height, stands for vertical growth pattern of individual

Fig. 3

Measuring image capture. The natural position of the head is adjusted in three dimensions. a The horizontal view. b The coronal view. c The sagittal view. A bunch of cutting lines (green) was vertical to incisor labial surface (d) and located at the central coronal view (e). The median sagittal views were established with nine layers (f–n), interval 0.10 mm, and the middle one was the measuring image (j)

Fig. 4

a The Collum angle is formed by the extension of the long axis of the crown and the long axis of the root. b Tangent L passes through upper and lower intersections of labial surface of crown and circle with the T center and radius of 0.5 mm. c The measuring example of Collum angle and labial surface angle

Fig. 5

The value of CA and LSA in maxillary incisor of Class II (a, b) and mandibular incisor of Class III (c, d) are significantly greater than other groups. There is no statistical difference among different vertical skeletal classifications

Fig. 6

Both in the maxilla (a) and mandible (b), the CA and LSA are significantly and positively correlated

Fig. 7

The various Collum angle in central incisor, the long axis of the root can deviate to the labial side (a) or lingual side (c) of the long axis of the crown, or coincidence (b). The schematic diagram indicates that the root bends toward lingual cortical alveolar because of Collum angle (d). The schematic diagram elucidates that the more obvious Collum angle accompanies with the greater labial surface curvature of the crown (e)