Crown-root shape of the permanent maxillary central incisor

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

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.

Galvanic coupling of steel and gold alloy lingual brackets with orthodontic wires: Is corrosion a concern?

OBJECTIVES

The aim of this research was to assess galvanic behavior of lingual orthodontic brackets coupled with representative types of orthodontic wires.

MATERIALS AND METHODS

Three types of lingual brackets: Incognito (INC), In-Ovation L (IOV), and STb (STB) were combined with a stainless steel (SS) and a nickel-titanium (NiTi) orthodontic archwire. All materials were initially investigated by scanning electron microscopy / x-ray energy dispersive spectroscopy (SEM/EDX) while wires were also tested by x-ray diffraction spectroscopy (XRD). All bracket-wire combinations were immersed in acidic 0.1M NaCl 0.1M lactic acid and neutral NaF 0.3% (wt) electrolyte, and the potential differences were continuously recorded for 48 hours.

RESULTS

The SEM/EDX analysis revealed that INC is a single-unit bracket made of a high gold (Au) alloy while IOV and STB are two-piece appliances in which the base and wing are made of SS alloys. The SS wire demonstrated austenite and martensite iron phase, while NiTi wire illustrated an intense austenite crystallographic structure with limited martensite. All bracket wire combinations showed potential differences below the threshold of galvanic corrosion (200 mV) except for INC and STB coupled with NiTi wire in NaF media.

CONCLUSIONS

The electrochemical results indicate that all brackets tested demonstrated galvanic compatibility with SS wire, but fluoride treatment should be used cautiously with NiTi wires coupled with Au and SS brackets.

Comparative tomographic study of the maxillary central incisor collum angle between Class I, Class II, division 1 and 2 patients

INTRODUCTION:

It has been reported that Class II, division 2 maxillary central incisors frequently demonstrate increased collum angles, which indicates an excessive palatal “bend” of the crown. However, evidence supporting such observation is mostly derived from radiographic studies.

OBJECTIVE:

The objective of this study was to evaluate and compare the collum angle of maxillary central incisors in Class I, Class II, division 1, and Class II, division 2 cases using cone-beam computed tomography.

MATERIALS AND METHODS:

Forty-eight consecutive orthodontic cases (16 Class I, 16 Class II, division 1, and 16 Class II, division 2 malocclusion) with cone-beam computed tomography as part of their initial diagnostic records were evaluated. Cross-sections including maxillary right and left central incisors were used to calculate the angulation between the crown and root long axes (collum angle). Comparisons between groups were performed using analysis of variance for multiple and post-hoc Tukey for paired analyses.

RESULTS:

Mean collum angle observed in Class II, division 2 cases was significantly larger (5.2 ± 1.3°) than the ones obtained for Class I (1.1 ± 4.2°) (P = 0.034) or Class II, division 1 cases (0.1 ± 0.7°) (P = 0.014).

CONCLUSIONS:

Our findings suggest that Class II, division 2 individuals demonstrate accentuated lingual inclination of the maxillary central incisor crown compared to the other types of malocclusion studied here. Such morphological feature indicates the need for better tooth movement planning, especially in regard to root palatal torqueing of the maxillary central incisors.

Influence of Post Angulation between Coronal and Radicular Segment on the Fracture Resistance of Endodontically Treated Teeth

INTRODUCTION

The objectives of coronal restoration in the Endodontically Treated Teeth (ETT) include rehabilitation of aesthetics, function and prevention of coronal leakage. The long axis of root and the coronal segment in the maxillary anterior teeth varies according to the occlusal scheme. The restorative dentist is required to fabricate the post angulation in compatibility to contour of the adjacent teeth.

AIM

To evaluate the influence of the angle between the long axes of core facial surface and the radicular segment of the post on fracture resistance of ETT.

MATERIALS AND METHODS

Total of 30 maxillary intact canines was root canal treated, sectioned 2 mm above the CEJ. The custom made nickel-chromium endodontic posts were fabricated using direct method. The samples were divided among three groups of 10 each. The angle between coronal segment and radicular segment of post in Group-I, Group-II, Group-III were 5°, 10° and 15° respectively. The teeth samples were cemented with full veneer metal crown and tested under universal testing machine. The static load at the angle of 130° was applied until the fracture to record the fracture strength. The obtained data was statistically analysed with ANOVA and Tukey post-hoc test.

RESULTS

The Group-III showed the highest fracture strength with 666.15 N. The Group-II and Group-I recorded the mean fracture strength at 443.37 N and 276.74 N respectively.

CONCLUSION

The endodontic post with higher angle between the long axis of facial surface and radicular segment long axis had the positive correlation to the fracture strength in ETT.

The relationship between the incisor position and lingual surface morphology in normal occlusion

This study aimed to investigate the relationship between the morphological characteristics of maxillary incisors and the anterior occlusion. The study materials comprised dental casts and lateral cephalograms of 26 modern Mongolian females with Angle Class I normal occlusion (mean age, 21 years 5 months). Computed tomography (CT) images of the dental casts were taken with an X-ray micro-CT system (SMX-100CT, Shimadzu, Kyoto Japan). The thickness of the marginal ridges and incisal edges, and the overjet and overbite, was measured on the three-dimensional images of the dental casts. On the lateral cephalogram, maxillary incisor to sella-nasion plane angle (U1 to SN angle), maxillary incisor to nasion-point A plane distance (U1 to NA distance), mandibular incisor to nasion-point B plane distance (L1 to NB distance), incisor mandibular plane angle, and interincisal angle were measured by tracing the left incisors of the maxilla and mandible. Spearman's single rank correlation coefficients were used to investigate any correlation between measurement items for each maxillary incisor. The thickness of the marginal ridges and incisal edges was positively correlated with the overbite. The thickness of the incisal edges was positively correlated with the irregularity index of the maxilla. There were significant negative correlations between overbite and U1 to SN angle, U1 to NA distance, and L1 to NB distance. Significant positive correlations were noted between the overbite and the overjet. In conclusion, there was no strong relationship between the morphological characteristics of maxillary incisors and the anterior occlusion.

Effects of crown-root angle on stress distribution in the maxillary central incisors' PDL during application of intrusive and retraction forces: a three-dimensional finite element analysis

Background

Different crown-root angulations of maxillary central incisors can be assumed as a potential reason for many underscored outcomes of orthodontic treatments. The aim of this study was to determine the effects of the different crown-root angles on stress distributions in the maxillary central incisor’s periodontal ligament (PDL) during application of intrusive and retraction forces using a 3D finite element method.

Methods

Two models of a maxillary central incisor were constructed using ANSYS software: the first one with an angle of 166.7° (as a sample of the maxillary central incisor in a class II, division 2 patient) and the other one with an angle of 173.4° (normal angulation). Each of the samples was loaded twice by an intrusive force (0.25 N) and a retraction force (0.5 N) through the ideal position of brackets.

Results

FEM results showed little difference between stress distributions in the two models during intrusion (ten thousandth) compared to retraction (thousandth). In the application of retraction force, the stress concentration on the curved tooth was less than the other.

Conclusion

To produce similar patterns of stress in the PDL, orthodontists can apply 1.18 times heavier retraction forces on the maxillary central incisors in class II, division 2 patients compared to class I patients.