Assessment of shoulder dimensions and angles of porcelain bonded to metal crown preparations

Trueness of intraoral scanners according to subgingival depth of abutment for fixed prosthesis

This study aimed to compare the trueness of intraoral scanners (IOSs) according to the subgingival finish line depth of tooth preparation for fixed prostheses. The prepared maxillary right first molar was fabricated by using ceramic material. A computer-aided design (CAD) reference model (CRM) of the abutment was obtained by using a contact scanner. The subgingival finish line was located according to the depth at 0-mm, 0.25-mm, 0.5-mm, 0.75-mm, and 1-mm. CAD test models (CTMs) were obtained by using 2 IOSs (i500 and CS3600). CRM and CTM were superimposed and analyzed (Geomagic control X). The one-way analysis of variance (ANOVA) was used to compare the trueness according to the subgingival finish line depth. The paired t test was used to compare the trueness of IOSs with and without gingival retraction (α = .05). When the gingival displacement code was not used, it was observed that the trueness of both IOSs decreased significantly as the depth of the subgingival finish line increased (P < 0.001). When the subgingival finish line was positioned deeper than 0.5-mm, the trueness of both IOSs exceeded 100 µm in the marginal region. When the gingival displacement cord was used, the trueness of both IOSs did not exceed 100 µm regardless of the subgingival finish line depth. When gingival cord was used, it showed significantly higher trueness than when not used (P < 0.001). When the gingival displacement cord was not used, the trueness of IOSs decreased as the subgingival finish line depth increased. But the use of the gingival displacement cord improved the scanning trueness by 90%. Thus, it is necessary to use the gingival displacement cord according to the clinical situation to improve scan trueness at the subgingival finish line.

Preparation Techniques Used to Make Single-Unit Crowns: Findings from The National Dental Practice-Based Research Network

PURPOSE

To: (1) determine which preparation techniques clinicians use in routine clinical practice for single-unit crown restorations; (2) test whether certain practice, dentist, and patient characteristics are significantly associated with these techniques.

MATERIALS AND METHODS

Dentists in the National Dental Practice-Based Research Network participated in a questionnaire regarding preparation techniques, dental equipment used for single-unit crown preparations, scheduled chair time, occlusal clearance determination, location of finish lines, magnification during preparation, supplemental lighting, shade selection, use of intraoral photographs, and trimming dies. Survey responses were compared by dentist and practice characteristics using ANOVA.

RESULTS

Of the 2132 eligible dentists, 1777 (83%) responded to the survey. The top two margin configuration choices for single-unit crown preparation for posterior crowns were chamfer/heavy chamfer (65%) and shoulder (23%). For anterior crowns, the most prevalent choices were the chamfer (54%) and the shoulder (37%) configurations. Regarding shade selection, a combination of dentist, assistant, and patient input was used to select anterior shades 59% of the time. Photographs are used to communicate shade selection with the laboratory in about half of esthetically demanding cases. The ideal finish line was located at the crest of gingival tissue for 49% of respondents; 29% preferred 1 mm below the crest; and 22% preferred the finish line above the crest of tissue. Average chair time scheduled for a crown preparation appointment was 76 ± 21 minutes. Practice and dentist characteristics were significantly associated with margin choice including practice type (p < 0.001), region (p < 0.001), and years since graduation (p < 0.001).

CONCLUSIONS

Network dentists prefer chamfer/heavy chamfer margin designs, followed by shoulder preparations. These choices were related to practice and dentist characteristics.

Influence of preparation design on the quality of tooth preparation in preclinical dental education

Background/purpose

One of the major educational goals in preclinical dental education is to learn tooth preparation techniques. The purpose of this study was to evaluate the influence of different preparation designs on the development of the manual dexterity of students, in order to perform a state-of-the-art tooth preparation.

Material and methods

Seventy-two 1^st semester students were divided into two groups and educated in tooth preparation for a ceramic anterior single crown. One group received cylindrical burs with a rounded edge to prepare a typodont model with a shoulder finishing line, while the other group had cylindrical burs with round noses to prepare a chamfer finishing line. All preparations were digitized and evaluated using special software focusing on the parameters of preparation depth and preparation angle. In addition, violation of the adjacent teeth was estimated. Data was statistically evaluated at a level of significance of 5%.

Results

The preparation design used did not show a statistically significant influence on the preparation depth or on the preparation angle. A trend to a higher tooth structure removal as required was detected. Furthermore, no influence of the type of preparation design on the number of violated adjacent teeth was found.

Conclusion

In preclinical dental education, the type of preparation design was found to have no influence on the measured parameters representing the quality of the preparation.

Evaluation of three different tooth preparation techniques for metal ceramic crowns by comparing preparation depths: An in vitro study

Aim:

To determine the effect of three different tooth preparation techniques had on operator's ability to appropriately and consistently prepare teeth for metal ceramic crowns.

Materials and Methods:

Ninety maxillary left central incisor typodont teeth were allocated to three equal groups (A, B and C) of thirty and mounted in standard working model one at a time. A freehand approach was used to prepare the teeth in Group A, which acted as a control. Groups B and C were prepared with the assistance of silicon index and suitable depth gauge burs, respectively. A silicon index of unprepared teeth, into which contrasting colored silicon injected to occupy the space created by tooth preparation, was sectioned in the midline. Images of sectioned index were captured with optical microscope attached to a personal computer. A calibrated image analysis software was used to measure the depth of preparation (in millimeters) at five points (labial-cervical, mid-labial, incisal, mid-palatal and palatal cervical) on two occasions. These results were pooled and averaged to give a mean labial, incisal and palatal preparation depths in mm. The data were analyzed by one-way analysis of variance and Scheffe's post-hoc statistical test.

Results:

The mean depth of labial and incisal preparation for Groups A, B and C was 1.23 and 1.72 mm, 1.45 and 1.96 mm, 1.47 and 1.95 mm, respectively. The difference between the groups’ labial preparation depth was significant as well as the difference between groups’ incisal preparation depth. The mean palatal preparation was 0.46 mm for Group A, 0.54 mm for Group B and 0.59 mm for Group C.

Conclusion:

Teeth preparation for metal ceramic crowns without any assistance can lead to under-preparation of labial and incisal surface.

Clinical Significance:

Whenever possible, considerable importance should be given to the use of index or depth gauge burs for preparing teeth for receiving metal ceramic crowns.

Coordinate geometry method for capturing and evaluating crown preparation geometry

STATEMENT OF PROBLEM

A validated universal method requiring no human input is needed to capture and evaluate preparation geometries in a manner that can be used to see the correlation of different parameters.

PURPOSE

The purpose of this study was to present a method of capturing and evaluating crown preparation geometry.

MATERIAL AND METHODS

One manually machined acrylic resin block and 9 randomly selected preparations for ceramic complete crowns prepared by general dentists were selected and prepared. The specimens were scanned (3D scanner; Nobel Biocare), and buccolingual and mesiodistal cross section images were collected. The images were imported into digitizing software (Engauge Digitizer 4.1) to convert the outlines into x and y coordinates. Six points were chosen by using a set of algorithms, and the resulting parameters were calculated.

RESULTS

The acrylic resin block was milled with a 12 degree total occlusal convergence (TOC) instrument producing a 12.83 degree TOC. For the other specimens, average TOC values ranged from 18 degrees to 52 degrees. The mean average margin width was 0.70 mm, and the mean average base dimension was 6.23 mm. The surface area/volume ratio, resistance length, and limiting taper were also calculated.

CONCLUSIONS

The method described provides a basis for accurately evaluating preparation geometry without human input.

Alternative methods for determining shrinkage in restorative resin composites

OBJECTIVES

The purpose of this study was to evaluate polymerization shrinkage of resin composites using a coordinate measuring machine, optical coherence tomography and a more widely known method, such as Archimedes Principle. Two null hypothesis were tested: (1) there are no differences between the materials tested; (2) there are no differences between the methods used for polymerization shrinkage measurements.

METHODS

Polymerization shrinkage of seven resin-based dental composites (Filtek Z250™, Filtek Z350™, Filtek P90™/3M ESPE, Esthet-X™, TPH Spectrum™/Dentsply 4 Seasons™, Tetric Ceram™/Ivoclar-Vivadent) was measured. For coordinate measuring machine measurements, composites were applied to a cylindrical Teflon mold (7 mm × 2 mm), polymerized and removed from the mold. The difference between the volume of the mold and the volume of the specimen was calculated as a percentage. Optical coherence tomography was also used for linear shrinkage evaluations. The thickness of the specimens was measured before and after photoactivation. Polymerization shrinkage was also measured using Archimedes Principle of buoyancy (n=5). Statistical analysis of the data was performed with ANOVA and the Games-Howell test.

RESULTS

The results show that polymerization shrinkage values vary with the method used. Despite numerical differences the ranking of the resins was very similar with Filtek P90 presenting the lowest shrinkage values.

SIGNIFICANCE

Because of the variations in the results, reported values could only be used to compare materials within the same method. However, it is possible rank composites for polymerization shrinkage and to relate these data from different test methods. Independently of the method used, reduced polymerization shrinkage was found for silorane resin-based composite.

Consistency of labial finish line preparation for metal ceramic crowns: an investigation of a new bur

PURPOSE

Previous studies have reported on the difficulties inherent in preparing the labial aspect of teeth for metal ceramic crowns with consistency and also the implications for the definitive restoration of underprepared and overangled finish lines. In this study, a bur designed to prepare a 1.2-mm deep chamfer was tested and compared with two other bur kits.

MATERIALS AND METHODS

Seventy-two teeth were prepared to receive metal ceramic crowns in vitro by students using one of the two bur kits or the new bur. Using a coordinate measuring machine (CMM), replicas of the prepared teeth were scanned in the mid-labial plane, and shoulder width and cavosurface angles were measured.

RESULTS

The new bur produced preparations with a mean shoulder width and cavosurface angle of 1.146 +/- 0.241 mm and 108 +/- 11 degrees, respectively, compared with 0.626 +/- 0.234 mm and 128 +/- 14 degrees produced by a commercially available standardized crown cutting bur system and 0.626 +/- 0.218 mm and 124 +/- 15 degrees produced by the bur kit in use at our center at the time of the study. These differences were statistically significant at the 5% level, using Tukey's comparison of means.

CONCLUSIONS

Teeth prepared by the new bur had wider shoulders and lower cavosurface angles than teeth prepared either with a standardized crown cutting system or locally selected bur kit. The quality of labial preparations produced by the new bur in terms of closeness to the postulated "ideal" width and angulation warrants further investigation.

Dentin exposure in tooth preparations for porcelain veneers: a pilot study

STATEMENT OF PROBLEM

The various clinical techniques available for even reduction of a tooth surface to receive a porcelain veneer restoration do not produce a consistently even conservative reduction. In addition, it is not known which technique is most conservative of enamel.

PURPOSE

The purpose of this pilot study was to assess the effectiveness of 3 clinical techniques, namely, dimple, depth groove, and freehand, in producing an intraenamel preparation. The relation between overpreparation beyond the commonly accepted depth of preparation of 0.5 mm and dentin exposure was also examined.

MATERIAL AND METHODS

A single operator prepared 3 groups of 5 extracted maxillary central incisors to a depth of 0.5 mm using dimple, depth-groove, and freehand methods of tooth preparation. The prepared teeth were scanned using an x-ray microtomography scanner. The reconstructed images were studied using software that provided a volume-rendering routine so that, by choosing suitable x-ray linear attenuation coefficient thresholds, enamel (2.78 cm(-1) at 40 keV) and dentin (1.63 cm(-1) at 40 keV) surfaces could be viewed. The percentage area of enamel conserved was analyzed from these images. Coordinate metrology was used to produce color-coded images depicting the depth of preparation. The Kruskal-Wallis test was used to determine the statistical significance (alpha=.05) in the difference between the mean percentage area of enamel conserved in the 3 technique groups. The coordinate metrology and x-ray microtomography images were visually compared to study the correlation between overpreparation and dentin exposure.

RESULTS

The Kruskal-Wallis test did not demonstrate significant difference (P=.07) between the 3 techniques in conserving enamel. However, the dimple technique showed a greater trend to retaining a larger mean percentage area of enamel (77.5% +/- 14.2) compared to depth-groove (50.1% +/- 17.5) and freehand (76.8% +/- 24.4) techniques. Preparation depth in the range of 0.4 to 0.6 mm was largely seen to be intraenamel, except in the cervical region.

CONCLUSION

Within the limitations of this pilot study, the 3 different techniques tested did not differ significantly in conserving enamel.

Mathematical analysis of tooth and restoration contour using image analysis

OBJECTIVES

The aim of this study was to develop a methodology for comparison of the contour of artificial crowns in the mid bucco-lingual plane with their equivalent natural teeth on the opposing side of the same arch (antimeres) using a novel application of image analysis software. The objective was to determine whether artificial crowns were overcontoured.

METHODS

Specimens consisted of thin sections of silicone putty impressions of the buccal and lingual surfaces of 55 full crown restorations and their natural antimeric teeth. A thin slice of the putty was obtained in the mid-tooth bucco-lingual plane and a digital image was captured and this was analysed to produce a data set (x, y) representing the curvature of the tooth surface. Further analysis was performed in order to describe the profile in optimum mathematical terms.

RESULTS

The curves were best represented by three equations: y = a + bx(0.5), ln(y) = a + bx2, and y2 = a + bx. In all equations parameter (b), which expresses the contour curvature, was used as a deciding factor in comparing the degree of contour of the crown restorations with their natural antimeres. Most artificial crowns were found to be either similarly or undercontoured when compared with their natural antimeres. When overcontouring was present in the artificial crowns this tended to occur on the lingual aspects of anterior and posterior crowns.

SIGNIFICANCE

Simplifying tooth contour into a mathematical model can be useful in determining whether restorations are overcontoured. Clinically, particular attention should be directed towards the lingual aspects of restorations which were more likely to be overcontoured.

Designing tooth preparations for optimal success

The form of prepared teeth and the amount of tooth structure remove dare important contributors to the mechanical, biologic,and esthetic success of the overlying crown or fixed partial denture.Therefore, it is important to develop clinical guidelines that can be used to optimize success in fixed prosthodontics.

Geographic distribution of porcelain veneer preparation depth with various clinical techniques

STATEMENT OF PROBLEM

Various clinical techniques have been advocated for uniform reduction of the tooth surface before a porcelain veneer restoration. Often these techniques do not produce a consistently uniform labial reduction.

PURPOSE

The purpose of this study was to identify the degree of inconsistency on a geographic scale in the depth of labial reduction for porcelain veneers, resulting from the use of 3 clinical techniques. The technique of co-ordinate metrology was used to map the variations in the depth of the preparation.

MATERIAL AND METHODS

A single operator using 3 techniques (dimples as depth guides, freehand, or depth grooves as depth guides) prepared 90 noncarious, unrestored extracted teeth to receive porcelain veneers (n=30). Impressions of the prepared and unprepared teeth were scanned with a co-ordinate measuring machine. In-house software was used to color-code the plotted images on the basis of the depth of preparation. Profile measurements were also made along the mid-labial sagital plane at the mid-labial, incisal, and cervical regions, as well as along the mesial proximal and distal proximal areas along the mid-labial horizontal plane. The ideal depth range for the labial reduction was chosen to be 0.4 to 0.6 mm. One-way analysis of variance and the Bonferroni test were performed to determine the significance (P<.05) in the difference between the means of reductions achieved with the 3 techniques.

RESULTS

There was no statistically significant difference in the mean percentage area prepared to the ideal depth range (0.4 mm-0.6 mm), between dimple (44.59%), freehand (36.35%), and depth groove (38.43%) techniques. The difference in the mean percentage area of reduction greater than 0.6 mm between dimple (12.98%), freehand (29.66%), and dimple and depth groove (37.32%) techniques were statistically significant (P=.0000), but not between freehand and depth groove techniques. With the profile measurements it was seen that there were statistically significant differences in the mean depth between dimple (0.45 mm) and depth groove (0.63 mm), and freehand (0.51 mm) and depth groove in the mid-buccal (P<.0004) and cervical (dimple = 0.48 mm, freehand = 0.52 mm, depth groove = 0.63 mm) (P<.0005) regions. There was statistically significant difference (P<.0000) in the mean depth between the dimple (0.39 mm) and freehand (0.30 mm), dimple and depth groove (0.50 mm), and freehand and depth groove techniques in the incisal area. In the mesial proximal region statistically significant difference (P<.0034) in the mean depth was found between the dimple (0.52 mm) and freehand (0.68 mm), and dimple and depth groove (0.64 mm) techniques only. In the distal proximal region, there was no statistically significant difference in the mean depth between dimple (0.55 mm), freehand (0.66 mm), and depth groove (0.64 mm) techniques.

CONCLUSION

The use of dimple technique showed a trend to greater consistency and fidelity in labial reduction to a depth of 0.4 to 0.6 mm. The 3 techniques for veneer preparations studied were associated with varying degrees of inconsistency in the distribution of depth of preparation within a tooth and between teeth in the same technique group.

A comparison of the depths produced using three different tooth preparation techniques

STATEMENT OF PROBLEM

Inadequate preparation of teeth for metal ceramic crowns can significantly influence the ultimate form and longevity of the definitive restoration, as well as adversely affect the supporting tissues.

PURPOSE

The purpose of the study was to determine the effect that 3 different tooth preparation techniques had on an operator's ability to appropriately and consistently prepare teeth for metal ceramic crowns.

MATERIAL AND METHODS

Thirty typodont maxillary central incisor teeth were mounted individually and randomly allocated to 3 equal groups (A, B, and C). One operator was asked to prepare each tooth for a metal ceramic crown. A freehand approach was used to prepare the teeth in Group A, which acted as a control. Groups B and C were prepared with the assistance of a silicone index and a suitable depth gauge bur, respectively. A silicone index of the unprepared tooth, into which contrasting silicone was injected to occupy the space created by tooth preparation, was sectioned in the midline. Images of the sectioned index were captured with an optical microscope (resolution +/- 0.02 mm), which was attached to a personal computer. A calibrated image analysis program was used to measure the depth of preparation (in millimeters) at 5 points (labial cervical, mid-labial, incisal, mid-palatal, and palatal cervical) on 2 occasions. These results were pooled and averaged to give a mean labial and palatal preparation depth (in millimeters) and incisal edge reduction. The data were analyzed by use of a 1-way analysis of variance and Scheffe's post hoc statistical test (P<.05).

RESULTS

The mean depths of labial and palatal preparation for Groups A, B, and C were 1.28 and 0.47 mm; 1.46 and 0.56 mm; and 1.45 and 0.63 mm, respectively. The difference between the groups' labial preparation depth was not significant (P=.06), but the difference for palatal preparation depth was significant (P=.01). The mean incisal reduction was 3.00 mm for Group A, 2.74 mm for Group B, and 2.13 mm for Group C (P=.00).

CONCLUSION

Within the limitations of this study, it was concluded that preparation of teeth for metal ceramic crowns without the use of devices to help gauge reduction depth can result in insufficient labial reduction and incisal overreduction.

Tooth preparations for complete crowns: an art form based on scientific principles

STATEMENT OF THE PROBLEM

No recent literature has reviewed the current scientific knowledge on complete coverage tooth preparations.

PURPOSE

This article traces the historic evolution of complete coverage tooth preparations and identifies guidelines for scientific tooth preparations.

MATERIAL AND METHODS

Literature covering 250 years of clinical practice was reviewed with emphasis on scientific data acquired during the last 50 years. Both a MEDLINE search and an extensive manual search were used to locate relevant articles written in English in the last 50 years.

RESULTS

Teeth should be prepared so that they exhibit the following characteristics: 10 to 20 degrees of total occlusal convergence, a minimal occlusocervical dimension of 4 mm for molars and 3 mm for other teeth, and an occlusocervical-to-faciolingual dimension ratio of 0.4 or greater. Facioproximal and linguoproximal line angles should be preserved whenever possible. When the above features are missing, the teeth should be modified with auxiliary resistance features such as axial grooves or boxes, preferably on proximal surfaces. Finish line selection should be based on the type of crown/retainer, esthetic requirements, ease of formation, and personal experience. Expectations of enhanced marginal fit with certain finish lines could not be validated by recent research. Esthetic requirements and tooth conditions determine finish line locations relative to the gingiva, with a supragingival location being more acceptable. Line angles should be rounded, and a reasonable degree of surface smoothness is desired.

CONCLUSIONS

Nine scientific principles have been developed that ensure mechanical, biologic, and esthetic success for tooth preparation of complete coverage restorations.