The taper of clinical preparations for fixed prosthodontics

The effect of virtual reality simulators on tooth preparation skills of dental students

BACKROUND

It has been considered that the preclinical education may be inadequate in the development of preparation skills of dental students. With technological developments, there is an increase in the use of computer-aided simulations in dental education. Virtual reality (VR) patient simulators promise practice in a realistic environment with detailed, frequent and objective feedback, but it remains unclear whether these features lead to better or faster skill acquisition. This study aims to investigate the impact of virtual reality simulations on students' clinical preparation skills.

METHODS

In this study, tooth preparation data of 100 fourth-year students who practiced preparation with a virtual reality device and had real patient experience, as well as a group of 100 fifth-year students who had no prior virtual reality experience but had real patient experience, were transferred to the exocad® DentalCAD program, a CAD software, for the measurement of TOC (Total Occlusal Convergence) angles. The data obtained from these measurements were analyzed using the SPSS program. In addition to this study, 25 random individuals from both groups underwent preparation in a phantom model in order to make a comparison between different experience environments. Finally, a questionnaire was administered to the group trained in the simulator to evaluate the students' perspective on this new training method. Independent sample T test and single sample analysis of variance (ANOVA), which are parametric test methods, and Tukey HSD, which is a post hoc test, were used in statistical analyses. With the t test, which is used to investigate whether there is a difference between two sample groups in terms of averages, we aimed to investigate the significance between class level and tooth cutting surface by using group statistics and descriptive analysis data of the preparations. With the ANOVA test, which is a statistical analysis method used to compare the averages between at least three groups, we looked at whether the preparations we made in 3 different experience environments produced a significant result according to the tooth cutting surface, and then we investigated which experience group was the source of the significance with the tukey test.

RESULTS

In the descriptive statistical analysis, it was determined that the average preparation angles of the 4th-year students were lower than those of the 5th-year students. Accordingly, it is possible to say that grade levels affect all surface preparation results in a statistically significant way. In descriptive statistics regarding patient preparation experience, the averages of the preparation angles taken according to the tooth surfaces of the 4th and 5th Grades are; The averages of the mesial, distal and buccal surfaces are close to each other, but only on the palatal surfaces, there is a significant difference between the averages of the 4th Grade (12.46) and the current average of the 5th Grade (15.22). The results of the Independent Samples T-Test conducted in the context of grade levels and four surface areas, is examined, it is seen that there are statistically significant differences between grade levels and the preparation results of all surfaces (p < 0.05).

CONCLUSIONS

According to the results of this study, students who used virtual reality simulators were more successful in terms of tooth preparation angles compared to students who did not use virtual reality simulators. This study demonstrated that the use of virtual reality simulators contributes to tooth prepartion skills.

Evaluating the Effectiveness of the "Double Finish Line Technique" on Full-Crown Retention for Short Abutments: An In Vitro Study

BACKGROUND

Achieving clinically adequate retention for cast crowns in prepared short abutments represents a major challenge for practicing dentists. Despite the important developments with adhesive cement, only a few teeth can be treated this way, and conventional preparation techniques are still employed for most crowns. Numerous options for auxiliary features exist; however, there is no consensus about one preferred method. This study aimed to evaluate the effect of a new innovative technique called the "double finish line technique" on full-crown retention for short abutments and to compare it with another modified preparation method.

MATERIAL AND METHODS

A comparative in vitro experimental study was conducted at the Department of Fixed Prosthodontics, Damascus University. The study sample consisted of 30 chromium-cobalt abutments and 30 metal crowns. The sample was divided into three equal groups. The first group was a control group (CG) with a 20-degree total occlusal convergence, 3 mm height, and a 0.5 mm chamfer finish line. The second group had a reduced total occlusal convergence (TOC) in the cervical 1.5 mm of the axial wall from 20 to 8 degrees. The third group implemented the so-called "double finish line technique," adding another 0.5 mm chamfer finish line 1 mm above the first finish line (DFL group). The metal crowns were cemented to metal dies with zinc-phosphate cement. Pull-off tests were applied until failure. Data were analyzed, and the differences between the three groups were detected using one-way ANOVA followed by Bonferroni's post-hoc tests (p<0.05).

RESULTS

The mean tensile strength values for the specimens were 115.36 (SD=14.59), 149.60 (SD=11.10), and 42.46 (SD=11.54) for the TOC, DFL, and CR groups, respectively.

CONCLUSION

The reduced total occlusal convergence and double finish line techniques effectively increased full crown retention and resistance cemented on short abutments.

Comparison of retention of monolithic zirconia crowns with alumina airborne-particle abraded and nonabraded intaglio using three different cements: A clinical simulation

STATEMENT OF PROBLEM

The necessity of roughening the intaglio surface of zirconia crowns to achieve adequate retention is unclear.

PURPOSE

The purpose of this clinical simulation study was to evaluate the retention of airborne-particle-abraded and nonabraded monolithic zirconia crowns using 3 different cement types.

MATERIAL AND METHODS

Extracted human molars were used and prepared with a 10-degree taper. Impressions were made of the prepared teeth with a polyvinyl siloxane (PVS) material, and dies were made with Type 4 gypsum. Each die was scanned with a NobelProcera 1G Scanner, and the standard tessellation language (STL) files were transferred electronically to the Nobel Biocare production site, where a bar was added virtually on top of each crown and parameters were set for milling. Seventy-two Procera zirconia crowns were generated, of which half were airborne-particle abraded on the intaglio surface with 50-µm alumina particles at 400 to 500 kPa for 15 seconds. The other 36 received no intaglio treatment other than cleaning. Both groups of 36 crowns were divided into 3 subgroups of 12 specimens. The area of each preparation was calculated using a computer-aided design software program. The specimens were distributed to attain similar mean surface areas among the cementation groups. The crowns were cemented onto the specimen with a controlled force of 196 N. The 3 cements used were self-adhesive, modified resin RelyX Unicem Aplicap, resin-modified glass ionomer RelyX Luting, and a composite resin, Panavia F2.0 with ED Primer A & B. All specimens were thermocycled (5 °C to 55 °C) for 5000 cycles and then removed axially with a universal testing machine (Instron Model 5585H) at a crosshead speed of 0.5 mm/min. The removal force was recorded, and stress of dislodgement was calculated for each crown. A 2-way analysis of variance was used for statistical analyses. The type of failure was analyzed with the chi-squared test of association for independent samples (α=.05 for all tests).

RESULTS

The mean dislodging force for crowns with airborne-particle abraded intaglio was 5.4 MPa, which was statistically greater than the mean of 3.2 MPa for nonabraded specimens (P<.001). No significant differences related to the dislodging stresses were detected among the 3 cements (P=.109). The mode of failure was similar whether abraded or not, with 50% of specimens retaining cement in the crown after separation.

CONCLUSIONS

Alumina airborne-particle abrasion of the intaglio of zirconia to create surface roughness is beneficial in retaining the crowns, regardless of the cement type. The nonabraded crowns demonstrated significantly lower retentive stress with crown removal. The principal mode of failure was similar whether the zirconia intaglio was airborne-particle abraded or not. The most common mode of failure (>50% of specimens) was at least three-fourths of the cement remaining within the crown.

A micro-computed tomography analysis of internal and marginal fits of fixed partial dentures: Effect of preparation finish line designs on monolithic zirconia and heat-pressed zirconia-reinforced lithium disilicate

PURPOSE

To evaluate the effect of finish line design (chamfer and feather-edge) and ceramic type on the internal and marginal fits of fixed partial dentures on abutment teeth.

MATERIALS AND METHODS

Two typodont mandibular casts, missing right first premolar tooth, received tooth preparation on canine and second premolar abutments (one cast with chamfer finish line and the other cast with feather-edge finish line). The preparation segment of each typodont model was scanned, 3D printed in resin, and then invested and casted in metal to obtain two metal models. Polyvinyl siloxane impressions were made for the metal models and poured in type IV stone. The stone models (n = 40) were randomly assigned into four groups (n = 10): chamfer finish line with heat-pressed zirconia reinforced lithium disilicate fixed partial denture (CL), chamfer finish line with monolithic zirconia fixed partial denture (CZ), feather-edge finish line with heat-pressed zirconia-reinforced lithium disilicate fixed partial denture (FL), and feather-edge finish line with monolithic zirconia fixed partial denture (FZ). After the fabrication of ceramic restoration, micro-computed tomography was used to evaluate the internal and marginal fits of each fixed partial denture. Data were statistically analyzed with three-way ANOVA (α = 0.05).

RESULTS

There were no significant interactions between preparation type, material type, and tooth type at any of the areas assessed. There was significant difference (p = 0.01) between CZ (59.15 ± 4.6 µm) and FZ (73.6 ± 17.1 µm) groups at the finish line area. Regarding the horizontal marginal discrepancy area, there were significant differences between CZ (62.65 ± 10.5 µm) and FZ (90.05 ± 5.6 µm) groups (p < 0.001), CL (77.45 ± 8.1 µm) and CZ (62.65 ± 10.5 µm) groups (p < 0.001), and FZ (90.05 ± 5.6 µm) and CL (77.45 ± 8.1 µm) groups (p < 0.001). At finish line area, there was a significant difference (p = 0.018) between feather-edge with canine (72.75 ± 13.3 µm) and chamfer with canine (59.05 ± 5.8 µm); however, there was no significant difference (p = 0.774) between feather-edge with premolar (69.45 ± 12 µm) and chamfer with premolar (65.1 ± 7.4 µm). Moreover, there was no significant difference (p = 0.886) between feather-edge with canine and feather-edge with premolar.

CONCLUSIONS

The internal and marginal fits of the ceramic fixed partial dentures can be affected by the finish line design and ceramic type. The feather-edge finish line had a negative impact on the marginal and internal fits of ceramic fixed partial dentures at certain measurement points. Regarding the effect of finish line design on abutment teeth, the difference in fit was only detected at the finish line area of the anterior abutment (canine) with the feather-edge finish line.

[Analysis of the fit quality of the metal frames on implants depending on the abutment taper]

OBJECTIVE

The objectivewas to study the effect of changing the anglconvergence of the abutment walls, depending on their height, as well as the number of supports, on the accuracy of fitting metal-ceramic prostheses to the abutments.

MATERIAL AND METHODS

It was studied 72 constructions: prostheses on single abutments with abutment heights from 3 mm to 9 mm with an increase in height of 2 mm, an angle of convergence of the abutment walls from 0° to 10° with an increase in convergence of 2° in the amount of 24 prostheses; 24 framework designs supported by two abutments, with the same parameters (height and angle of inclination of the abutment walls) and prostheses supported by three abutments in the amount of 24 designs. The abutments were milled from a titanium alloy Ti-6Al-4V, and the prostheses were cast from a cobalt-chrome alloy. During fabrication of the prostheses, a compensation varnish was applied to the respective abutments to create a gap between the framework and the abutment for a 50 μm thick cement film. The crown prostheses were cemented onto the abutments with GC Fuji + glass-ionomer cement, followed by vertical sawing in the medio-distal direction. After sawing, the gaps between the inner edges of the prostheses walls and the outer edges of the abutment walls were measured using a «Levenhuk DTX 90» digital microscope (Russia) at x100 magnification.

RESULTS

Abatement height 3 mm with any number of supports and 5 mm with up to 3 supports do not need taper for precise fitting. In 5 mm abatements and 3 and more supports minimal required taper is 2°. Seven mm singular abatement requires 4° taper, but the increased number of supports enhances it up to 6°. For 9 mm abatement optimal abatement taper for the best fitting is 8°.

CONCLUSION

The study showed the relationship between the fitting accuracy and the change in the abutment taper, their height, and the number of supports in the structure.

Influence of intraoral scanning on the quality of preparations for all-ceramic single crowns

Objectives

To evaluate the influence of intraoral scanning on the quality of preparations for all-ceramic single crowns.

Material and methods

A total of 690 randomly selected and anonymized in vivo single crown preparations were examined. Three hundred twenty-three preparations were directly recorded with an intraoral scanner (group IS). Data from plaster casts digitized by a laboratory scanner (group ID; N = 367) served as control. Comparisons included convergence angle, marginal design, marginal substance reduction, homogeneity of the finish line, and undercuts. Evaluation was performed using fully automated specialized software. Data were analyzed applying Kolmogorov-Smirnov, Mann-Whitney U test, and Fisher’s exact test. Level of significance was set at p < 0.05.

Results

Convergence angle was above optimum in both groups, but significantly larger for group IS (p < 0.001). Marginal design was more ideal in group IS concerning the absence of featheredge design (p < 0.001) and reverse bevel (p = 0.211). Marginal substance reduction was closer to prerequisites for all-ceramic restorations in group IS (p < 0.001). Finish lines were more homogeneous in group IS regarding the uniformity of their course (p < 0.001). Undercuts were more frequently found in group ID than in group IS (p < 0.001).

Conclusions

Intraoral scanning of prepared teeth has positive impact on the quality of preparations for all-ceramic single crowns regarding marginal substance reduction, marginal design, homogeneity of the finish line, and undercuts.

Clinical relevance

Accurate preparation design represents a fundamental condition for success of ceramic crowns. Since there is potential for optimization, intraoral scanning might enhance preparation quality providing instant visual feedback.

Long-term retention of zirconia crowns cemented with current automixed cements

STATEMENT OF PROBLEM

Automixing and dispensing cements is a straightforward approach with consistent dosing. Previous studies have demonstrated clinically significant differences in crown retention between power-liquid and paste-paste forms of the same cement, as the composition between the 2 differs. A self-adhesive modified-resin (SAMR) and a resin-modified glass ionomer (RMGI) cement, originally offered as a powder-liquid, are now in common use as paste-paste automixed cements. With the increased use of zirconia restorations, the long-term retention of zirconia crowns for these 2 automixed cements should be evaluated.

PURPOSE

The purpose of this in vitro study was to determine whether zirconia crowns cemented with 2 automixed cements provided clinically acceptable retention after 6 months of aging with monthly thermocycling.

MATERIAL AND METHODS

Extracted molars were mounted in resin and prepared with a flat occlusal surface, 20-degree taper, approximately 4-mm axial length, and with the axio-occlusal line angle slightly rounded. Prepared teeth were equally distributed into 3 cementation groups (n=12) to achieve nearly equal mean preparation surface areas for each group. Zirconia crowns (IPS ZirCAD LT) were fabricated with an added occlusal bar to facilitate removal of the cemented crowns. Cement space was set at 45 μm axially and 55 μm occlusally. After sintering and before delivery, the intaglio surfaces were airborne-particle abraded with 50-μm alumina at 275-kPa pressure for 3 seconds and then steam cleaned. Cements were the original powder-liquid RelyX Luting (RMGI; RXL) as the control, paste-paste, automixed systems RelyX Luting Plus Automix (RMGI; RXLA), and RelyX Unicem 2 Automix (SAMR; RXUA). Crowns were cemented under 196 N force, placed in an oven at 37 °C and 100% humidity during setting and then thermocycled (5 °C-55 °C) for 5000 cycles monthly for 6 months. The crowns were removed axially with a universal testing machine at 0.5 mm/min. Removal forces were recorded and dislodgement stress calculated by using the surface area of each preparation. One-way ANOVA was used for dislodgement stress and force. Chi-square test was used for cement location after testing (α=.05).

RESULTS

RXLA demonstrated considerably lower crown retention (1.3 MPa) and differed significantly (P<.001) from RXUA (3.1 MPa) and RXL (3.1 MPa). Modes of failure showed most of the cement remaining only in the crown intaglio for RXLA for all specimens, whereas half of the crowns for RXL and RXUA demonstrated cement adhesion to both dentin and the intaglio surface, indicating cohesive failure of the cement at separation. As the Levene test was significant, the Games-Howell test was used for mean differences. The χ² analysis was significant.

CONCLUSIONS

After long-term aging with monthly thermocycling, high-strength zirconia crowns were strongly retained by 2 (RXL, RXUA) of the 3 cements. Crown retention for RelyX Luting Plus Automix was less than half in comparison and with cement found only on the intaglio surface after separation.

Development of a dental handpiece angle correction device

Background

Preparation of a uniform angle of walls is essential for making an ideal convergence angle in fixed prosthodontics. We developed a de novo detachable angle-correction apparatus for dental handpiece drills that could help the ideal tooth preparation.

Methods

We utilized a gyro sensor to measure the angular velocities to calculate the slope of an object by integrating the values, acceleration sensor to calculate the slope of an object by measuring the acceleration relative to gravity, and Kalman filter algorithm. Converting the angulation of the handpiece body to its drill part could be performed by a specific matrix formulation set on two reference points (2° and 6°). A flexible printed circuit board was used to minimize the size of the device. For convergence angle investigation, 16 volunteers were divided randomly into two groups for performing tooth preparation on a mandibular first molar resin tooth. All abutments were scanned by a 3D scanner (D700^®, 3Shape Co., Japan), the convergence angle and tooth axis deviation were analyzed by a CAD program (SolidWorks 2013^®, Dassault Systems Co., USA) with statistical analysis by Wilcoxon signed-rank test (α = 0.05) using SPSS statistical software (Version 16.0, SPSS Inc.).

Results

This device successfully maintained the stable zero point (less than 1° deviation) at different angles (0°, 30°, 60°, 80°) for the first 30 min. In single tooth preparation, without this apparatus, the average bucco-lingual convergence angle was 20.26° (SD 7.85), and the average mesio–distal (MD) convergence angle was 17.88° (SD 7.64). However, the use of this apparatus improved the average BL convergence angle to 13.21° (SD 4.77) and the average MD convergence angle to 10.79° (SD 4.48). The angle correction device showed a statistically significant effect on reducing the convergence angle of both directions regardless of the order of the directions.

Conclusions

The angle correction device developed in this study is capable of guiding practitioners with high accuracy comparable to that of commercial navigation surgery. The volume of the angle correction device is much smaller than that of any other commercial navigation surgery system. This device is expected to be widely utilized in various fields of orofacial surgery.

Crown Taper Angles Achieved by Dental Students: A Systematic Review

The aim of this systematic review was to examine the literature on clinical taper angles achieved by dental students during crown preparation to determine the theoretical and clinically acceptable values identified in research studies. Medline, Embase, Web of Knowledge, the Cochrane Library, the British Dental Journal, and the Journal of the American Dental Association were searched to identify relevant studies. Studies were included if they were in vivo research on full crown preparations by dental students and published in English. Data extracted were country, year of publication, model selection and measurement methods, tests for reproducibility, tooth type, number of teeth assessed, and tapers achieved. The search resulted in 12 included articles from 11 countries published between 1978 and 2014 featuring a total of 2,306 preparations. In those studies, students failed to achieve ideal convergence angles (between 4° and 14°) but produced clinically acceptable results (between 10° and 20°). These findings should be taken into account when assessing dental students during their training programs.

Effect of silica-containing glass-ceramic liner treatment on zirconia coping retention

STATEMENT OF PROBLEM

Zirconia prostheses show a high rate of retention loss, and more information is needed regarding the treatment of the intaglios of zirconia prostheses to improve the bond strength between zirconia and resin cement.

PURPOSE

The purpose of this in vitro study was to evaluate the retention of titanium abutments and zirconia prostheses treated with silica-containing glass-ceramic liners. The effect of the abutment convergence angle on retention force was also examined.

MATERIAL AND METHODS

Titanium abutments and zirconia copings (n=90) were fabricated with 3 different convergence angles (6, 12, and 24 degrees). Specimens were divided into 2 groups according to surface treatment: an airborne-particle-abraded control group and a group that was coated with a silica-containing glass-ceramic liner after airborne-particle abrasion (liner group). The thickness of the liner was measured, and the inner spacing of the zirconia coping was modified in the liner group. The coping was cemented, and the retention was evaluated with a tensile bond strength test by using a universal testing machine. Retention was analyzed by using a 2-way analysis of variance. Modes of failure were evaluated by examining the fracture surface. Elemental analysis of the interface was also performed.

RESULTS

The liner group showed a significantly higher retention force for the zirconia prosthesis (P<.05). Regardless of surface treatment, the retention force at a convergence angle of 6 degrees was higher than that at 24 degrees (P<.05). The control group showed mostly adhesive type failures, whereas the liner group exhibited mostly mixed type failures.

CONCLUSIONS

Silica-containing glass-ceramic liner treatment increased the retention compared with airborne-particle abrasion treatment alone. Tensile bond strength increased with decreasing abutment convergence angle.

Assessment of Retention and Resistance Form of Tooth Preparations for All Ceramic Restorations using Digital Imaging Technique

AIM

This in vitro study evaluated the resistance form of die preparations for all ceramic restorations and, thereby, explored the concept of effective taper and its correlation between the ideal in theory and actual in the clinical situation by analyzing the digital images of the die preparations.

MATERIALS AND METHODS

Scanned digital images of 114 die preparations for all ceramic restorations (n = 114) were collected from a dental laboratory. All the images were also analyzed digitally using Adobe Photoshop® software to analyze the degree of taper (angle of convergence) of each preparation and then applied the Zuckerman's circle, and the Lewis perpendicular methods were used to measure the resistance form.

RESULTS

For the current study, the overall average degree of taper was found to be 20.9° (range, 2-80°), which is more than what is recommended by most previous studies and also sharply greater than the textbook ideal of 3 to 6°. Mean degree of taper for maxillary was 17.56° (anterior-10.50°, posterior-23.7°), and for mandibular teeth, it was 25.22° (anterior-15°, posterior- 28.45°). Out of the 64 analyzed images of maxillary teeth, 61 presented resistance form, while 3 were without it. Out of the 50 mandibular teeth analyzed, 38 possessed resistance form, whereas 12 were without. All the anterior teeth showed resistance form irrespective of the arch.

CONCLUSION

The degree of taper showed a significant relationship with resistance and retention form, which was inversely proportional to each other. The recommended "degree of taper" is not always the clinically achievable as advocated in textbooks, as it is modified by various factors in the actual clinical situation.

CLINICAL SIGNIFICANCE

The study provides scientific background regarding the relationship between the degree of taper with resistance and retention form, and the relationship was found to be inversely proportional to each other. The recommended "degree of taper" is not always the clinically achievable as advocated in textbooks, and it is modified by various modifying or limiting factors in the actual clinical situation.

Marginal and Internal Fit of CAD/CAM Crowns Fabricated Over Reverse Tapered Preparations

PURPOSE

Intraoral digital scanning can accurately record single abutment tooth preparations despite their geometry, and the algorithms of the CAD software can be set to manage different abutment forms. The purpose of this in vitro study was to evaluate the marginal and internal fit of CAD/CAM zirconia crowns fabricated over conventional and reverse-tapered preparations.

MATERIALS AND METHODS

Crown preparations with known total occlusal convergence (TOC) angles (-8°, -4°, 0°, 8°, 12°, 16°, and 22°) were digitally created from a maxillary left central incisor and printed in acrylic resin. Next, casts were scanned with a TRIOS intraoral scanner, and crowns were designed with KaVo multiCAD software using default parameters (50 μm cement space) on abutments with positive TOC angles, whereas reverse-tapered abutments (negative TOC angles) were digitally blocked out at 0° and had an extra mesiodistal gap set to 50 μm. Then, zirconia crowns were fabricated, and their marginal and internal discrepancies were recorded with the silicone replica technique. All replicas were examined under a stereomicroscope at 50× magnification. Collected data were analyzed with one-way ANOVA and post hoc Tukey test for marginal fit. For the axial and incisal fit, measured values did not follow a normal distribution; therefore, the Kruskal-Wallis and the Dunn/Bonferroni multiple comparison tests were applied (p = 0.05).

RESULTS

The mean marginal fit of -8° crowns (58.2 ± 6.0 μm) was statistically different (p < 0.0001) from all the remaining crowns (range 42.1-47.3 μm). Also, the internal fit was statistically significant when comparing crowns fabricated over abutments with positive and negative TOC angles (p < 0.0001). The largest median axial discrepancies were found in the -8° (165.5 μm) and -4° (130.8 μm) groups; however, when evaluating the incisal fit, they showed the smallest discrepancies (67.3 and 81.8 μm, respectively).

CONCLUSIONS

Under the conditions of this study, the marginal and internal fit of zirconia crowns fabricated over inverse-tapered preparations is within clinically accepted values.

Theoretical axial wall angulation for rotational resistance form in an experimental-fixed partial denture

PURPOSE

The aim of this study was to determine the influence of long base lengths of a fixed partial denture (FPD) to rotational resistance with variation of vertical wall angulation.

MATERIALS AND METHODS

Trigonometric calculations were done to determine the maximum wall angle needed to resist rotational displacement of an experimental-FPD model in 2-dimensional plane. The maximum wall angle calculation determines the greatest taper that resists rotation. Two different axes of rotation were used to test this model with five vertical abutment heights of 3-, 3.5-, 4-, 4.5-, and 5-mm. The two rotational axes were located on the mesial-side of the anterior abutment and the distal-side of the posterior abutment. Rotation of the FPD around the anterior axis was counter-clockwise, Posterior-Anterior (P-A) and clockwise, Anterior-Posterior (A-P) around the distal axis in the sagittal plane.

RESULTS

Low levels of vertical wall taper, ≤ 10-degrees, were needed to resist rotational displacement in all wall height categories; 2–to–6–degrees is generally considered ideal, with 7–to–10–degrees as favorable to the long axis of the abutment. Rotation around both axes demonstrated that two axial walls of the FPD resisted rotational displacement in each direction. In addition, uneven abutment height combinations required the lowest wall angulations to achieve resistance in this study.

CONCLUSION

The vertical height and angulation of FPD abutments, two rotational axes, and the long base lengths all play a role in FPD resistance form.

Post and core build-ups in crown and bridge abutments: Bio-mechanical advantages and disadvantages

Dentists often place post and core buildups on endodontically treated abutments for crown and bridge restorations. This article analyzes the bio-mechanical purposes, advantages and disadvantages of placing a core or a post and core in an endodontically treated tooth and reviews literature on post and core biomechanics. The author assesses the scientific rationale of the claim that the main purpose of a post is to retain a core, or the claim that posts weaken teeth. More likely, the main function of a post is to help prevent the abutment, on which a crown is cemented, from fracturing such that the abutment separates from the tooth root, at a fracture plane that is located approximately and theoretically at the level of the crown (or ferrule) margin. A post essentially improves the ferrule effect that is provided by the partial fixed denture prosthesis. This paper also explores the difference between bio-mechanical failures of crowns caused by lack of retention or excess taper, versus failures due to a sub-optimal ferrule effect in crown and bridge prostheses.

The Effect of Temporary Cement Cleaning Methods on the Retention of Crowns

PURPOSE

To evaluate the effect of temporary cement cleaning methods on the retention of cemented crowns using zinc phosphate cement and resin-modified glass ionomer cement.

MATERIALS AND METHODS

Forty titanium specimens were fabricated to simulate prepared molars with minimally retentive taper. The Ni-Cr cast crowns were fabricated, temporarily cemented, and separated. The specimens were divided into four groups according to the temporary cement cleaning method (n = 10) as follows: control group (no temporary cementation), orange solvent group, ultrasonic cleaning group, and air-abrasion group. After the cleaning procedures, the specimens were cemented with definitive cements (zinc phosphate cement and resin-modified glass ionomer, RMGI, cement) and subjected to thermocycling (5000 cycles, 5-55°C, dwell time, 10 seconds). The tensile bond strength of each specimen was measured using a universal testing machine, and the results were analyzed using the Kruskal-Wallis and Mann-Whitney U test (α = 0.05).

RESULTS

When cemented with zinc phosphate cement, the statistical analysis showed that the value of the air-abrasion group was significantly higher than those of the other groups (p < 0.01). There was no statistically significant difference among the other groups. When cemented with RMGI cement, the air-abrasion group showed the lowest value, and the control group showed the highest value (p < 0.01). The difference between the ultrasonic cleaning group and the orange solvent group was not statistically significant.

CONCLUSION

The use of temporary cement did not have a significant influence on retention of permanently cemented crowns when zinc phosphate cement was used for permanent cementation. Airborne-particle abrasion after provisional cementation improved retention of crowns cemented with zinc phosphate cement; however, the use of temporary cement significantly decreased retention of permanently cemented crowns when RMGI cement was used regardless of the temporary cement cleaning method.

Analysis of tooth preparations for zirconia-based crowns and fixed dental prostheses using stereolithography data sets

STATEMENT OF PROBLEM

Under clinical circumstances, it can be challenging for a dentist to achieve an adequate tooth preparation, which is essential for the long-term success of fixed dental restorations. This is particularly true for zirconia restorations fabricated by computer-aided design and computer-aided manufacturing (CAD-CAM) because of the difficulties involved in the scanning and milling process.

PURPOSE

The purpose of this retrospective study was to use a digital approach to evaluate dental tooth preparations for zirconia-based crowns and fixed dental prostheses (FDP) performed by general dental practitioners in Germany and to find out whether the type of prepared tooth or the type of restoration influenced the outcome.

MATERIAL AND METHODS

Stereolithography data sets of 182 zirconia frameworks with a total of 305 abutment teeth obtained by the CAD-CAM process were analyzed regarding total occlusal convergence, abutment height, and finish line design using a 3-dimensional (3D) inspection and mesh processing software. The criteria for adequate preparations were defined based on current literature and compared with the measurement results. The Kruskal-Wallis 1-way analysis of variance and the Mann-Whitney U test were applied to detect significant differences (α=.05).

RESULTS

Only 13 teeth (4.3%) met the clinical requirements for adequate zirconia-based tooth preparations. The mean total occlusal convergence was 17.9 ±9.7 degrees. Molars (23.2 ±10.1 degrees) showed statistically significant difference (P<.001) when compared with premolars (14.2 ±8.2 degrees) and anterior teeth (16.4 ±8.1 degrees). No significant difference was observed between the crown and FDP preparations. The mean abutment height was 5.6 ±1.3 mm. Again, significant differences occurred when tooth types (4.8 ±1.0 mm for molars, 5.2 ±0.9 mm for premolars and 6.9 ±1.2 mm for anterior teeth; P≤.01) were compared. In 92.5% of teeth, the finish line design was not ideal for zirconia-based restorations.

CONCLUSION

The tooth preparations of general dental practitioners differ from the ideal clinical preparation recommended for zirconia-based restorations. Difficulties are primarily related to the total occlusal convergence and finish line design. Although the outcome is strongly influenced by the type of tooth, the type of restoration seems to have only a minor effect on the tooth preparation quality.

Effect of preparation convergence on retention of multiple unit restorations - An in vitro study

BACKGROUND AND AIM

Convergence angle (CA) is one of the major determinant factors in the retention of single as well as multiple units. Hence, the aim of the present study was to determine the effect of preparation convergence on the retention of multiple unit restorations.

MATERIALS AND METHODS

Nickel-chromium alloy single crowns as well as three, six and nine multiple unit fixed partial dentures (FPDs) were casted for standardized milled dies that simulate canine, premolar and molar teeth machined with two different degrees of convergence, 12 (n = 55) and 20 (n = 55). The dies were threaded on rectangular metal platforms with the help of retainer shaft in the proper position to model either single or multiple preparations. The casted crowns and FPDs were cemented and retention tested by securing the units in an Instron universal testing machine. The tabulated values were analyzed statistically using Mann-Whitney test.

RESULTS

Single and multiple units with 12° CA needed greater force to dislodge compared to 20°, but this difference was not statistically significant for single units of premolar (P > 0.05). Irrespective of the angle, there was a simultaneous statistically significant increase in retention as the number of units increased in FPDs, except for the difference between 3 and 6 units in 20° group (P > 0.05).

CONCLUSION

This study highlights the importance of emphasizing on CA, during canine and molar preparation for single units as well as preparation of canines for 6 units FPDs.

Reporting numeric values of complete crowns. Part 1: Clinical preparation parameters

STATEMENT OF PROBLEM

An implemented objective measuring system for measuring clinical tooth preparations does not exist.

PURPOSE

The purpose of this study was to compare clinically achieved tooth preparations for ceramic crowns by general dentists with the recommended values in the literature with an objective measuring method.

MATERIAL AND METHODS

Two hundred thirty-six stone dies prepared for anterior and posterior complete ceramic crown restorations (IPS e.max Press; Ivoclar Vivadent) were collected from dental laboratories. The dies were scanned and analyzed using the coordinate geometry method. Cross-sectioned images were captured, and the average total occlusal convergence angle, margin width, and abutment height for each preparation was measured and presented with associated 95% confidence intervals.

RESULTS

The average total occlusal convergence angles for each tooth type was above the recommended values reported in the literature. The average margin widths (0.40 to 0.83 mm) were below the minimum recommended values (1 to 1.5 mm). The tallest preparations were maxillary canines (5.25 mm), while the shortest preparations were mandibular molars (1.87 mm).

CONCLUSIONS

Complete crown preparations produced in general practice do not achieve the recommended values found in the literature. However, these recommended values are not based on clinical trials, and the effects of observed shortfalls on the clinical longevity of these restorations are not predictable.

Measuring abutment convergence angles using stereovision dental image processing system

PURPOSE

The purpose of this study was to develop a dental image processing system using a three-dimensional (3D) camera and stereovision technology. The reliability of the system for measuring axial wall convergence angles was evaluated.

MATERIALS AND METHODS

The new system predicted 3D coordinate points from 2D images and calculated distances and angles between points. Two examiners measured axial wall convergence angles for seven artificial abutments using a traditional tracing-based method (TBM) and the stereovision-based method (SVBM). Five wax abutment models of simplified abutment forms were made and axial wall convergence angles of wax models were measured by both methods. The data were statistically analyzed at the level of significance, 0.05.

RESULTS

Intraclass correlation coefficients showed excellent intra-examiner and inter-examiner reliabilities for both methods. Bland-Altman plots and paired t-tests showed significant differences between measurements and true values using TBM; differences were not significant with SVBM.

CONCLUSION

This study found that the SVBM reflected true angle values more accurately than a TMB and illustrated an example of 3D computer science applied to clinical dentistry.

Effect of Preparation Taper, Height and Marginal Design Under Varying Occlusal Loading Conditions on Cement Lute Stress: A Three Dimensional Finite Element Analysis

To assess the effect of preparation taper, height and margin design under different loading conditions on cement lute stress. A 3-D FE model of an upper second premolar and molar was developed from CT scan of human skull using software programmes (MIMICS, Hypermesh and ANSYS). 10° and 30° taper, 3 and 5 mm preparation height and shoulder and chamfer finish lines were used. Type 1 Glass ionomer cement with 24 μm lute width was taken and the model was loaded under 100 N horizontal point load, vertical point load distributed axial load. The maximum shear stress and Von Mises stress within the cement lute were recorded. The maximum shear stresses ranged from 1.70 to 3.93 MPa (horizontal point loading), 0.66 to 3.04 MPa (vertical point loading), 0.38 to 0.87 MPa (distributed loading). The maximum Von Mises stresses ranged from 3.39 to 10.62 MPa (horizontal point loading), 1.93 to 8.58 MPa (vertical point loading) and 1.49 to 3.57 MPa (distributed loading). The combination of 10° taper and 5 mm height had the lowest stress field while the combination of 30° taper and 5 mm height had the highest stress field. Distributed axial loading shows least stress, better stress homogenization and gives a favorable prognosis for the fixed prostheses. Smaller preparation taper of 10° is biomechanically more acceptable than a 30° taper. It is desirable to decrease taper as height increases. The chamfer margin design is associated with greater local cement stresses toward the margins that could place the cement at greater risk for microfracture and failure.

The total occlusal convergence of the abutment of a partial fixed dental prosthesis: A definition and a clinical technique for its assessment

The abutment(s) of a partial fixed dental prosthesis (PFDP) should have a minimal total occlusal convergence (TOC), also called a taper, in order to ensure adequate retention of a PFDP that will be made for the abutment(s), given the height of the abutment(s). This article reviews the concept of PFDP abutment TOC and presents an alternative definition of what TOC is, defining it as the extent to which the shape of an abutment differs from an ideal cylinder shape of an abutment. This article also reviews experimental results concerning what is the ideal TOC in degrees and explores clinical techniques of estimating the TOC of a crown abutment. The author suggests that Dentists use high magnification loupes (×6-8 magnification or greater) or a surgical operating microscope when preparing crown abutments, to facilitate creating a minimum abutment TOC.

The reliability of an easy measuring method for abutment convergence angle with a computer-aided design (CAD) system

PURPOSE

The purpose of this study was to evaluate the intra-rater reliability and inter-rater reliability of three different methods using a drawing protractor, a digital protractor after tracing, and a CAD system.

MATERIALS AND METHODS

Twenty-four artificial abutments that had been prepared by dental students were used in this study. Three dental students measured the convergence angles by each method three times. Bland-Altman plots were applied to examine the overall reliability by comparing the traditional tracing method with a new method using the CAD system. Intraclass Correlation Coefficients (ICC) evaluated intra-rater reliability and inter-rater reliability.

RESULTS

All three methods exhibited high intra-rater and inter-rater reliability (ICC>0.80, P<.05). Measurements with the CAD system showed the highest intra-rater reliability. In addition, it showed improved inter-rater reliability compared with the traditional tracing methods.

CONCLUSION

Based on the results of this study, the CAD system may be an easy and reliable tool for measuring the abutment convergence angle.

Crown preparation design: an evidence-based review

Evidence-based clinical guidelines for full metal crown preparation design are seldom encountered in the veterinary literature. The veterinary literature regarding prosthodontic treatment in companion animals is sparse and consists primarily of case reports and expert opinion based primarily on clinical experience. The goal of this manuscript is to present concepts of evidence-based practice, review common recommendations on crown preparation design from the veterinary literature, and to provide an assessment of the evidence for or against those recommendations.

Measurement of total occlusal convergence of 3 different tooth preparations in 4 different planes by dental students

STATEMENT OF PROBLEM

Total occlusal convergence of crown preparation is an important didactic and clinical concept in dental education.

PURPOSE

The purpose of this study was to compare the discrepancy between the total occlusal convergence of dental students' typodont crown preparations and the ideal range (4 to 10 degrees) in 3 different regions of the mouth and in 4 different planes of the teeth.

MATERIAL AND METHODS

The dental students of the Class of 2012 at Harvard School of Dental Medicine were asked to prepare typodont teeth for crowns on 3 different teeth, the maxillary left central incisor (ceramic), mandibular left first molar (complete metal), and mandibular left first premolar (metal ceramic), during their third year preclinical summative examination and the Comprehensive Clinical Examination in their fourth year. Eighteen students prepared 3 teeth in their third and fourth years, whereas 19 students participated only in their fourth year, for a total of 55 sets of 3 teeth. By using custom fit die bases to reproduce the position, a novel procedure of measuring each tooth was accomplished in 4 different planes: the faciolingual, mesiodistal, mesiofacial-distolingual, and mesiolingual-distofacial. The total occlusal convergence of each image was measured with a computer screen protractor. The gingival 2 mm of the axial wall was used to determine the taper of each wall. Linear mixed model analysis was used to estimate and compare the total occlusal convergences of different teeth and planes (α=.05). Bonferroni corrections were used to adjust for post hoc multiple comparisons.

RESULTS

The mean total occlusal convergence varied by tooth and plane (2-way interaction; P<.001). For the first molar, dental students excessively tapered in all 4 planes; the model-predicted 99% CIs for the total occlusal convergence were as follows: faciolingual (12.7, 19.4), mesiodistal (14.0, 19.3), mesiofacial-distolingual (13,4, 19.4), and mesiolingual-distofacial (13.7, 19.1). For the central incisor, 99% CIs for the total occlusal convergence were (15.9, 24.4) for the faciolingual measurement, providing strong evidence of excessive tapering, and (4.1, 8.0) for the mesiodistal measurement, which was within the ideal total occlusal convergence range. The mesiofacial-distolingual and mesiolingual-distofacial planes in the central incisor and all 4 planes in the first premolar had mean total occlusal convergences that exceeded 10 degrees; however, excessive tapering could not be statistically established, because their CIs included values within the ideal range.

CONCLUSIONS

The present study found significant evidence of excessive tapering in a study comparing the total occlusal convergence values of crown preparations with those of the ideal preparation for 3 different teeth in 4 different planes. The total occlusal convergence for the molar preparations had the highest mean values.

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.

Comparing the effect of a resin based sealer on crown retention for three types of cements: an in vitro study

To determine the effect of resin based sealer on retention of casting cemented with three different luting agents. 55 extracted molar teeth were prepared with a flat occlusal surface, 20° taper and 4 mm axial height. The axial surface of each specimen was determined. The specimen were then distributed into five groups based on decreasing surface area, so each cementation group contained 11 specimens with similar mean axial surface area. A two-step, single bottle universal adhesive system (One-Step-Resinomer, Bisco) was used to seal dentin after the tooth preparation. Sealer was not used on the control specimens except for the modified-resin cement (Resinomer, Bisco) specimens that required use of adhesive with cementation. Using ceramometal (Wirobond(®), BEGO), a casting was produced for each specimen and cemented with either zinc phosphate (Harvard), glass ionomer (Vivaglass) or modified resin cement (Resinomer) with single bottle adhesive. All the castings were cemented with a force of 20 kg. Castings were thermal cycled at 5 and 55 °C for 2,500 cycles and were then removed along the path of insertion using a universal testing machine at 0.5 mm/min. A single-factor ANOVA was used with a = 0.05. The nature of failure was also recorded. The mean stress removal for non sealed zinc phosphate, sealed zinc phosphate, non sealed glass ionomer, sealed glass ionomer and modified resin cement was found to be 3.56, 1.92, 2.40, 4.26, 6.95 MPa respectively. Zinc phosphate cement remained principally on the castings when the tooth surface was treated with the sealer and was found on both the tooth and the casting when the sealer was not used. Fracture of root before dislodgement was seen in 9 of 11 specimens with modified resin cement. Resin sealer decreases the retention of the castings when used with zinc phosphate and increases it when used with glass ionomer cement. The highest mean dislodgement force was measured with modified resin cement.

Computer-aided evaluation of preparations for CAD/CAM-fabricated all-ceramic crowns

OBJECTIVE

The aim of this study was the evaluation of preparations from general dental practitioners for zirconia crowns and their correlation with clinical recommendations using a digital approach.

MATERIAL AND METHOD

Seventy-five datasets of left first upper molars (FDI 16) prepared for single zirconia crowns by general dental practitioners were analyzed using a computer-aided design software (LAVA(TM) Design; 3M ESPE, Seefeld, Germany) and a 3D-inspection software (COMETinspect®plus version 4.5; Steinbichler Optotechnik, Neubeuern, Germany). Evaluated parameters were convergence angle, undercuts, interocclusal reduction, abutment height, and design of preparation margin.

RESULTS

The mean convergence angle was determined to be 26.7°. The convergence angle in the mesiobuccal to distopalatal dimension was significantly the highest (31.7°), and the abutment height showed a mean value of 4.1 mm. Convergence angle and abutment height showed a negative correlation. Seventy-three percent of the evaluated locations revealed a margin design conforming to ceramic restorations. In over 30 % of the cases, the interocclusal reduction was insufficient. Generally, no preparation fulfilled all recommendations. Five (6.66 %) of the preparations fulfilled four criteria, 16 (21.33 %) preparations fulfilled three criteria, 31 (41.33 %) fulfilled two criteria, 17 (22.66 %) preparations fulfilled one criterion, and 6 (8 %) fulfilled no criterion.

CONCLUSIONS

Within the limitations of this study, most general dental practitioners seem to have difficulties fulfilling all clinical recommendations given for the preparation of zirconia crowns. The presented digital approach seems to be a useful method to evaluate the preparation geometry.

CLINICAL RELEVANCE

The correct preparation geometry represents an important prerequisite for the success of all-ceramic full crowns. As preparations clearly need to be improved, the approach presented could be the basis of a future tool to increase preparation quality in practice and education by direct objective feedback.

In vitro marginal fit of three all-ceramic crown systems before and after cementation

STATEMENT OF THE PROBLEM

Full-coverage all-ceramic restorations are widely used. The impact of various classifications of luting agent on marginal discrepancies is not well understood.

PURPOSE

The purpose of this study was to evaluate the cervical fit of all ceramic crowns (IPS e.maxPress, Cergogold, and In Ceram) on bovine teeth with two luting agents before and after cementation.

MATERIALS AND METHODS

Ninety bovine incisors were embedded in resin. The coronal portions of the teeth were prepared to receive full-coverage crowns. Thirty crowns of 7.0 ± 0.5 mm height, 8.0 mm cervical diameter, and 4.2 mm incisal diameter were fabricated for each ceramic system. The crowns were seated on the teeth, and the marginal discrepancy was measured using a measuring microscope. Then, 15 crowns of each ceramic system were luted on the teeth with resin cement (Variolink II) or resin-modified glass ionomer cement (Rely X luting), and the marginal discrepancy was measured. The results were submitted to analysis of variance, t test and Tukey's test (p<0.05).

RESULTS

The three ceramic systems showed cervical fits after cementation statistically inferior to cervical fits before cementation for the two cements. The IPS e.maxPress showed values for cervical fit statistically superior to Cergogold before cementation. No statistically significant difference was found between IPS e.maxPress and In Ceram and In Ceram and Cergogold. After cementation, no statistically significant difference was found for the three ceramics systems when luted with resin or resin-modified glass ionomer luting agents.

CONCLUSIONS

Within the limitations of this study, it can be concluded that both cements studied increase the marginal discrepancy between the crown and the preparation for the three ceramic systems evaluated.

Achievable convergence angle and the effect of preparation design on the clinical outcome of full veneer crowns in dogs

It is widely accepted that the convergence angle of a full veneer crown preparation should be as close to parallel as possible to attain adequate retention/resistance. The shape of the dog's canine tooth limits the veterinary dentists' ability to achieve the recommended convergence angle. However, the clinically achievable convergence angle of the canine tooth in dogs has not been evaluated. In addition, the convergence angle and other physical properties of a preparation, such as height and base diameter, have been shown to affect the retention/resistance of full veneer crowns, in vitro. This effect has not been evaluated clinically in the dog. Physical properties of 32 stone dies from full veneer crowns of canine teeth were studied to evaluate the clinically achievable convergence angle and the potential effect physical properties of the preparation had on the clinical outcome of the restoration. The clinically achievable convergence angle was much higher than the current recommendation. There was an association, albeit not statistically significant, between physical properties of a preparation (convergence angle, height, base diameter) and the clinical outcome of the restoration.

The effect of tooth and foundation restoration heights on the load fatigue performance of cast crowns

STATEMENT OF PROBLEM

In badly damaged teeth and teeth with short clinical crown heights, the placement of foundation restorations has been advocated to permit the development of retention and resistance form. However, there is little information on the effect of these foundation restorations on the clinical performance of the definitive restoration.

PURPOSE

The purpose of this study was to evaluate the load fatigue performance of teeth restored with posts and cores, with varying tooth heights, and to compare them with similar groups having no posts and cores. A secondary purpose was to determine whether a critical tooth height existed at which the placement of a foundation restoration resulted in no significant difference in the load fatigue performance.

MATERIAL AND METHODS

Three test groups (n=10) with prepared tooth heights of 2, 3, and 4 mm were tested. These were compared with another 3 groups with similar tooth heights that were restored with prefabricated titanium posts and core heights of 4, 3, and 2 mm, respectively. Cast complete crowns were then fabricated and cemented with zinc phosphate cement. A fatigue load of 58.8 N was applied at an angle of 135 degrees to the long axis of each crown-tooth specimen. The number of cycles to preliminary failure was determined. Significant differences in cycles to preliminary failure were assessed with 1-way ANOVA, followed by Tukey HSD tests (α=.05).

RESULTS

The group with the greatest preparation height (4 mm) and a 2-mm post-retained foundation had the highest number of cycles to preliminary failure (437,701), while the group with the shortest preparation height and no foundation had the lowest number of cycles (53,806). The Tukey HSD multiple comparison tests showed that for all 3 tooth heights, groups with foundation restorations had a significantly higher number of cycles to preliminary failure than those without foundation restorations.

CONCLUSIONS

For a given tooth height, teeth restored with foundation restorations had a significantly better load fatigue performance than those with no foundation restoration.

The taper of cast post preparation measured using innovative image processing technique

BACKGROUND

No documentation in the literature about taper of cast posts. This study was conducted to measure the degree of cast posts taper, and to evaluate its suitability based on the anatomy aspects of the common candidate teeth for post reconstruction.

METHODS

Working casts for cast posts, prepared using Gates Glidden drills, were collected. Impressions of post spaces were made using polyvinyl siloxan putty/wash technique. Digital camera with a 10' high quality lens was used for capturing two digital images for each impression; one in the Facio-Lingual (FL) and the other in the Mesio-Distal (MD) directions. Automated image processing program was developed to measure the degree of canal taper. Data were analyzed using Statistical Package for Social Sciences software and One way Analysis of Variance.

RESULTS

Eighty four dies for cast posts were collected: 16 for each maxillary anterior teeth subgroup, and 18 for each maxillary and mandibular premolar subgroup. Mean of total taper for all preparations was 10.7 degree. There were no statistical differences among the total taper of all groups (P = .256) or between the MD and FL taper for each subgroup. Mean FL taper for the maxillary first premolars was lower significantly (P = .003) than the maxillary FL taper of the second premolars. FL taper was higher than the MD taper in all teeth except the maxillary first premolars.

CONCLUSIONS

Taper produced did not reflect the differences among the anatomy of teeth. While this technique deemed satisfactory in the maxillary anterior teeth, the same could not be said for the maxillary first premolars. Careful attention to the root anatomy is mandatory.

An in vitro study to evaluate the retention of complete crowns prepared with five different tapers and luted with two different cements

This study includes comparatively evaluating and drawing general conclusion about the best advisable taper and cement for maximal retention. Eighty extracted human maxillary premolar teeth with sound surfaces were selected using dial vernier caliper. Selected teeth were randomly divided into five different taper groups (0°, 3°, 6°, 9° and 12°). The crown preparations with different tapers respectively were achieved by graduated customized device. Preparations were verified with special set up. Crowns were cast with Co-Cr alloy; metal copings were luted with glass ionomer and zinc phosphate cement. Retention was measured (MPa) by separating the metal crowns from the teeth under tension on a universal testing machine, data was recorded and statistically analyzed. Glass ionomer cemented 0° and 12° taper group showed increase in retentive strength (p = 0.003 hs), when compared to zinc phosphate cement. 9° and 12° group showed decreased retentive strength (p = 0.001 vhs) when compared with 0° taper group. No significant difference found between 0° and 3° and 6° group. The choice of cement for crowns prepared within this ideal range (0°-6° taper) might be of limited clinical significance. 3° and 6° taper with zinc phosphate or glass ionomer cement shown to be ideal for maximum retention.

Effect of surface conditioning and taper angle on the retention of IPS e.max Press crowns

PURPOSE

The objective of this study was to evaluate the retentive strength of single-unit crowns with 10 degrees and 26 degrees taper angles cemented using two surface conditioning methods.

MATERIALS AND METHODS

Thirty-two freshly extracted sound human molars were divided into two groups (n = 16) and prepared in a standardized manner with 10 degrees and 26 degrees taper angles. All-ceramic (IPS e.max Press) single crowns were fabricated for the prepared teeth. The crowns were then subdivided into two groups (n = 8), according to type of surface conditioning for the intaglio surfaces. Half the groups were HF acid etched and silanized, and the other half were conditioned with tribochemical silica coating and silanization. The crowns were cemented using adhesive cement (Panavia F 2.0). Retentive strength was measured in a universal testing machine.

RESULTS

No significant difference was found between the mean retention forces for both 10 degrees and 26 degrees taper angles when the crowns were conditioned either with silica coating (613 +/- 190 N and 525 +/- 90 N, respectively), or with hydrofluoric (HF) acid etching and silanization (550 +/- 110 N and 490 +/- 130 N for 10 degrees and 26 degrees, respectively) (p= 0.32).

CONCLUSION

Neither the surface conditioning type, nor the taper angle affected the retentive strength of IPS e.max Press single-unit crowns when cemented adhesively. Since silica coating and silanization did not show significant differences from HF acid gel and silanization, the former can be preferred for conditioning intaglio surfaces of glass ceramic crowns to avoid the use of the hazardous compound HF acid gel chairside.

The effect of preparation height and luting agent on the resistance form of cemented cast crowns under load fatigue

STATEMENT OF PROBLEM

The minimum amount of resistance form required for the success of a clinical crown is unknown.There is little information on the fatigue performance of complete coverage restorations on natural tooth preparations cemented with different luting cements.

PURPOSE

The purpose of this study was to evaluate the effect of tooth preparation height and luting agent on resistance form using unidirectional load fatigue testing. For a given tooth preparation with a clinically relevant total occlusal convergence (TOC), the adequacy of resistance form was investigated.

MATERIAL AND METHODS

Sixty-four human maxillary premolars were prepared with occlusal-cervical dimensions of 2, 3,4, or 5 mm and a TOC of 20 degrees. Complete metal crowns were cemented using either zinc phosphate cement (HY Bond;ZP groups) or resin cement (Panavia F; PF groups). Cyclic load fatigue testing was done with an applied load of 6.0 kg at 2.6 Hz. Load cycles to preliminary failure were detected with a strain gauge at the palatal crown-tooth interface. Results were subjected to the Kruskal-Wallis test and the Wilcoxon post-hoc rank sum test (alpha=.05).

RESULTS

Groups ZP4, ZP5, PF2, PF3, PF4, and PF5 had the highest mean number of cycles to preliminary failure,while group ZP2 had the lowest mean number of cycles to failure. Group ZP2 was significantly different (P<.001) from all other test groups for the number of cycles to failure.

CONCLUSIONS

For the 2- and 3-mm preparation height groups, zinc phosphate cement exhibited a poorer fatigue performance compared to Panavia F. There was no significant difference in the number of cycles to failure for groups ZP4,ZP5, PF2, PF3, PF4, and PF5. For both cements, the number of cycles to failure increased with increasing resistance length. (J Prosthet Dent 2009;102:155-164)

Crowns Cemented on Crown Preparations Lacking Geometric Resistance Form. Part II: Effect of Cement

PURPOSE

This study evaluated the effect of different cements on resistance to dislodgment of crowns cemented on preparations lacking geometric resistance form.

MATERIALS AND METHODS

A preparation that offered no geometric resistance form, with 20 degrees total occlusal convergence (TOC), 0.9 mm wide shoulder finish line, and a 2.5 mm axial wall height was created on an ivorine tooth using a milling machine. Ten metal test specimen die replicas and 10 standardized metal crowns with recipient sites for the application of external forces through a universal testing machine were fabricated. The crowns were cemented on the dies under 5 and 10 kg external loads, the marginal openings measured, loaded to dislodgment, and cleaned of cement. The process was repeated using zinc oxide and eugenol (ZOE), zinc phosphate (ZPh), resin modified glass ionomer (RMGI), and composite resin (CR) cements.

RESULTS

Marginal openings under 5 kg cementation loads were 74.63 (+/-15.04) for ZOE, 75.98 (+/-18.20) microm for ZPh, 98.58 (+/-22.62) microm for RMGI, and 105.82 (+/-20.07) microm for CR cements respectively; under 10 kg cementation loads they were 57.62 (+/-15.86) microm, 59.55 (+/-15.41) microm, 95.00 (+/-19.52) microm, 101.30 (+/-12.52) microm respectively. Oblique dislodgment forces, measured with a Universal testing machine, were 40.18 (+/- 6.76) N for ZOE, 215.65 (+/-45.79) N for ZPh, 165.43 (+/-19.53) N for RMGI, and 181.54 (+/-30.75) N for CR respectively when crowns were cemented under 5 kg loads. The corresponding values for 10 kg loads were 38.62 (+/-4.19), 274.86 (+/-54.22), 139.70 (+/-21.71), and 160.40 (+/-21.21) respectively. Only zinc phosphate cement produced statistically enhanced resistance when crowns were cemented under 10 kg force (p value = 0.035).

CONCLUSIONS

Under the conditions of the present study only crowns cemented with zinc phosphate displayed increased resistance to dislodgment on preparations lacking resistance form.

Factors influencing bonding fixed restorations

INTRODUCTION Crown displacement often occurs because the features of tooth preparations do not counteract the forces directed against restorations. OBJECTIVE The purpose of this study was to evaluate the effect of preparation designs on retention and resistance of fixed restorations. METHOD The study was performed on 64 differently sized stainless steel dies. Also, caps which were used for evaluated retention were made of stainless steel for each die. After cementing the caps on experimental dies, measuring of necessary tensile forces to separate cemented caps from dies was done. Caps, which were made of a silver-palladium alloy with a slope of 60? to the longitudinal axis formed on the occlusal surface, were used for evaluating resistance. A sudden drop in load pressure recorded by the test machine indicated failure for that cap. RESULTS A significant difference was found between the tensile force required to remove the caps from the dies with different length (p<0.05) and different taper (p<0.01). The greatest retentive strengths (2579.2 N and 2989.8 N) were noticed in experimental dies with the greatest length and smallest taper. No statistically significant (p>0.05) differences were found between tensile loads for caps cemented on dies with different diameter. Although there was an apparent slight increase in resistance values for caps on dies with smaller tapers, the increase in resistance for those preparation designs was not statistically significant. There was a significant difference among the resistance values for caps on dies with different length (p<0.01) and diameter (p<0.05). CONCLUSION In the light of the results obtained, it could be reasonably concluded that retention and resistance of the restoration is in inverse proportion to convergence angle of the prepared teeth. But, at a constant convergence angle, retention and resistance increase with rising length and diameter.