Chamfer Finish Line Research Articles

The Comparison of Vertical Margin Discrepancy in Casting Fabricated With Metal Ring, Ringless and Metal Ring With Hygroscopic Expansion Investment Systems

Background: The use of casting ring to produce accurate casting has been challenged with the introduction of a ringless casting technique. This study compared the vertical margin discrepancy of castings fabricated with the ringless technique, conventional (ring and liner) technique and conventional technique with hygroscopic expansion investment systems. Objectives: The current study aimed to determine and compare the mean values of vertical margin discrepancies of castings in three investing procedures: the ringless technique and conventional investing technique. Materials and Methods: In the first step, 39 coping wax patterns for Porcelain fused to metal restoration (P.F.M). With chamfer finish line were fabricated on the metal die and then divided into three groups of 13. The first group used conventional investing (with ring and liner), the second group used the ringless technique and the third group used the conventional ring technique with liner and hygroscopic expansion. All groups were invested using Thermovest phosphate bonded investment. Then wax patterns were cast with Ni-Cr alloy (super cast). Castings were divested, cleaned and seated on the metal die after minimal internal surface adjustment. The vertical margin discrepancy was measured in four sites: on the buccal, lingual, mesial and distal surfaces of the die. Castings were measured on an optical microscope. The vertical margin discrepancy in each group was used for statistical analysis. Results: After statistical evaluation, the following results were obtained: the mean vertical margin discrepancy for conventional investing was 156.72 ± 64.67µ; for the ringless technique it was 117.3 ± 26.77µ; and for the conventional metal ring technique with hygroscopic expansion it was 96.15 ± 38.31 µ. There is a significant difference between castings of the conventional investing technique and the ringless and ring plus hygroscopic expansion techniques (P < 0.001). No significant difference was found between the ringless and ring plus hygroscopic expansion techniques (P = 0.183). Conclusions: The castings fabricated from the ringless technique with non-precious alloy are acceptable. This investing system is useable for the fabrication of fixed restorations.

Marginal Adaptation of Metal Ceramic Crowns Cast from Four Different Base Metal Alloys before and after Porcelain Application

Introduction: Marginal adaptation of a crown is an important factor affecting its clinical acceptability and durability; also, poorly fitting cast restoration enables bacterial plaque accumulation and leads to teeth caries and periodontal deterioration. Purpose: The purpose of this study was to evaluate marginal adaptation of 4all, Verabband2, T3 and Supremcast V base metal alloys before and after porcelain firing. Material and Method: In this experimental study, a sound maxillary first premolar was prepared with chamfer and shoulder-bevel finish lines in palatal and buccal surfaces, respectively. Then the metal die was fabricated. Six points were marked 1 mm apical to the margin in the buccal and palatal surfaces. Forty impressions were taken from metal die by polyether impression material. Then, 10 frameworks were fabricated with each alloy type on stone dies. The vertical margin discrepancy between the frameworks and the marked points on metal dies were measured before and after porcelain firing. Paired sample t-test and One-way ANOVA, followed by multiple comparison tests (Tukey test) were used to evaluate data. In this study, statistical significance was defined at p<0.05. Results: Before porcelain firing, T3 alloy showed the lowest marginal discrepancy on chamfer finish line (13.13 ± (1.26)); but, the highest values were for Supermcast V copings and shoulder-bevel finish after porcelain application (30.83 ± (2.01)). The mean marginal discrepancies of buccal and palatal surfaces before porcelain application were significantly lower for all alloy types (p<0.001). Conclusion: The marginal adaptation of all four metal alloys was clinically acceptable and the thermal cycles of porcelain firing affected marginal adaptation. Keywords: Crown; Marginal adaptation; Metal ceramic alloys

Durability and Weibull Characteristics of Lithium Disilicate Crowns Bonded on Abutments with Knife-Edge and Large Chamfer Finish Lines after Cyclic Loading.

The aim of this study was to evaluate the durability of lithium disilicate crowns bonded on abutments prepared with two types of finish lines after long-term cyclic loading. Pressed lithium disilicate all-ceramic molar crowns were bonded (Variolink II) to epoxy abutments (height: 5.5 mm, Ø: 7.5 mm, conicity: 6°) (N = 20) with either knife-edge (KE) or large chamfer (LC) finish lines. Each assembly was submitted to cyclic loading (1,200,000×; 200 N; 1 Hz) in water and then tested until fracture in a universal testing machine (1 mm/min). Failure types were classified and further evaluated under stereomicroscope and SEM. The data (N) were analyzed using one-way ANOVA. Weibull distribution values including the Weibull modulus (m), characteristic strength (0), probability of failure at 5% (0.05), 1% (0.01), and correlation coefficient were calculated. Type of finish line did not significantly influence the mean fracture strength of pressed ceramic crowns (KE: 1655 ± 353 N; LC: 1618 ± 263 N) (p = 0.7898). Weibull distribution presented lower shape value (m) of KE (m = 5.48; CI: 3.5 to 8.6) compared to LC (m = 7.68; CI: 5.2 to 11.3). Characteristic strengths (0) (KE: 1784.9 N; LC: 1712.1 N) were higher than probability of failure at 5% (0.05) (KE: 1038.1 N; LC: 1163.4 N) followed by 1% (0.01) (KE: 771 N; LC: 941.1 N), with a correlation coefficient of 0.966 for KE and 0.924 for LC. Type V failures (severe fracture of the crown and/or tooth) were more common in both groups. SEM findings showed that fractures occurred mainly from the cement/ceramic interface at the occlusal side of the crowns. Lithium disilicate ceramic crowns bonded onto abutment teeth with KE preparation resulted in similar fracture strength to those bonded on abutments with LC finish line. Pressed lithium disilicate ceramic crowns may not require invasive finish line preparations since finish line type did not impair the strength after aging conditions.

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 24μm lute width was taken and the model was loaded under 100N 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 5mm height had the lowest stress field while the combination of 30° taper and 5mm 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.

Marginal adaptation of zirconium dioxide copings: Influence of the CAD/CAM system and the finish line design

Marginal adaptation is essential for the long-term success of dental restorations. Studies comparing the marginal gaps of computer-aided design/computer-aided manufacturing (CAD/CAM) restorations made from conventional and digital impressions are limited. The purpose of this study was to evaluate the marginal adaptation of zirconium dioxide copings made with 2 different CAD/CAM systems, the Lava All-Ceramic System (scanning definitive dies made with conventional impressions) and the Lava Chairside Oral Scanner (scanning directly from the prepared tooth). In addition, the influences of 2 different finish line configurations were also compared. Forty human molar teeth were prepared to receive complete crowns. Twenty were prepared with a 90-degree round shoulder and the other 20 with a 45-degree chamfer finish line. Zirconium dioxide copings were fabricated by using CAD/CAM technology. The specimens were randomized; they included 10 teeth with round shoulder and 10 with chamfer finish lines with the Lava All-Ceramic System, and 10 teeth with round shoulder and 10 with chamfer finish lines with the Lava Chairside Oral Scanner. The marginal gaps were measured at 5 points on each side with stereomicroscopy at 40× magnification. Data were analyzed with 2-way ANOVA (α=.05). The mean values of the measured marginal gap for the round shoulder group were 52.66 μm with the Lava All-Ceramic System and 14.98 μm with the Lava Chairside Oral Scanner. The mean values for the chamfer group were 64.06 μm for the Lava All-Ceramic System and 18.45 μm for the Lava Chairside Oral Scanner. Differences were identified when the marginal gaps between the specimens with different CAD/CAM systems were compared. However, differences in the finish line design were identified only between the round shoulder and chamfer with the Lava All-Ceramic System. Restorations fabricated with the Lava Chairside Oral Scanner displayed smaller marginal gaps than those made with the Lava All-Ceramic System. Nevertheless, all marginal gaps were within the range of clinical acceptability for both groups.

Effect of Shoulder and Deep Chamfer Finish Lines on Marginal Fitness of Electroformed P.F.M. Restorations

Objective: Comparison of the effect of shoulder with deep chamfer finish lines in P.F.M restorations using electroforming. Method: This experimental study was performed on 20 samples that were randomly selected. Two cutting designs shoulder and deep chamfer were created on dies via a standard turning machine. The diameter rim was 8mm and the height was 6 mm and the depth of cutting edge in cervical area, was 1mm. Impressions were made from dies and 10 acetone dies were prepared for each group. Silver was used on the dies to the finish line and a copper wire was connected to the die by making a foramen 1 mm away from finish line. The collection of die and wire were attached to the head of electroforming machine and gold cooping was created with electroforming. The gap between the die and the crown was measured by electron microscope; data was recorded and evaluated by the t-test. Results: According to statistical data, the average gap in shoulder lathe was 24.42 microns and in deep chamfer was 20.66 microns. Conclusions: The deep chamfer design in electroforming method had a better marginal fitness in comparison to the shoulder design.

Students’ Perceptions of Materials and Techniques Used at European Dental Schools in the Education of Fixed Prosthodontics

The aim of this study was to explore the materials and procedures used by students in dental schools across Europe for teaching fixed prosthodontics. An online questionnaire, containing twenty-eight dichotomous, multiple-choice, and Likert scale rating questions, was sent to students in forty dental schools. After excluding dental schools in which less than 10 percent of the students responded, 775 questionnaires from ten schools remained for statistical analysis. Among these respondents, acrylic resin teeth were said to be the most commonly used material during preclinical practice (46-96 percent), and use of extracted teeth varied from 8 to 65 percent. At nine of the ten institutions, metal-ceramic was reported to be most commonly used for fixed dental prostheses. There was large variation in the type of finish line for a metal-ceramic fixed dental prosthesis: students at five institutions reported using a shoulder finish line, three a chamfer finish line, and two a shoulder-bevel finish line. A similar variation was observed with regard to the final cementation of metal-ceramic fixed dental prostheses: students at four institutions reporting most frequently using glass ionomer cement, with three using zinc phosphate cement and three using carboxylate cement. The responding European dental students varied considerably in their opinions about whether they were preclinically properly trained for the first preparation on a patient and in their overall rating of their education in fixed prosthodontics. Responding students in the United Kingdom, Sweden, and Nijmegen, The Netherlands, rated their fixed prosthodontics training overall the highest. Overall, this study found a wide variation amongst dental schools with regard to their education in fixed prosthodontics and their rating of this teaching.

Fracture resistance of soft tissue level implants after cyclic loading and external modification

The exposure of the implant collar in the esthetic zone is a challenging complication of implant dentistry. The preparation of the exposed implant collar to create room for restorative material may decrease the strength of the abutment/implant assembly. The purpose of this in vitro study was to evaluate the effect on the failure load of preparing a 0.5-mm chamfer finish line on an implant collar at various apical depths. Thirty solid abutments and 30 implants were screwed into a customized metal mold at a 30-degree angle to the vertical axis. An intact group (IN) (control) (n=10) had no preparation, experimental group P1 had a 0.5-mm chamfer margin placed 1 mm apical to the implant/abutment interface, and experimental group P2 had the margin placed 2 mm apical. The specimens were subjected to a fatigue loading of 200 N for 1 000 000 cycles. Unfractured specimens were then loaded until failure. Mean fracture load (MFL) values were compared with 1-way ANOVA, and the Tukey test was used to identify differences among groups (α=.05). One specimen from both the IN and P1 groups and 2 specimens from the P2 group failed after fatigue loading. Significant differences in the MFL values were found between the IN group (3825) and P1 group (2944), P<.001 and between the IN group and the P2 group (2553), P<.001. The mean load required to fracture the abutment/implant assembly decreased significantly after various amounts of implant collar preparation.

The Effect of Occlusal Surface Relief of Dies on Marginal Adaptation of Metal‐Ceramic Casting Copings

The purpose of this study was to evaluate the impact of occlusal relief of dies on internal adaptation of metal-ceramic casting copings. Standardized preparations were made on 80 extracted third molar teeth. Impressions were made with poly(vinyl siloxane), and stone dies were prepared. Dies were covered with four layers of die spacer, covering the entire preparation together with the occlusal surface excluding the apical 0.5 mm of the preparation in group 1 (40 specimens), and covering the same area excluding the occlusal surface in group 2 (40 specimens). Copings were cast using nickel-chromium-based metal ceramic alloy and cemented using zinc phosphate cement. The specimens were sectioned along the long axis. Internal discrepancies were recorded with a 0.001-mm resolution stereoscope at 6 points: the middle of the occlusal surface (MO), middle of the lingual wall (ML), middle of the buccal wall (MB), middle of the buccal shoulder finish line (MSH), middle of the lingual chamfer finish line (MCH), and middle of the buccal bevel finish line (MBL). Student's t-test was used for statistical analysis. Significance level was set at p < 0.05. The marginal discrepancies of group 1 were higher than those of group 2. Significant differences in discrepancies were found on MO (p < 0.0001), MSH (p = 0.012), and MBL (p = 0.035). The bevel margin showed the least marginal discrepancy following occlusal surface of the die with no relief. Leaving the occlusal part of the die uncovered with the die spacer improved the crown seating considerably in the occlusal surface as well as shoulder and bevel margins.

Evaluation of Subgingival Microflora in All Ceramic Restorations with Subgingival Heavy Chamfer Finish Lines

Microbial colonization in the gingival sulci of abutment teeth receiving all ceramic retainers with subgingival margins need to be studied to assess the prognosis of periodontal health, which determine the eventual success of fixed partial dentures. This prospective observational study was done to evaluate the quantitative alteration in the microbial flora in the gingival sulci of abutment teeth adjacent to the edentulous space prior and after receiving all ceramic retainers over varying time intervals of 1week, 1month and 2months respectively. Twenty, healthy partially edentulous patients, aged 20-50years with single missing central incisor were selected for this prospective observational study and their microbial samples were collected from the gingival sulci of abutments adjacent to edentulous space with sterile paper points and cultured and the estimated values for microflora served as controls. The same abutments were prepared to receive all ceramic retainers with subgingival heavy chamfer marginal finish lines. The patients were recalled after 1week, 1month, 2months intervals during which the collected subgingival microbial samples were cultured and the corresponding quantitative microbial alteration in the restored gingival sulci was recorded. The obtained data was statistically analysed using the student t test and repeated analysis of variance test. The results of the study inferred student t test expressed a statistically significant (p<0.001) progressive increase in gingival sulcular microbial colonisation in the abutment teeth before [M=2.52±SD 1.21(10(6))CFU/ml] and after receiving all ceramic retainers over varying time intervals of 1week [M=3.25±SD 1.21(10(6))CFU/ml], 1month [M=4.64±SD 1.13(10(6))CFU/ml] and 2months [M=4.75±SD 1.16(10(6))CFU/ml] respectively. The result of repeated analysis of variance test inferred that there was a statistically significant difference (p<0.001) in the subgingival microfloral count between the pre operative and post operative samples at 1week, 1month and 2months. Subgingivally placed all-ceramic retainers with heavy chamfer finish lines in the abutment teeth demonstrated a statistically significant increase in sulcular microbial colonization over varying time intervals of 1week, 1month and 2months respectively and this may affect periodontal health of abutment teeth progressively.

A prospective evaluation of zirconia posterior fixed dental prostheses: Three-year clinical results

Although the favorable mechanical properties of zirconium oxide-based ceramics have increased the acceptance of fixed dental prostheses for use in the posterior regions of the mouth in recent years, there are few clinical studies documenting the longevity of these restorations. Furthermore, certain complications must be resolved before the material is used more extensively. The purpose of this randomized prospective study was to evaluate the clinical performance of zirconia (Lava) 3-unit posterior fixed dental prostheses. Twenty 3-unit fixed dental prostheses were placed in 17 participants to replace a second premolar or a first molar. Eleven were placed in the maxilla and 9 in the mandible. All abutment teeth were prepared with a chamfer finish line of 0.8 to 1 mm, and frameworks were prepared with the Lava system. Restorations were cemented with a resin cement. Two calibrated examiners independently evaluated the fixed dental prostheses 1 week (baseline) and 1, 2, and 3 years after placement with the California Dental Association quality evaluation system. The periodontal parameters: the gingival index, plaque index, margin index, and the probing depths of abutment teeth and contralateral teeth were assessed. Data were analyzed by using descriptive statistics and the Wilcoxon signed-rank test (α=.05). All fixed dental prostheses were rated satisfactory after 3 years, and no fracture of the framework was observed during the observation period. One fixed dental prostheses was lost because of a biological complication at the 3-year examination, and a small degree of chipping of the veneering ceramic was observed in 2 participants. No significant differences among the periodontal parameters of the test and control teeth were observed except for the margin index. The results of a 3-year evaluation suggest that posterior zirconia 3-unit fixed dental prostheses are a reliable treatment.

Comparison of the Marginal Adaptation of Zirconium Dioxide Crowns in Preparations with Two Different Finish Lines

The purpose of this study was to evaluate the marginal adaptation of zirconium dioxide crowns in preparations with two different finish line configurations before and after porcelain firing cycles, after a glaze cycle, and after cementation. Twenty human molar teeth were prepared to receive full crowns; ten were prepared with a 90° round shoulder and another ten with a 45° chamfer finish line. Zirconium dioxide copings were fabricated using CAD/CAM technology (Lava™ system). They were then veneered with a low-fusing glass-ceramic (IPS e.max® Ceram). Finally, they were glazed and cemented with a resin-composite cement (RelyX™ Unicem, Aplicap™). Measurements for marginal adaptation using stereomicroscopy (40×) were performed at four stages: copings (S1), after porcelain firing cycles (S2), after glazing (S3), and after cementation (S4). One-way ANOVA was used to assess the influence of the finish line design on the marginal adaptation in each stage. Two-way ANOVA with repeated measurements was performed to assess the influence on the marginal adaptation of the porcelain firing cycles, glaze firing cycle, and cementation. The measured marginal gap mean values for the shoulder group (μm) were: 50.13 (S1), 54.32 (S2), 55.12 (S3), and 59.83 (S4). The values for the chamfer group were: 63.56 (S1), 71.85 (S2), 74.12 (S3), and 76.97 (S4). When comparing marginal gaps between specimens with two different finish lines, differences were noticed at the four studied stages (p = 0.0165, p = 0.0027, p = 0.0009, and p = 0.0009, respectively). No differences were manifested in the marginal gap measurements of the shoulder group at the different stages of fabrication (p = 0.4335); however, in the chamfer group, differences were noticed between S1 and S3 (p = 0.0042). Marginal adaptation was influenced by the finish line design. The firing cycles significantly affected the chamfer group; nevertheless, the marginal gap was within the range of clinical acceptability.

Fatigue and Fracture Resistance of Zirconia Crowns Prepared with Different Finish Line Designs

The aim of this study was to evaluate the effect of finish line design on the fatigue, fracture resistance, and failure type of veneered zirconia restorations. A CAD/CAM system (Cercon) was used to prepare zirconia frameworks (0.5 mm thick) for a maxillary central incisor. Three finish line designs were evaluated: a complete narrow chamfer, a narrow chamfer with a lingual ledge, and a complete ledge. The prepared frameworks were veneered using a press-on ceramic (Ceram Press) and were cemented on the corresponding prepared teeth using a resin cement (Panavia F2.0). The cemented specimens were thermocycled, subjected to dynamic fatigue, and finally loaded till fracture. Fractured specimens were examined under a scanning electron microscope to assess fracture type. One-way ANOVA and Bonferroni post hoc tests were used to analyze the data (α= 0.05). The finish line design did not have any significant statistical influence on the fracture resistance (F = 1.9, p= 0.346) or on the failure type of the tested specimens. Adjusted R squared value (R = 0.049) indicated a weak correlation between finish line design and fracture load of the tested specimens. All specimens failed due to cracking and fracture of the veneer ceramic. Meanwhile, the framework remained entirely intact. Three narrow chamfer finish line specimens demonstrated adhesive fracture of the veneer ceramic during dynamic fatigue testing, related to overextension of the veneer ceramic during the layering procedure. Within the limitations of this study, the finish line design did not influence the fatigue or the fracture resistance of veneered zirconia crowns. Selection of any of the finish line designs should be based on the clinical condition of the restored tooth.

The influence of auxiliary features on the resistance form of short molars prepared for complete cast crowns

Several factors exist which result in crown preparations that are less than ideal. In these situations, the clinician should find a practical way to overcome the lack of resistance of the compromised tooth preparation. The purpose of this study was to evaluate the effect of different auxiliary features on the resistance form of crowns with reduced axial wall height and increased total occlusal convergence. An Ivorine tooth was prepared on a milling machine with 22 degrees of total occlusal convergence (TOC), 3.0 mm of occlusocervical height, and a chamfer finish line (Group Ctrl). The crown preparation was subsequently modified to include proximal grooves (Group Grv), and reduced TOC from 22 to 4 degrees in the cervical 1.5 mm (Group Rdc). Ten standardized metal dies were fabricated for each group. Cobalt-chromium copings were fabricated for all specimens. The metal copings were cemented onto their corresponding metal dies with zinc phosphate cement. The resistance of each specimen was evaluated when force was applied at a 45-degree angle to the long axis of the die with a universal testing machine in a buccal to lingual direction. The maximum force (newtons) was applied before coping dislodgment was measured. Data from the 3 groups were compared with a 1-way ANOVA (α=.05) and a Post Hoc Bonferroni test. The mean (SD) force needed to dislodge crowns with proximal grooves was 156.75 (30.96) N and for those with reduced TOC, it was 221.06 (27.02) N, Both were effective in increasing the resistance form of the original shape (P=.002 and P<.001 respectively). The reduced TOC increased the resistance significantly more than the proximal grooves (P<.001). Within the limitations of this in vitro study, crown preparation modifications enhanced the resistance form; however, the reduction in cervical TOC proved to be more effective than proximal grooves.

Comparative analysis of the clinical techniques used in evaluation of marginal accuracy of cast restoration using stereomicroscopy as gold standard

PURPOSEThis study assessed the effect of preparation design on marginal adaptation and also compared the sensitivity and specificity of clinical evaluation techniques for marginal accuracy of cast restorations to stereomicroscopy.MATERIALS AND METHODSThree Ivorine molar teeth of different designs were prepared. (A)-A complete crown preparation with buccal shoulder and beveled finish line. (B)-A complete crown preparation with chamfer finish line. (C)-A three-quarter crown preparation with proximal boxes and beveled finish line. Twenty four castings were prepared with eight castings for each design respectively. Each casting underwent examination with an explorer, disclosing media, and a stereomicroscope. Stereomicroscopy at a value less than or equal to 30 microns was used as a gold standard to evaluate the significance of different designs on marginal adaptation. Chi-square tests of independence and Kruskal-Wallis were used to evaluate the effect of preparation design and compare the agreement between examination methods for detection of marginal gap size of greater than or equal to 30 microns (α=.05). Sensitivity and specificity for explorer and disclosing media as compared to stereomicroscope was calculated using statistical formula given by Park.RESULTSThe preparation design did not significantly affect overall marginal adaptation. Examination by explorer and disclosing media at 30 µm revealed 39% and 10.06% sensitivity and 91% and 82% specificity respectively.CONCLUSIONPreparation designs examined in this study did not significantly affect the marginal adaptation of the castings. Commonly used clinical evaluation techniques using explorer and disclosing media appeared to be inadequate for assessment of marginal accuracy.

Comparison of chamfer and deep chamfer preparation designs on the fracture resistance of zirconia core restorations.

Background and aims One of the major problems of all-ceramic restorations is their probable fracture under occlusal force. The aim of the present in vitro study was to compare the effect of two marginal designs (chamfer and deep chamfer) on the fracture resistance of all-ceramic restorations, CERCON. Materials and methods This in vitro study was carried out with single-blind experimental technique. One stainless steel die with 50’ chamfer finish line design (0.8 mm deep) was prepared using a milling machine. Ten epoxy resin dies were prepared. The same die was retrieved and 50' chamfer was converted into a deep chamfer design (1 mm). Again ten epoxy resin dies were prepared from the deep chamfer die. Zirconia cores with 0.4 mm thickness and 35 µm cement space were fabricated on the epoxy resin dies (10 chamfer and 10 deep chamfer samples). The zirconia cores were cemented on the epoxy resin dies and underwent a fracture test with a universal testing machine and the samples were investigated from the point of view of the origin of the failure. Results The mean values of fracture resistance for deep chamfer and chamfer samples were 1426.10±182.60 and 991.75±112.00 N, respectively. Student’s t-test revealed statistically significant differences between the groups. Conclusion The results indicated a relationship between the marginal design of zirconia cores and their fracture re-sistance. A deep chamfer margin improved the biomechanical performance of posterior single zirconia crown restorations, which might be attributed to greater thickness and rounded internal angles in deep chamfer margins.

The Chamfer Finish Line: Preclinical Student Performance Using Different Bur Designs

The primary purposes of this investigation were to evaluate sophomore dental student performance in the production of a chamfer finish line using two diamond bur types-a round-ended bur and a torpedo-shaped bur-and to gain student feedback about their preferences for bur type. Fifty students took part in the study, each of whom prepared the buccal surfaces of two mandibular molar typodont teeth, producing chamfer finish lines. Students prepared both teeth in the same laboratory session and were randomly assigned to two groups that were required to prepare the first of the two molars with a specific bur type. The prepared chamfer finish lines were scored and the data analyzed using the Wilcoxon signed-rank test. Student performance was significantly better when the round-ended bur was used (p=0.005). Student feedback was collected with a survey that consisted of four questions and the opportunity to provide write-in comments. In response to the question "Overall, was one bur type better?" 58 percent of the students preferred the round-ended bur for creating a chamfer finish line. The most frequent write-in comment, made by twelve of the fifty students, criticized the torpedo-shaped bur for creating finish lines that were too shallow or too long.

Optimizing preparation design for metal-free composite resin crowns

Although composite resin materials are used for posterior crown restorations, the influence of preparation design, material thickness, convergence angle, and method of cementation on fracture resistance remains unclear. The purpose of this in vitro study was to test the hypothesis that minimal preparation designs provide an acceptable level of fracture resistance for posterior composite resin crowns. Nonreinforced Artglass composite resin crowns (n=128) were fabricated on human molars in 16 test groups (n=8). Axial tooth preparation included a 1-mm-deep shoulder or a 0.5-mm chamfer preparation, whereas occlusal reduction was either 0.5 mm or 1.3 mm. The total angle of convergence was 4 or 11 degrees and the crowns were cemented either with glass ionomer cement (Ketac Cem) or resin cement (2bond2). After 10,000 thermal cycles, crowns were vertically loaded until failure occurred; load was measured in newtons. Statistical analysis was performed by a 4-way ANOVA (alpha=.05). Statistical analysis revealed significant effects of occlusal thickness (P<.001), cement (P<.001), preparation design (P=.011), and convergence angle (P=.001) on the fracture resistance of the composite crowns. For composite resin crowns with an occlusal thickness of 0.5 mm, the resistance to fracture was lower than for crowns with a 1.3-mm thickness. Fracture resistance was greater when resin cement was used, and greater for the chamfer finish line than for the shoulder finish line. Use of a greater total convergence angle reduced fracture resistance. Fracture resistance of composite resin crowns was significantly improved by increasing the occlusal thickness of the crowns, by using resin cement, and by reducing the total convergence angle.

The effect of finish line preparation and layer thickness on the failure load and fractography of ZrO 2 copings

To prevent tooth weakening or pulp irritation, there is a need for a minimally invasive method of preparing single anterior crowns. Restoration dimensions for reduced coping thicknesses or less invasive finish line preparations are required. The purposed of this in vitro was to study investigate the fracture performance of high-strength zirconia copings, compare knife-edge margins with chamfer finish lines, and examine the effect of reducing the layer thickness from 0.5 mm to 0.3 mm. Y-TZP zirconia copings were manufactured on brass dies of a maxillary central incisor. Forty copings, with 2 layer thicknesses (0.5 and 0.3 mm), and 2 finish line preparations (knife edge and chamfer; n=10) were cemented using a conventional glass ionomer cement and stored in distilled water at 37 degrees C for 24 hours. The copings were vertically loaded until fracture using a universal testing machine. Data were analyzed by 2-way ANOVA (alpha=.05). Fractographic examination was performed using scanning electron microscopy and confocal laser scanning microscopy. A significantly higher mean failure load was measured for knife-edge (0.5 mm, 1110 +/-175 N; 0.3 mm, 730 +/-160 N) versus chamfer (0.5 mm, 697 +/-126 N; 0.3 mm, 455 +/-79 N) preparations (P<.001), and for 0.5-mm versus 0.3-mm thickness layers (P<.001). Knife-edge preparations present a promising alternative to chamfer finish lines; the fracture load required for knife-edge preparations was 38% greater than that required for chamfer preparations, regardless of coping thickness. Reducing the thickness of a single crown coping from 0.5 to 0.3 mm resulted in a 35% reduction in fracture load required for either preparation type.

Effect of the Crown Preparation Margin and Die Type on the Marginal Accuracy of Fiber-reinforced Composite Crowns

The objective of this laboratory investigation was to determine the effect of different preparation designs (light chamfer, deep chamfer, and shoulder) and die-making materials (stone and epoxy) on the resulting margin misfit for fiber-reinforced composite crowns using a measuring microscope. Sixty standardized FibreKor crowns were made on stone and epoxy resin dies (n=30 each) duplicated from three metal master dies representing complete crown tooth preparation with a total convergence of 5 degrees . For each die group, three of the tooth preparation designs were established in relation to the type of finish line (n=0 each) as follows: Group A (0.5-mm light chamfer finish line); Group B (1.0-mm deep chamfer finish line); and Group C (1.0-mm shoulder finish line). Marginal accuracy was evaluated by measuring the distances between each of four pairs of indentations on the crowns and on the dies with a Nikon measuring microscope. Analysis of seating measurements with parametric analysis of variance and Tukey's Studentized Range (HSD) disclosed a statistically significant difference for both tooth preparation design and die material (p< 0.001). However, the interaction effect was not significant (p=0.9073). The least marginal opening value was for FibreKor crowns made on epoxy resin dies with a light chamfer finish line (57 microm), but the difference was not statistically significantly different from crowns made on epoxy resin dies with a deep chamfer light chamfer finish line (61 microm). However, crowns made on epoxy resin dies with a shoulder finish line (81 microm) had significantly higher values (p< 0.05). FibreKor crowns made on stone dies with the shoulder finish line (95 microm) had statistically higher marginal opening values (p< 0.05). The least marginal opening value was for crowns made on stone dies with a light chamfer finish line (66 microm), but the difference was not statistically significantly different from crowns made on stone dies with a deep chamfer light chamfer finish line (70 microm). Significant differences were found among the die material used for the shoulder margin design. However, there was no significant difference between light chamfer and deep chamfer margin designs for both die materials.