Effect of enamel preparations on fracture resistance of composite resin buildup of fractures involving dentine in anterior bovine teeth: an in vitro study

Sixty bovine teeth with simulated mesio-incisal angle fracture were randomly and equally divided in one untreated (control) group and three experimental groups (Bevel, Chamfer and newly introduced Stair-step Chamfer preparation group) to evaluate the effect of enamel preparations on the fracture resistance of composite resin. Post restoration, fracture mechanics approach was used to quantify the failure of composite resins in testing the samples in Instron testing machine. Mean peak failure load (Newton) of composite amongst experimental groups was observed in the order; Chamfer (326.09 +/- 72.73), Stair-step chamfer (315.21 +/- 81.77) and Bevel (253.83 +/- 67.38). Results of the One-Way ANOVA revealed significant difference in the mean peak failure load values of the four different groups. (P<0.001) Scheffe's Post-Hoc comparison test (Subset for alpha = 0.05) revealed that there was no significant difference in the mean peak failure load values of the bevel, stair-step chamfer and chamfer preparation when considered together, but the mean peak values of control group (605.22 +/- 48.96) were observed significantly higher. Failure mode evaluation revealed, majority of failures occurred as cohesive and mixed type for all the experimental groups. Adhesive type failure was observed maximum (33%) in the bevel group. Stair-step chamfer preparation showed greatest potential for application and use as it no only demonstrated comparable values to Chamfer preparation ['t' value (0.39) (P > 0.05)] but also involved sacrificing less amount of tooth structure adjacent to fractured edge.     

Marginal Adaptation of Cerec 3 CAD/CAM Composite Crowns Using Two Different Finish Line Preparation Designs

PURPOSE: The purpose of this study was to compare marginal discrepancies of Cerec 3 CAD/CAM composite crowns, fabricated on human prepared teeth with two different finish line designs, chamfer and shoulder. MATERIALS AND METHODS: Sixteen human molar teeth were used to prepare full crowns. Eight teeth were prepared with a 1-mm-wide chamfer finish line and the other eight with a 1.2- to 1.5-mm circumferential shoulder. Cerec 3 crowns were fabricated from optical impressions using Paradigm MZ100 composite polymer. Marginal adaptation was evaluated in two ways: (1) using modified United States Public Health Service (USPHS) criteria to evaluate eight preselected sites on each crown margin, and (2) using scanning electron microscopy (SEM) to measure marginal gaps on all four axial walls with 15 measurements on each wall (60 measurements per crown). An evaluation of the number of acceptable crowns, determined by having all measured sites per tooth with margin gap size less than 100 microm, as a function of finish line design was also conducted. RESULTS: In both chamfer and shoulder groups, there were only two crowns (out of eight) with clinically acceptable ratings for all eight measurement sites according to USPHS criteria. Fisher's chi-square analysis showed that there was no statistically significant difference in marginal adaptability as a function of finish line design ( p>0.05). With SEM imaging, overall mean marginal gaps for the chamfer group were 65.9+/-38.7 microm (range 35.0 to 130.0 microm), and for the shoulder group were 46.0+/-9.2 microm (range 26.3 to 55.6 microm); this difference was not found to be statistically significant ( p>0.05). While crown assessment based on mean marginal discrepancy measurements indicated that both the chamfer and shoulder groups were considered clinically acceptable (<100 microm); crown acceptability based on all measurement sites being less than 100 mum indicated that in the chamfer and shoulder groups there were four and three acceptable crowns out of eight, respectively. The Fisher's chi-square test indicated no statistically significant difference between the groups ( p>0.05). An agreement rate of 81.2% was calculated between the two evaluation methods, modified USPHS criteria and SEM measurements. CONCLUSIONS: Based on mean marginal discrepancy measurements, the typical marginal assessment technique, Cerec 3 Paradigm MZ100 crown restorations appear to have acceptable marginal adaptability (mean discrepancies <100 microm). Thus, the evidence from this investigation would suggest that the finish line preparation design had no effect on marginal adaptation for Cerec 3 composite crowns.     

Effect of finish line variants on marginal accuracy and fracture strength of ceramic optimized polymer/fiber-reinforced composite crowns

STATEMENT OF PROBLEM: Ceramic optimized polymer (Ceromer)/fiber-reinforced composite (FRC) crowns have been promoted as alternatives to conventional crowns. However, little is known regarding the ideal tooth preparation for this type of crown. PURPOSE: This in vitro study evaluated the marginal adaptation and fracture strength of ceromer/FRC crowns with respect to the various types of finish lines. MATERIAL AND METHODS: Four metal dies with different finish lines (0.9-mm chamfer, 1.2-mm chamfer, 1.2-mm rounded shoulder, and 1.2-mm shoulder) were prepared. Forty (10 for each finish line) Targis/Vectris crowns were fabricated on duplicated base metal alloy dies. The restorations were stereoscopically evaluated at 56 points along the entire circumferential margin for measuring the margin adaptation before and after cementation with a resin luting agent. The specimens were then compressively loaded to failure using a universal testing machine. The marginal adaptation (microm) was analyzed with the Kruskal-Wallis test and post-hoc Dunnett test (alpha=.05). The fracture load (N) was analyzed with a 1-way analysis of the variance and the Scheffe adjustment (alpha=.05). The fractured surfaces of the crowns were examined with a scanning electron microscope to determine the mode of fracture. RESULTS: The marginal adaptation of crowns with a shoulder finish line was significantly better than crowns with a chamfer finish line before and after cementation (P<.001). The increased marginal gap after cementation was the lowest in the 1.2-mm rounded shoulder group. The fracture strength of the crowns with the 0.9-mm chamfer and crowns with 1.2-mm chamfer was significantly greater than those of the crowns with the 1.2-mm shoulder or rounded shoulder (P=.011, P=.049, respectively). The mean fracture load of all crowns, regardless of the finish line design, was 1646 N. The fractured surface of the crown revealed adhesive failure and 3 types of cohesive failure (fracture of the Targis and Vectris, Targis fracture with a crack in the Vectris layer, and crushing without fracture). CONCLUSION: The marginal gaps were greater for the chamfer finish line specimens than in the shoulder finish line specimens. However, the fracture strength of the chamfer finish line specimens was greater than that of the shoulder finish line specimens.     

Clinical evaluation of marginal fit of a new experimental all-ceramic system before and after cementation

PURPOSE: The purpose of this in vivo study was to evaluate the marginal fit of inlay and crown abutments for fixed partial dentures (FPD) made from a new all-ceramic material. MATERIALS AND METHODS: Sixteen patients were selected for the study. After abutment preparation, impressions were made and master dies were fabricated. FPDs were made with the experimental heat-pressed lithium-disilicate glass-ceramic following the manufacturer's recommendations. At random, one abutment of each FPD was chosen for evaluation (11 crown and 8 inlay abutments). Impressions of the restoration margins were taken before and after adhesive cementation. Replicas of the abutment margins were investigated by SEM. The marginal discrepancies were evaluated in sections of 200 microm. The highest value of each section was applied, and the mean of all sections of each replica was used for statistical analysis. RESULTS: The median of the marginal discrepancies before cementation was 96 microm for crowns and 89 microm for inlays. After adhesive cementation, the median increased for crowns to 130 microm and for inlays to 92 microm, a significant increase in the marginal discrepancies caused by cementation for crowns, but not for inlays. There were no significant differences between the discrepancies of crowns and inlays before or after cementation. CONCLUSION: The maximum marginal discrepancies found in this study were between 50 and 265 microm before cementation and between 70 and 285 microm after cementation, with no significant differences between the means for crowns and inlays. Adhesive cementation caused a significant increase of the marginal discrepancies for crown, but not inlay, abutments.     

Fracture load and marginal fit of shrinkage-free ZrSiO4 all-ceramic crowns after chewing simulation

The aim of this in vitro study was to evaluate the fracture load and marginal accuracy of crowns made from a shrinkage-free ZrSiO4 ceramic cemented with glass-ionomer or composite cement after chewing simulation. Thirty-two human mandibular molars were randomly divided into two groups. All teeth were prepared for and restored with shrinkage-free ZrSiO4 ceramic crowns (Everest HPC, KaVo). The crowns of group A (N = 16) were luted to the teeth using KetacCem and group B (N = 16) were adhesively cemented using Panavia21EX. Measurements of the marginal accuracy before and after cementation were made using replicas and an image analysis system. All specimens were exposed to 1.2 million cycles of thermo-mechanical fatigue in a chewing simulator. Surviving specimens were subsequently loaded until fracture in a static testing device. Fracture loads (N) were recorded. All specimens survived chewing simulation. The mean fracture loads (+/-s.d.) were Group A, 1622 N (+/-433); group B, 1957 N (+/-806). There was no significant difference between the two groups (P > 0.05). The marginal gap values before cementation were (mean +/- s.d.): Group A, 32.7 microm (+/-6.8); group B, 33.0 microm (+/-6.7).The mean marginal gap values after cementation were (+/-s.d.): Group A, 44.6 microm (+/-6.7); group B, 46.6 microm (+/-7.7). The marginal openings were significantly higher after cementation for both groups (P < 0.05). All test groups demonstrated fracture load and marginal accuracy values within the range of clinical acceptability.     

The effect of oscillation,low seating force and dentine surface treatment on pulpward pressure transmission during crown cementation: a laboratory study

The aim of the present research was to evaluate intra-pulpal pressure with respect to a combination of factors that are known to reduce pulpward pressure transmission in the laboratory while maintaining acceptable seating of the crowns. Thirty premolars were prepared for full crowns and individual crowns were cast. Ten teeth were etched prior to crown cementation with 100 N seating force. Crowns of groups 2 and 3 were cemented with 10 N force, after dentine sealing with bonding agent. Group two received, in addition, oscillating seating force during cementation. Pulp chamber pressure during cementation and marginal seating discrepancy after cementation were recorded. Group 1 showed significantly higher pressure mean 600 Pa and better crown seating (mean -22 microm) compared with group two (mean 144 Pa and 24 microm) and group 3 (mean 102 Pa and 44 microm) (P=0.001). Low seating forces combined with die spacer and sealing of dentinal tubules resulted in significant reduction of pressure transmitted into the pulp chamber during crown cementation. It is suggested that pre-impression sealing of dentine be considered for crown preparations on vital teeth.     

激光熔覆技术烤瓷冠边缘适合性的实验研究

目的通过对传统失蜡铸造和激光熔覆两种技术制作的钴铬合金烤瓷冠以及浅凹型和直角肩台型两种牙体预备形态制作的激光熔覆技术烤瓷冠的边缘适合性进行比较,以探讨激光熔覆技术烤瓷冠的边缘适合性。方法选取上颌前磨牙30颗,随机分成3组,每组10颗,按烤瓷冠要求常规牙体预备。A、B组用浅凹型边缘,C组用直角肩台。A组为传统失蜡铸造钴铬合金烤瓷冠,B、C组为BEGO德贝尔激光熔覆技术钴铬合金烤瓷冠;3组全用聚羧酸锌水门汀粘结剂粘固,经环氧树脂包埋后,沿颊腭向中间剖开烤瓷冠和牙体,用金相显微镜及电子显微镜观察和测量各组的颊、腭侧冠边缘间隙的大小。结果 A组烤瓷冠的水平边缘间隙为79.61±11.45μm,垂直边缘间隙94.90±7.09μm,绝对边缘间隙114.94±8.60μm;B组激光熔覆技术钴铬合金烤瓷冠的水平边缘间隙67.76±9.20μm,垂直边缘间隙82.54±9.92μm,绝对边缘间隙105.35±9.84μm;C组激光熔覆技术钴铬合金烤瓷冠的水平边缘间隙72.72±9.03μm,垂直边缘间隙88.11±9.53μm,绝对边缘间隙102.48±9.15μm。B组与A组比较差异有统计学意义（P〈0.05）;B、C组之间的差异无统计学意义（P〉0.05）。结论激光熔覆技术钴铬合金烤瓷冠边缘水平边缘间隙、垂直边缘间隙、绝对边缘间隙均小于120μm,均达到临床可接受的范围内。激光熔覆技术钴铬合金烤瓷冠比传统失蜡铸造钴铬合金烤瓷冠边缘适合性更好。牙体预备采用浅凹型（B组）和直角肩台型（C组）制作的激光熔覆技术钴铬合金烤瓷冠边缘适合性无明显差异。     

Marginal adaptation of cast partial crowns made of pure titanium and a gold alloy under influence of a manual burnishing technique

The aim of this study was to determine the marginal adaptation of partial crowns from pure titanium and a gold alloy after two different cementation techniques. Forty freshly extracted human molars were prepared and randomly divided in four groups. Two groups were restored with partial crowns using the gold alloy Degulor M*. In one group, the crowns were fixed on the tooth by using a zinc phosphate cement. In the other group the margins were additionally burnished by using a hand burnisher No. 660. In the other two groups, partial crowns from pure titanium were cemented in the same way. The marginal quality was determined by quantitative margin analysis in the SEM using a replica technique. Partial crowns from a gold alloy showed significantly (P < 0.05) more margin quality A (vertical marginal discrepancy < 50 microm) while partial crowns from pure titanium had significantly (P < 0.05) more margin quality B (vertical marginal discrepancy 50-100 microm) and over-extended margins (quality D). No significant (P < 0.05) difference was found between the conventional cementation technique and the technique with manual burnishing in both material groups.     

Marginal fit of leucite-glass pressable ceramic restorations and ceramic-pressed-to-metal restorations

STATEMENT OF PROBLEM: Fabricating a feldspathic porcelain margin on a metal-ceramic restoration with a clinically acceptable marginal fit has proven to be a technique-sensitive procedure. Pressable ceramics are advocated to solve this problem. PURPOSE: The purpose of this in vitro study was to compare the marginal adaptation of a pressable ceramic system when used with both all-ceramic and metal-ceramic crowns, with a traditional metal-ceramic restoration. MATERIAL AND METHODS: A 1.5-mm, 360-degree chamfer margin was prepared on a typodont maxillary central incisor. Polyether impressions were made and poured in a Type IV dental stone, and the following crowns were fabricated on individual dies: 15 metal ceramic restorations (MCR) (Ceramco II, Ceramco, and Argelite 60), 15 pressed-to-metal restorations (PTM) (CPC-MK, and Argelite 60), and 15 pressed ceramic restorations (PCR) (CPC-MK). The marginal fit of the crowns was evaluated every 90 degrees around the crown margin circumference under a microscope at original magnification x 45. A 1-way analysis of variance (ANOVA) was used to compare data (alpha=.05). RESULTS: The mean marginal discrepancy for MCRs was 94 +/- 41 microm, for PTMs, 88 +/- 29 microm, and for PCRs, 81 +/- 25 microm. The 1-way ANOVA showed no significant difference between groups ( P =.568). CONCLUSION: The marginal fit of pressed-to-metal (PTMs) and pressed all-ceramic crowns (PCRs) was similar to that of traditional metal-ceramic crowns (MCRs).     

Effect of seating force,margin design,and cement on marginal seal and retention of complete metal crowns

PURPOSE: The purpose of this study was to determine the marginal discrepancy and retention of silver-palladium crowns cemented with zinc phosphate (Phosphacap) and glass-ionomer cement (Fuji Cap 1) using different seating forces on preparations with various margin designs. MATERIALS AND METHODS: Crown preparations with three finish lines--chamfer, shoulder, and shoulder with a 45-degree bevel--were sequentially prepared on a dentoform premolar. A metal die for each of the three finish lines was constructed. Complete metal crowns were fabricated for each metal die using a silver-palladium alloy. Three different seating forces--25, 100, and 300 N--were used to load the crowns until initial set of the cement. The marginal discrepancy was calculated by measuring the change in crown height before and after cementation using a digimatic indicator. Retention was determined by measuring the tensile strength using the Lloyd universal testing machine. RESULTS: The higher seating forces produced better crown seating but had no significant effect on crown retention. The shoulder and shoulder with bevel finish lines provided better crown retention than the chamfer. Glass-ionomer cement provided greater crown retention than zinc phosphate cement. No significant correlation between marginal seating and crown retention was revealed using a Pearson analysis. CONCLUSION: Marginal seal was not influenced by either margin design or type of luting cement, but was improved with higher seating force. Crown retention was affected by the margin finish line and the luting agent.     

Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth

The in vitro marginal fit of five different all-ceramic crown systems (In-Ceram, Empress staining technique, Empress veneering technique, Celay feldspathic system, Celay In-Ceram system) was evaluated before and after cyclic preloading in an artificial mouth. The crowns were adhesively luted to extracted natural maxillary incisors prepared with a 90 degrees shoulder. The results were compared to those for porcelain-fused-to-metal (PFM) crowns with circular porcelain-butt margins which were cemented with zinc phosphate cement. The analysis of the marginal discrepancies showed significant (P < 0.001) differences among the groups. Crown cementation increased the marginal gaps significantly (P < 0.01). Empress staining technique crowns showed the smallest marginal gaps (median 47 microm), followed by conventional In-Ceram crowns (median 60 microm) and Empress veneer technique crowns (median 62 microm). Celay In-Ceram crowns displayed marginal openings with a median of 78 microm, followed by Celay feldspathic crowns with a median of 99 microm. The marginal gap of the PFM control crowns showed a median of 64 microm. Ageing in the chewing simulator had no significant influence on the marginal fit of all specimens. The study indicates that all the tested all-ceramic crowns have clinically acceptable margins. However, in vivo investigations of all all-ceramic crown systems should be made before clinical routine.     

The effect of preparation design on the marginal stress of resin-bonded metal-free crowns: a finite element study

The aim of this study was to evaluate the stress distribution at the marginal edge of metal-free crowns using 2 different materials (ceramic and hybrid composite) and 4 different preparation designs (knife edge, chamfer, heavy chamfer, rounded shoulder). All marginal preparation designs for hybrid composite crowns showed lower compression and tensile stress than the ceramic crowns. The rounded shoulder preparation provided more even stress distribution than the knife-edge preparation. Heavy chamfer was an acceptable preparation design for ceramic crowns. The chamfer preparation was preferred for hybrid composite crown.     

桩核材料与纤维/树脂复合材料桩钉联合修复无髓牙的抗折性能研究

目的评价3种树脂类桩核材料与高强玻璃纤维/树脂复合材料(FRC)桩钉联合修复无髓牙的抗折性能。方法用FRC桩钉和3种经不同方法处理(热压和未热压)的树脂核(Artglass聚合体、Charisma复合树脂、AB组份复合树脂)联合修复离体无髓牙根,测试1300加载时的破坏载荷。结果Artglass聚合体组的抗折力最高(505.4N),经热压处理后强度明显提高(564.1N) ; Chari~复合树脂组经热压后脆性增加,强度降低(热压前后分别为411.3N和 315.3N); AB组份树脂组经热压后强度明显提高(热压前后分别为358.4N和423.4N)。实验中无1例出现桩钉折断和聚酷冠碎裂。结论不同桩核材料与FRC桩钉联合修复无髓牙的抗折力不同,其中Art乡ass聚合体组的抗折力最高,但是此3组的抗折力均超过了咀嚼过程中前牙的咬合力,树脂核与纤维/树脂复合材料桩钉联合修复无髓前牙残根可以满足临床需要。     

Effect of preparation convergence on retention and seating discrepancy of complete veneer crowns

The purpose of this study was to evaluate the retention and marginal seating discrepancy of complete veneer crown cemented with zinc phosphate using various preparation convergence designs. The method employed was that of cementing cast metal crowns onto 46 metal dies fabricated as complete veneer crown preparations with various convergence angles ranging from 0 degrees to 70 degrees and a shoulder finish line. The marginal discrepancy was calculated by measuring the change in crown height before and after cementation using an optical measuring microscope. The force required to remove the crowns from the dies in a vertical direction was determined using an Instron universal testing machine and recorded as retention. The increase in preparation convergence exhibited a wide variation of seating discrepancy between specimens, ranging from 4.58 +/- 1.13 to 73.13 +/- 78.32 microm. Significant tilting of crown was observed as the convergence angle of preparation increased. The retention values ranged from 4.03 +/- 0.61 to 12.12 +/- 0.33 MPa. The Pearson analysis revealed statistically significant correlations between preparation convergence and marginal discrepancy (r = +0.62), and retention (r = -0.91). Complete veneer crown retention and marginal discrepancy were influenced by the preparation convergence design.     

In vivo study of the marginal integrity of composite resin buildups after full crown preparation

PURPOSE: The purpose of this in vivo study was to evaluate the marginal integrity of composite resin buildups after full crown preparation, and to identify possible changes during the provisional phase of the manufacturing process of the final restorations. MATERIALS AND METHODS: Two test groups were determined for the use of an autocuring (Clearfil New Bond, Kuraray) and a light-curing (Optibond FL, Kerr) dentin adhesive. After rubber-dam application, both dentin adhesives were used with the total-etch technique. Then, an autocuring composite resin was applied for the core buildup. Impressions of the built-up teeth were taken directly after preparation and before cementation of the final restorations. During the provisional phase, which lasted from 7 to 28 days, the prepared teeth were restored with cemented provisional crowns. Replicas of the built-up teeth were manufactured and examined with a scanning electron microscope. RESULTS: For the autocuring dentin adhesive, only one buildup showed a gap 10 microm wide and 100 microm long. For the light-curing dentin adhesive, two samples revealed gaps that were 10 microm wide and had lengths of 100 microm and 75 microm, respectively. No change could be perceived when comparing the state of the composite/tooth interface after preparation and before final cementation. CONCLUSION: The results show that when using an autocuring composite resin in combination with dentin adhesives, nearly gap-free margins of composite buildups can be achieved and therefore might serve as a sound basis for the final restoration.     

Predictions of cement microfracture under crowns using 3D-FEA

PURPOSE: The objective of this research study was to test the effects of (1) crown margin type, (2) cement type, (3) cement thickness, (4) loading direction, and (5) loading magnitude on stress levels and distributions within luting cement that might lead to cement microfracture using three-dimensional Finite Element Analysis techniques. MATERIALS AND METHODS: Thirty-two three-dimensional computer models, as well as models for standards, were generated for a mandibular first premolar. Crown preparations exhibited shoulder or chamfer margin configurations, and zinc phosphate, zinc polycarboxylate, glass ionomer, and resin cements were used in thicknesses of 25 or 100 microm. Modeled crowns were loaded axially or obliquely at 10 and 100 MPa. Areas and levels of stress concentrations within the cement were determined. RESULTS: Stresses in the cement were low for all situations except 100 MPa oblique stressing. Stresses at the margins of crowns with chamfer marginal configuration were higher than those with shoulder margins. Stresses under oblique stressing were 10 to 150 times higher than under axial stressing. Except for Zn phosphate cement, cement thickness minimally affected stress levels and distributions. Greater stresses were found in cements with the greater Young's modulus. CONCLUSIONS: Although the chamfer margin design could lead to greater stresses near the margins that places the cement at risk for microfracture and possible crown failure, glass-ionomer and composite resin cements have more favorable mechanical properties for resisting microfracture.     

Effect of cement selection and finishing technique on marginal opening of cast gold inlays

STATEMENT OF PROBLEM: The standard cement for partial-veneer cast gold restorations has been zinc phosphate. With increasing interest in using resin and resin-modified glass ionomer cements for this purpose, marginal adaptation with use of these newer cements must be assessed. PURPOSE: The purpose of this in vitro study was to determine if the marginal adaptation of cast gold inlays is comparable for 3 different luting agents and 3 finishing techniques. MATERIAL AND METHODS: Cast gold inlays were cemented into triangular preparations on the buccal surface of 45 extracted human molars. Three operators each used a different technique for finishing margins: finishing during cementation (FD), finishing before cementation (FB), and finishing before and during cementation (FBA). The 15 teeth for each operator were divided into 3 subgroups of 5 each to accommodate 3 cements: zinc phosphate (Fleck's zinc phosphate: ZP), resin-modified glass ionomer (RelyX Luting: RXL), and self-adhesive modified resin (RelyX Unicem: RXU). For all specimens, fine sandpaper disks followed by fine cuttle disks were used for finishing the castings. The marginal gap was measured using a Nikon measuring microscope at x50 magnification. Data were analyzed with a 1-way ANOVA for each cement (alpha=.05). RESULTS: There were significant differences among finishing techniques for each cement (P<.05). For ZP, the smallest mean marginal gaps were for FD (31 microm) and FBA (42 microm). For RXL, FBA produced the smallest gap (19 microm). For RXU, FB (23 microm) and FBA (22 microm) were optimal. CONCLUSIONS: Within the limitations of this in vitro study, the marginal gap attained with cementation of partial-veneer cast gold restorations with ZP (31 microm) can also be obtained using RXL or RXU. The techniques of FD or FBA are best used for ZP, whereas the FBA technique is best for RXL, and either FB or FBA is optimal for RXU. The FBA technique yielded a small gap; this was common to all 3 cements.     

Using implant connectors to support clinical abrasion probes: a methodological study.

OBJECTIVES: To validate a procedure aimed at assessing the clinical wear rate of full crown veneering materials. To confirm its clinical applicability and alignment capacity in the ancillary profiling apparatus. To quantify the wear rate of a full crown veneering composite (Sinfony, 3M-ESPE) as compared to that of a ceramic (negative control) and an unfilled resin (positive control). METHODS: The technique consisted in utilizing implant supported crowns which were either screw-fastened to endosseous implants via ITI-Straumann Octa connectors or to an Octa analog on an x-y measuring table. After initial contour profiling, 14 crowns were placed in the mouths of seven patients. Six crowns were veneered with composite, four with ceramic and four with an unfilled resin. The crowns were removed at 3, 6, 9, 12, 15, 18 and 24 months and digitalized contour profiles were obtained. Contour profiling consisted in securing the restorations to the x-y table and assessing the z-coordinate using an LVDT stylus. At the end of the experimental period, loss of material was computed as the differences between the initial- and subsequent measurements. RESULTS: The total error of the method was estimated at +/-10 microm. The composite's annual wear rate was 75 microm/year, the ceramic wore at ca. 6 microm/year and the unfilled resin wore at 160 microm/year. Wear was unevenly distributed and concentrated on occlusal contact areas. SIGNIFICANCE: (1) A technique utilizing ITI-Straumann octagonal implant connectors is suitable for clinical use. (2) The veneering composite wears at a rate that is superior to ADA guidelines for composites intended for direct fillings.     

Determination of the minimum number of marginal gap measurements required for practical in-vitro testing

STATEMENT OF PROBLEM: Gap measurements along margins are frequently used to assess the quality of single crowns. However, the number of gap measurements required for clinically relevant results in laboratory studies is not known. PURPOSE: This study estimated the minimum number of gap measurements on margins of single crowns to produce relevant results for gap analysis. METHODS AND MATERIAL: Ten all-ceramic crowns were fabricated on a master steel die. Gaps along crown margins were investigated in a scanning electron microscope on the master steel die without cementation and on replica dies after conventional cementation. Measurements were made in 100 microm steps according to 3 gap definitions. The initial number of measurements per crown (n = 230) was reduced to smaller subsets using both systematic and random approaches to determine the impact on the quality of results. RESULTS: On the data of gap definition 1, reduction from 230 to about 50 measurements caused less than +/-5 microm variability for arithmetic means. Analysis of standard errors showed slowly increasing values smaller than 3 microm, both indicating no relevant impact on the quality of results. Smaller data sizes yielded accelerated increase of standard errors and divergent variabilities of mean. The minimum of 50 measurements did not depend on gap definition or on cementation condition. CONCLUSION: Fifty measurements are required for clinically relevant information about gap size regardless of whether the measurement sites are selected in a systematic or random manner, which is far more than what current in vitro studies use.     

Influence of cavity design on the biomechanics of direct composite resin restorations in Class IV preparations

This study evaluated the effect of cavity design on stress distribution and fracture resistance of direct composite resin restorations in Class IV preparations. A finite element analysis (FEA) model of the maxillary central incisor with a Class IV cavity was established. Five model variations were studied: (i) a 1-mm bevel (ii) a 2-mm bevel, (iii) a plain chamfer, (iv) a stair-step chamfer, and (v) butt joints (a control configuration). All FEA variations modeled a tooth restored with composite resin loaded under 100 N at an angle of 45° to the longitudinal axis. The interfacial von Mises stress was evaluated. The FEA was complemented with an in vitro assessment. Fracture resistance of direct composite resin restorations was tested with a universal testing machine and fracture patterns were observed. Finite element analysis showed that stress in chamfer and stair-step chamfer models was more homogenously distributed, while stress in bevel models was relatively concentrated at lingual regions. Fracture resistance of a 1-mm bevel preparation was lower than for the 2-mm bevel, plain chamfer, and stair-step chamfer preparations, but was higher than for butt joints. The stair-step chamfer group presented the most favorable failure pattern. Considering biomechanics and esthetics, the present study indicates that the stair-step chamfer and 2-mm bevel should be recommended for clinical restoration.