Failure behavior of zirconia crowns subjected to air abrasion with different particle sizes.

This study aimed to investigate the failure behavior of 3 mol.% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) prosthetic crowns air-abraded with aluminum oxide (AO) particles of different sizes. Ninety ceramic premolar crowns were produced with 3Y-TZP frameworks veneered with porcelain. Crowns were randomly divided into three groups, according to the size of the air abrasion AO particles (n = 30): (GC) untreated (control); (G53) 53 µm; (G125) 125 µm. Air abrasion was performed with 0.25 mpa pressure, 10-mm distance, for 10 s. Crowns were adhesively cemented to dentin analog abutments. Specimens were loaded in compression to failure, in 37oC distilled water, using a universal testing machine (n = 30). Fractographic analysis was performed using a stereomicroscope and SEM. The roughness of the crown's inner surface was evaluated using an optical profilometer (n = 10). Fracture load data were statistically analyzed with Weibull analysis and roughness data with Kruskal-Wallis (α = 0.05). GC had the lowest characteristic fracture load (L0), while G53 and G125 had higher and statistically similar L0 values. The Weibull modulus (m) was similar among groups. The failure modes observed were catastrophic failure and porcelain chipping. There were no differences between the roughness parameters for the experimental groups (p > 0.05). The size of the AO particles did not affect the fracture load and failure mode of 3Y-TZP crowns. Air abrasion with 53 µm and 125 µm particles resulted in a higher fracture load of ceramic crowns than the untreated group while maintaining their reliability and surface characteristics.

Comparison of marginal accuracy in two different materials used in provisional crown and bridge - an in vitro experimental study.

OBJECTIVE: To determine the difference in the marginal accuracy at buccal, lingual, mesial and distal margins of temporary crowns fabricated with bisacryl-based temporary crown material. METHODS: The in-vitro, experimental, laboratory-based study was conducted at the Aga Khan University, Karachi, from September to December 2019, and comprised two bisacryl-based temporary crown material, Integrity and Protemp 4, which were used to fabricate a sample of 24 temporary crowns. A pre-operative polyvinyl siloxane impression served as a template for temporary crown fabrication. A right mandibular molar tooth on a typodont was prepared to receive a crown. The provisional crown material was syringed onto the template and was allowed to cure. All four surfaces of the crown were observed under a stereomicroscope equipped with digital single-lens reflex camera at 25.6x magnification. An image of each surface was captured and a photographic record was maintained. An image processing software was used for the measurement of marginal discrepancy. Marginal accuracy among the four surfaces was assessed. Data was analysed using SPSS 23. RESULTS: Mean marginal discrepancy for provisional crowns fabricated with Protemp 4 and Integrity was 410±222µm and 319±176µm, respectively. The marginal discrepancy between the two groups was statistically significant (p=0.027), with buccal margin exhibiting the most discrepancy (p<0.01). CONCLUSIONS: Integrity showed less microleakage than Protemp 4. Among all the walls, the buccal wall showed the most microleakage. Marginal accuracy was found to be dependent upon the type of provisional crown material and the side of the prepared axial wall.

How does the occlusal contact region influence the mechanical fatigue performance and fracture region of monolithic lithium disilicate ceramic crowns?

OBJECTIVE: To characterize the effect of the occlusal contact region on the mechanical fatigue performance and on the fracture region of monolithic lithium disilicate ceramic crowns. MATERIALS AND METHODS: Monolithic lithium disilicate ceramic crowns were machined in a CAD/CAM system and adhesively luted onto glass-fiber reinforced epoxy resin preparations with resin cement. The crowns were divided into three groups (n = 16) according to load application region (cusp tip: restricted to cusp tips; cusp plane: restricted to cuspal inclined plane; or mixed: associating tip cusp and cuspal inclined plane). The specimens were submitted to a cyclic fatigue test (initial load: 200 N; step-size: 100 N; cycles/step: 20,000; loading frequency: 20 Hz; load applicator: 6 mm or 40 mm diameter stainless steel) until observing cracks (1st outcome) and fracture (2nd outcome). The data were analyzed by the Kaplan-Meier + Mantel-Cox post-hoc tests for both outcomes (cracks and fracture). Finite element analysis (FEA), occlusal contact region, contact radii measurements, and fractographic analyzes were performed. RESULTS: The mixed group presented worse fatigue mechanical behavior (550 N / 85.000 cycles) compared to the cuspal inclined plane group (656 N / 111,250 cycles) (p < 0.05) for the first crack outcome, while the cusp tip group was similar to both groups (588 N / 97,500 cycles) (p > 0.05). The mixed group had the worst fatigue behavior (1413 N / 253,029 cycles) in relation to the other groups (Cusp tip: 1644 N / 293,312 cycles; Cuspal inclined plane: 1631 N / 295,174 cycles) considering the crown fracture outcome (p < 0.05). FEA showed higher tensile stress concentration areas just below the load application region. In addition, loading on the cuspal inclined plane induced a higher tensile stress concentration in the groove region. The most prevalent type of crown fracture was the wall fracture. Groove fracture was observed in 50% of the loading specimens exclusively on the cuspal inclined plane. CONCLUSION: Load application on distinct occlusal contact regions affects the stress distribution pattern and consequently the mechanical fatigue performance and fracture region of the monolithic lithium disilicate ceramic crowns. A combination of loading at distinct regions is recommended to promote better evaluation of the fatigue behavior of a restored set.

Aesthetic and functional rehabilitation of collapsed occlusal vertical dimension using an advanced digital workflow.

This report presents a clinical case that follows a comprehensive digital workflow to increase occlusal vertical dimension of a patient presenting with occlusal wear. A 56-year-old patient with aesthetic and functional problems presented to the authors' clinic. An intraoral scanner was used to obtain digital models of the patient's existing maxillary and mandibular arches. Advanced sculpting software and digital tooth libraries were used to design the patient's ideal smile. A three-dimensional (3D) printer was used to fabricate temporary restorations and resin casts. Full-mouth restoration was successfully completed with zirconia crowns using an advanced digital workflow.

Restoration longevity: Q: "How long will my crown last?" A: "78 years, Madame" - A case report.

This case report describes a post-retained crown placed on an anterior tooth that survived 78 years, with restoration repairs at 40 years and 58 years following placement. The history of the patient's other dental restorations and care showed a similar longevity.

Evaluation of marginal and internal fit of lithium disilicate and zirconia all-ceramic CAD-CAM crowns using digital impressions: A systematic review.

PURPOSE: A range of materials for single-tooth computer-aided design and computer-aided manufacturing (CAD-CAM) restorations have been introduced that may affect CAM accuracy. This study aimed to review articles evaluating marginal and internal fit of lithium disilicate (LD) and zirconia (Z) crowns fabricated by CAD-CAM systems using intraoral optical scanners (IOS). MATERIALS AND METHODS: Under the guidelines of Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA), a systematic review was performed along with an electronic article search in the Medline/Pubmed database. The articles were limited to those in the English language that were published within the past ten years. RESULTS: The initial search resulted in 50 articles and of those, a total of 18 articles were selected for full-text review following abstract evaluation. Eight articles that did not meet the inclusion criteria were excluded and the remaining ten articles, which provided internal and marginal gap values, were used in this review. For LD crowns, marginal gap values ranged between 45µm and 190.2µm. For Z crowns, the values varied between 39µm and 126.4µm. For LD crowns, the internal gap values were between 57.8µm and 475.4µm, and for Z crowns, the values were between 79µm and 205.8µm. CONCLUSION: The outcome of this review suggests that clinically acceptable marginal and internal fit can be attained with LD and Z all-ceramic CAD-CAM crowns using digital impressions. Additionally, it has been found that LD and Z ceramics provide similar marginal gap values, but LD material provides better internal fit than Z.

A new method assessing predicted and achieved mandibular tooth movement in adults treated with clear aligners using CBCT and individual crown superimposition.

The purpose of this study was to demonstrate a new method for quantifying the difference between predicted and achieved tooth movement with Invisalign using stable three-dimensional (3D) mandibular landmarks and dental superimposition. Cone-beam computed tomography (CBCT) scans before (T1) and after (T2) the first series of aligners, their corresponding digital models (ClinCheck initial of the first series as T1 and ClinCheck initial of the refinement series as T2), and the ClinCheck final model of the first series as the predicted were obtained from 5 patients treated with non-extraction Invisalign therapy. After segmentation of the mandible and its dentition, T1 and T2 CBCTs were superimposed on stable anatomic structures (Pogonion and bilateral mental foramen) along with the pre-registered ClinCheck models. The 3D prediction differences between the predicted and achieved tooth position for 70 teeth with four types (incisor, canine, premolar and molar) were measured using a combination of software. The method employed in this study was tested to be reliable and repeatable with a very high intraclass correlation coefficient (ICC) for both intra- and inter-examiner reliability. Premolar Phi (rotation), Incisor Psi (mesiodistal angulation), and Molar Y (mesiodistal translation) showed a significant prediction difference (P < 0.05), which is also clinically relevant. The method involving CBCT and individual crown superimposition to measure the 3D positional changes in the mandibular dentition is a robust and novel one. While, our finding in terms of the predictability of Invisalign treatment in the mandibular dentition mainly served as a crude, cursory examination, which warrants further and more rigorous investigations. With this novel methodology, it is possible to measure any amount of 3D tooth position difference in the mandibular dentition either between the simulated and the actual or with treatment and/or growth. Deliberate use of overcorrection of which specific type of tooth movement with clear aligner treatment and to what extent, might be possible with future studies.

Occlusal contact and clearance of posterior implant-supported single crowns designed by two different methods: a self-controlled study.

BACKGROUND: Precise occlusal design of implant-supported fixed prostheses is difficult to achieve by the conventional wax-up method, often requiring chairside adjustments. The computer-aided design (CAD) method is promising. This study aims to compare the occlusal contacts and clearance of posterior implant-supported single crowns designed by the CAD and conventional methods. METHODS: Sample size calculation indicated fourteen samples per group. Two sets of type-IV plaster casts with a single implant analog inserted in the posterior teeth region were mounted as master casts in a mechanical articulator in maximal intercuspal position (MIP). Seven working cast sets were obtained from each master cast by a closed tray technique, and mounted in MIP. Two implant-supported single crowns were designed with an occlusal clearance to achieve light occlusal contact in each working cast set by CAD and conventional method, separately. For the CAD group, the crown was designed in digital models obtained by scanning the working casts. For the conventional group, wax-up of the crown was prepared on the working casts and scanned to generate a STL file. In the working and master casts, mean and minimum occlusal clearances in the designed occlusal contact area of the both finished prostheses were calculated using the occlusal clearance (OC) and occlusal record (OR) method. The prostheses' occlusion was evaluated in master casts. RESULTS: For the evaluation in the working casts, both design methods had similar mean occlusal clearances by the OC method (195.4 ± 43.8 vs. 179.8 ± 41.8 µm; P = 0.300), while CAD group resulted in a significantly larger minimum occlusal clearance in the designed occlusal contact area (139.5 ± 52.3 vs. 99.8 ± 43.8 µm; P = 0.043). Both design methods had similar mean and minimum occlusal clearances by the OR method (P > 0.05). For the evaluation in the master casts, both design techniques had similar mean and minimum occlusal clearances, number and distribution of occlusal contacts, and lateral interference ratios (P > 0.05). CONCLUSION: Occlusal contact and clearance of posterior implant-supported single crowns designed by the CAD method can be at least as good as those designed by the conventional wax-up method.

Wear Resistance Comparison of Prefabricated Primary Crowns Using a Novel 3D Computed Tomography Method.

PURPOSE: The purpose of this study was to compare the wear resistance of stainless steel crowns, (SSCs), zirconia crowns (ZRCs), and nanohybrid crowns (NHCs) using a 3D tomography method. METHODS: Prefabricated SSCs, ZRCs, and NHCs (n equals 80) were worn for 400,000 cycles, equivalent to three years of simulated clinical wear, at 50 N and 1.2 Hz using the Leinfelder-Suzuki wear tester. Wear volume, maximum wear depth, and wear surface area were computed using a 3D superimposition method and 2D imaging software. Data were statistically analyzed using one-way analysis of variance with the least significant difference post hoc test (P<0.05). RESULTS: After a wear simulation of three years, NHCs had a 45 percent failure rate; NHCs also had the greatest wear volume loss (0.71 mm³), maximum wear depth (0.22 mm), and wear surface area (4.45 mm²). SSCs (0.23 mm³ , 0.12 mm, 2.63 mm²) and ZRCs (0.03 mm³ , 0.08 mm, 0.20 mm ²) had less wear volume, area, and depth (P<0.001). ZRCs were the most abrasive to their antagonists (P<0.001). The NHC (against SSC wearing group) had the greatest total wear facet surface area (4.43 mm²). CONCLUSIONS: Stainless steel crowns and zirconia crowns were the most wear-resistant materials. Based on these laboratory findings, in the primary dentition, nanohybrid crowns are not recommended as long-term restorations beyond 12 months (P=0.001).

Morphology and mechanical performance of dental crown designed by 3D-DCGAN.

OBJECTIVES: This study utilised an Artificial Intelligence (AI) method, namely 3D-Deep Convolutional Generative Adversarial Network (3D-DCGAN), which is one of the true 3D machine learning methods, as an automatic algorithm to design a dental crown. METHODS: Six hundred sets of digital casts containing mandibular second premolars and their adjacent and antagonist teeth obtained from healthy personnel were machine-learned using 3D-DCGAN. Additional 12 sets of data were used as the test dataset, whereas the natural second premolars in the test dataset were compared with the designs in (1) 3D-DCGAN, (2) CEREC Biogeneric, and (3) CAD for morphological parameters of 3D similarity, cusp angle, occlusal contact point number and area, and in silico fatigue simulations with finite element (FE) using lithium disilicate material. RESULTS: The 3D-DCGAN design and natural teeth had the lowest discrepancy in morphology compared with the other groups (root mean square value = 0.3611). The Biogeneric design showed a significantly (p < 0.05) higher cusp angle (67.11°) than that of the 3D-DCGAN design (49.43°) and natural tooth (54.05°). No significant difference was observed in the number and area of occlusal contact points among the four groups. FE analysis showed that the 3D-DCGAN design had the best match to the natural tooth regarding the stress distribution in the crown. The 3D-DCGAN design was subjected to 26.73 MPa and the natural tooth was subjected to 23.97 MPa stress at the central fossa area under physiological occlusal force (300 N); the two groups showed similar fatigue lifetimes (F-N curve) under simulated cyclic loading of 100-400 N. Designs with Biogeneric or technician would yield respectively higher or lower fatigue lifetime than natural teeth. SIGNIFICANCE: This study demonstrated that 3D-DCGAN could be utilised to design personalised dental crowns with high accuracy that can mimic both the morphology and biomechanics of natural teeth.

Synchronous Fabrication of Custom One-piece Glass Fiber Post-and-core and Zirconia Crown by a Fully Digital Workflow.

Combined crown and post-and-core treatment often requires multiple intermittent appointments and takes a long time. This article describes a novel method for simultaneously fabricating a custom glass fiber post-and-core and the corresponding crown. The critical step, defined as a post-and-core virtual try-in, is to construct a virtual crown abutment that mimics the profile of a real one gained after post-cementation and tooth preparation. The fully digital workflow optimizes the treatment and saves time by accomplishing restorations on the first visit.

Assessment of the fit of lithium disilicate crowns at various locations fabricated by three different methods using the triple-scan protocol.

AIM: The present study aimed to evaluate the marginal and internal fit of lithium disilicate crowns at various locations. MATERIALS AND METHODS: A typodont maxillary left first molar was prepared for a lithium disilicate crown, scanned, and a master die fabricated. Three groups were created according to fabrication method (n = 10): conventional impression and press method (group C); scanning of definitive cast and milling method (group D); and intraoral scanning and milling method (group I). Assessment was performed using the triple-scan protocol. At the buccopalatal and mesiodistal sections, the absolute marginal discrepancy (AMD), marginal gap (MG), axial internal gap (AI), and line angle internal gap (LI) were measured. One-way analysis of variance (ANOVA) and post hoc Tukey HSD tests were used for statistical analysis (α = 0.05). RESULTS: AMD values were significantly lower in group C than in groups D and I (P < 0.05). MG values in group C were significantly lower than those in the buccal and distal areas in group D and all areas in group I (P < 0.05). AI values in the buccal and palatal areas in group D were significantly lower than those in the mesial and distal areas in group D and all areas in groups C and I (P < 0.05). LI values were significantly lower in group C than in groups D and I (P < 0.05). CONCLUSION: All three methods were clinically acceptable except for the marginal fit of the intraoral scanning and milling method, which was on the borderline of a clinically acceptable fit. (Int J Comput Dent 2023;26(1): 37-0; doi: 10.3290/j.ijcd.b3818305).

Effect of pulp chamber depth on failure load and mode of failure of CAD/CAM endocrowns.

AIM: The present study aimed to investigate the effect of pulp chamber depth on the failure load and mode of failure of CAD/CAM endocrowns. MATERIALS AND METHODS: Thirty mandibular molars were sectioned above the cementoenamel junction (CEJ), followed by root canal treatment. Teeth were sectioned at a level of 1.5 mm above the CEJ, arranged from the lowest to the highest depths, and divided into three groups (n = 10): group SE: Shallow pulp chamber (1.42 to 2.17 mm); group IE: Intermediate pulp chamber (2.25 to 3.17 mm); group DE: Deep pulp chamber (3.33 to 5.17 mm). CAD/CAM endocrowns were fabricated by milling lithium disilicate ceramic blocks and were cemented using resin cement. Teeth were embedded in acrylic resin at 2 mm below the CEJ, and a compressive load was applied to create a 45-degree angled functional loading simulation until the occurrence of failure. RESULTS: The mean failure loads were highest in group DE (1893.75 ± 496.08 N) compared with groups IE (1103.71 ± 254.59 N) and SE (1084.63 ± 240.92 N), with statistically significant differences between group DE and both groups IE and SE (P < 0.001). Pearson correlation coefficient (PCC) revealed a strong positive correlation between the pulp chamber depth and failure load of the endocrowns. The failure mode for all samples was catastrophic failure. CONCLUSION: The pulp chamber depth affected the failure load of the teeth restored with endocrowns. The failure loads were higher in teeth with a greater pulp chamber depth. (Int J Comput Dent 2023;26(1): 31-0; doi: 10.3290/j.ijcd.b3818295).

Influence of thermo-mechanical aging on fracture resistance and wear of digitally standardized chairside computer-aided-designed/computer-assisted-manufactured restorations.

OBJECTIVES: To investigate the influence of thermal cycling and mechanical loading (TCML) aging on fracture resistance and wear behavior of various chairside computer-aided-designed/computer-assisted-manufactured (CAD/CAM) premolar crowns cemented on standardized tooth abutments. METHODS: Eighty chairside CAD/CAM crowns were prepared using lithium disilicate (IPS e.max CAD; EM), zirconia-infiltrated lithium silicate (Celtra Duo; CD), polymer-infiltrated ceramic network (Vita Enamic; VE), and resin nanoceramics (Cerasmart; CS) (n = 20). The specimens were divided into two groups (n = 10). In one group, they were subjected to TCML: thermocycling (6000 cycles in distilled water at 5-55 °C) and mechanical loading (50 N for 1.2 × 106 cycles), while in control group they were stored in distilled water (37 °C for 24 h). The fracture load, height loss, and volume wear of the crowns were measured after TCML. Fractography was performed on fractured specimens. Data were analyzed using analysis of variance and multiple comparison tests (α=0.05). RESULTS: The mean fracture loads of EM and CD were significantly higher than those of EC and CS (p<0.05). There was no significant change in the fracture load of any CAD/CAM crowns after TCML (p>0.05). CS exhibited a significantly higher volume wear than the other materials investigated. The wear tracts of all TCML crowns acted as failure origins during the fracture test. CONCLUSIONS: The fracture resistance of glass-ceramic CAD/CAM crowns was significantly higher than that of resin composite crowns. A 5-year TCML aging did not affect the fracture resistance of CAD/CAM crowns investigated. However, TCML treatment produces a larger wear track in CS than in other materials. CLINICAL SIGNIFICANCE: Appropriate chairside CAD/CAM restorative material should be selected for successful clinical practice after considering the fracture and wear resistance of the crowns.

A Study of 42 Partially Edentulous Patients with Single-Crown Restorations and Implants to Compare Bone Loss Between Crestal and Subcrestal Endosseous Implant Placement.

BACKGROUND The purpose of the study was to evaluate the influence of dental implant placement at different bone levels upon the resultant postoperative peri-implant bone loss. MATERIAL AND METHODS Forty-two partially edentulous patients seeking implant-supported single-crown restorations were screened followed by segregation into 2 groups (GP), GP E (equicrestal) and GP S (subcrestal) (n=21 each). Sixty endosseous implants (30 each) (Adin Tourage-S, Israel), size 3.5/8 and 4/10 mm for mandibles, were placed using a 2-stage surgical procedure. At 4 to 6 months, straight abutments were attached followed by restoration (Vita Zahnfabrik, Germany). Crestal bone levels (mesial/distal) of implant fixtures were assessed at 5 time intervals (after surgery, and at 3, 6, 9, and 12 months) using digital radiography. Means and standard deviations were calculated, following which the differences were statistically analyzed using ANOVA at P value of <0.05. RESULTS The mean annual bone loss for GP S (1.96 mm) was higher than GP E (1.10 mm). At all studied time intervals, the bone loss for implants in GP S was higher than in GP E (P<0.05). Between time intervals, lowest bone loss was observed on the distal side in GP E (0.11 mm/6-9 month) and the highest bone loss was observed on the distal side of GP S (0.6 mm/9-12 month). Differences in the means between the 2 groups on mesial and distal sides were statistically significant at all time intervals (P<0.05). CONCLUSIONS Subcrestal implant placement was associated with more bone loss than when implants are placed at the crestal level.

Computed tomography assessment of hypodontia and crown size in hemifacial microsomia.

OBJECTIVE: Our aims were to assess the prevalence of hypodontia in unilateral hemifacial microsomia (HFM), and to compare tooth (crown) size between affected and unaffected sides. DESIGN: In a retrospective cross-sectional study of South Australians, computed tomography (CT) scans were used to assess hypodontia and crown size (mesiodistal length, buccolingual width and crown height). The inclusion criteria were the absence of other congenital anomalies and the availability of CT scans. The exclusion criteria were the lack of extraction history or reproducible landmarks for morphometric assessment. The final sample comprised 41 participants in both dentitions, including 32 children and 9 adults (median age 13.9 years, range 0.4 - 47.6 years; 19 males and 22 females). Hypodontia was assessed in all participants, and the permanent crown size in 30 (73.2%) participants. Linear mixed-effects models were performed to determine if crown size was significantly different between the two sides, controlling for sex, HFM severity, and tooth and jaw type. RESULTS: Hypodontia occurred in none of the participants in the primary dentition, but in 6/30 (20%) participants in the permanent dentition (3/30 each on the affected and unaffected sides). There was no significant difference in the mean crown dimensions between the two sides, but the crown size was larger in males (p < 0.05), except for mesiodistal length, and became progressively smaller with increased HFM severity (p < 0.05). CONCLUSIONS: Hypodontia spared the primary dentition but featured prominently in the permanent dentition. The permanent crown dimensions were unaltered between the two sides.

[The digital impression under rubber dam: clinical insights for an accurate scan]. / Digitaal afdrukken onder rubberdam: klinische inzichten voor een nauwkeurige scan.

During the past 15 years, the advent of digital innovations has completely changed the dental practice. Tools like intraoral scanners, computer-aided design (CAD) and computer-aided manufacturing (CAM) are considered perfectly reliable and affordable, today. The use of these digital techniques greatly facilitates restorative and prosthetic treatment. It is particularly interesting to combine isolation under the rubber dam with digital impressions in adhesive restorative dentistry. In this article, a clinical protocol is given on the basis of a case for making a digital impression under the rubber dam.

[Changing scanning angles on improving finish line trueness of the full crown preparation].

Objective: To explore the effect of scanning methods on finish line trueness of the full crown preparation. Methods: The standard full crown preparation model of the right maxillary first molars was prepared by using the maxillary standard resin dentition model. The standard preparation was scanned by imetric scanner and data were used as the true value. CEREC Omnicam and 3Shape TRIOS were used to scan the standard preparation. According to the scanning methods, they were divided into parallel scanning group, occlusal wave scanning group and buccolingual wave scanning group. Each group was scanned repeatedly 6 times. The data were imported into Geomagic Studio 2013 software, and the local finish line image data of the mesial, distal, buccal and lingual regions of the full crown preparation were extracted respectively. Three-dimensional deviation analysis was performed with the reference true value, and the root- mean-square error (RMSE) was the evaluation index of scanning trueness. The statistical method was one-way ANOVA. Results: Parallel scanning group: in general, the RMSE value of complete finish lines of scanner B [(35±6) µm] was significantly lower than that of scanner A [(44±7) µm](P<0.05). After scanner A occlusal wave scanning, the RMSE values of the mesial and distal finish lines [(33±5) and (50±12) µm] were significantly lower than those of parallel scanning group (P<0.05). After buccal and lingual wave scanning, the RMSE values of local finish lines in the mesial, distal, buccal and lingual regions [(37±3), (50±6), (28±6) and (29±8) µm] were significantly lower than those in parallel scanning group [(45±9), (63±7), (38±3) and (40±3) µm] (P<0.05). No significant difference was found in the RMSE values of the mesial, distal, buccal and lingual regions of scanner B between parallel scanning group, occlusal wave scanning group and buccolingual scanning group (P>0.05). Conclusions: The scanning trueness of the full crown preparation finish line obtained by the active triangulation scanning equipment can be improved by changing the scanning method to wave scanning.

Fracture strength of implant-supported monolithic crowns after aging in a mastication simulator.

The fracture strength of implant-supported monolithic CAD/CAM crowns under approximate clinical conditions and how their fracture strength is affected by cement type remain unclear. The present study investigated the fracture resistance of implant-supported polymer-infiltrated ceramic and zirconia-reinforced lithium silicate glass-ceramic crowns cemented with two different cement types after aging in a mastication simulator. RelyX Ultimate Clicker (3M ESPE) as adhesive resin cement, Panavia SA Cement Plus (Kuraray Noritake) as self-adhesive resin cement, and two monolithic materials (A1-T for Enamic and M1-HT for Suprinity; both Vita Zahnfabrik) were tested. Forty CAD/CAM-produced monolithic implant-supported crowns for a maxillary right second premolar were prepared, and crowns were cemented to the straight implant abutments with each cement, then subjected to dynamic load cycling (480,000 cycles) and thermocycling (about 4,000 cycles) in a mastication simulator. Crown fracture resistance was analyzed by the load-to-failure test. Crown-abutment samples were loaded until fracture. No significant difference was observed in the fracture load of the crowns among the groups. The results suggest that both monolithic restoration materials could be an alternative for implant-supported cement-retained restorations, regardless of cement type.

Influence of the Operator`s Experience, Working Time, and Working Position on the Quality of the Margin Width: In Vitro Study.

Background and Objectives: Appropriate tooth preparation is mandatory to obtain a perfect marginal fit of fixed restorations. The heavy chamfer is the most commonly used finish line, especially for minimally invasive tooth preparation. The aim of the study was to compare the width of the finish line obtained during tooth preparation performed by experienced (university lecturers) and inexperienced persons (dental students) in different working times and positions. Materials and Methods: Forty left upper-second molars were prepared on the simulator by each participant, totalizing 160 prepared teeth. A new round-end tapered diamond was used to obtain the 0.5 mm width of the heavy chamfer. The prepared teeth were photographed using a Canon D5300 camera with a macro lens attached to a tripod. The measurements were made with the Image-Pro Insight software selecting the same eight reference points. From these points, perpendicular lines were drawn above the finish line to the axial walls and the distance between the chamfer's outer edge and the axial wall's inner edge was measured. GraphPad Instat and NCSS Dowson Edition software were used. The statistical significance was set at p < 0.05. The mean (M) and standard deviation (SD) were calculated. The used tests: one sample t-test, ANOVA test, and Tukey-Kramer Multiple Comparisons Test. Results: Statistically significant differences were obtained according to the experience of the participant, preparation time, patient's position, and the chamfer width on the prepared tooth different surfaces. Conclusions: Daytime or weeklong tiredness and patient position do not affect the width of the heavy chamfer prepared by experienced and inexperienced persons. The experience and the operator's working position influence the width of the prepared finish line.