Effect of Repeated Use of Different Types of Scan Bodies on Transfer Accuracy of Implant Position.

Statement of the Problem: Some components of implant treatment are reusable. Therefore, possible changes during fixation, removal, and sterilization process should be tested. Many studies have examined the reuse of implant parts, but the impact of repeated use of scan bodies on the accuracy of implant position has not been well investigated. Purpose: The aim of this in vitro study was to compare the effect of repeated use of two different types of scan bodies on the accuracy of implant position. Materials and Method: In this in vitro experimental study, two acrylic resin maxillary models, each with two implant analogues inserted at the site of missing first and second molars were used. Two types of scan bodies including titanium and polyetheretherketone (PEEK) were used for digital impression. Then they were ten times removed and autoclaved for sterilization. The first scan was considered as a reference to be compared with the other next nine scans. Values of linear distance between two scan bodies, diameter changes of each scan body, and three-dimensional linear displacement (ΔR) were measured. These values were compared between the two types of scan bodies using t-test (α=.05). Results: There was significant difference between titanium and PEEK scan bodies regarding inter-implant distance variation (p=.006) and diameter change (p< .001) in repeated use. However, for the ΔR, there was no significant difference between them (p= 0.759). Conclusion: The results demonstrated that type of scan body could affect the accuracy of implant position transfer after repeated use. PEEK scan body performed better after 9 cycles of reuse in comparison with titanium scan body.

Evaluation of One- and Two-Step Impression Techniques and Vertical Marginal Misfit in Fixed Prothesis.

Methods: 12 impressions were made of a resin maxillary model (second premolar and second molar) with two prepared abutment teeth using vinyl polysiloxane (VPS); the margin of the second premolar was 0.5 mm subgingivally; and the margin of the second molar tooth was at the level of the gingiva. Impressions were made using two techniques: one-step and two-step putty/light materials. A three-unit metal framework was fabricated on the master model using the computer-aided design/computer-aided manufacturing (CAD/CAM) technique. The vertical marginal misfit was evaluated in the buccal, lingual, and mesial and distal surfaces of the abutments on the gypsum casts using a light microscope. Data were analyzed using the independent t-test (α < 0.05). Results: The results showed significantly lower vertical marginal misfit in all six areas evaluated around the two abutments in the two-step impression technique compared with the corresponding values in the one-step technique. Conclusion: Vertical marginal misfit in the two-step technique with a preliminary putty impression was significantly lower than in the one-step putty/light-body technique.

The effect of preparation design on the fracture resistance and adaptation of the CEREC ceramic endocrowns.

OBJECTIVE: The purpose of this experimental in vitro study was to assess the effect of having one or two intact axial walls on the improvement of the fracture resistance, and marginal and internal adaptation of computer-aided design/computer-aided manufacturer fabricated ceramic endocrowns. MATERIALS AND METHODS: Thirty-six endodontically treated mandibular molars were divided into three groups (n = 12). Group A, represented teeth that all of their axial walls were reduced till they all had 3 mm height. In group B, the buccal wall had 5 mm height and the others were reduced to 3 mm. Specimens of the group C had 5-mm-height buccal and one 5 mm-height proximal wall, with all other walls of 3 mm height. All teeth were scanned using intraoral scanner, and endocrowns were milled from zirconia lithium silicate ceramics blocks. The marginal and internal discrepancy of restorations were evaluated with the replica technique. Fracture resistance was also measured after cementation and modes of failure were observed. One-way analysis of variance and Tukey HSD multiple comparisons were used to analyze the data (α = .5). RESULTS: Significant differences were observed within the groups in terms of the axial discrepancy (p = .022); group A had significantly higher amounts of axial discrepancy compared to group B (p = .001) and group C (p = .003). Preservation of the intact axial walls did not result in a statistically significant decrease in marginal (p = .21) and pulpal (p = .31) discrepancy values. Also, concerning the fracture resistance no significant difference was observed among the groups (p = .51). CONCLUSION: Preservation of at least one of the axial walls could reduce the amount of the axial discrepancy and, therefore, improves the adaptation of the restoration. However, based on this study, it did not improve fracture resistance.

Fully Digital versus Conventional Workflows for Fabricating Posterior Three-Unit Implant-Supported Reconstructions: A Prospective Crossover Clinical Trial.

This study assessed the clinical variables influencing the success of three-unit implant-supported fixed dental prostheses (ISFDPs) fabricated using either fully digital or conventional workflows. The clinical trial evaluated 10 patients requiring three-unit ISFDPs in the posterior mandible. Maxillomandibular relation records, and digital and conventional impressions were obtained from each patient using an intraoral scanner (IoS) and polyvinylsiloxane (PVS), and the frameworks were fabricated using zirconia and cobalt-chromium, respectively. A 2 µm accuracy scanner scanned the conventional master casts and standard reference models. The stereolithography (STL) files of the digital and conventional impressions were superimposed on the standard model file, and the accuracy was calculated with the best-fit algorithm. The framework adaptation and passivity were assessed using the one-screw and screw resistance tests. The time required for occlusal adjustment of both types of reconstructions, including the duration of the whole treatment, was recorded. The aesthetic appearance of ISFDPs was rated by each patient and clinician using a self-administered visual analogue scale questionnaire and the FDI World Dental Federation aesthetic parameters, respectively. The sample size was based on the power calculation, and alpha was set at 0.05 for the statistical analyses. The impression accuracy, framework adaptation and passivity, and reconstructions aesthetics did not significantly differ between the digital and conventional approaches. The total fabrication time was significantly shorter using the digital workflow. Within the limitations of this clinical study, the fully digital workflow can be used for the fabrication of ISFDPs with a clinical outcome comparable to that of the conventional workflow.

Smile Design: Mechanical Considerations.

Smile designing refers to the cosmetic and esthetic dental reconstruction that is visible during smiling. The use of modern digital tools requires adequate knowledge about the tooth shape and shade principles. Mechanical, biological, and psychological factors should be understood and tailor an individualized treatment accordingly to achieve pleasing esthetic outcomes. Dental therapy is becoming more appearance-driven, and thus, both patients and dental clinicians mainly emphasize on cosmetic dental and facial aspects of treatments.

Evaluation of the effect of milling mode and margin definition on adaptation of chairside digitally designed and fabricated ceramic veneers.

AIM: The present study aimed to evaluate the effect of the milling mode and margin defining method on the marginal and internal adaptation of digitally designed and fabricated ceramic veneers. MATERIALS AND METHODS: Forty extracted central incisors were prepared for ceramic veneers. The veneers were categorized into four groups (n = 10) based on the milling mode (fine or extra-fine) and design (automatic or manual defining of cervical and incisal margins). The veneers were cemented to the teeth using resin cement and then polished. After cementation, the teeth were embedded in clear polyester resin, then cut into three sections. Absolute marginal gap at the cervical margins (AMGC), marginal gap at the incisal (MGI) and cervical (MGC) margins, and internal gap (IG) at the axial wall were evaluated using a scanning electron microscope. Two-way analysis of variance (ANOVA) and t tests were used (α = 0.05) for the statistical analysis. RESULTS: The margin defining method affected the adaptation of the internal and incisal margin; the automatic (software) margin defining method resulted in a smaller MGI (P = 0.038) and IG (P = 0.009) than the manual defining method. CONCLUSION: The type of milling mode did not affect the marginal gap. The margin defining method affected the marginal gap at the incisal margin and the internal gaps. (Int J Comput Dent 2022;25(3):277-285; doi: 10.3290/j.ijcd.b2588153).

Biomechanical behavior of endocrown restorations with different cavity design and CAD-CAM materials under a static and vertical load: A finite element analysis.

STATEMENT OF PROBLEM: The influence of different restorative materials, the divergence angle, and the preparation design of the pulp chamber floor in endocrowns on stress distribution is unclear. PURPOSE: The purpose of this finite element analysis study was to evaluate the stress distribution and maximum von Mises stress values in endocrown restorations and remaining teeth. MATERIAL AND METHODS: An endodontically treated first mandibular molar was prepared with 2-mm occlusal reduction, a butt-joint margin, and a mesial box with a 4-mm buccolingual extension to receive an endocrown restoration. The model was then scanned, and the exported standard tessellation language file was designed according to the anatomic references by using a computer-aided design software program. The reference model was duplicated, and 27 variations were formed: restorations made of IPS e.max, Vitablocs MarkII, and Vita Enamic, as well as different divergence angles of 6, 12, and 16 degrees prepared with 3 variations of pulp chamber floor designs, keeping the natural form, adding a 1-mm glass ionomer pulp chamber base, and adding a 2-mm extension into the distal canal. Stress distribution under axial loading and maximum von Mises stress values were analyzed and calculated by using the Ansys software program. RESULTS: The endocrown restoration presented the lowest von Mises stress values when a 6-degree divergence angle was used with Vitablocs MarkII. However, in other layers such as enamel, resin cement, and pulpal floor, the lowest von Mises stress values were observed in IPS e.max restorative material when the divergence angle was increased to 16 degrees. No difference was found in the stress distribution pattern and maximum von Mises stress values in the alveolar bone or periodontal ligament. CONCLUSIONS: Endocrowns fabricated with lithium disilicate ceramics without intraradicular extension exhibited lower stresses on tooth structure. An increase in the divergence angle improved the stress distribution on tooth structure; however, sound dental tissue should not be removed to achieve better stress distribution.

Additional Manufactured Interim Restorations: a Review on the Literature.

Interim restorations are essential in many clinical scenarios in which indirect restorations are administered. Additional manufacturing (AM) technology has recently been introduced and applied in different fields of dental practice. This study aimed to collect relevant information from published papers regarding different aspects of AM temporary restorations with a focus on the most relevant technical properties. An electronic search was performed on Medline/ PubMed/ Scopus databases up to April 2020 to find relevant, peer-reviewed articles about AM provisional restoration. Although promising results for AM temporary restorations were obtained, comprehensive application of this technology in making provisional restorations requires information to address the missing properties concerning the short time of application.

Evaluating the effect of repeated use of milling burs on surface roughness and adaptation of digitally fabricated ceramic veneers.

OBJECTIVES: This study aimed to evaluate how repeated use of milling diamond burs with different coarseness affects surface roughness, and marginal and internal adaptation of CAD/CAM veneers. METHODS: Forty leucite-reinforced glass-ceramic veneers were milled in 2 groups based on the milling mode (with fine or extra-fine bur sets). In each group, every 10 veneers were milled with a new bur set. All veneers were cemented to bovine teeth and then polished. Labial surface roughness was measured before cementation, and after polishing. Marginal and internal discrepancies were measured using a field emission scanning electron microscope. Three-way and two-way mixed repeated measures ANOVA were applied to assess changes in surface roughness values of veneers and discrepancy values, respectively. The Bonferroni correction was applied for multiple comparisons. RESULTS: Repeated use of a milling diamond bur set had a significant effect on surface roughness of the veneers (P < .001). Mean surface roughness of the fine milling mode was significantly higher in comparison to that of extra-fine mode before (P = .002) and after (P = .01) polishing. After polishing a significant decrease in surface roughness occurred in fine (P = .02), but not in extra-fine milling mode (P = .99). Repeated use of milling burs significantly affected marginal and internal adaptation between some repeated uses. CONCLUSIONS: Marginal and internal adaptation were significantly affected by repeated use of milling diamond burs up to 10 times between some repeated uses. However, no specific pattern could be established. CLINICAL SIGNIFICANCE: Repeated use of milling burs could affect surface roughness, surface microcracks, critical defects, and adaptation of CAD/CAM restorations. Therefore, it plays a major role in clinical success of the restorations.

Comparison of peri-implant clinical outcomes of digitally customized and prefabricated abutments: A systematic review and meta-analysis.

BACKGROUND: Digitally customized abutments are increasingly used in contemporary implant prosthodontics. PURPOSE: This systematic review and meta-analysis aimed at comparing the peri-implant clinical outcomes of digitally customized and prefabricated abutments. MATERIALS AND METHODS: The search strategies included electronic databases (PubMed, Embase, Scopus, and Cochrane clinical trials database) and related journals up to September, 2020. A qualitative and quantitative synthesis was performed on data extracted from the included studies. RESULTS: Three RCTs (number of patients = 120; number of dental implants = 120) and two prospective cohort studies (number of patients = 144; number of dental implants = 144) with one to three-year follow-up periods were included. The quantitative analyses did not demonstrate a significant difference between digitally customized and prefabricated abutments for peri-implant pocket depth (P = 0.62), plaque index (P = 0.67), bleeding on probing (P = 0.43), keratinized mucosa width (P = 0.75), and pink aesthetic score (P = 0.30) at one-year follow-up visit. The qualitative analyses for marginal bone level change, calculus accumulation, implant survival rate, implant success rate, white aesthetic score, and patient-reported outcomes did not demonstrate a significant difference between two groups during 1 to 3-year follow-up visits. CONCLUSION: The current data do not provide evidence of significant differences between two abutment fabrication methods in terms of peri-implant clinical outcomes within short-term period (CRD42020170807).

Marginal Adaptation of Implant Prostheses Fabricated by Different Materials in Excessive Crown Height Space before and after Veneering.

Objectives: This study aimed to investigate the marginal adaptation of implant-supported three-unit fixed restorations fabricated in excessive crown height by various frameworks namely zirconia, nickel-chromium (Ni-Cr) alloy, and Polyetheretherketone (PEEK) before and after veneering. Materials and Methods: A basic model with two implant fixtures was made to receive posterior three-unit fixed partial dentures (second premolar to second molar) in 15 mm crown height. A total of 30 frameworks were fabricated using Ni-Cr, zirconia, and PEEK (n=10). All specimens were veneered and vertical marginal discrepancy was evaluated before and after veneering using a stereomicroscope (×75). The effect of framework material and veneering on marginal discrepancy was evaluated by repeated-measures and one-way ANOVA, and paired t test (α=0.05). Results: There was a significant difference between the groups (P<0.001) before and after veneering. The vertical marginal discrepancy of zirconia frameworks was significantly lower than that of other groups both before and after veneering (P<0.001). Statistical analysis revealed that the veneering process had a significant effect on marginal adaptation (P<0.001). Conclusion: In implant prostheses with excessive crown height, zirconia had the greatest marginal adaptaion significantly, followed by Ni-Cr. Veneering caused a significant increase in marginal discrepancy of all the materials.

The Effect of Sandblasting on Bond Strength of Soft Liners to Denture Base Resins: A Systematic Review and Meta-Analysis of In Vitro Studies.

OBJECTIVES: This study aimed to evaluate the effect of sandblasting on the bond strength of denture base resin to soft liners. MATERIALS AND METHODS: This report follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. PubMed, Embase, Cochrane, Scopus, and OpenGrey databases were searched for in vitro studies that compared sandblasting with no treatment in terms of the tensile, shear, and peel bond strength of resilient lining materials (acrylic-based or silicone-based) to polymethyl methacrylate denture base resin. Based on the outcome, the analysis was carried out in three groups of tensile, shear, and peel bond strength. Subgroup analysis was done for the effect of size of particles on sandblasting, blasting pressure, and type of soft liner whenever possible. Heterogeneity was evaluated among the studies, and meta-analysis was performed with random effect models (p < .05). RESULTS: After screening, 16 articles met the inclusion criteria for meta-analyses. No treatment showed significantly higher tensile (p < 0.001) or peel (p=0.04) bond strength, although shear bond strength of sandblasted resin was significantly better (p=0.008). Results of subgroup analyses of particle size favored the control group in 50 µ Al2O3 particle size (p < 0.001). In analyses of blasting pressure, the control group had significantly better tensile bond strength than specimens with blasting pressure ≤1 bar (p < 0.001) while specimens with blasting pressure beyond 1 bar showed significantly more tensile strength than control group (p=0.03). In silicon-based liners, groups without any surface treatment had significantly higher tensile bond strength (p < 0.001). CONCLUSION: According to the in vitro studies, sandblasting would not lead to significant increase in bond strength of soft liner to the denture base resin.

Comparative Evaluation of the Internal and Marginal Adaptations of CAD/CAM Endocrowns and Crowns Fabricated from Three Different Materials.

PURPOSE: To evaluate and compare the internal and marginal adaptations of chairside CAD/CAM (CEREC) endocrowns and crowns fabricated from lithium disilicate glass-ceramic (IPS e.max CAD), zirconia-reinforced lithium silicate glass-ceramic (VITA Suprinity), and hybrid ceramic (VITA Enamic). MATERIALS AND METHODS: Dental models of the two first maxillary molars were selected. One was prepared for an endocrown, and the other for a standard all-ceramic crown. A total of 72 CAD/CAM restorations, including 36 endocrowns and 36 crowns made of IPS e.max CAD, VITA Suprinity, and VITA Enamic (n = 12 each), were fabricated. Discrepancies were measured in the buccal, mesial, lingual, and distal aspects of three sites (marginal, mid-axial wall, and occlusal/floor) using the noncontact ATOS scanner. Statistical analysis was performed using MANOVA and between-subject effects tests (α = .05). RESULTS: Mesial axial wall discrepancy was significantly lower in endocrowns compared to occlusal discrepancy in crowns, while distal axial wall discrepancy was significantly higher. Moreover, floor discrepancy was found to be significantly lower in endocrowns compared to crowns. However, type of material had no significant effect on any kind of discrepancy. CONCLUSION: The marginal and internal adaptation values were within a clinically acceptable range for both kinds of restoration and all three materials. However, restoration type (crown vs endocrown) was significantly different in the mesial and distal axial wall and occlusal/floor discrepancies, regardless of restoration material.

Marginal and internal adaptation of single crowns and fixed dental prostheses by using digital and conventional workflows: A systematic review and meta-analysis.

STATEMENT OF PROBLEM: Digital and conventional options for definitive impressions and for the fabrication of fixed dental prostheses (FDPs) have been compared in previous studies. However, a comprehensive review with concluding data that determined which method provided the minimal internal and marginal adaptation is lacking. PURPOSE: The purpose of this systematic review and meta-analysis of in vivo and in vitro studies was to compare the marginal and internal adaptation of complete-coverage single-unit crowns and multiunit FDPs resulting from digital and conventional impression and fabrication methods. MATERIAL AND METHODS: The review protocol was registered in International Prospective Register of Systematic Reviews (PROSPERO) and followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. PubMed, Cochrane Trials, Scopus, and Open Grey databases were used to identify relevant articles. Based on fixed prostheses impression and fabrication methods, groups from each study were categorized into 4 groups: conventional impression and fabrication (CC), conventional impression and digital fabrication (CD), digital scanning and conventional fabrication (DC), and digital scanning and fabrication (DD). The risk of bias was assessed by using the Cochrane Collaboration tool for clinical trials and the modified Methodological Index for Non-Randomized Studies (MINORS) for in vitro studies. Heterogeneity was evaluated among studies, and meta-analysis was performed with random-effect models (α=.05). Subgroup analysis was conducted when possible. RESULTS: Eight clinical trials and 21 in vitro studies were eligible for analysis. There was no significant difference between the CD and DD clinical groups for marginal adaptation (P=.149); However, the DD group had significantly less internal discrepancy than the CD group (P=.009). The in vitro studies found no significant difference in marginal adaptation among the CC-CD, CC-DC, and CC-DD pairs (P=.437, P=.387, P=.587), but in the comparison CD versus DD group, a significantly better marginal adaptation was observed for the DD group (P=.001). All the compared in vitro groups were similar in terms of internal adaptation. CONCLUSIONS: Impression and fabrication techniques may affect the accuracy of fit of complete-coverage fixed restorations. A completely digital workflow yielded restorations with comparable or better marginal adaptation than the other methods.

Comparison of three-dimensional digital technique with two-dimensional replica method for measuring marginal and internal fit of full coverage restorations.

PURPOSE: This study compared digital (reference point matching) and replica methods for measuring marginal and internal fit of full coverage restorations. MATERIALS AND METHODS: A maxillary left first molar typodent was fixed on to an aluminum base and prepared to receive all-ceramic full coverage restoration. The model was scanned with an intraoral scanner (CEREC Omnicam, Sirona, York, PA, USA). Twelve crowns were fabricated from lithium disilicate blocks (IPS emax CAD, Ivoclar Vivadent) and then crystalized. Marginal and internal fit of each restoration was measured by two examiners using replica and a new digital three-dimensional technique. Reliability between the two methods and two examiners was assessed by correlation and Cronbach's Alpha coefficient (P<.05). A Bland-Altman assessment for agreement was used to compare the two methods. RESULTS: Bland-Altman assessment showed that the mean of difference for marginal, absolute marginal, and axial gap was respectively -1.04 µm, -41.9 µm, and -29.53 µm with limit of agreement (LOA) between -37.26 to 35.18 µm for marginal, -105.85 to 22.05 µm for absolute marginal and -80.52 to 22.02 µm for axial gap. Positive correlation for repeatability (P<.05) in determining marginal and internal gaps by the two examiners in both techniques was revealed. Reliability of both techniques in all sites of measurements was at least good (0.8 ≤ α < 0.9). CONCLUSION: Both measuring techniques appeared highly reliable for evaluating fit of fixed dental restorations, while reference point matching provided higher values in axial and absolute marginal gap assessment.

Effect of irrigation solutions on the coronal discoloration induced by mineral trioxide aggregate cements containing different radiopacifiers.

BACKGROUND: The aim of this study was to assess the discoloration of coronal tooth structure irrigated with different irrigation solutions and filled with calcium silicate-based materials containing bismuth oxide or calciumzirconia complex as radiopacifier. MATERIALS AND METHODS: In this ex vivo study, 72 bovine enamel-dentin blocks were prepared and divided into three groups. The dentinal cavities in each group were irrigated with 5.25% sodium hypochlorite (NaOCl), 2% chlorhexidine (CHX), or normal saline for 30 min. After that, irrigation solutions were removed using a cotton pellet. Each group was then randomly divided into two subgroups according to the cavity-filling materials (ProRoot mineral trioxide aggregate [MTA] and RetroMTA). The color assessments were performed before filling the cavities and 1 month and 6 months after filling the cavities. Data were analyzed with two-way ANOVA. The level of statistical significance was set at P < 0.05. RESULTS: The effect of irrigation solution on the color change of calcium silicate-based materials was not statistically significant at none of the time intervals (P = 0.334 and P= 0.252, respectively, for ProRoot MTA and RetroMTA). ProRoot MTA caused a significantly higher color change compared with RetroMTA exposed to different irrigation solutions at each time interval (P < 0.001). Color change of both materials exposed to each irrigation solution significantly increased over time (P < 0.05). CONCLUSION: Under the condition of this ex vivo study, irrigation of dentin with NaOCl and CHX and then removing the excess solution might be ineffective in increasing the tooth color change potential of either bismuth oxide or zirconium-containing calcium silicate-based materials. Furthermore, calcium silicate-based material, which contained bismuth oxide, caused higher tooth discoloration.

How adjustment could affect internal and marginal adaptation of CAD/CAM crowns made with different materials.

PURPOSE: Recently introduced hybrid and reinforced glass ceramic computer-aided design/computer-aided manufacturing (CAD/CAM) materials have been used for full-coverage restorations. However; the effect of adjustment and type of materials on internal and marginal adaptation are unknown. This study aimed to evaluate and compare the marginal and internal adaptations of crowns made of three different CAD/CAM materials before and after adjustment. MATERIALS AND METHODS: One acrylic resin maxillary first molar was prepared and served as the master die. Thirty-six restorations were fabricated using CAD/CAM system (CEREC Omnicam, MCXL) with three materials including lithium disilicate (IPS e.max CAD), zirconia-reinforced lithium silicate (Suprinity), and hybrid ceramic (Enamic). Internal and marginal adaptations were evaluated with the reference point matching technique before and after adjustment. The data were analyzed using mixed ANOVA considering α=.05 as the significance level. RESULTS: The effect of adjustment and its interaction with the restoration material were significant for marginal, absolute marginal, and occlusal discrepancies (P<.05). Before adjustment, Suprinity had lower marginal discrepancies than IPS e.max CAD (P=.18) and Enamic (P=.021); though no significant differences existed after adjustment. CONCLUSION: Within the limitations of this study, crowns fabricated from IPS e.max CAD and Suprinity resulted in slightly better adaptation compared with Enamic crowns before adjustment. However, marginal, axial, and occlusal discrepancies were similar among all materials after the adjustment.

Effect of Serrating Abutment-Implant Mating Surface on Torque Stability of Implant-Abutment Connection, Before and After Cyclic Loading.

This study evaluated the effect of adding serration to the abutment-implant connection on torque maintenance before and after loading. Two implant systems with the same dimensions and connection design (internal 8° Morse taper octagon) were selected: one with nonserrated abutments (Simple line II) and the other one with serrated abutments (F & B). The removal torque value (RTV) was measured in 2 groups for each system: one group with one-piece abutments and the other group with 2-piece abutments, before and after cyclic loading (n = 10 in each group). The initial RTV of the abutment screw was measured with a digital torque meter. Each abutment received a cement-retained metal crown with 30° occlusal surface. Cyclic axial peak load of 75 ± 5 N was applied to the implants for 500 000 cycles at 1 Hz. The post-load RTV was then measured. Two-way and repeated-measures analysis of variance (ANOVA), and independent t test were applied to assess the effects of cyclic loading, connection design, abutment type, and their interaction on the percentage of torque loss (α = .05). Two-way ANOVA showed that serration of mating surfaces had a significant effect on torque maintenance before (P < .001) and after (P = .004) cyclic loading. Repeated-measures ANOVA also showed that loading had a significant effect on the torque loss percentage (P < .01). Comparison of the groups with t test showed that the torque loss of the serrated groups was lower than that of non- serrated groups. Despite the limitations of this study, the stability of the implant-abutment connection in the serrated design was higher than that of non-serrated group.

Comparison of Retention and Seating of Implant-Supported Hard and Soft Metal Copings.

Objectives: Implant-supported restorations are generally used for the replacement of the lost teeth. Stability against masticatory forces and proper retention are critical for optimal durability of restorations. The aim of this experimental study was to compare the retention of cobalt-chromium (Co-Cr) copings made by different techniques. Materials and Methods: Twenty-four solid abutment analogs were mounted and scanned with a desktop scanner. They were divided into two groups (n=12) and received metal copings fabricated by either soft or hard Co-Cr alloy. Soft Ceramill Sintron Co-Cr patterns were milled and sintered. Hard Co-Cr blocks were milled in a milling machine. The copings were sandblasted, polished, adjusted, and placed on the respective abutments. The frequency of adjustments was recorded for each abutment. The copings were cemented with zinc phosphate cement and underwent tensile test by a universal testing machine. The Mann-Whitney test and t-test were used to compare the two groups (α=0.05). Results: There was no significant difference in retention of copings between the experimental groups. The mean retentive force was 559.58±115.66 N and 557.13 ±130.48 N for the soft and hard metal groups, respectively (P=0.96). Considering the non-normal distribution of adjustment frequency data, the Mann-Whitney test showed that the frequency of adjustments was significantly higher in the hard metal group than the soft metal group (9.5 versus 0.1667; P<0.001). Conclusion: Although hard metal copings required more adjustments, retention of soft and hard Co-Cr copings was not significantly different.