Cast Cobalt-chromium Research Articles

Comparison of Retention of Clasps Fabricated with Cast Cobalt Chromium and Acetal Resin in Maxillary Obturator Prosthesis: An In Vitro Study

Comparison of Retention of Clasps Fabricated with Cast Cobalt Chromium and Acetal Resin in Maxillary Obturator Prosthesis: An In Vitro Study

Application of digital impression and model in removable partial dentures for Kennedy classⅠandⅡdentition defects.

This study aimed to evaluate the application of digital impression and resin model technology in removable partial dentures (RPD) for Kennedy classⅠandⅡdentition defects. Patients with Kennedy classⅠorⅡdental defect were selected and grouped in accordance with the following denture production processes: digital impression/resin model/cast cobalt-chromium alloy framework group (group A), digital impression/resin model/laser printed titanium framework group (group B), alginate impression/plaster model/cast cobalt-chromium alloy framework group (group C), and alginate impression/plaster model/laser printed titanium framework group (group D), with 40 cases in each group. The final RPD was examined in place in the mouth, and the evaluation indicators included the retention force of clamp ring, the tightness of connector and base, and the accuracy of occlusion. The evaluation scores of each index were used for analysis on the Kruskal-Wallis rank-sum test. No statistically significant difference in the score of each index was found among the four groups in RPD. The cast cobalt-chromium alloy and laser-printed titanium framework RPD using digital impression and resin model can meet the clinical restoration requirements of patients with Kennedy classⅠandⅡdentition defects.

Comparison of Bond Strength Between Two Resin Cement Types and Additive Manufacturing or Cast Cobalt-Chromium Alloys.

To compare the bond strength of two types of resin cement to that of additive manufacturing (AM) or cast cobalt-chromium (Co-Cr) alloys. Two types of resin luting cement, composite resin and methyl methacrylate (MMA), were bonded to AM or cast Co-Cr alloys, and shear bond tests were performed after seven days of storage in distilled water at 37°C. Co-Cr alloy adhesive elements AM to the enamel surface of the labial aspect of a bovine mandibular central incisor crown were bonded with two types of resin luting cement and subjected to 1,000 cycles of storage in water for one day and 28 days or thermal cycling, followed by shear bonding tests. Residual cement on the metal and enamel surfaces after the bonding tests was evaluated using an optical microscope. The normality of the results was evaluated using statistical software Statcel4, analysis of variance, or Kruskal-Wallis test, depending on normality, and multiple comparison tests were performed using the Tukey-Kramer or Steel-Dwass tests. After one day, the shear bond strength (SBS) was 25.9 MPa for Panavia V5 (PV; Kuraray Noritake Dental Corporation, Niigata, Japan) and 23.5 MPa for Super-Bond (SB; Sun Medical Corporation, Shiga, Japan), with no significant difference between the two cement types (P > 0.05). After 28 days, the SBS decreased to 4.1 MPa for PV and 6.7 MPa for SB, showing a significant difference between the two cements (P < 0.05). Following 1,000 thermal cycles, the SBS was 2.0 MPa for PV and 5.6 MPa for SB, with SB exhibiting a significantly higher value (P < 0.05). The adhesive strength was significantly lower after 28 days of storage and thermal cycling compared to after one day of storage (P < 0.05). The Co-Cr alloy exhibited more residual cement on the enamel surface due to interfacial fracture with the resin cement. The Co-Cr alloy showed more residual cement on the enamel surface due to interfacial fracture with the resin cement. MMA-based resin cement showed optimal bond strength and may be suitable for clinical use in computer-aided design (CAD)/computer-aided manufacturing (CAM) orthodontic appliances.

Influence of Post Length, Post Material, and Substance Loss on the Fracture Resistance of Endodontically Treated Teeth: A Laboratory Study

To evaluate the effects of post length, post material, and substance loss on the fracture resistance of endodontically treated mandibular premolars. A total of 96 extracted human mandibular first premolars were endodontically treated and divided into 12 test groups (n = 8 each) based on the number of residual walls (one/two), post material (glass-fiber/titanium), and post length (5 mm, 7.5 mm, and 10 mm). After luting the posts, specimens received a composite resin core and a crown preparation with a 1.5-mm ferrule. Cast cobalt-chromium crowns were cemented using glass-ionomer cement. After 1,200,000 chewing cycles with a load of 49 N and simultaneous thermocycling (5°C to 55°C), specimens were quasi-statically loaded at 30 degrees to the longitudinal axis of the tooth until fracture. Fracture loads were analyzed using three-way, two-way, and one-way ANOVA (α = .05). Fracture modes were examined under a stereomicroscope (×25 magnification) and recorded. The mean ± SD fracture loads ranged from 642 ± 190 N (one wall, glass fiber, 5 mm) to 1,170 ± 130 N (two walls, titanium, 7.5 mm). The mean fracture load of titanium posts was significantly higher than that of glass-fiber posts (P < .001), and the 7.5-mm post length exhibited significantly higher fracture loads than groups with 5-mm and 10-mm post length (P = .008). Teeth restored with titanium posts revealed considerably higher fracture resistance than teeth restored with glass-fiber posts, especially if 7.5-mm-length posts were used.

Feasibility study and material selection for powder-bed fusion process in printing of denture clasps

Background and objectiveThe retention of selective laser melting (SLM)-built denture clasps is inferior to that of cast cobalt-chromium (Co–Cr) clasps engaging 0.01-in undercuts, which are commonly used in clinical practice. Either the clasps engage in excessively deep undercuts or inappropriate printing process parameters are applied. With appropriate undercut engagement and levels of process parameters, the retention of SLM-built clasps (including Co–Cr, commercially pure titanium [CP Ti], and Ti alloy [Ti–6Al–4V] ones) may be comparable to that of cast Co–Cr clasps. Therefore, this feasibility study aimed to evaluate their retention to guide dentists during material selection for the powder-bed fusion process during the printing of denture clasps. MethodsWe engaged the clasp arm at an appropriate undercut depth (0.01 or 0.02 in), built clasps at the orientation of their longitudinal axes approximately parallel to the build platform, generated square prism support structures at a critical overhang angle of 30°, applied optimized laser parameters (laser power, scan speed, and hatch space), and adopted annealing treatment for Co–Cr, CP Ti, and Ti–6Al–4V clasps. After postprocessing and accuracy measurement, an insertion/removal test of the clasps for 15,000 cycles was performed to simulate 10 years of clinical use, and the retentive force was recorded every 1500 cycles. Permanent deformation of the retentive arms of the clasps was measured. Cast Co–Cr clasps engaging 0.01-in undercuts were designated the control group. ResultsThe initial retentive forces of the SLM-built Co–Cr clasps engaging 0.01-in undercuts and CP Ti and Ti–6Al–4V clasps engaging 0.02-in undercuts were comparable to those of the control group. SLM-built Co–Cr clasps engaging 0.01-in undercuts and Ti–6Al–4V clasps engaging 0.02-in undercuts had similar final retentive force and less permanent deformation compared with those of the control group; SLM-built CP Ti clasps engaging 0.02-in undercuts had lower final retentive force and greater permanent deformation. ConclusionsConsidering the long-term retention and permanent deformation of the retentive arms, Co–Cr and Ti–6Al–4V alloys, except CP Ti, are recommended for printing denture clasps. SLM-built Co–Cr clasps should engage 0.01-in undercuts, and Ti–6Al–4V clasps should engage 0.02-in undercuts.

Accuracy of the landing of the original and non-original supraconstructions on implants

BACKGROUND: An integral part of modern orthopedic dentistry is the rehabilitation of patients with partial or completely missing teeth using intraosseous implant prosthetics. Despite the high success rate of implant integration, practicing dentists continue to encounter biological and technical complications during prosthetic treatment. One of the reasons for the resulting complications is inaccuracies in the design of orthopedic components. Since today the Russian dental market offers a wide range of suprastructures from different manufacturers and price categories, there is a need to compare the accuracy of their designs.&#x0D; AIM: To identify the difference between the accuracy of fit of original and non-original suprastructures presented in the Russian dental materials market by studying microgaps between these components and original Straumann implants.&#x0D; MATERIAL AND METHODS: Seven bone-level implants and six tissue-level Straumann implants were used. The original Straumann titanium bone-level and tissue-level bases were used as control superstructures. The following non-original components were used as test specimens: cast cobaltchromium bone and tissue-level bases from GeoMedi, plastic burnout bone-level abutments from NT-trading, and non-original titanium bone- and tissue-level abutments from GeoMedi, Zirkonzahn, and NT-trading. Cobalt-chromium abutments for tissue-level to a soft state were obtained from GeoMedi. Cobaltchromium specimens, namely, cast-to-fabricate and pre-smooth abutments from GeoMedi and a plastic burnout abutment from NT-trading, underwent a full technical cycle of metal-ceramic crown fabrication. All specimens were pressed in epoxy using a SimpliMet 1000 automatic hot press. Specimens were ground using a Buehler Beta-1 grinding and polishing machine with an automatic Vector attachment. Grinding was conducted layer by layer in three steps with a step of 1 mm. The contact area was examined with a Tescan Mira LMU scanning electron microscope.&#x0D; RESULTS: Segments between the implant and abutment and between the abutment and screw were considered in calculating the study results. Using a scanning electron microscope (Tescan Mira LMU), the length of the gap sections not exceeding 1 m and the length of each gap section exceeding 1 m were measured on each segment. For each gap, the area percentage was calculated, with the gap width not exceeding 1 m.&#x0D; CONCLUSION: For Straumann bone- and tissue-level implants, the original Straumann titanium bases had the highest fit accuracy, but for Straumann bone-level implants, the non-original titanium abutments from GeoMedi and Zirkonzahn also had a similar fit. The fit of the non-original cobaltchromium bases from GeoMedi and the burnout plastic abutment from NT-trading did not meet the criteria for an accurate fit of the superstructures.

Strength of Polyether Ether Ketone Composite as a Major Connector Material for Removable Partial Dentures

PEEK composite was tested mechanically as a maxillary removable partial denture (RPD) framework material instead of cast cobalt chromium alloy. Methods: Partial edentulous upper jaw cast was scanned using structured-light 3D scanner, palatal strap (PS) designs for RPD were designed by a designing dental laboratory software. CAD/CAM machine fabricated PSs patterns using their designs’ STL software files. PSs were made from poly-ether-ether ketone (PEEK) reinforced by ceramic fillers using thermal injection press technique and Co-Cr alloy using centrifugal casting, each material group was subdivided into two subgroups according to storage conditions (stored and non-stored subgroups), storage took place in deionized water for about 180 days at 37.5 ± 2 o C. All specimens were subjected to fracture resistance test using universal testing machine then maximum compression load (MCL) result values were subjected to statistical analysis. PEEK composite specimens were scanned by field emission microscope (FEM) and energy dispersive spectroscopy. Storage water of PEEK composite was analyzed using atomic absorption spectroscopy (AAS). Results: In either stored or non-stored subgroups of PEEK composite straps they showed significant lower mean MCL values than corresponding alloy subgroups (p=0.0001). FEM scanning showed fillers agglomerations in non-stored PEEK composite and their nearly absence from stored PEEK composite specimens. AAS detected Al element in PEEK composite storage water. Conclusion: Mechanically thermally injected ceramic reinforced PEEK composite could not replace cast Co-Cr alloy as PSs material for maxillary RPD. Biocompatibility concerns raised in this study due to suspected Al leaching and ceramic fillers dissolution from PEEK composite matrix.

Evaluation of Retention Force of Polyetheretherketone (PEEK) and Cast Cobalt-Chromium Circumferential Clasps: A comparative study

Aims: The aim of this in-vitro study was to compare the retention force of Polyetheretherketone (PEEK) and Cobalt-Chromium (Co-Cr) clasps by mechanical cycling test simulating ten years of use. Materials and Methods: 72 clasps samples were fabricated on a standardized e-max upper right first premolar, it is divided into 24 Co-Cr clasps, and 48 PEEK clasps which were subdivided into 24 clasps (2.4mm width), and 24 clasps (3.00mm width).Each group was further subdivided (n=8 clasps) into amounts of abutment undercut 0.25mm, 0.50, and 0.75mm undercuts. Mechanical cycling of each clasp was performed for 15000 times on its specific abutment crown. The retention force of each clasp in newton was measured every 1500 cycles by applying a tensile force using a universal tensile machine. Results: The retention force of Co-Cr clasps was higher significantly than the PEEK clasps retention force for the three undercuts. Deeper undercut showed a significantly higher retention force for both materials. All subgroups (except 0.75mm Co-Cr group) exhibit an increase in retention force after first 1500 cycles followed by gradual decrease till the end of 15000 cycles. Conclusions: The retention force of PEEK clasps with both widths along 15000 cycles of insertion/removal were lower than Co-Cr clasps but still sufficient for clinical use at 050, and 0.75mm undercut but not enough at 0.25mm undercut.

Short-term comparative evaluation of BioHPP and cast cobalt–chromium as framework for implant supported prostheses: A split mouth clinical randomized trial

Aim: The purpose of this parallel randomized clinical trial was to evaluate the effects of BioHPP versus cobalt–chromium (Co–Cr) frameworks in screw-retained implant-supported fixed dental prostheses (FDPs) on peri-implant soft and hard tissues clinically and radiographically. Materials and Methods: Fifteen patients with bilateral partial edentulism in the posterior mandible received two implants at the positions of the second premolar and second molar on both sides: one side was restored with BioHpp-based screw-retained FDP (test group) and the other side was restored with Co–Cr-based screw-retained FDP (control group). All patients were clinically examined at the time of prosthesis insertion, and 6 and 12 months later for fracture of implant or framework, fracture, or looseness of the screw, veneer chipping, and fractures, modified bleeding index, modified plaque index, peri-implant probing depth, as well as radiographically for marginal bone loss. Results: At 12 months, both groups showed no statistically significant differences in the modified bleeding index (P = 1.00), modified plaque index (P = 0.383), probing depth (P = 0.925), and marginal bone loss (P = 0.222). No technical complications occurred during the evaluation period in either group. Conclusion: BioHPP framework represents a viable nonmetallic alternative to cast Co–Cr framework, shown by good soft and hard tissue responses.

Marginal Accuracy of Milled Versus Cast Cobalt Chromium Alloys in Long Span Implant-Supported Frameworks: A Systematic Review and Meta-analysis

Aim: To assess if milled cobalt chromium (Co-Cr) alloy offers significantly better marginal accuracy than cast Co-Cr alloy for screw-retained long-span dental implant framework. Materials and Methods: A search PICO was formulated using suitable keywords and an electronic search was initiated. The databases of PubMed, Cochrane Library, Google Scholar and Embase were searched for related articles. Bibliographies of randomised control trials and reviews, identified in the electronic search, were analysed for studies published outside the electronically searched journals. Electronic search identified 26 studies. A total of 16 studies were eliminated after reading the abstracts. Out of the remaining 10 studies, 3 were eliminated based on the inclusion and exclusion criteria, and finally 7 studies were finalised for systematic review. Results Data were extracted from the included studies and analysed. The obtained data were suitable for meta-analysis, which showed an overall effect size z = 4.97 ( P &lt; .001) at 95% CI showing a significant statistical difference between milled and cast Co-Cr frameworks. Conclusion: Milled cobalt chromium frameworks are significantly more accurate than cast frameworks for long-span implant-supported frameworks used in dental prosthetic rehabilitation. More randomized controlled trials need to be conducted with a larger sample size to get a more authentic conclusion in a clinical scenario.

Evaluation of bond strength of porous cobalt-chromium metal and pressed ceramic fabricated by additive manufacturing

Statement of problemA high metal-ceramic bond strength is important for successful dental restorations. However, studies on the evaluation of the bond strength of porous metal–ceramic restorations compared with conventional ones are scarce. PurposeThe purpose of this in vitro study was to compare the bond strength of a porous metal–ceramic structure with conventional metal-ceramic restorations. Material and methodsTen porous specimens were fabricated by selective laser sintering (SLS) according to the International Organization for Standardization 9693-1:2012, 10 specimens without porosity by SLS, and 10 cobalt-chromium (Co-Cr) cast specimens without porosity. The ceramic was applied with the pressing method. A 3-point bend test was performed with a universal testing machine, and the data were statistically analyzed by using 1-way analysis of variance (α=.05). ResultsThe mean ±standard deviation bond strength of the porous group fabricated by SLS was the lowest (35.6 ±9.1 MPa), that of the group without porosity in the casting method was 43.5 ±7.8 MPa, and that of the group without porosity fabricated by SLS was the highest (47.7 ±4.5 MPa); these were statistically similar (P=.058). ConclusionsAlthough the porous structure reported the lowest bond strength among the 3 sets of test specimens, its bond strength was higher than the International Standards Organization standard of 25 MPa.

FREQUENCY OF INADEQUATE DIMENSIONS OF OCCLUSAL REST SEAT PREPARED ON 2ND MOLAR ABUTMENTS FOR CAST REMOVABLE PARTIAL DENTURES

Objective: To evaluate occlusal rest seat preparations for cobalt chromium cast removable partial denture. Materials and Methods: A cross-sectional study was carried out in the department of Prosthodontics, Khyber College of Dentistry, Peshawar starting from June 2014 to June 2015 using consecutive non-probability sampling technique. Data was collected from 127 partially dentate patients having a bounded saddle with a distal abutment as second molar with normal occlusal surface morphology and mesio-occlusal rest seats prepared on the mentioned molar teeth. Patients having partially dentate areas with no principal abutment as second molar and rest seat other than mesio-occlusal rest seat were excluded from the study. Casts were evaluated for aspects of rest seat like the general outline form of the rest seat (visually assessed from the casts), the bucco-lingual and mesio-distal dimensions were measured using a divider and scale. Depth of the rest seat was measured by placing a piece of warm modelling wax with the casts in occlusion and measuring the thickness using Iwenson gauge. Data was analyzed using SPSS 20. Results: Out of 127 patients, mean age recorded was 38 years with Standard deviation ± 2.15. Males (62%, n=79) reported more than females (38%, n=48). The most frequently reported inadequate dimension was the depth of rest seat (46%, n=58), followed by the general outline form and mesio-distal width (40%, n= 51). The bucco-lingual dimension was recorded to be inadequate in 37% (n=47) of the cases. Conclusion: Most of The participants tend to under-prepare rest seat in terms of outline form, width, length and depth.

Comparative evaluation of wear resistance of CAD-CAM zirconia and cast cobalt chromium alloy for indirect restorations against human enamel - An In Vitro study.

To comparatively evaluate the wear resistance of two different posterior indirect restorative materials against human enamel. Two different posterior indirect restorative materials of ten in each Group-I (Monolithic zirconia) (n = 10) and Group-II (Cast cobalt chromium) (n = 10) were formed into disc and used as a substrate for the wear test. Freshly extracted mandibular first premolars were used as a pin (antagonist) (n = 20). Pin-on-disc wear testing machine was used to simulate the masticatory parameters and evaluation of the wear parameters was done after 1,20,000 cycles, with load of 40N for specific duration. Data related to wear testing procedure were tabulated and evaluated. Results were statistically analyzed using Mann-Whitney test and Wilcoxon signed rank test. Mean surface roughness value (Ra value) obtained for Group-I disc test samples showed no statistical significance (P value < 0.05). Mean wear rate value of test disc samples was statistically significant between Group-I and Group-II (P value < 0.05). Mean surface roughness value was statistically significant between Groups-I and II before and after wear test (P value < 0.05). From the results obtained, it was found that the Group-I (Monolithic zirconia) exhibited higher wear resistance than Group-II (cast cobalt chromium) and causes less wear to the opposing enamel antagonist.

Cavitation erosion resistance of a non-standard cast cobalt alloy: Influence of solubilizing and cold working treatments

The wear behavior of a non-standard cast cobalt chromium alloy (Co30Cr19Fe), during vibratory cavitation test, was investigated. Low stacking fault energies (SFE) cobalt alloys deform by complex plastic deformation mechanisms, which enhance the cavitation erosion (CE) resistance, since the onset of localized stress, leading to fatigue failure and material removal, is delayed. The purpose of this work is to characterize the main operating deformation mechanisms during cavitation erosion testing of the non-standard Co30Cr19Fe alloy. As cast, solution treated, and 15% and 30% cold rolled specimens were tested. The as cast microstructure consisted of ~2 mm fcc alpha grains and hcp epsilon-martensite. The 1473 K solubilization treatment led to primary recrystallization and formation of 250 µm new grains. The 15% cold worked microstructure consisted of heavily deformed fcc phase containing deformation twins and epsilon-martensite. Ultrasonic cavitation testing was carried out, during 40 h, according to ASTM G32-09. The solubilized specimens presented the worst behavior, whereas, the 30% cold worked specimens were the most CE resistant. The CE results are discussed based on the microstructural parameters: amount of alpha fcc and epsilon-hcp phases, grain sizes, relative amounts of twinning, slip lines and strain induced martensite formation.

Formation of a nanocrystalline recrystallized layer during microabrasive wear of a cobalt-chromium based alloy (Co-30Cr-19Fe)

The tribological behavior of a cast cobalt-chromium alloy, during microabrasion testing, was investigated. The alloy's microstructure contains coarse grains of fcc cobalt rich dendrites, interdendritic segregations and almost no precipitates. The tests were done using a 0.1 g/cm3 Al2O3/distilled water slurry, 0.3 N applied load and 20 rpm rotational speed. Two-body abrasion (grooving wear) was the dominating wear micromechanism. A severely deformed nanocrystalline layer containing random oriented ∼20 nm grains was observed, immediately below the worn surface. Finite element modeling of a single Al2O3 particle scratching the Co-Cr alloy surface showed that severe plastic deformation occurs in a region 1–2 μm beneath the worn surface, giving rise to the observed nanocrystalline layer.

Precision Fit of Screw-Retained Implant-Supported Fixed Dental Prostheses Fabricated by CAD/CAM, Copy-Milling, and Conventional Methods

The aim of this study was to evaluate the effectiveness of fabrication methods (computer-aided design/computer-aided manufacture [CAD/CAM], copy-milling, and conventional casting) in the fit accuracy of three-unit, screw-retained fixed dental prostheses. Sixteen three-unit implant-supported screw-retained frameworks were fabricated to fit an in vitro model. Eight frameworks were fabricated using the CAD/CAM system, four in zirconia and four in cobalt-chromium. Four zirconia frameworks were fabricated using the copy-milled system, and four were cast in cobalt-chromium using conventional casting with premachined abutments. The vertical and horizontal misfit at the implant-framework interface was measured using scanning electron microscopy at ×250. The results for vertical misfit were analyzed using Kruskal-Wallis and Mann-Whitney tests. The horizontal misfits were categorized as underextended, equally extended, or overextended. Statistical analysis established differences between groups according to the chi-square test (α = .05). The mean vertical misfit was 5.9 ± 3.6 μm for CAD/CAM-fabricated zirconia, 1.2 ± 2.2 μm for CAD/CAM-fabricated cobalt-chromium frameworks, 7.6 ± 9.2 μm for copy-milling-fabricated zirconia frameworks, and 11.8 (9.8) μm for conventionally fabricated frameworks. The Mann-Whitney test revealed significant differences between all but the zirconia-fabricated frameworks. A significant association was observed between the horizontal misfits and the fabrication method. The percentage of horizontal misfits that were underextended and overextended was higher in milled zirconia (83.3%), CAD/CAM cobaltchromium (66.7%), cast cobalt-chromium (58.3%), and CAD/CAM zirconia (33.3%) frameworks. CAD/CAM-fabricated frameworks exhibit better vertical misfit and low variability compared with copy-milled and conventionally fabricated frameworks. The percentage of interfaces equally extended was higher when CAD/CAM and zirconia were used.

In vitro evaluation of marginal, axial, and occlusal discrepancies in metal ceramic restorations produced with new technologies

Statement of problemMarginal and axial discrepancies of metal ceramic restorations are key to their long-term success. Little information is available for metal ceramic restorations fabricated with soft metal milling and laser sintering technologies. PurposeThe purpose of this in vitro study was to compare the marginal, axial, and occlusal discrepancies in single-unit metal ceramic restorations fabricated with new production techniques with those in a single-unit restoration fabricated using a conventional technique. Material and methodsAfter the artificial tooth was prepared, impressions were made, and 40 dies were obtained. Dies were randomly divided into 4 groups (n=10). Cobalt-chromium (Co-Cr) cast (C), hard metal milled (HM), laser sintered (LS), and soft metal milled (SM) copings were fabricated. Marginal, axial, and occlusal discrepancies of these copings were measured using the silicone replica technique before and after the application of veneering ceramic. Data were analyzed with repeated measurements 2-way ANOVAs and Bonferroni post hoc tests (α=.05). ResultsSignificant differences were found in the increase of marginal discrepancy after the application of veneering ceramic in the LS group (P=.016). However, no significant differences in marginal discrepancy were found whether veneering ceramic was applied to copings before or after in the other groups (P>.05). With regard to marginal and occlusal discrepancies, significant differences were found among the production techniques (P<.001 and P<.05, respectively). No significant differences in axial discrepancies were found among the groups (P>.05). ConclusionThis in vitro study showed that metal ceramic restorations produced with HM and newly introduced SM techniques exhibited better marginal adaptations than those produced with the LS or C technique.

Influence of the Tool Geometry on the Machining of Cobalt Chromium Femoral Heads

In the industrial processing of cobalt-chromium (CoCr) cast alloys, tungsten carbide and CBN cutting materials are mainly used. Ceramic cutting materials have a high hardness and only suffer a decrease in hardness of 50 % at 1200°C compared to room temperature. This effect occurs with carbide at 800°C. In addition, the pressure resistance is temperature dependent and decreases with increasing temperature. At room temperature, the compressive strength of alumina ceramic (Al2O3) is about 3000 N/mm2 and at 1500°C only 100 N/mm2. The edge preparation has also an important influence on the chip formation, the chip transport, the tool wear, the surface integrity of the material and on the roughness of the workpiece surface. To investigate a new process, the influences of different cutting edge preparations and different cutting materials have been tested.

Effects of posterior ridge resorption and attachment wear on periimplant strain in mandibular two-implant-supported overdentures

Statement of problemProgressive resorption of the posterior residual ridge and attachment wear increase the rotational movement of mandibular 2-implant-supported overdentures (IODs). Limited information is available regarding the biomechanical effects of rotational movement on anterior implants during mastication. PurposeThe purpose of this in vitro investigation was to analyze the effects of posterior ridge resorption and attachment wear, using simulated IODs to examine periimplant strain changes under dynamic loading conditions. Material and methodsTwo dental implants were positioned in the canine regions of a mandibular edentulous cast. Two mandibular edentulous models were fabricated from the cast by using a fixture-level impression technique, and IODs reinforced with a cobalt-chromium cast framework were prepared using the laboratory models. Two different types of stud attachments (Locator and O-ring) were connected to each paired model and to the IOD. Using a dynamic load of 100 N, continuous stress-strain changes were recorded under 3 conditions: the original condition of the paired laboratory models and the IODs, following a 1-mm reduction of the posterior residual ridge support, and after performing a fatigue test to simulate attachment wear after reduction of the posterior ridge support. After these measurements, a scanning electron microscope (SEM) was used to analyze attachment wear. ResultsIn all implants, the 1-mm posterior ridge reduction and attachment wear procedure did not remarkably elevate tensile forces compared with their original condition. All stress-strain curves showed phased strain changes caused by the rotational movement of the IODs. After the fatigue test, the shape of the stress-strain curve changed from a smooth curve to a polygonal line compared with that in the previous records. The Locator matrices showed more wear than the other attachment components. ConclusionsNeither attachment wear nor an increase of the IOD rotational movement caused by an approximately 1-mm decrease in the posterior mucosal support led to significant changes in the periimplant strain in IODs.

A comparative evaluation of shear bond strength of porcelain fused to metal substructure fabricated using conventional and contemporary techniques: an in vitro study

Base metal alloys due to their low cost, are being used more often as the substructure because of their good mechanical properties, excellent metal ceramic bonding and biocompatibility. The bonding of porcelain to metal is an important point to be considered for the success of the restoration. Aim: To compare and evaluate the shear bond strength of porcelain fused to metal substructure fabricated using conventional and contemporary techniques. Methods and Material: Thirty sample discs were fabricated – 10 of cast nickel chromium alloy, 10 of cast cobalt chromium alloys and 10 of laser sintered cobalt chromium alloy. Conventionally used feldspathic porcelain was used and fired over the metal discs. These samples were placed in a specially fabricated jig, which was held in a universal testing machine. The samples were subjected to shear stress until they fractured and the readings were noted. The fractured surface of the sample was then viewed under stereomicroscope. Results: The mean shear bond strength was highest in group C (porcelain fused to laser sintered cobalt chromium), followed by group A (porcelain fused to cast nickel chromium) and group B (porcelain fused to cast cobalt chromium) which was the least. The level of significance was fixed at p < 0.05. After applying Student’s Unpaired ‘t’ test there is no significant difference in shear bond strength in group A compared with group B, highly significant in group A and group C and very highly significant in group B and group C. Conclusions: All the three groups showed adequate, but laser sintered cobalt chromium alloy had the highest shear bond strength to porcelain. Nickel chromium alloy fabricated by conventional casting method showed lesser values of the shear bond strength, followed by cobalt chromium alloy fabricated by conventional casting, which had the least shear bond strength.