Wax Copings Research Articles

Modified CAD/CAM Wax: The Dimension Accuracy of Wax/Metal Copy Patterns

Background: In dentistry, different researchers in various studies have tried various methods and techniques to reuse, recycle and manage the waste of dental materials. CAD/CAM wax could be recycled effectively without any degeneration of its original properties by refreshing with new materials using simple techniques. The objective of the current study is to evaluate the dimensional accuracy of the new modified machinable dental wax for CAD/CAM purposes.
 Materials and Methods: In the present study three experimental blank discs were performed using the same technique for each type, but with different weight/compositions of dental and mineral waxes. A master die was scanned and designed for the crown copy patterns. Ten identical wax copings were milled for each experimental group (Type I, Type II, and Type III) to compare with the control (Dentify-Germany and Zotion™-China) discs. Wax patterns were cast into metal copies to assess the differences in accuracy values of the axial, proximal, and occlusal surfaces between the wax patterns and metal coping.
 Result: Non- significant differences were noticed in the thickness accuracy between the wax pattern and the metal copy of the experimental discs of Type (I) and Type (II) that were manually produced compared to the commercial discs (P>0.05). While, Type III showed statistically significant differences were noticed between the wax and metal patterns, at different surfaces except in mesial surface (P<0.05).
 Conclusion: Wax patterns fabricated using a new version of machinable wax produced by recycling some CAD/CAM wax with different fresh materials showed acceptable dimensional accuracy. Also, it seems to have few significant differences for CAD/CAM purposes in terms of patterns thickness than those obtained available and conventional waxTM.

Evaluation of the marginal fit of cobalt-chrome coping with three different fabrication.

This study was done to evaluate and compare the marginal gap of metal coping fabricated in cobalt-chrome alloy using three different techniques of fabrication Casting, Milling and Direct Metal Laser Sintering. Typhodont premolar tooth was prepared to receive a porcelain-fused-to-metal crown. Metal replica (DIE) of this prepared premolar tooth was obtained and 6 pre-designated points were marked below the finish line on the die. The die was scanned digitally and coping was designed using the CAD software and STL file was obtained. The same STL file was used to fabricate 45 copings by3 different fabrication techniques of 15each. Group-1-Milled wax copings were casted, Group-2-Direct Laser Metal Sintered Copings and Group-3-Copings Milled directly from Chrome-Cobalt block. The marginal gap of each coping was evaluated on the metal die at 6 pre-designated points under stereo microscope at 40X magnification using custom made spring loaded jig assembly specifically made for this study. A total of 270 readings in µm obtained were statisticalyl analysed by One-way ANOVA and Turkey’s multiple post hoc procedures using SPSS software. A significant difference was found in the marginal gap of the copings fabricated by the three different techniques. The least marginal gap in the direct metal laser sintered copings Group-1: 95.12 (5.77 µm) followed by milled copings Group-2: 61.32 (2.94 µm) and casted copings Group-3: 83.50 (4.29 µm). Within limitations of the study best fit was in the direct laser metal sintering (DLMS) group followed by milled copings and milled wax coping which were casted.

To Evaluate the Marginal Adaptation of Porcelain Fused to Metal Crown with Different Base Metal Alloys.

Introduction:The success of any restoration depends on the marginal seal. The adaptation of castings, luting cement, and the surface structures of the margins are all important factors in achieving marginal seal.Aim:The aim of this study was to evaluate the vertical marginal discrepancy of cast copings obtained by employing conventional casting technique with two different base metal alloys with two different finish lines before and after porcelain firing.Material and Methods:A total of forty wax copings were fabricated with stainless steel die assembly and divided into four groups with ten specimens for each metal and each finish line. Measurements were recorded from coping margin to the stainless steel die margin for vertical marginal gap recordings. Each metal coping was finished, and porcelain application was completed. Copings with porcelain were placed on their respective dies, again subjected to the same measuring microscope for checking the vertical marginal discrepancy by the same operator and results.Results:The results of the present study showed that the mean vertical marginal gaps of all the cast copings obtained in each group (G1–G8) were within clinically acceptable limits. The mean vertical marginal gap of G1 was 135.36 ± 2.30 μm, G2 was 67.22 ± 4.25 μm, G3: 39.47 ± 2.98 μm, G4: 71.00 ± 3.97 μm, G5: 109.57 ± 2.98 μm, G6: 109.57 ± 2.98 μm, and G8: 114.58 ± 2.40 μm.Conclusion:The difference in the vertical marginal gap of cast copings obtained in different groups was statistically highly significant at 0.005 level, while the difference in the vertical marginal gap of cast copings obtained at different points was statistically nonsignificant.

Effect of firing and fabrication technique on the marginal fit of heat-pressed lithium disilicate veneers

Statement of problemStudies on the fit of heat-pressed anterior lithium disilicate veneers are sparse, and whether fit is affected by glaze firing or ceramic addition is unclear. PurposeThe purpose of this in vitro study was to evaluate and compare the marginal fit of heat-pressed anterior lithium disilicate veneers (IPS e.max Press) fabricated with the staining or cutback technique by using 3-dimensional analysis. Material and methodsTwo groups of heat-pressed maxillary left central incisor lithium disilicate veneers were fabricated (n=10) differing only in core thickness and the fabrication process. The tooth preparation was standardized at 0.6 mm cervical and middle third and 0.7 mm incisal third, with 1.5 mm of incisal reduction. Group S (staining) was an anatomic contour veneer with 1 glaze firing. Group CB (cutback) had a cutback core of 0.6 mm on the cervical and middle third and 0.5 mm on the incisal areas for ceramic application with 3 firings (wash, incisal, and glaze firing). The amount of ceramic application was standardized, and all veneers had identical final dimensions. Marginal fit was evaluated at 2 stages: after pressing the copings (control) and after glaze firing or ceramic addition using the virtual replica technique and 3-dimensional analysis. The wax copings were invested, eliminated, and pressed with IPS e.max lithium disilicate high translucency ingots, and the overall marginal fit and change in marginal fit after firing were measured in the cervical, mesial, distal, and incisal areas. The Shapiro-Wilk test was used to evaluate normality. Repeated-measures ANOVAs were used to explore differences between the 2 groups (S and CB) as per time (before and after firing) for each location (cervical, mesial, distal, and incisal) (α=.05). ResultsA statistically significant change in marginal fit after firing was found for the mesial and distal areas of the CB group (P<.05). In all other areas of measurements, no statistically significant differences were found (P>.05). The mean ±standard deviation marginal fit of group S was 63 ±13 μm and 62 ±9 μm for group CB. ConclusionsThe results suggest that firing affects the marginal fit of heat-pressed anterior lithium disilicate veneers fabricated with the cutback technique but not for the staining technique.

Production tolerance of conventional and digital workflow in the manufacturing of glass ceramic crowns

ObjectivesTo measure and compare the size of the cement gap of wax and polymer copings and final glass-ceramic crowns, produced from conventional and digital workflows, one additive and one subtractive. MethodsThirty wax copings were made by conventional manual layering technique and modeling wax on stone models with spacer varnish simulating a cement spacer. The wax copings were embedded and press-cast in lithium disilicate glass ceramic. Thirty wax copings were produced by milling from a wax blank, i.e. subtractive manufacturing, and thirty polymer burn-out copings were produced by stereolithography, i.e. additive manufacturing. These copings were embedded and press-cast in lithium disilicate glass ceramic in the same manner as the conventional group. The fit of the wax/polymer copings and subsequent crowns was checked using an impression replica method. Mean values for cement gap for marginal, axial, and occlusal areas were calculated and differences were analyzed using Student’s t-test. ResultsThere were significant differences in mean values for accuracy/production tolerance among different manufacturing techniques for both production stages: wax and polymer copings and final pressed glass-ceramic crowns. In general, crowns produced from a digital additive workflow showed smaller mean cement gaps than crowns produced from a conventional workflow or a digital subtractive workflow. Additive polymer copings showed significantly smaller cement gaps than milled wax copings (p≤.001) and conventional wax copings (p≤.001) in the axial area. In the occlusal area, both additive polymer copings and conventional wax copings showed significantly smaller cement gaps than milled wax copings (p=.002 and p≤.001 respectively). Crowns produced from conventional manual build-up wax copings showed significantly larger mean cement gaps than crowns produced from milled wax and additively manufactured polymer copings in the marginal and axial areas (p≤.001). Among the crowns with smaller cement gaps, crowns produced from additively manufactured polymer copings showed significantly smaller mean cement gaps than crowns produced from milled wax in the marginal and axial areas (p≤.001). In the occlusal areas, the differences in mean cement gaps were only statistically significant between crowns produced from conventional manual build-up wax copings and crowns produced from milled wax where the latter ones showed smaller mean cement gaps (p=.025). SignificanceThe present study suggests that an additive manufacturing technique produces smaller mean cement gaps in glass-ceramic crowns than a conventional or subtractive manufacturing technique.

The evaluation and comparison of marginal adaptation in metal ceramic and all ceramic restorations fabricated by two methods: CAD/CAM and conventional

Introduction: Marginal fit is a key factor for long term clinical success through any dental restorations. Poor marginal adaptation causes cement dissolution. This can lead to dental caries, gingival irritation, periodontal diseases, and finally treatment failure. The aim of this study was measurement and comparison of marginal gap quantities in metal ceramic and all ceramic dental restorations fabricated by various methods. Methods & Materials: A total of 60 complete crowns in 6 groups (n=10) were fabricated as. Follows: Group A: Conventional metal-ceramic collarless restorations. Group B: Metal-ceramic collarless restorations with CAD/CAM wax copings and porcelain layering. Group C: Metal-ceramic collarless restorations with Ceramill Sintron metal copings and porcelain layering. Group D: All ceramic e-max. Press (lithium disilicate) restorations. Group E: All ceramic restorations with CAD/CAM zirconia copings and porcelain layering. Group F: All ceramic CAD/CAM translucent zirconia (Zolid). Replica technique and optical microscope (60 x magnifications) used to gap measurement. Mann whitney and kruskal-wallis tests used to analyze the data. Results: The lowest mean marginal gap seen in group C (29.12) and the highest mean marginal gap seen in group E(78.19)The mean marginal adaptation was better in metal ceramic restorations than all ceramic restorations and the difference was significant (P˂0.001). Conclusion: According to our study, marginal gap of metal ceramic and all ceramic restorations was clinically acceptable (less than 120 microns).

An in vitro Comparative Study to evaluate the Marginal Fit of Castings using Ring and Ringless Casting Techniques with varying Sprue Diameter

ABSTRACT Introduction The ultimate success of fixed prosthesis is to pay meticulous attention to the detailed production of the missing tooth structure. Such an accurate replication is technically demanding. Hence, the aim of the study was to evaluate the accuracy of marginal fit of castings with varying sprue diameter and using ring and ringless techniques. Materials and methods Forty standardized wax copings were fabricated on a stainless steel die and divided into two groups. The first group was cast using the metal ring investment technique; the second group was cast using the ringless investment technique. Both the groups were further divided into two groups, each based on the diameter of the sprue (groups I and III: 4.5 mm and groups II and IV: 2.5 mm). The vertical marginal gap was measured at three sites per specimen, using a digital optical microscope at ×100 magnification. The results were analyzed using two-way analysis of variance and independent t-test to determine statistical significance. Results The vertical marginal gaps of castings fabricated using the ringless technique (0.145 ± 0.072 mm) were significantly less (p &lt; 0.001) than those castings fabricated using the conventional metal ring technique (0.264 ± 0.100 mm); however, the vertical marginal gaps of the 2.5 mm sprue diameter (0.199 ± 0.092 mm) and 4.5 mm sprue diameter (0.211 ± 0.118 mm) castings were not statistically significant (p &gt; 0.05). There was also statistically significant interaction (p &lt; 0.001) between group IIA (0.120 ± 0.038 mm) and group IA (0.302 ± 0.098 mm), which was analyzed using the independent t-test. Conclusion It can be concluded that the ringless casting technique in comparison to metal ring technique produces metal castings of better marginal adaptation. Furthermore, the sprue diameter had significant difference wherein the 4.5 mm diameter produced the least vertical discrepancies in the ringless casting technique and 2.5 mm produced lesser vertical discrepancies in the metal ring. Clinical significance One of the key factors that decides the success of a metal-ceramic restoration is the marginal fit. The accurate fit of the restoration greatly minimizes the plaque accumulation, offers good mechanical properties, and reduces micro-leakage and development of secondary caries. The techniques of casting procedure and sprue diameter can have an influence on the outcome. Hence, the current research was undertaken to study the influence of these factors on marginal fit. How to cite this article Dutta T, George VT, Balakrishnan D, Narayan AI. An in vitro Comparative Study to evaluate the Marginal Fit of Castings using Ring and Ringless Casting Techniques with Varying Sprue Diameter. World J Dent 2017;8(2):109-113.

Effect of production method on surface roughness, marginal and internal fit, and retention of cobalt-chromium single crowns

Statement of problemNew production methods have been developed for metal-ceramic restorations. Different production methods may show different surface roughness and fit, which may affect retention and long-term success. PurposeThe purpose of this in vitro study was to examine 3 different production methods with regard to surface roughness, marginal and internal fit, and retention of cobalt-chromium alloy single-crown copings. Material and methodsA master abutment of a premolar mandibular tooth preparation with 4-mm height and a 0.6-mm deep 120-degree chamfer finish line with a 12-degree angle of convergence was replicated in die stone and scanned. Thirty-six cobalt-chromium alloy copings were produced using 3 different production techniques. Twelve copings were produced by laser-sintering, 12 by milling, and 12 by milled wax/lost wax. The surface microstructure of 2 copings in each group was analyzed using interferometry. The remaining 10 copings in each group were used to evaluate marginal and internal fit by using an impression material replica method, and retention was evaluated by using a uniaxial tensile force pull-off test. The copings from each test group were cemented with zinc phosphate cement onto resin abutments. Statistical analyses of differences in marginal and internal fit were performed using 1-way ANOVA and the Mann-Whitney U test. Differences in surface topography were analyzed with the Kruskal-Wallis and Mann-Whitney U tests for nonparametric data. Differences in retentive values were analyzed using the Mann-Whitney U test for nonparametric data (all α=.05). ResultsDifferences in surface microstructure were seen. The laser-sintered copings showed increased surface roughness compared with milled and milled wax/lost wax copings. Differences in marginal and internal fit were noted. Laser-sintered showed significantly smaller spaces between coping and abutment than milled wax/lost wax copings (P=.003). At the margins, laser-sintered copings showed significantly smaller spaces than either the milled wax/lost wax group (P=.002) or the milled group (P=.002). At the chamfer, laser-sintered copings showed significantly smaller spaces than milled wax/lost wax copings (P=.005). At the center of the axial walls, laser-sintered copings showed significantly smaller spaces than those in the milled wax/lost wax (P=.004) and milled copings (P=.005). No significant differences were noted between milled and milled wax/lost wax copings (P>.05). No significant differences were detected regarding retentive forces in the pull-off tests (P>.05). ConclusionsLaser-sintered Co-Cr crown copings showed increased surface roughness and better internal and marginal fit than copings produced by milling or milled wax/lost wax technique. However, the crown pull-off tests did not reveal any significant differences.

구강 내 스캐너를 사용하여 CAD/CAM으로 제작된 왁스 코핑의 적합도 평가

Purpose: The aim of this study was to evaluate the marginal and internal gap of wax copings fabricated from extraoral scanning with intraoral scanning. Methods: Fifteen study models(abutment of teeth 16) were made by PMMA. STL files of thirty abutment were prepared by intraoral scanning with extraoral scanning. Wax copings of thirty fabricated using CAD/CAM system. Marginal and internal gap of wax copings of thirty were measured by silicone replica technique and digital microscope( 140). Date were analyzed by using independent samples t-test. Results: Mean (SD) of two group were measured 55.61(27.42) for totally gap of ES group and 60.67 (33.14) for totally gap of IS group. But marginal and internal gap of two group were not differences statistically significant(p>0.05). Conclusion: Evaluation of marginal and internal gap of two group showed that no differences statistically significant and clinically acceptable results.

Marginal adaptation of four inlay casting waxes on stone, titanium, and zirconia dies

Different inlay casting waxes do not produce copings with satisfactory marginal accuracy when used on different die materials. The purpose of this study was to evaluate the marginal accuracy of 4 inlay casting waxes on stone dies and titanium and zirconia abutments and to correlate the findings with the degree of wetting between the die specimens and the inlay casting waxes. The inlay casting waxes tested were Starwax (Dentaurum), Unterziehwachs (Bredent), SU Esthetic wax (Schuler), and Sculpturing wax (Renfert). The marginal opening of the waxes was measured with a stereomicroscope on high-strength stone dies and on titanium and zirconia abutments. Photographic images were obtained, and the mean marginal opening for each specimen was calculated. A total of 1440 measurements were made. Wetting between die materials and waxes was determined after fabricating stone, titanium, and zirconia rectangular specimens. A calibrated pipette was used to place a drop of molten wax onto each specimen. The contact angle was calculated with software after an image of each specimen had been made with a digital camera. Collected data were subjected to a 2-way analysis of variance (α=.05). Any association between marginal accuracy and wetting of different materials was found by using the Pearson correlation. The wax factor had a statistically significant effect both on the marginal discrepancy (F=158.31, P<.001) and contact angle values (F=68.09, P<.001). A statistically significant effect of the die material factor both on the marginal adaptation (F=503.47, P<.001) and contact angle values (F=585.02, P<.001) was detected. A significant correlation between the marginal accuracy and the contact angle values (Pearson=0.881, P=.01) was also found. Stone dies provided wax copings with the best marginal integrity, followed by titanium and zirconia abutments. Unterziehwachs (Bredent), wax produced the best marginal adaptation on different die materials. A significant correlation was found between the marginal accuracy and the contact angle values. As the contact angle value became smaller, the marginal accuracy improved. All combinations of waxes and stone and titanium dies presented a high wettability.

Accuracy of Ringless Casting and Accelerated Wax‐Elimination Technique: A Comparative In Vitro Study

The purpose of this study was to determine whether the ringless casting and accelerated wax-elimination techniques can be combined to offer a cost-effective, clinically acceptable, and time-saving alternative for fabricating single unit castings in fixed prosthodontics. Sixty standardized wax copings were fabricated on a type IV stone replica of a stainless steel die. The wax patterns were divided into four groups. The first group was cast using the ringless investment technique and conventional wax-elimination method; the second group was cast using the ringless investment technique and accelerated wax-elimination method; the third group was cast using the conventional metal ring investment technique and conventional wax-elimination method; the fourth group was cast using the metal ring investment technique and accelerated wax-elimination method. The vertical marginal gap was measured at four sites per specimen, using a digital optical microscope at 100× magnification. The results were analyzed using two-way ANOVA to determine statistical significance. The vertical marginal gaps of castings fabricated using the ringless technique (76.98 ± 7.59 μm) were significantly less (p < 0.05) than those castings fabricated using the conventional metal ring technique (138.44 ± 28.59 μm); however, the vertical marginal gaps of the conventional (102.63 ± 36.12 μm) and accelerated wax-elimination (112.79 ± 38.34 μm) castings were not statistically significant (p > 0.05). The ringless investment technique can produce castings with higher accuracy and can be favorably combined with the accelerated wax-elimination method as a vital alternative to the time-consuming conventional technique of casting restorations in fixed prosthodontics.

Effect of Base Metal Alloys Recasting on Marginal Integrity of Castable Crowns

Base metals have a wide use in casting methods. Sometimes they are reused in laboratories which may have an adverse effect on the restoration marginal integrity. This study aimed to investigate the effect of recasting of alloys on marginal integrity of restorations. Models with two types of finishing lines shoulder bevel 45° and shoulder 135° were produced and 15 wax copings were formed on each one of them. Each group containing 15 copings was divided into three subgroups A, B and C. Group A was casted with 100% new alloy, group B with 50% new and 50% recasted alloy and group C with 100% recasted alloy. Obtained metal copings were placed on dies and marginal gap size between restoration margin and the dies finishing line was measured using metric microscope and Moticam camera in four points, buccal, lingual, mesial and distal. A significant difference in mean marginal gap size exists among three types of alloys used (p-value = 0.036). A significant difference is observed between mean marginal gap size of two types of finishing lines for different alloys (p-value = 0.001). Using 100% recasted alloy is not recommended for any of the two types of finishing lines.

The influence of short-heating-cycle investments on the quality of commercially pure titanium castings

A new short-cycle, spinel-based investment was developed to minimize the long heating cycle generally required for conventional investments for titanium castings, but the quality of castings made using this material has yet to be evaluated. The purpose of this study was to compare the Vickers hardness, castability, surface roughness, in vitro marginal misfit, and internal porosity of castings made using silica phosphate-based and spinel-based investments. The specimens were commercially pure titanium cast using silica phosphate-based investment, Rematitan Plus (RP), and spinel-based investments, Rematitan Ultra (RU) or short-heat-cycle Trinell (TR). Disc-shaped patterns (6 × 3 mm) were cast, and Vickers hardness was measured at the surface, and 50, 100, 150, and 200 μm beneath the surface (n=10). Microstructure was analyzed by optical microscopy (×100). Mesh patterns (14 × 14 × 0.5 mm) were used for castability testing (n=10). Surface roughness (Ra) was measured on disc-shaped patterns (13 × 3 mm) with a profilometer (n=8). Wax copings were cast, screwed to implant abutments, and the marginal misfit was measured using image analysis software (n=10). Internal porosity of the copings was evaluated by density comparisons (n=10). Hardness data were analyzed by 2-way ANOVA and the Tukey HSD test. Castability, surface roughness, and marginal misfit were analyzed by 1-way ANOVA and the Tukey HSD test, and internal porosity by Kruskal-Wallis and Student-Newman-Keuls tests (α=.05). RP had the highest hardness values at the surface and 50 μm beneath, but the hardness values of TR, RU, and RP were statistically equivalent at 100, 150, and 200 μm. Significant differences were seen when comparing RU and TR with RP for castability (P<.001), surface roughness (P<.001), and marginal misfit (P<.001). No significant differences were seen for internal porosity. The quality of castings made from the new investment, TR, was similar to the quality of those made using the conventional spinel-based investment, RU, and superior to those made using RP.

Adherence of investment to Au–Pd–Ag alloy using a vacuum–argon pressure casting machine

Objective. The removal of adhered investment material from marginal metal surfaces represents a possible risk of poor marginal adaptation. The aim of this work was to evaluate the effects of argon on investment adherence to the casting surface and on the alloy surface microstructure. Methods. A die of stainless steel was used. 10 wax copings were cast by vacuum–argon pressure, while 10 copings by vacuum–air pressure using a Combilabor CLG-77 A in a graphite crucible; supply was direct with curved sprues of ∅ 3.5 mm. The alloy used was Herabond. After coarse investment, removal of all specimens were investigated with scanning electron microscopy and semi-quantitative digital densitometry. Results. The specimens cast under vacuum–argon pressure showed less amount of investment adhered (unpaired t test of grouped specimens, difference 2.896×10 5 pixel; p=0.002) while the grain size on the casting surface increased significantly p=0.000 (difference 5.968 (μm). Significance. Vacuum–argon pressure supply during casting reduces the investment adherence to the metal surface, and increases the packing density and size of the grains, especially on the casting surface. These minimize the risk of marginal distortions due to casting cleaning procedures and facilitate the finishing and polishing procedures.

The effect of sprue attachment design on castability and porosity

Sprue attachment design is an important and controversial variable that can affect casting characteristics of castability and porosity. Four sprue attachment designs—straight, flared, abrupt constriction, and gradual constriction—were investigated. Standardized wax copings simulating complete veneer metal ceramic crowns with knife-edge margins were sculpted on refractory investment dies, sprued, invested, and cast with Olympia alloy. The specimens were embedded, sectioned, and polished. Castability was analyzed by measuring the width of the cast meniscus of the margins. The margin widths of the flared and the straight sprue attachment groups were significantly less than the abrupt or gradual constriction attachment group ( p < 0.05). Photomicrographs revealed discernible differences in the relative quantity and location of porosity in the sprue-coping junctions. The straight and flared sprue attachment groups were less porous than the abrupt or gradual constriction groups ( p < 0.01). Flared and straight sprue attachments optimized castability and minimized porosity.

Directly measuring the galvanic current of different metal prostheses in the oral cavity : Guo Qing Xue et al. (1987) Chin. J. Stomatol., 22, 348–350

New production methods have been developed for metal-ceramic restorations. Different production methods may show different surface roughness and fit, which may affect retention and long-term success.The purpose of this in vitro study was to examine 3 different production methods with regard to surface roughness, marginal and internal fit, and retention of cobalt-chromium alloy single-crown copings.A master abutment of a premolar mandibular tooth preparation with 4-mm height and a 0.6-mm deep 120-degree chamfer finish line with a 12-degree angle of convergence was replicated in die stone and scanned. Thirty-six cobalt-chromium alloy copings were produced using 3 different production techniques. Twelve copings were produced by laser-sintering, 12 by milling, and 12 by milled wax/lost wax. The surface microstructure of 2 copings in each group was analyzed using interferometry. The remaining 10 copings in each group were used to evaluate marginal and internal fit by using an impression material replica method, and retention was evaluated by using a uniaxial tensile force pull-off test. The copings from each test group were cemented with zinc phosphate cement onto resin abutments. Statistical analyses of differences in marginal and internal fit were performed using 1-way ANOVA and the Mann-Whitney U test. Differences in surface topography were analyzed with the Kruskal-Wallis and Mann-Whitney U tests for nonparametric data. Differences in retentive values were analyzed using the Mann-Whitney U test for nonparametric data (all α=.05).Differences in surface microstructure were seen. The laser-sintered copings showed increased surface roughness compared with milled and milled wax/lost wax copings. Differences in marginal and internal fit were noted. Laser-sintered showed significantly smaller spaces between coping and abutment than milled wax/lost wax copings (P=.003). At the margins, laser-sintered copings showed significantly smaller spaces than either the milled wax/lost wax group (P=.002) or the milled group (P=.002). At the chamfer, laser-sintered copings showed significantly smaller spaces than milled wax/lost wax copings (P=.005). At the center of the axial walls, laser-sintered copings showed significantly smaller spaces than those in the milled wax/lost wax (P=.004) and milled copings (P=.005). No significant differences were noted between milled and milled wax/lost wax copings (P>.05). No significant differences were detected regarding retentive forces in the pull-off tests (P>.05).Laser-sintered Co-Cr crown copings showed increased surface roughness and better internal and marginal fit than copings produced by milling or milled wax/lost wax technique. However, the crown pull-off tests did not reveal any significant differences.

Fixed partial dentures for periodontally involved teeth

The involved teeth are prepared for the reception of the copings. The preparations are made more or less conical in shape to compensate for the slight error that may occur in parallelism. The gingival tissue should be reduced as little as possible to allow a maximum coverage of the exposed roots. An Alginate (irreversible hydrocolloid) impression is made of all the prepared teeth, and two casts are poured: one of a refractory investment material and the other of artificial stone. A cast of the opposing teeth is made and articulated with the lower artificial stone cast (Fig. 1) . The refractory investment die is soaked in water until saturated, and the surplus water is removed by shaking it. Then, the coronal portions of the cast of the prepared teeth are dipped into a dish of molten wax and withdrawn instantly to form wax copings on all prepared teeth in the die. The thickness of the copings depends upon the temperature of the molten wax as well as the number of times the die is dipped into the molten wax. The surplus wax is cut away all around the wax copings, including the interproximal spaces to isolate each wax coping. The wax will not stick to the wet die. A flat wax wire, 0.5 to 1 mm. in thickness and 2 mm. in width, is attached on the lingual surface of each wax coping running parallel to the long axis of the tooth. The wax wire is attached at the gingival and incisal ends of each coping (Fig. 3). The backing is formed from a double thickness of 28 guage pink wax. The height of the backing should equal the length of the teeth involved. This rectangular sheet of wax is softened in a dish of warm water, and adapted to the wax wires which are attached to the lingual surfaces of the wax copings (Fig. 2). Sprues are