The fit of cobalt–chromium three-unit fixed dental prostheses fabricated with four different techniques: A comparative in vitro study

Abstract

Objectives

This study sought to evaluate and compare the marginal and internal fit in vitro of three-unit FDPs in Co–Cr made using four fabrication techniques, and to conclude in which area the largest misfit is present.

Methods

An epoxy resin master model was produced. The impression was first made with silicone, and master and working models were then produced. A total of 32 three-unit Co–Cr FDPs were fabricated with four different production techniques: conventional lost-wax method (LW), milled wax with lost-wax method (MW), milled Co–Cr (MC), and direct laser metal sintering (DLMS). Each of the four groups consisted of eight FDPs (test groups). The FDPs were cemented on their cast and standardised-sectioned. The cement film thickness of the marginal and internal gaps was measured in a stereomicroscope, digital photos were taken at 12× magnification and then analyzed using measurement software. Statistical analyses were performed with one-way ANOVA and Tukey’s test.

Results

Best fit based on the means (SDs) in μm for all measurement points was in the DLMS group 84 (60) followed by MW 117 (89), LW 133 (89) and MC 166 (135). Significant differences were present between MC and DLMS ( p < 0.05). The regression analyses presented differences within the parameters: production technique, tooth size, position and measurement point ( p < 0.05).

Significance

Best fit was found in the DLMS group followed by MW, LW and MC. In all four groups, best fit in both abutments was along the axial walls and in the deepest part of the chamfer preparation. The greatest misfit was present occlusally in all specimens.

Introduction

To achieve a clinically acceptable result for fixed dental prostheses (FDPs) the fit of the construction is one important requisite for a good long-term prognosis . A study performed by Foster on 142 failed FDPs concluded that one important reason for this technical complication was an unacceptable fit. An unacceptable marginal gap could result in cement washout with subsequent biological complications such as secondary caries, periodontal problems and pulpitis .

The definition of the terminology fit varies between different studies. Further, different techniques for measuring the marginal and internal gaps are available . However, current techniques for these measurements are not optimal, as discussed by others , and reports in this field have not been entirely consistent . Even though results are somewhat inconsistent, both older and more recent studies have consistently shown that fabricated fixed prosthodontics have failed to produce an optimal fitting crown or FDP . However, with the increasing number of computer-aided design/computer-aided manufacturing (CAD/CAM) techniques in restorative dentistry , digitalised information and digitised techniques could also be a valuable resource for the future development of dentistry in the area of fit.

Today, there is no consensus regarding cement film thickness and clinical acceptance. However, long-term follow-up and laboratory studies discuss different levels of gaps for clinical acceptance . McLean and von Fraunhofer concluded that for single tooth restorations to be clinically acceptable, the maximum gap should be 120 μm. Studies also show that the longer the FDPs are, the larger is the risk of distortion . In addition, the heavier a metal construction, the more distortion is present . The latter is probably not a risk for cobalt–chromium (Co–Cr) constructions because of their low weight.

The few published studies on the fit of constructions fabricated in Co–Cr have demonstrated marginal discrepancies of 74–99 μm, with internal gaps ranging from 250 to 350 μm using laser melting technology on single crowns , and with laser-sintered Co–Cr crowns with a mean internal gap of 63 μm . Furthermore, in a recent study on cement-retained implant supported cast Co–Cr frameworks, the mean vertical misfit was 78 μm . Dental interest in Co–Cr has increased due to its low price and different fabrication methods . However, there are few published studies on properties such as biocompatibility, long term effects and the area of fit on FDPs for this material and the new fabrication methods . Several techniques for making a Co–Cr construction for fixed prosthodontics are available today, and after the conventional lost-wax technique CAD/CAM was introduced. Two fabrication methods are primarily used with this new digitised technique, either by milling the frameworks from a block of Co–Cr or by using direct laser metal sintering (DLMS) . Unlike the milling technique, DLMS sinters a metal powder in layers, which is then fused together by laser welding . The advantages of CAD/CAM techniques are simplicity, reduced costs and manufacturing time .

The aims of this in vitro study were to evaluate and compare the marginal and internal fit between three-unit FDPs in Co–Cr made using four fabrication techniques, and to conclude in which area the largest distortion on the FDPs are present. The null hypothesis was that there would be no differences in fit between the tested groups.

Materials and methods

Fabrication of models

One master model with two posterior abutment teeth, and 360° chamfer preparations with 16° total occlusal convergence (TOC), was made in epoxy resin (EpoFix Resin & EpoFix Hardner, Struers A/S, Ballerup, Denmark). To standardise impression-making, a device was constructed in type IV stone, and tracks were milled in the device for positioning of the trays. Eight silicon impressions (Colténe ® President, putty soft/light body, Colténe Whaledent, Altstätten, Switzerland) were performed on the master model with a metal frontal tray. Each impression was wet with Wax Pattern Cleaner (Jelenko Dental Health Products, NY, USA) and poured under vibration; for one master model and one working model for each method. The 32 master models ( Fig. 1 ) were fabricated using a type IV stone (GC Fujirock ® , GC Corporation, Leuven, Belgium). Eight of the working models (group Conventional lost-wax method (LW) were poured with the same type IV stone, and the remaining 24 (milled wax with lost-wax method (MW)), milled Co–Cr (MC), direct laser metal sintering (DLMS)) were poured with type IV stone (Everest ® Rock, KaVo, Biberach, Germany). All models were standardised-trimmed, and the working models to be scanned were sectioned, prepared with pins (Dowel Pin & Kunst, Edenta GmbH, AU/SG, Switzerland) and based with type III stone (Class III, BK Giulini GmbH, Ludwigshafen, Germany). The working models for the LW group were coated with one layer of Die Hardener (Stumpflack Klar, S&S Scheftner GmbH, Mainz, Germany) and five layers of Die Spacer (Stumpflack Die Spacer blau, 10 μm, S&S Scheftner GmbH). Each layer was approximately 10 μm with a total thickness of 50 μm . The dies were applied with spacer within 0.5 mm of their cervical margins.

Fig. 1
Dimensions in mm of the master model in plaster.

Fabrication of frameworks

In total, 32 three-unit Co–Cr FDPs were fabricated with four different production techniques, with eight specimens in each group. The frameworks were dimensioned with a thickness of 0.5 mm, with a mean connector area of 9 mm 2 . The outer surfaces of all frameworks were polished with a metal bur (Hartmetall-Fräser, Edenta GmbH), and the frameworks were cleaned using airborne particle abrasion (Basic Quatro IS, Renfert GmbH, Hilzingen, Germany) using 125-μm aluminum oxide with 3 bars of pressure. No other adjustments of the frameworks were performed.

Conventional lost-wax method (LW) : To obtain the same outer surface, a silicone impression of a Co–Cr framework from the method study was performed. Wax isolate (Kleen Lube, KerrLab, Orange, USA) was applied to the models. Through an occlusal opening in the silicone form, melted wax (Ultra-Waxer™, KerrLab) was poured, the form was removed and the wax was adjusted by an electric wax knife. Wax patterns were examined by two of the investigators to ensure that there were no visible gaps between the patterns and die margins. The wax patterns were connected with three 3-mm long wax spruces (Deton Ø 3 mm, Yeti Dental GmbH, Engen, Germany) on the abutments and the connector to the base of the sprue former. A ring free technique (Rapid-Ringless-System, Bego, Bremen, Germany) was used, and the wax patterns were invested with a phosphate bonded investment (GC Stellavest ® , GC Corporation) according to the manufacturer’s instructions. The patterns were casted in Co–Cr-alloy (Wirobond C; Co 61, Cr 26, Mo 6, W 5, Si < 2, Fe < 2, Ce < 2, C < 2, Bego) in an automatic vacuum and pressure casting machine (Nautilus CC, Bego). The castings were sectioned from the spruces by a grind disc (Grind Disc 3000, Forshaga, Sweden).

Milled wax with lost-wax method (MW) : The dies were read by a scanner (D-640™, 3Shape A/S, Copenhagen, Denmark). The scanner software program (DentalDesigner 2008-1, 3Shape A/S) transferred the data points into 3D CAD data. In the CAD process, modeling was performed on the digitalised abutments, and parameters in the program were given for milling in wax (LunaCast ® , ACF GmbH, Amberg, Germany). The cement film thickness was set to 50 μm with no space 0.5 mm from the margin. Data was sent to a milling center for computerised milling (Modified I-Mes Premium 4820, I-Mes Wieland, Wieland, Germany) from one piece of wax. The casting technique and adjustments were performed similarly to that described above for the LW method. However, because of the wax composition it was burned out for 10 min longer than in the LW method.

Milled Co–Cr (MC) : The CAD/CAM technique ( Fig. 2 ) was performed as described above, with adjustments for milling of Co–Cr alloy blocks (Co 64, Cr 21, Mo 6, W 6, Si 1.2, Mn 0.7, Nb 0.5, B 0.25, C 0.2, N 0.15; LunaNEM, ACF GmbH).

Fig. 2
Screen shot on CAD frameworks. CAD for milled framework (MC). Note the occlusal differences due to “drill compensation” (left), CAD for sintering (DLMS) (right).

Direct laser metal sintering (DLMS) : The same CAD technique was used in this method as described above with software adjustments for this technique ( Fig. 2 ). Data was sent for production of the frameworks with the Co–Cr powder (CoCrMoWSi; Co 63, Cr 25, Mo 5, W 6, Si 1) in a laser sintering machine (Biomain AB, Helsingborg, Sweden) with a laser processed density of 8.7 g/cm 3 . The thickness of the sintered layers was between 0.02 and 0.04 mm. The manufacturer (Biomain AB, Helsingborg, Sweden) used airborne particle abrasion using 250-μm aluminum oxide with 3 bars of pressure only at the outer surface.

Cementing and sectioning of the specimens

Frameworks were steam cleaned (Elmasteam ES3, Elma GmbH, Singen, Germany), dried, and primed (Monobond-S, Ivoclar Vivadent, Schaan, Liechtenstein) before cementation. A blue color (Dr. Oetker, Bielefeld, Germany) was mixed into the cement before the frameworks were cemented on their respective master model with dual cured resin cement (Variolink ® II, Ivoclar Vivadent). The frameworks had a pressure of 50 N during cementation with a loading device. A UV lamp (Bluephase ® , Ivoclar Vivadent) was used and excess cement was removed. The cemented frameworks on the master casts were embedded in epoxy resin for 12 h to stabilise the position, and the blocks were glued to a metal plate and screwed onto the saw device. The frameworks were sectioned with a low speed saw (IsoMet ® , Buehler LTD, Lake Bluff, USA) centrally in the mesiodistal direction ( Fig. 3 ). The half of the frameworks not left in the saw device was used to analyze the cement film thickness.

Fig. 3
Sectioned framework from the direct laser metal sintering (DLMS) group.

Analyses and measurements of the specimens

All measurements were on the cemented frameworks on the master casts. Analysis was performed using a stereo microscope (Wild M7A, Wild Heerbrugg LTD, Heerbrugg, Switzerland) that was calibrated by one experienced engineer according to the manufacturers instructions, before the study started. Furthermore digital photos (FC 420, Leica Microsystems GmbH, Wetzlar, Germany) were taken with a magnification of 12×, and analyzed in a measuring program (Leica Application Suite v. 3.3.1, Leica Microsystems GmbH). For each abutment, 11 reference measurement points were analyzed ( Fig. 4 ) . In total, 704 measurements were performed on the 32 frameworks by one blinded observer.

Fig. 4
Eleven reference points were measured on each abutment.

Statistical analysis

Differences between the four fabrication methods were submitted to one-way ANOVA and Tukey’s test on a mean of the discrepancy on all frameworks. All tests were performed with a confidence interval of 95%. Furthermore, a regression analysis was performed on fabrication method, tooth, position, adjusted cement film thickness, and measurement point to evaluate differences within these parameters. The level of significance was set at 5%.

Materials and methods

Fabrication of models

One master model with two posterior abutment teeth, and 360° chamfer preparations with 16° total occlusal convergence (TOC), was made in epoxy resin (EpoFix Resin & EpoFix Hardner, Struers A/S, Ballerup, Denmark). To standardise impression-making, a device was constructed in type IV stone, and tracks were milled in the device for positioning of the trays. Eight silicon impressions (Colténe ® President, putty soft/light body, Colténe Whaledent, Altstätten, Switzerland) were performed on the master model with a metal frontal tray. Each impression was wet with Wax Pattern Cleaner (Jelenko Dental Health Products, NY, USA) and poured under vibration; for one master model and one working model for each method. The 32 master models ( Fig. 1 ) were fabricated using a type IV stone (GC Fujirock ® , GC Corporation, Leuven, Belgium). Eight of the working models (group Conventional lost-wax method (LW) were poured with the same type IV stone, and the remaining 24 (milled wax with lost-wax method (MW)), milled Co–Cr (MC), direct laser metal sintering (DLMS)) were poured with type IV stone (Everest ® Rock, KaVo, Biberach, Germany). All models were standardised-trimmed, and the working models to be scanned were sectioned, prepared with pins (Dowel Pin & Kunst, Edenta GmbH, AU/SG, Switzerland) and based with type III stone (Class III, BK Giulini GmbH, Ludwigshafen, Germany). The working models for the LW group were coated with one layer of Die Hardener (Stumpflack Klar, S&S Scheftner GmbH, Mainz, Germany) and five layers of Die Spacer (Stumpflack Die Spacer blau, 10 μm, S&S Scheftner GmbH). Each layer was approximately 10 μm with a total thickness of 50 μm . The dies were applied with spacer within 0.5 mm of their cervical margins.

Fig. 1
Dimensions in mm of the master model in plaster.

Fabrication of frameworks

In total, 32 three-unit Co–Cr FDPs were fabricated with four different production techniques, with eight specimens in each group. The frameworks were dimensioned with a thickness of 0.5 mm, with a mean connector area of 9 mm 2 . The outer surfaces of all frameworks were polished with a metal bur (Hartmetall-Fräser, Edenta GmbH), and the frameworks were cleaned using airborne particle abrasion (Basic Quatro IS, Renfert GmbH, Hilzingen, Germany) using 125-μm aluminum oxide with 3 bars of pressure. No other adjustments of the frameworks were performed.

Conventional lost-wax method (LW) : To obtain the same outer surface, a silicone impression of a Co–Cr framework from the method study was performed. Wax isolate (Kleen Lube, KerrLab, Orange, USA) was applied to the models. Through an occlusal opening in the silicone form, melted wax (Ultra-Waxer™, KerrLab) was poured, the form was removed and the wax was adjusted by an electric wax knife. Wax patterns were examined by two of the investigators to ensure that there were no visible gaps between the patterns and die margins. The wax patterns were connected with three 3-mm long wax spruces (Deton Ø 3 mm, Yeti Dental GmbH, Engen, Germany) on the abutments and the connector to the base of the sprue former. A ring free technique (Rapid-Ringless-System, Bego, Bremen, Germany) was used, and the wax patterns were invested with a phosphate bonded investment (GC Stellavest ® , GC Corporation) according to the manufacturer’s instructions. The patterns were casted in Co–Cr-alloy (Wirobond C; Co 61, Cr 26, Mo 6, W 5, Si < 2, Fe < 2, Ce < 2, C < 2, Bego) in an automatic vacuum and pressure casting machine (Nautilus CC, Bego). The castings were sectioned from the spruces by a grind disc (Grind Disc 3000, Forshaga, Sweden).

Milled wax with lost-wax method (MW) : The dies were read by a scanner (D-640™, 3Shape A/S, Copenhagen, Denmark). The scanner software program (DentalDesigner 2008-1, 3Shape A/S) transferred the data points into 3D CAD data. In the CAD process, modeling was performed on the digitalised abutments, and parameters in the program were given for milling in wax (LunaCast ® , ACF GmbH, Amberg, Germany). The cement film thickness was set to 50 μm with no space 0.5 mm from the margin. Data was sent to a milling center for computerised milling (Modified I-Mes Premium 4820, I-Mes Wieland, Wieland, Germany) from one piece of wax. The casting technique and adjustments were performed similarly to that described above for the LW method. However, because of the wax composition it was burned out for 10 min longer than in the LW method.

Milled Co–Cr (MC) : The CAD/CAM technique ( Fig. 2 ) was performed as described above, with adjustments for milling of Co–Cr alloy blocks (Co 64, Cr 21, Mo 6, W 6, Si 1.2, Mn 0.7, Nb 0.5, B 0.25, C 0.2, N 0.15; LunaNEM, ACF GmbH).

Fig. 2
Screen shot on CAD frameworks. CAD for milled framework (MC). Note the occlusal differences due to “drill compensation” (left), CAD for sintering (DLMS) (right).

Direct laser metal sintering (DLMS) : The same CAD technique was used in this method as described above with software adjustments for this technique ( Fig. 2 ). Data was sent for production of the frameworks with the Co–Cr powder (CoCrMoWSi; Co 63, Cr 25, Mo 5, W 6, Si 1) in a laser sintering machine (Biomain AB, Helsingborg, Sweden) with a laser processed density of 8.7 g/cm 3 . The thickness of the sintered layers was between 0.02 and 0.04 mm. The manufacturer (Biomain AB, Helsingborg, Sweden) used airborne particle abrasion using 250-μm aluminum oxide with 3 bars of pressure only at the outer surface.

Cementing and sectioning of the specimens

Frameworks were steam cleaned (Elmasteam ES3, Elma GmbH, Singen, Germany), dried, and primed (Monobond-S, Ivoclar Vivadent, Schaan, Liechtenstein) before cementation. A blue color (Dr. Oetker, Bielefeld, Germany) was mixed into the cement before the frameworks were cemented on their respective master model with dual cured resin cement (Variolink ® II, Ivoclar Vivadent). The frameworks had a pressure of 50 N during cementation with a loading device. A UV lamp (Bluephase ® , Ivoclar Vivadent) was used and excess cement was removed. The cemented frameworks on the master casts were embedded in epoxy resin for 12 h to stabilise the position, and the blocks were glued to a metal plate and screwed onto the saw device. The frameworks were sectioned with a low speed saw (IsoMet ® , Buehler LTD, Lake Bluff, USA) centrally in the mesiodistal direction ( Fig. 3 ). The half of the frameworks not left in the saw device was used to analyze the cement film thickness.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on The fit of cobalt–chromium three-unit fixed dental prostheses fabricated with four different techniques: A comparative in vitro study
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