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Comparative evaluation of marginal fit and axial wall adaptability of copings fabricated by metal laser sintering and lost-wax technique: An in vitro study

Purpose: The present study aims to compare and evaluate the marginal fit and axial wall adaptability of Co-Cr copings fabricated by metal laser sintering (MLS) and lost-wax (LW) techniques using a stereomicroscope. Materials and Methods: A stainless steel master die assembly was fabricated simulating a prepared crown; 40 replicas of master die were fabricated in gypsum type IV and randomly divided in two equal groups. Group A coping was fabrication by LW technique and the Group B coping fabrication by MLS technique. The copings were seated on their respective gypsum dies and marginal fit was measured using stereomicroscope and image analysis software. For evaluation of axial wall adaptability, the coping and die assembly were embedded in autopolymerizing acrylic resin and sectioned vertically. The discrepancies between the dies and copings were measured along the axial wall on each halves. The data were subjected to statistical analysis using unpaired t-test. Results: The mean values of marginal fit for copings in Group B (MLS) were lower (24.6 μm) than the copings in Group A (LW) (39.53 μm), and the difference was statistically significant (P < 0.05). The mean axial wall discrepancy value was lower for Group B (31.03 μm) as compared with Group A (54.49 μm) and the difference was statistically significant (P < 0.05). Conclusions: The copings fabricated by MLS technique had better marginal fit and axial wall adaptability in comparison with copings fabricated by the LW technique. However, the values of marginal fit of copings fabricated that the two techniques were within the clinically acceptable limit (<50 μm).

Base of the die had two cylindrical projections on two sides which helped in precise orientation of the counter die [ Figure 1].

FABRICATION OF STONE DIES
The SS die was duplicated in type IV die stone (Ultra rock, Kalabhai) [ Figure 2] by polyvinylsiloxane impression material (3M ESPE, Germany). Forty die stone replicas were fabricated and checked for the fitting of SS counter die. The dies were randomly divided into two equal groups, i.e., Group A (LW) and Group B (MLS). All the dies were coated with die hardener (Heart-Man Dental Laboratory, Korea) to avoid loss of surface detail during coping fabrication.

INTRODUCTION
The marginal fit is of paramount importance for a successful fixed dental prosthesis (FDP). [1][2][3] Literature is replete with clinical trials underlining the importance of marginal accuracy for clinical success. [1,4] The axial wall adaptation affects the seating of a prosthesis in turn affecting the marginal fit, rendering it equally important. Incomplete marginal fit has been associated with the dissolution of luting cement, development of secondary caries, adverse pulpal reactions, and periodontal inflammation. The marginal fit of castings relies on perceptive tooth preparation, accurate impressions, precision castings with careful finishing, and cementation procedures. [5] Literature revealed that the clinically acceptable marginal discrepancy for a cast restoration ranges from 10 to 160 µm. [5] However, most of the authors have considered marginal discrepancies exceeding 100 µm as unacceptable. [6] Various computer-aided designing/computer-aided manufacturing (CAD/CAM)-based systems are available for rapid production of FDP and are available in dental laboratories. One such technology is the metal laser sintering (MLS). The MLS is an additive technique, based on the 3-dimensional information received from the CAD and the prosthesis is fabricated in CAM machine. The main advantage of MLS technique is that it eliminates the drawbacks of the lost-wax (LW) technique. In addition, the MLS technique renders easy fabrication of prosthesis with complex design. The technology is automated and has shorter working time due to elimination of procedures involved in the LW technique, i.e., wax pattern, investment, wax burnout, and casting works.
To consider a technique clinically acceptable, the technique has to undergo comparative evaluation of critical parameters with a gold standard. Thus, purpose of the present study was to compare and evaluate the marginal fit and axial wall adaptability of Co-Cr coping fabricated by the LW and MLS techniques.

Custom stainless steel master die
The stainless steel (SS) master die simulated a prepared crown with a 6° total axial wall taper. The axial height and occlusal diameter were 6 mm with a 90° shoulder finish line of 1 mm. Occlusal crosshairs were placed for precise reorientation of respective coping. A computer numerical control reference markings were scribed below the margin at 4 sites which were 90° apart (0°, 90°, 180° and 360°).   (Sigmadent, India) was applied on the die as well as the counter die. Standardized wax patters were made by flowing molten wax in the space between the stone die and counter die for all the samples. After the pattern wax completely solidified, counter die was removed carefully and wax pattern was inspected and carved to attain a uniform thickness of 0.5 mm; correction of defective pattern was done as well. Wax patters were invested (Bellasun, BEGO, Germany) (Bego Sol, BEGO, Germany) individually following the manufacturers instruction implementing ringless casting technique [7] to obtain Co-Cr (Wirobond, BEGO, Germany) copings for Group A.

Fabrication of Co-Cr copings by metal laser sintering technique (Group B)
The copings for Group B were fabricated using MLS technique in which individual dies were scanned (ESPE Lava scan ST scanner). Using the CAD software (Lave design software, 3M ESPE), coping design was made to be 0.5 mm in thickness and internal relief of 30 µm for each coping [ Figure 3].

Assessment of marginal fit
The copings obtained from Group A and Group B were seated on their respective dies and evaluated for marginal fit. To measure the marginal fit a stereomicroscope (Stereo Zoom S300) at ×40 magnification and image analysis software was used (Chroma Systems, India). The marginal fit was [ Figures 4 and 5] determined as the maximum distance between the margin of the die and the most apical part of the casting margin in a plane parallel to the long axis of the die. The values were recorded at 0°, 90°, 180°, and 360° for each die, respectively, and mean marginal fit value was obtained in µm for all the specimens. [8] Assessment of axial wall adaptation Using modeling wax, boxing was done of the die-coping assembly to provide uniform and adequate space for autopolymerized acrylic resin tray material (Instant tray material, Asia special, India). Proportional mixture of autopolymerizing acrylic resin was poured in the space while maintaining the position of coping over the dies. After the acrylic resin had polymerized, individual die-coping assembly was sectioned vertically through the  center by a diamond disc (DFS, Germany). Each section of individual die was finished by gently sliding over sandpaper which was laid on a flat surface to remove metal bur. Three markings were made on each axial wall, 1.5 mm above the axiomarginal line angle, and 1.5 mm below the occlusoaxial line angle. For each sample, there were in total of twelve points at which the values for axial wall adaptability were recorded in µm. Mean value was calculated for each prepared specimen [ Figure 6].

Statistical analysis
The acquired data were subjected to unpaired t-test for comparison of marginal fit and axial wall adaptability to test the level of significance between Group A and Group B.

RESULTS
In view of the current study, the null hypothesis stated that there would be no statistically significant difference between Group A and Group B when compared for marginal fit and axial wall adaptability. The copings obtained from Group A and Group B exhibited clinically acceptable values of marginal fit (<50 µm). Data from both the groups were subjected to comparative evaluation and showed a statistically significant difference (P < 0.05) [ Table 1]. The mean values for axial wall adaptability from Group A and Group B were 54.49 µm and 31.03 µm, respectively. On carrying out comparative evaluation, statistical data exhibited statistically significant difference between Group A with Group B (P < 0.05) [ Table 2].

DISCUSSION
Marginal and internal fit are considered as important criteria for clinical success of crowns and FDP. Lack of adequate fit can be potentially detrimental because of the intraoral degradation of cements which invariably causes loss of marginal seal and promotes retention of plaque and food debris. [7,9] Studies have showed that castability of Co-Cr alloy was within the range of Ni-Cr alloys and had better corrosion resistance. [10,11] In addition, Co-Cr alloys are less frequently associated with allergic reactions as compared to Ni-Cr alloys and are a common alternative for patients allergic to Nickel. [12,13] Considering these facts, in the present study, use of Co-Cr alloy can be justified and considered for crowns or FDP. [14] It is a known fact that with increase in steps of a procedure, the technique becomes more susceptible to errors. The objective of any casting procedure is to provide a metallic duplication of missing tooth structure with as much accuracy as possible. [15] A casting cannot be more accurate than the wax pattern from which it is made; thus, a flawless wax pattern should be accurately formed to have a precise casting. [16] The wax patterns can be developed either by additive technique, subtractive technique, or combination of both. [17] In the present study, the wax patterns for Group A samples were fabricated by flowing molten wax between the SS former and die followed by retrieval and carving to 0.5 mm thickness.
Ringless casting technique has proved to deliver better fitting copings when compared with conventional metal ring technique. [7] The ringless investment procedure ensured uniform expansion of the refractory mold by setting and thermal expansion. [15] Therefore, in the present study, ringless casting technique was implemented.
Problems with the fit of casting, either too large or small, can usually be traced to not following the instructions of investment manufacturer. [18] It is not possible to prescribe a single correct technique since many variables and environmental conditions are involved during the conventional LW procedure. To conclude, the casting procedure is partly empirical and a matter of routine procedure and routine procedure should be rigidly followed to achieve precise castings and consistent results. [15] The MLS is a CAD/CAM-based technology in which designing of the prostheses is done in the software, after which the information is transferred to the MLS unit followed by fabrication of prostheses. The prosthesis is fabricated by incremental layering of the special Co-Cr alloy powder of approximately 20 µm thick; the alloy particles are sintered by a high powered laser and the process is repeated till the entire prosthesis is formed.
Various studies have been conducted involving the new MLS procedure and have proved to be promising for dental applications when compared with the LW technique. [19][20][21][22]   The fit of a casting can be defined in terms of "misfit," measured at various points between the casting surface and tooth. The perpendicular measurement from the internal surface of the casting to the axial wall of the preparation is called the internal gap, and the same measurement at the margin is called the "marginal gap." [9] The vertical marginal misfit measured parallel to the path of withdrawal of the casting is called the vertical marginal discrepancy. [23] The literature revealed that the range of clinically acceptable marginal discrepancy for cast restoration was from 10 to 160 µm. [24] Previous studies concerning different materials and techniques resulted in a wide range of reported values of marginal and internal fit. [25][26][27][28][29] Various authors have evaluated the marginal accuracy of cast and CAD/CAM-fabricated crowns. [21,30,31] Most clinicians would be contended with marginal openings of 50 µm or less and probably of 100 µm clinically acceptable. [32] Others stated that marginal discrepancies in the range of 100 µm seem to be clinically acceptable with regard to the longevity of the restorations. [33][34][35][36][37] Data from the present study showed that copings from both groups had achieved the marginal fit well within 50 µm (mean marginal discrepancy of copings in Group A was 40.79 µm and that of Group B was 24.46 µm). The results were statistically significant between the two groups (P < 0.05).
It is said that restorations need a theoretical luting cement film thickness of 20-40 µm. [38] The cement space is critical as the luting agent will impart hydraulic pressure between the tooth and the restoration rendering incomplete seating with marginal discrepancy greater than before cementation. If the axial walls are not well relieved, there will be premature contact, further preventing the seating of coping. [39] In a study, it was observed that when the luting space was set to 10 µm, the marginal gaps of the crowns were greater than when it was set to 30 or 50 µm. [31,40] There are different ways to study and analyze the fit of dental restorations. [9,20,41,42] The die-coping assembly in the present study was embedded in autopolymerizing acrylic resin tray material and sectioned longitudinally followed by recording the observations. The intended area for observation was decided because when a die spacer is used, it is painted 1-1.5 mm above the marginal line angle. Thus, this area is more critical, i.e., vertical walls in the relief for internal fit.
The results for axial wall adaptation were in accordance with previous studies for both the groups (mean axial wall adaptation for Group A 54.49 µm and Group B 31.06 µm respectively) and there was a statistically significant difference between the two groups (P < 0.05). However, currently, there is no consensus of the clinically acceptable cement film thickness in FDP. [43] The present study was an in vitro study conducted under controlled conditions; thus, the results achieved in dental laboratory will vary. Within the limitations of the study, the copings fabricated by MLS technique were found to have less marginal discrepancy and consistent axial wall adaptability values as compared with copings fabricated by LW technique. However, further studies are needed to evaluate the parameters after the ceramic layering procedure. In the present study, copings were fabricated on standardize dies; therefore, there is also a need for addition investigations pertaining to a clinical situation.

CONCLUSIONS
The results obtained in the present study assured that the MLS technique could be an alternative for conventional LW technique,; however, the LW technique will still be considered as a gold standard for comparing the new techniques introduced in the field of dental laboratories to fabricate FPDs, as it has still stood the test of time and is widely practiced.

Financial support and sponsorship
Nil.

Conflicts of interest
There are no conflicts of interest.