Effect of repeated porcelain firing cycles on the marginal fit of cobalt chromium copings obtained by lost wax and DMLS techniques-an in-vitro comparative study

Fixed prosthodontics is the branch of prosthodontics concerned with the replacement and /or restoration of the teeth by artificial substitutes that cannot be removed from the mouth by patient. Fixed dental prostheses range from the restoration of single tooth to whole mouth rehabilitation restoring function, patient comfort, masticatory ability, maintaining the health and integrity of the dental arches and in many instances elevates the patient’s self-esteem by improving the esthetics. The pursuit of this success involves an intricate procedure called dental castings which has been an integral part of prosthodontics for more than a century. Precise marginal adaptation is necessary to achieve better mechanical, biological and esthetics prognosis of the restorations.1 Generally marginal fit and internal fit of restorations are very much influenced by clinical and laboratory factors. Clinical factors are geometry of tooth preparation, including type of finish line and degree of taper, impression materials, and lastly cement used to lute the restoration in dental office.2,3 Laboratory factors that affect marginal gap and internal gap are incompatibility of dental materials such as wax, die stone and casting investment, die spacer and the casting technique.4

One of the new techniques for fabrication of alloy copings reported in the literature is  Direct Metal Laser Sintering (DMLS) system. A typical fixed restoration, manufactured using a dental CAD system undergoes a process of scanning and design, and CAM systems are divided into those using subtractive manufacturing (SM) and those using additive manufacturing (AM) methods.5 Additive manufacturing require a layer-by-layer free-form fabrication using various materials. DMLS is a CAD/CAM based technique in which frame works and metal copings can be designed and fabricated using cobalt chromium. 6 Direct metal laser sintering (DMLS) is an additive metal fabrication technology uses a high temperature laser beam to selectively heat a substructure metal powder based on the CAD data with framework figure. Benefits of the DMLS system include easy fabrication of complex shapes, operation of an automatic system, and short working time due to removal of the procedures of fabricating a wax pattern, investing, burn out, and casting. The DMLS system could lessen metal waste by selectively shooting the required amount. . One cons of the DMLS system is the cost of the equipment. While an important condition for a successful dental prosthesis is good marginal fit, there is a minute data on the marginal fit of fixed dental prostheses manufactured by the DMLS system.7 Thus, the present study was planned to compare and evaluate the marginal fit of (Co-Cr) copings fabricated by computer aided designing /computer aided milling technique, direct metal laser sintering technique and conventional lost wax technique at three production stages

The marginal discrepancy values at each production stage—coping, porcelain build-up, were statistically analyzed and compared among the two groups: Group A (Direct Metal Laser Sintering or DMLS), and Group B (Lost-Wax technique).

At the coping stage, the mean marginal discrepancy was 20.91 ± 2.92 μm for Group A, and 65.82 ± 6.35 μm for Group B. Unpaired t test was used to find significance and suggested significant difference between both groups.At the porcelain build-up stage, mean marginal discrepancy increased slightly to 22.86 ± 2.21 μm (Group A), and 74.52 ± 4.61 μm (Group B). The differences remained highly significant among groups (p < 0.001).

Fig 1

Fig 2

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Table 1: Comparative statistics  between Group A ( DMLS) and Group   B during coping stage

**p<0.001 – highly significant difference

Table 2: Comparative statistics  between Group A ( DMLS) and Group   B during build -up stage

**p<0.001 – highly significant difference

The marginal fit of dental restorations is a crucial factor in determining the long-term clinical success of crowns. Numerous clinical failures have been linked to marginal discrepancies, with Schwartz et al.8 identifying defective margins as the cause in 11.3% of unserviceable crowns and bridges. While the conventional lost wax technique has been the standard for over a century, emerging technologies such as Direct Metal Laser Sintering (DMLS) offer the advantages of automation, reduced processing time, and potentially improved accuracy. The present study was designed to compare these two techniques—DMLS, and lost wax—by evaluating the marginal discrepancies of cobalt-chromium (Co-Cr) copings fabricated using each method at three clinically relevant stages: the coping stage, after porcelain build-up, and following cementation.

A standardized stainless steel master model was fabricated in accordance with the design recommended by Gaikwad et al.9, featuring a 6° total axial wall taper, a 6 mm axial height, and a 1 mm shoulder finish line. This design aimed to simulate a prepared tooth and was consistent with previous research advocating shoulder or heavy chamfer preparations for optimal marginal fit. Fourty Co-Cr copings were fabricated—twenty from each technique—and marginal discrepancies were measured at four standardized reference points using a stereo microscope at 100x magnification.

At the coping stage, the mean marginal discrepancy was found to be  20.91 μm for the DMLS group, and 65.82 μm for the lost wax group. The relatively high discrepancy in the lost wax group was likely due to dimensional changes in the inlay wax, including shrinkage and stress relaxation. The DMLS technique produced the lowest mean discrepancy, which can be attributed to its additive process that eliminates the casting and manual steps inherent in traditional methods.

These findings align with previously published results. McLean et al.10 reported post-casting marginal gaps in the range of 40 to 61.5 μm and established a clinically acceptable limit of 120 μm. Hung et al. similarly suggested that marginal gaps within 50 to 75 μm are acceptable in clinical practice. The current study’s findings for the CAD/CAM and DMLS techniques fall well below these thresholds. While the ADA recommends a marginal gap of 25 μm, this standard is rarely achieved in practice. Quante et al.11 noted that DMLS copings fabricated using laser sintering showed marginal gaps ranging from 76 to 93 μm, while Kim et al.12 reported values of 81.1 μm for DMLS, and 68.6 μm for lost wax. The results from the present study, particularly for the DMLS group, suggest improved accuracy due to technological advancements in laser sintering and digital design.

Following porcelain build-up, the mean marginal discrepancies increased slightly to  22.86 μm for DMLS, and 74.52 μm for lost wax. This increase in gap values is attributed to the distortion caused by the porcelain firing process. Zeng et al.13 documented marginal gap values of approximately 36 to 38 μm in SLM-fabricated copings after repeated porcelain firings. The DMLS group in this study demonstrated similar performance, suggesting the reliability of this method even after multiple firings. The five porcelain firing cycles used in this study reflect a typical clinical protocol, supporting the validity of the design.

Statistical analysis confirmed that the differences in marginal discrepancy among the two techniques remained highly significant after the porcelain build-up stage. While some studies, such as that by Kocaagaoglu et al.14, found no significant impact of porcelain firing on marginal fit, the present study clearly demonstrated an effect, especially in the lost wax group. Despite this, all marginal values remained within clinically acceptable limits. The increase in marginal discrepancy during porcelain build-up has been attributed to stress generated during the cooling phase of the firing cycle. Differences in thermal contraction between the metal coping and porcelain layer may create internal stresses that distort the restoration. Historical studies by Howell and others noted changes in marginal fit after firing, with theories suggesting that porcelain shrinkage or metal-porcelain interfacial stress plays a role in distortion. More rigid restorations are generally less susceptible to this effect.

The majority of marginal discrepancy appears to develop during the oxidation cycle, which may release residual stresses accumulated during the casting, grinding, or polishing phases. Additional discrepancy may result from the difference in contraction rates between porcelain and metal as the restoration cools from the firing temperature. Anusavice et al.15 suggested that thermal incompatibility stresses could lead to distortion, although there is some debate about whether metal-porcelain incompatibility or other factors are primarily responsible. Buchanan et al.16 observed that base metal alloys tend to develop greater marginal discrepancies compared to noble metal alloys, possibly due to thicker oxide layers formed during firing.

Intragroup analysis revealed a statistically significant increase in marginal discrepancy across the three stages in all groups.. The DMLS group also showed significant increases, particularly between the coping and cementation stages. The increase between build-up and cementation, however, was not statistically significant. The lost wax group showed a highly significant increase in marginal discrepancy at each stage, confirming that this technique is more prone to cumulative dimensional changes during processing.

The results of this study demonstrate that the DMLS technique consistently produced the lowest marginal discrepancies at all stages.. The lost wax technique, while producing the highest discrepancies, still delivered results within clinically acceptable limits. The superior performance of the DMLS group may be due to the elimination of casting procedures, manual wax-up, and investment burnout, which are inherent in the lost wax technique. DMLS consolidates metal powders at a microscopic level, enhancing precision and marginal adaptation.

As for the two stages evaluated, there was a consistent trend of increasing marginal discrepancy from the coping stage to porcelain build-up and finally to cementation, regardless of the fabrication technique used. This trend highlights the cumulative effects of multiple processing steps on the fit of the final restoration.

Each technique has its advantages and drawbacks. The DMLS technique, although expensive, minimizes material waste, removes the need for manual intervention, and offers high precision. Its wider adoption is currently limited by setup costs. In contrast, the lost wax technique remains a cost-effective and time-tested method, suitable for practice in regions with limited technological access despite its technique sensitivity and labor demands.

Some limitations must be acknowledged. The Co-Cr alloy composition varied across the three fabrication techniques, which may have influenced the results. Only marginal fit was evaluated; including internal fit analysis could have provided a more comprehensive assessment. The 100x magnification used may have limited the detection of finer discrepancies; a scanning electron microscope (SEM) might yield more accurate results. Also, only the cementation stage was standardized using a constant load, while other stages relied on finger pressure, which may introduce variability. The sample size of 20 per group and the in vitro nature of the study also limit generalizability to clinical practice. Future research with larger sample sizes, higher-resolution imaging, and in vivo studies is recommended to validate these findings.

Within these limitations, it can be concluded that marginal discrepancies of metal-ceramic copings fabricated using, DMLS, and lost wax techniques remain within clinically acceptable limits across all three production stages. Among the techniques, DMLS demonstrated the best marginal adaptation, Although the lost wax technique showed comparatively higher discrepancies, the results were still within the acceptable range. These findings support the potential for DMLS techniques to serve as viable, and in some cases superior, alternatives to conventional casting. Increased accessibility and reduced costs could further promote their adoption in routine prosthodontic practice.

Within the inherent limitations of this study, it can be inferred that the findings of this study can be used in clinical prosthetic dentistry in the following ways. Marginal discrepancy of metal ceramic copings at all the production stages, viz. coping stage, porcelain build up stage and post cementation stage were well within the clinically acceptable range of 120μm, hence either of the three can be used to obtain cast copings. Direct metal Laser sintering of Co-Cr alloy crowns seem to be the most promising technique, producing the lowest marginal discrepancy. With decreased in production cost and establishment of DMLS laboratories on a wider scale would make it the most preferred technique of obtaining cast copings. However, the repeated porcelain firings and cementation procedures do affect the fitting accuracy of metal-ceramic restorations fabricated with these technologies.

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