Key words: direct metal laser sintering, CAD/CAM, cobalt chromium copings, marginal fit, thickness.

Introduction:

The accuracy of marginal fit and the uniformity of coping thickness are fundamental determinants of the long-term success of fixed dental prostheses. Inaccurate adaptation at the margins of restorations can lead to microleakage, plaque accumulation, secondary caries, and periodontal breakdown, thereby compromising clinical outcomes and patient satisfaction. Similarly, variations in coping thickness influence the strength of the restoration, aesthetic performance of veneering porcelain, and overall biomechanical stability.^1,2

Traditionally, the lost-wax casting technique was the mainstay for fabricating metal copings. Although widely used, this method is prone to inaccuracies due to multiple manual steps and technical sensitivity. With advancements in digital dentistry, such as subtractive manufacturing technique like CAD/CAM milling and additive manufacturing technique like DMLS, have emerged as alternatives that promise superior precision, reproducibility, and efficiency but the necessity to compare and analyse the best among these methods lead to this study.^3-5

The CAD/CAM system fabricates copings by milling a pre-sintered or solid metal block based on digital designs generated through intraoral or laboratory scanners. This subtractive method ensures standardized production with reduced operator dependence. However, limitations such as tool wear, difficulty in replicating intricate geometries, and material wastage may affect marginal adaptation and uniformity of thickness.^6,7

DMLS, an additive manufacturing technique, builds copings layer by layer using a high powered laser to sinter cobalt-chromium powder according to CAD designs. The layer-wise fabrication allows for accurate replication of complex anatomical features, minimal material waste, and potentially enhanced marginal precision. Studies have demonstrated that additive techniques like DMLS can overcome the shortcomings of subtractive milling, providing restorations with improved fit and predictable thickness.^8-10

Cobalt-chromium alloys continue to be the material of choice for metal copings in fixed prosthodontics owing to their excellent mechanical strength, corrosion resistance, and biocompatibility. The introduction of digital fabrication technologies has expanded the potential of these alloys by reducing errors and ensuring clinical acceptability of restorations.^11-14 Given the clinical relevance of marginal adaptation and coping thickness, it is imperative to compare the performance of different fabrication methods. Although multiple studies have evaluated CAD/CAM and DMLS techniques individually, comparative data regarding Co-Cr copings remain limited. This in vitro study was therefore designed to assess and compare the marginal fit and thickness of Co Cr copings fabricated using CAD/CAM milling and DMLS, with the null hypothesis that there would be no significant difference between the two methods of fabrication.

Materials and Methods:

An experimental study was conducted to compare the marginal fit and thickness of cobalt-chromium (Co-Cr) copings fabricated by CAD/CAM milling and direct metal laser sintering (DMLS). This study was done in an invitro setting over a period of six months from the procurement of materials, fabrication of the copings, statistical analyses and arriving at the final result. The ethical clearance for this study was obtained from the Institutional Ethical Committee (No:21/ETHICS/2023).

A standardized typodont molar tooth was prepared with a chamfer finish line and a 6° taper according to accepted prosthodontic guidelines by Shillingburg. The tooth was scanned with a laboratory scanner (Trios Scanner, 3Shape, Denmark) to obtain a STL file. A coping was designed using CAD software (Exocad, 3Shape, Germany).¹⁵

The sample size was calculated by using OpenEpi software by which the mean and standard deviation for the sample size calculation were taken from the study done by Gunsoy S et al., the sample size of 16 per group was achieved.⁸

A total of 32 Co-Cr copings were fabricated and divided into two groups:

Group I (CAD/CAM): 16 copings milled from CoCr alloy blocks (CORiTEC CoCr) using a 5-axis milling unit (CoriTec 350i, Imes-icore GmbH, Germany). [Figure 1]



Group II (DMLS): 16 copings fabricated layer by layer from Co-Cr metal powder (EOS Co-Cr) using a DMLS machine (EOS M100, EOS GmbH, Germany). [Figure 2]

Each coping was seated on the master die and evaluated under a stereomicroscope (Leica M205, Germany) at 30X magnification. Marginal gaps were measured at four predetermined sites—mid-buccal, mid-palatal, mid-mesial, and mid-distal. The mean value for each coping was calculated and expressed in micrometres (µm).^3-6

Coping thickness was measured at the same four sites using a digital thickness gauge (Kreoplin, Germany). Values were recorded in millimetre (mm), and mean thickness for each coping was calculated.⁸

Data were entered in Microsoft Excel and analysed using IBM SPSS (Version 25.0, IBM Corp., USA). Descriptive statistics (mean ± SD) were calculated. Independent t-tests were used for intergroup comparisons, with a significance level set at p < 0.05.

Results:

A total of 32 Co-Cr copings were evaluated: 16 fabricated by CAD/CAM milling and 16 by DMLS.



The mean marginal gap values of CAD/CAM copings were significantly higher compared to DMLS copings at all four measurement sites (p < 0.05). The mean marginal discrepancy for CAD/ CAM ranged from 88.9 to 90.1 ± 4.2 µm, whereas for DMLS it ranged from 65.4 to 67.0 ± 5.6 µm. All values fell within the clinically acceptable range (<120 µm). [Table 1] [Figure 3]



The mean thickness of DMLS copings was significantly lower and more uniform than that of CAD/CAM copings (p < 0.0001). CAD/CAM copings exhibited a mean thickness between 0.415–0.425 mm, while DMLS copings showed more consistent values of 0.351–0.362 mm across all sites. [Table 2] [Figure 4]

Marginal fit: DMLS copings had significantly better adaptation than CAD/CAM.

Thickness: DMLS copings demonstrated more uniform and consistent coping thickness.

Discussion

The present study evaluated and compared the marginal fit and thickness of cobalt-chromium metal copings fabricated by CAD/CAM milling and DMLS methods. Within the limitations of this in vitro design, the results showed that DMLS copings exhibited significantly superior marginal fit and more uniform thickness compared to CAD/CAM copings.^16-20

Marginal adaptation is a key determinant of the longevity of fixed prostheses. Poor marginal fit predisposes to microleakage, secondary caries, and periodontal disease. The results of this study demonstrated mean marginal gaps of 65–67 µm for DMLS and 88–90 µm for CAD/CAM copings, both within the clinically acceptable range of ≤120 µm.

These findings are consistent with previous studies. Vojdani et al. and Lövgren et al. reported that laser-sintered copings showed smaller marginal discrepancies compared to milled or conventionally cast copings. Similarly, Park et al. and Gautam et al. observed that DMLS provided improved marginal adaptation compared to casting techniques. In contrast, Doddy et al. reported superior accuracy for CAD/CAM zirconia copings compared to DMLS Co-Cr copings, highlighting that material type and processing parameters may influence outcomes.^1-5

The improved adaptation of DMLS in the present study can be attributed to its additive manufacturing process, where copings are built layer by layer, minimizing distortions associated with milling burs or casting shrinkage.

Uniform coping thickness is critical for ensuring adequate strength and providing sufficient space for veneering porcelain. In this study, DMLS copings demonstrated consistent thickness (0.35–0.36 mm), whereas CAD/CAM copings showed slightly higher and more variable values (0.41–0.42 mm).

Shah et al. highlighted that coping thickness directly influences marginal adaptation, especially after porcelain firing cycles. Our findings support this by demonstrating that the precision of layer-by-layer fabrication in DMLS ensures dimensional stability and reproducibility. Kim et al. and Zeng et al. similarly reported that selective laser sintering and melting techniques produce copings with uniform thickness and acceptable marginal accuracy.^6-8

The superior performance of DMLS copings in both fit and thickness has practical significance:

Better marginal adaptation reduces microleakage and risk of secondary caries.

Uniform thickness ensures structural durability and optimal esthetic veneering space.

Additive manufacturing enables fabrication of complex geometries, offering customized restorations.

Thus, DMLS may be a preferred option for posterior crowns and multi-unit fixed prostheses where precision and durability are paramount.

The strengths of this study are the use of standardized preparation, fabrication, and evaluation methods, which minimized variability. Both stereomicroscope and thickness gauge measurements allowed objective evaluation.

Limitations include the in vitro design, which does not replicate intraoral factors such as saliva, occlusal loading, and thermal cycling. Additionally, only a single alloy (Co-Cr) and coping design were evaluated. Future research should incorporate ceramic veneering, long term mechanical testing, and clinical follow-up to validate these findings.

Overall, the findings of this study align with the majority of published literature indicating that DMLS provides enhanced marginal adaptation compared to subtractive and conventional techniques. However, the clinical acceptability of CAD/CAM copings remains high, and the choice of technique may depend on laboratory infrastructure, cost, requirements.^21-3

Conclusion

Within the limitations of this in vitro study, cobalt chromium copings fabricated using direct metal laser sintering (DMLS) demonstrated superior marginal adaptation and more uniform coping thickness compared to those fabricated using CAD/CAM milling. Both techniques produced values within clinically acceptable ranges; however, the enhanced precision and consistency of DMLS suggest it may offer better clinical outcomes for fixed prosthodontic restorations.

References