In the field of restorative dentistry, tooth defects are common and often addressed using inlay restorations. A critical factor influencing the success of these restorations is marginal microleakage, which occurs when oral fluids, enzymes, and acidic substances penetrate the microscopic gaps between the tooth structure and the bonding medium. This phenomenon can lead to secondary caries, pulpitis, and eventual restoration failure. Traditional methods like lost wax casting have been widely used for fabricating metal inlays, but they are prone to errors due to manual steps and material shrinkage. Recently, selective laser melting (SLM) has emerged as an advanced additive manufacturing technique that offers high precision and reduced human error. This study aims to compare the marginal microleakage of cobalt-chromium and titanium alloy inlays produced by SLM and lost wax casting, providing insights into the optimal fabrication method for non-precious metal inlays.
The experiment involved 40 extracted human premolars, which were stored in physiological saline at 4°C to maintain hydration. Each tooth was prepared with a standardized mesio-occlusal rectangular cavity measuring 4 mm in bucco-lingual width, 3 mm in mesio-distal width, and a depth of 2 mm above the cementoenamel junction. The axial walls were slightly diverged occlusally at 6°–8°, with rounded internal line angles and no undercuts. The teeth were randomly divided into four groups of 10 each: Group 1 – SLM cobalt-chromium inlays, Group 2 – lost wax casting cobalt-chromium inlays, Group 3 – SLM titanium alloy inlays, and Group 4 – lost wax casting titanium alloy inlays. For the lost wax casting groups, wax patterns were created, invested, and cast using conventional techniques, while the SLM groups utilized digital scans and laser melting to build the inlays layer by layer. All inlays were bonded using Single Bond 2 adhesive after cleaning with 95% ethanol.
Following bonding, the specimens underwent thermocycling to simulate oral conditions, with 500 cycles between 4°C and 60°C, each lasting 30 seconds with a 15-second interval. This process accelerates aging and tests the durability of the restoration interface. After thermocycling, the teeth were coated with nail varnish except for a 1 mm area around the restoration margins and immersed in a 0.5% basic fuchsin dye solution at 37°C for 24 hours. The specimens were then rinsed, and the dye was removed from the surface with 95% ethanol. Each tooth was sectioned mesio-distally into three halves using a precision cutter, and the sections were polished for examination under a stereomicroscope at 50x magnification. Dye penetration depths at the axial and gingival walls were measured using image analysis software, and the average values were recorded for statistical analysis.
The marginal microleakage was quantified as the depth of dye penetration in millimeters. The data were analyzed using one-way ANOVA to compare the groups, followed by post-hoc q-tests for pairwise comparisons. Paired t-tests were employed to assess differences between axial and gingival walls within the same material group. The significance level was set at P < 0.05. The results revealed that all inlay types exhibited some degree of microleakage, but the extent varied significantly between fabrication methods and wall locations.
| Inlay Material | Fabrication Method | Axial Wall Microleakage (Mean ± SD) | Gingival Wall Microleakage (Mean ± SD) |
|---|---|---|---|
| Cobalt-Chromium | SLM | 0.6896 ± 0.0824 | 1.4760 ± 0.1400 |
| Cobalt-Chromium | Lost Wax Casting | 1.7940 ± 0.0626 | 2.5855 ± 0.3200 |
| Titanium Alloy | SLM | 0.6064 ± 0.1211 | 1.5007 ± 0.1104 |
| Titanium Alloy | Lost Wax Casting | 1.5937 ± 0.0096 | 2.6177 ± 0.2038 |
The statistical analysis showed a significant effect of fabrication method on microleakage for both axial walls (F = 38.33, P < 0.05) and gingival walls (F = 90.95, P < 0.05). Pairwise comparisons indicated that SLM-fabricated inlays, whether cobalt-chromium or titanium alloy, had significantly lower microleakage than those made by lost wax casting (P < 0.05). However, there was no significant difference between SLM cobalt-chromium and SLM titanium alloy inlays (P > 0.05). Within each group, the gingival walls consistently exhibited higher microleakage than the axial walls, with paired t-tests confirming statistical significance (P < 0.01). For instance, in the lost wax casting cobalt-chromium group, the gingival wall microleakage was approximately 44% higher than the axial wall, highlighting the vulnerability of the gingival region.
To further understand these findings, we can model the microleakage depth (D) as a function of material properties and fabrication parameters. Let D be influenced by the coefficient of thermal expansion (α), casting shrinkage (S), and bonding strength (B). For lost wax casting, the shrinkage factor plays a crucial role, and the microleakage can be expressed as:
$$ D = k_1 \cdot S + k_2 \cdot \alpha \cdot \Delta T + k_3 \cdot \frac{1}{B} $$
where \( k_1 \), \( k_2 \), and \( k_3 \) are constants, and \( \Delta T \) is the temperature change during thermocycling. In contrast, for SLM, the layer-by-layer construction minimizes shrinkage, so the equation simplifies to:
$$ D = k_4 \cdot \alpha \cdot \Delta T + k_5 \cdot \frac{1}{B} $$
with \( k_4 \) and \( k_5 \) as constants. This theoretical framework explains why lost wax casting inlays, with their higher S values, exhibit greater microleakage. The inherent shrinkage in lost wax casting, often ranging from 1.5% to 2.5% for non-precious alloys, creates gaps that compromise the marginal seal. Additionally, the manual steps in lost wax casting, such as wax pattern fabrication and investment, introduce variability that SLM avoids through digital precision.
The difference in microleakage between axial and gingival walls can be attributed to the anatomical and structural variations in tooth tissues. The axial wall primarily consists of enamel, which, when acid-etched, forms microporosities that enhance mechanical bonding with resins. The bond strength (B) at the enamel interface is higher due to micromechanical interlocking, reducing microleakage. In contrast, the gingival wall often involves dentin, which has permeable tubules that facilitate dye penetration and reduce bonding efficacy. The bonding strength at the dentin interface is lower, as dentin bonding relies more on chemical adhesion and is susceptible to moisture. This can be quantified using the following relationship for microleakage in gingival walls (D_g) compared to axial walls (D_a):
$$ D_g = D_a + \Delta D \cdot \exp(-\beta \cdot B_d) $$
where \( \Delta D \) is the additional microleakage due to dentin permeability, \( \beta \) is a constant, and \( B_d \) is the dentin bonding strength. In our experiment, the use of Single Bond 2, a two-step etch-and-rinse adhesive, provided consistent bonding, but the inherent limitations of dentin adhesion led to higher gingival microleakage across all groups.
Another factor contributing to the superiority of SLM is the control over metal powder properties and laser parameters. In SLM, the laser power (P_l), scan speed (v_s), and layer thickness (t_l) are optimized to achieve dense structures with minimal porosity. The energy density (E_d) can be calculated as:
$$ E_d = \frac{P_l}{v_s \cdot t_l \cdot h_s} $$
where \( h_s \) is the hatch spacing. Higher energy density promotes better fusion and reduces defects, directly improving marginal adaptation. For lost wax casting, the process depends on mold filling and solidification dynamics, which are less controllable. The shrinkage compensation in lost wax casting is often empirical, leading to inconsistencies, whereas SLM uses digital compensation to account for thermal effects, resulting in inlays that fit more precisely without manual adjustment.
The clinical implications of these findings are significant. Lost wax casting has been a cornerstone of dental prosthetics for decades, but its susceptibility to microleakage due to shrinkage and human error necessitates careful technique and post-casting adjustments. In contrast, SLM offers a reproducible, high-precision alternative that minimizes microleakage and enhances restoration longevity. However, it is important to note that other factors, such as surface finish and adhesive protocols, also influence microleakage. Future studies should explore the interplay between fabrication methods and various adhesives to optimize clinical outcomes.
In conclusion, this study demonstrates that selective laser melting produces metal inlays with significantly lower marginal microleakage compared to traditional lost wax casting. The digital workflow of SLM reduces errors associated with wax patterns and casting shrinkage, leading to better adaptation at the tooth-restoration interface. Both cobalt-chromium and titanium alloy inlays fabricated by SLM performed similarly, indicating that material choice may be less critical than fabrication method in controlling microleakage. The gingival wall remains a challenge due to dentin-related factors, emphasizing the need for improved bonding strategies in this region. As additive manufacturing technologies like SLM advance, they hold promise for enhancing the precision and durability of dental restorations, potentially surpassing conventional methods like lost wax casting in clinical applications.
| Comparison | Test Statistic | P-value | Significance |
|---|---|---|---|
| Axial Walls: SLM vs. Lost Wax Casting | F = 38.33 | < 0.05 | Significant |
| Gingival Walls: SLM vs. Lost Wax Casting | F = 90.95 | < 0.05 | Significant |
| SLM Co-Cr vs. SLM Ti (Axial) | q = 42.39 | > 0.05 | Not Significant |
| Lost Wax Co-Cr vs. Lost Wax Ti (Gingival) | q = 15.99 | < 0.05 | Significant |
| Axial vs. Gingival in SLM Co-Cr | t = 15.38 | < 0.01 | Significant |
| Axial vs. Gingival in Lost Wax Ti | t = 11.18 | < 0.01 | Significant |
The results underscore the importance of selecting appropriate fabrication techniques to mitigate microleakage in restorative dentistry. While lost wax casting remains a viable option, its limitations in precision call for complementary approaches like SLM, especially in cases where marginal integrity is critical. Further research could integrate finite element analysis to model stress distributions at the restoration interface or explore hybrid methods that combine the benefits of both techniques. Ultimately, advancing fabrication technologies will contribute to more reliable and long-lasting dental restorations, reducing the incidence of failure due to microleakage.