Article Type : Original Article
Title : Evaluation of Marginal Fit of Single Unit Lithium Disilicate Crown with Shoulder Finish Line by two Different Methods of Fabrication – An in Vivo study
Authors : Madhavi Selukar
Abstract : Aim- Comparative evaluation of marginal fit of single unit lithium disilicate full veneer crown with two different methods of fabrication. Study design- This study compared CAD/CAM and pressed lithium disilicate crowns in 54 patients. Marginal fit for pressed method was evaluated using Geomagics Freeform software and for the crowns fabricated with CAD CAM technology the marginal fit was pre-set in exocad software. CAD/CAM showed superior marginal fit, with data analysed using the unpaired t-test for statistical significance. Material and method- This study compared CAD/CAM and conventional pressed lithium disilicate crowns. CAD/CAM crowns were digitally designed, milled, and glazed while pressed crowns involved the use of die spacer, wax pattern fabrication and heat pressing with the help of ceramic furnace. Marginal fit of crowns fabricated with pressed method was evaluated using the replica technique, the samples were scanned using Geomagics Freeform software that helped for precise comparison. Results- The study revealed that 56% of participants were men, with most aged 36-45. CAD/CAM crowns showed better axial marginal fit (0.0019) than conventional crowns (0.0248), though this difference was not statistically significant (p = 0.12). CAD/CAM crowns showed superior precision in both dimensions, with a significantly tighter radial fit (p = 0.003), confirming its enhanced accuracy over conventional methods in crown fabrication. Conclusion- The study highlights that the CAD/CAM fabricated crowns showed superior marginal fit, precision, and efficiency. Future research should optimize systems and materials for improved outcome.
Introduction : Ceramic restorations have revolutionized modern dentistry, offering exceptional aesthetics, durability, and precise fit. Lithium disilicate crowns, fabricated using CAD/CAM systems or the traditional pressing method, have gained popularity due to their strength and natural appearance. Previously studied literature indicates that pressed lithium disilicate crowns are 11% stronger than CAD/CAM-fabricated ones1. The null hypothesis of the current study states that the crowns fabricated by conventional method show good marginal fit compared to CAD CAM. The alternate hypothesis of the current study states that the crowns fabricated by CAD CAM technology have better marginal fit over conventional method. Marginal fit is the crucial parameter for the longevity of ceramic restorations. Poor marginal adaptation can lead to biofilm build up, secondary caries compromising the Longetivity of prosthesis. Studies by Von Fraunhofer et al. and McLean et al. suggested that a marginal gap of 120 ?m is clinically acceptable. However, even minor discrepancies can allow bacterial infiltration, impacting the success of restorations2. CAD/CAM technology enhances precision by reducing human error and help in improving efficiency while fabrication of prosthesis. Digital impressions and in-office milling streamline the process in designing and fabrication, minimizing inaccuracies caused by impressions during transportation and material distortions. Hence, the conventional pressing method is technique-sensitive and prone to errors3. The IPS E.max system, a lithium disilicate-based ceramic, combines aesthetic excellence with high flexural strength (400 MPa). It is fabricated either by CAD/CAM milling of partially crystalline "blue state" ingots or through heat-pressing using pre-sintered ingots. Crystallization at 850°C strengthens the final restoration4,5,6,7. Both methods offer clinical success, but CAD/CAM systems provide improved efficiency, reducing procedural steps and enhancing accuracy in lithium disilicate crown fabrication7,8,9.
Method : The experimental study was approved by the institutional ethics committee (IEC no: DMIHER(DU)/IEC/2023/851) and registered under the Clinical Trials Registry (CTRI ref no: REF/2023/06/068926). Conducted in the Department of Prosthodontics and Crown & Bridge at Sharad Pawar Dental College, Wardha, the study included 54 patients in the age group of 18-54 years from the Vidarbha region of Maharashtra. Out of these, 12 females and 15 males were assigned to each of the two groups. The subjects were divided into 2 groups. Group A involved subject who received lithium disilicate crowns fabricated using CAD/CAM, while Group B involved crowns fabricated using the conventional pressing technique. Inclusion criteria included patients requiring full veneer maxillary incisors. Exclusion criteria included cantilever cases, mobile abutments, abutments shorter than 4mm, discoloured teeth and patients allergic to dental materials. Sample size calculation compared two means using ? = 0.05 and ? = 0.1, resulting in 27 patients per group to achieve 90% statistical power. The test used to perform statistical analysis was unpaired t test. The armamentarium for crowns fabricated with conventional methods included ceramic (lithium disilicate) ingots from Ivoclar Vivadent. Additional tools such as Bausch articulating paper (40µm thickness), PKT instruments, die spacer (10µm thickness), lubricants, investment material and ceramic furnace were used. CAD/CAM procedures utilized armamentarium like InLab laboratory scanner, Dentsply Sirona milling and sintering machine and Exocad software. The conventional pressing method involved Ivoclar Vivadent lithium disilicate pressed ingots and a ceramic furnace. This comprehensive methodology ensured accurate fabrication, evaluation, and analysis of lithium disilicate crowns in both CAD/CAM and pressed techniques. Subjects divided in group A were lithium disilicate crowns fabricated using the CAD/CAM method, while subjects categorized in Group B were crowns fabricated using the conventional/ pressed method. Patients selected from the OPD of the Prosthodontics and Crown & Bridge department were randomly assigned to these groups after meeting inclusion criteria and the confounding factors were eliminated on the basis of gender and age, the extent of intrinsic staining and TMJ disorders. An informed consent was taken from the subject. A detailed case history and diagnostic assessment was done followed by tooth preparation. Putty index was made using either the direct or indirect technique. The direct technique involved adapting heavy-body elastomeric impression material directly on the tooth, while the indirect technique included mock preparation and diagnostic wax-up before the putty index was made. Tooth preparation involving reduction of incisal, proximal, and lingual axial wall with diamond burs were used and a shoulder finish line was prepared. Impressions were taken with elastomeric impression material, followed by cast preparation10. In the CAD/CAM method, casts were scanned using a Dentsply Sirona lab scanner, and the data was saved in STL/G-code format. The digital design was created in Exocad software with virtual articulation and dynamic simulation. Crown parameters for marginal fit were predefined in the software and finish line was marked. The crown was milled from a lithium disilicate block using a 5-axis milling machine. Glazing and finishing were performed at 770°C11. For the crown fabricated with conventional method, die spacer (red die spacer, 8-12 ?m thickness) was applied on tooth surface except at margin. Die spacer was not applied at margin to attain precision around margins. Wax patterns were fabricated, invested and burned out in a ceramic press furnace at 1550°C. Lithium disilicate ingots were heat-pressed into the mould cavity and glazing was carried out at 770°C for aesthetic enhancement12. To measure marginal fit, the replica technique was employed. Light body impression material was applied over the intaglio surface of the crown, followed by a heavy body impression to retrieve the impression. The replicas were scanned, and the data was saved in STL format. Using Geomagics Freeform software, the STL files were analysed by creating cross-sections and measuring marginal and internal fit dimensions. No superimpositions were done for the images and the crows fabricated by conventional methods were used as controls. Results were documented as 2D drawings for comparison between the two groups. Fig 1, 2 and 3 shows the replica, scanned replica sample and marginal fit calculation in geomagic free form software.

Fig 1 - replica of the sample for crowns calculated by conventional method

Result : Table 1 and Graph 2 compares axial and radial marginal fit between the two methods. For the axial dimension, the CAD/CAM method showed a lower mean marginal fit value of (0.0019) compared to the conventional method (0.0248), suggesting improved precision. The unit of measurement is in mm. Table 1 represents the tabular representation of marginal fit of lithium disilicate crowns fabricated by conventional and digital method. Graph 2 represents the graphical representation of marginal fit of lithium disilicate crowns fabricated by conventional and digital method. The unit of measurement is in mm. In the radial dimension, CAD/CAM technology achieved a significant precision with a mean marginal fit of 0.0022 versus 0.0185 for the conventional method. The p-value of 0.003 confirms this difference was statistically significant, supporting the superior precision of CAD/CAM technology in the radial dimension. While, CAD/CAM technology demonstrated better precision in both dimensions, the difference in radial dimensions were statistically significant the confidence interval observed was 95%. These findings highlighted enhanced accuracy in crown fabricated with CAD CAM technology.
Discussion : The study provides a comprehensive evaluation of the marginal fit of lithium disilicate crowns fabricated using CAD/CAM technology and the conventional pressed method. These parameters are crucial for the clinical success and longevity of dental restorations, as poor fit can lead to secondary caries, periodontal issues, and eventually prosthetic failure13,14,15,16. Marginal fit is a key determinant for success of an restoration, referring to the adaptation of the crown to the prepared tooth margin. An inadequate marginal fit can result in micro leakage, plaque accumulation and failure of the prosthesis. To achieve optimal aesthetics and functionality, a shoulder finish line with an eqi-gingival margin was prepared. CAD/CAM technology demonstrated superior performance in achieving precise marginal fit. Digital impressions, unlike traditional materials, eliminate errors linked to dimensional instability and distortions during stone pouring. The CAD software enables precise crown design, and the milling process ensures consistently accurate margins. This digital workflow minimizes manual errors, enhancing prosthetic outcome17,18,19. Conventional method, despite being a reliable technique, showed variability due to manual intervention in wax pattern fabrication, investing and pressing in ceramic furnace. The wax pattern may deform during handling and shrinkage of ceramic during pressing can compromise the marginal fit. Additionally, traces of die spacer on the crown margins may further hamper marginal accuracy20,21,22,23. In the present study, CAD/CAM-fabricated crowns displayed superior marginal fit compared to conventionally pressed crowns. CAD/CAM technology achieved the desired marginal fit due to its uniform and digitally controlled cement space distribution. Although CAD/CAM fabricated crowns may occasionally face challenges such as over-contouring around the margins, these factors had minimal impact on the overall accuracy. Conversely, pressed crowns showed greater variability in marginal fit due to manual errors and die spacer traces near the margin. The study measured marginal fit in both axial and radial sections, with CAD/CAM crowns achieving consistently better outcomes. Similar studies by Kokubo et al. confirmed that CAD/CAM-fabricated crowns consistently achieved superior marginal fit compared to pressed crowns. These findings reinforced that CAD/CAM technology minimized risks of micro leakage and secondary caries, improving clinical success rates 24,25. The study results strongly supported that CAD/CAM technology is superior method for fabricating lithium disilicate crowns. This is due to its enhanced precision, minimum manual intervention resulting in reduced errors and improved marginal fit. This minimizes the risk of complications such as plaque accumulation, micro leakage, and prosthetic failure, ultimately improving long-term restorative outcomes 26,27.
Conclusion : In conclusion, CAD/CAM technology offers promising results in restorative dentistry with superior marginal fit and improved accuracy. Future research should focus on optimizing CAD/CAM systems, exploring compatible materials and evaluating long-term performance in clinical settings to enhance precision and patient care.
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