Marginal Integrity of Glass Ceramic Laminate Veneers Produced Through Different CAD/CAM Milling Protocols: In-Vitro Study

Statement of the problem: The conservative nature of the labial ceramic veneers necessitates minimum amount of tooth reduction with thin peripheral margins. Since construction time of restorations is becoming a determinant factor together with accuracy, the effect of three different CAD/CAM milling protocols that depend on the time factor deserves to be assessed.
Purpose: The purpose of this investigation was to evaluate the effect of three different CAD/CAM milling protocols; namely normal, fast, and two-step milling on the peripheral marginal accuracy of glass-ceramic laminate veneers.
Materials and Methods: A maxillary central incisor acrylic tooth was chosen to serve as a die for veneer construction. Acrylic tooth preparation was done to receive incisal feather edge design for the ceramic veneer. A total of thirty laminate veneer restorations were constructed from IPS e.max CAD glass ceramic using the CAD/CAM technology with three different milling protocols. The veneers were divided into the following groups according to the selected milling protocol;
Group 1: Ten veneers constructed using the normal milling protocol, Group 2: Ten veneers constructed using the fast milling protocol, Group 3: Ten veneers constructed using the two-step milling protocol. The milled veneers were subjected to crystallization process according to manufacturer’s instructions then they were checked on their corresponding die for proper adaptation and seating. For proper assessment of the vertical marginal gap, the veneers were fixed on the master die using one drop of adhesive placed centrally to stabilize the veneers in their place during the measuring procedure. The vertical marginal gap distance of each ceramic was measured using a stereomicroscope at magnification X45. The measurements were done along the peripheral circumference for all the veneer margins (mesial, distal, cervical and incisal). Measurement at each point was repeated five times. A digital image analysis system was used for assessing and evaluating vertical marginal gap width. The mean vertical marginal gap distance was calculated and then tabulated for the statistical analysis of the data. Results: Effect of milling protocol: The two-step milling protocol showed the statistically significant lowest mean marginal gap distance (33.2 ± 6.3 μm) while both the normal and fast milling protocols showed the statistically significant highest mean marginal gap distances with no statistically significant difference between them. (40 ± 7.5 μm and 41.2 ± 4.9 μm respectively) Effect of surface: The cervical surface recorded the statistically significant lowest mean marginal gap distance (34 ± 6.5 μm) while the distal surface recorded the highest mean marginal gap distance (41.6 ± 8.6 μm) with non-statistically significant difference from incisal (37.2 ± 4.6 μm) and mesial surface (39.7 ± 6.4 μm).
Conclusions: Within the limitations of this in-vitro study, the following conclusions were drawn: 1) Marginal discrepancy of all groups fell within the clinically acceptable value, thus the three milling protocols can be safely used. 2)Veneers fabricated using the two-step milling protocol exhibited the least marginal discrepancy among other experimental groups, thus it can be used as the protocol of preference if restoration accuracy is of utmost importance. 3) Since there was no significant difference between normal and fast milling protocols regarding marginal discrepancy, therefore fast milling can be recommended for veneer fabrication if chairside time is of prime importance in some cases. 4) Cervical marginal discrepancy was found to have the least value compared to other surfaces