Can Tooth Preparation Design Affect the Fit of CAD/CAM Restorations?

Renato Cassio Roperto, DDS, MSc, PhD; Marina Piolli Oliveira, DDS; Thiago Soares Porto, DDS, MSc; Lais Alaberti Ferreira, DDS; Lucas Simino Melo, DDS; and Anna Akkus, PhD

Modification to the classic tooth-preparation design has recently been investigated by researchers and used by clinicians for full-contour ceramic computer-aided design and computer-aided manufacturing (CAD/CAM) cases.^1-6 One alternative is a simpler, practical occlusal reduction design with a flat pulpal floor, so that the overall clinical process is faster and more predictable, which may result in less chair time. Such an improved design may reduce milling times, improve accuracy, and expedite the treatment process, enhancing overall user-friendliness of chairside CAD/CAM systems.⁷

Margin fit among other factors can impact the long-term survival of CAD/CAM restorations,8 and margin differences greater than 100 µm can cause microleakage, staining, tooth sensitivity, recurrent caries, and periodontal problems.^9,10 Several studies have investigated the margin accuracy of some CAD/CAM systems; the vertical margin gap has been compared with conventional laboratory production.^11-21 However, these studies were confined to margin designs and those authors overlooked crucial factors, such as the overall preparation design.

Objective

The present study was designed to evaluate the marginal fit of ceramic CAD/CAM crown restorations manufactured with a current CAD/CAM acquisition and milling system using two preparation protocols: flat-pulpal-floor design and curved-pulpal-floor design. First, the authors determined whether the marginal fit would be affected by the different preparation protocols and then examined if the different regions of the tooth (mesial, distal, buccal, and lingual) differ in terms of marginal fit, irrespective of the preparation protocol used.

Materials/Methods/Design

Twenty-six maxillary typodont teeth (plastic teeth) (N = 26) with unrestored and intact teeth were separated into two groups (n = 13). For group 1 (G1), or the control group, the upper left second premolars received a full-contour crown preparation for use with CAD/CAM systems. A 6° to 8° of axial wall conversion, 1.0-mm axial reduction, and a 2.0-mm flat occlusal reduction on both functional and nonfunctional cusps were utilized. For group 2 (G2), the upper left second premolars received full-contour crown preparations. A 6° to 8° of axial wall conversion, 1.0-mm axial reduction, a 1.5-mm occlusal reduction on both functional and nonfunctional cusps, and a 2.0-mm occlusal reduction on the central fossa was employed. This resulted in a curved pulpal floor with a lower central fossa area. A flat 90° shoulder margin design was used for both sets of preparations.

For both groups, preparations were made using cone-shaped diamond points followed by the use of a finishing diamond point, also cone shaped. Then, a flat-end diamond point was used to finish the gingival margin, resulting in a 1.0-mm width circumferential flat chamfer. A single investigator prepared all preparations with 0.5-mm supragingival margins. A new set of diamond points was used for every 5 prepared teeth (Figure 1).

A current chairside CAD/CAM system (acquisition and milling unit) was used in this study. For each group of teeth, the intraoral camera was recalibrated using the calibration kit. A uniform layer of antireflective spray was used on all teeth surfaces, including the surrounding soft tissue. Special care was taken to avoid overpowdering the prepped teeth and adjacent surfaces, which were then scanned with the intraoral camera. Before generating the 3D model, scanned images for both G1 and G2 were evaluated to confirm preparation metrics. A single milling unit was used for all the crown restorations utilizing feldspathic ceramic blocks in size I12.

Teeth were then submitted to undergo the micrographic silicon replica technique for margin-discrepancy evaluations. Buccal, mesial, lingual, and distal surfaces of each group were evaluated separately. The light-body silicon-based polymer was pooled in a shallow container. The tooth specimen was gently positioned into the uncured, low-viscosity polymeric mixture without full submergence and allowed to remain inside the polymer until the substrate was fully cured. Subsequently, the tooth was gently separated from the polymeric material to expose the region of interest. The polymeric imprint of a particular side of the tooth was cut in the middle with a fresh razor blade, and subsequently 1-mm-thick slices were cut from the specimen to the right and left of the midsection, yielding 8 slices overall for analysis. Each slice was carefully marked to identify its position and viewed in a reflective optical 40x magnification microscope to measure the variations in the marginal fit. A linear glass scale with 10-µm resolution was used to identify the measured length in the teeth specimens.

Data were analyzed by using statistical software. Differences in the preparation design among groups, along with the individual sites, were analyzed by 2-way analysis of variance (ANOVA) with Tukey’s post hoc test comparisons for any statistically significant differences. The significance level was α = 0.05.

Results

The mean and standard deviation associated with vertical gap (μm) related to preparation type was: flat type preparation 83.61 ± 50.72 and curved type preparation 79.04 ± 30.97. Among the sites (buccal, mesial, lingual, and distal), the curved type preparation showed less discrepancy compared with flat design. However, the 2-way ANOVA analysis did not show statistical differences between both preparation types (Table 1). Statistics associated with individual sites for flat and curved preparations are shown in Table 2. No difference was found on flat preparations among mesial, buccal, and distal sites (I < .05). However, the lingual site had no difference from the distal site but showed a statistically significant difference from the mesial and buccal sites (P < .05).

For the curved type, the analysis associated with individual sites did not demonstrate statistically significant differences (P < .05). For each preparation type, 1-way analysis with Tukey’s post hoc test was made individually to identify where the differences among the sites occurred.

Discussion

Minimally invasive dentistry is becoming more popular, and clinicians are applying some of these principles.⁴ Differences in cavity-preparation design for full-contour all-ceramic restorations can directly impact the amount of enamel and dentin reduced during tooth preparation. This study shows minimally invasive dentistry can be performed when working on CAD/CAM crowns and that less-invasive preparations, in terms of occlusal reduction (curved preparation design), should be used when preparing teeth to receive CAD/CAM full-contour restorations.

Crown preparations milled for flat-pulpal-floor preparations had a faster milling time compared with the curved ones. This can be explained by the fact that preparations with flat pulpal floor involve less data processing, therefore requiring fewer milling steps. More complex milling steps for the curved pulpal floor preparations resulted in generating similarly accurate margins. As previously reported by Cho et al,²² the quality of the final CAD/CAM restoration does not increase with the number of steps and/or bur axes; instead, it depends, to a greater extent, on digitalization, data processing, and production process. In contrast, Hamza et al²³ observed a 5-axis milling unit can improve productivity and precision by utilizing the machine’s additional axis. However, comparison between that study and the present one is difficult, as the chairside CAD/CAM machines were at least 3-axis units. A possible explanation would be the fact that chairside CAD/CAM milling units can minimize imperfections and yet be accurate when a more complex milling process is required.

The authors of the present study noted lingual and distal surfaces of teeth had greater margin discrepancies than other surfaces, possibly because indirect vision made these areas more difficult to prepare. The buccal and mesial surfaces could be easily viewed by direct access, enabling better margin quality during the preparation phase.

The authors opted to use vertical cervical marginal gap measurement because this is a common way to quantify fit accuracy.^12,24

Conclusions and Clinical Implications

Within the limitations of this in vitro study, it was concluded that similar margin accuracy was found for both curved and flat-pulpal-floor design preparations when using current CAD/CAM systems. Buccal and mesial marginal areas exhibited less discrepancy when compared with lingual and distal surface areas because of easier location and access during the preparation of the samples. It seems likely that the current chairside CAD/CAM systems (acquisition and milling units) can facilitate uniformity of restorations and eliminate unintentional variability on the part of the practitioner.

About the Authors

Renato Cassio Roperto, DDS, MSc, PhD
Associate Professor
Case Western Reserve University
School of Dental Medicine
Cleveland, Ohio

Marina Piolli Oliveira, DDS
Researcher
Case Western Reserve University
School of Dental Medicine
Cleveland, Ohio

Thiago Soares Porto, DDS, MSc
Researcher
Case Western Reserve University
School of Dental Medicine
Cleveland, Ohio

Lais Alaberti Ferreira, DDS
Researcher, Case Western Reserve University
School of Dental Medicine
Cleveland, Ohio

Lucas Simino Melo, DDS
Researcher, Case Western Reserve University
School of Dental Medicine
Cleveland, Ohio

Anna Akkus, PhD
Assistant Professor
Case Western Reserve University
School of Dental Medicine
Cleveland, Ohio

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