Abstract

Purpose: To evaluate the fracture resistance of zirconia surveyed crowns with different rest seat designs.

Materials and methods: Forty surveyed crowns (n = 20 / group) were made with different rest seats as follows: group 1; zirconia-based material with narrow media-occlusal rest seat; group 2; zirconia-based material with wide media-occlusal rest seat. Then 3M RelyX Luting Cement was used to bond the printed crowns to resin dies. Using Instron 5569 Universal, compression force was applied to the crowns until fractured, and then fracture resistance at maximum load was recorded. Statistical analysis was conducted using the Wilcoxon signed-rank test (α = 0.05) in SPSS (IBM, Armonk, NY, USA).

Results: There was nostatistically significant difference in fracture resistance for both groups. Conclusion: Different designs of rest seats CAD/ CAM surveyed crowns can be used; however more research is necessary to measure clinical outcomes.

Keywords

Crown; Rest seat; Indirect restorations; Fracture

Introduction

Recently, the use of digital dentistry has increased among clinicians [1-3]. The advanced development of CAD/CAM dentistry can provide more accurate and predictable results in less time compared to conventional techniques [4-7]. A removable partial denture can be used to treat partial edentulism [8,9]. It takes less treatment and fewer prices than dental implants [10]. This treatment often requires the use of a surveyed crown for the abutment tooth [10]. A surveyed crown helps to improve the design of the abutment tooth before the insertion of the removable partial denture [10]. The design of the rest seat is critical for the removal of the partial denture as it can keep forces within the long axis of the tooth, thus preventing movement of the denture base, and preserving the planned design. The rest seat of the removable partial denture (RPD) provides support and prevents the impinging of soft tissues [10].

Several dental materials can be used for surveyed crowns [11]. Gold was used in the past as it has high strength, high biocompatibility great marginal fit. However, it is expensive and not a highly esthetic material [12]. Porcelain fused to metal (PFM) has been widely used as surveyed crowns; however, there are some disadvantages like porcelain chipping, extensive tooth preparation, and the high cost of noble metals [12]. Recently, CAD-CAM has been used to fabricate surveyed crowns with better outcomes [11]. Zirconia has been widely used due to its high fracture resistance, high biocompatibility, high esthetics properties, and adequate survival rates [13-16].

Different designs for rest seats are available for surveyed crowns [16-18]. Limited research has evaluated the fracture resistance of CAD/ CAM surveyed crowns. The present study aimed to assess the fracture resistance of zirconia surveyed crowns with a different rest seat design. Our hypothesis was there was no difference in fracture resistance between surveyed crowns with different widths.

Materials and Methods

One right mandibular second premolar typodont teeth (1560 Dentoform, Columbia Dentoform, Lancaster, PA, USA) was prepared for full coverage crowns to accommodate an a surveyed crown with mesio-occlusal rest seat. Tooth preparation was scanned with a chair side intraoral scanner (3Shape, Kobenhavn, Denmark). The surveyed crowns were prepared with a 1 mm rounded shoulder finish line and 1.5mm occlusal reduction.

Then dies were fabricated using a 3D printer (Asiga MAX™; ASIGA, Sydney, Australia) using model resin materials (Asiga Denta MODEL, Asiga, Sydney, Australia).

Forty surveyed crowns (n = 20 / group) were fabricated with different designs as follows:

Group 1: Zirconia-based material with narrow media-occlusal rest seat.

Group 2: Zirconia-based material with wide media-occlusal rest seat.

Each group was designed using Exocad software (Exocad GmbH, Darmstadt, Germany) in three dimensions, according to ISO 4949- 2019. Data was converted to standard tessellation language. Crowns were polished with pumice stone to increase surface quality. Then the crowns were sandblasted with 50 µm aluminum oxide for 10 seconds. Finally, crowns were bonded to resin dies using Relyx Luting Cement ((Multilink Automix, IvoclarVivadent Schaan, Liechtenstein, Switzerland). Then compression using a force of 50 N was applied in a vertical direction using a 4 mm diameter steel bar at the center of each crown using Instron 5569 Universal (contacting two cusps of lower second premolar). Restorations were loaded until fracture, and then fracture resistance at maximum load was recorded as the maximum compressive force. Statistical analysis was conducted using the Wilcoxon signed-rank test (α = 0.05) in SPSS (IBM, Armonk, NY, USA) (Figure 1).

Figure 1: A Diagram shows the sequence of the steps used.

Results

The mean fracture resistance for crowns with narrow rest seats (group 1) was 2250 ± 335 N, while the mean fracture resistance for crowns with wide rest seats (group 2) was 2242 ± 293 N. While crowns with narrow rest seats demonstrated slightly higher fracture resistance, statistical analysis showed no significant difference between the two groups (p > 0.05) (Table 1).

	n	Mean	Standard Deviation	Minimum	Median	Maximum	p-value
Narrow Rest Seats	20	2250	335	1400	2250	2800	0.179 (Wilcoxon signed-rank test; α = 0.05
Wide Rest Sears	19	2142	293	1400	2100	2600

Table 1: Descriptive statistics of the fracture resistance by narrow vs. wide rest seats (N).
Although the narrow rest seat group exhibited higher mean fracture resistance, statistical analysis revealed no significant difference between the two groups (p = 0.179).

Discussion

Porcelain fused to metal (PFM) crowns have been widely used as surveyed crowns till the evolution of high-strength ceramics [18,19]. The fracture resistance of porcelain fused to metal crowns (PFM) is higher than sintered monolithic zirconia crowns [20].

The aim of the study was to evaluate the width of the rest seats of zirconia surveyed crowns on fracture resistance. The mean fracture resistance for crowns with narrow rest seats (group 1) was 2250 ± 335 N, while the mean fracture resistance for crowns with wide rest seats (group 2) was 2242 ± 293 N. Although crowns with narrow rest seats demonstrated slightly higher fracture resistance, there was no statistical significance between groups. Our results were concurrent with a study that found Surveyed crowns with disto-occlusal extended rest seats (3283 N) were slightly higher than crowns with disto-occlusal rest seats (3257 N)[21].

Our results concur with another study that compared the fracture resistance of surveyed crowns using different zirconia brands. That study found that crowns with narrow rest seats have higher resistance ZirCAD 3476 N; ZLHT 2666.70 N) than crowns with wider rest seats (ZirCAD 2968 N; ZLHT 2083 N) [22].

The results of this study showed that chair side zirconia surveyed crowns demonstrated clinically acceptable strength values, exceeding the typical chewing and swallowing forces of 100 N [23]. Our study had several limitations as we did not use natural teeth, instead we used printed dies. One of the crowns was damaged and marked as an outlier. Another limitation of this study was the absence of thermal cycling and fatigue cycling, which can predict the restoration’s performance. More research is needed to determine guidelines to compare different designs, different materials, and cement selection.

Conclusion

The zirconia surveyed crowns with narrow base occlusal rest seat design had higher fracture resistance than surveyed crowns with wide occlusal rest seat design. The type of rest seat influences the fracture resistance of the chair side CAD-CAM zirconia surveyed crowns.

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Article Information

Article Type: SHORT COMMUNICATION

Citation: Mekled S, Gomez M, Shei G, Iıus O, Ngo D, et al. (2025) Fracture Resistance of Different Width of Zirconia Surveyed Crowns: an In vitro Study. Int J Dent Oral Health 11(2): dx.doi.org/10.16966/2378-7090.432

Copyright: ©2025 Mekled S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Publication history: 

Received date: 20 Apr, 2025

Accepted date: 12 May, 2025

Published date: 16 May, 2025