Influence of Ferrule Design and Occlusal Thickness on the Fracture Resistance and Failure Patterns of Zirconia Endocrowns

ABSTRACT

Background and Objective:

Zirconia endocrowns are a popular restorative option due to their fracture resistance and aesthetic appeal, but the influence of different preparation designs on their performance remains underexplored. This study evaluated the effects of preparation design and the occlusal thickness on the fracture strength and failure modes of zirconia endocrowns in endodontically treated teeth.

Materials and Methods:

Thirty-two extracted human mandibular first molars were endodontically treated and divided into four groups (n = 8), based on margin design and occlusal thickness: Group A: 2 mm butt joint with 3 mm occlusal thickness, Group B: 1 mm butt joint with 4 mm occlusal thickness, Group C: 2 mm ferrule with 3 mm occlusal thickness, and Group D: 1 mm ferrule with 4 mm occlusal thickness. Zirconia endocrowns were fabricated using CAD/CAM technology and cemented with self-adhesive resin cement. The specimens were subjected to thermocycling and then tested for fracture strength using a universal testing machine. Failure modes were analyzed under a stereomicroscope. Data were statistically analyzed.

Results:

Group B showed the highest mean fracture strength (6655.66 N), followed by Group D at 6091.00 N. Group C exhibited the lowest fracture strength (4867.41 N), although the differences were not statistically significant (P = 0.121). Type V failure was the most common failure mode across all groups, with 100% occurrence in both 2 mm margin groups (Groups A and C).

Conclusion:

Margin design and occlusal thickness appeared to influence the fracture strength and failure modes of zirconia endocrowns; the differences were not significant.

INTRODUCTION

Endodontically treated teeth with extensive structural loss present a significant challenge for restoration. While post-and-core techniques have been traditional, endocrowns have emerged as a more conservative alternative. Endocrowns are one-piece ceramic restorations that utilize the pulp chamber for retention without requiring a post.[1] These restorations have shown promise in preserving the remaining tooth structure and providing adequate stability and fracture resistance. Various materials, including zirconia, have been used to fabricate endocrowns, offering high strength and aesthetic properties.[2] However, the optimal design parameters for endocrowns, particularly regarding margin configuration and occlusal thickness, remain unclear. The present study aimed to address this research gap by assessing the effect of different margin designs and occlusal thicknesses on the fracture strength and failure modes of zirconia endocrowns restoring endodontically treated mandibular molars.

MATERIALS AND METHODS

Thirty-two extracted human mandibular first molars were selected, including teeth with intact crowns extracted for periodontal reasons or decayed crowns with fully formed root apices and at least 1-2 mm of remaining crown height from the cementoenamel junction (CEJ). Teeth with gross decay and insufficient dentin or enamel support were excluded. After endodontic treatment, the teeth were randomly divided into four groups (n = 8): Group A: 2 mm butt joint with 3 mm occlusal thickness, Group B: 1 mm butt joint with 4 mm occlusal thickness, Group C: 2 mm ferrule with 3 mm occlusal thickness, and Group D: 1 mm ferrule with 4 mm occlusal thickness. Tooth preparations were standardized using a CNC milling machine. Zirconia endocrowns were fabricated using CAD/CAM technology and cemented with self-adhesive resin cement.

Specimens underwent thermocycling (2500 cycles, 5-55°C) to simulate intraoral conditions. Fracture strength testing was performed using a universal testing machine with a 2.5 mm diameter steel ball at a crosshead speed of 0.5 mm/min until fracture occurred. Failure modes were examined under a stereomicroscope and classified into five categories, ranging from cohesive failure to irreparable tooth fracture. Statistical analysis was conducted to compare fracture strengths and failure modes across the groups.

RESULTS

Group B exhibited the highest mean fracture strength (6655.66N), followed by Group D (6091.00N), while Group C showed the lowest fracture strength (4867.41N). However, these differences were not statistically significant (P = 0.121) [Table 1]. Post-hoc Tukey’s test revealed no significant differences between any pair of groups (P > 0.05) [Table 2]. Regarding failure modes, Type V was the most prevalent across all groups. Notably, both 2 mm groups experienced 100% Type V failures, while the 1 mm groups showed a mix of failure types, with 62.5% Type V, 25% Type III, and 12.5% Type IV failures [Table 3].

Table 1.

Comparison of mean fracture strength

Design type	Number	Mean (N)	SD	F	P value
2 mm Butt	8	5046.25	1793.5	2.11	P=0.121
1mm Butt	8	6655.66	1048.9		NS
2mm Ferrule	8	4867.41	1895.7
1 mm Ferrule	8	6091.00	1757.8

SD, standard deviation; NS, not significant

Table 2.

Post-hoc Tukey’s test

Groups	MD	P
2 mm Butt vs. 1 mm Butt	-1609.4	P=0.23
2 mm Butt vs. 2 mm ferrule	178.83	P=0.99
2 mm Butt vs. 1 mm ferrule	-1044.75	P=0.59
1 mm Butt vs. 2 mm ferrule	1788.25	P=0.16
1 mm Butt vs. 1 mm ferrule	564.66	P=0.99
2 mm ferrule vs. 1 mm ferrule	-1223.58	P=0.46

MD, mean difference

Table 3.

Failure modes

Groups	Type I	Type II	Type III	Type IV	Type V
A	0	0	0	0	100%
B	0	0	25%	12.5%	62.5%
C	0	0	0	0	100%
D	0	0	25%	12.5%	62.5%

DISCUSSION

In the present study, thinning of enamel was anticipated with ferrule preparation in groups C and D. This is consistent with the study report of Govare et al.[3] where he emphasized that the loss of enamel was more detrimental when the margins of the preparation approximated to CEJ in the ferrule design preparation.

The current study investigated two different occlusal thicknesses, 3 mm and 4 mm. Higher fracture strength mean values were observed with the 1 mm group, but the difference was not statistically significant. Otto et al.[4] and our study observed that increasing occlusal thickness led to higher fracture strength; Turkistani et al.[5] found that a 3mm thickness outperformed thicker options.

Endocrowns demonstrated exceptional fracture strength in this study. This superior strength is attributed to several factors: the conservative nature of endocrowns, the properties of the restorative material, increased bonding surface area utilizing the pulp chamber, preservation of tooth structure, and the monolithic design. The monoblock effect created by directly bonding the restoration to the tooth structure results in a better biomechanical union compared to traditional multi-component restorations-like post and core-supported crowns.[6]

Lastly, zirconia was used as the restorative material. Zirconia has been compared to a wide range of restorative materials in previous studies, including feldspathic ceramic, lithium disilicate, resin nano-ceramic, polymer-infiltrated ceramic, leucite ceramic, PEEK, metal-reinforced glass ceramic, and composite resin.[7] Despite zirconia’s high fracture strength, studies by Dartora et al.[8] found that monolithic zirconia endocrowns showed the highest rates of catastrophic failure.

In conclusion, while endocrowns, particularly those made of zirconia, offer a promising option for restoring endodontically treated molars, careful consideration must be given to the design features such as occlusal thickness and margin design.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.