Fracture resistance of premolars restored with inlay and onlay ceramic restorations and luted with two different agents

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Technical procedure

Fracture resistance of premolars restored with inlay and onlay ceramic restorations and luted with two different agents

Gloria Beatriz de Azevedo Cubas DDS MSc, Luciano Habekost DDS MSc, Guilherme Bria˜o Camacho DDS MSc, PhD, Tatiana Pereira-Cenci DDS MSc PhD*

Department of Restorative Dentistry, School of Dentistry, Federal University of Pelotas, Brazil Received 10 June 2009; received in revised form 14 June 2010; accepted 1 July 2010

Available online 12 October 2010

Abstract

Purpose: The purpose of this study was to evaluate the fracture resistance of human maxillary premolars restored with 2 ceramic systems (Vitadur Alpha and In Ceram) comparing 3 preparation designs and 2 luting agents.

Methods: Seventy sound teeth were prepared to receive ceramic restorations (Vitadur Alpha; n = 14) as follows: (1) control, sound premolars, with no preparation, (2) inlays, (3) partial onlays (palatal cuspid coverage), (4) total onlays (both cuspids coverage), and (5) total onlays with an In Ceram core. The ceramic restorations were cemented using Enforce or RelyX ARC (half restorations with each cement), placed into the cavity and held under pressure, except for the control group. The teeth were subjected to compressive axial loading at 0.5 mm min 1using a 9 mm steel ball until fracture. Data were analyzed by 3-way ANOVA and post hoc Tukey’s test (a = .05).

Results: There was a significant difference between cements and among preparation designs (P < .05). All restorations cemented with Enforce exhibited significantly higher fracture resistance (P < .05). Inlay restorations showed similar fracture resistance when compared to control group (P > .05). Partial and total onlays did not statistically differ and showed the weakest performance. The use of an In Ceram core did not produce higher fracture resistance.

Conclusions: Within the limitations of this study, the cements tested had different mechanical properties, while cuspid coverage did not result in improved fracture resistance of the restored teeth.

# 2010 Japan Prosthodontic Society. Published by Elsevier Ireland.

Keywords: Ceramic; Fracture resistance; Inlays; Onlays

1. Introduction

The loss of dental structure caused by caries, trauma, or cavity preparation designs drastically diminishes teeth resis- tance when compared to sound teeth [1–5]. These partially destructed teeth may be restored using several adhesive systems and restorative techniques, which aims to reestablish the lost health[6–11]. However, little is known regarding the amount of the original resistance restored with dental treatment [1,2,6].

This way, it becomes important to study the factors that influence the fracture resistance of restored teeth, especially considering the occurrence of catastrophic fractures as a recurrent problem[3,11–14]. The main determinants in fracture

are the restoring material, the type of cementing agent, and the extension and conformation of the cavity preparation [2–

5,7,11,15].

There are several designs of cavity preparation of MOD type proposed for posterior teeth [3,4,16–18]. Indirect restorations usually consider preparations with or without cuspid reduction, width and depth of the dental structure reduction increasing the cuspids height and isthmus width, enhancing the generated stress to the cuspids base, which increases the fracture risk [3,19–21]. For this reason, the application of non-adhesive restorations is not indicated in deep and wide cavity preparations, such as amalgam or gold base inlays, as they do not promote reinforcement of the weakened cuspids[19,22].

Some studies also consider the fracture resistance of restored teeth increment when resin cements are used in detriment to conventional cements [8,19,23].

The esthetic demand for restoring treatments that reestablish the natural teeth aspects is rising [8,10,23–25]. Among the

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* Corresponding author at: School of Dentistry, Federal University of Pelotas, Rua Gonc¸alves Chaves, 457, 2nd floor, Pelotas-RS 96015560, Brazil.

Tel.: +55 53 32226690; fax: +55 53 32226690.

E-mail address:tatiana.cenci@ufpel.tche.br(T. Pereira-Cenci).

1883-1958 # 2010 Japan Prosthodontic Society. Published by Elsevier Ireland.

doi:10.1016/j.jpor.2010.07.001

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available options, ceramic restorations are an excellent alternative for the retrieval of posterior teeth with extensive dental structure losses and aesthetic needs[7,9,23,26]. These restorations present aesthetical advantages, chemical durabil- ity, fluorescence, resistance to compression and wear, thermal expansion coefficient similar to the structure, biocompatibility and when compared to resin composite restorations, restrain the polymerization contraction problem to the cement pellicle [7,10,23,27].

Despite technological advance of resin cements and adhesive systems, which permitted the increase of the clinical application of all ceramic restorations[7,15,26,28], there is a rising concern related to these restorations longevity [7,9,16,29–33]. Several studies point to a tendency of catastrophic fractures through time[29,33,34] related to their brittleness and low resistance to tensions provoked by antagonistic occlusal forces[10,23,24].

The aim of this study was to evaluate the fracture resistance of human maxillary premolars restored with two ceramic systems, comparing three types of cavity preparations and two different luting cements. Three null hypotheses were tested. (1) Different conformations of the cavity preparation would not influence fracture resistance of the restored teeth. (2) The utilization of different resin-based cementing agents would not lead to different performances on the fracture resistance of the restored teeth. (3) An In Ceram core would lead to higher fracture resistance of a total onlay when compared to a total onlay without the In Ceram core.

2. Materials and methods

This work was approved by the Ethics Committee of Federal University of Pelotas. Seventy caries-free human premolars extracted for orthodontic reasons were used. The removal of periodontal soft tissues was conducted, and the teeth were immersed in choramin-T 0.5% during 72 h[36]. Teeth were examined under magnification (10), to check possible fissures in the teeth. Teeth were washed in running water for 24 h and stored in distilled water at 37 8C for 5 days. These teeth were then randomly divided into five groups (n = 14), according to the following cavity preparation designs: control group (no preparation), Vitadur Alpha ceramic (VITA Zahnfabrik, Bad Sa¨ckingen, Germany) MOD inlays, Vitadur Alpha ceramic partial onlays with palatine cuspid coverage, Vitadur Alpha

ceramic total onlays with both cuspids coverage, and Vitadur Alpha ceramic total onlay manufactured on an In Ceram (VITA Zahnfabrik) core. Materials used in this study are described in Table 1.

The teeth were fixed 1 mm above the cementum–enamel junction limit and imbedded in autopolymerized acrylic resin (Artigos Odontolo´gicos, Classico Ltd., Sa˜o Paulo, Brazil) blocks, according to previously published protocols [35,36].

Standardized preparations were manufactured with proximal boxes 1.5 mm at the cervical wall in a proximal–proximal direction and occlusal boxes at a depth of 2 mm (half of the intercuspid distance—buccal-lingual dimension). The cervical walls stayed 1 mm above the cementum–enamel junction limit.

A 1.5 mm cuspid reduction was performed in the protected cusps (extended 2 mm in the cervical direction at the buccal surface) and a 2 mm reduction in the functional cuspid (extended 2 mm in the cervical direction at the lingual surface) [35] (Figs. 1–3). Diamond rotary cutting instruments (4138;

KG Sorensen, Barueri, Brazil) were used and discarded at every four preparations. The cavity preparations were copied with polyvinylsiloxane (Express; 3 M ESPE, St Paul, Minn, USA) working as a guide and cast with type IV Gypsum (Durone, Dentsply, RJ, Brazil).

The ceramic restorations were manufactured with the Vitadur Alpha ceramic using the refractory mould technique, with three burnings at (600–960 8C). Finishing and polishing were carried out with Sof-Lex system (Sof-Lex Pop-On, 3 M ESPE) and glaze at 930 8C. The ceramic restorations were sprayed with glass particles at 1 bar pressure for the internal surface cleaning. The acid conditioning of the ceramic surface was carried out with 10% hydrofluoridric acid during 4 min and a silane was applied with the assistance of a micro-brush. The dental surface treatment was conducted using 37% phosphoric acid (Scotchbond Etchant; 3 M ESPE) applied during 15 s.

Next, the dental cavity was abundantly washed with water during 15 s, and the tooth was slightly dried with absorbing paper. The adhesive (Single Bond; 3 M ESPE) was applied in two layers using a micro-brush with a light air jet after the first layer followed by one more layer. At the end of the applications, the adhesive was light polymerized for 20 s with a light curing unit XL 3000 (3 M ESPE) with energy higher than 450 mW cm 2.

Half of the restorations of each group were cemented with the Enforce resin cement and the other half with the RelyX

Table 1

Composition of materials used in the study.

Material Composition Batch # Manufacturer

Single bond Water, alcohol, HEMA, BisGMA, dimethacrylate, photoinitiator, copolymers of the polyacrylic acid and polyitaconic acid

3HL 3M ESPE, St Paul, MN, USA

RelyX ARC BisGMA, TEGDMA, silica and zirconium glass (67.5 wt.%)

BHBH 3M ESPE, St Paul, MN, USA

Enforce BisGMA, BHT, EDAB, TEGDMA, fumed,

silica, silanized barium, aluminum, borosilicate glass (66 wt.%)

731 Dentsply, Petro´polis RJ, Brazil

Silane Silane, ethanol and acetic acid 178 3M ESPE, St Paul, MN, USA

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ARC cement. Equal portions of base and catalyst of each cement were proportioned for cementation and after the overflow of cement excess, the restoration was placed under pressure (1 kgf charge using a Vicat needle for 2 min). Next the polymerization was conducted during 30 s. The mesial and distal polymerizations of each tooth was conducted during 40 s.

Finishing and polishing were done with the Sof-lex system. The restored teeth were kept in distilled water during seven days at 37 8C. Restorations with a core were manufactured with a ceramic core of high resistance (In Ceram) through the slip- casting technique and with the traditional Vitadur Alpha ceramic, as previously described. The fracture resistance test

was performed on the teeth’s occlusal surfaces in a universal testing machine MEM-2000 (EMIC Ltd., Sao Jose dos Pinhais, Brazil) with a 9 mm stainless steel sphere (in diameter) and 0.5 mm min 1speed (Fig. 4). The sphere was applied in the center of the occlusal surfaces with a 500 kgf load until specimen fracture[36].

The failure mode was assessed based on previous publications [35,36], where mode I was a fracture restrict to the restoration; mode II was a fracture of the dental structure, but not through the long axis of the tooth; mode III was a fracture of the tooth and restoration, but not through the long axis of the tooth and mode IV was a fracture through the long axis of the tooth, being in the tooth/restoration or only at the tooth.Fig. 5shows the failure mode and its distribution among the groups tested.

3. Results

With the application of ANOVA for three variation sources, a significant difference was found among the different cavity design groups (P < .001) and between the types of cement studied (P < .05). Mean and standard deviation values of the fracture strength obtained in the axial compression test are described inTable 2.Table 3shows the different performances among the restored premolars and control group, and cements.

Regarding the type of resin-based cement used, the Enforce system leaded to a greater pressure of the restorations.

The inlay ceramics had similar resistance when compared to control group (P > .001), and greater resistance when compared to the other restored groups (P < .001). The total onlay restorations with In Ceram core, although presenting greater resistance to fracture, did not statistically differ compared to the other groups (total and partial onlays without the In Ceram core), with P = 0.063.

[(Fig._2)TD$FIG]

Fig. 2. Onlays with palatine cuspid coverage.

[(Fig._1)TD$FIG]

Fig. 1. MOD inlays.

[(Fig._3)TD$FIG]

Fig. 3. Total onlays with both cuspids coverage.

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4. Discussion

Tissue reduction drastically reduces the teeth structural resistance, reaching up to a 50% loss, which increases their fracture risks[1]. The indication of partial ceramic restorations such as inlays, when compared to total crowns, save healthy dental tissue[6], reducing from around 67.5–72.3% to about 5.5–27.2% the tissue loss [25]. Partial ceramic and gold restorations, when associated to new adhesive systems and resin cements[6,25,26], develop greater resistance to fracture when compared to the traditional cements, such as glass ionomer and zinc phosphate[7,18,22].

The resin cements represent a good option for aesthetic restorations due to their adhesive capability not only to the dental structure, but also to the restoring material. Additionally, resin cements present enhanced properties in relation to solubility and lack of adhesion, if compared to the traditional cements [6,14,25,27]. In several studies the properties and performance of these cements are discussed, showing different types of cement and application techniques may influence bond strength of ceramic restorations and fracture resistance of the restored teeth [7,14,22,25,27,32]. In this study, there was a different performances between the two cementing agents used, with Enforce promoting greater values of fracture resistance in all types of cavity preparation evaluated when compared with the ones restored with RelyX ARC. Habekost et al. [36]

evaluated 3 cementing agents and reported that cements with higher flexural modulus exhibits higher values of fracture resistance for the ceramic/tooth assembly, what may help to explain the possible reason why there was a difference between cements in our study.

Although the teeth cemented with Enforce have shown a slight increase in fracture resistance varying from 10 to 30%

when compared with RelyX, the various restoration types showed, in general, similar fracture modes. Only the inlay group cemented with Enforce presented a prevalence of failure mode II, when compared with RelyX. There was a similar performance in the failure mode of the restorations cemented with the 2 luting agents. In the control group, 57% were failure mode IV (catastrophic), followed by failure mode II with 44%

of fracture of the palatal cuspid. In the inlay group, teeth cemented with RelyX presented higher prevalence of failure modes I and III, while teeth restored with Enforce presented higher prevalence of failure modes II and III. On the partial onlay group, teeth presented higher prevalence of mode failure mode I for both cements, with 70% cohesive and 30% cohesive and adhesive failure. The total onlay group resulted in failure mode I mode for both cements, with cohesive failure of the ceramic. For the In Ceram group, both cements presented 100%

of failure mode I mode with adhesive failure of the ceramic restoration.

Onlays presented higher prevalence of fracture of the palatal cuspid. In the partial onlay groups with palatal cuspid coverage, only one tooth restored with Enforce presented failure mode III with fracture of the palatine cuspid. The control group presented 42% of the restored tooth with fracture of the palatal cuspid with an II failure mode. A higher prevalence of catastrophic fractures by the failure mode IV occurred in control teeth. Considering that when fracture occurs in restored tooth, it is more desirable that it is restricted to the restoration.

In this study, the groups with cuspid coverage presented a higher prevalence of fractures restricted to the restoration (failure mode I), in general.

All ceramic restorations are brittle and stiff materials and have little capacity to deform; therefore they are luted and supported by less stiff materials such as adhesive resin, dentin bonding agent, and human dentin [37]. This combination results in a resin ceramic hybrid layer that acts as an inherent elastic buffering layer that is able to absorb stress during load application[38]. Information regarding the role of the resin in transferring stress from the loaded restoration to the underlying tooth structure is limited. The elastic modulus is related to the stress transmission between the restoration and the tooth

[(Fig._5)TD$FIG]

Fig. 5. Failure modes for each group.

[(Fig._4)TD$FIG]

Fig. 4.Schematic illustration of the stainless steel ball in contact with teeth.

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structure. It also indicates the ability of the cement to resist elastic deformation which could jeopardize the integrity of the bonded interface[39]. Ceramic strengthening enhancement is dependent upon the elastic modulus of the resin cement chosen [38]. Cements with higher flexural modulus exhibits higher values of fracture resistance for the ceramic inlays/tooth assembly[36], what may help to explain the reason why there was a difference between cements in our study. The elastic modulus of the luting agents RelyX ARC and Enforce were 5.70 and 7.29 GPa respectively [36]. Thus, the increase of fracture resistance values of the ceramic restorations cemented with Enforce may be due to its higher elastic modulus.

An intermediary elastic modulus for cements between dentin and the ceramic has been proposed, because this can reduce interfacial stress concentrations without causing excessive strains[40]. Alternatively, it is also argued that as the cement/tooth interfaces are weaker and of greater biologic importance than the cement/restoration interfaces, the elastic modulus of cements should be closer to that of dentin than of indirect restorative materials. Although neither argument supports the use of luting cements with elastic modulus lower than dentin, many commercially available cements presents elastic modulus ranging from 5 to 12 GPa. In this study, the cements had elastic modulus below the lower limits of the elastic modulus of dentin (11–19 GPa).

The clinical longevity of the ceramic restorations is a concern [6,8,16,23,28,32], since the long-term fracture resistance is a mandatory quality, as fractures are among the main flaw causes of these restoring treatments [16,29, 31,33,34]. Taking into account that posterior teeth are exposed to high occlusal charges when associated to extensive cavity preparations, the fracture risk becomes critical [3,4]. As previously described by Bader et al. [11], the incidence of fracture in previously restored posterior teeth is high. Molin et al.[29], in a randomized clinical evaluation with 3 ceramic inlays systems, reported 8% of fractures while 92% of ceramic

inlays based on CDA criteria were rated as satisfactory after 5 years. In a clinical evaluation of feldspathic inlays, Hayashi et al.[34], reported longevity of 80% after eight years, and the majority due the failures were due to bulk fracture. Clinical evaluations of leucite-reinforced glass ceramic inlays and onlays and CAD/CAM (Cerec) ceramic inlays have reported a 8–11% failure rate, with fractures as the main reason for failure [31,33]. This way, it becomes important to study fracture resistance among various preparation designs in order to identify which preparation would show lower fracture resistance, which may lead to higher longevity of the restorations.

It is difficult to determine which restorative material would be ideal for the restoration of posterior teeth[6–9]. Reviews of the literature considering ceramic inlays and ceramic compared with gold inlays concluded that there are few randomized clinical trials capable of defining which material presents better performance to restore posterior teeth in a long-term basis [28,30]. In the past decades, indirect metal or amalgam restorations were the first choice for the restoration of partially destructed teeth[18,21], however, many studies have demon- strated that there is a higher occurrence of fractures in these types of restoration through time[7], probably because indirect metal or amalgam restorations provide only primary mechan- ical retention without increasing dental structure resistance, which may ultimately result in cuspid fractures [12,21].

Although ceramic restorations present disadvantages in relation to friability and the antagonist wear [9,22,24], they are considered a clinically viable option that fulfills the aesthetics and resistance needs[7,22,26].

Premolar teeth are an interesting option for the indication of inlays due to their aesthetical necessity, when considering their occlusal requests, and for representing a frequently used teeth group in studies, allowing comparisons [1,3,5,10,18]. The major prevalence of palatine cuspid fractures of upper premolars has already been reported [1,5,10,13,20], and justified as it deals with centric contention cuspids that are subjected to higher masticatory forces. Additionally, functional cusps of maxillary premolars fracture with greater frequency than their counterparts probably because in this group of teeth the functional cusp is significantly smaller than the nonfunc- tional cusp[20]. Eakle et al.[19], reported in his study that the lingual cusps (53.1%) of maxillary premolars fractured slightly more frequently than buccal cusps.

Some authors have previously recommended cuspids reduction with their inclusion in the cavity preparation in order to enhance fracture resistance, converting inlays to onlays to protect cuspids without support, especially to eliminate

Table 2

Mean SD (kgf) of fracture resistance for different experimental groups.

Cements Groups

Control Inlays Partial onlays Total onlays Total onlays + In Ceram

RelyX 178.2 32.8 158.7 17.0 86.0 5.4 84.0 15.7 100.4 14.7

Enforce 173.9 12.8 116.5 21.2 104.7 20.4 108.0 41.9

Table 3

ANOVA in accordance with fracture resistance results.

Sources of variation df F value P

Ceramic system 2 6.18 .063 ns

Cement 4 4.01 <.001

Cavity design 4 0.92 <.05

Ceramic system cement 8 2.59 0.55 ns

Ceramic system cavity design 8 2.03 0.19 ns

Ceramic system cement  cavity design 32 0.30 0.20 ns df = degrees of freedom; ns = not significant.

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contacts that occur at the interface between the restoration and tooth structure[6,12]. For this reason, fracture resistance tests with designs of cavity preparations involving only the palatine cuspid are important. Additionally, in order to compare fracture resistance of preparations that involves one or both cuspids, studies aiming to evaluate the real need of greater dental loss are needed. There is no consensus regarding cuspids reduction in cavity preparation with regard to structural resistance of the tooth-restoration complex [15,17]. While some studies have shown that cuspal coverage of posterior teeth with ceramic partial coverage inlays did not result in increased fracture resistance[15,17], marginal integrity and longevity were not affected by cuspal reduction[15,31].

In the present study, inlay restorations were the only restorations to show resistance performance similar to the control group, as the less invasive preparation. The more invasive preparations were incapable of restoring the fracture resistance of the restored teeth when compared to the control group. This may explain why neither the inclusion of the protective cuspid nor the cuspid of support resulted in increased fracture resistance of restored teeth, corroborating with other findings[15]. Maintenance of the buccal cuspid contributes to the maintenance of the restored teeth aesthetic. Therefore, inlay restorations should be preferred, as they are more conservative since there is no occlusal reduction and they presented the highest fracture resistance values.

The evolution of the ceramic systems in the latest years is overwhelming[22,23,25]. The reinforced ceramics are largely used in total and partial metal-free crowns, associating both aesthetic and resistance[7,26]. They present a ceramic core of high resistance manufactured through the slip-casting techni- que [25]. Conversely, in this work, the utilization of an In Ceram core with the total onlay preparation did not differ from the traditional onlay, except for the inlays. The limited number of randomized, controlled and well designed longitudinal studies is a significant problem when evaluating ceramic inlay and onlay performance in a long-term perspective, turning difficult to compare and asses the overall effectiveness of different ceramic inlay systems[30].

Although this work does not recreate the mouth clinical conditions, this type of test with application of loads is currently a feasible option for reproduction of the answers in relation to the charge powers and may be a clue on how these restorations may perform clinically. It is important to conduct future studies to determine cavity preparation designs with minimal tissue reduction, which may help to promote adequate resistance of the tooth and restoration array, avoiding long-term restorations flaws.

5. Conclusions

Within the limitations of this study, the following conclu- sions were drawn:

1. There was a significant difference between the used cements in fracture resistance of the restored teeth with the different cavity preparations.

2. The ceramic inlay restorations presented the highest axial compression resistance values and did not differ from premolars used as control.

3. The superiority of the specimens restored with partial onlay restorations when compared to total onlays was not statistically confirmed.

4. The increase of dental reduction with the cuspids protection did not result in increased fracture resistance.

5. There were no differences between the total onlays restored with an In Ceram core compared to those restored with the traditional ceramic system.

Conflict of interest statement

The authors declare that they have no conflict of interest.

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