Fracture strength of minimally prepared all-ceramic CEREC crowns after simulating 5 years of service

Abstract

Objectives

To examine the strength and mode of fracture of traditionally and minimally prepared all-ceramic resin bonded CAD/CAM crowns after fatigue loading.

Methods

Thirty human maxillary molars were used and divided in three groups namely; traditional crown preparation group (I), minimal crown preparation group (II) and occlusal veneer preparation group (III). A leucite reinforced glass ceramic (IPS Empress CAD) was used for fabricating the crowns. The CEREC InEOS system (v3.10) was used for scanning, designing and milling. Five years of clinical service were simulated and the fracture strength of the crowns was measured. One-way ANOVA and Kruskal–Wallis test were used for data analysis ( α = 0.05).

Results

The mean fracture strength and SD in Group I was 1070 N (±181) and in Group II 1110 N (±222). One-way ANOVA analysis showed no statistically significant difference between the two groups ( p > 0.05). In Group III all restorations developed cracks during TCML and were not subjected to fracture loading. Three of the traditionally designed crowns (Group I) and 4 of the minimally designed crowns (Group II) developed surface cracks during TCML.

Significance

Minimal all-ceramic resin-bonded crowns can potentially form a viable restorative option as they demonstrated comparable strength to traditional all-ceramic crowns. However, this should be interpreted with caution as a number of crowns showed cracks after 5 years of simulated service. All the occlusal veneers developed cracks during simulation and further investigation is needed for this to be considered a viable option.

Introduction

During the past few decades the evolution in dental materials and techniques, along with more predictable adhesive technologies have led to a great interest in Minimally Invasive Dentistry. Maximum preservation of sound tooth structure and maintenance of the vitality of the restored teeth are its major advantages . This can be especially important in cases where a large amount of tooth tissue has been lost, due to tooth wear, trauma or hypoplastic defects, or when malformed/malpositioned or discolored teeth are restored and excessive removal of tooth tissue is not a justified reason for placing a restoration. In addition, the increasing demand from patients for esthetic restorations as well as the concerns that have been raised about the possible side-effects of mercury released from amalgam restorations have increased the use of esthetic restorations .

All-ceramic restorations are amongst the most esthetically pleasing restorations. They exhibit life-like appearance due to the lack of metal substructure and also their ability to allow natural light to pass through them in a similar way to enamel–dentin . Metal–ceramic restorations on the other hand do not exhibit any degree of translucency as they are only capable of diffusion and reflection of light . Thus matching with natural teeth is very difficult as enamel and dentin are able to transmit light in the degree of 70% and 30% respectively . In the case of gingival recession in the esthetic zone, the exposed margin of an all-ceramic crown is less objectionable esthetically than the exposed margin of a metal–ceramic restoration. Additionally, metal–ceramic restorations tend to cause graying of the free gingival margin due to the metal substrate .

Ceramics are biocompatible and resistant to dissolution in the mouth and in contrast to certain metals, they are not susceptible to corrosion phenomena . There are no reports of allergy connected with dental ceramic whereas metals used in metal–ceramic restorations may cause allergic or toxic reactions within adjacent soft or hard tissues .

All-ceramic resin-bonded crowns have been well established for the restoration of anterior teeth. For posterior teeth though, the preferred method is to place either a conventional metal–ceramic crown or a high strength reinforced ceramic core restoration as these restorations can survive the high occlusal loads imposed on posterior teeth.

However, all-ceramic resin bonded restorations have many advantages. One of these is that due to the lack of a substructure the application of minimal preparation designs are possible. Furthermore, there is evidence that the strength of the ceramic restoration is increased when bonded to the available tooth tissue , as the tooth-restorative system acts as one and the forces acting on the crown are being distributed through the tooth in a similar way that enamel–dentin complex works .

A small number of studies investigating the fracture resistance of posterior glass ceramic crowns fabricated with CAD/CAM exist in the literature and the testing involved only static loading. Further to that in a previous study it was found that the structural integrity of minimally prepared adhesively cemented all-ceramic posterior crowns was equal to that of traditionally prepared all-ceramic restorations when subjected to static loading . It was therefore of interest to evaluate the fracture strength of these restorations after simulating oral function in vitro as this will give valuable information and would be of greater clinical significance.

The aim of this study was to compare the fracture strength of traditionally and minimally designed all-ceramic resin bonded crowns fabricated by CAD/CAM technology after simulating 5 years of clinical service.

The null hypothesis was that traditionally and minimally designed crowns would demonstrate comparable fracture strength after simulating 5 years of service.

Materials and methods

Thirty maxillary first molar teeth were used for the purpose of this study. The teeth were chosen after careful examination with 2.5× magnifying loupes (Keeler Ltd., UK) to ensure they were free from any defects and cracks. Three groups were formed with ten teeth in each group (Groups I, II and III). Teeth were selected so that the mean measurement of the bucco-palatal width (BPW, the distance from the maximum convexity on the buccal and palatal surfaces) between the groups varied by no more than 2.5% . An ultrasonic scaler was used to remove calculus deposits, debris and soft tissue remnants.

The teeth were stored in thymol solution (0.5%) at room temperature. Each tooth was fixed with yellow die stone blocks (Dentstone KD, Saint-gobain Formula, France), crown uppermost and long axis vertical extending to within 2 mm of the cemento-enamel junction (CEJ).

Teeth in Group I were prepared for traditional all-ceramic crowns with a 2 mm occlusal reduction, a 6° convergence angle and a round shoulder of 1.5 mm. Teeth in Group II were prepared minimally with a 1.2 mm occlusal reduction, a 6° convergence angle and a chamfer of 0.8 mm. The teeth in Group III were ultra-minimally prepared. The preparation was intra-enamel with a 0.8 mm occlusal reduction, a 6° convergence angle and a chamfer of 0.5 mm just above the contact point to improve positioning of the restorations. The drawing in Fig. 1 demonstrates the three designs.

Fig. 1
Drawing of the three preparation designs: the traditional crown design (a), the minimal crown design (b) and the occlusal veneer design (c) (not to scale).

All stages were carried out using a high-speed hand piece operating with water coolant and 2.5× magnifying loupes. The handpiece was attached to a custom made paralleling device and gauged burs were used to standardize preparations (828Y, 828W, Meisinger, Germany; Intensiv Advanced CEREC Kit, Intensiv SA, Switzerland).

The CEREC InEos system (Sirona, Bensheim, Germany) was used for scanning the preparations. Each tooth was powdered with the CEREC powder (VITA Zahnfabric Gmbh, Bad Säckingen-Germany) using a propellant to provide an evenly thin layer of powder. The preparations were then scanned using the CEREC InEos system and the crowns were designed using the software CEREC 3D v.3.10 (Sirona, Bensheim, Germany).

A machinable leucite reinforced glass ceramic material [IPS Empress CAD, Ivoclar Vivadent (LT A3/I 12)] was used for the fabrication of the restorations. The Endo milling mode was used for all three groups and the default milling burs (1.2 mm cylinder bur, Step bur) were used for the milling of the crowns. The spacer was set at 50 μm. Ten restorations of each design were milled producing 30 restorations in total.

Following milling, the restorations were examined for defects or cracks and their fit was assessed on the tooth using a 2.5× magnification.

The restorations were glazed by two cycles in a vacuum furnace (Programat CS, Ivocaler-Vivadent) at 850 °C according to the manufacturer’s instructions. After completion of glazing, the fit of the crowns was reassessed. Prior to the adhesive cementation of the restorations, the roots of the human maxillary molars were coated with 1 mm thick layer of polyether material (Impregum, 3 M Espe, Seefeld, Germany) in order to simulate the human periodontium, and inserted into PMMA resin (Palapress Vario, Heraeus Kulzer, Hanau, Germany) . The molars used had comparable crown and root dimension to ensure a similar modulus of elasticity.

Conditioning of the internal surface of the restorations and their adhesive cementation was undertaken according to the manufacturer’s instructions. The internal surface of the restorations was rinsed with water and air dried, etched with 4.9% hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar Vivadent) for 60 s, then neutralized with ethylacetate for 3 min and air dried. The internal surface of the restorations was then silanised (Monobond-S, Ivoclar Vivadent) for 60 s. The prepared teeth were washed with water and dried with air taking care not to over dry the tooth surface. Etching with phosphoric acid gel (Total etch 37% wt) for 30 s followed. A multiple-component adhesive system was used (Syntac Adhesive System, Ivoclar Vivadent). A primer (Syntac Primer, Ivoclar Vivadent) and adhesive (Syntact Adhesive, Ivoclar Vivadent) were applied, followed by the bonding agent (Heliobond, Ivoclar Vivadent) according to manufacturer’s instructions. The restorations were placed in position using a gentle finger pressure using a dual cure luting composite system (Translucent, Base/low viscosity Catalyst, Variolink II, Ivoclar Vivadent), which was applied to the internal surface of the crowns. Excess luting material was removed using a brush and then the pressure was increased and maintained for 15 s. Any further excess cement was removed with a brush. The luting material was cured using a light-curing unit (800 mW/cm 2 , Elipar 2500, 3M ESPE, USA) for 40 s from mesial, distal, buccal, palatal and occlusal directions.

After cementation the specimens were stored for 24 h in normal saline and then placed in a masticatory simulator for thermal cycling and mechanical loading (TCML, Chewing Simulator, EGO, Regensburg, Germany) ( Fig. 2 ). Human molars were adjusted as antagonists using a dental articulator to create 3 point contacts occlusally; mesiobuccal, distobuccal and lingual (Amann Girrbach, Koblach, Austria). The loading parameters were 1,200,000 mechanical loadings [ML] of 50 N (frequency 1.66 Hz) and 6000 thermal cycles [TC] – 2 min each cycle – with distilled water between 5° and 55 °C to simulate 5 years of oral service .

Fig. 2
Thermocycling and mechanical loading of the cemented crowns took place using a TCML apparatus.

The restorations that survived fatigue loading without developing cracks were then loaded until failure using a fracture testing machine ( v = 1 mm/min) (Zwick 1446s, Ulm, Germany). The force was applied using a steel sphere ( d = 12 mm) while a 1 mm thick tin foil between the crown and antagonist was used to prevent force peaks.

After fracture strength testing all restorations were examined in order to establish the mode of fracture. This was recorded using Burke’s classification , which consists of the following categories according to the pattern of crown failure:

  • (I)

    Minimal fracture or crack in crown

  • (II)

    Less than half of crown lost

  • (III)

    Crown fracture through midline; half of crown displaced or lost

  • (IV)

    More than half of crown lost

  • (V)

    Severe fracture of tooth and/or crown.

Burke’s classification was used to assess the restorations after TCML and fracture testing. The restorations in Group III were not subjected to fracture strength testing as it was found that they failed after TCML.

Scanning electron microscopy (SEM; Quanta FEG 400, FEI Company, Eindhoven, NL, low vacuum 0.08 Torr, working distance up to 41.6 mm, magnification of 30–4000 at 10–20 keV) was used for fractographic failure analysis of all specimens. For the fractographic analysis of the crowns features such as arrest lines, hackle, wake hackle and compression curl were searched for. The description of these fracture patterns were based on ASTM standards and previous studies . An arrest line is a well-defined line produced when the crack comes to a pause, before recommencing its propagation commonly in a different direction. The beginning of the crack event is always on the concave side of the arrest line and for that they can indicate the crack propagation. Hackle lines are lines on the fracture surface that run in the local direction of cracking and separate parallel portion of the crack that are on slightly different plane . Wake hackle is a trail starting from an irregularity, often a pore, and is created by the crack front proceeding along either side of the pore before continuing on slightly different planes . They can are also indicate the crack propagation. The compression curl results from flexural stress and is the curved lip just before total fracture of the body .

The statistical package MINITAB 13.31 was used for the statistical analysis of the results and one-way ANOVA analysis was performed to assess the effect of the design (traditional/minimal) on the fracture strength of the restorations. Kruskal–Wallis test was used for the analysis of the mode of fracture.

Materials and methods

Thirty maxillary first molar teeth were used for the purpose of this study. The teeth were chosen after careful examination with 2.5× magnifying loupes (Keeler Ltd., UK) to ensure they were free from any defects and cracks. Three groups were formed with ten teeth in each group (Groups I, II and III). Teeth were selected so that the mean measurement of the bucco-palatal width (BPW, the distance from the maximum convexity on the buccal and palatal surfaces) between the groups varied by no more than 2.5% . An ultrasonic scaler was used to remove calculus deposits, debris and soft tissue remnants.

The teeth were stored in thymol solution (0.5%) at room temperature. Each tooth was fixed with yellow die stone blocks (Dentstone KD, Saint-gobain Formula, France), crown uppermost and long axis vertical extending to within 2 mm of the cemento-enamel junction (CEJ).

Teeth in Group I were prepared for traditional all-ceramic crowns with a 2 mm occlusal reduction, a 6° convergence angle and a round shoulder of 1.5 mm. Teeth in Group II were prepared minimally with a 1.2 mm occlusal reduction, a 6° convergence angle and a chamfer of 0.8 mm. The teeth in Group III were ultra-minimally prepared. The preparation was intra-enamel with a 0.8 mm occlusal reduction, a 6° convergence angle and a chamfer of 0.5 mm just above the contact point to improve positioning of the restorations. The drawing in Fig. 1 demonstrates the three designs.

Fig. 1
Drawing of the three preparation designs: the traditional crown design (a), the minimal crown design (b) and the occlusal veneer design (c) (not to scale).

All stages were carried out using a high-speed hand piece operating with water coolant and 2.5× magnifying loupes. The handpiece was attached to a custom made paralleling device and gauged burs were used to standardize preparations (828Y, 828W, Meisinger, Germany; Intensiv Advanced CEREC Kit, Intensiv SA, Switzerland).

The CEREC InEos system (Sirona, Bensheim, Germany) was used for scanning the preparations. Each tooth was powdered with the CEREC powder (VITA Zahnfabric Gmbh, Bad Säckingen-Germany) using a propellant to provide an evenly thin layer of powder. The preparations were then scanned using the CEREC InEos system and the crowns were designed using the software CEREC 3D v.3.10 (Sirona, Bensheim, Germany).

A machinable leucite reinforced glass ceramic material [IPS Empress CAD, Ivoclar Vivadent (LT A3/I 12)] was used for the fabrication of the restorations. The Endo milling mode was used for all three groups and the default milling burs (1.2 mm cylinder bur, Step bur) were used for the milling of the crowns. The spacer was set at 50 μm. Ten restorations of each design were milled producing 30 restorations in total.

Following milling, the restorations were examined for defects or cracks and their fit was assessed on the tooth using a 2.5× magnification.

The restorations were glazed by two cycles in a vacuum furnace (Programat CS, Ivocaler-Vivadent) at 850 °C according to the manufacturer’s instructions. After completion of glazing, the fit of the crowns was reassessed. Prior to the adhesive cementation of the restorations, the roots of the human maxillary molars were coated with 1 mm thick layer of polyether material (Impregum, 3 M Espe, Seefeld, Germany) in order to simulate the human periodontium, and inserted into PMMA resin (Palapress Vario, Heraeus Kulzer, Hanau, Germany) . The molars used had comparable crown and root dimension to ensure a similar modulus of elasticity.

Conditioning of the internal surface of the restorations and their adhesive cementation was undertaken according to the manufacturer’s instructions. The internal surface of the restorations was rinsed with water and air dried, etched with 4.9% hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar Vivadent) for 60 s, then neutralized with ethylacetate for 3 min and air dried. The internal surface of the restorations was then silanised (Monobond-S, Ivoclar Vivadent) for 60 s. The prepared teeth were washed with water and dried with air taking care not to over dry the tooth surface. Etching with phosphoric acid gel (Total etch 37% wt) for 30 s followed. A multiple-component adhesive system was used (Syntac Adhesive System, Ivoclar Vivadent). A primer (Syntac Primer, Ivoclar Vivadent) and adhesive (Syntact Adhesive, Ivoclar Vivadent) were applied, followed by the bonding agent (Heliobond, Ivoclar Vivadent) according to manufacturer’s instructions. The restorations were placed in position using a gentle finger pressure using a dual cure luting composite system (Translucent, Base/low viscosity Catalyst, Variolink II, Ivoclar Vivadent), which was applied to the internal surface of the crowns. Excess luting material was removed using a brush and then the pressure was increased and maintained for 15 s. Any further excess cement was removed with a brush. The luting material was cured using a light-curing unit (800 mW/cm 2 , Elipar 2500, 3M ESPE, USA) for 40 s from mesial, distal, buccal, palatal and occlusal directions.

After cementation the specimens were stored for 24 h in normal saline and then placed in a masticatory simulator for thermal cycling and mechanical loading (TCML, Chewing Simulator, EGO, Regensburg, Germany) ( Fig. 2 ). Human molars were adjusted as antagonists using a dental articulator to create 3 point contacts occlusally; mesiobuccal, distobuccal and lingual (Amann Girrbach, Koblach, Austria). The loading parameters were 1,200,000 mechanical loadings [ML] of 50 N (frequency 1.66 Hz) and 6000 thermal cycles [TC] – 2 min each cycle – with distilled water between 5° and 55 °C to simulate 5 years of oral service .

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Fracture strength of minimally prepared all-ceramic CEREC crowns after simulating 5 years of service
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