The purpose of this in vitro study was to evaluate the effect of intaglio surface air-abrasion on the retention of CAD/CAM produced zirconia ceramic crowns cemented with three different types of cement. In addition the influence of artificial aging in masticatory simulator and thermocycling was tested.
Extracted human premolars were prepared for all-ceramic crowns (12° taper, 3 mm axial length). CAD/CAM zirconia crowns were manufactured. Half of the crowns were air-abraded with 50 μm alumina particles at 0.25 MPa, the rest was left as machined. The crowns were luted with zinc phosphate cement (Hoffmann), glass ionomer cement (Ketac Cem), or composite resin (Panavia 21), subgroups were either stored for 3 days in 37° water bath or stored for 150 days in 37° water bath, with additional 37,500 thermal cycles (5–55°) and 300,000 cycles dynamic loading with 5 kg in a masticatory simulator. Then crown retention was measured in tension at a crosshead speed of 2 mm/min using a universal testing machine. Statistical analysis was performed with three-way ANOVA.
Mean retention values were ranged from 2.8 to 7.1 MPa after 3 days and from 1.6 to 6.1 MPa after artificial aging. Air-abrasion significantly increased crown retention ( p < 0.001), while artificial aging decreased retention ( p = 0.017). In addition, the luting material had a significant influence on retention ( p < 0.001) with the adhesive luting resin providing the highest retention.
The use of phosphate monomer containing composite resin on air-abraded zirconia ceramic can be recommended as most retentive luting method.
In recent years, the increasing demand for all-ceramic restorations led to the development of ceramic materials with optimized mechanical properties like densely sintered alumina and zirconia ceramics . These high-strength ceramics offer a wide variety of clinical applications, e.g. posts, fixed dental prostheses, implant abutments and even Maryland type resin-bonded fixed dental prostheses. A long-term durability of the resin bond to these ceramics is indispensable for many applications and can be obtained with various bonding methods when using air-abrasion .
Although clinical studies showed considerable failure rates related to loss of retention when using conventional cements , manufacturers claim that zirconia ceramic restorations can be cemented successfully with either conventional or adhesive cements. Also, various authors suggested the use of conventional cements for zirconia ceramic restorations and stated that retention of conventional cements can be equal or even better than with adhesive resins .
Nevertheless, some manufacturers do not recommend air-abrasion prior cementation taken into consideration that air-abrasion might affect the ceramic surface by creating microcracks which might reduce the fracture strength of the ceramic . Some authors suggested that the machining process generated a surface roughness what was at least as rough as a machined and air-abraded surface in some machinable all-ceramic systems . Other authors suggested that resin luting agents providing durable resin bonds significantly strengthen ceramic materials by “healing” minor surface defects caused by air-abrasion . However, many studies showed improved bond strength to zirconia ceramic after air-abrasion . Even more, there are authors who showed that air-abrasion might even strengthen zirconia ceramics . The question arises whether air-abrasion can be omitted when cementing crowns in order to avoid weakening of the ceramic. Some studies tested the zirconia crowns retention like Palacios et al. and Ernst et al. , but both studies did not test air-abrasion versus no air-abrasion. Besides, no long-term aging was performed. However, data on the retention of zirconia ceramic crowns when luted with conventional or adhesive cements and whether crown retention is influenced by air-abrasion are missing. In addition the effect of artificial aging on crown retention is not known.
Therefore, the purpose of this in vitro study was to evaluate the retention of zirconia ceramic crowns luted either with two conventional cements (zinc phosphate cement, glass ionomer cement) or with composite resin to human teeth. The influence of air-abrasion on crown retention and the effect of artificial aging were also tested. It was the null hypothesis of the study that crown retention is not influenced by luting materials, air-abrasion or artificial aging.
Materials and methods
Ninety-six caries free extracted human maxillary premolars were collected from dental orthodontic clinics in Damascus, Syria. They were mostly extracted from young patients for orthodontic reasons. Only carious free premolars were selected, cleaned with scalpel, and stored in thymol 0.1% solution (Caelo, Hilden, Germany) at room temperature. The teeth were used within one month after the extraction.
The roots of the teeth were embedded in custom made standard copper cylinders (25 mm long, Ø 15 mm) positioned along their long axis with auto-polymerizing acrylic resin material (Technovit 4000, Heraeus Kulzer, Wehrheim, Germany). The enamel–cement junction was located 1 mm above the level of the embedded resin. The roots were retained in the resin by a thin steel bar (10 mm long, Ø 1 mm) inserted in the apical third of the root. After embedding the teeth were stored in 0.1% thymol solution at room temperature .
The premolars were prepared for all-ceramic crowns with water spray cooling in standard manner (12° taper, approximately 3 mm axial length, and 0.8 mm right angle rounded marginal shoulder). The shoulder depth was defined with wheel diamond bur (Meisinger 908032, Neuss, Germany). The axial surfaces of the teeth were prepared using a hand piece secured in a parallelometer (Amann, Vienna, Austria) with 6° (one side to the long axis) standard cylindrical diamond bur at a speed of 55,000 rpm. A fine bur with the same angle was used for finishing of the preparation at the speed 10,000 r/m. The occlusal surface was prepared with occlusal double cone diamond bur (811.314.037, Komet Dental, Lemgo, Germany). Then the specimens were stored in 0.1% thymol solution.
The surface area of each preparation was determined by adapting a plastic foil (Adapta 0.5 mm, Bego, Bremen, Germany) to the master die and cutting foil excess at the line angles. This measurement foil was cut into pieces and digitized on a flat bed scanner (Epson Expression 1680 pro). The digitalized images were used to determine the surface area of the foil with a computer program (Photoshop CS3 extended, Adobe Systems Inc., CA, USA). This surface corresponded to the total surface of the preparation including the occlusal surface . The measured surface area ranged from 44.9 to 99.2 mm 2 . To minimize the effect of different surface areas, teeth were randomly allocated to groups.
CAD/CAM zirconia ceramic crowns (Vita In-Ceram YZ for inLab, Vita, Bad Säckingen, Germany) were manufactured in the dental laboratory of the prosthetic department of Kiel University, using a CAD/CAM machine (Cerec 3, Sirona, Bensheim, Germany). The upper occlusal surface of the crowns was designed with 60° (to the long axis of the tooth), and with two wings on the distal and mesial surfaces of the crown to provide retention for the dislodgment apparatus.
Two conventional luting cements (zinc phosphate cement; Hoffmann quick setting, Hoffmann, Berlin, Germany, and glass ionomer cement; Ketac Cem Maxicap, 3 M Espe, Seefeld, Germany), and one adhesive resin luting cement (Panavia 21 TC, Kuraray, Osaka, Japan), were used in this study ( Table 1 ). The uniformly prepared human premolars were divided by random into three cement groups with 32 teeth each. Half of the crowns in each group were air-abraded (alumina particles of size 50 μm, 0.25 MPa for 15 s) (Harnisch-Rieth P-G400, Winterbach, Germany) 1 h before cementation, while the other half of the specimens was left unconditioned “as received”. All the crowns were cleaned in the ultrasonic device (Sonorex super RK 510H, Bandelin Corp., Berlin, Germany) for 5 min with 99% ethanol directly before cementation.
|Zirconia ceramic (Vita In-Ceram YZ for inLab)||Vita, Bad Säckingen, Germany||19033|
|Zinc phosphate cement (Hoffmann quick setting)||Air-abraded||Hoffmann Dental, Berlin, Germany||P. 1104 A 16|
|As-received||L. 1116 B 04|
|Glass ionomer cement (Ketac Cem Maxi Cap)||Air-abraded||3M ESPE, Seefeld, Germany||249006|
|Risen composite cement (Panavia 21 TC)||Air-abraded||Kuraray, Osaka, Japan||41279|