The purpose of this study was to evaluate the fracture load of single zirconia abutment restorations using different veneering techniques and materials.
Materials and methods
The abutment restorations were divided into 6 groups with 20 samples each: test abutments (control group A), lithium disilicate ceramic crowns bonded on incisor abutments (group B), leucite ceramic crowns bonded on incisor abutments (group C), premolar abutments directly veneered with a fluor apatite ceramic (group D (layered) and group E (pressed)) and premolar abutments bonded with lithium disilicate ceramic crowns (group F). The fracture load of the restorations was evaluated using a universal testing machine. Half of each group was artificially aged (chewing simulation and thermocycling) before evaluating the fracture load with the exception of the test abutments.
The fracture load of the test abutments was 705 ± 43 N. Incisor abutments bonded with lithium disilicate or leucite ceramic crowns (groups B and C) showed fracture loads of about 580 N. Premolar restorations directly veneered with fluor apatite ceramic (groups D and E) showed fracture loads of about 850 N. Premolar restorations bonded with lithium disilicate ceramic crowns (group F) showed fracture loads of about 1850 N. The artificial ageing showed no significant influence on the strength of the examined restorations.
All ceramic crowns made of lithium disilicate glass-ceramic, adhesively bonded to premolar abutments showed the highest fracture loads in this study. However, all tested groups can withstand physiological bite forces.
Implant abutments made of yttria-stabilized zirconia (Y-TZP) are an interesting alternative to metallic abutments because of their optical properties. Especially in the visible anterior region this type of restoration provides esthetic advantages. This is mainly due to the white color of the abutments which may avoid gray shadows or even a metal-colored part that emerges from the gingiva .
The veneering and completion of those ceramic abutments to full crowns is a very important issue. Depending on the individual preferences of dentists and dental technicians, different crown systems come into consideration. One possibility to veneer a ceramic abutment is a separately produced all ceramic crown, which can be adhered to the abutment . Different ceramic materials such as high strength lithium disilicate or a weaker leucite glass-ceramic can be used.
Due to the fact that this superstructure is rather difficult to remove from the implant body in case of complications some dentists prefer a direct veneering of the ceramic abutment . In this case a screw cavity through the crown is needed to allow fixation and eventually removal of the implant crown. The cavity is filled with a composite. This leads to the possibility to remove or even replace the abutment crown very easily. However, direct veneering onto abutments is rarely indicated for upper incisor restorations because a prone implant axis would lead to a hole in the labial surface of the incisors created by the screw cavity. This is unacceptable due to the high esthetic requirements in this area.
In contrast, the direct veneering of a ceramic implant abutment is more often indicated in the premolar region due to the axial congruence of the implant compared to the replaced tooth. For this purpose different techniques, such as layering or pressing are available. In addition, the veneering of the ceramic abutment with a separately produced ceramic crown is of course also indicated in the premolar region.
The different materials and techniques mentioned can have an immense influence on the strength of the entire restoration. However, it is primarily important to investigate whether or not those techniques and materials can be used to produce reliable dental restorations. The hypothesis of this study was that all restorations can withstand the physiological forces in the oral environment.
Materials and methods
120 Bone Level implants (RC, 4.1 mm, Straumann AG, Basel, Switzerland) were embedded in polymethyl methacrylate (PMMA) (Technovit 4071, Heraeus Kulzer, Weinheim, Germany) to imitate the elastic reaction of the surrounding bone during loading. The upper edge of the embedding was set 3 mm beneath the implant shoulder to simulate bone resorption according to DIN EN ISO 14801 .
Special test abutments with a hemispherical attachment were used in group A according to DIN EN ISO 14801. In groups B–F Straumann Anatomical IPS e.max Abutments (Straumann, Basel, Switzerland) were used as mesostructure ( Fig. 1 ). For the tested premolar and incisor specimens, the abutment angle was 0° and 15°, respectively. The gingival height was 3.5 mm. The shape of those abutments was individualized to match the anatomical situation of a standard gypsum model used. To ensure comparability of the specimens, this adaption was realized industrially with a CNC milling machine (Röders, RXP 500 DSC). The emergence profile of the abutments was additionally individualized using a fluor apatite ceramic (IPS e.max Ceram, Ivoclar Vivadent AG, Schaan, Principality of Liechtenstein) and a silicone key.
The fabrication of crowns and direct veneerings was performed according to the instructions for use of the manufacturer. Incisor abutments were bonded to lithium disilicate glass-ceramic crowns (group B, IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and leucite glass-ceramic crowns (group C, IPS Empress CAD, Ivoclar Vivadent, Schaan, Liechtenstein). The veneering of the premolar abutments was carried out with a fluor apatite glass-ceramic by layering (group D, IPS e.max Ceram) or direct pressing (group E, IPS e.max ZirPress, Ivoclar Vivadent, Schaan, Liechtenstein). Additionally, lithium disilicate glass-ceramic crowns (group F, IPS e.max CAD) were adhered to premolar abutments. Each group consisted of 20 specimens. To investigate the influence of aging on the implant restorations (groups B–F), each group was divided into 10 aged and 10 non-aged specimens. An overview of all groups is given in Table 1 .
|Group||Restoration||Chewing simulation||Veneering technique||Veneering material||Biaxial flexural strength (MPa)|
|Bs||Incisor abutment||No||CAD/CAM||Lithium disilicate||360 ± 60|
|Bc + s||Incisor abutment||Yes||CAD/CAM||Lithium disilicate||360 ± 60|
|Cc + s||Incisor abutment||Yes||CAD/CAM||Leucite||160|
|Ds||Premolar abutment||No||Direct layering||Fluor apatite||90 ± 10|
|Dc + s||Premolar abutment||Yes||Direct layering||Fluor apatite||90 ± 10|
|Es||Premolar abutment||No||Direct pressing||Fluor apatite||110 ± 10|
|Ec + s||Premolar abutment||Yes||Direct pressing||Fluor apatite||110 ± 10|
|Fs||Premolar abutment||No||CAD/CAM||Lithium disilicate||360 ± 60|
|Fc + s||Premolar abutment||Yes||CAD/CAM||Lithium disilicate||360 ± 60|
Each all-ceramic crown was fabricated using a commercial dental CAD/CAM System (Sirona Cerec inLab MC XL, Sirona Dental Systems, Bensheim, Germany). Based on these dimensions, the shape of the directly veneered abutments was controlled using a silicone key. For the directly pressed veneerings CAD/CAM milled acrylic crowns (IPS AcrylCAD, Ivoclar Vivadent, Schaan, Liechtenstein) were used. Their dimensions were based on the previously designed data for all ceramic crowns. The incisor abutment veneering thickness was 2.2 mm at the incisal edge, 1 mm lingual and 1.4 mm labial. The premolar veneering thickness was 2.4 mm in the lingual cusp region, 2.7 mm in the buccal cusp region and 1.5 mm in the fissure region ( Fig. 2 ).
All abutments were attached to the implants with SCS occlusal screws (Straumann, Basel, Switzerland) and tightened at 35 Ncm with an electronic torque screwdriver (BMS, Limerick, Ireland). The screw cavities were filled with foam pellets and Fermit N (Ivoclar Vivadent, Schaan, Liechtenstein).
The bonding areas of the indirectly veneered abutments were sandblasted with Al 2 O 3 abrasive (100 μm, 1 bar). The inner surfaces of ceramic crowns were etched with 4.5% hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Liechtenstein). Bonding areas of abutments and crowns were silanized with Monobond Plus (Ivoclar Vivadent, Schaan, Liechtenstein) before adhesive placement with Multilink Implant (Ivoclar Vivadent, Schaan, Liechtenstein) according to the instructions of the manufacturer. Subsequently, the restorations were stored at 37 °C for 24 h before testing.
Chewing simulation of incisal and premolar crowns
The embedded implants were mounted in a steel holder and fixed to the chamber of the chewing simulator (eGo Kältetechnik, Regensburg, Germany) with a jig. The chewing simulator with integrated thermocycling uses pneumatic cylinders as force actuators that are regulated by a proportional valve. The whole process is driven and controlled by a modified industrial software (Zenon, COPA-DATA, Ottobrunn, Germany). The sinusoidal force profile was verified with an oscillograph and a force sensor. A hemispherical and a plane die (stainless steel) were used as antagonists for the premolar and incisal crowns, respectively. The antagonists were positioned on the premolar crowns to achieve contact on both the buccal and the lingual cusps. The contact points were checked with occlusion foil. Force transmission was parallel to the implant axis for the premolar restorations ( Fig. 3 ). For the incisor restorations the force was applied in a 30° angle to the implant axis. A 0.2 mm tin foil was placed between restorations and antagonists. During the cyclic loading at 1 Hz and 100 N maximum load the stylus was not lifted. Between two loading cycles, the restorations were unloaded for 100 ms. The chewing simulation was carried out in deionized water with simultaneous thermocycling (5–55 °C, 105 s per cycle). After chewing simulation, the restorations were analyzed with a stereomicroscope to ensure that no cracks occurred during aging.