Abstract
Objectives
The purpose of this in vitro study was to compare the centric and eccentric quasi-static and fatigue fracture strength of industrially prefabricated resin-bonded three-unit inlay-retained fixed dental prostheses (IPIRFDPs). The IPIRFDPs consisted of industrial manufactured yttria-stabilized tetragonal zirconia (Y-TZP) frameworks with an industrially added microhybrid composite veneering.
Methods
Identical IPIRFDP-models consisted of a second premolar, a missing first molar and a second molar (CoCrMo alloy) integrated in a low melting alloy base. Roots were covered with a soft silicone layer to simulate an artificial parodontium. Premolars had an occlusal–distal inlay-preparation and molars a mesial–occlusal inlay-preparation. Forty-two IPIRFDPs with a connector size of 9 mm 2 and a framework connector size of 4.7 mm 2 were cemented adhesively to the IPIRFDP-models. Quasi-static fracture strength was tested with centric ( n = 12) and eccentric ( n = 6) loading in a universal testing machine at a cross-head speed of 1 mm/min. Fatigue fracture strength was tested at 1200 N with centric loading ( n = 12) and at 600/500 N with eccentric loading ( n = 6) at a frequency of 0.5 Hz. Statistical comparison of groups was performed with the Mann–Whitney U test.
Results
Quasi-static fracture strength differed significantly between centric (1749 N) and eccentric loading (880 N, p < 0.001). Mean loading cycles until fracture were 4432 for centric loading at 1200 N compared to only 3 and 410 loading cycles for eccentric loading at 600 and 500 N, respectively.
Significance
Considering the maximum chewing forces in the molar region, it seems clinically possible to use prefabricated IPIRFDPs with Y-TZP as a core material with a framework connector size of 4.7 mm 2 .
1
Introduction
Among the permanent dentition, the first molar has the highest incidence of tooth loss due to caries, endodontic reasons, tooth fractures and periodontal diseases . However the first molar has a significant role in maintaining the intercuspal and condylar position . Therefore the replacement of a missing molar might be an important consideration to prevent further functional problems .
The traditional way of molar replacement is either a fixed dental prosthesis (FDP) or an implant retained crown . Irrespective of the type of FDP (porcelain fused to metal vs. all-ceramic crown) the clinician uses, a crown preparation is always a risk to pulp vitality and may lead to pulpal reactions in the long term . Approximately 63–73% of the coronal tooth structure is removed when teeth are prepared for crowns . Regarding these facts it seems desirable to adapt the type of abutment preparation to the extent of sound tooth structure after caries removal not only for a single tooth restoration, but also for abutment preparations for FDPs. Therefore, if a patient rejects an implant treatment and enough sound tooth structure is available it would be desirable to restore a missing tooth with an inlay-retained FDP instead of a crown-retained FDP.
Clinical evaluations of inlay-retained three-unit FDPs showed a failure rate of 10% after 9 months (Empress II) and 13% after 37 months (IPS e.max Press, Ivoclar-Vivadent, Schaan, Liechtenstein). In both studies the failure was evoked by debonding or a combination of both debonding and fracture. Despite these failure rates the inlay-retained FDP could be a favorable treatment option with respect to biological and economic reasons .
Recently, yttria-stabilized tetragonal zirconia (Y-TZP) has been made available to dentistry through CAD/CAM-techniques and provides excellent mechanical performance, superior strength and fracture resistance compared to other ceramics . Therefore Y-TZP ceramic might be an alternative material for the fabrication of inlay-retained FDPs to minimize the risk of fracture.
In the present study the quasi-static and fatigue fracture strengths of industrially prefabricated inlay-retained FDPs, consisting of Y-TZP frameworks with a microhybrid composite veneering, were determined. The null hypothesis tested was that centric and eccentric loading does not influence their fracture strength. In addition, it should be evaluated whether these FPDs provide fracture strengths, which make them a viable treatment option to replace a missing first molar during a single patient appointment.
2
Materials and methods
All materials used and batch numbers are summarized in Table 1 .
| Product | Material type | Manufacturer | Batch no. |
|---|---|---|---|
| Gapless hydrofluoric etchant | Hydrofluoric etchant | Gapless GmbH, Umkirch, Germany | LOT 07161 |
| Calibra Silane Coupling Agent | Silane | Dentsply DeTrey, Konstanz, Germany | LOT 070511 |
| XP-Bond | Bonding agent | Dentsply DeTrey, Konstanz, Germany | LOT 0702001786 |
| SCA | Self-curing activator | Dentsply DeTrey, Konstanz, Germany | LOT 070119 |
| Calibra esthetic resin cement | Self-curing resin based dental luting cement | Dentsply DeTrey, Konstanz, Germany | LOT 070525 (base) |
| Base/Catalyst | LOT 0705291 (cat.) | ||
| Airblock | Oxygen blocking gel | Dentsply DeTrey, Konstanz, Germany | LOT 072161 |
| Gapless FPD | Prefabricated all-ceramic inlay-retained three-unit FPDs | Gapless GmbH, Umkirch, Germany | LOT 10084-1-2+3/88, LOT 10084-1-2+3/89 |
2.1
Prefabricated all-ceramic three-unit inlay-retained fixed dental prostheses
In this study industrially prefabricated three-unit inlay-retained fixed dental prostheses (IPIRFDPs) were used (Gapless, Umkirch, Germany, Fig. 1 ). They are designed to restore upper or lower first molars chairside in one session. Three sizes (9, 10, and 11 mm pontic size) and three different colors (A, B, C, Gapless colors) are available.
The IPIRFDPs consist of an industrially manufactured yttria-stabilized tetragonal zirconia (Y-TZP) framework, a thin layer of feldspathic ceramic and microhybrid composite veneering . The Y-TZP framework has a connector height of 2.5 mm (4.7 mm 2 ) and provides a high fracture strength of IPIRFDPs, which was in previous tests 1200 N . A thin layer of feldspathic ceramic is fired to the framework in order to increase the bond strength between the Y-TZP framework and the veneering microhybrid composite and to enable the dentist to etch the surface of the connector area with hydrofluoric acid in preparation for the adhesive cement.
The microhybrid composite veneering is added industrially to the framework. The composite has a connector height of 2.3 mm above the Y-TZP framework, which leads to an overall connector height of 4.8 mm (9 mm 2 ). The height of the composite veneering in the pontic region of 2.9 mm above and 2.3 mm below the framework allows the dentist to adjust the base and the occlusal area of the pontic. The veneering microhybrid composite contains 24% (weight) organic matrix (Bis-GMA and Diurethandimethacylat) and 75% silane coated, inorganic siliciumdioxid filler particles, with a size of 0.04–3 μm and an average filler particle size of 0.7 μm. According to the manufacturer, the microhybrid composite resin has a fracture strength of 145 MPa and an elastic modulus of 9700 MPa (three-point flexural strength (DIN 13922/EN 24049)). The shear bond strength of the veneering microhybrid composite to the feldspathic layer on the Y-TZP framework was 900 N . The IPIRFDPs have been used as delivered without any modification.
2.2
Master models
Artificial teeth (No. 15 and 17, KaVo, Biberach, Germany) were integrated in an otherwise toothless KaVo model. A box-shaped inlay-preparation (3° divergence, 4.8 mm height, 1 mm minimal depth) was performed with a special diamond bur (100 μm grained) according to the manufacturer’s instructions (Gapless preparation set, Gapless). The teeth were duplicated (Speedy-duplication-technique ) and afterwards cast in a CoCrMo alloy (Wironit, Bego, Bremen, Germany). To create the master models one IPIRFDP was cemented provisionally (Freegenol, GC, Tokyo, Japan) to the prepared artificial teeth in the KaVo model and teeth and IPIRFDP afterwards duplicated together (Transpaduplisil, Wichnalek, Augsburg, Germany). The roots of the manufactured metal teeth were covered with a 200 μm thick soft silicone layer (Erkoskin, Erkodent, Pfalzgrafenweiler, Germany) to provide an artificial periodontal membrane, which allowed a periodontal movement of 50 μm . Tooth mobility was controlled with an electronic displacement transducer under a horizontal load of 20 N (W1T3, HBM, Darmstadt, Germany). Next, the prepared teeth were inserted into the duplicated form of the master models and the duplicated form filled with a low melting alloy (MCP 70, HEK, Lübeck, Germany). Fig. 2 exemplarily shows the schematic diagram of one specimen (IPIRFDP-model) and Fig. 3 explains the workflow from master model to specimen.
According to this procedure, 42 identical specimens with resilient supported teeth were fabricated and divided into the following four groups: centric (group CQSFS) and eccentric (group EQSFS) quasi-static fracture strength testing and centric (group CFFS) and eccentric (group EFFS) fatigue fracture strength testing.
2.3
Adhesive cementation
For all groups the surfaces of the IPIRFDPs were conditioned by etching with 5% hydrofluoric acid (Gapless) for 30 s and an application of silane coating (Calibra Silane, Dentsply DeTrey, Constance, Germany) for 1 min. The inlay cavities of the metal abutments were air-abraded with 110 μm alumina particles for 10 s at 0.3 MPa pressure. To ensure that the air-abraded surface was free of loose alumina particles, the metal abutments were ultrasonically cleaned for 3 min in isopropanol (96%) . In previous studies this procedure showed no negative effect on the bond strength of the luting resin. Finally, XP-BOND and SCA (Dentsply DeTrey) were mixed (1:1), applied to the IPIRFDPs and the inlay cavities for 3 min, but not light cured. After these conditioning procedures IPIRFDPs of all groups were cemented to the metal abutments with composite resin cement (Calibra, Dentsply DeTrey) according to the manufacturer’s recommendations. After removing excess luting resin, an air-blocking gel (Airblock, Dentsply DeTrey) was applied to the bonding margins and the luting resin was then light cured for 40 s at each of three different sites. The cemented IPIRFDPs were stored in distilled water for 3 days at 37 °C.
2.4
Testing procedures
A summary of all testing procedures and parameters is given in Fig. 4 . The centric quasi-static fracture strength (group CQSFS, n = 16) was tested in a universal testing machine (Zwick BZD10/TNZA, Ulm, Germany). The load was applied perpendicular to and at the centre of the IPIRFDPs using a stainless steel ball (diameter 5 mm) at a cross-head speed of 1 mm/min until fracture occurred. To avoid primary cracks at the point of loading a 0.5 mm foil of Makrolon (Macrolon, Bayer, Leverkusen, Germany) was inserted for all groups.
The eccentric quasi-static fracture strength (group EQSFS, n = 8) was also tested in the universal testing machine (Zwick BZD10/TNZA), but this time the load was applied 3 mm eccentric to the mesio-distal direction at the fissure between the mesiobuccal and distobuccal cusp of the IPIRFDPs. A stainless steel stamp (diameter 1.5 mm) was used at a cross-head speed of 1 mm/min until fracture occurred.
The centric fatigue fracture strength (group CFFS, n = 12) was tested in a universal testing machine (Zwick 1435, Ulm, Germany) and the specimen subjected to a dynamic load between 10 and 1200 N with a frequency of 0.5 Hz until the first fracture event occurred. As before, the load was applied perpendicular to and at the centre of the IPIRFDP pontic using a stainless steel ball (diameter 5 mm) and a 0.5 mm foil of macrolon. The steel ball was not lifted from the IPIRFDPs during loading. With these settings and the application of a basic loading of 10 N an impact pulse could be avoided.
The eccentric fatigue fracture strength (group EFFS, n = 6) was also tested in a universal testing machine (Zwick 1435, Ulm, Germany). The specimens were subjected to a dynamic load between 10 and 600 N with a frequency of 0.5 Hz until the first fracture event occurred. As in group EQSFS the load was applied 3 mm eccentric to the mesio-distal direction at the fissure between the mesiobuccal and distobuccal cusp of the IPIRFDPs using a stainless steel stamp (diameter 1.5 mm). At the top load of 600 N the fractures occurred for the first two specimens after a few trials and therefore the top load was reduced to 500 N ( n = 4) for group EFFS.
2.5
Statistical analyses
Due to the small group sizes and abnormally distributed data in some groups non-parametric testing was calculated by comparison of medians with the Mann–Whitney U test. All hypothesis testing was conducted at a 95% level of confidence.
2
Materials and methods
All materials used and batch numbers are summarized in Table 1 .
| Product | Material type | Manufacturer | Batch no. |
|---|---|---|---|
| Gapless hydrofluoric etchant | Hydrofluoric etchant | Gapless GmbH, Umkirch, Germany | LOT 07161 |
| Calibra Silane Coupling Agent | Silane | Dentsply DeTrey, Konstanz, Germany | LOT 070511 |
| XP-Bond | Bonding agent | Dentsply DeTrey, Konstanz, Germany | LOT 0702001786 |
| SCA | Self-curing activator | Dentsply DeTrey, Konstanz, Germany | LOT 070119 |
| Calibra esthetic resin cement | Self-curing resin based dental luting cement | Dentsply DeTrey, Konstanz, Germany | LOT 070525 (base) |
| Base/Catalyst | LOT 0705291 (cat.) | ||
| Airblock | Oxygen blocking gel | Dentsply DeTrey, Konstanz, Germany | LOT 072161 |
| Gapless FPD | Prefabricated all-ceramic inlay-retained three-unit FPDs | Gapless GmbH, Umkirch, Germany | LOT 10084-1-2+3/88, LOT 10084-1-2+3/89 |
2.1
Prefabricated all-ceramic three-unit inlay-retained fixed dental prostheses
In this study industrially prefabricated three-unit inlay-retained fixed dental prostheses (IPIRFDPs) were used (Gapless, Umkirch, Germany, Fig. 1 ). They are designed to restore upper or lower first molars chairside in one session. Three sizes (9, 10, and 11 mm pontic size) and three different colors (A, B, C, Gapless colors) are available.
