Effects of different surface treatments on the cyclic fatigue strength of one-piece CAD/CAM zirconia implants
Introduction
In the recent years, yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), a high-strength zirconia ceramic, has become an attractive new material for dental implants. Zirconia has tooth-like color and the ability to transmit light, improving the overall esthetic outcome (Oliva et al., 2010). Moreover, it has a high chemical resistance, high flexural strength (900–1200 MPa), a favorable fracture toughness (KIC, 7–10 MPa m1/2), and a suitable Young's modulus (210 GPa) (Ozkurt and Kazazoglu, 2011). Another advantage is the less affinity for dental plaque, minimizing the risk of inflammatory changes in the surrounding soft tissues (Scarano et al., 2010, Tete et al., 2009).
Both the implant body and the perimucosal portion can be individually designed to fit the local anatomical conditions and digitally machined. One-piece implants have the advantage of no implant/abutment movement or gap causing the leakage of harmful bacteria, as observed with conventional implant/abutment connections (Assenza et al., 2012). Because of the abovementioned advantages, Y-TZP implants have the potential to be an effective alternative of titanium implants in certain clinical situations (Andreiotelli et al., 2009, Oliva et al., 2010) as well as an appropriate material for computer-aided design and computer-aided manufacturing (CAD/CAM).
Previous clinical studies have reported promising survival rates of 98%, 96.5%, and 95% at 1, 3, and 5 years, respectively, after the placement of zirconia implants with rough surface topographies in partially edentulous patients (Brull et al., 2014, Oliva et al., 2010, Oliva et al., 2007). However, one study also reported a low survival rate of 77.3% after a mean follow-up period of 5.94 years. Furthermore, 18 implants failed because of fracture, mostly occurring in narrow implants with 3.25 mm in diameter (15/18) (Roehling et al., 2015).
The fracture of a dental implant is always a severe complication, leading to a high level of patient discomfort and many clinical problems such as the difficulty in removing the fractured implant and significant bone loss (Gahlert et al., 2012). Implant fractures in clinical use were caused by fatigue under physiological loads, and those failures were aggravated by the resorption of bone around the implants (Gahlert et al., 2012, Morgan et al., 1993). Static loading may have very slight clinical relevance as mechanical failures are more probably related to the application of repeated loads rather than an acute overload (Lee et al., 2009). As an important part of the mechanical properties of zirconia implants, fatigue is considered to be one of the key factors affecting the long-term clinical effect of the implants and their restoration (Rita Depprich, 2012).
Pure zirconium dioxide has three crystallographic forms that are stable at different temperature ranges under atmospheric pressure: the monoclinic phase, which is stable up to 1170 °C, where it transforms to the tetragonal phase, which in turn is stable up to 2370 °C, and the cubic phase, which exists up to its melting point of 2680 °C (Denry and Kelly, 2008). However, the tetragonal phase is in fact ‘‘metastable’’ at room temperature and may transform into the monoclinic phase, which can be triggered by mechanical stress, thermal aging, chemical aging, and intermittent forces during mastication in the oral environment (Guazzato et al., 2004, Pittayachawan et al., 2009)
Previous studies have demonstrated that surface modifications enhanced the integration of zirconia implants and resulted in a better osseointegration compared to the unmodified surface (Gahlert et al., 2007, Sennerby et al., 2005). Surface modifications such as sandblasting and acid etching trigger tetragonal-to-monoclinic (t → m) phase transformation (Karakoca and Yilmaz, 2009). This transformation is associated with 3–4% phase volume expansion and induces compressive stresses that shield the crack tip from the applied stress (Porter and Heuer, 1977). This unique characteristic is known as transformation toughening and may explain the increased fracture strength and fracture toughness of Y-TZP ceramics compared to other dental ceramics (Piconi and Maccauro, 1999).
On the other hand, the surface flaws introduced by sandblasting and acid etching act as stress concentrators and may become potential sites for crack initiation and propagation, causing strength degradation (Wang et al., 2008). The macroscopic and microscopic failure analysis in a clinical research showed that the presence of microcracks on a surface due to the manufacturing process, including machining and surface treatments, may be one of the primary reasons for zirconia implant fracture (Gahlert et al., 2012).
However, studies focused on the fatigue of zirconia implants after different surface treatments and the related mechanism are rare and are urgently required prior to consideration for routine clinical applications. Therefore, the aim of this study was to evaluate the effects of four different surface treatments on the cyclic fatigue strength and phase transformation of CAD/CAM Y-TZP implants.
Section snippets
Fabrication of zirconia implants
One-piece cylindrical screw-type zirconia implants were designed using the three-dimensional (3D) CAD software (Catia V5R19, Dassault System, France; Geomagic Studio 12.0, Geomagic, USA) and fabricated using the following procedure. To reduce processing time and wear of the cutting instruments during the milling process, partially sintered Y-TZP blocks instead of fully sintered ones were used in this study. First, partially sintered zirconia milling blocks (Y-TZP, Wieland, Germany) were cut
Microscopic observation and surface roughness of implants
The SEM images show that the CTRL surface is relatively flat with flaws such as notches and dents due to the CAM process (Luthardt et al., 2004). The surface of SB exhibited a macrorough topography with grooves and holes with sharp margins. After acid etching with experimental hot etching solution, the surfaces appeared uneven with peaks and valleys evident and nanoscale irregular pores. In contrast, the hydrofluoric acid etched surfaces exhibited a more regular porous topography in both the
Effect of sandblasting or sandblasting and acid etching on the fatigue strength of zirconia implants
In this study, sandblasting with 110-μm Al2O3 particles or sandblasting and acid etching with an experimental hot solution improved the fatigue and fracture resistance of the CAD/CAM zirconia implants, which can be attributed to the transformation toughening of Y-TZP. The sandblasting and acid etching triggered t → m phase transformation that induced compressive stresses, thus closing the crack tip and preventing further crack propagation (Ozcan et al., 2013, Porter and Heuer, 1977). The
Conclusion
Within the limitations of this study, it can be concluded that a CAD/CAM zirconia implant has a favorable fracture and fatigue resistance. Compared to sintering, sandblasting or sandblasting and experimental hot acid solution etching achieved moderately rough surfaces (Ra, 1–2 µm) and resulted in a higher fracture and fatigue resistance of CAD/CAM zirconia implants. However, sandblasting and hydrofluoric acid etching resulted in the roughest surfaces without increasing fracture and fatigue
Acknowledgements
This work was supported by the National Natural Science Foundation of China [grant number 81671026].
Declarations of interest
None.
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