Highly-translucent 3Y-TZP is more susceptible to in vivo ageing than classic 3Y-TZP.
Rapid sintering increases the susceptibility to in vivo ageing.
Airborne-particle abrasion suppresses in vivo ageing.
Surface degradation after 4 years in vivo was within clinically acceptable range
3Y-TZP ceramics with reduced alumina content have improved translucency and are used in monolithic dental restorations without porcelain-based veneers. The workflow can be further streamlined with rapid sintering. This study was designed to assess how these approaches affect ageing when the materials are exposed to the oral environment in vivo .
43 discs were fabricated from 3Y-TZP powder with 0.05% Al 2 O 3 and sintered with conventional or rapid regimens (1450 °C 2 h, 1530 °C 2 h, or 1530 °C 25 min). Their surfaces were polished or airborne-particle abraded with 50 μm Al 2 O 3 . The discs were incorporated in complete dentures of 16 volunteers and worn continuously for up to 48 months. Ageing changes on disc surfaces were monitored every 6 months by X-ray diffraction, scanning electron microscopy and atomic force microscopy. Data was statistically analysed with linear models.
The amount of monoclinic phase on polished surfaces increased linearly, reaching up to 40% after 48 months in vivo . The ageing process observed for rapid sintering was 1.6 times faster compared to conventional sintering. A nano-scale increase in roughness with microcracking was also detected on polished surfaces. Airborne-particle abraded surfaces did not exhibit clear signs of ageing during the course of the study.
Highly-translucent 3Y-TZP ceramics are more susceptible to ageing than classic 3Y-TZP. After 4 years in vivo , the extent of degradation did not yet constitute grounds for clinical concern, but was more pronounced in materials prepared with rapid sintering.
This is the second of our two co-published articles about ageing of 3Y-TZP ceramics in vivo . The aim was to build on the conclusions of the previous study [ ], to move beyond traditional, biomedical grade 3Y-TZP ceramics and investigate ageing properties when modern material variants and processing techniques are applied.
Traditional dental 3Y-TZP ceramics are very opaque and have to be veneered with glass-based porcelain veneers to achieve acceptable aesthetics. As veneers are prone to chipping [ ], there is a strong drive to circumvent this problem by using monolithic zirconia instead. The full-contour technique requires specially formulated versions of zirconia ceramics with improved light transmittance. This can be achieved by decreasing the grain size, reducing the porosity [ ] and modifying the amount of dopants such as yttria and alumina [ ]. Dopants are normally added to aid sintering and control the transformability, and consequently also affect the susceptibility to low temperature degradation (LTD) [ ]. Typically, the alumina content in zirconia ceramics is 0.25%, but highly-translucent variants have it reduced to 0.05%. Such concentration of Al 3+ is not sufficient for the isolated alumina grain formation in the 3Y-TZP matrix during sintering. A decrease in its protective effect against ageing can therefore be expected [ ]. Another strategy to improve optical properties is by increasing the yttria content and decreasing the tetragonality of the system. But although cubic Y-TZP ceramic is both translucent and ageing-resistant, it is a very different, mechanically weaker material due to the absence of the t-m toughening mechanism [ , ] and was not considered in this study.
As monolithic zirconia restorations do not require veneering, less steps are needed in production, which can be a considerable advantage. A complementary approach to further streamline the workflow is to shorten the sintering times. Sintering typically takes about 12 h and is the most time-consuming component of working with zirconia ceramics. Modern rapid sintering protocols only take a fraction of this time and are currently gaining traction [ ], but knowledge on their effect on the final material’s properties is still limited. Increased heating rates and shorter dwell times result in specific material microstructures with larger grain sizes, and the implications for their mechanical performance and translucency are not always beneficial [ ]. Our main goal was to assess the susceptibility of such ceramics to ageing when exposed to the oral environment in vivo . The ceramics were prepared with conventional or rapid sintering, and their surfaces were either polished or airborne-particle abraded (sandblasted) to emulate the relevant preparation procedures in a clinical setting. The experimental design used our established methodology with complete dentures as vehicles to hold the ceramic samples in patients’ mouths for up to 48 months [ ].
Materials and methods
Clinical study background
This clinical study was designed as a collaboration between the Department of Prosthodontics, Faculty of Medicine, University of Ljubljana and Faculty of Dentistry, Ss. Cyril and Methodius University. The study protocol consisted of inserting the ceramic specimens in lower complete dentures worn by patients, as also described in our previous publication [ ]. The study was granted approval by the Republic of Slovenia National Medical Ethics Committee (approval no. 61/04/1011). All the procedures were in accordance with the 1975 Declaration of Helsinki, as revised in 1983. The following inclusion criteria were used to select the patients:
good general satisfaction with the existing complete dentures based on the Patient Denture Assessment (PDA) questionnaire [ ],
personal preference and willingness to wear the dentures 24 h a day,
no medical history that might interfere with the planned 6-month recall during the course of the study.
The present series consisted of 16 volunteers (5 men and 11 women). They received verbal and written explanation on the study design and the purpose of the research. The participation was confirmed by signing an informed consent form. The participants agreed to take part in a regular 6-month recall program and were free to stop participating at any time. The target time frame was 24 months. Patients were then given the option to continue in the study extension for up to 48 months.
Preparation of the intraoral ageing devices
The studied ceramic materials to be incorporated into dentures were prepared from commercially available, translucent grade 3Y-TZP granulated powder containing 3 mol% of yttria, 0.05 wt.% of alumina and 3 wt.% of an organic binder (TZ-PX-242A, Tosoh, Tokyo, Japan).
|Material||Sintering regimen||Average grain size in μm (SD)||Surface treatment|
|CS1450||Conventional sintering: heat 5 °C/min, dwell 1450 °C 2 h, cool 5 °C/min until ambient temperature||0.26 (0.02)||Polishing (n = 10)|
|APA (n = 10)|
|CS1530||Conventional sintering: heat 5 °C/min, dwell 1530 °C 2 h, cool 5 °C/min until ambient temperature||0.32 (0.03)||Polishing (n = 12)|
|RS1530||Rapid sintering: heat 60 °C/min, dwell 700 °C 2 min, heat 60 °C/min, dwell 1300 °C 2 min, heat 40 °C/min, dwell 1530 °C 25 min, cool 90 °C/min until 700 °C, cool 60 °C/min until 400 °C, cool 40 °C/min until ambient temperature||0.47 (0.06)||Polishing (n = 11)|
After sintering, the final diameter of the discs was 8 mm and their thickness 1 mm. The relative density was determined by the Archimedes’ method, using deionized water as the immersion liquid. A theoretical density of ρ T = 6.08 g/cm 3 for the tetragonal phase was used in the calculations. The relative density exceeded 99% of the theoretical value. The grain sizes were estimated from FE-SEM images (GeminiSEM, Carl Zeiss AG, Jena, Germany) using the linear-intercept procedure based on the ASTM E112-13 standard without introducing any correction factors. The examined ceramic surfaces were mirror polished, thermally etched (1300 °C for 30 min in air) to expose the grain boundaries and examined without any surface coating applied ( Fig.1 ).
Ten CS1450 specimens were airborne-particle abraded with 50 μm Al 2 O 3 particles at a pressure of 2.5 bar, using an air-abrasion unit (Basic IS, Renfert Dental, Hilzingen, Germany). The rest of the specimens were mirror polished. After surface treatments, the specimens were ultrasonically cleaned with acetone and deionized water.
Prior to inserting the ceramic specimens in the dentures, the necessary space was prepared in suitably flat regions of the denture’s sublingual flanges. The patients were instructed to wear their dentures continuously, removing them only for daily cleaning with soapy water and a denture brush. Regular recall visits were scheduled to ensure the patients were in good oral health and could use their dentures without difficulties. If any denture-related lesions were visible on the mucosa, the dentures were adjusted accordingly or relined.
Ceramic specimen analyses
At every 6-month recall appointment, ceramic specimens were temporarily removed from the dentures and submerged in 2.5% sodium hypochlorite for 10 min to remove the organic debris from the surface. After rinsing, the specimens were cleaned ultrasonically in acetone, ethanol and deionized water for 10 min to prepare them for surface characterizations with XRD, FE-SEM and AFM. XRD patterns were collected using Cu-Kα radiation at 45 kV and 40 mA over the range of 25°–40° 2θ (X’Pert PRO X-Ray diffractometer, PANalytical, Almelo, The Netherlands). The X m was estimated using the Garvie and Nicholson method [ ].
Scanning electron micrographs of ceramic surfaces were taken with FE-SEM (GeminiSEM, Carl Zeiss AG, Jena, Germany) without any surface coating applied. AFM analysis of the polished surfaces was performed in the contact mode with the scan size of 10μm × 10μm (Dimension 3100, Veeco Instruments, Plainview, USA). The acquired AFM data was processed with the WSXm 4.0 Beta 8.1 software [ ], utilizing three measurements to estimate the mean roughness (Ra).
When the analyses were complete, the specimens were re-inserted in the corresponding patients’ dentures using cold-curing acrylic resin. At the end of the study, the discs were permanently removed from the dentures and cross-sections perpendicular to aged surfaces were prepared by FIB machining (Helios Nanolab 650, FEI, Hillsboro, USA). A 0.5 μm layer of platinum film was sputtered on the area of interest, using the ion-beam-assisted gas injection system at 30 kV and 0.43 nA to prevent the extensive curtain effect. FIB trenches were cut at 30 kV and 65 nA and finalized by ion polishing at 30 kV and 21 nA. The transformation depth and changes in the immediate subsurface zone were observed in situ , at an angle of 52°, using the electron probe at 2 kV and 100 pA.
The data on X m was statistically analysed with linear models using the statistics software package R 3.1.2 [ ]. Ageing was treated as a numerical variable expressed as months in vivo . Sintering regimen was treated as a descriptive variable with three levels: conventional sintering at 1450 °C, conventional sintering at 1530 °C and rapid sintering at 1530 °C. Surface treatment was treated as a descriptive variable with two levels: polishing or airborne-particle abrasion. Pairwise differences between treatment groups were further examined with Tukey’s HSD test. The significance level was set to α = 0.05.
The retention rate and compliance of study participants was very good. One participant was lost to follow-up after 6 months, one moved abroad after 12 months, and one could not continue wearing their dentures due to suffering a stroke 12 months into the study. After 24 months, 6 participants elected to continue with the study extension for up to 48 months. These participants were wearing polished CS1450 and RS1530 specimens. We were therefore able to obtain 48-month results for these groups, and 24-month results for polished CS1530 and airborne-particle abraded CS1450 specimens.
XRD analyses revealed changes in phase composition, reflecting the rate and the extent of ageing in vivo, as presented in Fig. 2 and Table 2 . Before ageing, polished samples were monoclinic phase-free, whereas X m of the airborne-particle abraded samples was 2.5%. The latter also exhibited low-2θ-angle asymmetric broadening of peak (111) t’ /(101) t , the reversed intensity of peaks (002) t and (110) t , and an increased full width at half maximum (FWHM) value of peak (002) t .
|Ageing in vivo (months)||Material||Surface treatment||X m in % (SD)|
|0||CS1450||Polished||0.1 p (0.14)|
|0||CS1530||Polished||0.0 p (0.00)|
|0||RS1530||Polished||0.1 p (0.13)|
|0||CS1450||APA||2.5 n (0.31)|
|6||CS1450||Polished||2.7 n (0.47)|
|6||CS1530||Polished||5.2 m (0.47)|
|6||RS1530||Polished||9.0 j (0.66)|
|6||CS1450||APA||3.6 n (0.62)|
|12||CS1450||Polished||5.4 lm (0.72)|
|12||CS1530||Polished||6.7 kl (0.64)|
|12||RS1530||Polished||13.4 h (1.37)|
|12||CS1450||APA||3.5 n (0.68)|
|18||CS1450||Polished||8.4 j (0.87)|
|18||CS1530||Polished||8.2 jk (0.49)|
|18||RS1530||Polished||18.8 f (1.22)|
|18||CS1450||APA||4.0 mn (0.61)|
|24||CS1450||Polished||11.9 i (1.22)|
|24||CS1530||Polished||9.3 j (0.71)|
|24||RS1530||Polished||23.3 e (0.85)|
|24||CS1450||APA||4.1 mn (0.61)|
|30||CS1450||Polished||15.5 g (2.02)|
|30||RS1530||Polished||28.2 d (0.92)|
|36||CS1450||Polished||19.5 f (1.76)|
|36||RS1530||Polished||32.6 c (1.10)|
|42||CS1450||Polished||21.7 e (2.48)|
|42||RS1530||Polished||36.9 b (0.84)|
|48||CS1450||Polished||27.3 d (0.49)|
|48||RS1530||Polished||39.7 a (0.80)|