Long-term stability of an injection-molded zirconia bone-level implant: A testing protocol considering aging kinetics and dynamic fatigue

Influence of loading and aging on the fracture strength of an injection-molded two-piece zirconia implant restored with a zirconia abutment

OBJECTIVE

To investigate the fracture strength and potential phase transformation of an injection-molded two-piece zirconia implant restored with a zirconia abutment after loading and/or aging.

METHODS

Thirty-two two-piece zirconia implants (4.0 mm diameter) restored with zirconia abutments were embedded according to ISO 14801 and divided into four groups (n = 8/group): Three groups were either exclusively hydrothermally treated (group HT; 85°C), dynamically loaded (group DL; 10⁷  cycles; 98 N), or subjected to both treatments simultaneously (group DL/HT). One group remained untreated (group 0). A sample from each group was cross-sectioned and examined by scanning electron microscopy for possible crystal phase transformation. The remaining samples were then loaded to fracture in a static loading test. A one-way ANOVA was used for statistical analyses.

RESULTS

During dynamic loading, three implants of group DL and six implants of group DL/HT fractured at a load of 98 N. The fracture strength of group DL/HT (108 ± 141 Ncm) was significantly reduced compared to the other groups (group 0: 342 ± 36 Ncm; HT: 363 ± 49 Ncm; DL: 264 ± 198 Ncm) (p < .05). Fractures from group 0 and HT occurred at both implant and abutment level, whereas implants from group DL and DL/HT fractured only at implant level. A shallow monoclinic transformation zone of approximately 2 μm was observed following hydrothermal treatment.

CONCLUSIONS

Within the limitations of this study, it can be concluded that dynamic loading and the combination of loading and aging reduced the fracture strength of the implant abutment combination. Hydrothermal treatment caused a shallow transformation zone which had no influence on the fracture strength.

3D printed zirconia dental implants with integrated directional surface pores combine mechanical strength with favorable osteoblast response

Dental implants need to combine mechanical strength with promoted osseointegration. Currently used subtractive manufacturing techniques require a multi-step process to obtain a rough surface topography that stimulates osseointegration. Advantageously, additive manufacturing (AM) enables direct implant shaping with unique geometries and surface topographies. In this study, zirconia implants with integrated lamellar surface topography were additively manufactured by nano-particle ink-jetting. The ISO-14801 fracture load of as-sintered implants (516±39 N) resisted fatigue in 5-55°C water thermo-cycling (631±134 N). Remarkably, simultaneous mechanical fatigue and hydrothermal aging at 90°C significantly increased the implant strength to 909±280 N due to compressive stress generated at the seamless transition of the 30-40 µm thick, rough and porous surface layer to the dense implant core. This unique surface structure induced an elongated osteoblast morphology with uniform cell orientation and allowed for osteoblast proliferation, long-term attachment and matrix mineralization. In conclusion, the developed AM zirconia implants not only provided high long-term mechanical resistance thanks to the dense core along with compressive stress induced at the transition zone, but also generated a favorable osteoblast response owing to the integrated directional surface pores. STATEMENT OF SIGNIFICANCE: : Zirconia ceramics are becoming the material of choice for metal-free dental implants, however significant efforts are required to obtain a rough/porous surface for enhanced osseointegration, along with the risk of surface delamination and/or microstructure variation. In this study, we addressed the challenge by additively manufacturing implants that seamlessly combine dense core with a porous surface layer. For the first time, a unique surface with a directional lamellar pore morphology was additively obtained. This AM implant also provided strength as strong as conventionally manufactured zirconia implants before and after long-term fatigue. Favorable osteoblast response was proved by in-vitro cell investigation. This work demonstrated the opportunity to AM fabricate novel ceramic implants that can simultaneously meet the mechanical and biological functionality requirements.

Influence of different zirconia surface treatments on biofilm formation in vitro and in situ

OBJECTIVES

To determine if the surface treatment of zirconia affects biofilm formation in an in vitro three-species biofilm model and in situ.

MATERIAL AND METHODS

Zirconia surfaces considered for the transmucosal portion of a zirconia implant were compared with polished pure titanium grade 4 (Tp). Discs 13 mm in diameter of either polished (Zp), polished and heat-treated (Zpt), machined (Zm), machined and heat-treated (Zmt) and sandblasted, etched and heat-treated (Z14) zirconia were fabricated. Surface roughness and wettability of specimens was measured. Biofilm formation was evaluated by safranin staining and scanning electron microscopy (SEM) using a three-species model, and intraorally with 16 volunteers carrying oral splints in two independent experiments. Relative biofilm formation was compared with Kruskal-Wallis followed by Bonferroni post-hoc test (α=0.05).

RESULTS

In vitro biofilm formation with optical density values on Zp (0.14±0.01), Zpt (0.14±0.02), Zm (0.13±0.01) and Zmt (0.13±0.01) was significantly lower than on Tp (0.21±0.05) and Z14 (0.20±0.04) (p<0.05). In situ biofilm formation was significantly higher on Z14 (0.56±0.45) (p<0.05), while no significant differences in optical density were observed among Zp (0.25±0.20), Zm (0.36±0.34) and Tp (0.28±0.22). SEM analysis supported quantitative findings.

CONCLUSIONS

In the in vitro three-species biofilm model differences in material and surface roughness affected biofilm formation. In situ biofilm formation was mainly affected by the surface roughness of the specimens. Polishing of zirconia is recommended to reduce biofilm formation while heat-treatment has no significant effect.

Fracture resistance and crystal phase transformation of a one- and a two-piece zirconia implant with and without simultaneous loading and aging-An in vitro study

OBJECTIVE

To evaluate the influence of artificial aging on the transformation propagation and fracture resistance of zirconia implants.

METHODS

One-piece (with integrated implant abutment, 1P; regular diameter [4.1mm]; n = 16) and two-piece (with separate implant abutment, 2P; wide diameter [5 mm]; n = 16) zirconia implants were embedded according to ISO 14801. A two-piece titanium-zirconium implant (Ti-Zr; 4.1 mm diameter) served as a control (n = 16). One subgroup (n = 8) of each system was simultaneously dynamically loaded (10⁷ cycles; 98N) and hydrothermally aged (85°C, 58 days), while the other subgroup (n = 8) remained untreated. Finally, specimens were statically loaded to fracture. Potential crystal phase transformation was examined at cross sections using scanning electron microscopy (SEM). A multivariate linear regression model was applied for statistical analyses.

RESULTS

The fracture resistance of 1P (1,117 [SD = 38] N; loaded/aged: 1,009 [60] N), 2P (850 [36] N; loaded/aged: 799 [84] N), and Ti-Zr implants (1,338 [205] N; loaded/aged: 1,319 [247] N) was not affected significantly by loading/aging (p = .171). However, when comparing the systems, they revealed significant differences independent of loading/aging (p ≤ .001). Regarding the crystal structure, a transformation zone was observed in SEM images of 1P only after aging, while 2P showed a transformation zone even before aging. After hydrothermal treatment, an increase of this monoclinic layer was observed in both systems.

CONCLUSIONS

The Ti-Zr control implant showed higher fracture resistance compared to both zirconia implants. Loading/aging had no significant impact on the fracture resistance of both zirconia implants. The wide-body 2P zirconia implant was weaker than the regular body 1P implant.

A Novel Zirconia-Based Composite Presents an Aging Resistant Material for Narrow-Diameter Ceramic Implants

A novel ceria-stabilized zirconia-alumina-aluminate composite (Ce-TZP-comp) that is not prone to aging presents a potential alternative to yttrium-stabilized zirconia for ceramic oral implants. The objective of this study was to evaluate the long-term stability of a one-piece narrow-diameter implant made of Ce-TZP-comp. Implant prototypes with a narrow (3.4 mm) and regular (4.0 mm) diameter were embedded according to ISO 14801, and subgroups (n = 8) were subsequently exposed to dynamic loading (10⁷ cycles, 98N) and/or hydrothermal treatment (aging, 85 °C). Loading/aging was only applied as a combined protocol for the 4.0 mm diameter implants. One subgroup of each diameter remained untreated. One sample was cross-sectioned from each subgroup and evaluated with a scanning electron microscope for phase-transformation of the lattice. Finally, the remaining samples were loaded to fracture. A multivariate linear regression model was applied for statistical analyses (significance at p < 0.05). All samples withstood the different loading/aging protocols and no transformation propagation was observed. The narrow diameter implants showed the lowest fracture load after combined loading/aging (628 ± 56 N; p < 0.01), whereas all other subgroups exhibited no significantly reduced fracture resistance (between 762 ± 62 and 806 ± 73 N; p > 0.05). Therefore, fracture load values of Ce-TZP-comp implants suggest a reliable intraoral clinical application in the anterior jaw regions.

Hydrothermal-aging-induced lattice distortion in yttria-stabilized zirconia using EBSD technique

The destabilization problem is of importance in the application of yttria-stabilized zirconia (YSZ) bio-ceramic in the oral environment due to phase transformation between tetragonal to monoclinic. Thus, in this study, the lattice distortion induced by hydrothermal aging in yttria-stabilized zirconia (YSZ) was investigated, in which YSZ specimens were subjected to hydrothermal-aging treatment for 0-48 h. The Kikuchi-band based method was employed to calculate the lattice distortion after phase transformation and the results from EBSD were compared with these obtained by X-ray diffraction (XRD). Both measurement methods showed a similar tendency of the lattice change in the a- and c- axes. EBSD results showed that the strain rates were 7.98 % and -5.03 % at the a- and c-axes, respectively. A significant decrease in the c/a ratio from 1.429 to 1.257 for the tetragonal matrix after 48 h aging is observed, which indicated Kikuchi-band based method using EBSD technique can successfully determine the local strain in tetragonal matrices.

Reliability of an injection-moulded two-piece zirconia implant with PEKK abutment after long-term thermo-mechanical loading

Zirconia implants are appreciated in some clinical indications in light of their aesthetic appearance and good biocompatibility. The aim of this work was to evaluate the performance of a newly developed two-piece zirconia/polyether ketone ketone (PEKK) implant-abutment combination after long-term cyclic loading in a hydrothermal environment, using a new protocol adapted from two available ISO standards. Sixteen implants (n = 8/group) were embedded according to ISO 14801 and divided into two groups: implants in the Observational Group (OG) were cyclically loaded for 60 days (98 N, 10 million loading cycles, 2 Hz) in 85 °C water in a chewing simulator, while non-loaded/non-aged implants (as-received) constituted the Control Group (CG). After 4.7 million loading cycles, one OG implant fractured in the chewing simulator. The surviving implants were compared to CG implants by X-ray diffraction (XRD) to investigate potential ageing as suggested by ISO 13356, but also μ-Raman spectroscopy, Focused-Ion-Beam - Scanning-Electron-Microscopy (FIB-SEM), and load-to-fracture. Ageing was shown to have limited influence on the evaluated zirconia implant, with increased monoclinic content after loading/ageing being to a shallow transformed zone of ~2 μm at the implant surface. However, OG implants showed a significantly decreased fracture load of 751 ± 231 N (CG: 995 ± 161 N; p = .046). These values enable clinical application, but the fact that one failure was recorded during cyclic fatigue along with the significant decrease in strength after cyclic loading/ageing suggest that there may be room for further optimization of especially the PEKK abutment. Furthermore, good agreement was observed between the fracture modes of the implant that failed during the cyclic fatigue experiment and the in vivo failure of one implant during pre-clinical trials, validating the interest of the in vitro protocol used in this work to check the reliability of zirconia implant.

Fracture Resistance of Zirconia Oral Implants In Vitro: A Systematic Review and Meta-Analysis

Various protocols are available to preclinically assess the fracture resistance of zirconia oral implants. The objective of the present review was to determine the impact of different treatments (dynamic loading, hydrothermal aging) and implant features (e.g., material, design or manufacturing) on the fracture resistance of zirconia implants. An electronic screening of two databases (MEDLINE/Pubmed, Embase) was performed. Investigations including > 5 screw-shaped implants providing information to calculate the bending moment at the time point of static loading to fracture were considered. Data was extracted and meta-analyses were conducted using multilevel mixed-effects generalized linear models (GLMs). The Šidák method was used to correct for multiple testing. The initial search resulted in 1864 articles, and finally 19 investigations loading 731 zirconia implants to fracture were analyzed. In general, fracture resistance was affected by the implant design (1-piece > 2-piece, p = 0.004), material (alumina-toughened zirconia/ATZ > yttria-stabilized tetragonal zirconia polycrystal/Y-TZP, p = 0.002) and abutment preparation (untouched > modified/grinded, p < 0.001). In case of 2-piece implants, the amount of dynamic loading cycles prior to static loading (p < 0.001) or anatomical crown supply (p < 0.001) negatively affected the outcome. No impact was found for hydrothermal aging. Heterogeneous findings of the present review highlight the importance of thoroughly and individually evaluating the fracture resistance of every zirconia implant system prior to market release.

High-translucent yttria-stabilized zirconia ceramics are wear-resistant and antagonist-friendly

OBJECTIVES

To evaluate two-body wear of three zirconia ceramics stabilized with 3, 4 and 5mol% yttria and to compare their wear behavior with that of a lithium-disilicate glass-ceramic.

METHODS

Sixteen rectangular-shaped specimens made from three grades of zirconia ceramics and a lithium-disilicate glass-ceramic were polished and dynamically loaded in a chewing simulator (2kg vertical load, 2.1Hz) under water at 90°C for 1.2×10⁶ cycles (about 7 days) in the ball-on-plate mode against steatite antagonists. Surface roughness was measured before and after wear testing. Wear tracks were scanned with a non-contact 3D profilometer and super-impositions were used to determine wear loss of the antagonists. Wear surfaces were imaged by SEM. XRD and micro-Raman spectroscopy were used to characterize phase transformation and stress status in the worn and unworn areas of the zirconia ceramics.

RESULTS

Independent of fracture toughness, strength and aging-susceptibility, the three zirconia ceramics showed a similar and limited amount of wear (∼10μm in depth) and were more wear-resistant than the lithium-disilicate glass-ceramic (∼880μm in depth). Abrasive wear without obvious cracks was observed for all investigated zirconias, whereas the glass-ceramic with a lower fatigue threshold and high susceptibility to surface dissolution exhibited significant abrasion, fatigue and corrosion wear. All three zirconia ceramics yielded a lower antagonist wear than the glass-ceramic and no significant differences were found between the zirconia ceramics.

SIGNIFICANCE

In the context of this study, high-translucent zirconia ceramics stabilized with a higher yttria content, recently introduced in the dental field, were as wear-resistant and antagonist-friendly as conventional high-strength zirconia and suitable for monolithic restorations.

Is a Zirconia Dental Implant Safe When It Is Available on the Market?

The market share of zirconia (ZrO2) dental implants is steadily increasing. This material comprises a polymorphous character with three temperature-dependent crystalline structures, namely monoclinic (m), tetragonal (t) and cubic (c) phases. Special attention is given to the tetragonal phase when maintained in a metastable state at room temperature. Metastable tetragonal grains allow for the beneficial phenomenon of Phase Transformation Toughening (PTT), resulting in a high fracture resistance, but may lead to an undesired surface transformation to the monoclinic phase in a humid environment (low-temperature degradation, LTD, often referred to as ‘ageing’). Today, the clinical safety of zirconia dental implants by means of long-term stability is being addressed by two international ISO standards. These standards impose different experimental setups concerning the dynamic fatigue resistance of the final product (ISO 14801) or the ageing behavior of a standardized sample (ISO 13356) separately. However, when evaluating zirconia dental implants pre-clinically, oral environmental conditions should be simulated to the extent possible by combining a hydrothermal treatment and dynamic fatigue. For failure analysis, phase transformation might be quantified by non-destructive techniques, such as X-Ray Diffraction (XRD) or Raman spectroscopy, whereas Scanning Electron Microscopy (SEM) of cross-sections or Focused Ion Beam (FIB) sections might be used for visualization of the monoclinic layer growth in depth. Finally, a minimum load should be defined for static loading to fracture. The purpose of this communication is to contribute to the current discussion on how to optimize the aforementioned standards in order to guarantee clinical safety for the patients.

Stability and aging resistance of a zirconia oral implant using a carbon fiber-reinforced screw for implant-abutment connection

OBJECTIVE

To investigate the long-term stability of a metal-free zirconia two-piece implant assembled with a carbon fiber-reinforced (CRF) screw by means of transformation propagation, potential changes in surface roughness, the gap size of the implant-abutment connection, and fracture load values.

METHODS

In a combined procedure, two-piece implants made from alumina-toughened zirconia were dynamically loaded (10⁷ cycles) and hydrothermally aged (85°, 60days). Implants made from titanium (Ti) and a titanium-zirconium (TiZr) alloy with a titanium abutment screw served as control. Transformation propagation (ATZ) and gap size of the IAC were monitored at cross-sections by scanning electron microscopy (SEM). Furthermore, changes in surface roughness of ATZ implants were measured. Finally, implants were statically loaded to fracture. Linear regression models and pairwise comparisons were used for statistical analyses.

RESULTS

Independent of the implant bulk material, dynamic loading/hydrothermal aging did not decrease fracture resistance (p=0.704). All test and control implants fractured at mean loads >1100N. Gap size of the IAC remained stable (<5μm) or decreased. None of the CFR screws fractured during static or dynamic loading. Monoclinic layer thickness of ATZ implants increased by 2-3μm at surfaces exposed to water, including internal surfaces of the IAC. No changes in surface roughness were observed.

SIGNIFICANCE

Combined hydrothermal aging and dynamic loading did not affect the above-mentioned parameters of the evaluated two-piece ATZ implant. Mean fracture loads >1100N suggest a reliable clinical application.