Long-Term Stability of Hydrothermally Aged and/or Dynamically Loaded One-Piece Diameter Reduced Zirconia Oral Implants

Abstract

The aim of this in vitro study was to evaluate the long-term stability of one-piece diameter reduced zirconia oral implants under the influence of loading and artificial aging in a chewing simulator as well as the fracture load in a static loading test. Thirty-two one-piece zirconia implants with a diameter of 3.6 mm were embedded according to the ISO 14801:2016 standard. The implants were divided into four groups of eight implants. The implants of group DLHT were dynamically loaded (DL) in a chewing simulator for 10⁷ cycles with a load of 98 N and simultaneously hydrothermally aged (HT) using a hot water bath at 85 °C. Group DL was only subjected to dynamic loading and group HT was exclusively subjected to hydrothermal aging. Group 0 acted as a control group: no dynamical loading, no hydrothermal ageing. After exposure to the chewing simulator, the implants were statically loaded to fracture in a universal testing machine. To evaluate group differences in the fracture load and bending moments, a one-way ANOVA with Bonferroni correction for multiple testing was performed. The level of significance was set to p < 0.05. In the static loading test, group DLHT showed a mean fracture load of 511 N, group DL of 569 N, group HT of 588 N and control group 0 of 516 N. The average bending moments had the following values: DLHT: 283.5 Ncm; DL: 313.7 Ncm; HT: 324.4 Ncm; 0: 284.5 Ncm. No significant differences could be found between the groups. Hydrothermal aging and/or dynamic loading had no significant effect on the stability of the one-piece diameter reduced zirconia implants (p > 0.05). Within the limits of this investigation, it can be concluded that dynamic loading, hydrothermal aging and the combination of loading and aging did not negatively influence the fracture load of the implant system. The artificial chewing results and the fracture load values indicate that the investigated implant system seems to be able to resist physiological chewing forces also over a long service period.

Keywords: fracture strength; loading/aging; one-piece; oral implants; phase transformation; thermomechanical aging; zirconia implant.

Figure 1

The one-piece zirconia implant ZiBone^®: (a) conical abutment part, (b) transmucosal area, (c) endosseous cylindric-conical part, (d) bone chip reservation groove.

Figure 2

Set-up of the experiment. Seven specimens per group were intended to be exposed to the static load until fracture; one specimen per group was intended for surface microstructural and subsurface phase and microstructural characterization.

Figure 3

Investigated one-piece zirconia implant embedded according to the ISO 14801 standard with a dual-curing composite in a PEEK tube and a loading hemisphere attached.

Figure 4

Fractured implant (DLHT4) after dynamic loading and hydrothermal aging. The fracture occurred inside the tube.

Figure 5

FE-SEM micrographs of the control group 0 implants (a,b) transmucosal and (c,d) endosseous cylindric areas. Arrows indicate isolated alumina grains in the zirconia matrix. The circled areas are showing flattened areas as a consequence of sandblasting.

Figure 6

High-magnification FE-SEM micrographs showing microstructure of the (a) control group 0 implant specimen and (b) after thermomechanical ageing (DLHT).

Figure 7

Representative grazing-incidence X-ray patterns of the (a) as-received control group 0 implant and after (b) hydrothermal ageing (HT) and (c) dynamic loading plus hydrothermal ageing (DLHT). t-ZrO₂ and t-ZrO₂ phases are labelled with letters m and t, respectively.

Figure 8

SEM-FIB micrographs showing ion-milled cross-sections of implant specimens: (a) control group 0, (b) hydrothermally aged (HT) and (c) dynamically loaded and hydrothermally aged (DLHT). White arrows and dotted lines indicate the formation of martensitic variants and microcracks, respectively. The transition to the unaffected bulk is indicated by the white dashed line.

Figure 9

The calculated fracture load is visualized with boxplots (see Table 1 for detailed data). A whisker shows all samples which are within 1.5 times of the interquartile range, all other data are plotted as outliers.

Figure 10

The calculated bending moment is visualized with boxplots (see Table 1 for detailed data). A whisker shows all samples which are within 1.5 times of the interquartile range, all other data are plotted as outliers.

Figure 11

The different implant fracture localizations after static loading: (a) fracture occurred inside the composite material in the tube, (b) the only fracture at the abutment level.