Fracture Resistance of a Bone-Level Two-Piece Zirconia Oral Implant System-The Influence of Artificial Loading and Hydrothermal Aging

Abstract

Preclinical and clinical research on two-piece zirconia implants are warranted. Therefore, we evaluated the in vitro fracture resistance of such a zirconia oral implant system. The present study comprised 32 two-piece zirconia implants and abutments attached to the implants using a titanium (n = 16) or a zirconia abutment screw (n = 16). Both groups were subdivided (n = 8): group T-0 comprised implants with a titanium abutment screw and no artificial loading; group T-HL was the titanium screw group exposed to hydro-thermomechanical loading in a chewing simulator; group Z-0 was the zirconia abutment screw group with no artificial loading; and group Z-HL comprised the zirconia screw group with hydro-thermomechanical loading. Groups T-HL and Z-HL were loaded with 98 N and aged in 85 °C hot water for 10⁷ chewing cycles. All samples were loaded to fracture. Kruskal-Wallis tests were executed to assess the loading/bending moment group differences. The significance level was established at a probability of 0.05. During the artificial loading, there was a single occurrence of an implant fracture. The mean fracture resistances measured in a universal testing machine were 749 N for group T-0, 828 N for group Z-0, 652 N for group T-HL, and 826 N for group Z-HL. The corresponding bending moments were as follows: group T-0, 411 Ncm; group Z-0, 452 Ncm; group T-HL, 356 Ncm; and group Z-HL, 456 Ncm. There were no statistically significant differences found between the experimental groups. Therefore, the conclusion was that loading and aging did not diminish the fracture resistance of the evaluated implant system.

Keywords: abutment screw; artificial chewing simulation; dental implants; loading/aging; stability; two-piece; zirconia.

Figure 1

The investigated zirconia oral implant with the attached zirconia abutment.

Figure 2

Left: Zirconia abutment screw (BL-OSC-H). Right: Titanium abutment screw (BL-OST-H).

Figure 3

Experimental setup of the investigation. L = length of the implants; GH = gingival height of the abutment; T = titanium; Z = zirconia; 0 = no exposure to the chewing simulator; HL = hydrothermal aging and mechanical loading in the chewing simulator.

Figure 4

A sample (implant with screw-retained abutment plus loading hemisphere) embedded according to ISO standard 14801 in a PEEK tube (y = lever arm, 5.5 mm; I = distance from tube rim to the loading center, 11 mm; α = embedding angle, 30°).

Figure 5

An ion-milled cross-section (white arrow) prepared by FIB in the implant head (red arrow head) of a loaded hydrothermally aged sample.

Figure 6

SEM micrographs of cross-sections showing a subsurface region of ∼5 µm in depth. White dotted lines roughly indicate the transformed region. The white arrows indicate distinct altered individual grains containing intragranular domains and monoclinic variants with lath-like geometry. Transformation changes gradually become less pronounced going deeper into the specimen. The non-loaded/non-aged groups (T-0, K-0) showed a 1 µm deep and the loaded and hydrothermally aged groups (T-HL, K-HL) an approximately 3 µm deep transformation. * in Z-HL: During microstructural inspection with FIB-SEM several distinct and isolated pores were detected (as expected), representing the residual porosity of the dense zirconia implant material. The pores had a low coordination number (≤5) and were in the size range of a single grain (~300 nm). As such, these pores usually do not act as critical flaws initiating failure. Pore sizes and/or surface and volume flaws in the size range 5–10 µm or larger are critical flaws initiating a crack/failure under stress.

Figure 7

Raman depth profiles of the monoclinic-to-tetragonal phase ratios between the different groups based on the peak intensities at 178 cm⁻¹ (monoclinic) and 260 cm⁻¹ (tetragonal) for the AB samples before and after thermomechanical loading.