Residual Stress Homogenization of Hybrid Implants

Abstract

Objectives: Hybrid implants commonly exhibit decreased corrosion resistance and fatigue due to differences in compressive residual stresses between the smooth and rough surfaces. The main objective of this study was to investigate the influence of an annealing heat treatment to reduce the residual stresses in hybrid implants.

Methodology: Commercially pure titanium (CpTi) bars were heat-treated at 800 °C and different annealing times. Optical microscopy was used to analyze the resulting grain growth kinetics. Diffractometry was used to measure residual stress after heat treatment, corrosion resistance by open circuit potential (E_OCP), corrosion potentials (E_CORR), and corrosion currents (I_CORR) of heat-treated samples, as well as fatigue behavior by creep testing. The von Mises distribution and the resulting microstrains in heat-treated hybrid implants and in cortical and trabecular bone were assessed by finite element analysis. The results of treated hybrid implants were compared to those of untreated hybrid implants and hybrid implants with a rough surface (shot-blasted).

Results: The proposed heat treatment (800 °C for 30 min, followed by quenching in water at 20 °C) could successfully homogenize the residual stress difference between the two surfaces of the hybrid implant (-20.2 MPa). It provides better fatigue behavior and corrosion resistance (p ˂ 0.05, ANOVA). Stress distribution was significantly improved in the trabecular bone. Heat-treated hybrid implants performed worse than implants with a rough surface.

Clinical significance: Annealing heat treatment can be used to improve the mechanical properties and corrosion resistance of hybrid surface implants by homogenizing residual stresses.

Keywords: annealing heat treatment; corrosion; finite element analysis (FEA); hybrid implants; mechanical behavior; microhardness; residual stress.

Figure 1

Microstructure of the α-phase of titanium, obtained by optical microscopy.

Figure 2

Implants in the study. S: machined surface. H: hybrid surface. R: rough surface (SLA).

Figure 3

Corrosion resistance equipment.

Figure 4

(Left): SS implant. (Center): SR implant. (Right): RR implant (dimensions in mm).

Figure 5

Upper-cortical model. (Left): implant model. (Center): loads. (Right): finite element mesh (56,227 elements and 109,328 nodes. Dimensions in mm.).

Figure 6

Lower-cortical model. (Left): implant model. (Center): loads. (Right): finite element mesh (79,234 elements and 101,452 nodes).

Figure 7

Mean grain size diameter as a function of annealing heat treatment time at 800 °C.

Figure 8

S-N curve of rough dental implants produced by grit blasting treatment, hybrid dental implants, and hybrid dental implants with annealing heat treatment.

Figure 9

Implant Von Mises stresses (MPa) in upper-cortical model. Left: implant. Right: bone.

Figure 10

Implant Von Mises stresses (MPa) in lower-cortical model. Left: implant. Right: bone.

Figure 11

Implant microstrains in upper-cortical model. Left: implant. Right: bone.

Figure 12

Implant microstrains in lower-cortical model. Left: implant. Right: bone.

Figure 13

Comparison of von Mises stress values (MPa) and resulting microstrains (mm) in the upper- cortical and lower-cortical bones of the four implant groups (implant with machined surface, rough SLA, hybrid without heat treatment, and hybrid with heat treatment) using finite element analysis [16].