Fracture and Fatigue of Titanium Narrow Dental Implants: New Trends in Order to Improve the Mechanical Response

Abstract

Sixty-four fractured commercially pure titanium (cp-Ti) narrow dental implants (NDIs) with similar macrogeometry and connection designs were studied after different implantation times in humans in order to determine their reliability and to evaluate the causes of the fracture. These NDIs were compared with other similar implants, made with alloyed titanium with 15% Zr and with 12% strained titanium. Original implants were tested under static and fatigue conditions, simulating the tri-axial loads in the mouth by means of a Bionix hydraulic test machine. Fractography was studied using field-emission scanning electron microscopy (FSEM). The results showed that cp-Ti NDI exhibits low strength for mechanical cycling, and the alloyed Ti and strained titanium increase the mechanical strength, guaranteeing long term mechanical behavior. NDIs fractured due to fatigue, and, in some cases, the presence of cracks in the original NDIs quickly led to fracture. These cracks were attributed to plastic deformation during machining were found to be exacerbated due to acid etching in the passivation process. All cases of fracture were cp-Ti dental implants due to the low fatigue limit. The results show that, when titanium is alloyed or cold-worked, the fatigue limit is higher than cp-Ti. This in vitro research will help clinicians to select a better NDI system for safer treatment.

Keywords: fatigue; fracture; narrow dental implants; plastic deformation; titanium.

Figure 1

SEM micrographs of a narrow dental implant (NDI) fractured 17 months after implantation in a patient. The fracture is located in the connection zone where the cross-sectional surface is lower.

Figure 2

S–N curves (stress vs. number of cycles) of the different NDIs studied.

Figure 3

SEM fractography of explanted cp-Ti grade 4 NDIs: (a) SEM micrograph of an explanted implant, top view; (b) SEM micrograph of hexagonal inner connection (side view) showing longitudinal fracture cracks; the white arrows mark the places of crack nucleation; (c,d) SEM micrographs showing crack propagation at different magnifications.

Figure 4

SEM micrographs of the cpTi grade 4 NDI showing crack nucleation and propagation: (a) SEM micrograph showing crack nucleation starting point, (b) SEM micrograph showing crack cyclic propagation due to fatigue.

Figure 5

SEM micrographs of inner cp-Ti grade 4 implant connection, showing some plastic deformation and crack formation on the connection surface due to “broaching”. (a). Defects in the internal surface. (b). Defects with more detail from 5a. These defects are the place of the crack nucleation. (c). Defects in the internal surface with more plastic deformation. (d). These defects are cracks.

Figure 6

Roughness of the different surfaces studied. (a) cp-Ti grade 4 dental implant; (b) Ti-15%Zr dental implant; (c) cold-worked (12%) dental implant. These topographies were obtained by sandblasting with Al₂O₃ and acid etching.

Figure 7

NDIs studied. TiG4: cp titanium grade 4; TiZr: titanium alloyed with 15% Zr; TiG4CW: cp titanium grade 4 cold-worked at 12%.

Figure 8

(a) Overall scheme of the mechanical testing described by the ISO 14801 standard. (b) Front view of representative drawing with lengths and distances of the testing samples.

Figure 9

Detail of the fatigue testing grips used by servo-hydraulic testing machine: (a) lower clamping grip; (b) upper articulated grip.