Fatigue properties of UFG Ti grade 2 dental implant vs. conventionally tested smooth specimens

Abstract

Complicated geometry in combination with surface treatment strongly deteriorates fatigue resistance of metallic dental implants. Mechanical properties of pure Ti grade 2, usually used for dental implant production, were shown to be significantly improved due to intensive grain refinement via Conform SPD. The increase of the tensile strength properties was accompanied by a significant increase in the fatigue resistance and fatigue endurance limit. However, the SLA treatment usually used for the implants' surface roughening, resulted in the fatigue properties and endurance limit decrease, while this effect was more pronounced for the ultrafine-grained comparing to the coarse-grained material when tested under tensile-tensile loading mode.

The testing of the implants is usually provided under the bending mode. Even though different testing condition for the conventional specimens tests and implants testing was adopted, a numerical study revealed their comparable fatigue properties. The fatigue limit determined for the implants was 105% higher than the one for coarse-grained and only by 4 % lower than the one for ultrafine-grained Ti grade 2.

Based on the obtained results, conventional specimens testing can be used for the prediction of the fatigue limit of the implants.

Introduction

Titanium and its alloys are materials widely used for medical applications. The reasons are mainly the good corrosion resistance, good weight-to-mechanical properties ratio, and especially, the very low toxicity for the human body (Lütjering and Williams, 2007; Le Guéhennec et al., 2007; Medvedev et al., 2016; Ganesh et al., 2014; Lederer et al., 2019; Pazos et al., 2010; Aparicio et al., 2011; Elias et al., 2008, 2019). Mostly the grade 4 (Le Guéhennec et al., 2007; Medvedev et al., 2016; Lederer et al., 2019; Elias et al., 2019; Marenzi et al., 2019) and grade 2 (Medvedev et al., 2016; Elias et al., 2019; Masrouri et al., 2020) are used for the dental implants from CP (Commercially Pure) Ti grades. Very often used Ti6Al4V alloy (marked as Ti grade 5) (Ganesh et al., 2014; Elias et al., 2019; Marenzi et al., 2019) exhibits better tensile strength and mainly ductility compared to the CP Ti grades, however, its usage is slightly controversial. The content of Al and V is potentially dangerous for the human organism (especially the neurological system) (Masrouri et al., 2020). Alternatively, also due to the quite increasing demand of the industry on titanium, stainless steel dental miniaturized implants are more often used in some countries (Pan et al., 2012). Good mechanical properties of commercially pure Ti grades, including the fatigue properties, were shown to be even improved via severe plastic deformation (SPD) (Medvedev et al., 2016; Elias et al., 2019; Kim et al., 2006; Estrin and Vinogradov, 2010; Vinogradov et al., 2001; Um et al., 2017; Semenova et al., 2009; Fintová et al., 2017, 2020; Palán et al., 2017, 2018; Mertová et al., 2020). Such improvement of material mechanical properties allows further miniaturization of the dental implants ensuring sufficient mechanical stability and providing a less invasive alternative to conventional implants. The drawback of miniaturized implants application is the requested insertion depth, the most critical factor to ensure the implant fixation in the bone.

In the case of the dental implant, the geometrical notches due to screw-thread are a typical feature. Besides the specific geometry of dental implants, also the surface treatment increases the implant roughness. This treatment is important to ensure the implant's biomechanical stability and allows bone ingrowth and tissue regeneration. Sandblasting in combination with acid etching (denoted as SLA) is one of the most used methods for Ti dental implant surface treatment (Le Guéhennec et al., 2007; Medvedev et al., 2016; Pazos et al., 2010; Aparicio et al., 2011; González-García et al., 2014; Kim et al., 2020; Shi et al., 2016; Jiang et al., 2006; Kopf et al., 2015; Pippenger et al., 2019; Rong et al., 2009; Marenzi et al., 2019; Masrouri et al., 2020). A combination of SLA treatment and the dental implant geometry result in three levels of roughness: macro- (>10 μm), micro- (1–10 μm), and nano-sized topologies (Le Guéhennec et al., 2007). Micro- and nanoscale surface roughness are a consequence of the SLA treatment, sandblasting and acid etching, respectively. The second mentioned is necessary for adsorption of proteins, adhesion of osteoblastic cells, and thus the rate of osseointegration (Le Guéhennec et al., 2007; Medvedev et al., 2016; González-García et al., 2014; Kopf et al., 2015; Pippenger et al., 2019; Masrouri et al., 2020).

Several studies were aimed at the analysis of the influence of sandblasting, acid etching, and SLA on fatigue properties of commercially pure coarse-grained (CG) Ti grades (Medvedev et al., 2016; Pazos et al., 2010; Fintová et al., 2020; Kim et al., 2020; Jiang et al., 2006), while the application of the treatment was proven to be adequate also for the ultrafine-grained (UFG) Ti (Fintová et al., 2020; Pippenger et al., 2019; Masrouri et al., 2020). Surface roughness is, however, a detrimental factor for fatigue properties.

The negative effect of the increased surface roughness on the Ti fatigue properties due to the acid etching was shown to be potentially eliminated via the compression stresses introduced by surface blasting. However, it was proven only for the CG Ti grades. For these grades, the surface layer grain refinement and compressive residual stresses were observed to be a consequence of blasting and explained the improved or unchanged fatigue properties of SLA treated material compared to the basic state (Medvedev et al., 2016; Pazos et al., 2010; Jiang et al., 2006). Refined microstructure in combination with the compressive stresses was assumed in (Jiang et al., 2006) to be responsible for CG Ti grade 1 fatigue endurance limit increase by approximately 10 %. Sandblasting resulted in the creation of a severely deformed zone (20 μm, characterized by grain refinement and twinning) followed by a region deformed mainly by twinning (no grain refinement occurred) and finally by the undeformed material. It also caused compressive residual stresses to the depth of ~70 μm (maximum of 480 MPa at about 10 μm below the surface) and low compensatory tensile residual stresses deeper in the material. According to (Pazos et al., 2010), the combination of grit-blasting with acid etching resulted in a slight increase of CG Ti grade 4 fatigue endurance limit. However, no influence on the material fatigue strength was observed when compared to the as-machined material. Also, in (Medvedev et al., 2016), the increase of the fatigue properties observed for the CG Ti grades 2 and 4 was explained by the surface layer grain refinement (2–4 μm for grade 2 and 1–2 μm for grade 4), and surface compressive residual stresses introduced into the material due to the grit-blasting. On the other hand, in (Fintová et al., 2020) deterioration of fatigue properties due to the SLA treatment for CG Ti grade 4 was observed. Even though the SLA affected zone (grain refined) of approximately 20–30 μm on the specimen surface was reached, the surface roughness and defects such as notches and cracks in the SLA-affected zone were stress concentrators resulting in the fatigue crack initiation. SLA treatment resulted in a decrease of fatigue life by one order of magnitude at the same stress level comparing to the as-machined specimens. The same behavior was observed by authors, (Fintová et al., 2020), also for UFG Ti grade 4 where the SLA affected zone reached a thickness bellow 10 μm. Even though the authors in (Medvedev et al., 2016) did not observe any grain refined zone for UFG Ti grade 2 and 4, also their results revealed a decrease in the fatigue properties and fatigue endurance limit for the UFG Ti grades. This decrease of fatigue properties was reported to be a consequence of nanoscale surface roughness increase due to the acid etching, acting as the fatigue crack initiation sites. The fatigue properties and fatigue endurance limit of SLA-treated UFG grades were anyway higher than in the case of the CG grades in (Medvedev et al., 2016; Fintová et al., 2020).

As different SLA treatment conditions (blasting with SiO₂ particles with 0.2–0.3 mm in size blasted by pressurized air (Jiang et al., 2006), Al₂O₃ particles with 0.425–0.6 mm in size (Pazos et al., 2010), Al₂O₃ particles with 0.5 mm in size (Medvedev et al., 2016), SiO₂ particle with a size from 0.2 to 0.4 mm at a pressure of 8 atm (Fintová et al., 2020), etc. in combination with etching with H₂SO₄ solution (Pazos et al., 2010), a mixture of HCl and H₂SO₄ (Medvedev et al., 2016), 30% solution of H₂SO₄ (Fintová et al., 2020), etc.) for Ti specimens treatment are used, different surface morphology and affected zone is reached in literature and as a subsequence, different results are obtained. Also, only the fatigue behavior of the conventional specimens or implants is compared to each other. The present study provides not only the comparison of the fatigue behavior of the CG and UFG Ti grade 2 with the as-machined and SLA treated surface, however, also the correlation of the conventionally obtained data of the behavior of the real dental implants from UFG SLA Ti grade 2 tested according to the ISO 14801-07 standard (ISO 14801, 2016). The obtained data are discussed in terms of the microstructure, surface character, and fracture mechanism taking into account the specificity of the implants.