**1. Introduction**

Ti and its alloys (Ti–6Al–4V, Ti–6Al–7Nb, Ni–Ti and others) have a long record of applica‐ tions in dental field. [1–3]. Although Ti is well known for its biocompatibility and excellent corrosion resistance, there are still concerns for the ionic release of Al and V from Ti alloys as they are connected with adverse biological consequences [4–9]. To overwhelm this complica‐

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tion, most dental implants are manufactured of commercially pure titanium (cpTi: grade II and IV), although a few implants are still produced by the stronger Ti–6Al–4V alloy.

In first place corrosion of dental implants is not a primary concern as the implant surface is not exposed to oral fluids. Ideally after the implant placement the collar will be covered by the soft tissue at cervical region while root region will be covered by the attached bone. However, under inflammatory conditions like peri-implantitis, the environment can be very acidic and thus much more aggressive. In general, peri-implantitis establishes two changes at the region. The first is a significant decrease in pH value at the region resulting in a more aggressive environment for Ti surfaces. Both cpTi and Ti6Al4V alloys showed inferior corrosion resistance in lower pH while the corrosion rate and kinetic is accelerated [10]. The second is the direct contact of collar and root regions with oral fluids due to the resorption of soft and hard tissues has to be considered. Under these conditions, different corrosion mechanisms can be activated:

**Uniform corrosion:** Ti surfaces cannot withstand the corrosive action of oral fluids and a uniform regular removal of metal from implant surface is occurred [11].

**Pitting corrosion:** A form of localized corrosion, where small surface fissures are developed on the metal surface.

**Crevice corrosion:** Corrosion takes place between two close metallic surfaces as in the case of implant and abutment [12]. Crevice corrosion can be also developed on a deep surface crevice where stagnant conditions of the solution are achieved and oxygen exchange between surface and environment is impossible.

**Galvanic corrosion:** A galvanic couple is developed when dissimilar metallic materials are placed in contact.

**Microbial corrosion:** Microbial corrosion or microbiologically-influenced corrosion is the corrosion form caused or promoted by the metabolic actions of microorganisms which reduce the pH levels.

**Fretting corrosion:** Fretting corrosion is caused due to micro movements of mechanically connected parts of an implant structure.

Recently, a research study claims that corrosion of dental implants might be not the result but the triggering factor for peri-implantitis [13]. In 2009, Alberkston et al [13] claimed that corrosion along with the presence of aggressive bacteria, lesion of peri-implant attachment and excessive mechanical loading, among the four triggering factors of peri-implantitis. They concluded that "peri-implantitis is a general term dependent on a synergy of several factors, irrespective of the precise reason for first triggering of symptoms" and thus corrosion resist‐ ance might be associated with the failure of dental implants.

Although Ti oxide can be instantly rebuilt after an unexpected damage, a recent study has pointed out that the breakdown of the oxide film is followed by a dissolution process which finally deteriorates the corrosion resistance if this happens repeatedly [14]. In a retrieval analysis study, the corrosion and pitting potential of an intra-oral aged implants were found lower compared to unused ones. The retrieved implants showed lower passivation range and polarization resistance, indicating that in vivo aging deteriorates the electrochemical proper‐ ties of Ti implants [15]. Moreover, a retrieval study of four failed dental implants showed that all had been corroded during intra-oral service [16]. The authors concluded that surface oxidation of dental implants might be changed due to the acidic environment developed by bacteria biofilms and/or the inflammatory conditions at the region. This process may perma‐ nently breakdown the oxide film facilitating the release of debris and metal ions around the implant. The latter might also hinders the re-integration of bone on implant surface. [16]

tion, most dental implants are manufactured of commercially pure titanium (cpTi: grade II and

In first place corrosion of dental implants is not a primary concern as the implant surface is not exposed to oral fluids. Ideally after the implant placement the collar will be covered by the soft tissue at cervical region while root region will be covered by the attached bone. However, under inflammatory conditions like peri-implantitis, the environment can be very acidic and thus much more aggressive. In general, peri-implantitis establishes two changes at the region. The first is a significant decrease in pH value at the region resulting in a more aggressive environment for Ti surfaces. Both cpTi and Ti6Al4V alloys showed inferior corrosion resistance in lower pH while the corrosion rate and kinetic is accelerated [10]. The second is the direct contact of collar and root regions with oral fluids due to the resorption of soft and hard tissues has to be considered. Under these conditions, different corrosion mechanisms can be activated:

**Uniform corrosion:** Ti surfaces cannot withstand the corrosive action of oral fluids and a

**Pitting corrosion:** A form of localized corrosion, where small surface fissures are developed

**Crevice corrosion:** Corrosion takes place between two close metallic surfaces as in the case of implant and abutment [12]. Crevice corrosion can be also developed on a deep surface crevice where stagnant conditions of the solution are achieved and oxygen exchange between surface

**Galvanic corrosion:** A galvanic couple is developed when dissimilar metallic materials are

**Microbial corrosion:** Microbial corrosion or microbiologically-influenced corrosion is the corrosion form caused or promoted by the metabolic actions of microorganisms which reduce

**Fretting corrosion:** Fretting corrosion is caused due to micro movements of mechanically

Recently, a research study claims that corrosion of dental implants might be not the result but the triggering factor for peri-implantitis [13]. In 2009, Alberkston et al [13] claimed that corrosion along with the presence of aggressive bacteria, lesion of peri-implant attachment and excessive mechanical loading, among the four triggering factors of peri-implantitis. They concluded that "peri-implantitis is a general term dependent on a synergy of several factors, irrespective of the precise reason for first triggering of symptoms" and thus corrosion resist‐

Although Ti oxide can be instantly rebuilt after an unexpected damage, a recent study has pointed out that the breakdown of the oxide film is followed by a dissolution process which finally deteriorates the corrosion resistance if this happens repeatedly [14]. In a retrieval analysis study, the corrosion and pitting potential of an intra-oral aged implants were found lower compared to unused ones. The retrieved implants showed lower passivation range and

uniform regular removal of metal from implant surface is occurred [11].

on the metal surface.

154 Dental Implantology and Biomaterial

placed in contact.

the pH levels.

and environment is impossible.

connected parts of an implant structure.

ance might be associated with the failure of dental implants.

IV), although a few implants are still produced by the stronger Ti–6Al–4V alloy.

Given that corrosion has not yet considered among the risk factors of implant failure there are no specific guidelines to clinicians to minimize the possibility of in vivo corrosion (i.e minimize galvanic coupling between implant and supestructure alloys). Unfortunately, till today, there are no comparative studies on the electrochemical behavior of contemporary dental implants with different surface treatments. A few studies have employed advanced techniques such as electrochemical impedance spectroscopy to characterize the electrochemical properties of anodized and machined dental implants [17–19]. However, the diversity of the applied methods and solutions used for testing makes comparison between the surface treatments rather invalid. A few papers have studied the galvanic coupling of Ti with different dental alloys used for the preparation of implant-retained superstructures providing data for suitable and nonsuitable combinations [20–23]. However, the aforementioned results cannot be directly extrapolated in clinical practice as the experimental conditions are far from intra-oral envi‐ ronment.
