Elsevier

Dental Materials

Volume 19, Issue 3, May 2003, Pages 188-198
Dental Materials

In vitro corrosion behavior of bioceramic, metallic, and bioceramic–metallic coated stainless steel dental implants

https://doi.org/10.1016/S0109-5641(02)00029-5Get rights and content

Abstract

Objectives: The most common metals and alloys used in dentistry may be exposed to a process of corrosion in vivo that make them cytotoxic. The biocompatibility of dental alloys is primarily related to their corrosion behavior. The aim of this work was to evaluate the corrosion behavior and thus the biocompatibility of the uncoated and coated stainless steels and compare the effect of type of coatings on corrosion behavior.

Methods: Three types of coatings, hydroxyapatite (HA), titanium (Ti), and a double-layer HA/Ti on AISI 316L stainless steel were made. HA coating was produced using plasma-spraying technique and Ti coating was made using physical vapor deposition process. In order to perform a novel double-layer composite coating, a top layer of HA was plasma-sprayed over a physical vapor deposited Ti layer on AISI 316L stainless steel. Structural characterization techniques including XRD, SEM and EDX were used to investigate the microstructure, morphology and crystallinity of the coatings. Electrochemical potentiodynamic tests were performed in physiological solutions in order to determine and compare the corrosion behavior of the coated and uncoated specimens as an indication of biocompatibility.

Results: Double-layer HA/Ti coating on AISI 316L SS had a positive effect on improvement of corrosion behavior. The decrease in corrosion current densities was significant for these coated specimens and was much lower than the values obtained for uncoated and single HA coated specimens. Ti coating on AISI 316L SS also has a beneficial effect on corrosion behavior. The results were compared with the results of corrosion behavior of HA coated commercially pure titanium (cpTi) and uncoated cpTi.

Significance: These results demonstrated that the double-layer HA/Ti coated 316L SS can be used as an endodontic implant and two goals including improvement of corrosion resistance and bone osteointegration can be obtained simultaneously.

Introduction

Biocompatibility is the ability of a material to perform with an appropriate host response in a specific application [1]. This means that the tissue of the patient that comes into contact with the materials does not suffer from any toxic, irritating, inflammatory, allergic, mutagenic, or carcinogenetic action [2], [3], [4], [5]. Since the oral environment is particularly favorable for the biodegradation of metals due to its ionic, thermal, microbiological, and enzymatic properties, it can be presumed that the patient is exposed to a certain extent to the products of the corrosion process [6]. The biocompatibility of dental alloys is primarily related to their corrosion behavior [7]. The higher the corrosion of an alloy, the more its elements will be released and the risk of unwanted reactions in the oral tissues may be increased. These unwanted reactions include unpleasant tastes, irritation, allergy or other reaction [7].

Surgical implants are usually made of metallic materials, such as austenitic stainless steel, cobalt–chromium alloys, and titanium and its alloys. Among all the metallic materials, the austenitic stainless steels are the most popular materials because of their relatively low cost, ease of fabrication and reasonable corrosion resistance [8]. However, the austenitic stainless steels are prone to localized attack in long-term applications due to the aggressive biological effects. The corrosion products include iron, chromium, nickel and molybdenum, etc. Ions can accumulate in tissues surrounding the implant or be transported to distant parts of the body [9]. It has been demonstrated that metallic ions resulting from the in vitro corrosion of austenitic stainless steels cause alteration of the expression of human lymphocyte-surface antigens and inhibit the immune response as assessed by lymphocyte proliferation [10]. The presence of these ions in vivo not only causes toxic effects in mouse testicular seminiferous epithelium but also alterations in the spleen cellular population [11].

A number of investigations have demonstrated that metal ions can be released from metallic materials as the result of corrosion [2], [3], [8], [9], [10], [12], [13], [14]. Local adverse tissue reactions or elicit allergy reactions caused by metallic implants originate from the release of metal ions. This release of ions depends upon the corrosion rate of the alloy and the solubility of the first formed corrosion products [15]. Intensive investigations have been carried out on biomaterials in order to study their corrosion behavior, ion release and its effects [14], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], and toxicity and immunity of alloys [6], [16], [17], [18], [26], [27], [28], [29]. The studies have been more intensive for stainless steels due to their corrosion behavior [14], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. It has been suggested that metal ions associated with 316L stainless steel are toxic to osteogenic cells, affecting their proliferation and differentiation [19]. Stainless steel corrosion products above certain concentrations may disturb the normal behavior of osteoblast-like bone marrow cell cultures [20], [21], [22], [25]. Advanced techniques have been used for determining toxicity and tissue reaction around metal implants [24], [27], [29]. For this purpose, in vivo animal tests have been performed [28].

Ion release rates in vitro can be used to determine the proportionality of release in vivo [30], [31], [32] but long-term in vivo studies have been made and results indicate that metal ion release increases with the exposure time [33], [34]. Release of metal ion in vivo analysis did not demonstrate any consistent differences in the concentrations of metallic elements next to stressed and nonstressed miniplates and screws of stainless steels [35].

A previous in vivo study by the present authors has shown that the type of metallic implants had significant effects on the clinical success, bone tissue response and histopathological results of hydroxyapatite coated/uncoated metallic implants in animals [36]. In this previous work, endodontic implants consisting of plasma-sprayed HA coated and uncoated substrates of various materials, namely 316L stainless steel and Co–Cr–Mo alloy, were prepared. These implants were subsequently implanted in the mandibular canine of cats. After a healing period of 4 months, osteointegration evaluation and histopathological interpretation were carried out by using scanning electron microscopy (SEM). The results showed that different substrates had pronounced effects on the histopathological response to different HA coated implants [36].

The aim of the present work was to evaluate the corrosion behavior and therefore biocompatibility of the uncoated and coated stainless steels. In order to design and produce a desired coating for improvement of histopathological response, tissue reaction and bone osteointegration around dental implants, the effects of three types of coatings on the behavior of substrates were compared.

Section snippets

Substrates and coatings

Commercially pure titanium (cpTi) and AISI 316L stainless steel were used as substrates. The composition (wt%) of stainless steel was C 0.03, Si 0.80, Mn 1.2, Cr 17.55, Ni 13.65, Mo 3.1, P≤0.040, S≤0.030 and Fe as the balance. The composition (wt%) of cpTi was Ti>99.5%.

Ti coating (3–7 μm thickness) was made using physical vapor deposition process on AISI 316L stainless steel. Crystalline hydroxyapatite coatings with 40–60 μm were produced using plasma-spraying technique on two types of substrate,

Structural characterization

The XRD patterns of the plasma-sprayed HA coatings on cpTi and 316L SS are shown in Fig. 1(a) and (b), respectively. In the plot of intensity vs. 2θ, there are numerous sharp peaks and a low background indicative of highly crystalline HA coating. Fig. 1(c) also shows the XRD pattern of a double-layer HA/Ti coating on 316L SS which is very similar to the XRD pattern of HA coating on 316L SS.

The SEM micrograph showing a typical plasma-sprayed HA coating on a metallic substrate is shown in Fig.

Discussion

The single HA coating had a positive effect on corrosion resistance of metallic substrate, i.e. 316L SS and cpTi and decreased the corrosion current density of each type of coated metallic substrate. The polarization curve, (b) in Fig. 4, of the HA coated 316L SS was shifted to the left upper area, compared with curve (a), which is the uncoated 316L SS specimen. It means that the HA coated 316L SS (Ecorr=−105 mV, icorr=68 nA) was more corrosion resistant than the uncoated 316L SS (Ecorr=−174 mV, i

Conclusion

Double-layer HA/Ti coating can have a beneficial and desired effect on corrosion behavior of 316L SS and decrease the corrosion current density that is a distinct advantage for prevention of ion release. Corrosion current density of double-layer HA/Ti coated 316L SS was similar and equal to single HA coated cpTi and it suggests that double-layer HA/Ti coated 316L SS can be used as endodontic implant.

Acknowledgements

The authors are grateful for the support of this research by Isfahan University of Technology.

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