Cell response to plasma electrolytic oxidation surface-modified low-modulus β-type titanium alloys

Highlights

• PEO application to β-type titanium alloys for load-bearing orthopaedic implants.

• PEO treated samples promote osteoblast attachment, spreading and apatite deposition.

• Cell interdigitation into the porous structure of the coatings was observed.

• PEO does not impede osteoblast activity or induce an untoward inflammatory response.

Abstract

Plasma electrolytic oxidation (PEO) has been demonstrated to be an effective surface treatment for enhancing the osteoconduction and osseointegration of commercially pure α-Ti (CP α-Ti) dental implant materials for clinical application. To explore the feasibility of extending the application of PEO to low-modulus β-type titanium alloys for load-bearing orthopaedic implants, a thorough understanding of the effect of substrate material on the biological performance of the PEO-treated surface is required. A 10 kW 50 Hz KeroniteTM processing unit was used to modify the surface of low-modulus near β-Ti13Nb13Zr and β-Ti45Nb substrates. CP α-Ti and (α + β)-Ti6Al4V were also used in parallel as reference materials. In vitro culture of foetal human osteoblast (fHOb) cells on PEO-treated low-modulus near β-Ti13Nb13Zr and β-Ti45Nb alloys revealed comparable behaviour to that seen with CP α-Ti and (α + β)-Ti6Al4V with respect to metabolic activity, collagen production, matrix formation and matrix mineralisation. No difference was observed in TNF-α and IL-10 cytokine release from CD14+ monocytes as markers of inflammatory response across samples. Cell interdigitation into the porous structure of the PEO coatings was demonstrated and cell processes remained adherent to the porous structure despite rigorous sonication. This study shows that PEO technology can be used to modify the surface of low-modulus β-type titanium alloys with porous structure facilitating osseointegration, without impeding osteoblast activity or introducing an untoward inflammatory response.

Introduction

In order to optimise the osseointegration of implanted biomaterials, an in-depth understanding of the physical and chemical characteristics of the bone–implant interface is required, since attributes such as surface topography and chemistry play an important role in cell attachment, spreading and proliferation as well as the initial inflammatory response [[1], [2], [3]]. The mechanisms underpinning the interactions between the implant and the surrounding cells are still the subject of considerable interest and debate; however, several studies have shown that osseointegration can be improved by modifying the surface roughness of the implanted biomaterial [2,[4], [5], [6]]. Following implantation, the surface of the implant comes into contact with a number of proteins and cell types, including neutrophils, monocytes, macrophages, stromal cells and osteoblasts, which is in turn affected by the surface physicochemical properties of the implant material such as roughness, wettability, and porosity. Therefore, the cell behaviour at the bone–implant interface is influenced by the biomaterial surface characteristics [7,8] which, in turn, influence the outcome of the implantation procedure [9].

Metals are probably still the materials most frequently used in orthopaedic surgery for bone interfacing applications including fracture fixation, and reconstructions of damaged tissue in patients with degenerative and inflammatory conditions of bone and joints. Thus, metallic biomaterials must exhibit specific properties such as biological compatibility, enhanced mechanical performance and superior corrosion resistance [10,11]. Titanium and titanium alloys are of particular interest due to the passive titanium oxide layer that forms readily on their surface when exposed to the atmosphere providing corrosion resistance and chemical stability [12,13]. This native oxide layer is thin (approximately 2–7 nm) and bioinert, but its ability to induce apatite formation is dependent on the surface properties of the oxide layer, rather than on the bulk titanium itself [14]. Several methods have been used to introduce a bioactive layer to promote long-term osseointegration [[15], [16], [17], [18]]. Plasma electrolytic oxidation (PEO), also called anodic spark oxidation or micro-arc oxidation, has been shown over the last decade to be a highly effective methods to produce a bioactive surface with a rough and porous structure. The coating layer produced using PEO has been shown extensively to enhance the bioactivity and osseointegration of commercially pure α-Ti (CP α-Ti) dental implant materials, e.g. TiUnite^® (Nobel Biocare) [[19], [20], [21]], BioSpark^™ (Keystone Dental) [[22], [23], [24], [25], [26], [27]], and Ticer^® (ZL Microdent) [28,29].

A wide variety of low-modulus β-type titanium alloys have been developed for potential use in load-bearing orthopaedic applications to ameliorate the effects of stress shielding on bone [[30], [31], [32], [33]]; however, the applicability of PEO treatment to low-modulus titanium alloys in a long term in vitro study has not yet been investigated. Hence, we considered that knowledge transfer of the PEO technique from CP α-Ti to low-modulus β-type titanium alloys would be a valuable assessment for orthopaedic research. The aim of this work was to explore the effect of PEO-treated near β- and β-type titanium alloys compared with PEO-treated CP α-Ti and (α + β)-Ti6Al4V on foetal human osteoblasts (fHOb) and investigate the early inflammatory response using CD14⁺ monocytes.