Bone bonding bioactivity of Ti metal and Ti–Zr–Nb–Ta alloys with Ca ions incorporated on their surfaces by simple chemical and heat treatments

Abstract

Ti15Zr4Nb4Ta and Ti29Nb13Ta4.6Zr, which do not contain the potentially cytotoxic elements V and Al, represent a new generation of alloys with improved corrosion resistance, mechanical properties, and cytocompatibility. Recently it has become possible for the apatite forming ability of these alloys to be ascertained by treatment with alkali, CaCl₂, heat, and water (ACaHW). In order to confirm the actual in vivo bioactivity of commercially pure titanium (cp-Ti) and these alloys after subjecting them to ACaHW treatment at different temperatures, the bone bonding strength of implants made from these materials was evaluated. The failure load between implant and bone was measured for treated and untreated plates at 4, 8, 16, and 26 weeks after implantation in rabbit tibia. The untreated implants showed almost no bonding, whereas all treated implants showed successful bonding by 4 weeks, and the failure load subsequently increased with time. This suggests that a simple and economical ACaHW treatment could successfully be used to impart bone bonding bioactivity to Ti metal and Ti–Zr–Nb–Ta alloys in vivo. In particular, implants heat treated at 700 °C exhibited significantly greater bone bonding strength, as well as augmented in vitro apatite formation, in comparison with those treated at 600 °C. Thus, with this improved bioactive treatment process these advantageous Ti–Zr–Nb–Ta alloys can serve as useful candidates for orthopedic devices.

Introduction

Titanium (Ti) and its alloys are the most popular materials for orthopedic and dental implants because of their superior biocompatibility, excellent corrosion resistance, and good mechanical properties. However, they are essentially bioinert materials that, after implantation in the living body, are merely encapsulated by fibrous tissue that isolates them from the surrounding tissue. On the other hand, orthopedic load-bearing devices such as total hip prostheses require direct bonding between living bone and the implant. Hence, various methods have been developed to promote bone in-growth and implant fixation for Ti and its alloys [1], [2], including physical modification of the implant design, modification of the surface topography, and chemical modification of the material composition and structure. Among these methods, plasma sprayed hydroxyapatite coating is one of the most extensively investigated methods, and its efficiency has been confirmed by many reports [3], [4].

In the past decade we have developed a chemical and heat treatment method to produce bioactive Ti [5], [6], [7]. This method can be used to create a long-lasting bioactive layer on the surface of Ti and its alloys, allowing bonding with living bone via a spontaneously formed apatite layer. In this method the implants are simply immersed in aqueous solutions before heat treatment, and the bonding effects extend homogeneously throughout the irregular structure of the implant. This method is considered superior to the conventional hydroxyapatite plasma spray method, wherein the coating tends to be applied to the most superficial areas, thereby resulting in uneven and inadequate treatment. This alkali and heat treatment was applied to a porous commercially pure Ti (cp-Ti) surface layer on an artificial hip prosthesis made of a Ti6Al2Nb1Ta alloy, and its effectiveness was confirmed in clinical trials in Japan [8]. In fact, this bioactive artificial hip joint was approved for clinical use in 2007 (AHFIX, Japan Medical Materials Co., Japan).

We have also reported that our chemical and heat treatment is effective for Ti alloys (Ti6Al4V, Ti15Mo5Zr3Al, and Ti6Al2Nb1Ta) [9], [10], [11]. However, these Ti alloys contain aluminum (Al) and vanadium (V), which are suspected of being cytotoxic [12], [13], [14]. In this context, the new generation of Ti alloys without V and Al [14], such as Ti15Zr4Ta4Nb and Ti29Nb13Ta4.6Zr, offers a promising alternative. Ti15Zr4Ta4Nb has been reported to show much better corrosion resistance, mechanical properties, and cytocompatibility than Ti6Al4V [15]; furthermore, Ti29Nb13Ta4.6Zr has been reported to show a lower Young’s modulus and cytotoxicity than Ti6Al4V and the same cytotoxicity as cp-Ti [14], [16]. Unfortunately, these new generation Ti alloys cannot be endowed with in vitro apatite forming ability by conventional chemical and heat treatment.

Instead, we recently found that Ti15Zr4Ta4Nb and Ti29Nb13Ta4.6Zr can be endowed with in vitro apatite forming ability by treatment with NaOH, CaCl₂, heat, and water (ACaHW). In vitro examination showed faster and greater apatite formation on the obtained calcium-modified titanate surface in simulated body fluid (SBF), with ion concentrations nearly equal to those of human blood plasma [17], [18]. In this treatment calcium hydrogen titanate is formed after treatment of the Ti surface with NaOH and CaCl₂. Subsequent heat treatment transforms the calcium hydrogen titanate into calcium titanates and rutile [17], [19]. The final water treatment causes a remarkable increase in in vitro apatite forming ability on account of the increasing mobility of the Ca²⁺ ions via incorporation of H₃O⁺ ions in the calcium titanate [17]. These results lead us to expect superior in vivo bioactivity when the ACaHW treatment is applied [20]. In the present study, to confirm the in vivo bioactivity of ACaHW-treated cp-Ti, Ti15Zr4Ta4Nb, and Ti29Nb13Ta4.6Zr alloys, the biomechanical performance was investigated by histological examination and tensile strength testing using animal models [21].