A biodegradable polymer-based coating to control the performance of magnesium alloy orthopaedic implants

Abstract

Magnesium and its alloys may potentially be applied as degradable metallic materials in orthopaedic implantations due to their degradability and resemblance to human cortical bone. However, the high corrosion rate and accumulation of hydrogen gas upon degradation hinders its clinical application. In this study, we adopt a new approach to control the corrosion rate by coating a controllable polymeric membrane fabricated by polycaprolactone and dichloromethane onto magnesium alloys, in which the pore size was controlled during the manufacturing process. The addition of the polymeric membrane was found to reduce the degradation rate of magnesium, and the bulk mechanical properties were shown to be maintained upon degradation. The in-vitro studies indicated good cytocompatibility of eGFP and SaOS-2 osteoblasts with the polymer-coated samples, which was not observed for the uncoated samples. The in-vivo study indicated that the uncoated sample degraded more rapidly than that of the polymer-coated samples. Although new bone formation was found on both samples, as determined by Micro-CT, higher volumes of new bone were observed on the polymer-coated samples. Histological analysis indicated no inflammation, necrosis or hydrogen gas accumulation on either of the samples during degradation. Collectively, these data suggest that the use of polymeric membrane may be potentially applied for future clinical use.

Introduction

The most commonly used materials for bone fracture fixation are usually made of medical-grade metals such as 316L stainless steel, pure titanium and its alloys, and cobalt–chromium-based alloys [1], [2] which are non-biodegradable. However, one desirable characteristic of an implant is its ability to be degraded after the bone has healed as problems may arise if the implants are not degradable. Long-term adverse effects or even an increased risk of local inflammation may occur after long-term implantation since the metallic implant is a foreign body to human tissues [3]. If this is the case, second surgery is subsequently conducted for implant removal. However, repeated surgery not only increases the morbidity rate of the patients, but also results in an increase of health care costs and longer hospitalization [1]. To reduce such complications, the use of biodegradable metallic implants has been investigated [1], [4], [5], [6], [7].

Magnesium and its alloys are the most commonly used metal amongst all the degradable metallic materials. However, the major obstacles of the clinical use of magnesium-based materials are its rapid degradation rate and the release of hydrogen gas upon degradation [8], [9]. Troitskii and Tsitrin used a magnesium–cadmium alloy to secure various fractures, however, reported that the mechanical integrity of the magnesium alloy was only maintained for 6–8 weeks with the release of hydrogen during the corrosion process [10]. Hence, in order to make use of magnesium-based materials feasible for surgical implantation, the corrosion rate must be controlled.

The enhancement of the corrosion resistance of magnesium can be achieved by using different modification methods such as alloying [11] and various surface treatments [8]. Witte et al. [4], [12] suggested that magnesium alloys, especially those containing rare earth elements seemed to be suitable for use as orthopaedic implants. However, in addition to the alteration of its original mechanical properties, the addition of rare earth metals such as zirconium and cerium into the magnesium substrate may potentially add toxic effects to cells [13], [14], as the cytocompatibility of these elements is not known. In the studies by Li et al. (2008) [5], Zhang et al. (2009) [15] and Zberg et al. (2009) [16]; a magnesium–calcium alloy, magnesium–zinc alloy and magnesium–zinc–calcium alloys, were fabricated respectively and used as biodegradable implants, however, the change in mechanical properties of these alloys during degradation were not addressed.

Apart from alloying, surface modifications such as micro-arc oxidation (MAO), ion implantation and plasma anodisation to improve the corrosion properties of magnesium alloys have been investigated [17], [18], [19], [20], [21], [22]. Electrochemical tests were conducted with those surface-treated samples, and an increase in corrosion resistance was reported [17], [18], [19], [20], [21], [22], however, as the biological integrity of those surface-treated samples was not reported, there was insufficient data to draw any conclusion before applying in clinical use.

In this paper, we improve the properties of magnesium implants via the deposition of a biodegradable polymer-based porous membrane made of polycaprolactone (PCL) and dichloromethane (DCM) onto a commercially available magnesium alloy in order to control its degradation rate. This paper aims at investigating the feasibility of these polymeric membranes in controlling the degradation of magnesium alloy under in-vitro and in-vivo conditions, and addresses the cytocompatibility and mechanical integrity of the deposited samples during degradation.