The effect of strontium-substituted hydroxyapatite coating on implant fixation in ovariectomized rats
Introduction
Post-menopausal osteoporosis, characterized by low bone mass and deterioration of bone microarchitecture, leading to enhanced bone fragility and a consequent increase in fracture risk, is a kind of abnormal bone metabolism condition induced by estrogen deficiency [1]. Due to much more enhanced bone resorption than new bone formation, osteoporosis could lead to depressed implant stabilization, even result in implant loosing and failure [2], [3]. Hydroxyapatite (HA) was a widely used coating material in orthopaedic and dental applications due to its excellent biocompatibility and bioactive properties to enhance peri-implant bone tissue formation [4]. HA coating has also been demonstrated to improve implant osseointegration in osteoporotic bone, but the results were still unsatisfactory and new strategy with anti-resorptive and anabolic ability was still desired [5], [6], [7].
Strontium (Sr) is a trace element found in calcareous rocks and ocean water [8], and a natural component of food and beverages [9]. Because of its chemical and physical similarity to calcium (Ca), Sr is also a natural bone-seeking element and about 98% of the total body Sr content is localized in bone tissue [10], [11]. Moreover, a substantial amount of research has indicated the dual effects of stable strontium ions (Sr2+) to promote bone formation and reduce bone resorption in various experimental studies and clinical trials over the years [12], [13], [14]. Recently, this original mechanism of Sr2+ has provoked an increasing interest in preparation and characterization of Sr-substituted hydroxyapatite (SrHA) [15], [16], [17]. Subsequently, these SrHA with different extents of Ca2+ substituted by Sr2+ was demonstrated to stimulate osteoblast activity and inhibit osteoclast proliferation [18], [19]. Some in vivo studies even investigated the interfacial behavior of SrHA cement with cancellous and cortical bone in rabbit or goat, and indicated the beneficial effect of SrHA on bone mass around the bone cement interface [20], [21], [22]. Surface coatings of SrHA on titanium (Ti) or Ti alloy substrate have also been synthesized via pulsed-laser deposition [23], plasma spray [24], or biomimetic method [25], which showed beneficial effect on osteoprecursor cells and osteoblast-like MG63 cells and negative influence on osteoclasts. These beneficial results of Sr2+ even when incorporated into apatites promote us to investigate the effects of SrHA coatings on implant osseointegration in osteoporotic animals when compared to HA coatings. We hypothesize that the local presence of Sr2+ at bone-to-implant interface after implant insertion holds promise for enhancing implant osseointegration, especially in animals with osteoporosis or low bone density.
Thus, the present study was designed (1) to synthesize HA and 10% SrHA (with 10 mol% Ca2+ replaced by Sr2+) coatings on titanium (Ti) implant using sol-gel dip-coating method, (2) to analyze the morphological and chemical characterizations of the coatings using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), and X-ray Photoemission Spectroscopy (XPS), and (3) to investigate the effects of 10% SrHA coatings on implant fixation in ovariectomized (OVX) rats when compared to HA coatings, which was evaluated by histological, micro-computed tomography (micro-CT) and biomechanical test 12 weeks after implant insertion in the tibial metaphysis and medullary canal.
Section snippets
Implants and treatments
Fifty rod-shaped Ti implants (supplied by National Engineering Research Center for Biomaterials, Sichuan University), with a diameter of 1.0 mm and a length of 12 mm, were included in this study. Before coating preparation, all implants were grit-blasted with 75 μm aluminium oxide (Al2O3) particles and sequentially washed in acetone, alcohol and de-ionized water in an ultrasonic bath, and then dried at 45 °C.
Preparation of HA and 10% SrHA coatings
In this study, the relative molar content of Sr was 10 mol%, and the 10 mol% SrHA (with
Morphology and thickness of coatings by AFM and SEM
The SEM and AFM examination clearly revealed the morphological characteristics of the HA and 10% SrHA coatings made by so-gel dip-coating method and calcined at 700 °C (Fig. 1, Fig. 2). The microstructure of powders consisted of tightly agglomerated crystallite in nano-size. The crystallite in 10% SrHA coatings seemed a little larger than that in HA coatings from SEM micrographs. In quantitative roughness analysis by AFM, the HA and 10% SrHA coatings showed no significant difference in RMS, Z
Discussion
In this study, HA and 10% SrHA coatings on Ti implants were prepared by sol-gel dip-coating method as previous reports described [32], [33]. The surface of HA and 10% SrHA coatings showed similar morphology from SEM micrographs and no significant difference of RMS, surface area diff. and Z range was detected in quantitative roughness analysis by AFM. The XRD patterns of 10% SrHA revealed single phase spectrum similar to that of HA. The molar ratio of Sr/(Ca + Sr) in 10% SrHA coatings determined
Conclusions
This study indicated that the 10% SrHA coatings prepared by sol-gel dip-coating method improved implant osseointegration in OVX rats 12 weeks after implantation. When compared to HA coatings, the 10% SrHA coatings showed similar morphology, surface roughness, coating thickness, phase composition and adhesion strength. The promising results from histomorphometry, micro-CT evaluation and biomechanical test indicated the beneficial effects of 10% SrHA coatings on implant fixation in OVX rats. It
Acknowledgements
This study was supported by a grant from National Science Funds for Distinguished Young Scholars of China (No. 30825040). We appreciate the assistance of Prof. Ying Fan from State Key Laboratory of Polymer Materials Engineering in Sichuan University of China.
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