Nanoporous hydroxyapatite/sodium titanate bilayer on titanium implants for improved osteointegration
Introduction
Titanium (Ti) and its alloys have been extensively used in dental implants since the discovery of the osteointegration of Ti in bones [1], [2]. However, the native metallic devices are bioinert and are capsulated by fibrous tissue preventing tight contact to the surrounding host bone tissue [3], [4]. Many improvements have been made to decrease the healing time by increasing the surface biocompatibility and the osteoconductive properties [5]. Some studies [6], [7], [8] have demonstrated that the bone to implant contact and the biomechanical interaction between them can be improved at an early implantation stage when the surface roughness is increased [9], [10]. Acid-etching or grit-blasting/acid-etching are common methods used for commercial implants to obtain roughness values ranging from 1 to 2 μm that, in most cases, lead to acceptable bone integration [11], [12]. Combining the latter with chemical changes by incorporating bioactive cations (Ca, Si, Zn, Sr and Mg) enhances the osteogenic differentiation of stem cells [13].
Despite the success of these implant surfaces over several decades, further progress is of particular importance in treating the growing number of patients having poor bone quality due to pathologies or in the elderly [14]. The addition of calcium- and phosphorous-based materials, such as calcium phosphate and particularly hydroxyapatite (HA), as coatings has received significant attention due to the similarities of these elements with the basic components of natural bone. Most commercially available bioceramic coatings are plasma-sprayed HA coatings of 20–50 μm thick [15]. They provide a higher osteoconductivity in comparison to uncoated implants [16], [17], [18]. However, a high adhesion strength requires high plasma energy and temperature that in turn causes increased thermal decomposition of HA [19] and limitations, such as residual stresses, may lead to coating delamination. In an attempt to mitigate the plasma-sprayed coating limitations, other physical processes (e.g. pulsed laser deposition, sputtering coating techniques, ion-beam assisted deposition, electrophoretic deposition), or chemical routes (e.g. sol–gel method, growth in simulated body fluid, anodic oxidation) have been developed. The main advantages and limitations of these techniques for HA-based coatings are presented elsewhere [20], [21]. Among them, the chemical or sol–gel methods are simple, low cost procedures and they provide a method for growing bioactive layers.
Nanoporous structures of a bioactive titanate layer were grown in concentrated NaOH and subsequently heat treated in air [22]. The bone’s response to the Ti surface has been studied [23]. The heat treatment temperature played a crucial role in promoting superior cell adhesion. In other reported studies, an apatite layer was grown on sodium titanate after the immersion in simulated body fluid [4], [24], [25], [26], [27]. The sol–gel method was also used to grow thin HA films having a homogeneous chemical composition [28], [29], [30]. In the present study, we combine the alkaline treatment of Ti and the sol–gel coating processes to obtain a nanoporous HA/sodium titanate bilayer [31]. The Ti/HA interface displays a micro/nano topography favouring a firm HA to Ti bond. The biological properties of this novel functional Ti surface are evaluated in vitro by tests with murine MC3T3-E1 preosteoblasts and human SaOs-2 cells. Its efficacy is investigated using an in vivo dog mandible implantation model. Bone to implant contact and torque measurements of the novel functional Ti surface are compared to those of sandblasted commercial implants.
Section snippets
Fabrication and characterization of hydroxyapatite (HA)/sodium titanate coatings
For the in vitro studies, bars of commercial titanium (Ti Gr4) were cut into disks (18 mm diameter, 1.5 mm height). Commercial Ti implants with and without sand-blasting treatments were purchased from Biotech Dental. To reach a final roughness of approximately 500 nm, the disks were mechanically ground and polished to a mirror-like surface finish using plate series (P120 to P4000 grit, 125–2.5 μm grades) and colloidal silica (0.05 μm grade). Homogeneous polishing levels were controlled by using the
Results and discussion
It is widely accepted that prosthesis osteointegration is favoured by calcium phosphate coatings. However, the torsional forces applied to the implant surface during insertion, load-bearing forces, and frictional interactions occurring during the implant life cause damage to the coating such as crumbling, fracture or delamination [39]. Our strategy to overcome this problem involved improving the strength and quality of the metal-coating interface. The novelty was to combine thermochemical
Conclusions
Our strategy to improve the strength and quality of the titanium–hydroxyapatite interface is to combine thermochemical treatments of the acid-etched metallic support with sol–gel HA coating processes to obtain a nanoporous hydroxyapatite/sodium titanate bilayer. Firstly, a sodium titanate layer is created by incorporating sodium ions onto the Ti surface during a NaOH alkaline treatment and stabilizing using a heat treatment. Secondly, a HA layer is added by dip-coating in a sol–gel solution.
Acknowledgements
We thank SATT-Conectus Alsace (Strasbourg, France) for its financial support, Biotech Dental for supplying commercial implants, Alexis Tilly for his help in preparing the Ti-supports, Eric Mathieu, Jacques Faerber for scanning electron microscopy observations and MiNaMec platform of the ICS Strasbourg for the scratch tests. We would also like to acknowledge INSERM, CNRS and Université de Strasbourg for institutional research funding.
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