Does implant surface hydrophilicity influence the maintenance of surface integrity after insertion into low-density artificial bone?
Introduction
Oral rehabilitation with titanium (Ti) implants routinely achieves survival rates of ≥94% over 10 years of follow-up [1]. The biocompatibility and corrosion resistance due to the stable superficial oxide layer [2] combined with the surface microtopography [3] and implant geometry [4] are responsible for the successful osseointegration [5] and long-term mechanical stability of Ti implants. At the same time, studies showing high prevalence rates of peri-implantitis [6], progressive marginal bone loss [7], and high risk of implant failure in low-density bone [8,9] highlight that field of implant dentistry still faces challenges that require further innovation.
New implant designs, surface treatment procedures [[10], [11], [12]], and surgical milling protocols [13] primarily focus on maximizing and accelerating osseointegration to reduce the potential for implant failure. For this reason, modification of topographical features and optimization of surface roughness has been the focus of research on implant surfaces over the last decades [14]. Surface roughness affects initial events of osseointegration such as cell adhesion, proliferation, and differentiation, directly influencing biomaterial-bone interaction [3,15,16]. Implants with a moderately rough surface (Sa = 1–2 μm) are more attractive for new bone formation, presenting a higher bone-implant contact rate (BIC) and removal torque than those with smooth (Sa < 0.5 μm) or rough (Sa > 2 μm) surfaces [17,18].
Currently, much research focuses on the interaction between topographic features and wetting properties through physicochemical modifications [14]. These modifications jointly influence the topographic, mechanical, and surface chemical properties, resulting in varying surface roughness, increasing surface energy, reducing surface tensions [19], changing surface charge and chemical composition, which induces surface wettability and bioactivity [14,[20], [21], [22]] and may also increase resistance to the shear forces generated during implant insertion [23]. Wettability is quantified by the liquid–solid contact angle (Ɵ), and water Ɵ is classified as follows: superhydrophilic (Ɵ very close to 0°), hydrophilic (Ɵ < 90°), hydrophobic (Ɵ > 90°), and superhydrophobic (Ɵ > 150°) [24].
The synergism of micro and nano-roughness with surface wettability influences the molecular and cellular interactions of the implant surface with the bioliquids (e.g., blood), optimizing and accelerating the initial bone healing period [14,24,25]. Studies have shown that hydrophilic implants show superior clinical results than hydrophobic implants in the early bone healing period with higher BIC [[26], [27], [28]], bone area fraction occupied [27,28] and primary stability [19], which indicates their suitability for critical situations, such as in cases with low density bone and immediate loading [19,29].
The SLActive® surface (Straumann®, Basel, Switzerland) is the most widely investigated superhydrophilic surface [14]. More recently, the Acqua™ surface (Neodent®, Curitiba, PR, Brazil) has been made available on the market [27,28]. Both surfaces are created by abrasive sandblasting, combined with acid-etching and rinsing for surface neutralization in a protective nitrogen environment [20,[26], [27], [28]]. The obtained surface has a high surface energy, a positive surface charge, and a lower Ɵ, and is stored in an isotonic 0.9% NaCl solution [20,26] to preserve the surface hydroxylation and to avoid contamination with organic components and carbonates until installation [14,25].
From a tribological point of view, wettability is associated with the application of lubricant fluids between the contact surfaces and relative motion. The primary objective of lubrication is to prevent contacts between asperities on both surfaces, reducing the coefficient of friction and surface wear [30]. Rupp et al. suggested that bioliquids dynamically wet biomaterials during relative movements, such as when implants are inserted into the blood-filled surgical wound [14]. Blood is a viscous fluid constituted of plasma, an aqueous solution containing salts, gases, proteins, amino acids, and glycoproteins. Proteins and erythrocyte volume determine blood viscosity, which influences the formation of a lubricating film [31]. In addition, the apparent viscosity of blood depends on the shear rate [32]. Shear stress is generated from implant friction against bone during insertion, creating a dynamic stress concentration along the bone-implant interface, causing bone rupture [33], surface damage to the implants and release of wear particles in the peri-implant region [[33], [34], [35], [36], [37], [38]]. The surface area at the bone-implant interface affects the insertion torque and directly relates to roughness and implant geometry [39].
During the insertion of implants, Ti particles are assumed to be released [34,40]. In vitro studies revealed damage to implant surfaces and the presence of Ti particles in the bone bed after implant insertion [[34], [35], [36]41]. Surfaces with higher average roughness and predominance of peaks showed a greater reduction of roughness parameters and an increased Ti particle release [35,36]. However, there are currently few studies that investigated the impact of hydrophilicity on the implant surface integrity after insertion [36].
Ti particles are commonly detected around peri-implant tissues with the highest Ti concentration at the bone-implant interface [[41], [42], [43], [44], [45]]. At present, there are no studies that map Ti intensity along the bone bed using a benchtop micro X-Ray Fluorescence (μ-XRF) spectrometer. Ti debris can generate an exacerbated inflammatory process with increased expression of pro-inflammatory cytokines related to the osteolytic process [46]. Recent studies suggested that Ti particles at the bone-implant interface may influence the pathogenesis of peri-implantitis diseases and marginal bone resorption [[47], [48], [49], [50], [51]]. The evaluation of the deleterious effects of surface wear and Ti particles release by hydrophilic implants is difficult to assess since studies that quantify the surface integrity of hydrophilic implants [36] and the spatial distribution of Ti intensity around the implant by Synchrotron Radiation X-Ray Fluorescence (SRXRF) spectrometer are still scarce [41,52,53], in part because access to SRXRF is limited. Therefore, the aim of this study was to (1) analyze the effect of surface hydrophilicity of implants with different geometries on surface integrity after insertion, (2) to evaluate the Ti intensity distribution along the bone bed, and (3) to establish the benchtop μ-XRF system as an accessible methodology for spatial analysis of metallic particles.
Section snippets
Experimental design
This in vitro study evaluated the influence of hydrophilicity on surface changes after implant installation in artificial bone blocks (Sawbones, Pacific Research Laboratories Inc., Washington, DC, USA) with density compatible with bone type III and IV (combination of block grade 20 (0.32 g/cm3) and sheets grade 40 (0.64 g/cm³). Forty-eight commercial implants (Neodent®, Curitiba, PR, Brazil) with 4 different designs and 2 surface treatments were investigated (6 implants per group): 1 - Titamax
Surface topography characterization
The implant insertion torque (IT, Ncm) means ± standard deviation were 39.40 (±3.34), 20.00 (±2.54), 45.20 (±3.91) and 45.50 (±6.43) for Titamax Ex, Facility, Alvim, and Drive implants with NeoPoros® surface, respectively; and 42.00 (±2.11), 14.00 (±3.20), 49.70 (±5.21) and 51.70 (±5.17) for Titamax Ex, Facility, Alvim and Drive implants with Acqua™ surface, respectively.
The mean values and associated standard deviations are shown in Table 1, along with the relative reduction in the roughness
Discussion
Superhydrophilic characteristics of dental implant surfaces are linked to increased bone-biomaterial interaction [24,27,29,64] but should ideally also promote mechanical resistance against shear forces and long-term chemical stability [25] while minimizing harm to the tissues. Because preservation of the surface integrity of implants during insertion is a critical factor [[34], [35], [36]38] for successful osseointegration, the impact of physical-chemical modifications and hydrophilic
Conclusion
The results shown in this study provide evidence that the physicochemical surface treatment affects the integrity of the implant surface and the release of particles in the bone beds. Surface modifications to obtain superhydrophilic surfaces increase the surface roughness of implants, making them more susceptible to surface damage. Consequently, hydrophilic implants displayed higher relative roughness reductions than their hydrophobic counterparts and released more Ti particles into the bone
Acknowledgements
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors also thank Neodent, Curitiba, PR, Brazil for providing the dental implants and accessories used in the study; the Brazilian Nanotechnology National Laboratory (LNNano), Campinas, SP, Brazil; and the Nuclear Instrumentation Laboratory at CENA/USP, Piracicaba, SP, Brazil for the use of equipment and facilities.
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