Elsevier

Acta Biomaterialia

Volume 8, Issue 5, May 2012, Pages 1948-1956
Acta Biomaterialia

Osteoclast resorption of thermal spray hydoxyapatite coatings is influenced by surface topography

https://doi.org/10.1016/j.actbio.2012.01.023Get rights and content

Abstract

Coating characteristics such as composition, crystallite features and topography collectively impact the cell response. The influence from splats has not yet been assessed for hydroxyapatite (HAp) thermal spray coatings. The objective of this work is to (a) survey the topography on commercial implants, (b) ascertain topography formation from single splats, and (c) determine the osteoclast resorption pattern on a topographically refined coating compared to dentine. Coatings on dental implants, an orthopedic screw, a femoral stem and a knee implant were studied for reference. The effects of substrate pre-heat, roughness, spray distance and particle size on the coating roughness and topography were studied. Human-derived osteoclasts were placed on a coating with refined topography and compared to dentine, a polished coating and polished sintered HAp. A pre-heat of at least 200 °C on titanium was required to form rounded splats. The greatest influence on coating roughness and topography arose from particle size. A 2-fold increase in the mean particle size from 30 to 72 μm produced a significant difference (P < 0.001) in roughness from 4.8 and 9.7 μm. A model is shown to illustrate topography formation, nanostructure evolution on single splats, and the topography as seen in commercial implants. Osteoclasts showed a clear preference for activity on coatings with refined topography. A one-way ANOVA test revealed a significantly greater pit depth (P = 0.022) for dentine (14 μm) compared to the as-sprayed and polished coating (5 μm). Coatings with topography display a similar number of resorption pits with dentine, but a 10-fold greater number than polished coatings, emphasizing the importance of flattened droplet topography on implant surfaces.

Introduction

Orthopedic implants are exposed to osteoclasts, bone-resorbing cells, and osteoblasts, bone-forming cells. The individual contribution of both cell types dictates bone regeneration through the processes of resorption, and bone formation. Biomaterial surface characteristics impact the chain of biological events, leading to a successful clinical outcome. To date, there has been little or no attention on the topography of thermal spray hydroxyapatite (HAp) coatings. This work will survey the surface topography of implants and prostheses, investigate topography control from thermal spraying, and show the human osteoclast resorption of topographically improved HAp coatings.

In the past, attention has been focussed on providing a stable and strong interface with bone. The direct bonding capability of HAp was supplemented by mechanical interlocking of bone with porous surfaces made from sintered beads, wire-mesh or a titanium plasma coating bond-coat (Fig. 1). Pores larger than 100 μm allowed bone ingrowth or abutment into surface cavities. A study on thermally sprayed fluorhydroxyapatite with large changes in surface roughness (Ra of 6 μm vs. 21 μm) provided openings for interdigitation with bone, but did not show any difference in the osteoblast response [1], posing the question of whether more organized topography at the micro- and nano-scale could be important for cell response. The work developed here is motivated by the question, “Is it possible to control the surface topography with thermal spraying, and will this have any effect on the cell response?” This research is an extension of unpublished data from a Masters thesis that showed textured HAp coatings with a topography from flattened molten droplets [2].

Significant effort has been directed at improving the coating mechanical stability with approaches such as strengthening with a secondary phase or an increase in the HAp crystallinity. Inclusion of a second phase, such as zirconia, increases the fracture toughness [3], but zirconia should be retained within the coating to provide the sole purpose of reinforcing HAp. Use of a strengthening phase, however, requires a homogeneous crystalline coating. A crystalline coating has been obtained with (a) the use of a ceramic bond-coat under the HAp coating [4], [5], (b) a change from air plasma spraying to radio-frequency plasma spraying [6] or vacuum plasma spraying [7], or (c) post treatments such as hydrothermal processing [8]. The alternative has been to spray a thermally stable fluorapatite instead of the more thermally sensitive HAp. While the main focus has been the production of a uniform stable coating, these modifications influence the coating roughness and topography. This work will use increased heat input from the spray gun to encourage crystallization and columnar growth within the coating. Crystal orientation and a designed roughness offer new surface characteristics.

It is difficult to review the details on the coating surface due to the range of terms used in the scientific literature. “Surface characteristics” is used as a general term. “Roughness” implies a measured value from profilometry. “Morphology” describes the shape of features on a surface. “Topology” places attention to the height of features, but “topography” describes the shape and height of surface features. Further discussion will use “roughness” as a quantitative measure and “topography” to refer to the shape and height of surface features.

There has been little research on the effect of thermal spraying on the resulting surface topography. Chemistry has been separated from topography with a thin titanium film to reveal that 80% of the maximum bone-forming response is attributed to topography [9]. Since the coating surface contains numerous features, the most important topographical characteristic could not be determined. Improved cell adhesion and proliferation occur on sintered HAp ground with different SiC papers [10]. A recent literature review on surface topography the need to establish routine measurement methods and a report on specific surface features to allow a comparison with surfaces in journal articles and clearly show the cell response to topographical features to understand the effect of topography on cell response [11]. This work will generate a surface with reproducible features from molten droplets, a control of droplet spreading and crystallization within the splat.

The thermal spray process is an additive process that can control the shape and thickness of each flattened solidified droplet, herein referred to as a splat. This building block offers a new surface feature for which the cell response is presently not known. This work will (a) characterize the surface of commercial coatings, (b) investigate the conditions for producing a rounded splat, (c) determine coating parameters that influence the coating topography, and (d) determine the human osteoclast (OC) resorption of an as-sprayed coating compared to dentine, a polished coating and sintered HAp.

Section snippets

Assessment of commercial coatings

HAp coatings from three dental implants (Interpore, LifeCore, Biovent), an orthopedic screw (Osseotite®, Orthofix), a hip prosthesis (Margron) and a knee prosthesis (ASDM) were examined in a scanning electron microscope. Joint prostheses presented large areas that allowed the measurement of the coating roughness. The orthopedic screw and the dental implants did not present suitable surfaces for roughness measurement. Conditions for scanning electron microscopy (SEM) and roughness measurement

Topography of commercial hydroxyapatite coatings

Coatings on dental implants, an orthopedic screw, a femoral stem of the hip prosthesis and a femoral component from a knee implant all showed one of two different features. These include a smooth surface from well-molten particles, and scattered fine particulate or clusters (Fig. 2). The coating on the Interpore implant consisted only of smooth areas from molten particles. This was distinctly different from all other coatings and showed the highest degree of particle melting. A 10 μm wide raised

Commercial coating surfaces

Commercial coatings displayed a large variability in surface characteristics with a range in feature size, height and smoothness. These aspects collectively define the quantitative measure of roughness, a value frequently referred to without mention of the feature size, their smoothness and placement.

The majority of coatings contained a combination of microrough and smooth areas, with the exception of one coating that displayed only smooth areas. This high variability in topography arises

Conclusions

Topography varied amongst the commercially sprayed HAp coatings showing partially molten HAp more frequently than well-molten HAp particles. The single splat study revealed that a substrate pre-heat of at least 200 °C is required to produce rounded splats. Both spray distance and particle size had the largest influence on roughness. A model was developed to explain the different contributions to topography within a coating formed by solidified droplets.

The resorption of human-derived osteoclasts

Acknowledgements

This research was supported by a Small grant from Monash University. A contribution is also acknowledged from an ARC Discovery grant, nr. DP0774251. The first author was supported by a Marie Curie grant (#PIRG05-GA-2009-249306 and partly by an ESF grant # 2009/0199/1DP/1.1.1.2.0/09/APIA/VIAA/090) during the preparation of this manuscript. DM is acknowledged for examination of the implants and prostheses, producing single splats and a topographically modified coating. Osteoclasts work was

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