Inflammatory cell response to ultra-thin amorphous and crystalline hydroxyapatite surfaces

Louise Rydén,Anna Johansson,Peter Thomsen,Ryo Jimbo,Anders Palmquist,Omar Omar

doi:10.1007/s10856-016-5814-2

Abstract

It has been suggested that surface modification with a thin hydroxyapatite (HA) coating enhances the osseointegration of titanium implants. However, there is insufficient information about the biological processes involved in the HA-induced response. This study aimed to investigate the inflammatory cell response to titanium implants with either amorphous or crystalline thin HA. Human mononuclear cells were cultured on titanium discs with a machined surface or with a thin, 0.1 μm, amorphous or crystalline HA coating. Cells were cultured for 24 and 96 h, with and without lipopolysaccharide (LPS) stimulation. The surfaces were characterized with respect to chemistry, phase composition, wettability and topography. Biological analyses included the percentage of implant-adherent cells and the secretion of pro-inflammatory cytokine (TNF-α) and growth factors (BMP-2 and TGF-β1). Crystalline HA revealed a smooth surface, whereas the amorphous HA displayed a porous structure, at nano-scale, and a hydrophobic surface. Higher TNF-α secretion and a higher ratio of adherent cells were demonstrated for the amorphous HA compared with the crystalline HA. TGF-β1 secretion was detected in all groups, but without any difference. No BMP-2 secretion was detected in any of the groups. The addition of LPS resulted in a significant increase in TNF-α in all groups, whereas TGF-β1 was not affected. Taken together, the results show that thin HA coatings with similar micro-roughness but a different phase composition, nano-scale roughness and wettability are associated with different monocyte responses. In the absence of strong inflammatory stimuli, crystalline hydroxyapatite elicits a lower inflammatory response compared with amorphous hydroxyapatite.

Highlights

Hydroxyapatite (HA) is a principal example of biomaterials with osteoconductive and osteoinductive capacity [1]
We analyzed the secretion of the pro-osteogenic factors, bone morphogenetic protein-2 (BMP-2) and TGF-β1, from monocytes cultured on titanium and titanium with amorphous or crystalline thin HA coatings
Murine J774A.1 macrophages both expressed and secreted BMP-2, with higher BMP-2 secretion detected at HA and PS surfaces compared with titanium [30]

Summary

Introduction

Hydroxyapatite (HA) is a principal example of biomaterials with osteoconductive and osteoinductive capacity [1]. Hydroxyapatite closely resembles the mineral phase of bone and has demonstrated superior bone ingrowth, osseointegration and bone bonding [2,3,4]. One important aspect of the HA coating is its thickness. Thin HA coatings have been found to remain stable during bone matrix mineralization [13], while thicker HA coatings might crack and/or delaminate, with a risk of particulate release [14]. Thin HA coatings have been shown to promote the same osteogenic response as thicker coatings [8]. Experimental studies have shown promising results using thin sputtered HA coatings (0.1 μm thickness) on titanium implants [5, 8]. There is insufficient knowledge about the biological factors that could contribute to the improved bone response to the thin HA coating.

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