Elsevier

Biomaterials

Volume 23, Issue 3, February 2002, Pages 887-892
Biomaterials

Silver coated materials for external fixation devices: in vitro biocompatibility and genotoxicity

https://doi.org/10.1016/S0142-9612(01)00198-3Get rights and content

Abstract

The most significant complication in external fixation is pin tract infection causally related to the highly adaptive ability of bacteria to colonise the surfaces of “inert” biomaterials or of adjacent damaged tissue cells. The hypothesis that coating a pin with a silver-containing compound will decrease bacterial colonisation and/or pin tract infection has been confirmed in other studies in vitro and in vivo experiments.

In this work, biocompatibility of silver-coated orthopaedic external fixation pins was compared with stainless steel controls in an in vitro study. Human peripheral blood lymphocytes were used to assess the possible genotoxic effect of silver, studying the frequency of sister-chromatid exchanges and micronuclei while fibroblasts (NIH 3T3) and osteoblast-like cells were used for cytotoxicity and cytocompatibility studies. These studies have shown that silver is neither genotoxic nor cytotoxic as compared to stainless steel, a material in wide use as a metal implant. At 4 days cells cultured on the silver-coated material evidenced good cell spreading and a higher cell count with respect to the uncoated material. It appears that the addition of silver onto implantable medical devices could be beneficial when specific biological properties, such as antibacterial behaviour, are required. Based on these and the previous bacterial studies it seems like the toxicity towards bacteria was quite a bit greater than that towards the human cells.

Introduction

The external fixation is a biomechanical method for osteosynthesis based on the use of bone fixing elements (pins, wires), placed far from the fracture site, which connect through soft, medullar and bone tissue to external systems. The most significant complication in external fixation is pin tract infection [1] causally related to the highly adaptive ability of bacteria to colonise the surfaces of “inert” biomaterials or of adjacent damaged tissue cells [2]. Systemic antibiotics do not provide effective treatment for infection of implanted pins [3]; consequently, an alternative strategy is required to control infections. Modifications to biomaterial surfaces will allow the programming of cells to substratum events, thereby diminishing infection by inhibiting bacterial adhesion or by enhancing tissue compatibility or integration.

Silver has long been known to be a potent antibacterial agent with a very broad spectrum of activity and has been used safely in medicine for many years [4], [5], [6], [7], [8], [9]. Surface coating with silver, providing a thin layer of metal strongly bound to the underlying substratum, may limit the toxic effect of the silver metal towards the host organism while still allowing it to exercise infection control. The hypothesis that coating a pin with a silver-containing compound will decrease bacterial colonisation and/or pin tract infection has been confirmed in vitro and in vivo [3], [10], [11].

In this work, we have studied in vitro biocompatibility of silver-coated stainless steel external fixation pins. Cytogenetic, cytotoxic and cell physiological aspects were evaluated and compared with the results obtained using the uncoated stainless steel pins normally used in surgical practice.

Section snippets

Materials

1.5 cm×0.5 cm stainless steel AISI 316L rods and stainless steel rods were provided by Orthofix (Verona, Italy) then coated with silver by the SPI-ARGENTTM method (Spire Corporation, Bedford, MA). The silver film was prepared by ion-beam-assisted deposition of silver from the vapour-phase. Samples were ion implanted with silver ions at an energy of 80 kV. Total dose on the parts was approximately 6×1017 ions/cm2. Under these conditions, the maximum depth of penetration of the silver ions was

Cytogenetic results

The mean frequency of SCEs/cell for the 30 cultures we examined showed a slight difference between stainless steel and silver-coated stainless steel and between treated and controls untreated cultures (Table 1). Evaluating SCEs from each subject four of the ten donors evidenced increased exchanges when silver-coated rods were present. This difference was not statistically significant considering total mean values (n=300).

The trend of cell kinetics can be seen in Table 2.

No differences of cell

Discussion

External bone fixing elements are not sterile interfaces isolated from the environment but rather conduits between the surface of the skin, which is normally colonised by bacteria, and the bone tissue, which is normally not [1]. Biomaterial surfaces must be modified to improve compatibility and tissue integration [18] and to resist microbial colonisation that in vivo defeats host tissue cells in the race for the surface, causing infection instead of tissue integration [2]. It is reported in the

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