Zirconium Nitride Coating Reduced Staphylococcus epidermidis Biofilm Formation on Orthopaedic Implant Surfaces: An In Vitro Study
Authors: Pilz Magdalena , Staats Kevin , Tobudic Selma , Assadian Ojan , Presterl Elisabeth , Windhager Reinhard , Holinka Johannes 
Published: November 2018

Abstract

Background
One of the most commonly identified pathogens responsible for orthopaedic implant infection is Staphylococcus epidermidis, which can form biofilms on surfaces. Currently, orthopaedic implants made of various surface materials are available, each with features influencing osseointegration, biocompatibility, and adherence of bacteria to the surface, which is the first step in biofilm formation. The aim of this experimental study was to investigate the effect of a high tribologic-resistant 2.5-µm zirconium nitride top coat on an antiallergic multilayer ceramic-covered cobalt-chromium-molybdenum surface on the formation of S. epidermidis biofilm compared with other commonly used smooth and rough orthopaedic implant surface materials.

Questions/purposes
(1) When evaluating the surfaces of a cobalt-chromium-molybdenum (CoCrMo) alloy with a zirconium (Zr) nitride coating, a CoCrMo alloy without a coating, titanium alloy, a titanium alloy with a corundum-blasted rough surface, and stainless steel with a corundum-blasted rough surface, does a Zr coating reduce the number of colony-forming units of S. epidermidis in an in vitro setting? (2) Is there quantitatively less biofilm surface area on Zr-coated surfaces than on the other surfaces tested in this in vitro model?

Methods
To determine bacterial adhesion, five different experimental implant surface discs were incubated separately with one of 31 different S. epidermidis strains each and subsequently sonicated. Twenty test strains were obtained from orthopaedic patients undergoing emergency hip prosthesis surgeries or revision of implant infection and 10 further strains were obtained from the skin of healthy individuals. Additionally, one reference strain, S. epidermidis DSM 3269, was tested. After serial dilutions, the number of bacteria was counted and expressed as colony-forming units (CFUs)/mL. For biofilm detection, discs were stained with 0.1% Safranin-O for 15 minutes, photographed, and analyzed with computer imaging software.

Results
The lowest bacterial count was found in the CoCrMo + Zr surface disc (6.6 x 104 CFU/mL ± 4.6 x 104 SD) followed by the CoCrMo surface (1.1 x 105 CFU/mL ± 1.9 x 105 SD), the titanium surface (1.36 x 105 CFU/mL ± 1.8 x 105 SD), the rough stainless steel surface (2.65 x 105 CFU/mL ± 3.8 x 105 SD), and the rough titanium surface (2.1 x 105 CFU/mL ± 3.0 x 105 SD). The mean CFU count was lower for CoCrMo + Zr discs compared with the rough stainless steel surface (mean difference: 2.0 x 105, p = 0.021), the rough titanium alloy surface (mean difference: 1.4 x 105, p = 0.002), and the smooth titanium surface (mean difference: 7.0 x 104, p = 0.016). The results of biofilm formation quantification show that the mean covered area of the surface of the CoCrMo + Zr discs was 19% (± 16 SD), which was lower than CoCrMo surfaces (35% ± 23 SD), titanium alloy surface (46% ± 20 SD), rough titanium alloy surface (66% ± 23 SD), and rough stainless steel surface (58% ± 18 SD).

Conclusions
These results demonstrate that a multilayer, ceramic-covered, CoCrMo surface with a 2.5-µm zirconium nitride top coat showed less S. epidermidis biofilm formation compared with other surface materials used for orthopaedic implants.

Clinical Relevance
CoCrMo with a 2.5-µm zirconium nitride top coat seems to be a promising surface modification technology able to reduce bacterial attachment on the surface of an implant and, hence, may further prevent implant infection with S. epidermidis biofilm formation.