Interplay between surface properties of standard, vitamin E blended and oxidized UHMWPE for total joint arthroplasty and adhesion of Staphylococcus epidermidis, S. aureus and Escherichia coli.

The increasing number of total joint replacement (TJR) in orthopaedic surgery, led to the development of new diseases related to the biomaterials themselves: biomaterial associate infection (BAI). BAI is initiated by the adhesion and subsequent growth of microorganisms to the implant surface. Staphylococci are the most common microorganisms causing BAI, followed by streptococci and Gram-negative bacilli. It is widely known that the initial adherence of microorganisms to biomaterials is a process related to a series of physico-chemical interactions between substratum and microorganisms. The physical properties of the biomaterial surface, such as roughness, hydrophobicity, surface energy, electrostatic charge, and coating also influence the feasibility and kinetics of microbial adhesion. A complex strategy is required to efficaciously deal with the BAI. The best aseptic techniques cannot totally remove contamination risk and peri-operative antibiotic prophylaxis is not sufficient. Reducing microbial adhesion to the biomaterial could be an attractive mean to reduce BAI. Starting almost 50 years ago, Ultra High Molecular Weight Polyethylene (UHMWPE) has a long successful clinical record as bearing material in TJR. However, this long clinical history has revealed weak points of this polymer: in particular, it has been demonstrated that oxidation due to sterilization by gamma rays, resulting in a severe decrease in molecular mass, has been the main cause of many dramatic implant failures over the 1980s and 1990s. As a consequence, many efforts have been made to improve the quality and the performance of UHMWPE in vivo, for example through reducing or eliminating the oxidation: the newest frontier is the addition of antioxidants, such as the alphatocopherol (vitamin E). Aims The purpose of this interdisciplinary study was to assess the different adhesive strength of some of the most common bacteria associated with periprosthetic infection on various types of UHMWPE components, assuming that a different chemical composition of the surface of the biomaterial corresponds to a different bacterial adhesion. Methods We examined the adhesion of ATCC biofilm producing strains of Staphylococcus epidermidis, Staphylococcus aureus and Escherichia coli on standard GUR 1020 UHMWPE, vitamin E blended polyethylene (VE-PE) and oxidized polyethylene on purpose (OX-PE) at different incubation times (3, 7, 24 and 48 hours). Quantitative in vitro analysis of bacterial adhesion was performed by using a sonication protocol to dislodge adherent microorganisms. The biomaterials were physico-chemically characterized by means of scanning electron microscopy (SEM), water contact angle (CA) measurements and attenuated total reflectance (ATR)-fourier transform infrared (FTIR) spectroscopy, before and after adhesion assays. The experiments were assayed in triplicate and repeated a minimum of three times. A statistical analysis on results (T-Student test) was conducted. Results No significant difference of the surface roughness and CA was found among the different samples, while a lower CA was observed for OX-PE, if compared to the other two groups. Again, with ATR-FTIR spectroscopy, the only significant difference was observed in the spectrum of the aged group, OX-PE, indicating adsorption of protein-like substances on the polymer surface. This different protein adsorption has been shown to influence bacterial adhesion. In fact, adhesion assays, performed on the three polyethylenes by using the ATCC biofilm producing S. epidermidis, S. aureus and E. coli strains evidenced a significant (p<0.05) decrease in the number of adhered bacteria on VE-PE and a considerable enhancement of adhered bacteria on OX-PE, compared to standard UHMWPE after 24 and 48 hours of incubation. A similar trend of adhesion was observed testing both biofilm non producing ATCC bacterial strains and clinical biofilm producing strains. Conclusions Since it is widely known that BAI in TJR causes a decrease in the success rate of the implant with consequent enormous burden on the patients and high cost to the healthcare system, the results obtained in this interdisciplinary study may have important clinical implications concerning one aspect of the multifactorial septic loosening. We highlight that initial adhesion on inert surfaces is strain-dependent and it is strictly influenced by biomaterial surface chemistry: VE-PE reduces S. epidermidis, S. aureus and E. coli adhesive ability and its antioxidant properties, due to vitamin E addition, may be one of the key points.