KALÇA EKLEM PROTEZLERİ YAPIMINDA KULLANILACAK OLAN ULTRA MOLEKÜL AĞIRLIKLI POLİETİLEN (UMAPE) YÜZEYLERE KAYGANLIK KAZANDIRMAK İÇİN KONTROLLÜ OLARAK POLi(AKRiLiK ASiT) (PAA) AŞILANMASI

Abstract

At present, 80% of total hip prosthesis (THP) has achieved 20 years of usability [1]. In the meantime, due to the increase in the number of total joint artroplasty and continuous decrease in the average age of patients with the increase in the average age, there is a need to increase the lifetime of implant materials [2]. The most common reason of revision surgeries is aseptic loosening [3]. The reason for aseptic loosening is osteolysis caused by the released of UHMWPE wear particles over time [4]. The wear problem of ultra high molecular weight (UHMWPE) based implant materials is significantly reduced with crosslinking by using ionizing radiation. Radiation crosslinked and wear resistant UHMWPE implants has been clinically used since the 1990s and still in use for clinical applications. However, high energy radiation has critical disadvantages such as its adverse effects on mechanical properties and its high cost. On the other hand, the highly effective lubrication and low friction coefficient as natural articular joint surfaces has not been achieved so far. However, the mechanism of wear between metallic head and UHMWPE is due to the hard contact and deformation. More hydrophilic UHMWPE surface may provide lubricity as the one in between natural articular surfaces. Therefore, one of the approach that can be applied to extend the lifetime of artificial joint is mimicking their surface properties to the characteristic surface properties of natural articular joint [3]. It is thought that bone resorption problems due to the wear of implant materials can be minimized by modification of artificial joint surfaces. In addition, increasing the surface lubrication can reduce the rotational torque acting on the prostheses, thereby reducing the risk of aseptic loosening. Surface properties of polymeric materials can be modified using different methods (physical, chemical coating) [5]. Among these methods, surface initiated grafting method is adventegeous in terms of homogenous, high density and chemical bonding of polymers to the surface. Controlled graft polymerization methods are used to control the thickness of grafted polymers to the surface of polymeric materials [7]. The aim of this thesis is grafting acrylic acid (AA) by UV initiated polymerization to UHMWPE liner surface which is one of the component of total hip prosthesis. It is predicted that the synovial fluid layer which can be held on the surface by the grafted poly(acrylic acid) (PAA) can provide lubricious surface and reduce the friction force. PAA was grafted to UHMWPE by using conventional radical polymerization and reversible addition fragmentation chain transfer polymerization (RAFT) which is one of the controlled radical polymerization methods. In order to obtain grafted layers with different thickness on the surface of UHMWPE samples, grafting conditions were optimized by using various monomer concentrations, irradiation time, UV lamp distances and monomer/RAFT agent ratios. Grafting applied for both powder and consolidated forms of UHMWPE. Alterations in the surface properties after grafting PAA to UHMWPE surfaces have been investigated by using surface characterization methods, FT-IR/ATR, elemental analysis, X-ray photoelectron spectroscopy (XPS), contact angle. Changes in the mechanical properties after grafting PAA onto UHMWPE surfaces have been investigated by using tensile and impact strength tests. The wear test of samples were investigated by applying biaxial and rotational motion in bovine serum. We observed that consolidated samples prepared from PAA grafted UHMWPE powder samples have lower wear rates than samples prepared by grafting PAA to the surface of consolidated UHMWPE. Consolidated samples prepared from PAA grafted UHMWPE powder samples have higher amount of grafted hydrophilic polymers in the bulk and surface thus those samples absorb more fluid on the surface and exhibit lower wear rates. We obtained 55% percent decrease on wear rates for consolidated samples which prepared from PAA grafted UHMWPE samples and synthesized by conventional and controlled radical polymerization methods. Controlling the length of grafted chains by RAFT method does not provide significant differences in wear rates and friction coefficients for samples that have almost same percentage of grafting degree.