Abstract
Introduction
Total joint arthroplasty is frequently necessary when a traumatic or degenerative disease leads to develop osteoarthritis (OA). Nowadays, the main reason for long term prosthesis failure is due to osteolysys and aseptic loosening of the implant itself, that are related to UHMWPE wear debris [1–3]. Different solutions to overcome this issue have been proposed, including different couplings like metal-on-metal and ceramic-on-ceramic. Our hypothesis was that a hard ceramic thin film realized on the plastic component (i.e. UHMWPE) could improve the friction and wear performance in a prosthetic coupling. The purpose of the presented study was therefore to characterize from the point of view of structure and mechanical performance of this ceramic-coated plastic component. The thin films were specifically realized by means of the novel Pulsed Plasma Deposition (PPD) technique [4].
Materials and methods
PPD technique was used to deposit Yttria-stabilized zirconia (YSZ at 3%) films on medical-grade UHMWPE substrates [4]. The morphology and micro-structure were characterized by Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). By means of nanoindentation and scratch tests mechanical properties were investigated. Ball-on-disk tribological tests were carried out in air, deionized water and physiological solution against alumina balls (6 mm diameter, grade 200) used as counterpart; friction evaluation of the proposed approach and the corresponding worn track were analyzed by SEM-EDS.
Results
Deposited zirconia films up to several micron thickness exhibited a fully cubic structure and a smooth nanostructured surface.
Good high hardness and Young's modulus values (17 GPa and 154 GPa respectively) were revealed by nanoindentation tests, while no radial cracks, spalling or pile-up phenomena were observable at critical fracture tests. Assessment of the very strong interface adhesion was observed by scratch tests (with initial delamination at about 2 N load).
When the UHMWPE substrate was covered by a ceramic film as thin as 1.5 micron, an indentation depth reduction of about 330% was registered. Further, the material yielding under an applied constant load (creep) was larger for UHMWPE compared to coated UHMWPE, whose total creep being only the 19% of the total creep of UHMWPE, respectively. Finally, preliminary tribological tests carried out in air against an alumina ball counterpart showed wear rate as low as 3.2*10-6mm3N-1m-1 after 500.000 cycles, showing an average friction coefficient evaluated on unpolished materials ranging from 0.15 to 0.3 in air.
Conclusions
The proposed approach was able to directly coat the plastic insert of a commercial implant joint with hard ceramic materials, thus providing specific additional mechanical and superficial properties, while preserving the well established mechanical properties of UHMWPE. The results of this study showed an alternative and promising approach to improve UHMWPE mechanical properties in arthroplasty.
