Abstract
Introduction
When osteoarthritis occurs, joint replacement is the most frequent treatment. Currently, the mean survival rate for total joint arthroplasty is ∼90% after 10 years: the main reason for long-term implant failure, that generally required a revision surgery, are osteolysis and aseptic loosening of the implant, which are strongly correlated with wear debris formation from the UHMWPE insert [Ingham, 2005], as a consequence of the cyclic loading against the metallic or ceramic counterface [Dumbleton, 2002]. Wear debris bring to chronic inflammation of periprosthetic tissues causing an increase of bone reabsorption that finally provoke aseptic loosening, so implant failure[Holt, 2007]. Different solutions were proposed to reduce wear debris production but agreement has not been achieved yet. Our challenging approach prefigures the direct coating of the plastic component with a hard and well-adherent ceramic film, in order to drastically reduce wear debris formation from the plastic substrate while preserving its well-established bulk mechanical properties, especially under high local loads [Bianchi, 2013].
Methods
3%yttria-stabilized zirconia films were deposited by PPD technique. PPD is a new vapour deposition technique based on the ablation of a target material as a consequence of the impact of a high-energy electron beam. The plasma plume of ablated material is directed toward and deposited onto the substrate. Films were characterized by SEM-EDX, X-ray diffraction, nanoindentation, adhesion and tribological tests. Moreover, capability of ZrO2–UHMWPE system of carrying local loads – i.e. an estimation of the resistance to a third-body abrasion – was investigated.
Results
X-ray diffraction measurements revealed that zirconia films grew in cubic phase, while scanning electron microscopy images showed a dense, columnar film microstructure, exhibiting a nanostructured top surface (grain size <500 nm) (Fig. 1). Nanoindentation tests revealed rather high hardness and Young's modulus values (17 GPa and 154 GPa respectively) (Fig. 2), while critical fracture tests revealed that, even under loads as high as 500 mN (equivalent to ∼8 times the maximum pressure exercised on a femoral head during normal walking activity) no lateral cracks, spalling or pile-up phenomena were observable, revealing a high fracture toughness and a very high adhesion degree of the ceramic film to the plastic substrate, despite the large mechanical mismatch between the two materials.
Moreover, 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.
Conclusions
We have presented the preliminary results of a novel approach aiming to the drastic reduction of plastic debris release from common joint prostheses. The results showed suggested the feasibility of pursuing this alternative and completely new route to improve UHMPWE performances while preserving its well-established mechanical properties.
Hard and well-adherent zirconia films deposited directly on the surface of plastic component of a joint implant will allow a drastic reduction of wear debris production and plastic deformation of the component, without weakening primary and secondary stability of the implant.
