## 4 Discussion
Hence, Raman spectroscopy may represent a useful tool on monitoring and controlling the quality of UHMWPE components and would be a feasible choice for investigating the behavior of novel UHMWPE-derived materials with incorporated antioxidant agents like \(\alpha\)-tocopherol and others [36]. In our study, confocal Raman spectroscopy was employed for rapid screening of retrieved tibial component of knee prosthesis, evaluating the modifications related to aging process as possible reasons of failure. The metallic tibial component does not show any modification during the implantation period, as demonstrated by the well preserved anatase phase detected in retrieved component, compared to the unused piece. Generally, metals are subjected to corrosion when in contact with body fluid as the body environment is very aggressive due to the presence of chloride ions and proteins [38]. Specifically, titanium and titanium alloys that come into contact with biologic systems may undergo some degree of corrosion while metal ions released intra-articularly may form complexes with native proteins. These metal-protein complexes may act as antigens or allergens and cause an immunologic response in the body or synovial joint [39]. In this case, we found no evidence of corrosion, and hence, metal ions released cannot be a reason of failure. Evan if the protective and stable oxides on titanium surfaces are able to provide favorable osseointegration, in this case, an oxide-free surface is desired ensuring the best contact with the UHMWPE component. In the retrieved titanium component, the eventually expected oxidation (TiO\({}_{2}\)) layer with characteristic Raman bands specific for each polymorph (anatase, rutile, brookite) was not observed [20; 22]. To clarify the complex phenomena occurring in vivo in UHMWPE component, accurate structural analyses were performed in the present study, by assessing the crystallinity degree and oxidation behavior of UHMWPE component due to aging process. Polyethylene is a very transparent material to visible lasers, and hence, the penetration depth of the laser can be significant. A careful consideration of the main spectral region was made by comparing a new UHMWPE component with the retrieved one, highlighting important details such as slightly shifts accompanied by reduction of bands intensity due to aging process. Moreover, based on spectral deconvolution and curve fitting procedure, crystalline (C%) and amorphous (A%) percentages in each sample along with the oxidation index (OI) was calculated. An insignificant level of oxidation was demonstrated (OI less than one), while small modifications of crystallinity and amorphous phase were noticed. It is considered that values OI less than one and degrees of crystallinity in the range of 45%-50% does not affect the mechanical properties, but only the wear lifetime with respect to degradation [8; 9]. According to previous studies [24], both degrees of crystallinity and amorphous phase were altered by oxidation, but the mechanical strain was considered to be the main factor in alteration of crystallinity. Conversely, the chemical process (oxidation) is the only one affecting the amorphous phase. An interesting study applying Raman confocal spectroscopic technique to quantitatively assess the structural features of two kinds of UHMWPE acetabular cupsbelonging to different generations, and thus manufactured by different procedures, demonstrated that oxidation profiles of polyethylene cups belonging to different generations greatly differed after wear testing [8]. In a similar approach, micro-Raman spectroscopy was used to investigate the effects of the sterilization method (gamma and ethylene oxide treatment) on the crystallinity degree of UHMWPE acetabular cups, demonstrating that unworn gamma-sterilized cups were significantly more crystalline than the ethylene oxide-sterilized ones [26].