## 3 Results
### Aged UHMWPE Component  
Multiple Raman spectra were recorded on the surface of UHMWPE component of the retrieved prosthesis and compared with those recorded from a new one (unused, as received from the producer) as presented in Figure 4, highlighting the vibrational details in the main spectral regions. Spectral acquisition was performed from multiple points to comprise the contact interfaces, with the femoral and tibial sites. The corresponding micrographs are presented in Figure 5, showing deterioration signs due to the mechanical contact (scratching and pitting), but also upon interaction with the biologic tissue, as we noticed the presence of persistent organic spots on the surface of retrieved insert. The main vibrational Raman bands are characteristic to polyethylene: 1060, 1128 and 1168 cm\({}^{-1}\) assigned to the C-C symmetric and asymmetric stretching modes, 1294 cm\({}^{-1}\) assigned to CH\({}_{2}\) twisting vibrations and the triplet centered at about 1440 cm\({}^{-1}\) assigned to CH\({}_{2}\) bending modes. In the high wavenumber region, the characteristic peaks of symmetric and asymmetric CH\({}_{2}\), stretching vibration are located at 2846 and 2881 cm\({}^{-1}\), respectively [8; 24; 25]. Comparing the spectra of the new and old plastic, two main characteristics are observed: (i) the systematic slightly shift (2-3 cm\({}^{-1}\)) toward higher wavenumber in old polyethylene and (ii) a significant decrease of the relative intensities of the main vibrational bands due to aging process. Figure 3: Micro-Raman spectra recorded on the surface of tibial metallic component (titanium). (**a**) As supplied from the producer; (**b**) retrieved prosthetic component along with the corresponding microscopic image (inset). Scale bar 50 μm. Y spectral offset was applied for clarity. According to literature [8; 24], the most sensitive vibrational bands which involves the structural and morphologic modifications due to oxidation process, and consequently the degree of crystallinity, are 1440, 1294, 1080 and 1310 cm\({}^{-1}\) band which can be used for the calculation of crystalline and amorphous phase [8; 25]. For this purpose, computational analysis by spectral deconvolution and curve fitting procedure was performed for the intervals 1000-1200, 1250-1350 and 1400-1500 cm\({}^{-1}\), in order calculate the exact bands intensity, as presented in Figure 6, for both unused and old (retrieved UHMPE) component. Moreover, since the spectral region between 1250-1350 cm\({}^{-1}\) is considered to be independent of the polymer chain conformation, the area of the band at 1294 cm\({}^{-1}\) is accepted as standard to which all the other bands can be related and compared [25]. Figure 4: Micro-Raman spectra acquired from the surface of plastic (UHMWPE) component of a new (green spectra) and retrieved (red) knee prosthesis, showing the vibrational changes (band shifts, relative intensity change) during 10 years aging in human physiological environment. The details of significant intervals are shown in (**a**) 960–1200 cm\({}^{-1}\), (**b**) 1280–1310 cm\({}^{-1}\), (**c**) 1400–1500 cm\({}^{-1}\) and (**d**) 2820–2960 cm\({}^{-1}\) spectral ranges. Spectra acquired under similar excitation and optical collection conditions (excitation: 785, 30 mW, 10 s acquisition, 1 accumulation, 20\(\times\) objective). Multiple spectra collected from single points of old plastic are shown. Black spectra refer to color spotted plastic supposed to be due to organic micro-deposits, however, the decreased polyethylene signal only was observed. Figure 5: Micrographs recorded in real time on the surface of UHMWPE component during Raman measurements. (**a**,**b**) New (unused) UHMWPE component at the interface with (**a**) tibial and (**b**) femoral sites, respectively; (**c–f**) retrieved UHMWPE component—the interface with (**c**) femoral and (**d**) tibial sites, respectively; (**e**,**f**) persistent organic spots detected on the interface.