## Materials and Methods
### Retrieved Components of Knee Prosthesis and confocal Raman Spectroscopy  
In Figure 2 are presented the photographic images of retrieved tibial components after 10 years, showing the surface deterioration especially on the surface of UHMWPE component (delamination, scratching), along with the accumulation of pelleted synovial fluid in the stem of tibial component (Figure 2b). Figure 1: **(a)** Bilateral knee radiography—anteroposterior view, demonstrating advanced degenerative changes (gonarthrosis stadium IV) at the level of the right knee (white arrows), secondary to the malunion of tibial plateau fracture; **(b)** lateral view, showing depression of the lateral tibial plateau (white arrow); **(c)** anteroposterior and later view, eight years after surgery; both views highlight the loosening of the tibial component, minimal osteolysis of the medial tibial plateau (anteroposterior view) and minimal osteolysis around the distal region of the tibial implant (lateral view); **(d)** anteroposterior and lateral view after revision of the total knee arthroplasty using modular tibial implant with short stem and two screws for re-insertion of the patellar tendon (after tibial tuberosity osteotomy). Confocal micro-Raman spectra were acquired using an InVia Reflex Raman spectrometer (Renishaw, New Mills, UK) equipped with an upright microscope (Leica). Raman spectra were recorded from the surface of metallic and plastic components using a 785 nm laser diode for excitation. The retrieved plastic and metal were placed under different orientations on the Raman microscope stage to be able to collect distinct Raman signal from exposed surfaces to bone and metal, through a 20\(\times\) objective (NA 0.35), 30 mW excitation power, 1 accumulation and 1 to 10 s exposure, depending on the collecting mode (short range from 700 to 1800 cm\({}^{-1}\) or extended mode from 100 to 3200 cm\({}^{-1}\)). Different data collection modes were applied to control any spectral change associated with longer time exposure to laser of the raw, freshly retrieved biological material. The spectral resolution was 1 cm\({}^{-1}\). The spectral calibration was achieved using an internal silicon standard. The Raman spectral characterization was run to comprise at least 10 spectral acquisitions from different active sites of the knee prosthesis (i.e., the tibial metallic plateau in direct contact with UHMWPE or in contact with biologic tissue, interface between UHMWPE and femoral component, interface between metallic component and acrylic cement, synovial fluid collected from the stem, acrylic cement detached from the tibial component). A total of 159 data files and micrographs were acquired via Raman micro-spectroscopy. In the case of UHMWPE component, a computational analysis of the relevant spectral regions was applied in the 1000-1200, 1250-1350 and 1400-1500 cm\({}^{-1}\) intervals respectively. The second-derivative spectra of each interval were smoothed with a nine-point Savitsky-Golay function and the resulted components were used to locate the position of each relevant band component. The curve-fitting analysis was performed by means of Origin 8 software and the spectra were baseline corrected according to the algorithm of [PERSON] et al. [18], assuming a Gaussian profile. The intensity of each spectral band component was further used to calculate the percentage of crystalline (C%) and amorphous (A%) contribution along with the oxidation index (OI) and interpreted in terms of aging influence on the structure of UHMWPE. The Raman spectra of acrylic cement pieces detached from the metallic tibial component were analyzed in the range 600-3200 cm\({}^{-1}\) and compared with the abundant literature data on polymethyl methacrylate (PMMA)-based acrylic cement, while the synovial fluid collected from the stem of the tibial component was examined in terms of major Raman bands describing the chemical composition, such as proteins and lipids.