## 4 Discussion
All these previous works demonstrated the superiority of confocal Raman spectroscopy over other available techniques (such as differential scanning calorimetry or X-rays diffraction) use for quantitative evaluation of polymer crystallinity. In the present study, the micrographs recorded on the interface of UHMWPE with both metallic component and biologic tissue, evidenced some mechanical damage such as scratches and pitting, along with persistent organic spots. These aspects could be correlated to the decrementation observed based on the radiographic examination. PMMA-based acrylic bone cements has been traditionally used for fixation of total joint replacement prostheses to periprosthetic bone, based on radical polymerization of the MMA, which is an exothermic reaction resulting in a temperature increase in the curing bone cement [40]. It has been demonstrated that biodegradation of polymeric bone cements can result from the environment in vivo. For example, acrylic cement specimens retrieved six years after total hip arthroplasty showed significant decrease in fatigue strength compared with unused (control) specimens [41]. On the other hand, some previous works investigating antibiotic release from acrylic bone cements have postulated that water penetrates into the cement through surface cracks and voids, created by the release of gentamicin sulfate [42; 43]. The involvement of cement particles in the wear process of knee prosthesis was demonstrated in some previous works [12; 44]. For example, [PERSON] et al. [44] reported severe articular and third-body wear from cement debris in a retrieval analysis of 55 polyethylene tibial inserts with deep prismatic scratching of cobalt-chromium alloy condylar components. A more recent study developed in a knee wear simulator, evaluated the influence of third bodies (bone and PMMA-particles) on number, size and shape of wear debris generated [12]. These previous studies demonstrated that free cement debris can significantly increase the generation of wear particles in TKA. Hence, it is basically accepted that particle size, number and morphology will affect the biologic response, resulting in an osteolysis that finally leads to an aseptic loosening in TKA. In our case, the deterioration of acrylic cement due to aging, and third-body wear of cement particles could be a reason of failure, which is also supported by the radiographic examination and the mineralization process evidenced by the Raman spectra of acrylic cement debris. Pre-revision radiographs were evaluated for implant alignment as well as the presence or absence of extruded cement and periprosthetic osteolysis. It is well known that wear is very difficult to assess on conventional X rays, it can appear suddenly before the tenth years after implantation, in the form of early osteolysis, in the case of heavy patients [44; 45; 46]. It was previously demonstrated that resonance Raman (RR) and surface-enhanced Raman scattering (SERS) analysis are very useful tools in estimating knee osteoarthritis grading, based on the signature of proteins in synovial fluid [16; 17]. An accurate discrimination between low-grade and high-grade osteoarthritis was possible based the molecular changes taking place in the synovial fluid of patients [17]. Moreover, in situ techniques and real time measurements, using epidural needle Raman sensors were recently described, testing the ability of Raman spectroscopy to distinguish each tissue type [47]. In the present work, confocal Raman applied to synovial fluid collected from the stem of the tibial component demonstrated a lipidomic profile, based upon a comparison with commercial lipid sample, human serum albumin and collagen reference samples. According to literature, several clinical studies have revealed correlations between various lipid classes (mainly triglycerides and cholesterols) in serum and synovial fluid with different stage of osteoarthritis, synovitis and wound repair [48].