## 4 Discussion
### Practical Value  
The practical value of the research is to pay attention to the changes occurring within the subchondral bone layer in the course of degenerative disease. Taking into account the fact of lower mineralization and a decrease in the content of hydroxyapatite in the subchondral bone layer, it seems important to ask whether cementless knee arthroplasty in more advanced stages of osteoarthritis can provide a satisfactory and long-term therapeutic effect for the patient. It can be speculated that these changes may negatively affect the method of osseointegration of the endoprosthesis and accelerate the loosening process of the endoprosthesis. It should be noted that despite the clinical thickening of the subchondral bone layer and sclerosis visible in roentgenograms, this layer has a significantly changed microstructure. Studies show that patients should be qualified for single-compartment endoprosthesis with great caution when radiologically large changes occur in the subchondral bone layer at the same time.  
The latest research demonstrates the potential of NIR spectroscopy coupled with a deep-learning approach as a means of rapid PMI classification of human bone samples. This approach can distinguish between forensic and archaeological human bone samples [32].  
To summarize, in light of the presented studies, the bone mineralization index is the most useful parameter for imaging changes in the bone microstructure, and it is most sensitive to changes within unfixed bone tissue, as also indicated by the results of other authors, who used the range of 960-961 cm\({}^{-1}\), which corresponds to phosphorus groups. The ratio of carbonate apatite to hydroxyapatite also provides crucial information. Raman spectroscopy opens up new analytical and diagnostic possibilities due to the possibility of examining unfixed preparations. The sensitivity of the method, which is not only limited to the presence or absence of degenerative changes, along with the ability to determine the progression or severity of such changes, increases its usefulness.  
However, further clinical trials are necessary, which would enable the use of the method not only in degenerative disease but also in the diagnosis of necrotic bone, cancer diagnosis, or the possibility of predicting osteoporortic fractures.  
Conceptualization, P.K., M.G. and M.S.; methodology, M.G. and M.S.; software, M.S.; validation, P.K., M.S. and M.G.; formal analysis, P.K.; investigation, P.K.; resources, P.K.; data curation, P.K.; writing--original draft preparation, P.K.; writing--review and editing, P.K.; visualization, P.K.; supervision, M.G. and M.S.; project administration, M.G. and M.S.; funding acquisition, P.K. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The study was conducted in accordance with the Declaration of Helsinki and approved by the Bioethical Commission at the Poznan University of Medical Sciences no. 1199/17 on 7 December 2017. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the patients' data. The material was collected during the surgical procedure in the Hospital of Puszczykowo. Samples were prepared at the Poznan University of Medical Sciences. All the measurements were carried out using a Renishaw inVia microscope at the Poznan University of Technology. The authors declare no conflict of interest.