Specificity of Fourier Transform Infrared (FTIR) Microspectroscopy to Estimate Depth-Wise Proteoglycan Content in Normal and Osteoarthritic Human Articular Cartilage

Background: Fourier transform infrared (FTIR) microspectroscopy is a promising method for estimating the depth-wise composition of articular cartilage. The aim was to compare the specificity of two earlier introduced, presumably proteoglycan (PG)-specific FTIR parameters (i.e., absorption in the carbohydrate region with and without normalization with Amide I absorption) to estimate the reference PG content of normal and osteoarthritic human articular cartilage. This study is a direct continuation of our earlier studies, from which the presented data are reanalyzed. Design: Earlier, FTIR microspectroscopy, digital densitometry, histological analyses, and polarized light microscopy were conducted in vitro for articular cartilage samples of human patellae (n = 72). In the present study, earlier data were combined and statistically reanalyzed in a depth-wise manner to clarify the specificity of FTIR parameters introduced for the estimation of PG content of articular cartilage. Digital densitometry for Safranin O-stained samples was used to indicate reference PG content. Results: Direct absorption of the carbohydrate region estimated well the PG content in the middle and deep zones of articular cartilage and appeared to be superior compared to carbohydrate values normalized with Amide I absorption. However, in the superficial zone, the specificity of both FTIR-derived PG parameters was limited. Conclusions: Limitations of current FTIR-based PG parameters in the superficial zone of articular cartilage should be recognized and carefully taken into account in future studies using FTIR microspectroscopy for PG content estimation. Further research is needed to improve the specificity of FTIR parameters for the estimation of PG content of articular cartilage.