Spectrocolorimetric evaluation of human articular cartilage

Objective Colorimetric Evaluation of Aging-related Articular Cartilage Degeneration

BACKGROUND

Aging is a primary risk factor for the development of osteoarthritis (OA). Recently, advanced glycation end products (AGEs) have received much attention in relation to aging and OA. Some AGEs are reported to be brownish, but the association between AGE levels and browning in articular cartilage is unknown. The purposes of this study were first, to develop a colorimetric device to evaluate the quality and aging of the articular cartilage, and second, to investigate the relationship between AGE levels and color of articular cartilage by using this device without enzymatic digestion of the articular surface.

DESIGN

Open-label, single-center, prospective study Methods: Seven patients with OA (1 man, 6 women; mean age, 74.4 years; age range 58-81 years) who underwent primary total knee arthroplasty at our university hospital between July and December 2014 were enrolled in the study. Articular cartilage was harvested from the femur and tibia during surgery. The color and chromaticity of the articular cartilage was assayed by using a newly developed device with high validity. The characteristics of the articular cartilage were examined using the CIE XYZ Color Coordinate System (International Commission on Illumination, Vienna, Austria), and the color indicative of browning of the cartilage was defined on the X- and Y-axes. The brightness was adjusted and the specimen was photographed submerged in distilled water and the color was measured using a commercial luminance and color analyzer. Measurement of X and Y was repeated 3 times per site at 9 different points on the articular cartilage surface, and the mean value per specimen was calculated. Pentosidine (a well characterized biomarker of AGEs) levels and hydroxyproline content in articular cartilage were determined by high-performance liquid chromatography (HPLC). The correlation between age, articular cartilage AGE levels, and browning was analyzed using Spearman's rank correlation coefficient. The association between the degree of macroscopic degeneration and AGE levels was analyzed using one-way analysis of variance.

RESULTS

Age was positively correlated with pentosidine levels in articular cartilage (r_s=0.322) and browning of articular cartilage (r_s=0.261). However, a weak negative correlation was observed between pentosidine levels and browning of articular cartilage (r_s=-0.564, p=0.004). No positive relationship was observed between pentosidine levels and browning of articular cartilage in the visible spectrum.

CONCLUSIONS

This study developed an original colorimetric device with high validity. Browning of articular cartilage increased with age, but this study did not detect pentosidine-caused browning. Further study is needed to clarify the factors associated with browning of cartilage.

Intrinsic fluorescence and mechanical testing of articular cartilage in human patients with osteoarthritis

The degeneration of articular cartilage is the main cause of osteoarthritis (OA), a common cause of disability among elderly patients. The aim of this study is to understand the correlation between intrinsic fluorescence of articular cartilage and its biomechanical properties in patients with osteoarthritis. Cylindrical samples of articular cartilage 6 mm in diameter were extracted via biopsy punch from the femoral condyles of 6 patients with advanced OA undergoing knee replacement surgery. The mechanical stiffness and fluorescence of each cartilage plug were measured by indentation test and spectrofluorometry. Maps of fluorescence intensity, at excitation/emission wavelengths of 240-520/290-530 nm, were used to identify wavelengths of interest. The mechanical stiffness and fluorescence intensity were correlated using a Spearman analysis. The excitation/emission maps demonstrated three fluorescence peaks at excitation/emission wavelength pairs 330/390, 350/430 and 370/460 nm. The best correlation between the fluorescence intensity and stiffness of cartilage was obtained for the 330 nm excitation band [R=0.82, p=0.04]. The intrinsic fluorescence of articular cartilage may have application in optically assessing the state of cartilage in patients with osteoarthritis.

Optical spectral reflectance of human articular cartilage - relationships with tissue structure, composition and mechanical properties

The information from spectral reflectance of articular cartilage has been related to the integrity of the tissue. This study explores more in detail the interrelations between the cartilage composition, structure and mechanical properties, and optical spectral reflectance. Using human osteochondral samples the reflectance spectral images of articular cartilage were captured and analyzed by using CIELAB color space as well as principal component analysis. With both analysis methods statistically significant correlations were observed between the reflectance and histological integrity, as assessed by Mankin scoring, tissue proteoglycan content and dynamic modulus. In thick human cartilage, the reflectance was found to be independent of the cartilage thickness, suggesting negligible influence of the underlying subchondral bone. Based on the present results diagnostically relevant information on cartilage quality can be extracted using optical spectral reflectance measurements. These measurements could be feasible during arthroscopic surgery when more in-depth information of the properties of articular cartilage is needed.

Optical spectral imaging of degeneration of articular cartilage

Osteoarthritis (OA) is a common musculoskeletal disorder often diagnosed during arthroscopy. In OA, visual color changes of the articular cartilage surface are typically observed. We demonstrate in vitro the potential of visible light spectral imaging (420 to 720 nm) to quantificate these color changes. Intact bovine articular cartilage samples (n=26) are degraded both enzymatically using the collagenase and mechanically using the emery paper (P60 grit, 269 microm particle size). Spectral images are analyzed by using standard CIELAB color coordinates and the principal component analysis (PCA). After collagenase digestion, changes in the CIELAB coordinates and projection of the spectra to PCA eigenvector are statistically significant (p<0.05). After mechanical degradation, the grinding tracks could not be visualized in the RGB presentation, i.e., in the visual appearance of the sample to the naked eye under the D65 illumination. However, after projecting to the chosen eigenvector, the grinding tracks are revealed. The tracks are also seen by using only one wavelength, i.e., 469 nm, however, the contrast in the projection image is 1.6 to 2.5 times higher. Our results support the idea that the spectral imaging can be used for evaluation of the integrity of the cartilage surface.