Optical spectral imaging of degeneration of articular cartilage

Ultraviolet-Visible-Near Infrared Spectroscopy May Aid in the Qualitative Assessment of Early-Stage Cartilage Degradation

Purpose

To assess the potential of ultraviolet-visible near-infrared spectroscopy to provide quantitative information on the cartilage surface at early osteoarthritis.

Methods

We used a similar source and optical path to a standard arthroscope and constraining input to the range available to a standard detector/camera, further capturing and analyzing spectral information quantitatively in terms of specific electronic absorbance bands and scattering from the cartilage surface, with a focus on the early stages of degradation.

Results

The ratio of the 320-nm and longer than 500-nm absorbances produced a distinct change from the normal to diseased states. The slopes between the wavelengths of 600 and 980 nm may show the transition of the single fibril to fibril bundles that occurs during early stages disease.

Conclusions

Ultraviolet-visible near-infrared spectroscopy has good potential for use in integrated arthroscopic assessment.

Clinical Relevance

This raises the possibility of advancing arthroscopy from a qualitative to a quantitative tool, without requiring modification of either the radiation (the light source and path) or instrumentation (the arthroscope itself) delivered to the patient, thus allowing a low-cost yet potentially high-value technology.

Light Absorptive Properties of Articular Cartilage, ECM Molecules, Synovial Fluid, and Photoinitiators as Potential Barriers to Light-Initiated Polymer Scaffolding Procedures

OBJECTIVE

Many in vivo procedures to repair chondral defects use ultraviolet (UV)-photoinitiated in situ polymerization within the cartilage matrix. Chemical species that absorb UV light might reduce the effectiveness of these procedures by acting as light absorption barriers. This study evaluated whether any of the individual native biochemical components in cartilage and synovial fluid interfered with the absorption of light by common scaffolding photosensitizers.

MATERIALS

UV-visible spectroscopy was performed on each major component of cartilage in solution, on bovine synovial fluid, and on four photosensitizers, riboflavin, Irgacure 2959, quinine, and riboflavin-5'-phosphate. Molar extinction and absorption coefficients were calculated at wavelengths of maximum absorbance and 365 nm. Intact articular cartilage was also examined.

RESULTS

The individual major biochemical components of cartilage, Irgacure 2959, and quinine did not exhibit a significant absorption at 365 nm. Riboflavin and riboflavin-5'-phosphate were more effectual light absorbers at 365 nm, compared with the individual native species. Intact cartilage absorbed a significantly greater amount of UV light in comparison with the native species.

CONCLUSION

Our results indicate that none of the individual native species in cartilage will interfere with the absorption of UV light at 365 nm by these commonly used photoinitiators. Intact cartilage slices exhibited significant light absorption at 365 nm, while also having distinct absorbance peaks at wavelengths less than 300 nm. Determining the UV absorptive properties of the biomolecules native to articular cartilage and synovial fluid will aid in optimizing scaffolding procedures to ensure sufficient scaffold polymerization at a minimum UV intensity.

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.

Noninvasive assessment of articular cartilage surface damage using reflected polarized light microscopy

Articular surface damage occurs to cartilage during normal aging, osteoarthritis, and in trauma. A noninvasive assessment of cartilage microstructural alterations is useful for studies involving cartilage explants. This study evaluates polarized reflectance microscopy as a tool to assess surface damage to cartilage explants caused by mechanical scraping and enzymatic degradation. Adult bovine articular cartilage explants were scraped, incubated in collagenase, or underwent scrape and collagenase treatments. In an additional experiment, cartilage explants were subject to scrapes at graduated levels of severity. Polarized reflectance parameters were compared with India ink surface staining, features of histological sections, changes in explant wet weight and thickness, and chondrocyte viability. The polarized reflectance signal was sensitive to surface scrape damage and revealed individual scrape features consistent with India ink marks. Following surface treatments, the reflectance contrast parameter was elevated and correlated with image area fraction of India ink. After extensive scraping, polarized reflectance contrast and chondrocyte viability were lower than that from untreated explants. As part of this work, a mathematical model was developed and confirmed the trend in the reflectance signal due to changes in surface scattering and subsurface birefringence. These results demonstrate the effectiveness of polarized reflectance microscopy to sensitively assess surface microstructural alterations in articular cartilage explants.

Noninvasive surface damage assessment of bovine articular cartilage explants by reflected polarized light microscopy

Articular surface damage is a hallmark of cartilage degeneration. Noninvasive assessment of cartilage microstructural alterations has potential clinical value. In this study, we use bovine patellofemoral articular cartilage explants treated with mechanical scraping and collagenase to create cartilage surface disruption, and use polarized reflectance microscopy to quantify alterations to surface and sub-surface microstructure. Reflected polarized signal was sensitive to mild damage to the cartilage surface, and highlighted disruptive alterations. The results indicate the efficacy of reflected polarized light microscopy in assessing the microstructural status of superficial articular cartilage.

Optical spectroscopic determination of human meniscus composition

This study investigates the correlation between the composition of human meniscus and its absorption spectrum in the visible (VIS) and near infrared (NIR) spectral range. Meniscus samples (n = 24) were obtained from nonarthritic knees of human cadavers with no history of joint diseases. Specimens (n = 72) were obtained from three distinct sections of the meniscus, namely; anterior, center, posterior. Absorption spectra were acquired from each specimen in the VIS and NIR spectral range (400-1,100 nm). Following spectroscopic probing, the specimens were subjected to biochemical analyses to determine the matrix composition, that is water, hydroxyproline, and uronic acid contents. Multivariate analytical techniques, including principal component analysis (PCA) and partial least squares (PLS) regression, were then used to investigate the correlation between the matrix composition and it spectral response. Our results indicate that the optical absorption of meniscus matrix is related to its composition, and this relationship is optimal in the NIR spectral range (750-1,100 nm). High correlations (R(2) (uronic)  = 86.9%, R(2) (water)  = 83.8%, R(2) (hydroxyproline)  = 81.7%, p < 0.0001) were obtained between the spectral predicted and measured meniscus composition, thus suggesting that spectral data in the NIR range can be utilized for estimating the matrix composition of human meniscus. In conclusion, optical spectroscopy, particularly in the NIR spectral range, is a potential method for evaluating the composition of human meniscus. This presents a promising technique for rapid and nondestructive evaluation of meniscus integrity in real-time during arthroscopic surgery.

Optical absorption spectra of human articular cartilage correlate with biomechanical properties, histological score and biochemical composition

This study investigates the relationship between the optical response of human articular cartilage in the visible (VIS) and near infrared (NIR) spectral range and its matrix properties.Full-thickness osteochondral cores (dia. = 16 mm, n = 50) were extracted from human cadaver knees (N = 13) at four anatomical locations and divided into quadrants. Absorption spectra were acquired in the spectral range 400-1100 nm from one quadrant. Reference biomechanical, biochemical composition, histological, and cartilage thickness measurements were obtained from two other quadrants. A multivariate statistical technique based on partial least squares (PLS) regression was then employed to investigate the correlation between the absorption spectra and tissue properties.Our results demonstrate that cartilage optical response correlates with its function, composition and morphology, as indicated by the significant relationship between spectral predicted and measured biomechanical (79.0%  ⩽  R(2)  ⩽  80.3%, p  <  0.0001), biochemical (65.1%  ⩽  R(2)  ⩽  81.0%, p  < 0.0001), and histological scores ([Formula: see text] = 83.3%, p  < 0.0001) properties. Significant correlation was also obtained with the non-calcified cartilage thickness ([Formula: see text] = 83.2%, p  <  0.0001).We conclude that optical absorption of human cartilage in the VIS and NIR spectral range correlates with the overall tissue properties, thus providing knowledge that could facilitate development of systems for rapid assessment of tissue integrity.

What can biophotonics tell us about the 3D microstructure of articular cartilage?

Connective tissues such as articular cartilage have been the subject of study using novel optical techniques almost since the invention of polarized light microscopy (PLM). Early studies of polarized light micrographs were the main evidential basis for the establishment of quantitative models of articular cartilage collagen structure by Benninghoff and others. Even now, state of the art optical techniques including quantitative polarized light microscopy (qPLM), optical coherence tomography (OCT), polarization-sensitive optical coherence tomography (PS-OCT), second harmonic generation (SHG) microscopy, Fourier-transform infrared (FTIR) microscopy, Raman and optical hyperspectral reflectance and fluorescence imaging are providing new insights into articular cartilage structure from the nanoscale through to the mesoscale. New insights are promised by emerging modalities such as optical elastography. This short review highlights some key recent results from modern optical techniques.

Multispectral Imaging

The field of anatomic pathology has changed significantly over the last decades and, as a result of the technological developments in molecular pathology and genetics, has had increasing pressures put on it to become quantitative and to provide more information about protein expression on a cellular level in tissue sections. Multispectral imaging (MSI) has a long history as an advanced imaging modality and has been used for over a decade now in pathology to improve quantitative accuracy, enable the analysis of multicolor immunohistochemistry, and drastically reduce the impact of contrast-robbing tissue autofluorescence common in formalin-fixed, paraffin-embedded tissues. When combined with advanced software for the automated segmentation of different tissue morphologies (eg, tumor vs stroma) and cellular and subcellular segmentation, MSI can enable the per-cell quantitation of many markers simultaneously. This article covers the role that MSI has played in anatomic pathology in the analysis of formalin-fixed, paraffin-embedded tissue sections, discusses the technological aspects of why MSI has been adopted, and provides a review of the literature of the application of MSI in anatomic pathology.

Spectroscopic measurement of cartilage thickness in arthroscopy: ex vivo validation in human knee condyles

PURPOSE

To evaluate the accuracy of articular cartilage thickness measurement when implementing a new technology based on spectroscopic measurement into an arthroscopic camera.

METHODS

Cartilage thickness was studied by ex vivo arthroscopy at a number of sites (N = 113) in human knee joint osteoarthritic femoral condyles and tibial plateaus, removed from 7 patients undergoing total knee replacement. The arthroscopic image spectral data at each site were used to estimate cartilage thickness. Arthroscopically derived thickness values were compared with reference cartilage thickness as measured by 3 different methods: needle penetration, spiral computed tomography scanning, and geometric measurement after sample slicing.

RESULTS

The lowest mean error (0.28 to 0.30 mm) in the regression between arthroscopic and reference cartilage thickness was seen for reference cartilage thickness less than 1.5 mm. Corresponding values for cartilage thickness less than 2.0 and 2.5 mm were 0.32 to 0.40 mm and 0.37 to 0.47 mm, respectively. Cartilage thickness images--created by pixel-by-pixel regression model calculations applied to the arthroscopic images--were derived to demonstrate the clinical use of a camera implementation.

CONCLUSIONS

On the basis of this investigation on osteoarthritic material, when one is implementing the spectroscopic method for estimating cartilage thickness into an arthroscopic camera, errors in the range of 0.28 to 0.30 mm are expected. This implementation does not, however, influence the fact that the spectral method performs less well in the cartilage thickness region from 1.5 to 2.5 mm and cannot assess cartilage thicker than 2.5 mm.

CLINICAL RELEVANCE

Imaging cartilage thickness directly in the arthroscopic camera video stream could serve as an interesting image tool for in vivo cartilage quality assessment, in connection with cartilage diagnosis, repair, and follow-up.

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.

Fiber-optic Raman spectroscopy of joint tissues

In this study, we report adaptation of Raman spectroscopy for arthroscopy of joint tissues using a custom-built fiber-optic probe. Differentiation of healthy and damaged tissue or examination of subsurface tissue, such as subchondral bone, is a challenge in arthroscopy because visual inspection may not provide sufficient contrast. Discrimination of healthy versus damaged tissue may be improved by incorporating point spectroscopy or hyperspectral imaging into arthroscopy where the contrast is based on the molecular structure or chemical composition. Articular joint surfaces of knee cadaveric human tissue and tissue phantoms were examined using a custom-designed Raman fiber-optic probe. Fiber-optic Raman spectra were compared against reference spectra of cartilage, subchondral bone and cancellous bone collected using Raman microspectroscopy. In fiber-optic Raman spectra of the articular surface, there was an effect of cartilage thickness on recovery of signal from subchondral bone. At sites with intact cartilage, the bone mineralization ratio decreased but there was a minimal effect in the bone mineral chemistry ratios. Tissue phantoms were prepared as experimental models of the osteochondral interface. Raman spectra of tissue phantoms suggested that optical scattering of cartilage has a large effect on the relative cartilage and bone signal. Finite element analysis modeling of light fluence in the osteochondral interface confirmed experimental findings in human cadaveric tissue and tissue phantoms. These first studies demonstrate the proof of principle for Raman arthroscopic measurement of joint tissues and provide a basis for future clinical or animal model studies.