Quantification of stiffness change in degenerated articular cartilage using optical coherence tomography-based air-jet indentation.

Articular cartilage is a thin complex tissue that covers the bony ends of joints. Changes in the composition and structure of articular cartilage will cause degeneration, which may further lead to osteoarthritis. Decreased stiffness is one of the earliest symptoms of cartilage degeneration and also represents the imperfect quality of repaired cartilage. An optical coherence tomography (OCT)-based air-jet indentation system was recently developed in our group to measure the mechanical properties of soft tissues. In this study, this system was applied to quantify the change of mechanical properties of articular cartilage after degeneration induced by enzymatic digestions. Forty osteochondral disks (n = 20 × 2) were prepared from bovine patellae and treated with collagenase and trypsin digestions, respectively. The apparent stiffness of the cartilage was measured by the OCT-based air-jet indentation system before and after the degeneration. The results were also compared with those from a rigid contact mechanical indentation and an ultrasound water-jet indentation. Through the air-jet indentation, it was found that the articular cartilage stiffness dropped significantly by 84% (p < 0.001) and 63% (p < 0.001) on average after collagenase and trypsin digestions, respectively. The stiffness measured by the air-jet indentation system was highly correlated (R > 0.8, p < 0.001) with that from the other two indentation methods. This study demonstrated that the OCT-based air-jet indentation can be a useful tool to quantitatively assess the mechanical properties of articular cartilage, and this encourages us to further develop a miniaturized probe suitable for arthroscopic applications.

Characterization of center frequency and bandwidth of broadband ultrasound reflected by the articular cartilage to subchondral bone interface.

Osteoarthritis (OA) produces degenerative changes both in articular cartilage and subchondral bone. During OA, reflection of high frequency ultrasound from the cartilage-bone interface is affected by both changes in attenuation of the cartilage layer and acoustic properties of the interface. The objective of this study was to experimentally investigate the spectral content of ultrasound reflection from the cartilage-bone interface. Specifically, we analyzed the center frequency and -6 dB bandwidth of the broadband high-frequency (40 MHz) ultrasound signal. Intact bovine articular cartilage samples with and without the underlying subchondral bone (n = 6) were measured in vitro using a commercial high-frequency ultrasound scanner. Furthermore, the diagnostic potential of the measurement of center frequency and bandwidth for OA was studied with another series of bovine articular cartilage samples (n = 40) after enzymatic degradations of tissue proteoglycans and collagen. Compared with the reference spectrum at the same depth from a perfect reflector, a major downshift (>51%) of the center frequency and a reduction (>42%) of the bandwidth were observed in both sample groups when analyzing the ultrasound reflection from the cartilage-bone interface. The results suggest that attenuation in the cartilage layer primarily controls the observed downshift of the center frequency and acoustic properties of the subchondral bone play only a minor role in affecting the spectrum of the cartilage-bone interface. Changes in the ultrasound bandwidth of the cartilage-bone interface signals, compared with reference signals, were found to vary more than those in the center frequency in both cartilage sample groups. Compared with pretreatment values, a significant downshift in center frequency (p < 0.01) and a minor reduction in bandwidth of spectra from the cartilage-bone interface were recorded after chemical degradation of proteoglycans with trypsin. In contrast, center frequency and bandwidth of the echoes from the cartilage-bone interface did not change after the chemical degradation of cartilage collagen fibrils. The results suggest that proteoglycan loss, typical to OA, may be detected via the changes in the center frequency of the ultrasound reflected from the cartilage-bone interface.

Ultrasound evaluation of site-specific effect of simulated microgravity on articular cartilage.

Space flight induces acute changes in normal physiology in response to the microgravity environment. Articular cartilage is subjected to high loads under a ground reaction force on Earth. The objectives of this study were to investigate the site dependence of morphological and ultrasonic parameters of articular cartilage and to examine the site-specific responses of articular cartilage to simulated microgravity using ultrasound biomicroscopy (UBM). Six rats underwent tail suspension (simulated microgravity) for four weeks and six other rats were kept under normal Earth gravity as controls. Cartilage thickness, ultrasound roughness index (URI), integrated reflection coefficient (IRC) and integrated backscatter coefficient (IBC) of cartilage tissues, as well as histological degeneration were measured at the femoral head (FH), medial femoral condyle (MFC), lateral femoral condyle (LFC), patello-femoral groove (PFG) and patella (PAT). The results showed site dependence not significant in all UBM parameters except cartilage thickness (p < 0.01) in the control specimens. Only minor changes in articular cartilage were induced by 4-week tail suspension, although there were significant decreases in cartilage thickness at the MFC and PAT (p < 0.05) and a significant increase in URI at the PAT (p < 0.01). This study suggested that the 4-week simulated microgravity had only mild effects on femoral articular cartilage in the rat model. This information is useful for human spaceflight and clinical medicine in improving understanding of the effect of microgravity on articular cartilage. However, the effects of longer duration microgravity experience on articular cartilage need further investigation.

Quantitative assessment of articular cartilage with morphologic, acoustic and mechanical properties obtained using high-frequency ultrasound.

Osteoarthritis (OA) is one of the most common joint diseases among adults, and its early detection is still not possible. In this study, high-frequency ultrasound and ultrasound-assisted mechanical testing systems were used to quantitatively measure the morphologic, acoustic and mechanical properties of normal and enzymatically degraded bovine articular cartilages in vitro. A total of 40 osteochondral cartilage plugs were prepared from 20 bovine patellae (n=20x2) and divided into two groups for collagenase and trypsin digestions, respectively. A high-frequency ultrasound system (center frequency: 40 MHz) was used to analyze the surface integrity (ultrasound roughness index, URI), thickness and acoustic properties of the articular cartilages before and after enzymatic degradations. Acoustic parameters included the integrated reflection coefficient (IRC) from the cartilage surface, reflection from the cartilage-bone interface (AIB(bone)), integrated attenuation (IA) and integrated backscatter (IBS) of the internal cartilage tissue. A newly developed ultrasound water jet indentation system was used to assess the mechanical properties of the cartilage samples. The results showed that the URI increased significantly (p<0.05) after collagenase digestion while no significant change (p>0.05) was found after trypsin digestion. With regard to acoustic parameters, the IRC decreased significantly (p<0.05) after collagenase digestion while no significant change (p>0.05) was found after trypsin digestion. The AIB(bone) demonstrated an insignificant change after collagenase digestion (p>0.05) but a significant decrease after trypsin digestion (p<0.05). Both enzymatic degradation groups showed insignificant differences (p>0.05) in the IA but a significant increase (p<0.05) in the IBS after both enzymatic degradations. The apparent stiffness measured by ultrasound water jet indentation suggested that articular cartilage from both groups became significantly softer (p<0.05) after the enzymatic degradations. A significant relationship was found to exist between the IRC and URI (p<0.05). This study showed that high-frequency ultrasound can be a comprehensive tool to quantitatively and systematically analyze the morphologic, acoustic and mechanical properties of articular cartilage in association with its degeneration.

Assessment of depth and degeneration dependences of articular cartilage refractive index using optical coherence tomography in vitro.

In this study, optical coherence tomography (OCT) with an axial resolution of 15 mum was used to investigate the depth and degeneration dependences of the refractive index (RI) of articular cartilage collected from bovine patellae in vitro. Eighteen disks of articular cartilage with a diameter of 6.35 mm harvested from different patellae were successfully prepared. Each disk was cut into two halves and three horizontal cartilage slices (n = 18 x 2 x 3) with an approximately equal thickness of 0.5 mm were further prepared from each half disk. The cartilage slices were digested by two different enzymes, collagenase and trypsin, to disturb collagen fibrils and proteoglycans, respectively. The samples were submerged in the physiological saline and tested using OCT before and after the enzyme digestion and the RI for each specimen was calculated. The RI of articular cartilage from the superficial to deep regions was 1.361 +/- 0.032 (mean +/- SD), 1.338 +/- 0.036, and 1.371 +/- 0.041 for normal specimens; 1.357 +/- 0.036, 1.331 +/- 0.030, and 1.392 +/- 0.037 for trypsin digested specimens; and 1.361 +/- 0.032, 1.336 +/- 0.048, and 1.376 +/- 0.043 for those treated by collagenase, respectively. Two-factor repeated measure ANOVA revealed that for all the three groups of specimens, the RI in different depths was significantly different (p < 0.05). However, we found that the trypsin and collagenase treatments did not exert a significant effect on the RI (p > 0.05). The results suggested that the depth dependence of articular cartilage should be taken into account when OCT is used for related measurement.

Quantification of the optical surface reflection and surface roughness of articular cartilage using optical coherence tomography.

Optical coherence tomography (OCT) is a promising new technique for characterizing the structural changes of articular cartilage in osteoarthritis (OA). The calculation of quantitative parameters from the OCT signal is an important step to develop OCT as an effective diagnostic technique. In this study, two novel parameters for the quantification of optical surface reflection and surface roughness from OCT measurements are introduced: optical surface reflection coefficient (ORC), describing the amount of a ratio of the optical reflection from cartilage surface with respect to that from a reference material, and OCT roughness index (ORI) indicating the smoothness of the cartilage surface. The sensitivity of ORC and ORI to detect changes in bovine articular cartilage samples after enzymatic degradations of collagen and proteoglycans using collagenase and trypsin enzymes, respectively, was tested in vitro. A significant decrease (p < 0.001) in ORC as well as a significant increase (p < 0.001) in ORI was observed after collagenase digestion. After trypsin digestion, no significant changes in ORC or ORI were observed. To conclude, the new parameters introduced were demonstrated to be feasible and sensitive to detect typical OA-like degenerative changes in the collagen network. From the clinical point of view, the quantification of OCT measurements is of great interest since OCT probes have been already miniaturized and applied in patient studies during arthroscopy or open knee surgery in vivo. Further studies are still necessary to demonstrate the clinical capability of the introduced parameters for naturally occurring early OA changes in the cartilage.

An optical coherence tomography (OCT)-based air jet indentation system for measuring the mechanical properties of soft tissues.

A novel noncontact indentation system with the combination of an air jet and optical coherence tomography (OCT) was presented in this paper for the quantitative measurement of the mechanical properties of soft tissues. The key idea of this method is to use a pressure-controlled air jet as an indenter to compress the soft tissue in a noncontact way and utilize the OCT signals to extract the deformation induced. This indentation system provides measurement and mapping of tissue elasticity for small specimens with high scanning speed. Experiments were performed on 27 silicone tissue-mimicking phantoms with different Young's moduli, which were also measured by uniaxial compression tests. The regression coefficient of the indentation force to the indentation depth (N mm(-1)) was used as an indicator of the stiffness of tissue under air jet indentation. Results showed that the stiffness coefficients measured by the current system correlated well with the corresponding Young's moduli obtained by conventional mechanical testing (r = 0.89, p < 0.001). Preliminary in vivo tests also showed that the change of soft tissue stiffness with and without the contraction of the underlying muscles in the hand could be differentiated by the current measurement. This system may have broad applications in tissue assessment and characterization where alterations of mechanical properties are involved, in particular with the potential of noncontact micro-indentation for tissues.

Assessment of depth and degeneration dependences of articular cartilage refractive index using optical coherence tomography in vitro.

Measuring the depth and degeneration dependences of articular cartilage is important for the investigation of cartilage structure and the reason behind its degeneration. In this study, optical coherence tomography (OCT) was used to investigate the depth and degeneration dependences of the refractive index (RI) of articular cartilage collected from bovine patellae in vitro. Eighteen disks of articular cartilage with a diameter of 6.35 mm harvested from different patellae were prepared. Each disk was cut into two halves and three horizontal cartilage slices (n=18 x 2 x 3) with approximately equal thickness were further prepared from each half disk. The cartilage slices were digested by two different enzymes, collagenase and trypsin, to remove collagen fibres and proteoglycans, respectively. The samples were tested using OCT before and after the enzyme digestion and the RI for each specimen was calculated. Two-factor repeated measure ANOVA showed that for all the three groups of specimens, the RI in different depths was significantly different (p0.05). However, it was revealed that the trypsin and collagenase treatments did not exert a significant effect on the RI (p0.05). The results suggested that the depth dependence of articular cartilage should be taken into account when OCT is used for related measurement.