Real-time ultrasonic assessment of progressive proteoglycan depletion in articular cartilage

Surrogate modeling of articular cartilage degradation to understand the synergistic role of MMP-1 and MMP-9: a case study

A characteristic feature of arthritic diseases is cartilage extracellular matrix (ECM) degradation, often orchestrated by the overexpression of matrix metalloproteinases (MMPs) and other proteases. The interplay between fibril level degradation and the tissue-level aggregate response to biomechanical loading was explored in this work by a computational multiscale cartilaginous model. We considered the relative abundance of collagenases (MMP-1) and gelatinases (MMP-9) in surrogate models, where the diffusion (spatial distribution) of these enzymes and the subsequent, co-localized fibrillar damage were spatially randomized with Latin Hypercube Sampling. The computational model was constructed by incorporating the results from prior molecular dynamics simulations (tensile test) of microfibril degradation into a hyper-elastoplastic fibril-reinforced cartilage model. Including MMPs-mediated collagen fibril-level degradation in computational models may help understand the ECM pathomechanics at the tissue level. The mechanics of cartilage tissue and fibril show variations in mechanical integrity depending on the different combinations of MMPs-1 and 9 with a concentration ratio of 1:1, 3:1, and 1:3 in simulated indentation tests. The fibril yield (local failure) was initiated at 20.2 ± 3.0 (%) and at 23.0 ± 2.8 (%) of bulk strain for col 1:gel 3 and col 3: gel 1, respectively. The reduction in failure stress (global response) was 39.8% for col 1:gel 3, 37.5% for col 1:gel 1, and 36.7% for col 3:gel 1 compared with the failure stress of the degradation free tissue. These findings indicate that cartilage's global and local mechanisms of failure largely depend on the relative abundance of the two key enzymes-collagenase (MMP-1) and gelatinase (MMP-9) and the spatial characteristics of diffusion across the layers of the cartilage ECM.

Non-invasive Electroarthrography Measures Load-Induced Cartilage Streaming Potentials via Electrodes Placed on Skin Surrounding an Articular Joint

OBJECTIVE

We aimed to demonstrate that electroarthrography (EAG) measures streaming potentials originating in the cartilage extracellular matrix during load bearing through electrodes adhered to skin surrounding an articular joint.

DESIGN

Equine metacarpophalangeal joints were subjected to simulated physiological loads while (1) replacing synovial fluid with immersion buffers of different electrolyte concentrations and (2) directly degrading cartilage with trypsin.

RESULTS

An inverse relationship between ionic strength and EAG coefficient was detected. Compared to native synovial fluid, EAG coefficients increased (P < 0.05) for 5 of 6 electrodes immersed in 0.1X phosphate-buffered saline (PBS) (0.014 M NaCl), decreased (P < 0.05) for 4 of 6 electrodes in 1X PBS (0.14 M NaCl), and decreased (P < 0.05) for all 6 electrodes in 10X PBS (1.4 M NaCl). This relationship corresponds to similar studies where streaming potentials were directly measured on cartilage. EAG coefficients, obtained after trypsin degradation, were reduced (P < 0.05) in 6 of 8, and 7 of 8 electrodes, during simulated standing and walking, respectively. Trypsin degradation was confirmed by direct cartilage assessments. Streaming potentials, measured by directly contacting cartilage, indicated lower cartilage stiffness (P < 10-5). Unconfined compression data revealed reduced Em, representing proteoglycan matrix stiffness (P = 0.005), no change in Ef, representing collagen network stiffness (P = 0.15), and no change in permeability (P = 0.24). Trypsin depleted proteoglycan as observed by both dimethylmethylene blue assay (P = 0.0005) and safranin-O stained histological sections.

CONCLUSION

These data show that non-invasive EAG detects streaming potentials produced by cartilage during joint compression and has potential to become a diagnostic tool capable of detecting early cartilage degeneration.

An in vitro investigation to understand the synergistic role of MMPs-1 and 9 on articular cartilage biomechanical properties

Matrix metalloproteinases (MMPs) play a crucial role in enzymatically digesting cartilage extracellular matrix (ECM) components, resulting in degraded cartilage with altered mechanical loading capacity. Overexpression of MMPs is often caused by trauma, physiologic conditions and by disease. To understand the synergistic impact MMPs have on cartilage biomechanical properties, MMPs from two subfamilies: collagenase (MMP-1) and gelatinase (MMP-9) were investigated in this study. Three different ratios of MMP-1 (c) and MMP-9 (g), c1:g1, c3:g1 and c1:g3 were considered to develop a degradation model. Thirty samples, harvested from bovine femoral condyles, were treated in groups of 10 with one concentration of enzyme mixture. Each sample was tested in a healthy state prior to introducing degradative enzymes to establish a baseline. Samples were subjected to indentation loading up to 20% bulk strain. Both control and treated samples were mechanically and histologically assessed to determine the impact of degradation. Young's modulus and peak load of the tissue under indentation were compared between the control and degraded cartilage explants. Cartilage degraded with the c3:g1 enzyme concentration resulted in maximum 33% reduction in stiffness and peak load compared to the other two concentrations. The abundance of collagenase is more responsible for cartilage degradation and reduced mechanical integrity.

Near Infrared Spectroscopy Enables Differentiation of Mechanically and Enzymatically Induced Cartilage Injuries

This study evaluates the feasibility of near infrared (NIR) spectroscopy to distinguish between different cartilage injury types associated with post-traumatic osteoarthritis and idiopathic osteoarthritis (OA) induced by mechanical and enzymatic damages. Bovine osteochondral samples (n = 72) were subjected to mechanical (n = 24) and enzymatic (n = 36) damage; NIR spectral measurements were acquired from each sample before and after damage, and from a separate control group (n = 12). Biomechanical measurements were then conducted to determine the functional integrity of the samples. NIR spectral variations resulting from different damage types were investigated and the samples classified using partial least squares discriminant analysis (PLS-DA). Partial least squares regression (PLSR) was then employed to investigate the relationship between the NIR spectra and biomechanical properties of the samples. Results of the study demonstrate that substantial spectral changes occur in the region of 1700–2200 nm due to tissue damages, while differences between enzymatically and mechanically induced damages can be observed mainly in the region of 1780–1810 nm. We conclude that NIR spectroscopy, combined with multivariate analysis, is capable of discriminating between cartilage injuries that mimic idiopathic OA and traumatic injuries based on specific spectral features. This information could be useful in determining the optimal treatment strategy during cartilage repair in arthroscopy.

Introduction to Cartilage

Cartilage is a supporting connective tissue that, together with the bone, forms the framework supporting the body as a whole. There are many distinct types of cartilage, which exhibit numerous similarities as well as differences. Among them, articular cartilage is the best known and the most studied type. Articular cartilage is the thin layer of connective tissue that covers the articulating ends of bones in synovial (diarthrodial) joints. It provides a smooth surface for joint movement and acts as a load-bearing medium that protects the bone and distributes stress. The intense interest in articular cartilage is motivated by the critical role its degradation plays in arthritis and related joint diseases, which are the number one cause of disability in humans. This chapter discusses the physical, chemical and cellular properties of cartilage that give the tissue its extraordinary load-bearing characteristics.

Quantitative Ultrasound Assessment of Cartilage Degeneration in Ovariectomized Rats with Low Estrogen Levels

The aim of this study was to assess quantitatively the site-specific degeneration of articular cartilage in ovariectomized rats with low estrogen levels using a high-frequency ultrasound system. Fourteen female Sprague-Dawley rats were randomly divided into two groups (n = 7 per group): a sham group in which only the peri-ovarian fatty tissue was exteriorized and an ovariectomized group that underwent bilateral ovariectomy to create a menopause model with low estrogen levels. All animals were sacrificed at the end of the third week after ovariectomy. Hindlimbs were harvested. The articular cartilage from five anatomic sites (i.e., femoral caput [FC], medial femoral condyle [MFC], lateral femoral condyle [LFC], medial tibial plateau [MTP] and lateral tibial plateau [LTP]) was examined with ultrasound. Four parameters were extracted from the ultrasound radiofrequency data: reflection coefficient of the cartilage surface (RC1), reflection coefficient of the cartilage-bone interface (RC2), ultrasound roughness index (URI) and thickness of the cartilage tissue. The results indicated significant (p < 0.05) site dependence for cartilage thickness, URI and RC1 in the sham group. The 3-wk post-menopause ovariectomized rats exhibited significant increases (p < 0.05) in the URI at the LFC, MTP and LTP; significant decreases (p < 0.05) in RC1 at the FC, LFC and MTP; and significant decreases (p < 0.05) in cartilage thickness at the MFC, LFC, MTP and LTP. These results of this study suggest that post-menopausal estrogen reduction induces morphologic and acoustic alterations in the articular cartilage of the hip and knee joints in ovariectomized rats.

99mTc-NTP 15-5 Imaging for Cartilage Involvement in Experimental Rheumatoid Arthritis: Comparison with Routinely Used Molecular Imaging Methods and Sensitivity to Chronic Nonsteroidal Antiinflammatory Drug Treatment

This study determined, using the intraarticular complete Freund adjuvant arthritis mice model, whether the radiotracer (99m)Tc-N-(triethylammonium)-3-propyl-[15]ane-N5 ((99m)Tc-NTP 15-5) targeting proteoglycans has a pathophysiologic validity for in vivo imaging of rheumatoid arthritis (RA) and its response to chronic nonsteroidal antiinflammatory drugs.

METHODS

We investigated the time course of cartilage remodeling by (99m)Tc-NTP 15-5 scintigraphy, bone damages by (99m)Tc-hydroxymethylene diphosphonate imaging, inflammation by (18)F-FDG PET, and joint proteoglycan content and pain behavior in animals, without and with meloxicam treatment. Paw circumference, thermal pain behavior, and histology as well as proteoglycan content of the whole joint were determined.

RESULTS

(99m)Tc-NTP 15-5 showed specific tracer accumulation within RA joints, with a significant increase in scintigraphic ratio observed in RA versus shams from day 3 to day 28. (18)F-FDG evidenced uptake in RA joints from day 15 to day 29. Animals treated with meloxicam (5 mg/kg) exhibited a dose-dependent decrease in both (99m)Tc-NTP 15-5 and (18)F-FDG uptake ratios versus saline-treated animals. (99m)Tc-hydroxymethylene diphosphonate bone scans were only positive at day 14 in RA versus shams, with a significant effect of meloxicam. An increase in proteoglycans of RA joint and thermal pain behavior were observed and were dose-dependently reduced by meloxicam.

CONCLUSION

These experimental results bring data in favor of the (99m)Tc-NTP 15-5 radiotracer for assessing, in vivo, cartilage remodeling in RA that could be used to monitor therapy.

A superficial hyperechoic band in human articular cartilage on ultrasonography with histological correlation: preliminary observations

Purpose:

To demonstrate the superficial hyperechoic band (SHEB) in articular cartilage by using ultrasonography (US) and to assess its correlation with histological images.

Methods:

In total, 47 regions of interest (ROIs) were analyzed from six tibial osteochondral specimens (OCSs) that were obtained after total knee arthroplasty. Ultrasonograms were obtained for each OCS. Then, matching histological sections from all specimens were obtained for comparison with the ultrasonograms. Two types of histological staining were used: Safranin-O stain (SO) to identify glycosaminoglycans (GAG) and Masson’s trichrome stain (MT) to identify collagen. In step 1, two observers evaluated whether there was an SHEB in each ROI. In step 2, the two observers evaluated which histological staining method correlated better with the SHEB by using the ImageJ software.

Results:

In step 1 of the analysis, 20 out of 47 ROIs showed an SHEB (42.6%, kappa=0.579). Step 2 showed that the SHEB correlated significantly better with the topographical variation in stainability in SO staining, indicating the GAG distribution, than with MT staining, indicating the collagen distribution (P<0.05, kappa=0.722).

Conclusion:

The SHEB that is frequently seen in human articular cartilage on high-resolution US correlated better with variations in SO staining than with variations in MT staining. Thus, we suggest that a SHEB is predominantly related to changes in GAG. Identifying an SHEB by US is a promising method for assessing the thickness of articular cartilage or for monitoring early osteoarthritis.

An in vitro model for the pathological degradation of articular cartilage in osteoarthritis

The objective of this study was to develop an in vitro cartilage degradation model that emulates the damage seen in early-stage osteoarthritis. To this end, cartilage explants were collagenase-treated to induce enzymatic degradation of collagen fibers and proteoglycans at the articular surface. To assess changes in mechanical properties, intact and degraded cartilage explants were subjected to a series of confined compression creep tests. Changes in extracellular matrix structure and composition were determined using biochemical and histological approaches. Our results show that collagenase-induced degradation increased the amount of deformation experienced by the cartilage explants under compression. An increase in apparent permeability as well as a decrease in instantaneous and aggregate moduli was measured following collagenase treatment. Histological analysis of degraded explants revealed the presence of surface fibrillation, proteoglycan depletion in the superficial and intermediate zones and loss of the lamina splendens. Collagen cleavage was confirmed by the Col II-3/4Cshort antibody. Degraded specimens experienced a significant decrease in proteoglycan content but maintained total collagen content. Repetitive testing of degraded samples resulted in the gradual collapse of the articular surface and the compaction of the superficial zone. Taken together, our data demonstrates that enzymatic degradation with collagenase can be used to emulate changes seen in early-stage osteoarthritis. Further, our in vitro model provides information on cartilage mechanics and insights on how matrix changes can affect cartilage's functional properties. More importantly, our model can be applied to develop and test treatment options for tissue repair.

An Instrumented Bioreactor for Mechanical Stimulation and Real-Time, Nondestructive Evaluation of Engineered Cartilage Tissue

Mechanical stimulation is essential for chondrocyte metabolism and cartilage matrix deposition. Traditional methods for evaluating developing tissue in vitro are destructive, time consuming, and expensive. Nondestructive evaluation of engineered tissue is promising for the development of replacement tissues. Here we present a novel instrumented bioreactor for dynamic mechanical stimulation and nondestructive evaluation of tissue mechanical properties and extracellular matrix (ECM) content. The bioreactor is instrumented with a video microscope and load cells in each well to measure tissue stiffness and an ultrasonic transducer for evaluating ECM content. Chondrocyte-laden hydrogel constructs were placed in the bioreactor and subjected to dynamic intermittent compression at 1 Hz and 10% strain for 1 h, twice per day for 7 days. Compressive modulus of the constructs, measured online in the bioreactor and offline on a mechanical testing machine, did not significantly change over time. Deposition of sulfated glycosaminoglycan (sGAG) increased significantly after 7 days, independent of loading. Furthermore, the relative reflection amplitude of the loaded constructs decreased significantly after 7 days, consistent with an increase in sGAG content. This preliminary work with our novel bioreactor demonstrates its capabilities for dynamic culture and nondestructive evaluation.

In vivo scintigraphic imaging of proteoglycans

In this chapter, we present the methods developed in our lab for the scintigraphic imaging and direct quantitative evaluation of proteoglycan (PG) distribution in vivo. These methods relate to (1) the synthesis and radiolabeling of the NTP 15-5 with (99m)Tc, (2) preclinical scintigraphic imaging using laboratory animals, and (3) quantitative analysis of scintigraphic images.

Real-time electro-mechano-acoustic imaging for monitoring interactions between trypsin and different inhibitors in articular cartilage

The purpose of this study was to observe the real-time interactions between trypsin and various inhibitors in articular cartilage in vitro using a novel electro-mechano-acoustic imaging method. Monitored in real-time, articular cartilage specimens from bovine patellae were first treated with trypsin to reach half proteoglycan depletion (Phase I), then the trypsin solution was replaced with (i) physiological saline buffer (PS), (ii) fetal bovine serum (FBS), (iii) protease inhibitor cocktail (PI) and (iv) 10% formalin (F), respectively, to observe their effects on residual digestion (Phase II). Ultrasound radio frequency signals from the articular cartilage were used to form a M-mode image, where the interface between trypsin digested and intact cartilage tissues could be observed with an additional echo generated. The inhibition time, the digestion depth and digestion fraction were measured for each specimen. The results showed that the dilution of trypsin using saline solution was not sufficient to stop the enzyme action instantly. Although groups FBS and PI had a similar inhibition time of approximately 1.5 h, their digestion depth was obviously different (0.25±0.03 and 0.06±0.06 mm, respectively). In contrast, formalin only took <30 min to stop the trypsin digestion with almost no further digestion. The results demonstrated that the current system was capable of monitoring the trypsin digestion and inhibition process in real time. Also, different chemicals affected the residual trypsin digestion to different degrees.

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.

Mechanical effects of surgical procedures on osteochondral grafts elucidated by osmotic loading and real-time ultrasound

Introduction

Osteochondral grafts have become popular for treating small, isolated and full-thickness cartilage lesions. It is recommended that a slightly oversized, rather than an exact-sized, osteochondral plug is transplanted to achieve a tight fit. Consequently, impacting forces are required to insert the osteochondral plug into the recipient site. However, it remains controversial whether these impacting forces affect the biomechanical condition of the grafted articular cartilage. The present study aimed to investigate the mechanical effects of osteochondral plug implantation using osmotic loading and real-time ultrasound.

Methods

A full-thickness cylindrical osteochondral defect (diameter, 3.5 mm; depth, 5 mm) was created in the lateral lower quarter of the patella. Using graft-harvesting instruments, an osteochondral plug (diameter, 3.5 mm as exact-size or 4.5 mm as oversize; depth, 5 mm) was harvested from the lateral upper quarter of the patella and transplanted into the defect. Intact patella was used as a control. The samples were monitored by real-time ultrasound during sequential changes of the bathing solution from 0.15 M to 2 M saline (shrinkage phase) and back to 0.15 M saline (swelling phase). For cartilage sample assessment, three indices were selected, namely the change in amplitude from the cartilage surface (amplitude recovery rate: ARR) and the maximum echo shifts from the cartilage surface and the cartilage-bone interface.

Results

The ARR is closely related to the cartilage surface integrity, while the echo shifts from the cartilage surface and the cartilage-bone interface are closely related to tissue deformation and NaCl diffusion, respectively. The ARR values of the oversized plugs were significantly lower than those of the control and exact-sized plugs. Regarding the maximum echo shifts from the cartilage surface and the cartilage-bone interface, no significant differences were observed among the three groups.

Conclusions

These findings demonstrated that osmotic loading and real-time ultrasound were able to assess the mechanical condition of cartilage plugs after osteochondral grafting. In particular, the ARR was able to detect damage to the superficial collagen network in a non-destructive manner. Therefore, osmotic loading and real-time ultrasound are promising as minimally invasive methods for evaluating cartilage damage in the superficial zone after trauma or impact loading for osteochondral grafting.

Ultrasound biomicroscopy imaging for monitoring progressive trypsin digestion and inhibition in articular cartilage

This study reports an ultrasound biomicroscopy (UBM) imaging approach to monitor the progressive trypsin-induced depletion of proteoglycan (PG) and its inhibition in articular cartilage. Three fresh, normal bovine patellae were obtained and four full-thickness cartilage-bone specimens were prepared from the lower medial side of each patella. One sample was used as a control and the other three were divided into three groups: Groups A, B and C (n=3 for each group). After a 40min 0.25% trypsin digestion, samples from group A were continuously digested in trypsin solution, while those in groups B and C were immersed in physiologic saline and fetal bovine serum (FBS), respectively, for another 280min. The trypsin penetration front was observed by UBM and M-mode images were acquired using 50MHz focused ultrasound and custom-developed software. The results show that the 40min trypsin digestion degraded nearly the whole surface layer of the cartilage tissue. Further digestion in trypsin or residual digestion in saline for 280min depleted most of the PG content, as observed in groups A and B. The replacement of trypsin with a physiologic saline solution only slightly slowed the digestion process (group B), while trypsin inhibitors in FBS stopped the digestion in approximately 1.5h (group C). The normalized digestion fractions of the digested tissues were calculated from ultrasound data and histology sections, and then compared between the groups. Without the use of FBS, 80% to 100% of the full thickness was digested, while this number was only approximately 50% when using FBS. Our findings indicate that the UBM imaging system could provide two-dimensional (2-D) visual information for monitoring progressive trypsin-induced PG depletion in articular cartilage. The system also potentially offers a useful tool for preparing cartilage degeneration models with precisely controlled PG depletion.

Intravascular Ultrasound (IVUS): A Potential Arthroscopic Tool for Quantitative Assessment of Articular Cartilage

Conventional ultrasound examination of the articular cartilage performed externally on the body surface around the joint has limited accuracy due to the inadequacy in frequency used. In contrast to this, minimally invasive arthroscopy-based ultrasound with adequately high frequency may be a better alternative to assess the cartilage. Up to date, no special ultrasound transducer for imaging the cartilage in arthroscopic use has been designed. In this study, we introduced the intravascular ultrasound (IVUS) for this purpose. An IVUS system with a catheter-based probe (Ø approximately 1mm) was used to measure the thickness and surface acoustical reflection of the bovine patellar articular cartilage in vitro before and after degeneration induced by enzyme treatments. Similar measurement was performed using another high frequency ultrasound system (Vevo) with a probe of much larger size and the results were compared between the two systems. The thickness measured using IVUS was highly correlated (r = 0.985, p < 0.001) with that obtained by Vevo. Thickness and surface reflection amplitude measured using IVUS on the enzymatically digested articular cartilage showed changes similar to those obtained by Vevo, which were expectedly consistent with previous investigations. IVUS can be potentially used for the quantitative assessment of articular cartilage, with its ready-to-use arthroscopic feature.

More intrinsic parameters should be used in assessing degeneration of articular cartilage with quantitative ultrasound

During the last decade, the quantitative ultrasound technique has been widely employed as a versatile modality to investigate a thin but crucial tissue layer – the articular cartilage. Previous studies provide information about the morphology and mechanical and acoustic properties of the tissue derived from ultrasound measurements and correlate them with cartilage degeneration. In a previous issue of Arthritis Research & Therapy, Kuroki and colleagues presented a study about the relationship between International Cartilage Repair Society grading and ultrasound echo magnitude, duration, and interval in human knee cartilage. We think indirect measurements of the intrinsic physical characteristics of cartilage, as reported in this study, should be interpreted more carefully as they can be affected by many experimental and physical factors. In this editorial, we offer our opinion that more intrinsic material parameters should be selected for the assessment of degeneration states of cartilage using quantitative ultrasound.

Progressive trypsin digestion and serum inhibition in articular cartilage monitored using high-frequency ultrasound in situ

This study investigated the trypsin digestion and serum inhibition in articular cartilage monitored using high-frequency ultrasound system in situ. Three normal bovine patellae were obtained from a local butcher shop. Four full-thickness cartilage-bone specimens were prepared from the lower medial side of each patella and divided into 4 groups: Group A, B, C and D. Group A was treated as control. The samples from the other three groups were digested by trypsin for 40 minutes and then were immersed for 280 additional minutes in trypsin, physiological saline, or fetal bovine serum (FBS), respectively. It was measured in the M-mode ultrasound images that 30% to 54% of the full-thickness cartilage tissue for all the samples was digested by the 40-minute trypsin digestion. The continuous trypsin digestion depleted almost all proteoglycan (PG) content of the samples of Group B. For Group C, the residual enzyme digestion in saline generated a digestion extension only slightly less than that of Group B. The enzyme digestion in the samples of Group D was relatively quickly (approximately 1 hour) stopped by FBS. The final ultrasound data were compared to histology for assessing the PG depletion. Our findings provide important information of enzyme digestion and inhibition inside articular cartilage. Ultrasound represents a useful visual tool to evaluate the dynamics of models of OA in cartilage specimens in vitro.