Estimation of articular cartilage properties using multivariate analysis of optical coherence tomography signal

SummaryObjectiveThe aim was to investigate the applicability of multivariate analysis of optical coherence tomography (OCT) information for determining structural integrity, composition and mechanical properties of articular cartilage.DesignEquine osteochondral samples (N = 65) were imaged with OCT, and their total attenuation and backscattering coefficients (μt and μb) were measured. Subsequently, the Mankin score, optical density (OD) describing the fixed charge density, light absorbance in amide I region (Aamide), collagen orientation, permeability, fibril network modulus (Ef) and non-fibrillar matrix modulus (Em) of the samples were determined. Partial least squares (PLS) regression model was calculated to predict tissue properties from the OCT signals of the samples.ResultsSignificant correlations between the measured and predicted mean collagen orientation (R2 = 0.75, P < 0.0001), permeability (R2 = 0.74, P < 0.0001), mean OD (R2 = 0.73, P < 0.0001), Mankin scores (R2 = 0.70, P < 0.0001), Em (R2 = 0.50, P < 0.0001), Ef (R2 = 0.42, P < 0.0001), and Aamide (R2 = 0.43, P < 0.0001) were obtained. Significant correlation was also found between μb and Ef (ρ = 0.280, P = 0.03), but not between μt and any of the determined properties of articular cartilage (P > 0.05).ConclusionMultivariate analysis of OCT signal provided good estimates for tissue structure, composition and mechanical properties. This technique may significantly enhance OCT evaluation of articular cartilage integrity, and could be applied, for example, in delineation of degenerated areas around cartilage injuries during arthroscopic repair surgery

Effects of Ultrasound Beam Angle and Surface Roughness on the Quantitative Ultrasound Parameters of Articular Cartilage

High-resolution arthroscopic ultrasound imaging provides a potential quantitative technique for the diagnostics of early osteoarthritis. However, an uncontrolled, nonperpendicular angle of an ultrasound beam or the natural curvature of the cartilage surface may jeopardize the reliability of the ultrasound measurements. We evaluated systematically the effect of inclining an articular surface on the quantitative ultrasound parameters. Visually intact (n = 8) and mechanically degraded (n = 6) osteochondral bovine patella samples and spontaneously fibrillated (n = 1) and spontaneously proteoglycan depleted (n = 1) osteochondral human tibial samples were imaged using a 50-MHz scanning acoustic system. The surface of each sample was adjusted to predetermined inclination angles (0, 2, 5 and 7°) and five ultrasound scan lines along the direction of the inclination were analyzed. For each scan line, reflection coefficient (R), integrated reflection coefficient (IRC) and ultrasound roughness index (URI) were calculated. Nonperpendicularity of the cartilage surface was found to affect R, IRC and URI significantly (p < 0.05). Importantly, all ultrasound parameters were able to distinguish (p < 0.05) the mechanically degraded samples from the intact ones even though the angle of incidence of the ultrasound beam varied between 0 and 5° among the samples. Diagnostically, the present findings are important because the natural curvature of the articular surface varies, and a perfect perpendicularity between the ultrasound beam and the surface of the cartilage may be challenging to achieve in a clinical measurement. (Email: [email protected])

Minimally Invasive Ultrasound Method for Intra-Articular Diagnostics of Cartilage Degeneration

Quantitative ultrasound imaging (QUI) can be used to evaluate the integrity of articular cartilage and for diagnosing the early signs of osteoarthritis (OA). In this study, we applied a minimally invasive ultrasound imaging technique and investigated its ability to detect superficial degeneration of bovine knee articular cartilage. Intact (n = 13), collagenase-digested (n = 6) and mechanically degraded (n = 7) osteochondral samples (dia. = 25 mm) and custom-made phantoms with different degrees of surface roughness (n = 8) were imaged using a high-frequency (40 MHz) QUI system. For each sample and phantom, the ultrasound reflection coefficient (R), integrated reflection coefficient (IRC) and ultrasound roughness index (URI) were determined. Furthermore, to evaluate the clinical applicability of intra-articular ultrasound (IAUS) in diagnostics, one intact bovine knee joint was investigated ex vivo using a simulated arthroscopic approach. Differences in the surface characteristics of the phantoms were detected by monitoring changes in the reflection and surface roughness parameters. Both mechanically- and enzymatically-induced degradation were sensitively diagnosed by decreased (p < 0.05) reflection (R and IRC) at the cartilage surface. Furthermore, mechanical degradation was detected in the increased (p < 0.05) surface roughness (URI). The intra-articular investigation of a bovine knee joint suggested that the IAUS technique may enable minimally invasive, straightforward diagnostics of the degenerative status of the articular surfaces. We conclude that quantitative IAUS imaging can be used for detecting collagen disruption and increased roughness of the articular surface. This quantitative in vivo ultrasound technique could have great clinical value in the diagnostics of joint diseases. (E-mail: [email protected])

Effect of bone inhomogeneity on tibiofemoral contact mechanics during physiological loading

It is not known how inhomogeneous mechanical properties of bone affect contact mechanics and cartilage response during physiological loading of the knee joint. In this study, a finite element model of a cadaver knee joint was constructed based on quantitative computed tomography (QCT). The mechanical properties of bone were altered and their effect on tibiofemoral contact mechanics and cartilage stresses, strains and pore pressures were evaluated during the first 20% of stance. For this purpose, models with rigid, homogeneous and inhomogeneous bones were created. When bone was modeled to be rigid, the resulting contact pressures were substantially higher in the medial side of the joint, as compared to the non-rigid bones. Similar changes were revealed also in stresses, strains and pore pressures throughout the cartilage depth at the cartilage-cartilage contact area. Furthermore, the mechanical response of medial tibial cartilage was found to be highly dependent on the bone properties. When Young's modulus in the model with homogeneous bone was 5 GPa, cartilage mechanical response approached to that of the model with inhomogeneous bone. Finally, when the apparent bone mineral densities were decreased globally in the inhomogeneous bone, stresses, strains and pore pressures were decreased at all layers of medial tibial cartilage. Similar changes were observed also in cartilage-cartilage contact area of the lateral compartment but with a lesser extent. These results indicate that during physiological loading Young's modulus of bone has a substantial influence on cartilage stresses and strains, especially in the medial compartment

Arthroscopic ultrasound technique for simultaneous quantitative assessment of articular cartilage and subchondral bone: an in vitro and in vivo feasibility study

Traditional arthroscopic examination is subjective and poorly reproducible. Recently, we introduced an arthroscopic ultrasound method for quantitative diagnostics of cartilage lesions. Here we describe our investigation of the feasibility of ultrasound arthroscopy for simultaneous measurements of articular cartilage and subchondral bone. Human osteochondral samples (n= 13) were imaged using a clinical 9-MHz ultrasound system. Ultrasound reflection coefficients (R, IRC), the ultrasound roughness index (URI) and the apparent integrated backscattering coefficient (AIB) were determined for both tissues. Mechanical testing, histologic analyses and micro-scale computed tomography imaging were the reference methods. Ultrasound arthroscopies were conducted on two patients. The ultrasound reflection coefficient correlated with the Mankin score and Young's modulus of cartilage (|. r| > 0.56, p 0.70, p 0.59, p 0.65, p < 0.05). Arthroscopic ultrasound examination provided diagnostically valuable information on cartilage and subchondral bone in vivo