Relationship between T1rho magnetic resonance imaging, synovial fluid biomarkers, and the biochemical and biomechanical properties of cartilage

Abstract

Non-invasive techniques for quantifying early biochemical and biomechanical changes in articular cartilage may provide a means of more precisely assessing osteoarthritis (OA) progression. The goals of this study were to determine the relationship between T1rho magnetic resonance (MR) imaging relaxation times and changes in cartilage composition, cartilage mechanical properties, and synovial fluid biomarker levels and to demonstrate the application of T1rho imaging to evaluate cartilage composition in human subjects in vivo. Femoral condyles and synovial fluid were harvested from healthy and OA porcine knee joints. Sagittal T1rho relaxation MR images of the condyles were acquired. OA regions of OA joints exhibited an increase in T1rho relaxation times as compared to non-OA regions. Furthermore in these regions, cartilage sGAG content and aggregate modulus decreased, while percent degraded collagen and water content increased. In OA joints, synovial fluid concentrations of sGAG decreased and C2C concentrations increased compared to healthy joints. T1rho relaxation times were negatively correlated with cartilage and synovial fluid sGAG concentrations and aggregate modulus and positively correlated with water content and permeability. Additionally, we demonstrated the application of these in vitro findings to the study of human subjects. Specifically, we demonstrated that walking results in decreased T1rho relaxation times, consistent with water exudation and an increase in proteoglycan concentration with in vivo loading. Together, these findings demonstrate that cartilage MR imaging and synovial fluid biomarkers provide powerful non-invasive tools for characterizing changes in the biochemical and biomechanical environments of the joint.

Introduction

Osteoarthritis (OA) is a joint disease characterized by the degeneration of articular cartilage, osteophyte formation, and joint space narrowing (Duvvuri et al., 1997). Articular cartilage is composed of chondrocytes in a dense extracellular matrix that is primarily made up of water, collagen, and proteoglycans, and has a limited capacity for repair following damage. OA progression is associated with increases in synthesis and breakdown of the extracellular matrix components, although the precise changes in the biochemical and biomechanical environment of the tissue as OA progresses remain unclear.

Recent data suggests that major contributing factors to early OA are altered mechanical loading and abnormal cartilage physiology (Guilak, 2011, Rivers et al., 2000). However, the specific contributions and interactions of these factors are not fully understood (Guilak, 2011). Both altered loading patterns and abnormal physiology can contribute to degradation of the cartilage matrix, which is reflected in the biochemical and biomechanical properties of the extracellular matrix. Specifically, one of the earliest detectable changes in animal models of OA is a loss of biomechanical properties, including decreased compressive stiffness and increased permeability (Appleyard et al., 1999, Setton et al., 1994). Additionally, in OA cartilage, proteoglycan content decreases and the collagen network becomes disrupted (Keenan et al., 2011). These breakdown products are evident in synovial fluid with varying proteoglycan concentrations and increased collagen type II cleavage (C2C) neoepitope concentrations, depending on the OA severity (Bolam et al., 2006, Prink et al., 2010, Ratcliffe et al., 1988). Additionally, matrix metalloproteinases (MMPs) are elevated in OA cartilage, promoting the enzymatic digestion of extracellular matrix components and forming degradation fragments that can be detected in synovial fluid (Catterall et al., 2010, Janusz et al., 2002, Settle et al., 2010).

While molecular biomarkers in the synovial fluid can provide measures of overall joint health (Kraus et al., 2010), it is unclear how these biomarkers relate to local changes in the biochemical or biomechanical properties of cartilage. In this regard, quantitative magnetic resonance (MR) imaging techniques have been used to track early OA in vivo (Tang et al., 2011). T1rho and T2 relaxation times in particular have been shown to be sensitive to disruptions in the organization of proteoglycan and collagen within the cartilage extracellular matrix, respectively (David-Vaudey et al., 2004, Duvvuri et al., 1997). Specifically, previous studies have shown that increased T1rho relaxation times correspond to decreased proteoglycan concentration and increased water content (Keenan et al., 2011, Li et al., 2011, Regatte et al., 2006, Wheaton et al., 2005). T2 relaxation times are sensitive to changes in collagen content, collagen organization, and water content (Choi and Gold, 2011, Chou et al., 2009, Dunn et al., 2004, Jazrawi et al., 2011).

However, there is limited data providing a comprehensive assessment of the compositional, biochemical, and biomechanical properties of OA cartilage related to T1rho relaxation times. Additionally, while synovial fluid biomarkers have been used to reflect the health status of the entire joint, their relationship to T1rho relaxation times is unclear. A comprehensive evaluation relating T1rho relaxation times to the properties of OA cartilage and synovial fluid biomarkers would provide critical information for evaluating the changes in the biochemical and biomechanical environment of the joint as OA progresses in vivo. In addition, these measurements could be used to establish molecular biomarkers that reflect changes in local cartilage biomechanical function and composition.

The goals of this study were to relate T1rho relaxation times to changes in cartilage composition, cartilage mechanical properties, and synovial fluid biomarker levels in vitro. In order to demonstrate the utility of these findings to the study of human subjects, we sought to quantify the effects of in vivo mechanical loading on T1rho relaxation times. We hypothesized that T1rho relaxation times, water content, and permeability would be increased in OA regions compared to normal regions of OA porcine joints, while cartilage sGAG content and aggregate modulus would be decreased in the OA regions. Additionally, we hypothesized that OA joints would contain decreased concentrations of synovial fluid sGAG and increased levels of C2C and MMP activity compared to normal joints. Finally, in human subjects, we hypothesized that in vivo mechanical loading experienced during walking would cause decreased T1rho relaxation times due to compression of the cartilage matrix leading to exudation of water and increased proteoglycan concentrations.