Changes in MR relaxation times of the meniscus with acute loading: An in vivo pilot study in knee osteoarthritis

THE MENISCUS, A fibrocartilaginous tissue found within the knee joint, is responsible for joint congruity, shock dissipation, load transmission, lubrication, and stability of the joint (1,2). The mechanical function of the meniscus largely depends on the structural and molecular integrity of its matrix, composed of a network of collagen fibers (type I and smaller amounts of type II) immobilizing proteoglycans (PG) (2,3). While PGs in the healthy meniscus resist large loads, it is the collagen fibrils arranged in fibrous lamellae that make meniscus efficient shock absorbers (4). Because of these functional roles, damage to or loss of the menisci affects the articular cartilage, as shown by the increased risk of developing osteoarthritis (OA) after meniscectomy (5). Traditionally, structural damage to the articular cartilage is considered as an initiator for the knee OA, but structural changes in the meniscus, which plays a critical role in the normal biomechanics and stability of the knee joint, may be a precursor to OA. Studies have also demonstrated that meniscal pathologies, such as meniscal degeneration and meniscal tears (6–8), are associated with change of the articular cartilage extracellular matrix (ECM) in patients in early or late stages of OA (9,10). The meniscus is increasingly being recognized as an important tissue in knee osteoarthritis (OA) (1–5). Morphological defects of meniscus and cartilage visualized with MRI are preceded by early degeneration of their ECM (8,10,11). In early stage osteoarthritis, the meniscal ECM exhibits increased proteoglycan content, loss of collagens, and less organized collagen fiber networks than those of normal menisci (12–18), and swelling, resulting in loss of mechanical properties. Changes in the matrix are an early and key finding in the evolution of OA as they likely alter the load distribution transmitted to the underlying articular cartilage. Thus, quantitative measures to detect and monitor changes that occur within the meniscus, both in unloaded and loaded conditions, could provide powerful diagnostic measurements for early-stage OA. Recent studies have shown the potential of MR relaxation times (T1ρ and T2) in studying biochemical composition of meniscus. This technology is promising for quantifying early meniscal degeneration and injury (15,19). These measures are highly sensitive to alterations in composition and structural integrity of collagen in the ECM of articular cartilage in vivo (20–22). Previous studies have shown that the collagen content and its orientation is the major factor in changes of cartilage T2 relaxation times (21,22). T2 relaxation time mapping is sensitive to a wide range of water interactions in tissue and in particular depends on the content, orientation and anisotropy of collagen and may therefore also play a role in assessing meniscal degeneration. While the exact contributor to T1ρ and T2 relaxation times remain disputed it is generally agreed upon that these metrics are sensitive to alterations in ECM composition and macromolecular structure and integrity. Rauscher et al reported promising results using this technique to quantify degenerative changes in the meniscal matrix in subjects with early OA (15). While several studies have investigated the use of these quantitative measures (T1ρ and T2) in assessing the effect of acute loading and physical exercise on articular cartilage (23–27), there is a lack of information regarding the affects of acute loading on meniscus relaxation times. Understanding the behavior of the meniscus relaxation times to acute loading may provide valuable information regarding the transmission of forces to the articular cartilage in persons with knee OA. Thus the purpose of this study was to evaluate changes in MR (T1ρ and T2) relaxation times in the meniscal body with acute loading in osteoarthritic knees and to compare these findings with those of age-matched healthy controls. The underlying hypothesis was that healthy knees will exhibit greater T1ρ and T2 change with loading when compared with osteoarthritic knees.