Aging is associated with changes in the cellular composition of articular cartilage.1 These age-related changes in cartilage composition, combined with altered joint loading patterns, may help to explain the increased prevalence of osteoarthritis (OA) in older adults.2 Prior work using T1rho (proteoglycan content) and T2 (collagen structure) mapping to assess cartilage composition demonstrated that asymptomatic, older adults (≥45 years) exhibit worse cartilage composition compared to asymptomatic, young adults (18–44 years).3–5 This suggests that quantitative magnetic resonance imaging (QMRI) is sensitive enough to detect age-related changes in cartilage composition3–5 but it remains difficult to specify the primary contributor to these changes in cartilage composition.
Quantitative spatial mapping (QSM) is a technique that measures the magnetic susceptibility value in cartilage and is sensitive to collagen fiber orientation.6 Healthy cartilage exhibits a specific depth-dependent collagen fiber orientation7 and this natural collagen fiber orientation provides cartilage with its high tensile stiffness and strength.8 The impact of aging on collagen fiber orientation within articular cartilage is not well understood and therefore, warrants assessment as collagen fiber orientation may be associated with the increased prevalence of OA in the aging population.
In this issue of JMRI, author et al9 utilized QSM to measure the effects of aging on collagen fiber orientation within the tibiofemoral articular cartilage. Ninety-four asymptomatic, healthy adults were recruited into this study and were divided into three age groups: young adults (19–30 years), adults (31–50 years), and mature adults (51–66 years). Each study participant underwent a knee joint MR-scan consisting of a sagittal, 3D spoiled gradient echo sequence and QSM was performed to assess collagen fiber orientation. Depth wise susceptibility profiles were created by normalizing the cartilage thickness (0–1.0) within the weight-bearing regions of the lateral and medial tibial and femoral cartilage. This approach allowed for assessment of age-related effects on collagen fiber orientation in the deep (0–0.29), middle (0.3–0.89), and superficial layers (0.9–1.0) within each of the specified regions of articular cartilage. The authors also assessed the relationship between age and cartilage thickness within these specific regions of interest (ROI).
The authors demonstrated that there were no correlations between age and cartilage thickness in any of the ROI. Age-related differences in collagen fiber orientation were present within the deep and middle layers of the tibiofemoral cartilage. Compared to the other two study groups, the mature adults exhibited higher susceptibility values within the deep layer of the medial femoral cartilage. The adult group also exhibited higher susceptibility values compared to the young adult group within the middle layer of the medial femoral cartilage. Both of the two older adult groups exhibited higher susceptibility values within the deep layer of the lateral femoral cartilage compared to the young adult group. The mature adult group exhibited higher susceptibility values within the deep and middle layers of the lateral tibial cartilage compared to the other two study groups. In addition, the adult group demonstrated higher susceptibility values within the deep and middle layers of the lateral tibial cartilage compared to the young adult group. There were no age-related differences in susceptibility values within the medial tibial cartilage. These results suggest that cartilage biochemistry (collagen fiber orientation) may be altered prior to cartilage morphology (i.e., thickness) in aging adults.
In order to prevent the onset of OA in the aging population and to help aging adults maintain adequate function and activity levels throughout the lifespan, a better understanding of the overall impact of aging on cartilage biochemistry is needed. This study by author et al9 offers a unique methodology to assess the age-related changes that occur in collagen fiber orientation within the articular cartilage. This study also provides unique insight into the changes that occur in cartilage structure that may affect overall cartilage function during the natural aging process. Future studies could begin to incorporate QSM, in combination with biomechanical analyses, to understand the direct link between cartilage structure and function in the aging population. This information would provide clinicians and scientists with the information needed to develop effective interventions to mitigate cartilage degeneration and to reduce the prevalence of OA in the aging population.
Contributor Information
Michael A. Samaan, Department of Kinesiology and Health Promotion, University of Kentucky, Lexington, Kentucky, USA.
Kate N. Jochimsen, Division of Athletic Training, School of Medicine, West Virginia University, Morgantown, West Virginia, USA.
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