Background/Purpose: Osteoarthritis (OA) is the most common arthritic condition but its treatment still remains symptomatic. Understanding factors affecting the progression of this disease may enable the development of disease-modifying therapies. Vastus medialis (VM) muscle surface has been proposed in recent OA studies to be associated with cartilage volume loss over time. However, the VM may include a significant proportion of fatty infiltration, which could also influence knee metabolism and mechanics. These two parameters can be visualized and quantified by MRI. Using data from a recent randomized clinical trial in knee OA¹, this study contrasted the VM surface area and fatty infiltration (% Fat) with cartilage volume loss and changes in bone marrow lesions as assessed by MRI.

Methods: OA patients, diagnosed according to American College of Rheumatology clinical and radiological criteria, were from the according-to-protocol population (n=143) of a 2-year randomized clinical trial, and had MRI acquisitions at baseline and 2 years. MR images of the VM (mm²) were assessed semi-automatically and VM % Fat by a fully automated software. Univariate and multivariate analyses were performed to assess the relationship between VM area and its % Fat, cartilage volume loss, and bone marrow lesion changes over 2 years.

Results: The median baseline of the VM area and % Fat were chosen to stratify patients. Female (p≤0.001), higher BMI (p≤0.008), and disability (p≤0.040) were associated with both a higher baseline VM surface area and % Fat. Higher baseline VM area was associated with greater cartilage volume loss in the medial compartment, medial femur, and lower cartilage volume loss in the lateral plateau (all p≤0.048). Change in % Fat, but not in surface area, was associated with an increase in the bone marrow lesion score in the global knee and cartilage volume loss in the global knee, lateral compartment, lateral femur, and medial plateau (all p≤0.035). Multivariate analyses revealed correlations between % Fat, but not surface area, and cartilage volume loss in the global knee (p=0.011) and most subregions studied. Importantly, % Fat change was independently associated with bone marrow lesion change at 2 years (p=0.001). All of the above changes were found irrespective of the treatment the patients had during the clinical trial.

Conclusion: This study is the first to demonstrate that the % Fat in the VM is strongly associated with cartilage volume loss and the presence and progression of bone marrow lesions. Importantly, two different OA phenotypes were evidenced: i) low VM area phenotype comprising female patients with lower BMI, being more symptomatic at baseline, having more cartilage volume damage at baseline, and at 2 years less cartilage volume loss in the medial femur and more cartilage volume loss in the lateral plateau; ii) higher VM % Fat phenotype comprises female patients with higher BMI, having more disability at baseline, more cartilage volume damage at baseline, and at 2 years more cartilage volume loss in the medial plateau and lateral femur.

Reference: 1) Raynauld JP, et al. Ann Rheum Dis 2009;68:938-47.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/surface-area-and-fatty-infiltration-of-vastus-medialis-measured-by-magnetic-resonance-imaging-are-risk-factors-for-the-progression-of-knee-osteoarthritis-and-discriminate-two-osteoarthritis-phenotypes/