Background Muscle weakness is of clinical interest in knee osteoarthritis (OA). High proportion of non-contractile tissue including fatty tissue and connective tissue could contribute to muscle weakness in patients with knee OA.

Objectives The study aims were to establish the association of non-contractile tissue in the vastus medialis muscle, assessed by magnetic resonance imaging, to muscle strength in patients with established knee OA.

Methods A sample of 94 patients were included. Sagittal T1 weighted MRI, conducted at 3 Tesla, were performed using a phased array knee coil. Within a standardized region of interest located at the distal end of the vastus medialis, non-contractile tissue was distinguished from contractile tissue based on pixel grayscale values. The amount of non-contractile tissue was expressed as the percentage of non-contractile tissue pixels in proportion to the total amount of pixels. Muscle strength was assessed isokinetically (Nm). The GUG test was used as a measurement of physical function. Percentage of non-contractile tissue was dichotomized into low and high percentage non-contractile tissue groups. With Student's t-test, differences in demographics and clinical characteristics between the group of low percentage of non-contractile tissue versus a high percentage of non-contractile tissue were calculated. In regression analysis the associations of the percentage of non-contractile tissue with muscle strength and GUG test were determined, controlled for BMI, gender, age and radiographic severity.

Results Mean ± SD age and BMI were 61.68 ± 7.09 years and 29.13 ± 4.72kg/m², respectively. A K&L grade of 2 or higher was found in 68% of the participants. The mean ± SD of the percentage of non-contractile tissue was 16.03 ±15.31 with a median of 11%. The participants with a high percentage of non-contractile tissue (>11%) had on average less muscle strength, a longer GUG time and a higher BMI than participants with a low percentage of non-contractile tissue (≤11%) (P=0.031, P=0.021, P=0.026, respectively). In univariable regression analysis, associations of the percentage of non-contractile tissue to muscle strength and the GUG test were found (B= -19.04, 95% CI = -36.34 to -1.73, P=0.031; B=1.09, 95% CI = 0.17 to 2.01, P=0.021, respectively). Multivariate regression analysis, adjusting for BMI, gender, age and radiographic severity, showed that the percentage of non-contractile tissue was negatively associated with muscle strength (B= -13.94, 95% CI = -27.56 to -0.33, P=0.045), but no association was found with the GUG (B=0.674, 95% CI = -0.20 to 1.55, P=0.130).

Conclusions In patients with established knee OA, non-contractile tissue in the vastus medialis muscle measured by clinical MRI is associated with muscle strength, but not with physical performance (GUG test) after adjustment for BMI, gender, age and radiographic severity.

Disclosure of Interest None declared