Muscle Deformation of the Quadriceps Femoris in Individuals with Knee Osteoarthritis

Purpose: Skeletal muscle degeneration, which includes atrophy and intramuscular fat infiltration, is widely observed in individuals with knee osteoarthritis (OA). Although these changes in muscle volume and internal properties are likely accompanied by alterations in three-dimensional (3D) muscle shape, it remains largely understudied. This study aimed to determine variations in 3D muscle shape in individuals with knee OA using statistical shape modeling. Methods: Forty patients with knee OA and 51 control individuals underwent magnetic resonance imaging via T1-weighted and Dixon methods to examine volume and intramuscular fat of the quadriceps femoris. The 3D muscle shape was quantified using statistical shape models based on the principal components. Muscle volume, intramuscular fat content, and shape were compared between knee OA individuals and controls. Logistic regression analysis was used to distinguish individuals with knee OA from controls based on muscle volume, intramuscular fat, and shape, respectively. Results: Individuals with knee OA presented not only volume loss and intramuscular fat accumulation but also muscle shape changes in the vastus medialis and intermedius. The vastus medialis exhibited a flattened shape and anterior edge shrinkage, whereas the vastus intermedius presented anterior bulging along the length in the knee OA group; these were specific morphological changes associated with knee OA. These muscle deformations were compared to the degree of volume loss and intramuscular fat accumulation to accurately distinguish individuals with knee OA from controls, outperforming classifications based on volume and intramuscular fat. Conclusions: Our findings demonstrate muscle deformation of vastus medialis and vastus intermedius in individuals with knee OA, providing new insights into muscle morphological changes and their potential relevance as novel morphological biomarkers of knee OA.