Dynamic Changes in Muscle Thickness and Infrapatellar Fat Pad During Quadriceps Setting: An Ultrasound Echo Analysis

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Abstract
Purpose The infrapatellar fat pad (IFP) has the lowest pain threshold among all knee joint components and causes anterior knee pain after knee arthroplasty. It has been reported that selective muscle atrophy of the vastus medialis (VM) and fibrosis of the IFP may develop following knee joint surgery. Ultrasound enables visualization of IFP deformation (A1) from within the joint to the proximal area in response to muscle contraction, and this may be helpful in developing preventive and therapeutic strategies for IFP fibrosis. Therefore, this study aimed to clarify the relationship between quadriceps muscle thickness and dynamic changes in IFP during quadriceps setting (QS). Methods This study involved six participants (all men, 12 knees) with no history of knee joint problems, with a mean age of 36.7±8.7 years. We used ultrasound imaging to evaluate quadriceps muscle thickness and IFP dynamics. The muscle thicknesses of the VM, vastus lateralis (VL), vastus intermedius (VI) and rectus femoris (RF) were measured at rest and during QS with maximal contraction. The IFP measured the anterior-posterior width and the patellar tendon-tibia angle. The differences between conditions were examined using the Wilcoxon signed-rank test, and the correlation between the differences between measurement conditions was calculated using the Spearman rank correlation coefficient. Results The thickness of each muscle was measured and only the vastus lateralis muscle showed a significantly lower value during QS, while all other measurements showed higher values ​​(VM: p=0.0029, VL: p=0.0414, VI: p=0.0022, RF: p=0.0022, compared to resting). The mean anteroposterior width of the IFP increased by 0.9 mm medially (p=0.015) and 1.4 mm laterally (p=0.0076). Regarding the correlation between the measurements, a significant positive correlation was observed only between the VM difference and the lateral IFP difference (ρ=0.81, p=0.0071). Discussion The IFP provides a cushioning effect for the patellofemoral joint and reduces friction between the articular cartilage and the patellar ligament through functional deformation. In this study, a significant positive correlation was found between VM difference and lateral IFP difference. These findings suggest that the contractile force of the VM may be related to the extent by which the IFP is pushed outward, and changes in the flexibility of the soft tissues around the knee, such as IFP, may contribute to functional impairment in patients with knee joint disease. Conclusion This is the first study to quantitatively assess the extent of IFP deformation in the medial and lateral patella. The results of the study suggest that changes in soft tissue flexibility around the knee, such as in the IFP, may contribute to functional impairment in patients with knee joint disease.

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