Ultrasonography and Particle Image Velocimetry of Gliding Characteristics of Kager’s Fat Pad and Surrounding Tissues

Abstract Background: Kager’s fat pad (KFP), located at the posterior ankle joint, is divided into three regions: The Achilles-associated part (AT part), flexor hallucis longus - associated part (FHL part), and calcaneal wedge (Wedge part). Although the Wedge part has been studied several times, the gliding characteristics of the AT and FHL parts remain unclear. The purpose of this study was to clarify the gliding characteristics of KFP and the surrounding tissues using ultrasonography. Methods: 20 healthy adult patients performed active motion in the prone position with knee extension and an ankle neutral position with a metronome at 60 BPM. The gliding velocity and direction of movement of the AT and FHL parts of the KFP, Achilles tendon, and flexor hallucis longus muscles were calculated using ultrasonography and particle image velocimetry. Gliding velocity comparisons were made using one-way repeated measures ANOVA and multiple comparison tests. Results: In terms of gliding velocity, the AT part of the KFP was significantly lower than the FHL part (1.03 ± 0.15 vs 1.18 ± 0.15, P < 0.05) and the Achilles tendon (1.03 ± 0.15 vs 1.19 ± 0.20, P < 0.01). Regarding the direction of movement, the FHL part of the KFP moved distally and showed movement opposite to that of the AT part. Conclusion: The differences in gliding velocity between the AT part and Achilles tendon shown in the present study suggest that the AT part may be involved in damage around the Achilles tendon. The differences in gliding velocity and direction of movement between the AT and FHL parts, which have the same KFP, suggest that they may have movements according to their respective roles.