Ultrasound Analysis of Plantar Tissue Stiffness of Behavior Properties in the Midfoot
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Foot orthoses have been used for many years to treat various pathologies. While these orthoses can be effective, the mechanism of how plantar soft tissues and skeletal structures interface with the orthosis is not well understood. Studies have been conducted which examine tissue properties of the heel pad, including the use of ultrasound, but analysis of the midfoot is limited. This study aims to quantify displacement and stiffness of the plantar soft tissues of the midfoot under a known load through the use of ultrasonography in weight bearing and non- weight bearing conditions. Twenty -five subjects were recruited for the study ranging in age from 23-56 (Mean = 36.08) years old, consisting of 12 females and 13 males. Simple demographic information was recorded and a Foot Posture Index (FPI) examination was performed on all subjects. A force application apparatus was designed that utilizes a stepper-motor-driven linear actuator. The test set-up consisted of load cell mounted to the actuator which could drive an ultrasound transducer into the plantar surface of the midfoot to a maximum load of 40N. The results show plantar foot tissue stiffness exhibits a non-linear "toe" region at low force values, while increased force causes a shift to a linear stiffness profile. Midfoot plantar tissues are significantly stiffer in weight bearing conditions than in nonweight bearing conditions (p<0.05). Observations and measurements from ultrasound videos indicate that soft tissue between the skin and skeletal structure experiences greater change in compression in non-weight bearing. However, muscle tissue appears to have a greater change in deformation under non-weight bearing, while the connective tissue between skin and muscle display greater change in deformation under weight bearing conditions. The disparity in amount of deformation between tissues during different weight bearing conditions shows that stiffness properties in the foot are dynamic. The aim for future research is to develop a map of plantar soft tissue properties in all regions of the foot for clinical reference in selecting compatible orthotic interface materials.