Variations in heel pad mechanical properties variation between children and young adults

The purpose of this study was to compare the mechanical properties of the heel pads of adults and children, using ultrasound to assess the responses of the heel pad during compression. Thirty volunteers without heel problems, aged from 4 to 36 years, were recruited. There were 10 children (< 15 years old) and 20 adults (> 18 years old). A 7.5-MHz linear array ultrasound transducer was incorporated into a specially designed device to measure the thickness of the heel pad under different loads. The load on the heel pad was increased serially in increments of 0.5 kg, to a maximum of 3 kg, and then reduced sequentially. The load-displacement curve of the heel pad during a loading-unloading cycle was then plotted. Mechanical properties of the heel, including unloaded heel-pad thickness (UHPT), compressibility index, elastic modulus, and energy dissipation ratio (EDR), were calculated from the load-displacement curves. The average UHPT was 1.53 +/- 0.09 cm in children and 1.76 +/- 0.20 cm in adults (p < 0.001). The EDR, which represents the shock absorbency of the heel pad, was 13.5 +/- 2.0% in the children and 23.7 +/- 6.9% in adults (p < 0.001). The average compressibility index and elastic modulus were also higher in adults than in children, although the differences were not statistically significant. These findings suggest that the mechanical properties of the heel pad change from childhood to adulthood. Less energy is absorbed in the heel pad of children, which may partially explain why children tend to have fewer heel problems than adults.