The effect of loading conditions on stress in the barefooted heel pad

Purpose: High internal stress is considered to be a possible cause of heel-pad problems. External biomechanical measurements are used to attempt to understand the causes of heel pain. However, internal stress cannot be measured experimentally. Therefore, the purpose of this study was to quantify the relationship between magnitude of force, time to peak force, and sole angle with internal stresses in the heel using a finite element model. Methods: Computer tomography (CT) was used to create a nonlinear time-dependent three-dimensional finite element model of the heel pad. The material model was based on previously reported force-displacement data derived from in vitro experiments. Although it was not possible to compare internal calculations of stress with experimental data, good agreement was found for external plantar pressures and strains when compared with in vivo values. Internal stresses and external plantar pressures were then investigated for different forces, loading rates (i.e., time to peak force), and angles of foot inclination in the sagittal plane (i.e., sole angle). Results: The results of the model indicate that compressive stress is localized in the region inferior to the calcaneal tuberosity. Peak internal compressive stress was greater than external plantar pressure. Increasing the loading rate (i.e., reducing the time to peak force) caused plantar pressure to increase to a greater extent than internal stress. The general levels of stress were higher when the heel was loaded in an inclined position (i.e., greater sole angle). Conclusion: The finite element technique provides a useful step in bridging the gap between external measures and internal mechanics of the heel pad. A combined kinematic, kinetic, and modeling approach may be required when attempting to identify the biomechanical source of heel pain.