Minimum indentation depth for characterization of 2nd sub-metatarsal head and heel pad tissue properties

Most in-vivo indentation techniques are limited by the lack of adequate indentation into the plantar soft tissue. The purpose of this study is therefore to assess the effect of deformation depth on plantar soft tissue behavior and to establish a guideline for the minimum indentation depth that is sufficient to quantify critical plantar soft tissue behavior. Twenty young subjects (20-25 years) participated in this study. The test was conducted with equal weight borne on each of the participants' feet to mimic the static stance of the gait cycle. During the experiment, the indenter probed the 2nd sub-metatarsal head (MTH) and heel pad tissue at a constant rate of 12.3 mm/s. The maximum tissue deformation induced was varied from 1.2mm to 6.0mm, in steps of 1.2 mm. The tissue stiffness obtained from the tissue response curves was compared and fitted to the proposed viscoelastic model. As the probe tip indents deeper into the plantar soft tissue beyond a threshold depth, X_s, the force gradient increases notably. The absolute value of X_s was approximately 2.23 mm and 2.14 mm at the heel and 2nd sub-MTH respectively. Indentation depths which were less than this threshold depth might not be representative of the nature of plantar soft tissue nor reflect the critical deformation it experiences during physical activities that expose the tissue to risk of ulceration. Our study indicated the necessity to induce a minimum tissue indentation depth in order to describe its actual characteristics. By doing so, additional useful parameters can be obtained to identify potentially abnormal soft tissue.