Effect of loading history on material properties of human heel pad: an in-vivo pilot investigation during gait.

BACKGROUND: This study was aimed to develop a novel dynamic measurement technique for testing the material properties and investigating the effect of continuous compression load on the structural and mechanical properties of human heel pad during actual gait. METHODS: The dual fluoroscopic imaging system (DFIS) and dynamic foot-ground contact pressure-test plate were used for measuring the material properties, including primary thickness, peak strain, peak stress, elastic modulus, viscous modulus and energy dissipation rate (EDR), both at time zero and following continuous loading. Ten healthy pilot subjects, aged from 23 to 72 (average: 46.5 ± 17.6), were enrolled. A "three-step gait cycle" is performed for all subjects, with the second step striking at a marked position on the force plate with the heel to maintain the location of the tested foot to be in the view of fluoroscopes. The subjects were measured at both relaxed (time-zero group) and fatigue (continuous-loading group) statuses, and the left and right heels were measured using the identical procedures. RESULTS: The peak strain, peak stress, elastic modulus, and EDR are similar before and after continuous load, while the viscous modulus was significantly decreased (median: 43.9 vs. 20.37 kPa•s; p < 0.001) as well as primary thicknesses (median: 15.99 vs. 15.72 mm; p < 0.001). Age is demonstrated to be moderately correlated with the primary thicknesses both at time zero (R = -0.507) and following continuous load (R = -0.607). The peak stress was significantly correlated with the elastic modulus before (R = 0.741) and after continuous load (R = 0.802). The peak strain was correlated with the elastic modulus before (R = -0.765) and after continuous load (R = -0.801). The correlations between the viscous modulus and peak stress/ peak strain are similar to above(R = 0.643, 0.577, - 0.586 and - 0.717 respectively). The viscous modulus is positively correlated with the elastic modulus before (R = 0.821) and after continuous load (R = 0.784). CONCLUSIONS: By using dynamic fluoroscopy combined with the plantar pressure plate, the in vivo viscoelastic properties and other data of the heel pad in the actual gait can be obtained. Age was negatively correlated with the primary thickness of heel pad and peak strain, and was positively correlated with viscous modulus. Repetitive loading could decrease the primary thickness of heel pad and viscous modulus.

Biomechanical assessment of two types and two different locations of subtalar arthroereisis implants for flexible flatfoot: A cadaveric study.

BACKGROUND: Subtalar arthroereisis refers to the implantation of a sinus tarsi implant for the treatment of flexible flatfoot. The purpose of this study was to compare the ability to correct the flatfoot deformity and contact pressure of the posterior subtalar joint between two types of self-locking wedge implants and between two different positions for the same device in a cadaveric flatfoot model. METHODS: The flatfoot model was created in ten cadaver feet through ligament sectioning and cyclic loading. Three kinds of arthroereisis procedures were evaluated: Talar-Fit (type I self-locking wedge implant) anchored in the sinus portion of the tarsal sinus (T-sinus group), Talar-Fit in the canalis portion (T-canalis group), and HyProCure (type II) in the canalis portion (H group). Corrective ability in the sagittal and transverse planes were measured with clinometers. Contact pressure was measured with pressure-sensitive films. FINDINGS: T-canalis group provided more sagittal (mean difference for size 10 mm: 1.9°, P = 0.014; mean difference for size 11 mm: 3.1°, P = 0.037) and transverse (mean difference for size 8 mm: 1.8°, P = 0.049; mean difference for size 11 mm: 2.2°, P = 0.049) corrections than T-sinus group. The flattening process shifted the peak pressure of the posterior subtalar joint to the posteromedial side (P < 0.05) and arthroereisis helped the distribution of contact pressure restore uniformity (all P > 0.05). INTERPRETATION: A self-locking wedge implant inserted in the canalis portion of the tarsal sinus achieved better correction than an implant inserted in the sinus portion.

An Investigation of Regional Plantar Soft Tissue Hardness and Its Potential Correlation with Plantar Pressure Distribution in Healthy Adults.

BACKGROUND: The plantar soft tissue plays a critical role in absorbing shocks and attenuating excessive stresses during walking. Plantar soft tissue property and plantar pressure are critical information for footwear design and clinical assessment. The aim of this study was to investigate the relationship between plantar soft tissue hardness and plantar pressure during walking. METHODS: 59 healthy volunteers (27 males and 32 females, aged 20 to 82) participated in this study. The plantar surface was divided into five regions: lateral rearfoot, medial rearfoot, lateral midfoot, lateral forefoot, and medial forefoot, and the plantar tissue hardness was tested using Shore durometer in each region. Average dynamic pressures in each region were analyzed for the five regions corresponding to the hardness tests. The relationship between hardness and average dynamic pressure was analyzed in each region. RESULTS: The average hardness of the plantar soft tissue in the above five regions is as follows: lateral rearfoot (34.49 ± 6.77), medial rearfoot (34.47 ± 6.64), lateral midfoot (27.95 ± 6.13), lateral forefoot (29.72 ± 5.47), and medial forefoot (28.58 ± 4.41). Differences of hardness were observed between age groups, and hardness of plantar soft tissues in forefoot regions increased with age (P < 0.05). A negative relationship was found between plantar soft tissue hardness and pressure reduction at lateral rearfoot, medial rearfoot, and lateral midfoot (P < 0.05). CONCLUSION: The hardness of plantar soft tissues changes with age in healthy individuals, and there is a trend of increasing hardness of the plantar soft tissue with age. The plantar soft tissue hardness increases with plantar pressure.