A preliminary study of patient-specific mechanical properties of diabetic and healthy plantar soft tissue from gated magnetic resonance imaging

Subject-specific material properties of the heel pad: An inverse finite element analysis

Individuals with diabetes are at a higher risk of developing foot ulcers. To better understand internal soft tissue loading and potential treatment options, subject-specific finite element (FE) foot models have been used. However, existing models typically lack subject-specific soft tissue material properties and only utilize subject-specific anatomy. Therefore, this study determined subject-specific hindfoot soft tissue material properties from one non-diabetic and one diabetic subject using inverse FE analysis. Each subject underwent cyclic MRI experiments to simulate physiological gait and to obtain compressive force and three-dimensional soft tissue imaging data at 16 phases along the loading-unloading cycles. The FE models consisted of rigid bones and nearly-incompressible first-order Ogden hyperelastic skin, fat, and muscle (resulting in six independent material parameters). Then, calcaneus and loading platen kinematics were computed from imaging data and prescribed to the FE model. Two analyses were performed for each subject. First, the skin, fat, and muscle layers were lumped into a single generic soft tissue material and optimized to the platen force. Second, the skin, fat, and muscle material properties were individually determined by simultaneously optimizing for platen force, muscle vertical displacement, and skin mediolateral bulging. Our results indicated that compared to the individual without diabetes, the individual with diabetes had stiffer generic soft tissue behavior at high strain and that the only substantially stiffer multi-material layer was fat tissue. Thus, we suggest that this protocol serves as a guideline for exploring differences in non-diabetic and diabetic soft tissue material properties in a larger population.

Changes in sub-calcaneal fat pad composition and their association with dynamic plantar foot pressure in people with diabetic neuropathy

BACKGROUND

Diabetic foot disease is associated with physiological and biomechanical abnormalities in the foot that increase risk for ulceration. The objective was to assess MRI changes in the composition of sub-calcaneal fat pad tissue and its association with plantar pressure during walking.

METHODS

Fourteen people with diabetes and peripheral neuropathy and five age-matched healthy controls underwent T1-weighted sagittal plane spin-echo Dixon MRI of the rearfoot. Dixon Chemical Shift Imaging was used to create fat-only and water-only images from which the fat signal fraction in a defined ROI of the sub-calcaneal fat pad was calculated. Barefoot plantar pressure distribution during walking was assessed and associated with fat pad outcomes.

FINDINGS

Mean ± SD fat signal fraction was significantly lower in the neuropathic subjects than in the healthy controls (0.55 ± 0.11 vs. 0.72 ± 0.03, p < 0.005), and was explained by a lowering in fat signal (R² 0.87), more than an increase in water signal (R² 0.32). Mean ± SD peak pressure at the heel was 391 ± 119 kPa for the neuropathic subjects and 325 ± 53 kPa for the healthy controls (non-significantly different). Fat signal fraction and peak pressure were significantly inversely correlated (r = -0.59, p < 0.01).

INTERPRETATION

Dixon chemical shift MRI showed a reduced fat signal fraction in sub-calcaneal fat pad tissue in people with diabetic neuropathy. Both neuropathic and non-neuropathic factors may be attributed to this outcome. Fat pad function also seems to be compromised, as indicated by an associated increase in peak plantar pressures. This may increase risk for foot ulceration.

Using Elastographic Ultrasound to Assess Plantar Tissue Stiffness after Walking at Different Speeds and Durations

Exercise has been demonstrated to improve health in people with diabetes. However, exercise may increase risk for foot ulcers because of increased plantar pressure during most weight-bearing physical activities. To date, there is no study investigating the effect of various walking speeds and durations (i.e., the most common form of exercise in daily living) on the plantar foot. The objective of this study was to investigate the effect of various walking intensities on plantar tissue stiffness. A 3 × 2 factorial design, including three walking speeds (1.8, 3.6 and 5.4 mph) and two durations (10 and 20 min), was tested in 12 healthy participants. B-mode and elastographic ultrasound images were measured from the first metatarsal head to quantify plantar tissue stiffness after walking. Two-way ANOVA was used to examine the results. Our results showed that the walking speed factor caused a significant main effect of planar stiffness of the superficial layers (p = 0.007 and 0.003, respectively). However, the walking duration factor did not significantly affect the plantar stiffness. There was no interaction between the speed and duration factors on plantar tissue stiffness. Regarding the walking speed effect, there was a significant difference in the plantar stiffness between 1.8 and 3.6 mph (56.8 ± 0.8% vs. 53.6 ± 0.9%, p = 0.017) under 20 min walking duration. This finding is significant because moderate-to-fast walking speed (3.6 mph) can decrease plantar stiffness compared to slow walking speed (1.8 mph). This study suggests people at risk for foot ulcers walk at a preferred or fast speed (3.6 mph) rather than walk slowly (1.8 mph).