# Experimental Analysis of Plantar Fascia Mechanical Properties in Subjects with Foot Pathologies
## 4 Discussion  
In this study, the mechanical response of PF is investigated by experimental tests on the nonlinear stress-strain and time-dependent behavior, to understand if the Ledderhose disease and diabetes can affect PF biomechanics. Concerning the nonlinear stress-strain response, the PF is stiffer in patients affected by Ledderhose disease and diabetes, with respect to healthy subjects. Indeed, secant elastic moduli E\({}_{\text{S}}\) reach considerable values in some PF samples (up to about 30 MPa in L2 and about 26 MPa in D4, see Table 2), while they are limited in the range between 10 and 14 MPa in healthy subjects. The same comparison can be gathered from Figure 6a, where the values of mean E\({}_{\text{S}}\) are reported for each subject. This difference is statistically significant, as highlighted by ANOVA, with a source in the difference in the comparison of subjects affected by Ledderhose disease with healthy ones. Previous studies hypothesized a possible correlation of PF stiffness with BMI [26] and PF thickness [21], which is here analyzed in Figure 6c,d. In these charts, the mean values for all the subjects are plotted together, irrespectively of the pathology, since data from the three groups are randomly positioned and not well grouped in distinct regions of the plot. Based on the experimental data of the present work, it appears that patients with greater BMI have tissue with slightly higher stiffness than patients with lower BMI, even if a good correlation is not found. Regarding the correlation of PF stiffness with thickness, this is absent in the present study. Since the subjects included in the healthy group are significantly elderly with respect to subjects affected by Ledderhose disease and aging is generally considered responsible for variation in tissue mechanical properties [33], a possible correlation of PF stiffness with age is investigated. However, no correlation is found in the present work and this allows one to deduce that significant stiffness differences are mostly related to the pathology and not to the age of the subjects. Figure 6: Comparison among PF samples from subjects with Ledderhose disease (L), diabetes (D) and healthy subjects (H): comparison of secant moduli E\({}_{\text{S}}\) (**a**); comparison of normalized stress value Pnorm after 300 s of stress relaxation (**b**); comparison of age (**c**); correlation between secant modulus Es and BMI (**d**); correlation between secant modulus E\({}_{\text{S}}\) and PF thickness t (**e**); correlation between secant modulus E\({}_{\text{S}}\) and age (**f**). Differences among the data are indicated as statistically significant with \(p\)-value < 0.05 (*) or \(p\)-value < 0.01 (**). Concerning the time-dependent behavior, the PF of healthy subjects shows a lower stress reduction than the PF taken from patients with Ledderhose syndrome or affected by diabetes. A statistically significant difference is found between diabetic and healthy subjects. In the literature there are very limited data on the viscoelastic properties of plantar aponeurosis for the comparison of the findings of this work with healthy patients. In a previous work [34], stress-relaxation phenomena were evaluated on plantar aponeurosis tissue from donors with age ranging from 67 to 78 years and not reporting specific foot pathologies. At 240 s of relaxation time, an average decrease of 40% (confidence interval: 38% \(\div\) 42%) was estimated. This value is in line with that obtained in the present work, where a mean stress reduction of 44% and 33% of the initial stress value was measured at 240 s, respectively, in H1 and H2, representing the samples with higher and lower stress reduction in the healthy subjects' group. The findings of this study have to be seen in the light of some limitations. First, the number of involved subjects is limited.