The shear mechanical properties of diabetic and non-diabetic plantar soft tissue

Abstract

Changes in the plantar soft tissue shear properties may contribute to ulceration in diabetic patients, however, little is known about these shear parameters. This study examines the elastic and viscoelastic shear behavior of both diabetic and non-diabetic plantar tissue. Previously compression tested plantar tissue specimens (n = 54) at six relevant plantar locations (hallux, first, third, and fifth metatarsal heads, lateral midfoot, and calcaneus) from four cadaveric diabetic feet and five non-diabetic feet were utilized. Per in vivo data (i.e., combined deformation patterns of compression followed by shear), an initial static compressive strain (36–38%) was applied to the tissue followed by target shear strains of 50% and 85% of initial thickness. Triangle waves were used to quantify elastic parameters at both strain levels and a stress relaxation test (0.25s ramp and 300s hold) was used to quantify the viscoelastic parameters at the upper strain level. Several differences were found between test groups including a 52–62% increase in peak shear stress, a 63% increase in toe shear modulus, a 47% increase in final shear modulus, and a 67% increase in middle slope magnitude (sharper drop in relaxation) in the diabetic tissue. Beyond a 54% greater peak compressive stress in the third metatarsal compared to the lateral midfoot, there were no differences in shear properties between plantar locations. Notably, this study demonstrates that plantar soft tissue with diabetes is stiffer than healthy tissue, thereby compromising its ability to dissipate shear stresses borne by the foot that may increase ulceration risk.

A stereologic study of the plantar fat pad in young and aged rats

“…The IPFP is a seemingly random collection of adipocytes distributed in the loose connective tissue. It lacks the specialized organization of adipocytes with accompanying neural and vascular structures contained and separated by septa as in the PFP, observed in this study and by Pai & Ledoux (2012). The histologic organization is responsible for the mechanical properties of the PFP (Jahss et al 1992) and distinguishes the PFP from other fat pads in the body -even ones that undergo cyclical loading.…”

The Effect of Prior Compression Tests on the Plantar Soft Tissue Compressive and Shear Properties

“…This study demonstrates that prior compression testing of the plantar soft tissue may alter the compressive properties, but further investigation is needed. Of note, the shear properties were not affected by prior testing in compression, indicating that previously compression tested specimens [5] should provide representative shear properties. However, these results do not necessarily apply to diabetic specimens which were not tested in this study.…”

“…Furthermore, since only nondiabetic specimens were examined, it remains unclear whether these results may be extrapolated to diabetic tissue which may or may not be at greater risk of tissue deterioration and hence alterations in mechanical properties with excessive testing. We stipulate that our results may apply to diabetic tissue since prior compression testing did not diminish the anticipated increased tissue stiffness in shear for diabetic versus nondiabetic tissue [5].…”

“…Similar to Part A, nonlinear elastic parameters were computed except due to differently shaped loading curves, instead of a single modulus, three moduli were computed to accommodate the Sshaped curve [5]: initial modulus (to capture the short initial highly stiff region at low strains), toe modulus (to capture a long low stiffness toe region), and final modulus (to capture a final stiff region at higher strains). Additionally, the relaxation data were used to quantify viscoelastic parameters including the relaxation rate (slope of the force-time curve) at three time intervals, initial (t ¼ 0-0.5 s), mid (t ¼ 10-15 s), and final (t ¼ 290-300 s), and the relative total relaxation (normalized curve area).…”

“…Towards this goal, our group has evaluated diabetic changes in the compressive [2,3] and histomorphological [4] properties of the plantar soft tissue. We also recently obtained the corresponding shear properties from the specimens previously tested in compression [5]. However, it is unclear whether these shear properties are representative since prior compressive testing may have altered the tissue.…”