Ideal operating conditions for a variable stiffness transverse plane adapter for individuals with lower-limb amputation

Abstract

Transverse plane shear stress between the prosthetic socket and residual limb often results in soft tissue breakdown and discomfort for individuals with lower-limb amputation. To better understand the effects of reduced transverse plane stiffness in the shank of a prosthesis, a second-generation variable stiffness torsion adapter (VSTA II) was tested with individuals with a transtibial amputation (n = 10). Peak transverse plane moments, VSTA II deflection, range of whole body angular momentum (WBAM), ground reaction impulse, joint work, and personal stiffness preference were evaluated at three fixed stiffness levels (compliant: 0.25 Nm/°, intermediate: 0.75 Nm/°, stiff: 1.25 Nm/°) at three walking speeds (self-selected, fast and slow: +/− 20% of self-selected, respectively) while straight-line walking and performing left and right turns. Residual limb loading decreased and VSTA II displacement increased for reductions in stiffness and both metrics increased with increasing walking speed, while ground reaction impulse and joint work were unaffected. The range of WBAM increased with decreased stiffness, which suggests an increased risk of falling when using the VSTA II at lower stiffness settings. Preference testing showed no significant result, but trends for lower stiffness settings when turning and walking at self-selected speeds were noted, as were stiffer settings when walking straight and at faster speeds. These results show that a device with rotational compliance like the VSTA II could reduce loading on the residual limb during straight walking and turning activities and that factors such as walking speed, activity type and user preference can affect the conditions for optimal use.