A Synthetic Digital Anatomy Approach to Determine the Effects of Tissue Composition and Alignment on Transfemoral Residual Limb Mechanics

The soft tissue envelope within the residual limb influences prosthetic fit, function, and user comfort. The purpose of this study was to examine the influence of residual limb tissue composition and skeletal alignment on its mechanical properties during axial loading. We hypothesized that deviations in the adipose and muscle volume of the limb, as well as frontal plane alignment, will alter the mechanical stiffness and energy loss. We constructed multi-material synthetic residual limb models with varying tissue compositions and mechanically tested them under cyclical axial compression. Stiffness was quantified as the change in force divided by the change in displacement over the entire loading response, and energy loss was quantified as the mechanical hysteresis of the force-displacement loop. A large (~35%) reduction in soft tissue volume, representative of the change from pre- to post-limb recontouring surgery, resulted in an increase in stiffness of 57.6% out-of-socket and 17.3% in-socket relative to the pre-surgery model. Small (±2.5%) changes in tissue composition produced up to an 18.6% change in stiffness and up to a 59.2% change in hysteresis relative to the nominal post-surgery model. Finally, alignment in the frontal plane (±4° and $\pm 8^{\circ }\text {)}$ had significant effects on stiffness and hysteresis both pre- and post-surgery. We expect that clinical and technical approaches that optimize these factors could potentially contribute to improvements in whole-body outcomes such as metabolic cost and user comfort. In addition, these testing and verification methods offer a non-invasive and powerful technique to test patient-specific residual limb properties.