Effects of alignment changes on stance phase pressures and shear stresses on transtibial amputees: measurements from 13 transducer sites

Interface pressures and shear stresses were measured at 13 sites on two unilateral below-knee amputee subjects ambulating with lower-limb patellar-tendon-bearing prostheses. Interface stresses at the time of the first peak in the shank axial force-time curve were investigated at different socket-shank alignment settings. Stress magnitudes ranged from 1.2 to 214.7 kPa for pressure and 0.4 to 79.6 kPa for resultant shear stress, and changes in stress due to misalignment ranged from 1.3 to 80.7 kPa for pressure and from 0.2 to 38.0 kPa for resultant shear stress. For both subjects interface stress changes were much greater in the anterior socket region than in the lateral or posterior regions. Thus, alignment changes had a localized effect on interface stresses. Plots of alignment versus pressure or resultant shear stress were nonlinear for both subjects, in a number of cases maximizing or minimizing at the nominal alignment, indicating complex interface stress-alignment relationships. Variation (standard deviation/mean) was not significantly different for nominal versus misaligned steps, indicating that the subjects adapted well to the alignment changes. Session to session differences in interface stresses were typically larger than interface stress differences induced by alignment modifications. Thus, while these subjects compensated well for alignment changes to maintain consistent interface stresses within a session, they did not do so for different sessions conducted weeks apart.