Effects of fluid insert volume changes on socket pressures and shear stresses: case studies from two trans-tibial amputee subjects.

Fluid inserts potentially help to overcome prosthetic fit problems resulting from stump volume change. The purpose of this investigation was to add fluid to fluid inserts positioned on the inner socket walls of trans-tibial prostheses and to assess their influence on socket stresses. Pressures and shear stresses were measured at 13 sites on the sockets of two trans-tibial amputee subjects while they ambulated at their self-selected walking speeds. Stresses at the transducer sites generally increased with greater fluid addition and, interestingly, both subjects found relatively high fluid insert volumes most comfortable. The magnitudes of stress change were larger than those resulting from alignment, cadence, and componentry changes as reported in the literature. Possible explanations for why subjects found settings that induced higher measured socket stresses more comfortable than those that induced lower measured stresses include: A reduced shear: pressure ratio; the short duration of the study; and reduced stresses at sites not monitored with transducers.