Mechanical performance of inflatable inserts used in limb prosthetics

Inflatable insert products used to overcome residual limb shrinkage were evaluated in a bench test environment under compressive loading conditions. Pressure-loss tests under static loading demonstrated that after inserts were inflated to 43.4 to 45.6 kPa, insert pressures reduced from 0.09%/min to 1.36%/min in the first 5 min and from 0.00%/min to 0.27%/min in the subsequent 55 min. As inserts were inflated, they demonstrated at least two phases in their pressure versus thickness curves: A relatively low-slope portion (Phase I) was followed by a high-slope portion (Phase II). The transition from Phase I to Phase II occurred at more than a 12-mm thickness, a thickness greater than that considered acceptable for practical clinical use (10 mm). This result suggests that in a socket, stress to resist insert expansion is taken by the residual limb and socket more than by the insert itself. Cyclic-loading tests under constrained thickness conditions demonstrated that local stiffness was more sensitive to insert pressure than to constraint spacing (insert thickness). The static and dynamic test results help to explain why some users claim that inserts do not provide equal and consistent support unless inflated to a very high insert pressure. An insert that allowed adjustment of the location of the Phase I to Phase II transition point in the pressure versus thickness curve might help to overcome these limitations.