INTRODUCTIONOrthotic shoe insoles, which reduce elevated local pressures at the plantar foot/shoe interface, can be of substantial preventative and/or therapeutic value to a variety of patients (e.g., patients with diabetic neuropathy, who frequently demonstrate elevated plantar pressure, and patients with metatarsalgia). In this investigation, we assessed the efficacy of silicone gel-filled shoe insoles for relieving elevated plantar pressures, in a manner consistent with the potential mechanism by which these gel insoles can relieve plantar pressure.The mechanism by which the gel-filled insoles, that were used in this study, can relieve plantar pressure differs substantially from the mechanisms of other insole materials and designs. These gel insoles can maintain nearly uniform pressure within the fluid and transmit such pressure across the upper surface. Consequently, if without such an insole, elevated pressures exist adjacent to regions of low pressure, then a gel insole can reduce elevated pressures by distributing load equally across the contacting plantar surface. This mechanism for plantar pressure reduction, however, is effective only while fluid (i.e. silicone gel) is present beneath the entire contacting plantar surface. When no fluid exists beneath a portion of the contacting plantar surface, then that portion of the plantar surface is essentially in direct contact with the shoe sole, and substantial disparities in plantar pressure can persist.In addition to identifying peak plantar pressures at the heel and metatarsal head regions, our evaluation also involved computation and identification of peak plantar pressure gradients in the heel and forefoot regions, as well as the percentage of stance phase at which forefoot pressure gradients increased most dramatically. Plantar pressure data was collected with a device that measured pressure directly at the foot/insole interface within the shoe. Such data was believed to be more relevant to our purposes than data collected from a pressure platform.METHODOLOGYSeven young, healthy adults (3 males, 4 females, mean age = 26, mean weight = 142 lbs) were recruited for this study. We selected, for each subject, appropriate sized pairs of silicone gel-filled insoles (PGB Medical, Inc.), and pressure sensing inserts (PEDAR, Novel, Inc.). The quantity of gel within the gel-filled insoles was similar for all subjects. In order to negate potential effects of different footwear, each subject's shoes and socks were removed, and the pressure sensing inserts were taped directly to the bare plantar surfaces of each subject's feet. For each subject, we obtained pressure data for five steps under each of the following conditions: (1) slow cadence and (2) fast cadence while wearing only the pressure sensing insoles; and (3) slow cadence and (4) fast cadence with the gel-filled insoles taped over the pressure sensing insoles.Raw pressure data for each step of every subject was processed using software provided by the equipment manufacturer to obtain peak plantar pressures at the heel and five metatarsal heads (PPH, PPM1, PPM2, PPM3, PPM4, PPM5). For each footwear/cadence combination, the peak pressure data at the six regions were incorporated into a euclidean distance-base algorithm to select a representative trial. Each representative trial was processed further using ''in-house'' software to compute pressure gradients across the plantar surface for each frame of pressure data. Peak gradients at the heel and forefoot regions were identified at each frame; and these data were interpolated to 2% stance time intervals between 0% and 100% (Figure 1.). Overall heel and forefoot peak gradients (PGH and PGF) then were determined for each footwear/cadence combination. Additionally, the percentage of stance at which forefoot gradients appeared to rise most dramatically (TFGR) were determined by inspection of the interpolated plots (Figure 1.). Consequently, this final parameter was somewhat more subjective than the overall peak gradients. Using a regression technique, a two-way analysis of variance with repeated measures was performed for the parameters identified.RESULTS AND DISCUSSIONResults of the analysis of variance are summarized in the table below. These results indicate that peak pressures were not affected significantly by either insole use or cadence, except at the heel where peak pressure increased with increased cadence. Peak heel gradients were significantly higher for faster versus slower cadence. Peak heel gradients, however, were not affected by use of the gel-filled insole. Peak forefoot gradients were significantly lower with insole use. Peak forefoot gradients were not affected significantly by cadence. The percentage of stance phase, at which forefoot gradient appears to rise substantially, seems to be delayed somewhat by using a gel insole; however, this parameter was not affected by insole use or cadence.The results of this investigation suggest that these silicone gel insoles have little effect on the magnitude of peak plantar pressures, peak heel pressure gradients, or the time of forefoot gradient rise. Additionally, this work suggests that plantar pressure gradients may be added to the list of pressure parameter which are used commonly in clinical assessments (e.g. pressure-time-integral). Future work in this field should consider the interactive effects of different shoe wear used in conjunction with these gel-filled insoles.
