Shoe inlays (insoles) have become popular and can be purchased at most pharmacies and supermarkets. These do not need to be prescribed and in the simplest form offer comfortable cushioning. Insoles are made from a wide range of materials offering additional properties including shock absorption and insulation. Prices vary with the usual rule of thumb, ‘you get what you pay for. ’ Expense may equate to longevity, but not in all cases. On average over the counter shoe inlays will last approx., three months of normal wear before they bottom out. Some, usually the cheaper ones degrade more quickly. The following is a brief review of some of the materials commonly used in over the counter insoles.
The most common material used in over the counter insoles is cellular foams. There are two types, Open Cell and Closed Cell. The former is soft and spongey and closed cell foams are more rigid. Compressed open cell foam will almost return to original shape (elastic memory) whereas closed cell foams are resistant to compression and hence less comfortable for insoling material. Density determines longevity but because elastic memory of open cell foam is poor, repeated compression degrades the material quickly. New open cell insoles are very comfortable but soon degrade with regular use.
Plastazote® (expanded polyethylene) makes an ideal shoe insert and almost all trainers come with a standard insock made of Plastazote®. Manufacturers usually cover the material with a sock lining which reduces some of the benefits of the material such as insulation, but this gives the manufacturer a platform to advertise their wares.
Expanded polyethylene's properties are pretty unique and because the material is a poor conductor of heat this insulates the skin surface keeping the foot at a constant temperature making them ideal for both cold and hot climates. The material is waterproof so will not degrade in the presence of moisture. The non-cellular material exhibits thermoplastic properties which under pressure and heat plastically deform to give a perfect foot bed. Whilst the material condenses under pressure points it also reinforces its cross-mesh structure. Simply put, this has a water bed effect which reduces peak forces over bony areas (isotactic properties). Plastazote ® is cheap and can be easily replaced when signs of wear become apparent. Fitting a new insole is a simple process.
Space Age polymers are man-made solids that contain both fluid and gas which makes visco-elastic materials truly out of this world material. Pressures cannot pass through either fluid or gas which makes visco-elastic material ideal for dampening down peak shock at heel strike. Developed to enable space travel, the synthetic polymers have now been incorporated into shoe and insole manufacture. Over the counter precut insoles of visco-elastics such as Poron®, are available but are more expensive than simple cushioning devices.
By combining these materials the therapeutic effect of the insole can be enhanced Many commercial incoles are now available over the counter.