Protection of The Foot Tissues from Excessive Plantar Pressure

Historically, protection of the foot tissues from excessive plantar pressure was archived by use of wide available materials such as wood, cork and leather. With advances in material sciences in last four decades, a wide range of materials have been introduced in the market, including foam rubbers, plastics, warp-knitted fabrics and cellular polymers that possess suitable properties and characteristics for use in off-loading insoles. Latter the main properties characteristic for each material type are discussed. For clarity and easier comprehension of results all commercial materials mentioned in present report are grouped according to their polymeric nature

Foam rubbers

Rubber can be obtained in nature or be synthetically manufactured. Synthetic rubber materials are commonly used in numerous orthotic devices. Natural latex is lightweight and very soft but prone to rapid and permanent shape deformation. Orthopaedic insoles made of latex foam are mainly used to provide additional cushioning and to temporarily relieve pressure and painful symptoms. However, latex readily stains and deteriorates in practical uses so that the insoles easily ‘bottom out’ under low loads, which result in little cushioning or shock absorption for the wearer.

Neoprene is a special type of synthetic rubber made of poly(chloroprene) and it is primarily known for its use as an inlay or insole material in the form of either a closed-cell foam or an open-cell foam. Neoprene can reduce friction that is why It is often chosen as a covering material for sport orthoses when friction and shear forces are expected to be high. However, it has poor ventilation and heat retaining properties. Recently, neoprene cover of insoles was reported as a cause of allergy contact dermatitis. Thiourea compounds contained within neoprene can trigger allergic reactions as severe itching, eczematous eruptions, and in some cases blistering as reported by Hawkey et all., 2015 .

Nature-originated materials

The most commonly used natural materials are leather(6), cork and felted fabrics(7). Most of them possess many desirable characteristics for use in orthopaedic insoles, such as wide availability, light weight, low cost, air and water permeability, and ability to be easily shaped. Leather is extremely durable and conforms to the shape of the foot. It is not only used as an interface material for orthopaedic insoles to protect the skin from irritation, but also for supportive purposes when strength and resiliency are required. It is a frequently used material for many foot orthoses and shoe modifications due to its superior breathability characteristics.

Cork is a lightweight cushioning material made from the outer bark of a tree. It is usually used in combination with leather due to its rough surface and irritation to the skin. Currently, there are a new composite material that utilise both properties of the cork and polymer matrix adds extreme durability. Felted materials such as wool felt are adopted in orthopaedic insoles because of their softness, smoothness, resilience and excellent shock absorption properties. They also have the advantage of being easily moulded into shapes and give a great deal of warmth.

Cellular polymers

Cellular polymers can be made from a vast array of materials, such as polypropylene (PP), polyethylene (PE), ethylene vinyl acetate (EVA), poly(vinyl)chloride (PVC), polyurethane (PU) and segmented poly(ether) urethane (SPEU). They are generally available in a wide range of harnesses, thicknesses and densities, with structural and mechanical properties of diverse usefulness. Many cellular polymer materials are patented, and may cost up to USD$300 per sheet. The choice of materials with suitable structural and cushioning properties is closely associated with the intended use and efficacy of the orthopaedic insoles. PP and PE are both flexible olefin polymers that resist breakage. The materials are soft, tend to be non-brittle and heat-formable in applications, and offer reasonable cushioning and shock absorption performances.