Foot orthoses serve as important modalities for foot care in the diabetic patient. Their range of indications, while including the myriad of biomechanical pathologies commonly found in the human foot, is significant in preventing skin breakdown which can lead to infection and amputation. There is a direct relationship between high plantar pressures in the diabetic foot and the development of foot ulcers. During normal walking, as the body weight transfers from one foot to another, pressure equal to sixty percent body weight is thrust upon the plantar tissues of the foot in 0.02 seconds. These repetative forces, in combination with peripheral neuropathy, ischemia, foot deformity, or limited joint mobility, all common in diabetes, could lead to tissue breakdown and foot ulcerations. Problems such as blisters, corns, and calluses should be considered suspect, because this is where ulcers often begin. The presence of diabetes also appears to result in the increased prevalence of pedal fractures, especially in those individuals with a duration of diabetes greater than 25 years1.
Orthotic devices can be used to minimize the magnitude of the compression and shear to the foot as it comes in contact with the ground during ambulation. These devices can be incorporated as an interface between the plantar surface of the foot and the shoe sole. They provide shock attenuation, friction reduction, and weight dispersion to protect the normal, and especially the denervated, foot from these harmful forces, thus relieving pain and preventing potentially serious medical problems. Orthotic devices can also be used as an adjunct in the treatment of a diabetic ulcer as well as a platform for the fabrication of a shoe filler in cases of partial foot amputations.
When electing to utilize a foot orthosis in treatment of a patient with diabetes, the patient's level of risk must be assessed in order to properly select both the materials and design of the device. An orthotic device that may be helpful for a specific biomechanical pathology may be inappropriate or even detrimental if that biomechanical pathology is accompanied by poor pedal circulation or peripheral neuropathy.
Assessing a Patient's Level of Risk
A thorough lower extremity evaluation of the diabetic patient is an essential part of determining their level of pedal risk. The evaluation should include a thorough medical history, the detection of peripheral neuropathy, circulatory compromise, as well as their extremity structural (biomechanical) abnormalities. A history of prior infections or ulcerations of the foot has an important predictive value with regard to the potential for future ulcerations and other pedal complications. Structural pedal malalignments must be recognized and addressed in the diabetic, especially in the presence of sensory loss. Conditions such as hallux valgus, hammertoes, and plantar flexed metatarsals create areas of increased pressure, and potential tissue breakdown. Joint ranges of motion must also be assessed, since decreased joint mobility increases the risk of pedal ulcerations.
Selecting Orthotic Materials and Design
Polyethylene Foams
Polyethylene foams make up the most common group of materials used in the fabrication of pressure reducing insoles and orthoses for the diabetic patient. Included in this category are Plastizote and Evazote (Bakalite Xylonite, LTD (BXL), UK), Pelite, and Aliplast (Alimed, Needham, MA). These materials are closed cell foams that are nontoxic, resistant to chemicals and fluids, light in weight, and when heated to suitable temperatures, are moldable. They are readily washable and discourage bacteria growth. Orthotic shells may be fabricated using one type of foam, or by laminating different foams together.
Plastizote, the most commonly used and well known of the polyethylene foams, originally received fairly extensive clinical use in the 1960's by Dr. Paul Brand for the treatment of the insensate foot of leprosy patients. It was used as a shoe insert, or more frequently constructed into a shoe or sandal. It offered protection by eliminating high pressure points to the feet which have lost all sensation, as well as to feet which have actually undergone ulcerations. Plastizote foams are known for their memory or ability to retain moldable shape, as well as their smooth dry feel. They are generally categorized according to durometer or stiffness. The medium and firm durometers are commonly used for accommodative and dynamic inlays, while the rigid durometer is used for a semi functional orthotic device2.
Applications of polyethylene foams include the fabrication of orthoses for the relief of pain associated with burns, plantar fascial strain, synovitis, hyperesthesia, and high pressure points of the deformed foot. They are also used in the treatment of skin breakdown as seen in diabetic foot ulcers and as fillers on insoles for partial foot amputations.
While these polyethylene foams are excellent materials for orthotic fabrication, they are poor shock attenuators. After a period of time, they will experience a loss of thickness due to both compression and shear.compression stress and must be replaced. The clinical lifetime of these foams is affected by their ability to dissipate the force of walking through their compression rather than the breakdown of the plantar skin. For this reason, these foams are often combined with other materials3.
Spenco
In an attempt to produce an insole material that would better absorb lateral and oblique forces and decrease the shearing forces on skin which resulted in blisters, Wayman Spence, M.D. and Marlin Shields, P.T., developed Spenco (Spenco Medical Corporation, Waco, TX). Spenco is a neoprene closed cell foam with entrapped nitrogen bubbles and a nylon (polyamide) top cover. Insoles of this material are currently available in 1/8, 1/4, and 3/32 inch thicknesses. They absorb vertical forces, torque, and fore, aft, and lateral sheer, thus are useful in preventing neuropathic and rheumatoid ulcerations by reducing the increased plantar foot pressure responsible for skin breakdown. Spenco is commonly used as a durable as well as shock absorbing top cover on both custom and pre-fabricated orthotic devices4.
Poron
Poron (The Rogers Company, CT) is an open cell polyurethane foam made from a combination of polyether and polyester resins. The Langer Biomechanics Group was the first to medically market this foam and distributed it under the name of PPT (Professional Protective Technology). It is currently distributed by many vendors. Poron is used whenever the reduction of pressure from static and dynamic forces are to be achieved, as in the production of foot orthoses, prostheses, or shoe inlays. These polyurethane foams are stable in form; they are elastic, shock absorbing, odorless, and washable.
As an open cell foam, medical grade Poron dissipates heat well and, by allowing water vapor transmission, enables perspiration to be transmitted away from the foot. This open cell nature also allows the air to circulate according to the weight lying on the material, thus retaining its form stability and always returning to its original position, even after permanent load. When used for a shock attenuating, soft tissue supplement for the plantar foot, it is available in thicknesses ranging from 1/16 to 1/2 inch in a single durometer. The material is manufactured both perforated and non-perforated with a variety of surfaces and top covers. It can be used for both custom and prefabricated orthoses, either as a filler or as a top cover5.
Leather
Leather orthotic devices can be either functional or accommodative depending on material combinations and casting techniques. Leather devices may include various materials from firm to soft depending on the needs of the patient. The basic shell of a leather orthosis consists of adding layers of leather to one another to form a lamination that can be shaped to a positive cast of the patient's foot. The advantage of leather devices include the fact that leather conforms to foot contours and deformities and can be easily modified to disperse pressure over bony prominences. In order to give leather orthoses more shock attenuation properties, the underside of a leather shell is often filled with other softer materials. Rubber butter is commonly used in the fabrication of leather orthoses for this purpose. Rubber butter is a generic substance formulated by mixing liquid latex with either cork, wood, or leather shavings, each producing a slightly different material. The rubber butter which is composed of latex and leather grindings has more shock attenuation than the latex and cork mixtures, however, because of the leather content, this rubber butter is more susceptible to breakdown2.
Silicone
Silicone rubbers are compositions containing a high molecular weight dimethyl silicone linear polymer. These materials offer excellent viscoelastic properties capable of providing shock absorption by dissipating the energy invading the body during locomotion. They accomplish this task without bottoming out as quickly as other materials used for the same purpose, thus making them a good substitute for the natural viscoelastic body tissues in the manufacture of orthoses. Devices made from silicone can be heated and cooled without drying or cracking. They maintain both their shape and viscoelastic properties over wide ranges of temperature.
Other important characteristics include a high degree of chemical inertness, which allows the silicone devices to be soaked in many types of solutions for both cleaning and disinfecting without any loss of function. This chemical inertness also reduces the chances of allergies upon contact with the skin and does not support bacterial growth or odor. A variety of both heel and full foot prefabricated silicone orthoses are currently on the market. Although they differ somewhat in appearance, they are all designed primarily for both shock attenuation and weight dispersion6.
Polyethylene Thermoplastics
Polyethylene thermoplastics possess the properties of toughness and flexibility with good dimensional stability, are heat moldable and lightweight. They are classified as being low, medium, high, or ultra-high density. In the 1960's, polyethylene thermoplastics became available for medical use with the development of Ortholen and Sub-Ortholen by Teufel.
Ortholen is an extremely high density thermoforming PE with a molecular weight of one million. It is a very tough material, being resistant to chemical erosion and non-brittle at low temperatures, as well as being easily adjustable with heat. It is available in thicknesses of 3 to 6 mm and is heat molded by heating to 350 degrees F for 10 to 15 minutes. Orthoses fabricated out of Ortholen are indicated when a functional or semifunctional device is desired as it will provide efficient pronation control. Sub-Ortholen is an ultra-high density thermoforming PE with a molecular weight of 500,000. It is similar to Ortholen, but has less toughness and strength, although is easier to form under heat. It is available in thicknesses of 1 to 6 mm2.
When choosing a more rigid orthosis made from these materials for a patient with diabetes, the patient's level of risk must be carefully considered. By adding an appropriate shock absorbing top cover, however, many diabetic patients can tolerate a more functional device, thus striking a balance between controlling the patient's biomechanical problems and creating a device that can be tolerated.
With nearly 16 million diabetics in the United States, a podiatrist will most likely be placed in the position of treating diabetic patients with all types of biomechanical needs. The thorough knowledge of both orthotic materials and design will not only allow the practitioner to address these needs in an efficient and effective manner, but also to reduce the ever present risk of pedal tissue breakdown, infection, and amputation.
REFERENCES
1. Wolf SK. Diabetes mellitus and predisposition to athletic pedal fracture. The Journal of Foot and Ankle Surgery. 37(1): 16-22, 1988.
2. Levitz SJ, Whiteside LS, Fitzgerald TA. Biomechanical foot therapy. Clinics in Podiatric Medicine and Surgery. 5(3): 721-736. July 1988.
3. Brodsky J, Kourosh S, Stills M, Mooney V. Objective evaluation of insert material for diabetic and athletic footwear. Foot and Ankle. 9(3): 115, December 1988.
4. Spence WR, Shields MN. New insole for prevention of athletic blisters. Journal of Sports Medicine. August 1968.
5. Jones LS, Caselli MA. The foam zone. Biomechanics, January 1996, pp. 73-77.
6. Caselli MA. Silicone. Biomechanics, January 1995, pp. 55-58.