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The Role of Orthotics in the Management of the Diabetic Foot PDF Print E-mail

Debra L. Bartley, BSc, M.D.
Section of Orthopaedic Surgery, Department of Surgery
University of Manitoba
Winnipeg, MB

Introduction
The foot of the person with diabetes is at risk for trauma because of loss of protective sensation. When this loss of protective sensation is combined with vascular insufficiency and pre-existing or new morphologic abnormalities, serious complications such as skin ulceration, skin and soft tissue infections, osteomyelitis, fractures and Charcot arthropathy may result. Ultimately, these complications may lead to infections that cannot be treated resulting in the amputation of digits, part of the foot, or the entire lower extremity. Before these complications occur, measures must be taken to protect the neuropathic foot. In addition to appropriately fitted footwear, orthotics should be considered.  

The custom-moulded orthotic is recognized and recommended as an adjunctive measure in the management of the diabetic foot1,2. The custom-moulded orthotic functions by redistributing pressure, absorbing compressive or shear forces, reducing internal mechanical stress and immobilizing or controlling motion of the foot and ankle. The goal is to prevent trauma, foot ulceration and associated morbidity, specifically amputation of digits and lower extremities3.

Although the use of custom-moulded orthotics reduces the recurrence rate of neuropathic ulcerations in the feet of persons with diabetes from 83% to 26%, evidence does not show that custom-moulded insoles prevent the initial neuropathic ulceration from occurring2. Patient compliance with both protective footwear and custom-moulded insoles is critical for the prevention of ulceration recurrence. When patients wore their protective footwear including insoles, for greater than 60% of the day, the rate of ulcer recurrence was significantly reduced by 50% compared to shorter wearing times4.

Why are Diabetic Feet at Risk?
Normal feet are able to resist the cyclic mechanical stresses of normal gait including friction, plantar pressure and compression/shear forces. Friction may lead to blister formation when feet are subjected to constant, rapid friction forces and to calluses or corns when subjected to slow and intermittent friction5. Plantar pressure is equal to the weight-bearing forces divided by the total contact surface area5. Constant pressure on an area can cause localized ischemia and eventually necrosis of the tissue. It is important to minimize shear and compressive forces to minimize the risk of ulcer occurrence5.

In the diabetic foot, alteration of these mechanical stresses due to bony abnormalities, limited joint mobility and skin changes leads to increased plantar pressure, intolerance of friction and abnormal shear and compressive forces3. Increased plantar pressure may lead to initial foot ulceration and to recurrent ulcers due to local ischemia and tissue destruction3. The causes of increased plantar pressure include: increased body mass and structural abnormalities producing plantar prominences that alter the normal contact surface area of the foot3. Classic examples of structural abnormalities include claw toes, prominent metatarsal heads and charcot midfoot changes. Claw toes result from intrinsic muscle atrophy secondary to motor neuropathy which leads to unopposed hyperextension at the metatarsal phalangeal joints by the extrinsic muscles2,3,4. Hyperextension contributes to prominent metatarsal heads along with migration of the plantar fat pad distal and dorsally, further exposing the metatarsal heads to increased pressure2,3,5. This especially affects the first metatarsal head increasing the risk of ulceration. Gait and postural abnormalities may also add to the effects on plantar pressure3.

The skin and soft tissues in the person with diabetes are less pliable due to glycosylation. This can lead to skin breakdown and callus formation due to decreased tolerability to friction and restricted joint motion.

The combination of altered biomechanics and peripheral neuropathy significantly increase the risk of ulceration of the skin and soft tissue of the foot of the person with diabetes3. When protective sensation is intact, pain and discomfort provide an early warning that the pattern of gait must be modified, or alternatively, ambulation must cease6. Thus, loss of protective sensation results in increasing problems secondary to friction, compressive and shear forces. Neuropathic plantar ulcers normally develop at sites exposed to moderate to high repetitive pressure during ambulation6.

The Custom-Moulded Orthotic: How Does it Work?
There are a variety of different pressure off-loading devices known as insoles or orthotics. Off the shelf insoles which may be purchased at a pharmacy or athletic supplies store will provide some pressure relief, however, may not be satisfactory in cushioning, protecting and redistributing high plantar pressure in the person with a neuropathic or morphologically abnormal foot. It may therefore be necessary, at the discretion of the care provider, to recommend that custom-moulded orthotics be obtained as they are specifically moulded to the foot of their wearer to optimally protect and cushion the insensate foot and accommodate deformity if present1. Custom-moulded orthotics decrease plantar pressures, especially over bony prominences, to prevent the recurrence of ulcers and possibly treat ulcers in a fashion similar to the total contact cast (TCC)1,2,6,7,8. They are also considered to assist in the primary prevention of ulcers.

Off-loading of pressure from the affected areas is an established treatment option for diabetic ulcers. It allows healing by minimizing mechanical stresses and repetitive trauma9. This can also be accomplished by non-weight-bearing or TCC which allows for ambulation. However, these latter modalities may be unacceptable for some patients and unrealistic in those who are already unsteady on ambulation because of peripheral neuropathy7,8. The custom-moulded insole provides off-loading based on the same theory as TCC. It redistributes the plantar pressure and allows some range of motion4,6,7,8. Plantar pressure is directly proportional to weight-bearing forces and inversely proportional to surface area. Plantar pressures can therefore be altered, by decreasing body mass, which is difficult to change in the short-term, or by increasing the area of the weight-bearing surface as done with the TCC or the custom-moulded insole2,7,8. The TCC has been shown to heal a higher proportion of diabetic foot wounds than other therapeutic interventions as it leads to optimal weight off-loading, decreased patient mobilization, reduced edema, foot protection and it cannot be removed, thus ensuring compliance.10 The absolute threshold of plantar pressure which causes tissue damage is not known and likely varies among individuals. Therefore the goal is even distribution of force2.

In a study comparing the manufacturer's insole versus a custom-moulded insole made of viscoelastic, the latter demonstrated a statistically significant reduction in plantar pressure for ulcer sites involving any of the metatarsal heads, especially the first metatarsal head and great toe6. The viscoelastic insole showed a significant decrease in pressure at other ulcer sites and areas not associated with ulceration6.

What are the Indications for Prescribing Orthotics?
The person with diabetes with intact protective sensation without morphologic changes of the foot usually does not require custom-moulded orthotics or special footwear4,12. Once protective sensation is lost, the foot becomes at risk for trauma and ulceration as the appreciation of pain, an important indicator of increasing pressure, is absent2,4,12.

The National Hansen's Disease Centre in Carville, LA has outlined specific foot risk categories and suggested protective footwear appropriate for each risk category2. Once protective sensation is lost, prevention of the initial foot ulcer becomes critical, with orthotics as an essential part of the prevention strategy. Once a person with diabetes has been treated for his/her first ulcer, insoles gain even more importance in the prevention of ulcer recurrence2. Orthotics should also be considered for accommodating feet with abnormalities such as the Charcot changes or hammer and claw toes or feet with prominent metatarsal heads2,3,5,6,9.

How are Custom-Moulded Insoles Made?
To create the custom-moulded insole, the patient first stands in a foam impression box. The impression obtained is then used to make a plaster cast of the foot. The thermoplastic base of the orthotic is moulded using the plaster cast and the interference layer forms the top layer of the orthotic. Metatarsal mounds are usually added before completion. After the orthotic is completed, the final stage of the fitting process entails ensuring that the orthotic follows the contours of the foot. Modifications to the orthotic may be necessary prior to its final dispensing. The orthotic must be used in conjunction with a therapeutic or orthopaedic shoe to accommodate the orthotic and the foot, provide room for any foot deformities such as hammertoes, and avoid trauma to the dorsum of the foot. Custom-made insoles function to their highest efficiency in proper fitting footwear. Not only may the orthotic require modification, but the footwear may require changes to optimally accommodate both the orthotic and foot.

What Materials are Used to Make Custom-Moulded Orthotics?
The materials used to make the custom-moulded orthotics need to be sufficiently firm to support the foot and off-load the problem area, yet not too firm as to cause other problems or aggravate the pre-existing problem. The material must be able to endure cyclic compression, shear combined with compression and allow for force distribution of plantar pressure throughout the material1. No single material satisfies all of these requirements; therefore two layers of different materials are routinely used to make custom-moulded orthotics1. In a study comparing commercially available materials, medium density crossed linked polyethylene (PeliteTM) was determined to be the most suitable for the base of the orthotic as it can be moulded to the shape of the foot1,13. The materials used for the base of the orthotic must be carefully selected to ensure that they are lightweight, flexible, strong, resilient, and retain their shape despite compressive forces. The top cover, or the material which comes in contact with the foot, is routinely made of an expanded rubber (e.g., SpencoTM) or a polyurethane foam (e.g., PPTTM ) as they provide cushioning and decrease friction and shear forces1,13.

Summary
Custom-moulded orthotics are an important asset for the management of the foot of the person with diabetes but cannot be used in isolation. The custom-moulded orthotic must be used in conjunction with appropriately fitted footwear, education, regular foot maintenance for callus debridement and nail trimming, and ulcer management. Compliance with the orthotic and appropriately fitted shoes is critical to prevent an initial ulceration, or alternatively to facilitate the healing of a current ulceration and prevent future ulcerations. The footwear and orthotics can only prevent problems if they are worn at all times.

Acknowledgements
The author wishes to acknowledge the helpful comments of Dr. John Embil, Consultant, Infectious Diseases, University of Manitoba, Winnipeg, Manitoba, Canada; Mr. Brian Scharfstein, Canadian Footwear, Winnipeg, Manitoba, Canada and Mr. Ken Weber, Orthotist, Rehabilitation Engineering, Health Sciences Centre, Winnipeg, Manitoba, Canada, for their review and helpful comments. Ms. Carolyn Schlippert is also acknowledged for her secretarial skills in preparing this document.

References

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  13. Murphy E.F., Burstein A.H.: Physical Properties in Materials Including Solid Mechanics. Atlas of Orthotics. American Academy of Orthopaedic Surgeons. 2nd Edition, St. Louis, C.V. Mosby, 1985, 6-33.