Given the repetitive microtrauma experienced by patients with diabetic peripheral neuropathy, proper offloading is critical to the prevention of diabetic foot ulcers. Accordingly, these authors explore the potential of total contact casts, Charcot restraint orthotic walkers and other offloading devices, and discuss key factors that affect patient adherence.
The diabetic foot ulcer (DFU) that podiatric physicians commonly treat today does not simply represent a local dermal interruption but a clinical manifestation of what is happening systemically among our diabetic population.
The most common causal pathway in the development of the diabetic foot ulcer is repetitive microtrauma.1 The clinical presentation of DFUs arises from unopposed repetitive microtrauma exacerbated by the systemic comorbidities associated with diabetes mellitus including peripheral neuropathy, structural changes and ischemia.2
In 1963, an early offloading publication established that providing equal distribution of plantar pressure throughout the sole of the foot was the principal factor in healing trophic ulcerations occurring in a neuropathic population.3 Yet nearly 50 years later, practical offloading of the diabetic foot ulcer still remains a critical challenge for podiatric physicians.
Up to 50 percent of people with diabetes eventually lose sensation in their feet, which is sufficient for them to injure themselves unknowingly. It is vital to offload these areas but due to patients’ inadequate sensation, they are unable to judge whether they have obtained pressure relief. Plantar pressure is a causative factor in the development of DFUs and ulceration is frequently a precursor to amputations and increases the mortality rate.4
Research indicates that there is a mechanical cause of diabetic ulcerations associated with foot slap secondary to progressive motor neuropathy, demonstrating an increased rate of tissue deformation. Landsman and colleagues show high rates of tissue deformation resulting in elevated intracellular calcium concentrations, leading to cellular death.5
Extrinsic mechanical factors such as abnormal plantar pressure distribution and high plantar pressure contribute to DFUs. Intrinsic factors such as calluses, foot deformities and peripheral neuropathy also play roles.
Skin breakdown occurs with increased plantar pressure and more specifically, peak plantar pressure. Repeated loading of high peak plantar pressure during walking has been associated with skin breakdown although there is no definite threshold that predicts ulceration. One must also consider the duration and repetition of pressures. The rate of spatial change in plantar pressure distribution and the peak pressure gradient may be other markers of skin trauma.6
Contact pressure on the plantar surface of the foot generates local internal forces that cause the subsurface tissue to deform. Breakdown occurs when the foot contacts the ground force, giving rise to distortion of the tissue and the formation of localized tissue damage. The maximum shear stress is vital in the mechanical failure criteria and may predict the tissue injury and skin breakdown.6
Increased plantar pressure levels occur in patients with structural abnormalities such as Charcot’s neuropathy, clawtoes, hammertoes or other foot deformities. These irregularities can cause disruptions in the shape of the foot. Motor neuropathy is frequently associated with diabetic neuropathy. This in turn can lead to elevated foot pressures and ulceration.7 The combination of a deformed foot, loss of sensation, motor neuropathy and inadequate offloading may lead to tissue damage, ulceration, infection, amputations and death. Even in a patient with adequate perfusion, once an ulcer forms, studies indicate that healing is still delayed unless one catches the ulcer early and offloads it.4 Once an ulcer has healed, the rate of recurrence is 40 percent over four months.8
Furthermore, individuals with diabetes have a 30-fold higher lifetime risk of undergoing an amputation in the lower extremity in comparison to non-diabetics. Those with diabetes have at least a 10-fold greater risk of being hospitalized for bone and soft tissue infections in the lower extremity than non-diabetics.9 Infected foot wounds comprise about two-thirds of lower extremity amputations. Infection is exceeded only by gangrene as an indication for diabetic amputations of the lower extremity.9
There are many new possible solutions that one may use in conjunction with offloading modalities. These modalities include living skin substitutes, stem cells and topically applied growth factors. The key component with all of these treatment options is that the mechanical environment must be controlled so as not to destroy the modality. Decreasing pressure and strain rate are important factors for offloading.8 Pressure is the exertion of force upon a surface by an object in contact with it. This is measured as force per unit of area (P = F/A). Strain rate (ε.) is the rate of change in force with respect to time (dε/dt).
Many patients with diabetic foot ulcers are obese. Patients suffering with diabetic foot ulcers place 2 to 2.5 times their body weight on the wound, thereby increasing pressure and strain with each step.7 The easiest way to decrease strain rate is to decelerate the speed at which the foot hits the ground as well as shortening the time the foot is in contact with the ground. Nonetheless, most patients with diabetic foot ulcers suffer from neuropathy and are unable to decelerate as they step on the ground. Therefore, they tend to step more forcefully in comparison to those without neuropathy. Effective offloading modalities address both force and strain rate.7
Total contact casts. Several studies support the total contact cast (TCC) as the gold standard offloading device for diabetic foot ulcers. Researchers have shown that offloading methods such as custom insoles, shoes or pads heal fewer wounds in comparison to the total contact cast. In a 2000 study, Hanft and Surprenant found that the percentage of DFU closures after five weeks was 88 percent from TCC, 63 percent from 3-D walkers and custom insoles, and 55 percent from custom sandals with three layers of foam.10
However, some clinicians may argue that the meticulous detail and time required to prepare and apply a TCC is incompatible with their methodology of practice. Patients who are unable to readily identify pressure points within a TCC, frail individuals with motor difficulties or morbidly obese patients are not ideal candidates for this device.11
Alternatives to the TCC include: the Charcot restraint orthotic walker (CROW), a controlled ankle motion (CAM) walker, ankle foot orthoses (AFO) and wedge shoes.
Charcot restraint orthotic walker. The CROW boot is a device composed of polypropylene material lined with Plastazote. It incorporates a total contact custom orthotic and utilizes a rocker-bottom sole. The CROW is the treatment of choice for many during the second and third stages of Charcot neuroarthropathy to maintain joint stability and alignment. One main advantage is that the clinician can remove the boot, treat the ulcer and reapply the boot during office visits. One can control edema with the boot, which allows the patient to ambulate. A disadvantage of the CROW boot is removability as the patient may not be compliant and may remove the brace at any time. The CROW boot may also require frequent adjustments due to changes in lower extremity edema.12
The prefabricated walker was originally designed to treat fractures and sprains. One may also use the CROW to offload DFUs. Hanft and co-workers showed a healing rate of 85 percent within 13 weeks with an average time to closure of 5.51 weeks (+/- 1.02 weeks) during a retrospective study of more than 300 patients with plantar DFUs.12 Some of the prefabricated walkers contain polyethylene plugs that one can remove to offload a particular area where an ulcer may reside. Advantages of these devices are similar to the CROW boot because one can easily remove the prefab walkers.12
Integrated prosthetic and orthotic system (IPOS). The IPOS model is designed for forefoot ulcerations. This “half shoe” has 10 degrees of dorsiflexion and a heel that is elevated 4 cm so the forefoot does not contact the ground. Patients who wear the IPOS often have difficulty with balance. Due to the dorsiflexion of the device, many patients with diabetes are unable to wear the shoe.12
The OrthoWedge shoe (Darco) is very similar to the IPOS model with the exception of the sole extending to the toes. The patient’s ability to dorsiflex the ankle is an issue as well. One study has shown plantar pressure reduction of 64 to 66 percent although the OrthoWedge was less effective than the total contact cast.12 The healing sandal is a total contact orthotic made of Plastazote. The ulcerated area contains a cutout for offloading and the edges are skived to minimize stress. Hanft and colleagues showed a healing rate of 74 percent within 13 weeks and an average time to closure of 7.11 weeks (+/-2.35 weeks).10 The healing sandal is lightweight but has poor control over foot motion, which elevates the risk of increasing stress at the ulceration site.
While there are certainly offloading modalities our wound care patients cannot physically remove, the majority of what we provide are removable. In 2003, researchers discovered that the notion of non-adherence plays a much larger role in the delayed healing of diabetic ulcers than once believed. Armstrong and colleagues revealed that patients with removable offloading devices spent 40 percent of their daily steps without appropriate offloading in comparison to control patients with non-removable devices.14
Research suggests that several factors play contribute to non-adherent behavior in patients. For instance, patients may blame the size of the particular device while others place blame on the difficulty of applying the device. Other patients may have work-related shoe restrictions or jobs that require constant ambulation. Furthermore, socioeconomic status may play a role as many patients may be the sole provider for their families and may be unable to take time off from work. Other studies have supported the idea that the presence of an offloading device may negatively affect the patient’s ability to ambulate or maintain self-reliance.15 Whatever the case may be, researchers have recommended that early identification and evaluation of potential psychosocial issues may render a higher adherence rate and enhance clinical outcomes.15
Other clinical methods to monitor and improve patient adherence have emerged. One approach is using activity monitors that one can attach to offloading devices.16 This not only allows the physician to track how often patients are using the offloading device but may also encourage better patient adherence with them knowing they are being monitored. However, the practicality of implementing these devices in private practice may be the biggest shortcoming of this technique.
The ultimate goals in the treatment of the diabetic foot ulcers are salvaging the affected limb, rapid complete wound closure and maintenance of the protective skin envelope. Once wound closure has occurred, the often difficult and frustrating task of preventing recurrent ulceration should begin immediately. Studies suggest that 34 percent of patients will develop a recurrent or new pedal ulceration within one year of healing a wound. Greater than 50 percent of patients will develop a recurrent or new pedal ulcer within three years of healing.17 These findings strongly indicate that lifelong diabetic foot care is required for patients with a known history of diabetic foot ulcers.
How do we minimize the odds of patients developing new or recurrent pedal ulcers when evidence-based medicine reveals over 50 percent will relapse within a three-year period?17
We are beginning to understand that preventing recurrent ulcers is proving to be just as challenging as wound healing itself. Much like wound healing, the prevention of diabetic foot ulcers requires one to decrease the strain rate and control the environment in which the foot bears weight.
In order to properly protect and control the forces about the foot, we must continue to offload and decrease the strain rate once the wound has healed. Selecting an offloading device requires a crucial understanding of the patient’s needs as well as identifying potential risks that may lead to the failure of the device and recurrent ulceration. Patient foot type and/or deformity (i.e. amputation), the location of healed diabetic foot ulcer and potential pressure areas are among the most important factors to consider.
Patients with healed ulcers, no amputations or significant pedal deficiencies can transition to a shoe-based total contact device such as a custom-molded shoe with an insole. This can protect the skin surrounding the healed ulcer. The patient can transition into a custom-molded shoe after the tissue and healed ulcer are strong enough to bear the pressure and shear forces of weightbearing. Pressure is spread out across the plantar surface of the foot with total contact insoles so not all the pressure focuses on the previous ulceration site.
There are various offloading modalities that have insoles with removable diamond or hexagonal shaped pieces to selectively offload particular areas of the foot.18 Patients with digital amputations or partial ray amputations can use custom toe filler insoles in conjunction with custom shoes. However, toe fillers are not indicated in the dysvascular patient because they increase the risk of tissue breakdown.19
Single or multiple ray amputations or resections may result in a significant reduction of the weightbearing plantar surface. Custom-made multi-layer, multi-density insoles can help distribute pressure under the remainder of the foot. Patients with central ray resections usually need minor modifications to their molded insoles in order to ensure proper fit.19
Compared to a regular sneaker, extra-depth shoes have an extra vertical depth of 3/16 to 1/2 inch. Patients with mild deformities, such as hammertoes or bunions, are often unable to wear regular shoes due to their toes feeling crowded. When it comes to diabetic patients with pedal deformities, regular shoes are not good options. In this patient population, regular shoes may lead to the development of recurrent ulcerations due to a likelihood of neuropathy and a resulting inability to detect pressure points.
A regular shoe is also not a good option for the neuropathic patient with or without deformity because these patients require a custom molded insole. “Diabetic” shoes have enough extra depth to allow a custom insole for extra protection whereas regular shoes have trouble accommodating the custom insole. Custom insoles distribute pressure across the entire plantar foot, thereby decreasing shearing forces for the neuropathic patient. Different styles of insoles can be custom made from a mold of the patient’s foot in order to accommodate various deformities, including but not limited to hammertoe contractures, bunions and plantar foot ulcers.20
“Rocker sole” is a general term comprising a variety of rigid sole shoes but all these shoes unload pressure almost entirely at the forefoot. Research suggests that rocker soles effectively reduce peak plantar pressures in patients at risk for diabetic foot ulceration, particularly when one pairs these shoes with custom orthoses. Research has shown that this combination reduces pressure under the metatarsal heads by as much as 30 to 50 percent.21
The rigid sole limits foot movement at all joints, specifically extension at the metatarsophalangeal joints. In turn, this prevents movement of tissue at the plantar aspect and distributes forefoot load over a larger area.21 Patients with digital amputations, including the hallux, may benefit from the rocker sole devices. Following a digital amputation, the remaining adjacent digits naturally attempt to compensate for the deficit and this compensation can overload the remaining rays, increasing the risk of plantar foot ulcerations.19
The AFO with custom insoles is characterized by a rigid ankle design that patients can use for a variety of pedal problems including drop foot and ankle instability. Ankle foot orthoses provide the necessary decrease in strain rate required for maintenance and prevention of the diabetic foot ulcer.12 This is an excellent option when dealing with DFU prevention with either an underlying equinus component or multilevel foot amputations that require controlled ankle motion due to the loss of extensor tendon insertions.
The CROW boot not only serves to accommodate the Charcot foot but can also be a preventative offloading option for patients with a history of recurrent diabetic foot ulcers secondary to sensory neuropathy, pedal deformity or increased plantar pressure due to midfoot amputation.
Preventative offloading of the diabetic foot with amputation is difficult for several reasons. Amputation of the lower extremity leads to a considerable loss of weightbearing areas due to shortening of the foot, making footwear selection extremely important. One can provide adequate footwear under the condition that the remaining foot stump allows for proper weightbearing. The device needs to compensate for the loss of the weightbearing area, whether it is a custom shoe or prosthesis. Total contact of the foot is important in providing a snug fit of the stump with the prosthesis.20
So what does the future hold with regard to evolving our current offloading modalities for diabetic foot ulcers? How can we preserve a steady decrease in strain rate necessary for wound healing while improving patient adherence with offloading devices?
Future offloading devices should incorporate aspects that would increase adherence. Qualities of these offloading devices should include: comfort, durability and ease in application, all of which serve as a construct to prevent secondary ulcerations. While this may seem obvious to many, these core issues have not adequately been incorporated into one device. Perhaps we are underestimating the importance of these devices in limb and life salvage, and should redirect our attention toward new offloading technology.
Offloading is a very important aspect of wound healing that may often be undervalued. Diabetic foot ulcers are caused by a combination of pressure and strain. Effective offloading requires reduction of both of these forces. Currently, there is no perfect device. However, we do have the tools at our disposal that can yield very high healing rates. By recognizing the shortcomings of the offloading devices and tailoring the device to the specific needs of the patient along with proper patient education, we can drastically improve clinical outcomes.
Dr. Hanft is the Director of Podiatric Medical Education at Baptist Health in South Florida. He is a Fellow of the American College of Foot and Ankle Surgeons.
Dr. Hall is a first-year resident at South Miami Hospital.
Dr. Kapila is a second-year resident at South Miami Hospital.
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Editor’s note: For related articles, see “A Guide To Offloading The Diabetic Foot” in the September 2005 issue of Podiatry Today, “Inside Insights For Offloading Diabetic Neuropathic Ulcers” in the July 2004 issue or “Current Concepts In Offloading Diabetic Foot Ulcers” in the September 2009 issue.