Study findings suggest that rocker soles effectively reduce peak plantar pressures in patients at risk for diabetic foot ulceration, particularly when paired with custom orthoses. Whether or not rocker soles can play a role in healing active neuropathic ulcers, however, remains unclear.
Preventing foot ulcers in patients with diabetes is a goal that, when met, can save patient’s limbs, and often, their lives, as five-year mortality after lower extremity amputation is as high as that of many cancers.1 Diabetic footwear in general, and rocker soles in particular, can play an important role in offloading foot pressure to prevent ulceration, yet some experts note that such conservative interventions are increasingly underutilized.
In the current era of evidence-based medicine, the lack of high-quality research on rocker soles and diabetic footwear means practitioners often are working without the benefits of a large medical literature and expert consensus to inform decision-making.
“Although it’s clear that more studies need to be done to inform what we as practitioners recommend for our patients, rocker soles and diabetic footwear have been used successfully for many years to alleviate plantar pressure in patients with diabetes, as well as in people without diabetes who are at risk for ulceration from other causes,” said David Levine, DPM, CPed, who is in private practice as director of Physician’s Footwear and Walkright in Frederick, MD.
Practitioners also may face challenging reimbursement issues.
“Although the rocker sole is recognized by Medicare and such modifications are specifically covered in the Therapeutic Shoe Bill, practitioners are facing more reimbursement and documentation challenges than they have in the past,” said Dennis Janisse, CPed, president and chief executive officer of National Pedorthic Services in Milwaukee, WI, and one of the original authors of the 1993 Therapeutic Shoe Bill (see “The Therapeutic Shoe Bill: Making sure the benefits add up,” A real world approach to diabetic footwear, March [suppl], page 15).
Levine also suggested that in the last decade, diabetic footcare practitioners increasingly have been exploring surgical options and focusing less on shoes, orthotics, and other conservative modalities.
“While this is not necessarily a bad thing, these conservative modalities remain effective and we should not lose sight of them. As I tell my patients, ‘It’s a lot easier to operate on shoes than on your feet,'” Levine said.
Studies suggests rocker soles reduce pressure under the forefoot, and specifically under the metatarsal heads, by 30% to 50%.2,3 That finding is the most significant aspect of the research to date, said Peter Cavanagh, PhD, DSc, professor and vice chair for research in the Department of Orthopaedics and Sports Medicine at the University of Washington Medical Center in Seattle.
“That’s huge,” Cavanagh said. “And it’s the best option we have in terms of pressure reduction with shoes. Despite this, there has not been a full-scale randomized controlled trial with rocker soles that looks at ulceration rates. Until such evidence is available, practitioners have to focus on the available evidence, which shows plantar pressure is dramatically reduced with rocker shoes.”
“Rocker sole” is a generic term covering a range of rigid-sole designs, but all unload pressure almost exclusively at the forefoot, Cavanagh said.
“The rigid soles limit movement at foot joints, particularly extension of the metatarsophalangeal joints,” he said. “This prevents movement of tissue across the plantar aspect of the foot and distributes forefoot load over a larger area; this is where the primary benefit for prevention of tissue damage seems to lie.”
Using a custom insole in conjunction with rocker soles maximizes pressure reduction, Cavanagh said.
“Adding insoles with a design based on pressure distribution rather than conventional insoles based on foot shape optimizes the pressure reduction you get with rocker soles,” he said.
A 2008 Diabetes Care paper demonstrated this additive effect, finding that pressure reduction with shape-plus-pressure-based insoles (compared with shape-based insoles) in standard flexible shoes increased with the addition of rocker soles.4
Investigators identified 70 regions of elevated barefoot pressure (mean peak 834 kPa under metatarsal heads) in 22 patients with diabetes and loss of sensation. The researchers sent impressions of participants’ feet to three orthotic supply companies for fabrication of insoles; one company also received plantar pressure data and incorporated it into the insole design.
Investigators tested all participants during walking while wearing the flexible shoes with the three custom insoles and while wearing the rocker with three insoles. In 64 of the 70 regions of interest the shape-plus-pressure-based insole reduced peak pressure significantly more than the shape-based insoles (32% and 21%). The addition of rocker soles to shape-plus-pressure-based insoles reduced peak pressure by 37% and 29% more than the two shape-based insoles.
Despite the evidence of significant pressure reduction with rocker soles, important questions remain unanswered.
“We don’t know the level of pressure at which tissue damage occurs or how much pressure reduction is enough to prevent damage, problems compounded by variable results produced by the different pressure measuring systems in use today,” Cavanagh said. “And while most studies have looked at barefoot plantar pressure, in-shoe pressure is probably a much more important predictor of tissue damage for patients who wear prescribed footwear during most weight-bearing activities. Until recently, however, there hasn’t been a clearly defined target goal for that in-shoe pressure.”
Cavanagh and colleagues published a paper in 2009 in Diabetes Medicine to answer this question, and the epidemiological cohort study evaluated in-shoe plantar pressures and other characteristics in patients with diabetic neuropathy who had prior foot ulcers that remained healed.5
Investigators identified 190 individuals (part of a database of 2625 patients) seen in two Swedish diabetes clinics who had previous plantar ulcers of the hallux or metatarsal heads caused by repetitive stress; 40 participated in the study. Investigators measured barefoot and in-shoe plantar pressures during walking and collected data on foot deformity, activity profiles, and self-reported behavior. The researchers measured in-shoe pressure in patients wearing shoes they self-identified as most frequently worn, most often extra-depth shoes with custom-molded insoles.
Only patients aged 30 to 80 years with ulcers of predominantly neuropathic origin that had remained healed for at least 90 days were included. Participants’ ulcers had been healed for an average of 3.6 years. Previous studies have reported recurrence rates for neuropathic ulcers between 28% at 12 months and 100% at 40 months6,7 and Pound et al found that in 226 patients with healed ulcers, 40.3% developed recurrent or new ulcers within six months, at a median of 126 days.8
Cavanagh and colleagues hypothesize that successful offloading of the previous ulcer site likely enabled the patients in their study to stay healed for longer than patients in other studies.
“Although we saw great variability in the plantar pressure reduction among participants’ various prescribed footwear, we propose that the mean pressure we identified—approximately 200 kPa [kilopascals] at the previous ulcer site—is a reasonable goal for plantar offloading until there’s better evidence available,” Cavanagh said.
In the 2008 Diabetes Care study, neither of the shape-based insoles reduced mean peak plantar pressure below 200 kPa when used in flexible shoes, but one reduced mean values below the threshold when used in a rocker shoe. Mean peak plantar pressure for the least-effective shape-based insole was exactly 200 kPa when that device was combined with a rocker shoe; values for the other two rocker-sole conditions were well below 200 kPa.
As noted, rocker shoe types vary, and research shows three types reduce plantar pressure—toe-only, negative heel, and double rockers.9,10 There also are a number of design variables; a 2000 study in Foot & Ankle International compared nine different rocker shoe designs and found that no single configuration was optimal for all patients with diabetes.11 The authors described the four key variables to consider: rocker angle (angle of the front part of the shoe to the ground); shoe height; rocker axis position with respect to the long axis of the shoe; and rocker axis angle with respect to the long axis of the shoe.
“Rocker sole design must be individualized for each patient; not every patient with diabetes is going to benefit from the same design, and the wrong design can certainly make things worse for some,” said Janisse. “Different rocker-bottom designs transfer pressure in different ways. Toe-only rockers provide the best forefoot relief, but transfer pressure to the midfoot, something that’s undesirable for people with Charcot foot, for example. Double-sole rockers do a better job of relieving pressure at the midfoot and are primarily used for people with issues in that region. Toe-only rockers, the most commonly prescribed rocker type, are used for relief at the forefoot and heel.”
There is not yet good evidence to guide selection of design variables for individual patients, Cavanagh agreed.
“These decisions are part of the ‘art’ of designing therapeutic shoes,” he said. “Our research11 shows that different individuals have better pressure offloading with the axis placed in varying positions, although in every case, there is better reduction than with a flat shoe. Until we have better data, a good rule of thumb is to place the axis behind the metatarsophalangeal joints; this may not be optimal for every patient, but it’s certainly going to improve pressure reduction compared with a flat shoe.”
Levine noted that while choice of specific rocker sole design depends on a number of factors in an individual’s feet, he has developed a fairly simple test to see if patients can benefit from the kinematic effects of rocker soles.
“I have the patient stand up, lift up one foot, and see if they can raise up on their toes. Then they try this on the opposite foot,” Levine said. “If they can’t raise up on the toes of one foot, that means they probably have issues with pushing themselves off when they walk, and rocker soles can help provide this motion, which these people’s feet don’t have naturally.”
Rocker soles perturb balance, and this can create special concerns for patients with diabetic neuropathy who may already be at increased risk for instability and falling. A 2009 study in Gait & Posture of 20 healthy participants aged 22 to 25 found that rocker soles had a significant destabilizing effect even in these young individuals when they stood and walked on a curved, unstable surface.12
Both Janisse and Levine agreed that rocker soles can create stability concerns, particularly for people with diabetic neuropathy.
“People do accommodate to rocker soles, but I think it’s important to start with education, explaining to the patient how the shoes work, what issues the shoes can address, and what patients can expect in terms of balance and other issues,” Levine said. “Training patients to walk in the shoes should always be part of the process, but rocker soles can be designed in so many ways, from a fairly extreme rocker to one that has a much more mild effect, and the shoe must be tailored to the person’s individual needs, including issues of stability.”
Most individuals can overcome stability issues with appropriate training, said Janisse, who has his patients work with one-on-one with a physical therapist to learn how to let the shoe do the work of push-off for them.
“Again, it goes back to evaluating the individual,” he said. “For example, elderly patients with foot issues can sometimes feel they’re going to fall backward if the heel height is inappropriate or there’s too much rock in the heel.”
Prevention vs healing
Another unanswered question is whether rocker soles can help improve ulcer healing, given the documented shortcomings of standard therapeutic shoes in this area, though most experts say therapeutic shoes should be prescribed for prevention only.
In a 2010 review article published in both the Journal of Vascular Surgery and the Journal of the American Podiatric Medical Association, Cavanagh and coauthor Sicco Bus, PhD, noted that total contact casting has been associated with more than twice the plantar pressure reduction of rocker soles and seven times that of conventional extra-depth diabetic footwear.13 Evidence suggests uncomplicated plantar ulcers can be healed in about six to eight weeks using TCC.
“In the real world, this time frame is typically much longer. In studies of total contact casts for example, we’ve looked at how long patient have had ulcers and it’s often 12 to 18 months.14 But when you put these individuals in a total contact cast, they heal in six to eight weeks. And that’s all caused by the difference in pressure reduction; nothing else changed except that wound was adequately unloaded to allow for healing,” Cavanagh said.
But the literature also indicates that practitioners utilize TCC in only a small percentage of cases, the authors acknowledged. Wrapping a removable walker in fiberglass to render it irremovable—sometimes called an “instant” TCC —has been proposed as a more user-friendly alternative that maintains high levels of pressure reduction.
“It’s true that significantly more resources and time are required to provide patients with total contact casts, but newer methods of offloading have proven to be much more efficient than conventional total contact casts and can be used even in very busy clinics,” said David G. Armstrong, DPM, MD, PhD, professor of surgery and director at the Southern Arizona Limb Salvage Alliance at the University of Arizona College of Medicine in Tucson.
The Cavanagh and Bus article’s analysis of the literature suggests that standard therapeutic shoes are associated with mean healing times that are about twice as long as with TCCs, with the mean percentage of healed ulcers about two-thirds lower. Healing times are slightly longer with nonremovable walkers than with TCC but healing percentages are similar.
Ulcer healing has not been studied in rocker sole shoes. However, a recent Mayo Clinic study15 suggests that rocker soles may be appropriate for forefoot ulcer prevention and healing in certain situations.
Investigators looked at the offloading properties of provisional footwear for patients awaiting fabrication of custom solutions or for sufficient ulcer healing to occur before such custom solutions could be applied. Researchers analyzed dynamic plantar pressures in 15 subjects with diabetic peripheral neuropathy and either an active forefoot ulceration or a recent history of forefoot ulceration. A stiff-soled shoe with a rocker sole modification was compared with a flat stiff-soled shoe; investigators tested both footwear types with and without a 1.25-cm plastazote insert.
The rocker sole with insert most effectively decreased mean peak plantar pressures, associated with a 50% mean reduction overall compared to the flat shoe with no insert, and a 35% mean reduction at the metatarsal heads and hallux specifically.
“Total contact casting is the gold standard but is quite labor intensive and requires significant skill and resources. Many community hospitals and wound centers do not have the expertise to provide this service,” noted lead study author Steven Kavros, DPM, assistant professor of podiatric medicine at the Mayo Clinic, Rochester, MN. “Our study can be used in clinical practice for the average clinician treating foot ulcerations.”
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