Our expert panelists discuss whether reducing shear in patients with diabetes is comparable to reducing plantar pressure, when to change insoles and the optimum insole depth, and the future of computer-aided design for orthoses.
As custom orthotics have evolved, Eric Feit, DPM, notes that DPMs have traditionally emphasized designing diabetic orthoses to reduce plantar pressures. However, he says shear forces may be the most important factor in preventing ulcerations. Shear or frictional forces cause the skin to stretch or bunch up at the plantar aspect of the forefoot, which is most common at the plantar aspect of the first and fifth metatarsals, according to Dr. Feit. In an elderly patient with thin skin or a patient whose skin is thin from a prior ulceration, Dr. Feit says shear forces may create a break in the skin very easily.
James Wrobel, DPM, notes that peak shear and peak pressure are not necessarily the same. Citing a study by Yavuz and colleagues in patients with diabetic peripheral neuropathy (DPN), he says the authors found that peak shear and peak pressure only correlated about 20 percent of the time in patients with DPN and did not correlate at all in control patients.1 Dr. Wrobel adds that Yavuz and colleagues also reported peak shear at the hallux whereas peak pressure occurred underneath the second metatarsal head in the patients with DPN.
From a clinical perspective, Dr. Wrobel believes this makes sense.
“How many times have you seen an initial presentation of an untreated hallux ulcer, which was heavily macerated with subdermal bleeding compared with those ulcers presenting in other regions of the foot?” asks Dr. Wrobel.
Dr. Wrobel also cites studies that did not find a correlation between the location of pain and peak pressure in patients with pes cavus.2,3
Both Dr. Wrobel and Christopher Nester, BSc(Hons), PhD, are cautious about the potential impact of shear reduction.
“For sure, reducing shear is a laudable aim but achieving it is complex,” says Professor Nester.
With too little friction, Professor Nester says there would be a risk of the foot colliding with the sides of the shoes and this could lead to further problems. He also cites the importance of shoe fit in the equation.
“A great deal depends upon the fit of the shoe rather than the insole. A foot can be held very firmly by a shoe, so much so that any insole technology aimed at shear reduction can be negated,” explains Professor Nester.
Dr. Wrobel notes shear and vertical forces are important but different. In the gait lab, he notes the shear components of the ground reactive force represent about 5 to 20 percent of body weight compared to the vertical component representing up to 120 percent of body weight.
All panelists note the importance of careful observation rather than a strict timeline in determining when to change insoles. Dr. Wrobel says the decision to change insoles should be based on individual patients.
“One size simply doesn’t fit all in this case. I think we have to stay ‘old school’ and look and feel for wear patterns,” maintains Dr. Wrobel.
Dr. Feit concurs, saying he will change the insole when the device looks thinner in the forefoot or if the patient’s skin shows any signs of pre-ulceration or increased callus formation since the last office visit.
Although Medicare covers three pairs of orthotics every year for patients with diabetic neuropathy, Dr. Feit notes all orthotics do not wear out that fast. He says the orthotic wear depends on the stability of the foot, the weight of the patient and the activity level of the patient.
Dr. Wrobel concurs in regard to understanding the impact of varying activity levels. He recently co-authored an abstract that highlighted the variability in activity quality in a small study of patients with DPN.4 The study found that patient steps per day ranged from 4,013 to 17,856 and combined standing and walking activity ranged from 12 to 34 percent of total activity over two days.
Similarly, Professor Nester notes that the real “wear out date” is a combination of steps taken, loads applied and time passed. “However, the costs of recording this are probably greater than the costs of simply giving a new pair out every three months,” he says.
In addition, Dr. Feit says materials are a factor in changing insoles. Most foot specialists prescribe diabetic orthotics made of Plastazote®, NickelPlast™ and Poron®, according to Dr. Feit. He says these materials are soft and will not maintain their shape for more than three to four months in an active patient.
Professor Nester says pressure relief will occur with most compliant materials at a depth of 3/16 inch. However, he says it is unknown whether this depth is optimum for all patients. Professor Nester also notes that the depth of the insole is important only if one knows the “space” available in the shoe. Otherwise, he says even a shallow insole can increase plantar pressures in a shoe with insufficient depth or space.
Dr. Wrobel speculates that the CMS probably did not have much data available when it recommended an insole depth and had to rely on expert opinion and bench studies. As he points out, the data suggests that a fully arch contoured inner sole in addition to compliant material reduces plantar pressures more effectively than compliant material alone.5,6
“I believe that (CMS is) trying to prevent doctors and prosthetists from dispensing diabetic orthotics which are inadequate or ineffective,” says Dr. Feit. He notes the difficulty of designing a quality orthotic for an adult that is thinner than 3/16 inch in thickness unless one is using graphite, which is expensive.
Dr. Feit says CAD/CAM technology has helped DPMs prescribe orthotics that better conform to the patient’s foot and theoretically may be better in helping to reduce shear forces. He notes the use of experimental portable scanners can enable one to take a three-dimensional scan of the foot without the use of plaster and send the image directly to the lab. Dr. Feit says this reduces the lab time to create the device and helps return the orthotic more quickly to the DPM and patient.
“I think we will move beyond our obsession with static shape of the foot in due course,” says Professor Nester.
He advocates that CAD/CAM will need to combine information on foot dynamics to make any advance in the design and performance of orthoses. Professor Nester notes that his group is currently working on this via the Europe-wide project Special Shoes Movement (SSHOES), which is available at www.sshoes.eu .
While Dr. Wrobel sees some value for CAD approaches, he notes that CAM approaches usually involve the direct milling of one material. He suggests looking at differing materials by region. If one is moving shear from the skin and soft tissues into a layered device, he advocates measuring changes in spatial temporal gait. Dr. Wrobel says patients might compensate by slowing their gait speed and increasing their double support time, changes that physicians might miss if they are only looking to reduce shear.
Dr. Feit is a Fellow of the American College of Foot and Ankle Surgeons, and is in private practice in San Pedro and Torrance, Calif. He is the Past President of the Los Angeles chapter of the American Diabetes Association.
Professor Nester is the Director of the Centre for Health, Sport and Rehabilitation Sciences Research, and the Associate Head for Research and Innovation at the School of Health, Sport and Rehabilitation Sciences at the University of Salford in the United Kingdom.
Dr. Wrobel is an Associate Professor of Medicine and the Director of the Center for Lower Extremity Ambulatory Research (CLEAR) at the Dr. William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago. He is also a Clinical and Research Podiatrist at the Captain James A. Lovell Federal Health Care Center in North Chicago, Ill.
Dr. Armstrong is a Professor of Surgery at the University of Arizona College of Medicine and is the Director of the Southern Arizona Limb Salvage Alliance (SALSA). He is the Founder of CLEAR at the Dr. William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago.
1. Yavuz M, Erdemir A, Botek G, et al. Peak plantar pressure and shear locations: relevance to diabetic patients. Diabetes Care 2007; 30(10):2643-5.
2. Burns J, et al. The effect of pes cavus on foot pain and plantar pressure. Clin Biomech (Bristol, Avon) 2005; 20(9):877-82.
3. Crosbie J, Burns J. Are in-shoe pressure characteristics in symptomatic idiopathic pes cavus related to the location of foot pain? Gait Posture 2008; 27(1):16-22.
4. Najafi B, Crews RT, Wrobel JS. Incorporating standing time into physical activity assessment of patients at risk of diabetic foot ulceration. Abstract 185-OR, presented at American Diabetes Association Scientific Sessions, 2010. Available at: http://professional.diabetes.org/  Abstracts_Display.aspx?TYP=1&CID=79130.
5. Lord M, Hosein R. Pressure redistribution by molded inserts in diabetic footwear: a pilot study. J Rehabil Res Dev 1994; 31(3):214-21.
6. Bus SA, Ulbrecht JS, Cavanagh PR. Pressure relief and load redistribution by custom-made insoles in diabetic patients with neuropathy and foot deformity. Clin Biomech (Bristol, Avon) 2004; 19(6):629-38.