Wednesday, November 2, 2011

What Do Planes, Diabetics and Athletes Have in Common? Shear Force: The Common Dilemma that Diabetics and Athletes Face

For years the biomechanics community has been talking about pressure as a leading cause for ulcers, while its counterpart, shearing, was generally left out of the discussion. It turns out that the same force that causes blisters in runners may be the key to unlocking ulcer prevention.

What Does the Research Show?

Dr. Bijan Najafi at the Center for Lower Extremity ambulatory Research (CLEAR) in Chicago sums it up this way: The most dangerous period for a moving body is during gait initiation and termination because this is when the body is most subject to accelerating and decelerating forces. The same principal holds when considering an airplane taking off and landing. The greatest risk for a crash is during takeoff and landing, during which the plane must endure significant stress from the extreme accelerations and decelerations. Just as a plane reaches a steady state at its cruising altitude, people achieve a steady state during episodes of walking. Individuals with diabetic neuropathy take particularly long to reach a smooth walking gait. It typically takes over 4 steps for a diabetic with moderate neuropathy to reach steady state according to Dr. Najafi, which is in average 25% higher than healthy control subjects. Depending on the type of footwear or insole, this number could be increased or decreased. Although four steps may seem like a small number, when we look at it in the context of daily physical activity it becomes very important. A recent CLEAR study published in Diabetes Care revealed that diabetic patients initiated over 350 episodes of walking per day that are often short (less than 20 steps per episode). The 2010 Diabetes Care study noted that diabetics average 7,800 steps per day, and an average of 10-15 steps per episode of walking.

Various methods have been proposed to decrease shearing in shoes including the use of roller soles, bilayer insoles and orthotics, socks and skin lubricants. Dr. David Armstrong feels that traditional plastizote will eventually become passe in favor of emerging superior biomechanical products.

Brian Davis, PhD, has stated previously that one should not overlook shearing forces when noting the moderate success of therapeutic footwear. While it is likely frictional forces are a key variable, he acknowledges several other reasons for skin breakdown. These factors include alterations in collagen properties, autonomic nervous dysfunction that alters sweating responses, diminished microcirculation and superimposed aging. Hydration also cannot be ignored. Good skin turgor depends on proper hydration.

Dr. Najafi feels that there are some misconceptions regarding research on efficiency and friction. In some lab testing, the emphasis has been on the shoe ground interface instead of the foot shoe interface. But, currently this is a serious challenge, since there is no available technology to measure the shearing inside of shoes. An alternative solution is measuring the effect of reducing shearing on the biomechanics of walking. For example, if we could reduce shear force between the foot/shoe interface but not on the shoe/ground interface, this would allow us to increase the distance between center of pressure (COP) and center of mass (COM) in particular during gait initiation, which in turn would increase gait initiation velocity. Biomechanically, this is similar to a runner who tries to increase his take off velocity by leaning forward. Najafi’s recent study confirmed this hypothesis, in which significant improvement was noted in gait initiation velocity by wearing insoles that reduce shearing compared to sham insoles. He also notes that orthotics reduce the number of steps required to reach steady state. Reducing shear force does not reduce gait velocity.

He notes we should be careful about the balance of our patients, since increasing the distance between COP and COM may trigger falls in those patients with poor muscle performance. Therefore, an assessment of a patient’s balance and risk of falling is of key importance after reducing any shearing on the foot/shoe interface.

In the pipeline, Dr. Najafi is exploring differences in people with diabetic neuropathy versus healthy age matched controls under both shod and barefoot conditions. Gait parameters such as stability and gait fluctuation will be assessed.

How Can We Incorporate These Anti-shearing Methods in Our Practice?

Currently, there are ongoing investigations on various types of innovations in arch supports and orthotics. Lubricant creams are also under current study. Socks that have better wicking properties are something that we should be recommending to both runners and diabetics alike. It remains to be seen whether some of the double layer socks provide any therapeutic benefit over single layer socks. Some running store owners I interviewed related that some runners report bunching with some of the double layered socks.

It appears that a combination of following the research and clinical experience will guide us toward the best solutions for this issue.

David Armstrong, DPM. Personal Communication. 2011.
Davis, Brian. “Can Smart Orthotics Have An Impact in Preventing Diabetic Ulcerations?”. Podiatry Today. Volume 23. Issue 6. June 2010.
Bijan Najafi, PhD MS. Personal Communication. 2011.
Najafi, Bijan. “Importance of Time Spent Standing for Those at Risk of Diabetic Foot Ulceration”. Diabetes Care. Volume 33. Number 11. November 2010.

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