Focusing on Shear Force

Pedorthists have routinely associated pressure with skin trauma and discomfort. If a high pressure point on the foot is accepting more pressure than it should, the skin on the foot will be impacted. The link seemed obvious and the solution seemed simple – relieve the pressure. Less pressure equals less discomfort. But after identifying and healing the pressure areas, abrasions and calluses would frequently reappear. Reoccurrence of redness and skin abrasions following treatment were considered normal. This conclusion left many pedorthists, orthotists and prosthetists unsatisfied. After closer examination, a new culprit has emerged and has changed the way they treat their patients.

The shear force problem

“We have always focused on pressure. Pressure’s easy to measure, everyone understands pressure and that is a component to a lot of problems with the feet,” Dennis Janisse, CPed, president and chief executive officer of National Pedorthic Services Inc. said.

In his article, Functional Limitations from Pain Caused by Repetitive Loading on the Skin: A Review and Discussion for Practitioners, with New Data for Limiting Friction Loads, J. Martin Carlson, CPO, and president of Tamarack Habilitation Technologies Inc. stated, “The assumption that those problems [skin traumas] are directly governed by excess pressure is substantially in error, leading to partial solutions and missed opportunities to achieve much higher levels of safe, pain-free function.”

What is causing skin trauma in combination with excess pressure?

Skin trauma on a foot
Image reprinted with permission of Dennis Janisse.

“Shear force seems to be the main culprit,” Seamus Kennedy, CPed, president and co-owner of Hersco Orthotic Labs said. “But there is no question that both play a part.”

Shear force is an external force that acts parallel to a plane. Basically, it is a force that is caused by friction and is known to cause an object to move or slide. If there is a high pressure point or movement within the shoe, then the shear will occur where the pressure has existed. The higher the shear force, the quicker a blister is formed.

“Shear force plays an important role because the foot can slide inside the shoe,” Kennedy said. “This can result in blisters, calluses and eventually skin breakdown if you are not careful.”

According to Janisse, even if a pedorthist builds relief and reduces pressure there will always be some movement by the foot inside the shoe. Therefore it is important to not only reduce pressure in the shoe, but to manage the shear of the shoe.

“We used to use materials that stopped the foot from moving inside of the shoe and we believed that decreased shear,” Janisse said. “But in reality, what it did was not allow the foot to move and it increased the friction between the foot and the insole material.”

Janisse was decreasing movement inside of the shoe, but at the same time increasing the shear on the foot itself.

Shear force management

“We still don’t have a good way to really measure it,” Janisse said. “But we all knew that shear was taking place inside the shoe and that it was a major component when managing foot problems.”

There are materials on the market that help prevent blistering and callusing. Interface materials are used to help manage and reduce excess pressure and friction on the skin. Kennedy describes interfaces as materials that are inserted between the skin and the contact surface to help reduce shear. Interfaces can reduce the friction of the skin during repetitive loading.

According to Carlson, repetitive loading occurs after walking or running for a long amount of time. Janisse recommends an interface be placed on “hot spots” of the foot, especially for athletes. This will lower the likelihood of discomfort despite the repetitive loading and increase in pressure. Materials have been created that can be custom fit and placed on prevalent shear force areas.

Janisse pointed out that he initially did not know the proper way to use the materials. Sometimes the materials would have a positive effect on the patient, other times they did not. Janisse was providing relief only for pressure areas, not necessarily shear force areas. When he placed the materials in areas prevalent for shear, results greatly improved.

Janisse would take a foot orthosis for a patient that had an ulcer on their foot and provide relief in pressure areas. However, calluses and skin breakdown would continue to emerge. He added interface materials that reduced shear and eliminated reoccurrence.

“A well constructed foot orthotic, made properly, from a good mold of the foot, will relieve areas of pressure on the foot,” Kennedy said.

The cast duplicated any misalignment in the foot structure that would indicate a greater chance for excess pressure or shear force. Janisse would then place shear reducing materials at the top of the foot orthosis. The materials would eventually compress over time, but they would still protect the skin.

“Even though you take an impression of the foot, the bones and the foot changes over time” Janisse said. “We tend to use soft materials that will mold to the foot as a person breaks it in.”

Janisse recommends gradually increasing the time wearing the orthotic so the materials can mold to the foot.

Putting Your Best Foot Forward

The most basic way to avoid skin trauma on your feet is to inspect them every day. Finding redness or calluses on your feet is a tell-tale sign that you are in the early stages of developing a pressure point or shear force. At the first sign of any skin breakage, make an appointment with your practitioner so they can address the problem quickly. Ignoring the problem will not make the redness or ulceration go away. In fact, the skin will ultimately get worse.

According to Seamus Kennedy, CPed one should make sure there is no foreign body inside your shoe. A pebble or small stone in your shoe can be a slight aggravation or cause tremendous pain to your feet. Take the extra second and deposit it from your shoe right away.

Another practice that will help decrease foot blisters is wearing shoes that fit your feet appropriately. This simple task is often ignored. Before buying those expensive shoes you always wanted, try them on and make sure they are your size. Shoes that are too tight will cause high pressure areas in the shoe. Shoes that are too big will be loose and the skin will constantly rub against your shoe causing abrasions and increasing shear. Have your foot measured to see what size is appropriate and you may be surprised.

Kennedy suggests finding an orthosis that will relieve pressure in the shoe. Accommodating orthoses with pressure relief and sole stiffener will help avoid excess shear in the shoe. Patients that have more than one orthosis should switch their orthoses daily. This will reduce the risk of bacteria on your feet. Kennedy recommends having more than one orthosis. Although having two orthoses may cost a little more, it is a small price to pay for healthy and comfortable feet.


Risk factors

Dennis Janisse, CPed
Dennis Janisse

According to Carlson, shear forces only create a problem in limited areas. He believes shear force on the skin is useful for stability, suspension and the transmission of functional forces. Both Janisse and Kennedy agree with Carlson’s assessment. Shear forces, maintained properly and under the threshold of callusing and blistering are useful.

The challenge is finding the threshold which is different for each individual. A healthy individual may only get a slight blister on the back of their heel that will eventually heal on its own provided they take the necessary precautions like keeping the skin clean and perhaps finding a better fitting shoe.

But to the 23.6 million diabetics in the United States, a blister or ulceration on the foot is cause for greater concern. According to the Mayo Clinic, an estimated 5% of adults who have diabetes eventually require toe or foot amputation.

“An amputated toe can lead to half your foot and half your foot can lead to your lower leg,” Kennedy warned. “It is a progressive cycle.”

How does a blister on a toe or heel lead to toe or foot amputation?

If a diabetic wears an ill-fitting shoe that is too small and pays little or no attention to his or her feet, shear forces, coupled with excess pressure, will cause blistering on the foot. It is possible the nerve sensations can be damaged in the diabetics’ feet due to prolonged high blood sugar levels causing neuropathy.

“The nerve endings begin to deteriorate, so you’re not getting the sensation and the feedback from your skin and your foot,” Kennedy explained to O&P Business News.

According to, 30% of neuropathy cases are caused by diabetes. With neuropathy, the diabetic patient is now walking with a blister on their big toe, which they cannot feel. Considering the average person takes 8,000 to 10,000 steps a day, the blistering could eventually rupture causing ulceration. Left untreated, the ulceration can easily become infected due to various bacteria on the foot, in the shoe or on the sock. Infection may ultimately lead to amputation of the toe. From there, it only gets worse.

“Once you [have undergone] an amputation you have to be very careful because you are redistributing pressure where it didn’t exist before. The patient is put in a high risk category immediately,” Kennedy explained. “What five toes used to do, four toes are now doing. You are going to see a transfer in pressure. You see people lose the big toe, then they may lose the second and third, then they might lose the front of the foot, but at each stage you are still asking the remainder of the foot to do the same work as the whole foot.”

Janisse agrees with Kennedy that an amputated toe can cause severe damage.

“Basically, you’re going to have problems on a compromised foot,” Janisse said.

Maintain activity

Seamus Kennedy, CPed
Seamus Kennedy

The pedorthist needs the patient to be an active partner. A proactive patient will reduce the risk of escalation.

“They have to take an interest in their own feet immediately,” Kennedy said.

The difficulty in this is that many are not proactive while others simply cannot maintain their healthy lifestyle when their feet are blistered or ulcerated. Kennedy pointed out that there is no true healing timetable for patients with ulcerations.

“People with ulcers can take weeks, months or up to a year in some cases, just to heal,” Kennedy said. “In that time, they are not as active as they were, they get passive. When they get passive, they get heavy.”

A patient’s size, weight and activity level also plays a vital part in the amount of shear force inside a shoe.

“If you go for a run, you are increasing impact forces by three to five times on your feet,” Kennedy said. “If you gain ten pounds of weight, you are adding thirty to fifty pounds of impact force every time you run.”

For diabetics, maintaining a proper weight and healthy lifestyle is paramount.

“If we can train people to wear proper fitting shoes and wear good insoles that are providing support and protection for feet, you will keep those patients active,” Kennedy said. “If you keep them active, it will help them maintain their blood sugar levels correctly, which will reduce the progression of disease. There will be less chance of ulceration and amputation and they will then remain active longer.”

Small changes

For at-risk patients, those with advanced diabetes or neuropathy, the goal is protection. In order for pedorthists to protect their patients from serious skin damage, attention to detail from both the pedorthist and the patient is important. Minor changes in lifestyle can go a long way in reducing skin trauma in healthy individuals and those who are at risk.

Kennedy suggests moisturizing the skin, especially for people who are prone to dry heels. Callusing on heels can build up and eventually crack. Healthy athletes should wear moisture wicking socks in order to reduce shear and friction while they run. This will also reduce the risk of perspiration creating bacteria that could lead to infection on ulcerated skin. He also suggests finding socks that incorporate metals in the fabric, such as silver or copper. These types of socks promote odor reduction which is another indicator of bacteria. Janisse also recommends these types of metals because they help blood flow which will increase the feeling in the foot.

“Sock technology, it sounds kind of funny, but it is part of the total package of treating patients,” Kennedy said.

Shear force, combined with excess pressure can cause serious damage to the skin of an individual’s feet. Now that shear force has been identified as a significant culprit of skin trauma, pedorthists are more educated and the patient is better served.

For more information:

  • Carlson, Martin J. Functional Limitations from Pain Caused by Repetitive Loading on the Skin: A Review and Discussion for Practitioners, With New Data for Limiting Friction Loads. Journal of Prosthetics and Orthotics. 2006;18(4):93-101
  • 2007 amputation and diabetes: how to protect your feet. Available at: Accessed Nov. 11, 2008.

— by Anthony Calabro

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