The mechanism of soft tissue damage: It is all in the rub

Abstract

Understanding mechanisms of injury to the skin and soft tissue are an important feature in optimizing management strategies. As technology advances and innovative wound products evolve, the need for the awareness of such developments is key to enhancing knowledge and their clinical application. This article provides an overview of the role of repetitive loading of the skin and tissues and the influence of thickness and mobility of these structures. The role of friction and pressure strategies is also discussed along with a brief overview of new products.

Clinical relevance

Pressure and friction are important parameters in the management of wounds. Clinicians should be aware of these concepts and be cognizant of new technologies that are available for the reduction of these parameters.

SC:

Repetitive loading on the skin and soft tissues is an important parameter for many clinicians dealing with wounds to the lower limb. In 300 words, can you sum up the importance of this statement?

MC:

Yes, absolutely! It is a combination of peak load magnitude and number of load repetitions which govern tissue damage in a given time period. If the load repetitions are of low enough magnitude and/or spread over sufficient time, biological repair processes can keep ahead of damage. People without peripheral neuropathy will, almost unconsciously, alter their gait to lower and/or change the location of peak forces. They will, if sensible, avoid wounds. However, active ambulatory people with significant peripheral neuropathy lack the discomfort feedback that would have informed them to alter their gait as the day/activity progresses or limit their ambulation.

In addition to what is said above, the direction of the loading is important to clinicians. The compressive/pressure component of gait loads on soft tissue is virtually always accompanied by sizeable loading parallel to the skin surface. Those loads are some combination of fore and aft, transverse and rotational in nature. They are caused by friction forces preventing or resisting the skin from moving/sliding as the bone moves small amounts inside the shoe. Excellent experimental research extending from the 1950s from the work of Naylor^1,2 and Sulzberger et al. ³ in the 1960s to the present have repeatedly established the damaging effects of that component parallel to the skin surface. With the right materials and techniques, clinicians are able to reduce both load components in at-risk locations. ⁴

SC:

What about the role of soft tissue thickness and mobility?

MC:

Soft tissue thickness and mobility (low shear modulus, in engineering terms) are important because they reduce peak compressive and, especially, shear stresses within the skin and soft tissue. Repetitive loading tissue trauma is almost always confined to “boney” locations where soft tissue thickness (between bone and the supporting surface) is much thinner than in nearby areas. However, even thick, mobile soft tissue can be vulnerable when the number of load cycles becomes very large with insufficient periods for biological repair.

SC:

Using key clinical examples, what is your take on pressure versus friction?

MC:

Both pressure and friction are important and should be managed. Pressure (compression load perpendicular to the skin surface) management by cushioning and contouring at support surfaces is a well-established clinical practice. Shear distortions are very destructive and are significantly enhanced by the loads acting parallel to the skin surface. Those parallel loads are due to friction. The role of friction load repetitions in soft tissue damage has been repeatedly established by eminent researchers. We have seen many examples (look in my literature) of plantar wounds healing only after friction management materials were added to existing pressure management. Low-friction interface patches are routinely used by some high-performing athletes to avoid skin blisters from friction rubbing. The low-friction patches are adhered to the inside of their shoes in specific locations where they have had blister problems.

SC:

What new technology is available for 2015 and how does this differ from previous years?

MC:

Self-adhering low-friction interface patches have been available for some years. Very recently, a new fabric low-friction technology (GlideWear) has become available in oval shapes. The ovals are most often used between the skin and prosthetic gel liners. Placement is in specific locations such as the distal tibia, fibular head or kneecap where the liner may be repeatedly exerting damaging levels of friction/traction on the skin.

The polytetrafluoroethylene (PTFE) (ShearBan) and the fabric (GlideWear) products cited above were developed specifically to reduce friction-induced shear in, and only in, at-risk areas. Oils, creams, and powders have seen some use for many years, but the benefit is only very temporary and those substances cannot be confined to the at-risk area

SC:

Which are the most common friction management techniques?

MC:

Wearing a silk or nylon sheath/sock under a conventional sock has been a rather common practice for runners and amputees for some years. That double sock arrangement is usually seen as keeping the skin dryer by providing better wicking for perspiration. That may well be true, but it also creates an additional interface (nylon–cotton, for instance). That interface has a lower coefficient of friction (COF) than the skin–cotton or the cotton–insole interfaces. That lower COF, of course, releases the skin and inner sock to be released at a lower friction force, to glide along with the bone (distal anterior tibia or metatarsal head, for instance) as it cycles through its small movements during gait. Many people use this approach to reduce skin irritation/damage, but it has the drawback of reducing friction throughout the sock, rather than just in at-risk areas. That can cause some loss of control and/or suspension.

There is a Podiatrist in Australia, Rebecca Rushton, with whom I have had quite a bit of correspondence over the past year. She has experience with using friction management (ShearBan & Engo) to avoid and treat blisters in distance runners. She has taken on the challenge of explaining how friction management works.

SC:

Pressure management often uses cushioning material to distribute the load, and the role of gel liners is becoming very useful for the stump-to-socket interface. How best can you inform the role of the clinician with this modality—in terms of technology and understanding of the product to enhance patient education?

MC:

Gel materials have two characteristics that, in combination, make them excellent as cushions: They are typically constant volume materials (unlike foams) and they exhibit a low shear modulus. When a bony prominence presses against a gel material, those two material characteristics allow the gel some limited movement to form a sort of “cup” to cradle the impinging bony area. The low shear modulus also allows the gel to act like an extra layer of soft tissue. It absorbs some of the shearing stresses/strains that would otherwise be transmitted to and through the skin. However, the gel’s shear absorption is limited. When reciprocating movements of the bone are large enough in at-risk locations, such as the distal anterior tibia or patella, the tacky nature of the gel can exert damaging levels of skin traction. When such repetitive traction loads cause amputee discomfort, it can now be addressed strategically with low-friction fabric material (GlideWear). Suspension and cushioning characteristics of the gel liner are preserved.

SC:

Which is the most effective material currently available for reducing skin/soft tissue damage? How are they evolving and what future directions will these products take?

MC:

As noted earlier, excellent research going back to the 1950s has repeatedly established the significant role of friction-shear loading in skin trauma. In spite of that, wound prevention and treatment products have almost totally ignored the potential benefits of minimizing friction forces on at-risk tissue. I am not aware of any friction management product developed for use in the orthotic, prosthetic, and pedorthic profession before the introduction of ShearBan (and now we also have GlideWear). Both of these products are so thin, they do not impact shoe or socket volume. Cost is small because they serve their purpose for many months. They are designed for strategic installation/use which does not negatively affect useful (suspending and stabilizing) friction in areas not at-risk.

In time, we will see low-friction materials routinely installed or included in orthoses, prostheses, and shoes. The low-friction materials will be prelocated so as to reduce discomfort problems in areas formerly prone to irritation from rubbing. Greater comfort will lead to improved levels of function and satisfaction. For people with peripheral neuropathies, more utilization of friction management technologies will reduce the incidence of foot wounds and amputations.

SC:

So is there evidence to support their use, and if so, in which patient population?

MC:

Quality scientific evidence of the benefit of friction-induced shear is very difficult to produce. This is partly because instrumentation is not available to accurately map friction/shear load distribution at surfaces such as the insole of a shoe. Such friction load mapping would enable researchers to see and measure how effectively specialized materials and clinical techniques are able to redistribute friction loads off/away from at-risk locations. In contrast, thin, flexible instrumentation is available for mapping pressure distribution. Good clinical studies are very expensive because of wide variations in patient circumstances, lifestyle, and compliance. Reliable, significant data require large and lengthy clinical trials.

With regard to the efficacy of friction management, we are left with looking at the more fundamental experimental and theoretical research which has already established that the friction component of repetitive loading is a significant factor in soft tissue damage. I have taken that as sufficient reason to look for new and better ways/technologies to reduce that factor. We have seen overwhelming anecdotal evidence that friction management is very beneficial. The day will come when instrumentation advances and clinical trials bring forth better evidence. Until then, it seems clinically responsible to carefully utilize very low-risk technologies and techniques guided by existing high quality experimental research; observe the results in our patients and go from there.

SC:

Finally, what is your key message to all clinicians on the role of mechanism of soft tissue damage?

MC:

Management of pressure, the load component perpendicular to the skin surface, has proven beneficial to reduce repetitive loading soft tissue trauma. Experimental research and mathematical analysis have established that repetitive loads parallel/tangential to the skin surface are also a large factor generating soft tissue trauma. Those tangential forces can be reduced by introducing a low-friction material at any one of the material interfaces between skin and the support surface (shoe insole, prosthetic socket, etc.).

For better wound prevention and healing, we should use techniques and materials to strategically manage friction in addition to managing pressure.