T
he materials that form the interface between skin
and structural support surfaces of orthotic and
prosthetic devices are extremely important. Their
physical characteristics determine our patients’
comfort levels and, in many cases, the safe func-
tion of their devices. We are very accustomed to comparing
the cushioning characteristics of interface materials. We intui-
tively equate cushy with comfort. There is an equally impor-
tant second material characteristic, which is related to friction.
Repeated stresses upon and within the skin and soft tissues
give rise to “hot spots” that cause discomfort and limit func-
tion. These stresses occur because skeletal elements are mov-
ing within a containment structure. When our patients push on
in spite of the pain or because of sensation loss, open trauma
may result (Figure 1).
The everyday term for this is “rub-
bing.” We all have experienced this
from time to time inside our ski
boots, under the straps of a backpack,
or with our thumb against the handle
of a golf club, rake, canoe paddle,
shovel, etc. Gloves and socks help,
but rubbing can do damage even if
there is nothing sliding against the
skin surface. Rubbing just needs to
pull against the skin enough times
and with enough force to begin to
break it down. Friction is the force
that exerts pull/traction on the skin.
NAME_1, a dermatolo-
gist who specialized in the skin prob-
lems of amputees, cites rubbing, friction, and shear as being
causative factors in the generation of blisters, follicular hyper-
keratosis, callosities, and epidermoid cysts.1,2
NAME_2, the orthopedic surgeon who spent his life
caring for people with neuropathic wounds, wrote, “There are
two types of force which occur on the sole of the foot: one is
vertical force at right angles to the foot, which causes direct
pressure on the tissues. The other is horizontal force, or shear
stress, which is parallel to the surface of the foot and occurs
in association with acceleration and deceleration. Of the two
forces shear stress is more damaging than pressure....”3
When discussing forces upon the surface of the skin, the
terms “friction force,” “shear force,” and “traction force” are,
as used in medical literature, essentially the same. The term
“shear stress” is a bit different because it refers to force per
unit of area. The medical literature has been a little loose in
the usage of these terms, but that has not caused any signifi-
cant misunderstanding.
The pull or traction force that an interface material will ex-
ert on the ski There should be exactly 3 paragraphs separated by the markdown divider: ***.
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