Shear and Friction Management for Occupational Therapists:
The Role of Friction and Shear Forces in Pressure Ulcer Generation
Presented by: Mark Payette, CO
Tamarack Habilitation Technologies, Inc.
And Caroline Portoghese, OTR/L, ATP
Fairview University Medical Center
Minnesota Occupational Therapy Association 2011 MOTA Annual Conference: Inspire, Empower, and Engage
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The Role of Friction and Shear Forces in Pressure Ulcer Generation
Objectives
•Review the four extrinsic factors which contribute to pressure ulcer formation; pressure, friction, shear, and microclimate
•Focus on the friction and shear phenomenon:
•Definitions / Terminology
•The partnership between friction, shear and pressure
•Friction and shear in the static loading context
•Ideal Interface characteristics and how to assess •Techniques to manage the “tendency to slide”
•Friction Product review
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•2.5 Million pressure ulcers each year in the US (Reddy et al.2006)
•50% of all admissions and 8% of all deaths at specialized spinal cord injury hospitals are due to pressure ulcers (Thomas 2005) •25% of people with a spinal cord injury develop pressure ulcers each year (Salzberg 1996)
• According to the National Spinal Cord Injury Statistical Center (NSCISC 2010) there are app. 262,000 people with traumatic SCI – that means that app. 65,000 people may develop a pressure ulcer.
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Economic Cost •$43,180 Center for Medicare Services (CMS) fiscal year 2007 (Armstrong et al. 2008)
•65,000 people X $43,000 = $2.8 billion
•Expenditures for treatment of pressure ulcers in the US as high as $11 billion (Estimates by Reddy et al. 2006)
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Human Cost •Treatment time (Down time!)
•Destroy careers, upend lifestyles, reduce independence, depression
•Amputations (reaching the trans‐pelvic level)
•Sepsis / death
•Skin never fully recovers (scar tissue heightens future risks)
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Pressure Ulcer Generation Factors
•Intrinsic factors
•Vascular health, muscle tone, nutrition, age, etc.
•Global Factors
•Client education, motivation, lifestyle, program follow‐up, etc.
•Extrinsic factors
•Pressure
•Shear
•Friction
•Microclimate
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Pressure Ulcer Generation Factors
“Margin of Safety”
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Pressure Ulcer Generation Factors
International review:
Pressure, shear, friction and microclimate in context
A consensus document
London: Wounds International, 2010
http://www.eswell.eu/files/PressureUlcerprevention.pdf
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Traditional PU Generation Model:
“When contact pressure exceeds capillary pressure, cells are deprived of oxygen and nutrients”
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NPUAP definition prefers “Pressure Ulcer” – but recognizes that other factors exist…
“…a localized injury to the skin and/or underlying tissue usually over a bony prominence, as a result of pressure in combination with shear and/or friction. A number of contributing or confounding factors are also associated with pressure ulcers; the significance of these factors is yet to be elucidated.”
National Pressure Ulcer Advisory Panel
www.npuap.org
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Pressure
Support Surfaces (foam, pneumatic, liquid, gel)
•Spreads out pressure load over greater area
•Peak areas will remain in the same locations
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Pressure
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Microclimate: Temperature
•Local tissue temperatures are affected by heat transfer properties of the sitting support surface.
•Local increases in tissue temperature may increase due to hyperemia (natural healing response to slight amounts of tissue trauma).
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Microclimate: Temperature
•1°C (1.8°F) elevation of temperature of a human cell will increase metabolic rate by 10% (Kosiak, 1991)
•Increased metabolic rate increases need for nutrients and oxygen and produces greater volume of waste www.tamarackhti.com
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Microclimate: Temperature
•Temperature increases of 3.75° C (6.75 °F) were found after sitting for two hours on certain seat support surfaces (Ferguson‐Pell et al. 1985). This translates to a 37% higher metabolic rate. The “margin of safety” shrinks:
A small amount of trauma resulting in hyperemic temperature rise can destabilize a previously safe, but marginal, condition. 15
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Microclimate: Temperature
•Temperature affects the strength of the stratum corneum:
At 35°C the mechanical strength of the stratum corneum is 25% of that at 25°C
(Flam, Raab 2005)
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Microclimate: Moisture
•Reduces strength of epidermal tissues (maceration)
•Causes irritation (urine and /or bowel material)
•Increases the interface friction coefficients:
Increasing the possibility of shear induced tissue damage
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Friction and Shear
Research is beginning to establish that the traditional Pressure Ulcer Etiology Model we have been working with (ischemia) is seriously incomplete
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Friction and Shear
There are two Shear/Friction Force Mechanisms
1.Shear augments the ischemic effect of pressure 2.Shear stresses fracture fine biological structures
Pressure is important because it FACILITATES shear
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Definitions
•Stress •Strain
•Pressure (Compression)
•Friction (Frictional Force/Shear Force)
•Shear (noun and verb version)
•Coefficient of Friction (CoF)
•Limiting Friction Load (LFL)
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Stress: the ratio of a force to an area (e.g.; Lbs / sq In)
Strain: the amount of “dimensional change”
that occurs to a material by applying a stress divided by the original dimension.
(e.g.; inches per inch) 21
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Pressure: force per unit area exerted perpendicular to the plane of interest (compression, vertical force)
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Friction (Frictional Force): •the resistance to motion in a parallel direction (of surfaces in contact) relative to the common boundary of two surfaces •the force which resists an effort to slide one surface over another
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Shear: •the deformation of an object in which parallel planes remain parallel but are shifted in a direction parallel to themselves
•the distortion of a body by two oppositely directed parallel forces
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Shear Stress: •the force per unit area exerted parallel to the plane of interest
Shear Strain: •the amount of distortion or deformation of tissue as a result of shear stress
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Coefficient of Friction (CoF):
•A non‐dimensional ratio of the force required to slide two materials (in contact) to the force perpendicular to the surfaces
‐A measurement of the amount of friction existing between two surfaces
‐A lower friction coefficient indicates that there is less resistance to sliding/motion
‐The CoF is always related to two surfaces materials
‐“CoF = the Tangent of the slip angle in radians”
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Limiting Friction Load (LFL):
•The threshold at which motion (sliding) will begin to occur
•The LFL is a mathematical calculation: LFL = the “normal load” X the CoF
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“LFL”
or
“Threshold
of Motion”
Note: “Lpull” is the tangential force (can be thought of as the
“tendency to slide”)
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Conditions (related to shear issues) Static – Dynamic
Dry – Moist
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Dynamic Loading Condition
Dynamic Friction / Shear is associated with movement
•Sliding down in wheelchair (or on bed)
•Sliding during transfers &repositioning
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Static Loading Condition
Sliding (movement) is not yet occurring
•Lying in bed (level/horizontal < head/trunk tilted up)
•Slumped in a chair
•Settling onto a support surface
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Dry – Moist
•“Environmental” conditions; Important because the CoF of most materials increases in a moist environment 35
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“Pressure” (Compression)
Resting (no pressure or shear load)
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“Pressure” (Compression)
Static pressure (compression) load
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“Pressure” (Compression)
No load
Loaded
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Compression strain (distortion)
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“Shear” (static condition) Pressure (compression)
Tangential force (“Tendency to slide”)
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Force resisting motion (friction)
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“Shear” (static condition) No
tangential
load (just
pressure)
Loaded
Shear strain (distortion)
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“Tendency to Slide”
•Gradual sliding or settling into the supported position
•Friction/Shear forces may be very high and sustained after sliding motion stops
•If friction/shear forces are sufficient, they may prevent sliding from occurring in the first place…(incorrect understanding: “I don’t have any friction problems because I do not slip!”)
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Skin “Mobility”:
A Critical friction loading (shear strain) Issue
•Scarred / Adhered Areas
•Skin grafts
Requires special attention to avoid shear trauma
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Skin “Mobility”
Low Shear Modulus
High Shear Modulus
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Adhered scars and skin grafts will see the limiting friction load (threshold of motion) whatever it is.
So we must make that load as low as possible.
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Interface:
Interfaces transfer load to the skin…
…Load that is transmitted through soft tissue to bone.
We customarily think of the “interface” as a foam or gel liner, a cushion (support surface), etc.
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Seating interventions can reduce tissue damaging shear stress/strain
By reducing the “tendency to slide”
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Thigh ramp / orientation
Back recline
Orientation in space (tilt)
Alignment (cradling)
Componentry (straps, knee blockers)
Reducing the “tendency to slide” cannot be overemphasized!
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Interface technologies can reduce tissue damaging friction and shear stress/strain •Seat support surface covers having low CoF characteristics (best if friction reduction is targeted) www.tamarackhti.com
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Ideal Seat Interface Characteristics •Be thin, flexible and stretchable in all directions to allow the support surface to envelop the contours of the body (bony prominences) without developing tension loads from “tenting”
or “hammocking”
•Should not form ridges
•Be moisture and air permeable
•Should reduce friction/shear forces (to maintain stability, “target” the risk area)
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Ideal Seat Interface Characteristics •Avoid multiple layers of “extra padding”
(towels, blankets)
•Clothing is an interface material too –
underwear, outer wear, belt loops, buttons, rivets, restrictive fabric, etc. Old news; same challenge…(fashion)
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Interface technologies can reduce tissue damaging friction and shear stress/strain •What is the difference between “Global” friction reduction and “Targeted” friction reduction?
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Friction is not all bad! Friction is only “bad” where it peaks high enough to be a problem…It is otherwise helpful (for stability, control, etc.)
… Target CoF reduction in identified problem / at risk areas!
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Demonstration
•There are not very many products available that claim to address shear and friction •Most related to moving and transfers (task oriented)
•Assess the various samples used in the demonstration for yourself
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Demonstration
Several seat support surfaces are available for comparing ‐ assess for yourself:
1. First do a “hand test” (bench test)
•Can you feel the reduced friction (low CoF) area?
•Is there a direction to the low CoF area?
•How noticeable is the low CoF area (comparison between samples)
•Is the low CoF area global? Targeted?
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Demonstration
2. Do a “Sit Test”
Recognize and avoid being in a position on the test chair having a “tendency to slide” (this will also demonstrate if you are!)
•horizontal thigh ramp orientation (prop feet if necessary)
•erect trunk alignment
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Demonstration
“Sit Test”
•Can you feel the reduced friction (low CoF) area?
•Is the low CoF area global? Targeted? •Is there a direction to the low CoF area?
•How noticeable is the low CoF area (comparison between samples)
•Do you stay positioned on seat surface? (can you differentiate between “settling” and sliding downwards?)
•Is stability compromised?
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Thank You
For your attention!
Next part of workshop is “hands on”
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For more information
Mark Payette, CO
Tamarack Habilitation Technologies, Inc
Email: [email protected]
Website: www.tamarackhti.com
763‐795‐0057
Caroline Portoghese, OTR/L, ATP, MSCS
Fairview Rehabilitation Services
University of Minnesota Medical Center
Email: [email protected]
612‐237‐3179
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