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 www.tamarackhti.com
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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 www.tamarackhti.com
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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. 3
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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) www.tamarackhti.com
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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 www.tamarackhti.com
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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 www.tamarackhti.com
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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 www.tamarackhti.com
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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 11
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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) www.tamarackhti.com
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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 17
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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 19
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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) www.tamarackhti.com
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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 www.tamarackhti.com
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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” www.tamarackhti.com
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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”) 31
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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