Cause and Mechanisms of (pressure,friction,shear) pressure wounds

What are the three primary mechanical forces that lead to pressure injuries?

Pressure, friction, and shear.

What is the main cause of pressure injuries?

Prolonged external pressure that exceeds capillary closing pressure, leading to ischemia and tissue death.

What is the capillary closing pressure threshold?

Approximately 32 mmHg — pressure above this level impairs blood flow.

How does pressure duration affect tissue injury?

Longer periods of pressure increase tissue ischemia and necrosis risk.

Why does pressure over bony prominences cause injury?

Bone compresses underlying tissue and capillaries against external surfaces.

What happens to tissue when pressure is unrelieved?

Oxygen and nutrients cannot reach cells, causing hypoxia, acidosis, and eventual necrosis.

Which bony prominences are most at risk for pressure injuries in supine position?

Occiput, scapulae, elbows, sacrum, coccyx, and heels.

Which bony prominences are most at risk for pressure injuries in side-lying position?

Ears, shoulders, trochanters, knees, and ankles.

Which areas are most at risk for pressure injuries in sitting position?

Shoulder blades, sacrum, buttocks, and ischial tuberosities.

What is friction?

The mechanical rubbing of skin against another surface, causing superficial damage or abrasion.

How does friction contribute to skin breakdown?

It removes the protective stratum corneum and creates microtears in the epidermis.

What activities increase friction risk?

Dragging a patient across a bed sheet or improper repositioning.

How can nurses prevent friction injuries?

Use lift sheets, transfer devices, and avoid sliding patients across surfaces.

What is shear?

The force that occurs when skin remains stationary but underlying tissues move in opposite directions.

How does shear differ from friction?

Friction damages the surface layer; shear tears underlying blood vessels and connective tissue.

What happens at the tissue level during shear injury?

Deformation of capillaries causes occlusion and ischemia in deeper tissues.

When is shear most likely to occur?

When patients slide down in bed or are pulled up without proper lifting technique.

Why is shear particularly dangerous?

It damages deeper tissues even when the skin appears intact.

What combination of forces causes most pressure injuries?

Pressure plus shear.

What nursing practice can minimize shear?

Keep the head of the bed ≤30 degrees unless contraindicated and use draw sheets for movement.

What is microclimate in relation to skin?

The local temperature and moisture at the skin-support surface interface.

How does moisture contribute to pressure injury risk?

It increases friction and reduces tissue tolerance to pressure and shear.

What causes moisture-associated skin damage (MASD)?

Prolonged contact with urine, feces, sweat, or wound drainage.

What nursing intervention reduces moisture-related damage?

Frequent cleansing, moisture barriers, and absorbent underpads.

How does poor nutrition increase mechanical injury risk?

It weakens skin and connective tissue, lowering tissue tolerance to stress.

How does immobility relate to pressure injury formation?

Immobile patients cannot reposition to relieve pressure or shear.

What systemic factors can worsen mechanical injury effects?

Hypotension, dehydration, hypoxia, and anemia reduce tissue perfusion.

What is the purpose of pressure-redistributing surfaces?

They spread weight over a larger area to reduce localized pressure.

Why is shear often unnoticed in early stages of injury?

It occurs beneath the surface, causing deep tissue damage before visible signs appear.

How can repositioning prevent both pressure and shear injuries?

It restores blood flow and reduces prolonged compression of tissues.

What should nurses assess for during repositioning?

Redness, firmness, temperature change, and tenderness in high-risk areas.

Why are friction and shear especially harmful to older adults?

They have thinner dermal layers and reduced elasticity.

What nursing action best prevents mechanical stress to skin?

Gently lifting, not dragging, patients during transfers or turns.