Pressure Injury: Tissue Deformation, Sustained Ischaemia, and Progressive Skin Breakdown
Pressure injury is a localised injury to the skin and underlying tissue caused by sustained mechanical loading, most commonly over bony prominences. Although often identified by visible skin changes, pressure injury is fundamentally a disorder of deep tissue deformation and microvascular failure, with skin breakdown representing a late manifestation. Damage can occur beneath intact skin in some pressure injuries, which explains why injury may progress after pressure is relieved, and why prevention must focus on offloading and repositioning rather than wound care alone.
What You Need to Know
Pressure injury develops when sustained pressure or shear deforms soft tissue beyond its ability to recover. This deformation compresses cells, distorts cell membranes, and collapses capillaries, impairing oxygen delivery and waste removal. Unlike acute trauma, injury evolves silently as prolonged mechanical loading disrupts cellular metabolism and structural integrity, leading to ischaemia, energy failure, and eventual cell death even when the skin surface initially appears intact.
Tissue damage arises through overlapping mechanical and vascular mechanisms rather than pressure alone. Deformation directly injures cells while also compromising perfusion, creating a metabolic environment that accelerates tissue breakdown:
sustained compression that physically distorts cells and intracellular structures
capillary occlusion and impaired microcirculation leading to local hypoxia
accumulation of metabolic waste and depletion of ATP
progressive cell membrane failure and necrosis
Deeper tissues such as muscle and subcutaneous fat are affected first because they have higher metabolic demand and are more vulnerable to deformation between bone and external surfaces. As injury progresses upward, skin breakdown becomes visible only after significant underlying damage has already occurred. Pressure injury is therefore best understood as a deep tissue pathology driven by sustained mechanical loading and ischaemia rather than a superficial skin condition that begins at the surface.
Beyond the Basics
Tissue Deformation as the Primary Injury Mechanism
Pressure injury begins with physical deformation of soft tissue rather than with vascular occlusion alone. Sustained pressure and shear compress, stretch, and distort skin, fat, muscle, and the microvasculature, forcing cells into shapes they cannot tolerate. This deformation disrupts cell membranes, cytoskeletal architecture, and ion transport systems, leading to loss of cellular homeostasis and initiation of injury pathways.
Importantly, this direct mechanical injury can occur even when some blood flow persists. Cells may be structurally damaged before hypoxia becomes severe, explaining why tissue necrosis can develop rapidly under high load and why pressure injury can occur in situations where perfusion appears superficially adequate.
Pressure-Induced Capillary Occlusion
Alongside direct cellular deformation, external pressure compresses capillaries and small venules within affected tissue. These vessels operate at low pressure and collapse easily when exposed to sustained loading, particularly over bony prominences where soft tissue is trapped between bone and an external surface.
Capillary collapse reduces oxygen delivery and impairs removal of metabolic waste. When combined with mechanical distortion of cells, this creates a hostile microenvironment in which injury progresses more rapidly than with ischaemia or deformation alone. The interaction between these mechanisms explains the efficiency with which pressure injuries develop under sustained load.
Tissue Ischaemia and Cellular Hypoxia
Reduced perfusion forces affected cells to rely on anaerobic metabolism, resulting in ATP depletion, lactic acid accumulation, and failure of energy-dependent ion pumps. Intracellular calcium rises, activating proteases and phospholipases that damage membranes and organelles. Muscle tissue is particularly vulnerable because of its high metabolic demand and limited tolerance for oxygen deprivation. This vulnerability explains why pressure injuries frequently originate in deep tissue layers and why surface skin changes may lag behind extensive underlying damage.
Shear Forces and Deep Tissue Distortion
Shear occurs when the skin remains relatively stationary while deeper tissues shift, such as during sliding in bed or semi-recumbent positioning. This movement angulates and stretches blood vessels, worsening perfusion while simultaneously increasing internal tissue strain. Shear dramatically amplifies tissue deformation even when surface pressure appears modest. This mechanism explains the predilection for pressure injury over the sacrum and buttocks and highlights why internal tissue damage may be severe despite minimal visible surface change.
Reperfusion Injury After Pressure Relief
Relief of pressure restores blood flow to previously ischaemic tissue, which is necessary for recovery but can paradoxically worsen injury. Reperfusion introduces oxygen free radicals and inflammatory mediators that exacerbate cellular damage and vascular permeability. This delayed injury contributes to progression of tissue necrosis after offloading and explains why pressure injuries may continue to evolve even when pressure has been reduced. Injury therefore reflects cumulative mechanical and metabolic stress rather than a single reversible event.
Moisture, Friction, and Barrier Breakdown
Moisture weakens the stratum corneum by disrupting lipid organisation and increasing permeability, reducing the skin’s resistance to mechanical stress. Friction then removes superficial epidermal layers, exposing already compromised tissue. Although moisture and friction do not initiate pressure injury independently, they accelerate progression once deep tissue deformation and ischaemia are present. This interaction explains why pressure injuries often deteriorate rapidly in moist environments.
Impaired Healing and Chronicity
Once necrosis has occurred, healing requires restoration of perfusion, oxygenation, and mechanical stability. Continued deformation or microvascular compromise prevents resolution of inflammation and limits effective tissue repair. Chronic pressure injuries persist because the underlying mechanical and vascular drivers remain active. Failure of healing reflects ongoing tissue stress rather than inadequate surface care, reinforcing pressure injury as a disorder of load tolerance and tissue viability rather than a primary skin disease.
Clinical Connections
Pressure injuries most commonly develop over bony prominences such as the sacrum, heels, hips, and elbows, where soft tissue is compressed between bone and an external surface. Early indicators including non-blanching erythema, local temperature change, induration, or deep tissue discolouration reflect underlying deformation and ischaemic injury rather than superficial skin damage. These changes may precede visible breakdown because muscle and subcutaneous tissue fail before the epidermis, masking the true extent of injury in its early stages.
Prevention strategies are effective only when they interrupt the mechanical and vascular mechanisms driving tissue damage. Interventions are therefore designed to reduce deformation, preserve microcirculation, and support tissue tolerance:
regular repositioning to limit duration of sustained tissue deformation
pressure redistribution to reduce peak loads over vulnerable areas
shear reduction to prevent internal tissue distortion during movement
moisture control to preserve skin integrity and reduce friction
optimisation of perfusion and nutrition to support cellular resilience and repair
Early recognition of deep tissue injury is critical because once deformation-induced necrosis has occurred, damage may continue to progress even after pressure is relieved. Identifying subtle early changes allows intervention before injury becomes irreversible, reinforcing pressure injury as a disorder of tissue tolerance and mechanical load rather than a late-stage skin breakdown alone.
Pressure injuries are staged based on the depth of tissue damage and the structures involved:
Stage 1: intact skin with non-blanchable erythema
Stage 2: partial thickness loss involving epidermis and/or dermis, may present as a blister
Stage 3: full thickness skin loss extending into subcutaneous tissue
Stage 4: full thickness tissue loss with exposed bone, tendon, or muscle
Unstageable: depth obscured by slough or eschar
Deep tissue injury: persistent non-blanchable deep red or purple discoloration indicating underlying damage
Concept Check
Why is tissue deformation considered the primary mechanism of pressure injury?
How does pressure differ from shear in contributing to cellular injury?
Why does deep tissue injury often occur before skin breakdown?
How does reperfusion worsen tissue damage after pressure relief?
Why are dressings ineffective without addressing mechanical load?