The effect of compressive deformations on the rate of build-up of oxygen in isolated skeletal muscle cells

The complex interplay between mechanical forces, tissue response and individual susceptibility to pressure ulcers

OBJECTIVE

The most recent edition of the International Clinical Practice Guideline for the Prevention and Treatment of Pressure Ulcers/Injuries was released in 2019. Shortly after, in 2020, the first edition of the SECURE Prevention expert panel report, focusing on device-related pressure ulcers/injuries, was published as a special issue in the Journal of Wound Care. A second edition followed in 2022. This article presents a comprehensive summary of the current understanding of the causes of pressure ulcers/injuries (PU/Is) as detailed in these globally recognised consensus documents.

METHOD

The literature reviewed in this summary specifically addresses the impact of prolonged soft tissue deformations on the viability of cells and tissues in the context of PU/Is related to bodyweight or medical devices.

RESULTS

Prolonged soft tissue deformations initially result in cell death and tissue damage on a microscopic scale, potentially leading to development of clinical PU/Is over time. That is, localised high tissue deformations or mechanical stress concentrations can cause microscopic damage within minutes, but it may take several hours of continued mechanical loading for this initial cell and tissue damage to become visible and clinically noticeable. Superficial tissue damage primarily stems from excessive shear loading on fragile or vulnerable skin. In contrast, deeper PU/Is, known as deep tissue injuries, typically arise from stress concentrations in soft tissues at body regions over sharp or curved bony prominences, or under stiff medical devices in prolonged contact with the skin.

CONCLUSION

This review promotes deeper understanding of the pathophysiology of PU/Is, indicating that their primary prevention should focus on alleviating the exposure of cells and tissues to stress concentrations. This goal can be achieved either by reducing the intensity of stress concentrations in soft tissues, or by decreasing the exposure time of soft tissues to such stress concentrations.

Our contemporary understanding of the aetiology of pressure ulcers/pressure injuries

In 2019, the third and updated edition of the Clinical Practice Guideline (CPG) on Prevention and Treatment of Pressure Ulcers/Injuries has been published. In addition to this most up‐to‐date evidence‐based guidance for clinicians, related topics such as pressure ulcers (PUs)/pressure injuries (PIs) aetiology, classification, and future research needs were considered by the teams of experts. To elaborate on these topics, this is the third paper of a series of the CPG articles, which summarises the latest understanding of the aetiology of PUs/PIs with a special focus on the effects of soft tissue deformation. Sustained deformations of soft tissues cause initial cell death and tissue damage that ultimately may result in the formation of PUs/PIs. High tissue deformations result in cell damage on a microscopic level within just a few minutes, although it may take hours of sustained loading for the damage to become clinically visible. Superficial skin damage seems to be primarily caused by excessive shear strain/stress exposures, deeper PUs/PIs predominantly result from high pressures in combination with shear at the surface over bony prominences, or under stiff medical devices. Therefore, primary PU/PI prevention should aim for minimising deformations by either reducing the peak strain/stress values in tissues or decreasing the exposure time.

Role of microRNAs in Pressure Ulcer Immune Response, Pathogenesis, and Treatment

Pressure ulcers are preventable, yet highly prevalent, chronic wounds that have significant patient morbidity and high healthcare costs. Like other chronic wounds, they are characterized by impaired wound healing due to dysregulated immune processes. This review will highlight key biochemical pathways in the pathogenesis of pressure injury and how this signaling leads to impaired wound healing. This review is the first to comprehensively describe the current literature on microRNA (miRNA, miR) regulation of pressure ulcer pathophysiology.

Effects of Biowastes Released by Mechanically Damaged Muscle Cells on the Propagation of Deep Tissue Injury: A Multiphysics Study

Deep tissue injuries occur in muscle tissues around bony prominences under mechanical loading leading to severe pressure ulcers. Tissue compression can potentially compromise lymphatic transport and cause accumulation of metabolic biowastes, which may cause further cell damage under continuous mechanical loading. In this study, we hypothesized that biowastes released by mechanically damaged muscle cells could be toxic to the surrounding muscle cells and could compromise the capability of the surrounding muscle cells to withstand further mechanical loadings. In vitro, we applied prolonged low compressive stress (PLCS) and short-term high compressive stress to myoblasts to cause cell damage and collected the biowastes released by the damaged cells under the respective loading scenarios. In silico, we used COMSOL to simulate the compressive stress distribution and the diffusion of biowastes in a semi-3D buttock finite element model. In vitro results showed that biowastes collected from cells damaged under PLCS were more toxic and could compromise the capability of normal myoblasts to resist compressive damage. In silico results showed that higher biowastes diffusion coefficient, higher biowastes release rate, lower biowastes tolerance threshold and earlier timeline of releasing biowastes would cause faster propagation of tissue damage. This study highlighted the importance of biowastes in the development of deep tissue injury to clinical pressure ulcers under prolonged skeletal compression.

Stretching Affects Intracellular Oxygen Levels: Three-Dimensional Multiphysics Studies

Multiphysics modeling is an emerging approach in cellular bioengineering research, used for simulating complex biophysical interactions and their effects on cell viability and function. Our goal in the present study was to integrate cell-specific finite element modeling—which we have developed in previous research to simulate deformation of individual cells subjected to external loading—with oxygen transport in the deformed cells at normoxic and hypoxic environments. We specifically studied individual and combined effects of substrate stretch levels, O2 concentration in the culture media, and temperature of the culture media on intracellular O2 levels in cultured myoblasts, in models of two individual cells. We found that (i) O2 transport became faster with the increasing levels of substrate stretch (ranging from 0 to 24%), and (ii) the effect of a 3  °C temperature drop on slowing down the O2 transport was milder with respect to the effect that strains had. The changes in cell geometry due to externally applied deformations could, hence, theoretically affect cell respiration, which should be a consideration in cellular mechanics experiments.