Bioengineering models of deep tissue injury

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.

The Role of Shear Stress and Shear Strain in Pressure Injury Development

ABSTRACT

Although other patient safety indicators have seen a decline, pressure injury (PI) incidence has continued to rise. In this article, the authors discuss the role of shear stress and shear strain in PI development and describe how accurate assessment and management can reduce PI risk. They provide explanations of shear stress, shear strain, friction, and tissue deformation to support a better clinical understanding of how damaging these forces are for soft tissue. Clinicians must carefully assess each patient's risk factors regarding shear forces within the contexts of activity and mobility. The authors also provide a toolbox of mitigation strategies, including support surface selection, selection of materials that contact the individual, management of immobility using positioning techniques, and the use of safe patient handling techniques. With a clear understanding of how shear forces affect PI risk and mitigation strategies, clinicians will more accurately assess PI risk and improve PI prevention care plans, ultimately reducing PI incidence to become more aligned with other patient safety indicators.

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.

Best Practice in Pressure Injury Prevention Among Critical Care Patients

Pressure injuries are areas of damage to the skin and underlying tissue caused by pressure or pressure in combination with shear. Pressure injury prevention in the critical care population necessitates risk assessment, selection of appropriate preventive interventions, and ongoing assessment to determine the adequacy of the preventive interventions. Best practices in preventive interventions among critical care patients, including skin and tissue assessment, skin care, repositioning, nutrition, support surfaces, and early mobilization, are described. Unique considerations in special populations including older adults and individuals with obesity are also addressed.

Functional and Volumetric Analysis of the Pectoralis Major Muscle After Submuscular Breast Augmentation

BACKGROUND

Dual plane breast augmentation is a technical variation of the submuscular plane described as a technique that reduces contour deformities due to contraction of the pectoralis major muscle and lower risk of double-bubble deformity associated with breast ptosis. Despite improvement in the aesthetic aspect, there is still no consensus whether this technique affects the function of the pectoralis major muscle.

OBJECTIVES

The aim of this study was to correlate functional with volumetric changes associated with dissection of the muscle origin in submuscular breast augmentation.

METHODS

Thirty women who desired to undergo breast augmentation were selected prospectively and randomly allocated to 2 groups: 10 patients in the control group and 20 patients in the interventional group, who underwent submuscular breast augmentation. Magnetic resonance imaging and volumetric software were used to assess muscle volume and isokinetic dynamometry was used to assess function of the pectoralis major muscle. Preoperative measurements were compared with those at 3, 6, and 12 months after surgery.

RESULTS

Magnetic resonance imaging revealed significant decrease in muscle volume at 6 and 12 months follow-up. The isokinetic test conducted during adduction showed a significant difference in muscle strength between groups from baseline to the 12-month follow-up, and between the 3- and 12-month follow-up. No significant differences in muscle strength during abduction were observed from baseline to the 3-, 6-, and 12-month follow-up.

CONCLUSIONS

Submuscular breast augmentation reduced muscle strength during adduction 12 months after surgery, but without a significant correlation with volumetric muscle loss.

LEVEL OF EVIDENCE 2

Pressure Combined with Ischemia/Reperfusion Injury Induces Deep Tissue Injury via Endoplasmic Reticulum Stress in a Rat Pressure Ulcer Model

Pressure ulcer is a complex and significant health problem in long-term bedridden patients, and there is currently no effective treatment or efficient prevention method. Furthermore, the molecular mechanisms and pathogenesis contributing to the deep injury of pressure ulcers are unclear. The aim of the study was to explore the role of endoplasmic reticulum (ER) stress and Akt/GSK3β signaling in pressure ulcers. A model of pressure-induced deep tissue injury in adult Sprague-Dawley rats was established. Rats were treated with 2-h compression and subsequent 0.5-h release for various cycles. After recovery, the tissue in the compressed regions was collected for further analysis. The compressed muscle tissues showed clear cellular degenerative features. First, the expression levels of ER stress proteins GRP78, CHOP, and caspase-12 were generally increased compared to those in the control. Phosphorylated Akt and phosphorylated GSK3β were upregulated in the beginning of muscle compression, and immediately significantly decreased at the initiation of ischemia-reperfusion injury in compressed muscles tissue. These data show that ER stress may be involved in the underlying mechanisms of cell degeneration after pressure ulcers and that the Akt/GSK3β signal pathway may play an important role in deep tissue injury induced by pressure and ischemia/reperfusion.

Performance of different support surfaces during experimental resuscitation (CPR)

The relationship between the efficacy of resuscitation and the mattresses and backboards used in acute care units, has been studied previously. However, few reports focus on the relative efficacy of resuscitation when using mattresses with different modes of function. This study examines the performance of different support surfaces during experimental cardiopulmonary resuscitation (CPR). The surfaces included a hard surface, a higher specification foam mattress, a dynamic, alternating pressure mattress, and a dynamic, reactive minimum pressure air mattress system. A pressure sensitive mat was placed between the mattresses and each surface and the efficacy of resuscitation measured using differences in compression frequency, compression depth and hands-on time. Our results suggest that the efficacy of resuscitation is dependent on the mode of action of the mattress, while adequate compression frequency and depth do not have a significant effect. In the open system alternating mattress, deflation of the mattress using the CPR function improved the stability of the resuscitation in our study, especially in situations where the height of the air mattress is greater than 20–25 centimeters. Using our experimental system, resuscitation on a closed air system mattress optimally combined stability and effort, while the CPR function converts the air system of the mattress to open, which impairs its functionality during resuscitation. These results indicate that resuscitation is dependent of the mode of action of the mattress and whether the mattress-specific CPR function was used or not. However, the interactions are complex and are dependent on the interaction between the body and the mattress, i.e. its immersion and envelopment properties. Furthermore, this study casts doubt on the necessity of the CPR function in air mattresses.

Vasopressors and development of pressure ulcers in adult critical care patients

BACKGROUND

Vasopressors are lifesaving agents used to raise mean arterial pressure in critically ill patients in shock states. The pharmacodynamics of these agents suggest vasopressors may play a role in development of pressure ulcers; however, this aspect has been understudied.

OBJECTIVE

To examine associations between type, dose, and duration of vasopressors (norepinephrine, epinephrine, vasopressin, phenylephrine, dopamine) and development of pressure ulcers in medical-surgical and cardiothoracic intensive care unit patients and to examine predictors of the development of pressure ulcers in these patients.

METHODS

A retrospective correlational design was used in a sample of 306 medical-surgical and cardiothoracic intensive care unit patients who received vasopressor agents during 2012.

RESULTS

Norepinephrine and vasopressin were significantly associated with development of pressure ulcers; vasopressin was the only significant predictor in multivariate analysis. In addition, mean arterial pressure less than 60 mm Hg in patients receiving vasopressors, cardiac arrest, and mechanical ventilation longer than 72 hours were predictive of development of pressure ulcers. Patients with a cardiac diagnosis at the time of admission to the intensive care unit were less likely than patients without such a diagnosis to experience pressure ulcers while in the unit.

CONCLUSION

The addition of vasopressin administered concomitantly with a first-line agent (often norepinephrine) may represent the point at which the risk for pressure ulcers escalates and may be an early warning to heighten strategies to prevent pressure ulcers. Conversely, because vasopressors cannot be terminated to avert development of pressure ulcers, these findings may add to the body of knowledge on factors that potentially contribute to the development of unavoidable pressure ulcers.

Effectiveness of platelet-rich plasma and hyaluronic acid for the treatment and care of pressure ulcers

Platelet-rich growth factor (PRGF) is a natural source of growth factors (GF), while hyaluronic acid (HA) is a biopolymer present in the extracellular matrix of skin, cartilage, bone, and brain, among other tissues. Both are involved in the pathophysiological mechanisms underlying wound healing. The objective of this study was to evaluate the clinical efficacy (as measured by ulcer area) and safety (as measured by signs of infection) of PRGF and PRGF plus HA in the treatment of pressure ulcers (PUs). Patients (N = 100) with 124 Stage II-III PUs were randomized to a control group (n = 25 PUs) for standard care or to case groups for treatment with one (n = 34 PUs) or two (n = 25 PUs) doses of PRGF from their own peripheral blood, or two doses of PRGF plus HA (n = 40 PUs). All ulcers were followed up every 3 days for a 36-day period. At 36 days, a significant reduction in ulcer area (p ≤ .001) was observed in all treatment groups, with a mean reduction of more than 48.0% versus baseline. The greatest mean reduction (80.4% vs. baseline) was obtained with the PRGF plus HA regimen. Complete wound healing was observed in 32.0% of PUs treated with two doses of PRGF (p ≤ .002) and in 37.5% of those treated with two doses of PRGF plus HA (p ≤ .004). There were no signs of infection in any PUs during the 36-day follow-up period. The degree of wound healing was inversely correlated with the consumption of drugs such as statins and with the peripheral blood platelet levels of patients at baseline.

Laser Doppler imaging of skin microcirculation under fiber-reinforced composite framework of facial prosthesis

OBJECTIVE

Glass-fiber reinforced composite has been suggested to be used as framework material in silicone elastomer facial prostheses. The glass-fiber reinforced framework makes it possible to make the margin of the prosthesis very tight, so that it will lean tightly against the skin even during facial expressions and jaw movements. The purpose of this study was to study how the compression of the glass-fiber reinforced framework would affect the microcirculation of the facial skin.

MATERIALS AND METHODS

A face mask, with a compression pad corresponding to the outer margin of a glass fiber-reinforced composite framework beam of a facial prosthesis, was used to apply pressure on the facial skin of healthy volunteers. The skin blood flow during touch, light and moderate compression of the skin was measured by laser Doppler imaging technique.

RESULTS

None of the compressions had any marked effects on local skin blood flow. No significant differences between the blood flow of the compressed skin, compared to the baseline values, were found.

CONCLUSIONS

The pressure applied to the skin by the tight margins of a facial prosthesis, fabricated with a framework of glass-fiber reinforced composite, does not remarkably alter the skin blood flow.

The angiogenic peptide vascular endothelial growth factor-basic fibroblast growth factor signaling is up-regulated in a rat pressure ulcer model

The purpose of this study is to investigate the mRNA and protein expression levels of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) in pressure ulcers, and to elucidate the molecular mechanism by which VEGF and bFGF are involved in pressure ulcer formation. A rat model of ischemia-reperfusion pressure ulcer was established by magnetic disk circulating compression method. Real-time fluorescence quantitative PCR and Western blot assays were conducted to detect the mRNA and protein expression of VEGF and bFGF in the tissues of rat I-, II-, and III-degree pressure ulcers, the surrounding tissues, and normal skin. Our study confirmed that the mRNA and protein expression levels of VEGF and bFGF in the tissues of rat I-degree pressure ulcer were significantly higher than that in the II- and III-degree pressure ulcer tissues (P < 0.05). The expression of VEGF and bFGF in the tissues surrounding I- and II-degree pressure ulcers were higher than the rats with normal skin. The expression of VEGF and bFGF in the tissues of rat III-degree pressure ulcer was lower than that in the surrounding tissues and normal skin (P < 0.05). There was a significant positive correlation between change in the VEGF and bFGF. The results showed that with an increase in the degree of pressure ulcers, the expression of VEGF and bFGF in pressure ulcers tissue are decreased. This leads to a reduction in angiogenesis and may be a crucial factor in the formation of pressure ulcers.

Pressure ulcer staging revisited: superficial skin changes & Deep Pressure Ulcer Framework©

Deficiencies in the current pressure ulcer classification system create the impetus for the current discourse on the clinical, legal, and economic implications of staging and considering shifting the paradigm in pressure ulcer description and assessment.

The demographics of suspected deep tissue injury in the United States: an analysis of the International Pressure Ulcer Prevalence Survey 2006-2009

OBJECTIVE

Suspected deep tissue injury (sDTI) was identified in 2001 and added as a staging definition by the National Pressure Ulcer Advisory Panel in 2007. Clinical data on sDTI are sparse. This article reports the overall prevalence data and describes the demographics of subjects with sDTI from the International Pressure Ulcer Prevalence survey 2006-2009.

METHODS

Participating healthcare facilities performed prevalence surveys in their facility during a pre-determined 24-hour period within a pre-selected 2- to 3-day window. All generated data was incorporated into the database, even if specific data fields were absent.

RESULTS

Approximately 79,000 to 92,000 patients were surveyed each year from 2006 to 2009. The overall and nosocomial pressure ulcer (PrU) prevalence decreased by approximately 1% in 2009 (P < .001), after remaining fairly constant in the years 2006-2008. The proportion of ulcers identified as sDTI has increased 3 fold, to 9% of all observed ulcers in 2009 and is more prevalent than either Stage III or IV ulcers. Over the same period, the proportion of Stage I and II ulcers have decreased, and the proportion of Stage III and IV ulcers has remained nearly constant. Patients with sDTIs are older than patients with Stage III, IV, and Unstageable ulcers. The anatomic location of sDTIs are more commonly found at the heel (41%), the sacrum (19%), or the buttocks (13%). Compared with other staged ulcers, sDTIs are significantly more prevalent at the heel (P < .001) and the ankle and foot (P < .001) and less prevalent at the sacrum and coccyx (P < .001) and at the buttocks and ischial tuberosities (P < 0.001).

CONCLUSION

The survey data indicate that a decrease in overall prevalence of PrUs, as well as hospital-acquired PrUs, may have occurred in 2009. Suspected deep tissue injuries have become more commonly identified, which may be secondary to education of staging definitions.

Proteasome inhibition alleviates prolonged moderate compression-induced muscle pathology

BACKGROUND

The molecular mechanism initiating deep pressure ulcer remains to be elucidated. The present study tested the hypothesis that the ubiquitin proteasome system is involved in the signalling mechanism in pressure-induced deep tissue injury.

METHODS

Adult Sprague Dawley rats were subjected to an experimental compression model to induce deep tissue injury. The tibialis region of the right hind limb was subjected to 100 mmHg of static pressure for six hours on each of two consecutive days. The compression pressure was continuously monitored by a three-axial force transducer within the compression indentor. The left hind limb served as the intra-animal control. Muscle tissues underneath the compressed region were collected and used for analyses.

RESULTS

Our results demonstrated that the activity of 20S proteasome and the protein abundance of ubiquitin and MAFbx/atrogin-1 were elevated in conjunction with pathohistological changes in the compressed muscle, as compared to control muscle. The administration of the proteasome inhibitor MG132 was found to be effective in ameliorating the development of pathological histology in compressed muscle. Furthermore, 20S proteasome activity and protein content of ubiquitin and MAFbx/atrogin-1 showed no apparent increase in the MG132-treated muscle following compression.

CONCLUSION

Our data suggest that the ubiquitin proteasome system may play a role in the pathogenesis of pressure-induced deep tissue injury.

Update on Pressure Ulcer Management and Deep Tissue Injury

Objective:

To review recent literature regarding therapeutic management of pressure ulcers and to discuss the potential implications of the newly recognized entity, deep tissue injury (DTI).

Data Sources:

A MEDLINE search was conducted of pressure ulcer therapy published between January 2001 and October 2009. Key search terms included pressure ulcers, deep tissue injury, nutrition, antibiotics, and therapy.

Study Selection and Data Extraction:

Comparative clinical trials involving pharmacologic agents in the treatment of pressure ulcers and DTI were evaluated. Included trials were those with defined interventions and outcome parameters for wound healing.

Data Synthesis:

Pressure ulcers remain an important issue in the care of elderly and immobilized patients. DTI has been recently added by the National Pressure Ulcer Advisory Panel as a separate category in the staging of pressure ulcers. There is currently a lack of consensus on how to identify and treat DTI. but preventive measures typically employed in the management of all pressure ulcers have been recommended. Recent studies on topical phenytoin, silver preparations, and growth factors report benefit in the management of pressure ulcers. However, studies evaluating these treatment approaches often lack the sample size necessary to adequately support recommendations.

Conclusions:

Determining the extent of tissue damage in DTI is currently not possible. Therefore, management recommendations focus on limiting extension of the ulcer stage through preventive strategies. Routine use of topical phenytoin, silver preparations, or growth factors in therapy of pressure ulcers cannot be recommended until more data from rigorously designed studies are available.

Current progress in patient-specific modeling

We present a survey of recent advancements in the emerging field of patient-specific modeling (PSM). Researchers in this field are currently simulating a wide variety of tissue and organ dynamics to address challenges in various clinical domains. The majority of this research employs three-dimensional, image-based modeling techniques. Recent PSM publications mostly represent feasibility or preliminary validation studies on modeling technologies, and these systems will require further clinical validation and usability testing before they can become a standard of care. We anticipate that with further testing and research, PSM-derived technologies will eventually become valuable, versatile clinical tools.

Muscle apoptosis is induced in pressure-induced deep tissue injury

Pressure ulcer is a complex and significant health problem. Although the factors including pressure, shear, and ischemia have been identified in the etiology of pressure ulcer, the cellular and molecular mechanisms that contribute to the development of pressure ulcer are unclear. This study tested the hypothesis that the early-onset molecular regulation of pressure ulcer involves apoptosis in muscle tissue. Adult Sprague-Dawley rats were subjected to an in vivo protocol to mimic pressure-induced deep tissue injury. Static pressure was applied to the tibialis region of the right limb of the rats for 6 h each day on two consecutive days. The compression force was continuously monitored by a three-axial force transducer equipped in the compression indentor. The contralateral uncompressed limb served as intra-animal control. Tissues underneath the compressed region were collected for histological analysis, terminal dUTP nick-end labeling (TUNEL), cell death ELISA, immunocytochemical staining, and real-time RT-PCR gene expression analysis. The compressed muscle tissue generally demonstrated degenerative characteristics. TUNEL/dystrophin labeling showed a significant increase in the apoptotic muscle-related nuclei, and cell death ELISA demonstrated a threefold elevation of apoptotic DNA fragmentation in the compressed muscle tissue relative to control. Positive immunoreactivities of cleaved caspase-3, Bax, and Bcl-2 were evident in compressed muscle. The mRNA contents of Bax, caspase-3, caspase-8, and caspase-9 were found to be higher in the compressed muscle tissue than control. These results demonstrated that apoptosis is activated in muscle tissue following prolonged moderate compression. The data are consistent with the hypothesis that muscle apoptosis is involved in the underlying mechanism of pressure-induced deep tissue injury.

Internal mechanical conditions in the soft tissues of a residual limb of a trans-tibial amputee

Most trans-tibial amputation (TTA) patients use a prosthesis to retain upright mobility capabilities. Unfortunately, interaction between the residual limb and the prosthetic socket causes elevated internal strains and stresses in the muscle and fat tissues in the residual limb, which may lead to deep tissue injury (DTI) and other complications. Presently, there is paucity of information on the mechanical conditions in the TTA residual limb during load-bearing. Accordingly, our aim was to characterize the mechanical conditions in the muscle flap of the residual limb of a TTA patient after donning the prosthetic socket and during load-bearing. Knowledge of internal mechanical conditions in the muscle flap can be used to identify the risk for DTI and improve the fitting of the prosthesis. We used a patient-specific modelling approach which involved an MRI scan, interface pressure measurements between the residual limb and the socket of the prosthesis and three-dimensional non-linear large-deformation finite-element (FE) modelling to quantify internal soft tissue strains and stresses in a female TTA patient during static load-bearing. Movement of the truncated tibia and fibula during load-bearing was measured by means of MRI and used as displacement boundary conditions for the FE model. Subsequently, we calculated the internal strains, strain energy density (SED) and stresses in the muscle flap under the truncated bones. Internal strains under the tibia peaked at 85%, 129% and 106% for compression, tension and shear strains, respectively. Internal strains under the fibula peaked at substantially lower values, that is, 19%, 22% and 19% for compression, tension and shear strains, respectively. Strain energy density peaked at the tibial end (104kJ/m(3)). The von Mises stresses peaked at 215kPa around the distal end of the tibia. Stresses under the fibula were at least one order of magnitude lower than the stresses under the tibia. We surmise that our present patient-specific modelling method is an important tool in understanding the etiology of DTI in the residual limbs of TTA patients.

The effects of pressure and shear on capillary closure in the microstructure of skeletal muscles

Deep tissue injury (DTI) is a severe pressure ulcer, which initiates in muscle tissue under a bony prominence, and progresses outwards. It is associated with mechanical pressure and shear that may cause capillaries to collapse and thus, induce ischemic conditions. Recently, some investigators stipulated that ischemia alone cannot explain the etiology of DTI, and other mechanisms, particularly excessive cellular deformations may be involved. The goal of this study was to evaluate the functioning of capillaries in loaded muscle tissue, using animal and finite element (FE) models. Pressures of 12, 37, and 78 kPa were applied directly to one gracilis muscle of 11 rats for 2 h. Temperatures of the loaded and contralateral muscles were recorded with time using infrared thermography (IRT) as a measure of the ischemic level. In addition, a non-linear large deformation muscle-fascicle-level FE model was developed and subjected to pressures of 12-120 kPa without and with simultaneous shear strain of up to 8%. For each simulation case, the accumulative percentage of open capillary cross-sectional area and the number of completely closed capillaries were determined. After 2 h, temperature of the loaded muscles was 2.4 +/- 0.3 degrees C (mean +/- standard deviation) lower than that of the unloaded contralateral limbs (mean of plateau temperature values across all pressure groups). Temperature of the loaded muscles dropped within 10 min but then remained stable and significantly higher than room temperature for at least 30 additional minutes in all pressure groups, indicating that limbs were not completely ischemic within the first 40 min of the trials. Our FE model showed that in response to pressures of 12-120 kPa and no shear, the accumulative percentage of open capillary cross-sectional area decreased by up to 71%. When shear strains were added, the open capillary cross-sectional area decreased more rapidly, but even for maximal loading, only 46% of the capillaries were completely closed. Taken together, the animal and FE model results suggest that acute ischemia does not develop in skeletal muscles under physiological load levels within a timeframe of 40 min. Since there is evidence that DTI develops within a shorter time, ischemia is unlikely to be the only factor causing DTI.