Compression induced cell damage in engineered muscle tissue: an in vitro model to study pressure ulcer aetiology

An ex vivo animal model to study the effect of transverse mechanical loading on skeletal muscle

In many populations like wheelchair and prosthetic users, the soft tissue is subject to excessive or repetitive loading, making it prone to Deep Tissue Injury (DTI). To study the skeletal muscle response to physical stress, numerous in vitro and in vivo models exist. Yet, accuracy, variability, and ethical considerations pose significant trade-offs. Here, we present an ex vivo approach to address these limitations and offer additional quantitative information on cellular damage. In this study, skeletal muscle tissue from Sprague Dawley rats was isolated and transversely loaded. Histological analysis and fluorescence staining demonstrated that the setup was suitable to keep the tissue alive throughout the experimental procedure. Mechanically induced cell damage was readily distinguishable through morphological changes and uptake of a membrane impermeable dye. Our comparably simple experimental setup can be adapted to different loading conditions and tissues to assess the cell response to mechanical loading in future studies.

Use of a Shear Reduction Surface for Prehospital Transport: A Randomized Crossover Study

OBJECTIVE

To compare the effectiveness of an antishear mattress overlay (ASMO) with a standard ambulance stretcher surface in reducing pressure and shear and increasing patient comfort.

METHODS

In this randomized, crossover design, adults in three body mass index categories served as their own controls. Pressure/shear sensors were applied to the sacrum, ischial tuberosity, and heel. The stretcher was placed in sequential 0°, 15°, and 30° head-of-bed elevations with and without an ASMO. The ambulance traveled a closed course, achieving 30 mph, with five stops at each head-of-bed elevation. Participants rated discomfort after each series of five runs.

RESULTS

Thirty individuals participated. Each participant had 30 runs (15 with an ASMO, 15 without), for a total of 900 trial runs. The peak-to-peak shear difference between support surfaces was -0.03 N, indicating that after adjustment for elevation, sensor location, and body mass index, peak shear levels at baseline (starting pause) were 0.03 N lower for the ASMO than for the standard surface ( P = .02). The peak-to-peak pressure difference between surfaces was -0.16 mm Hg, indicating that prerun peak-to-peak pressure was 0.16 mm Hg lower with the ASMO versus standard surface ( P = .002). The heel received the most pressure and shear. Discomfort score distributions differed between surfaces at 0° ( P = .004) and 30° ( P = .01); the overall score across all elevations was significantly higher with the standard surface than with the ASMO ( P = .046).

CONCLUSIONS

The ASMO reduced shear, pressure, and discomfort. During transport, the ambulance team should provide additional heel offloading.

A pressure monitoring approach for pressure ulcer prevention

BACKGROUND

A pressure ulcer (PU) is a debilitating condition that disproportionately affects people with impaired mobility. PUs facilitate tissue damage due to prolonged unrelieved pressure, degrading quality of life with a considerable socio-economic impact. While rapid treatment is crucial, an effective  prevention strategy may help avoid the development of PUs altogether. While pressure monitoring is currently used in PU prevention, available monitoring approaches are not formalised and do not appropriately account for accumulation and relief of the effect of an applied pressure over a prolonged duration. The aim of this study was to define an approach that incorporates the accumulation and relief of an applied load to enable continuous pressure monitoring.

RESULTS

A tunable continuous pressure magnitude and duration monitoring approach that can account for accumulated damaging effect of an applied pressure and pressure relief over a prolonged period is proposed. Unlike classic pressure monitoring approaches, the presented method provides ongoing indication of the net impact of a load during and after loading.

CONCLUSIONS

The tunable continuous pressure magnitude and duration monitoring approach proposed here may further development towards formalised pressure monitoring approaches that aim to provide information on the risk of PU formation in real-time.

Long-Term Aesthetic and Functional Evaluation of Intramuscular Augmentation Gluteoplasty with Implants

BACKGROUND

The insertion of gluteal silicone implants by intramuscular technique leads patients to develop gluteus maximus muscle atrophy. The objective of the present study was to correlate the muscular atrophy of the gluteus maximus proportional to the volume of the silicone implants used. The secondary objectives were to assess volumetry of the gluteus maximus muscle in the late follow-up, to assess the positioning of the implants, and to verify the association between volumetric muscle recovery and practice of physical exercise.

METHODS

This is a prospective study. The sample was composed of 22 patients who were operated and followed up on an outpatient basis and through gluteus computed tomography at three different moments: preoperatively, 12 months postoperatively, and late postoperatively (≥96 months).

RESULTS

Computed tomographic three-dimensional reconstruction and volumetric analysis showed a median atrophy of 6.68% of the gluteus maximus muscle volume in 12 months and 7.47% in the late postoperative period. The correlation between relative volume of the implant and atrophy percentage of the gluteus maximus did not present statistically significant results. There was an association between the practice of physical exercise and volumetry recovery of the gluteus maximus. No patient presented gluteal implant rotation.

CONCLUSIONS

There is no correlation between proportional volume of implants and atrophy percentage of gluteus maximus muscle, when using implants up to 400 cm 3 . The gluteus maximus muscle presents atrophy in the late follow-up of augmentation gluteoplasty with implants. There is recovery of muscle volumetry in the patients who practice physical activities. Intramuscular plane implants demonstrated stability in their long-term positioning.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, IV.

The prevalence and risk factors of facial pressure injuries related to adult non-invasive ventilation equipment: A systematic review and meta-analysis

To systematically assess the prevalence of facial pressure injuries related to adult non-invasive ventilation equipment, and risk factors of facial pressure injuries. PubMed, Cochrane Library, Web of Science, Embase, China Knowledge Resource Integrated Database, Wanfang Database, Chinese Biomedical Database and Weipu Database were comprehensively searched for observational studies investigating the prevalence and risk factors of facial pressure injuries related to adult non-invasive ventilation equipment from inception to May 16th, 2022. Filter articles based on inclusion and exclusion criteria. The quality of the included studies was evaluated independently by two investigators. Meta-analysis was conducted using Stata 16.0 software package. In total, 2835 articles were screened and data from 12 studies were used in meta-analysis. The prevalence of facial pressure injuries related to adult non-invasive ventilation equipment was 25% (95% confidence interval, CI:15% to 37%, I²  = 97.34%, P < 0.0001). After controlling for confounding variables, the following risk factors of facial pressure injuries: use equipment form, with diabetes, fever, cumulative time of using equipment, facial skin oedema and Glasgow score. Understanding the risk factors of facial pressure injuries can provide the healthcare personnel with the theoretical basis for the management and treatment of the patients.

Investigation of the effect of a 15-degree tilt-in-space on the fluctuation of shear forces exerted on the buttocks when the back support is reclined

[Purpose] This study aimed to investigate the effect of the combination of 15° tilt-in-space and recline angles on the fluctuation of shear forces exerted on the buttocks. [Participants and Methods] The participants were 11 healthy adult males. The parameters of the shear forces were the parallel and perpendicular forces exerted on the buttocks as measured by a force plate. The two conditions tested were T0R100-130 and T15R100-130. The tilt-in-space angles were set to 0° and 15° in the T0R100-130 and T15R100-130 conditions, respectively. The reclining angles were determined to be 100° to 130° in both conditions. [Results] Upon comparing the two conditions, the parallel and the perpendicular forces exerted on the buttocks in the T15R100-130 condition were significantly lower than those in the T0R100-130 condition in all positions of back support. Upon comparing the fluctuation values of the parallel and perpendicular forces, those applied in the T15R100-130 condition were significantly higher than those in the T0R100-130 condition. [Conclusion] These results suggest that the fluctuation of shear forces exerted on the buttocks could be decreased by using a combination of 15° tilt-in-space and reclining functions.

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.

Prevention and treatment of pressure sore following spinal cord injury

Pressure sores or pressure injury is a serious complication of a spinal cord injury (SCI), representing a challenging problem for patients, their caregivers, and their physicians. Persons with SCI are vulnerable to pressure sores throughout their life. Pressure sores can potentially interfere with the physical, psychosocial, and overall quality of life. Outcomes directly depend on education and prevention along with conservative and surgical management. Therefore, it is very important to understand everything about pressure sores following SCI. This review covers epidemiology, cost, pathophysiology, risk factors, staging, evaluation tools, prevention, education, conservative wound care methods, surgical treatment, and future trends in wound healing related to post-SCI pressure sores. A change in nomenclature was adopted by the National Pressure Ulcer Advisory Panel in 2016, replacing “pressure ulcer”with “pressure injury.” New concepts of pressure injury staging, such as suspected deep tissue injuries and unstageable pressure injuries, were also introduced. A systematic evidence-based review of the prevention of and therapeutic interventions for pressure sores was also discussed.

Finite element analysis reveals an important role for cell morphology in response to mechanical compression

Mechanical loading naturally controls cell phenotype, development, motility and various other biological functions; however, prolonged or substantial loading can cause cell damage and eventual death. Loading-induced mechanobiological and mechanostructural responses of different cell types affect their morphology and the internal architecture and the mechanics of the cellular components. Using single, mesenchymal stem cells, we have developed a cell-specific three-dimensional finite-element model; cell models were developed from phase-contrast microscopy images. This allowed us to evaluate the mechanostructural response of the naturally occurring variety of cell morphologies to increase sustained compressive loading. We focus on the morphology of the cytoplasm and the nucleus, as the main mechanically responsive elements, and evaluate formation of tensional strains and area changes in cells undergoing increasing uniaxial compressions. Here, we study mesenchymal stem cells as a model, due to their important role in tissue engineering and regenerative medicine; the method and findings are, however, applicable to any cell type. We observe variability in the cell responses to compression, which correlate directly with the morphology of the cells. Specifically, in cells with or without elongated protrusions (i.e., lamellipodia) tensional strains were, respectively, distributed mostly in the thin extensions or concentrated around the stiff nucleus. Thus, through cell-specific computational modeling of mechanical loading we have identified an underlying cause for stiffening (by actin recruitment) along the length of lamellipodia as well as a role for cell morphology in inducing cell-to-cell variability in mechanostructural response to loading.

Wheelchair Tilt-in-Space and Recline Functions: Influence on Sitting Interface Pressure and Ischial Blood Flow in an Elderly Population

Pressure ulcers (PUs) result from localised injury to the skin and underlying tissue and usually occur over a bony prominence as a result of pressure, often in combination with shear forces. Both pressure magnitude and duration are thought to be key risk factors in the occurrence of PUs, thus exposing wheelchair-bound subjects to high risk of PU development. As a result, wheelchairs that incorporate tilt-in-space and recline functions are routinely prescribed to redistribute pressure away from their ischial tuberosities. The goal of this study was to analyse the role of full-body tilt and recline angles in governing sitting interface pressure and blood circulation parameters in elderly subjects and thereby investigate the efficacy of tilt-in-space wheelchairs for aiding pressure relief activity. Sitting interface pressure and ischial blood flow parameters were examined in 20 healthy elderly subjects while seated in a tilt-in-space and recline wheelchair. Five different angles of seat tilt (5°, 15°, 25°, 35°, and 45°) were assessed in combination with three different angles of backrest recline (5°, 15°, and 30°). The results of the study show that when compared to the upright reference posture, every position (except 15°T/5°R) resulted in a significant decrease in sitting interface pressure. Ischial blood flow also showed significant increases at four different positions (45°T/15°R, 15°T/30°R, 35°T/30°R, and 45°T/30°R) but only at larger tilt-in-space and recline angles. The results therefore suggest that small tilt-in-space and recline angles are indeed able to reduce sitting interface pressures, whereas changes in ischial blood flow only occur at larger angles. In the literature, cell deformation is thought to be dominant over tissue ischemia in the development of tissue necrosis and PUs. Therefore, together with our findings it can be concluded that frequently undertaking small adjustments in tilt-in-space and recline angle might be important for preventing cell deformation and any associated cell necrosis. Larger angles of tilt-in-space and recline seem to support blood flow returning to the tissues, which is likely to play a positive role in healing damaged tissue.

There is an individual tolerance to mechanical loading in compression induced deep tissue injury

BACKGROUND

Deep tissue injury is a type of pressure ulcer which originates subcutaneously due to sustained mechanical loading. The relationship between mechanical compression and damage development has been extensively studied in 2D. However, recent studies have suggested that damage develops beyond the site of indentation. The objective of this study was to compare mechanical loading conditions to the associated damage in 3D.

METHODS

An indentation test was performed on the tibialis anterior muscle of rats (n = 39). Changes in the form of oedema and structural damage were monitored with MRI in an extensive region. The internal deformations were evaluated using MRI based 3D finite element models.

FINDINGS

Damage propagates away from the loaded region. The 3D analysis indicates that there is a subject specific tolerance to compression induced deep tissue injury.

INTERPRETATION

Individual tolerance is an important factor when considering the mechanical loading conditions which induce damage.

Cuff Pressure Algometry in Patients with Chronic Pain as Guidance for Circumferential Tissue Compression for Wearable Soft Exoskeletons: A Systematic Review

In this article, we report on a systematic review of the literature on pressure-pain thresholds induced and assessed by computerized cuff pressure algometry (CPA). The motivation for this review is to provide design guidance on pressure levels for wearable soft exoskeletons and similar wearable robotics devices. In our review, we focus on CPA studies of patients who are candidates for wearable soft exoskeletons, as pain-related physiological mechanisms reportedly differ significantly between healthy subjects and patients with chronic pain. The results indicate that circumferential limb compression in patients most likely becomes painful at ∼10-18 kPa and can become unbearable even below 25 kPa. The corresponding ranges for healthy control subjects are 20-42 kPa (painful limits) and 34-84 kPa (unbearable levels). In addition, the increase of pain with time tends to be significantly higher, and the adaptation to pain significantly lower, than in healthy subjects. The results of this review provide guidance to designers of wearable robotics for populations with chronic pain regarding rates and magnitudes of tissue compression that may be unacceptable to users.

An advanced magnetic resonance imaging perspective on the etiology of deep tissue injury

Early diagnosis of deep tissue injury remains problematic due to the complicated and multifactorial nature of damage induction and the many processes involved in damage development and recovery. In this paper, we present a comprehensive assessment of deep tissue injury development and remodeling in a rat model by multiparametric magnetic resonance imaging (MRI) and histopathology. The tibialis anterior muscle of rats was subjected to mechanical deformation for 2 h. Multiparametric in vivo MRI, consisting of T2, T2*, mean diffusivity (MD), and angiography measurements, was applied before, during, and directly after indentation as well as at several time points during a 14-day follow-up. MRI readouts were linked to histological analyses of the damaged tissue. The results showed dynamic change in various MRI parameters, reflecting the histopathological status of the tissue during damage induction and repair. Increased T2 corresponded with edema, muscle cell damage, and inflammation. T2* was related to tissue perfusion, hemorrhage, and inflammation. MD increase and decrease was reported on the tissue’s microstructural integrity and reflected muscle degeneration and edema as well as fibrosis. Angiography provided information on blockage of blood flow during deformation. Our results indicate that the effects of a single damage-causing event of only 2 h of deformation were present up to 14 days. The initial tissue response to deformation, as observed by MRI, starts at the edge of the indentation. The quantitative MRI readouts provided distinct and complementary information on the extent, temporal evolution, and microstructural basis of deep tissue injury-related muscle damage.

NEW & NOTEWORTHY We have applied a multiparametric MRI approach linked to histopathology to characterize damage development and remodeling in a rat model of deep tissue injury. Our approach provided several relevant insights in deep tissue injury. Response to damage, as observed by MRI, started at some distance from the deformation. Damage after a single indentation period persisted up to 14 days. The MRI parameters provided distinct and complementary information on the microstructural basis of the damage.

Tapered Microfluidic for Continuous Micro-Object Separation Based on Hydrodynamic Principle

Recent advances in microfluidic technologies have created a demand for a simple and efficient separation intended for various applications such as food industries, biological preparation, and medical diagnostic. In this paper, we report a tapered microfluidic device for passive continuous separation of microparticles by using hydrodynamic separation. By exploiting the hydrodynamic properties of the fluid flow and physical characteristics of micro particles, effective size based separation is demonstrated. The tapered microfluidic device has widening geometries with respect to specific taper angle which amplify the sedimentation effect experienced by particles of different sizes. A mixture of 3-μm and 10-μm polystyrene microbeads are successfully separated using 20° and 25° taper angles. The results obtained are in agreement with three-dimensional finite element simulation conducted using Abaqus 6.12. Moreover, the feasibility of this mechanism for biological separation is demonstrated by using polydisperse samples consists of 3-μm polystyrene microbeads and human epithelial cervical carcinoma (HeLa) cells. 98% of samples purity is recovered at outlet 1 and outlet 3 with flow rate of 0.5-3.0 μl/min. Our device is interesting despite adopting passive separation approach. This method enables straightforward, label-free, and continuous separation of multiparticles in a stand-alone device without the need for bulky apparatus. Therefore, this device may become an enabling technology for point of care diagnosis tools and may hold potential for micrototal analysis system applications.

Accuracy of ultrasound, thermography and subepidermal moisture in predicting pressure ulcers: a systematic review

OBJECTIVE

Our aims were to: establish the clinical significance of ultrasound, thermography, photography and subepidermal moisture (SEM) measurement; determine the accuracy of ultrasound, thermography, photography and SEM measurement in detecting skin/tissue damage; determine the relative accuracy of one of these assessment methods over another; make recommendations for practice pertaining to assessment of early skin/tissue damage.

METHOD

The following databases, Cochrane Wounds Group Specialised Register, The Cochrane Central Register of Controlled Trials, Ovid MEDLINE, Ovid EMBASE, Elsevier version, EBSCO CINAHL, ClinicalTrials.gov , WHO International Clinical Trials Registry (ICTR) and The EU Clinical Trials Register were searched for terms including; thermography, ultrasound, subepidermal moisture, photograph and pressure ulcer.

RESULTS

We identified four SEM, one thermography and five ultrasound studies for inclusion in this review. Data analysis indicated that photography was not a method which allowed for the early prediction of PU presence. SEM values increased with increasing tissue damage, with the sacrum and the heels being the most common anatomical locations for the development of erythema and stage I PUs. Thermography identified temperature changes in tissues and skin that may give an indication of early PU development; however the data were not sufficiently robust. Ultrasound detected pockets of fluid/oedema at different levels of the skin that were comparable with tissue damage. Thus, SEM and ultrasound were the best methods for allowing a more accurate assessment of early skin/tissue damage. Using the EBL Critical Appraisal Tool the overall validities of the studies varied between 33.3-55.6%, meaning that there is potential for bias within all the included studies. All of the studies were situated at level IV, V and VII of the evidence pyramid. Although the methodological quality of the studies warrants consideration, these studies showed the potential that SEM and ultrasound have in early PU detection.

CONCLUSION

SEM and ultrasound are promising in the detection and prediction of early tissue damage and PU presence. However, these methods should be further studied to clarify their potential for use more widely in PU prevention strategies.

Strengthening of C2C12 mouse myoblasts against compression damage by mild cyclic compressive stimulation

Deep tissue injury (DTI) is a severe kind of pressure ulcers formed by sustained deformation of muscle tissues over bony prominences. As a major clinical issue, DTI affects people with physical disabilities, and is obviously related to the load-bearing capacity of muscle cells in various in-vivo conditions. It is important to provide a preventive approach to help muscle cells from being damaged by compressive stress. In this study, we hypothesized that cyclic compressive stimulation could strengthen muscle cells against compressive damage and enhance the cell plasma membrane resealing capability. Monolayer of myoblasts was cultured in the cell culture dish covered by a cylinder 0.5% agarose gel. The platen indenter was applied with 20% strain on the agarose gel in the Mach-1 micromechanical system. The vibration was 1Hz sinusoidal function with amplitude 0.2% strain based on 20% gel strain. Cyclic compressive stimulation for 2h could enhance the compressive stress damage threshold of muscle cells, the muscle cell plasma membrane resealing ratio and viability of muscle cell under static loading as preventive approach. This approach might help to reduce the risk of DTI in clinic.

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.

How does lateral tilting affect the internal strains in the sacral region of bed ridden patients? - A contribution to pressure ulcer prevention

BACKGROUND

Repositioning of individuals with reduced mobility and at risk of pressure ulcers is an essential preventive step. Manual or automatic lateral tilting is a way of doing this and the international guidelines propose a 30° to 40° side lying position. The goal of the present study was to determine the internal strains in individuals lying in a supine position and during tilting.

METHODS

Based on magnetic resonance imaging (MRI) of the sacral area of human volunteers, subject specific finite element models were developed. By comparing calculated contours of the skin, fat and muscle with MRI measurements on a flat surface the models were validated. A parameter study was performed to assess the sensitivity of the model for changes in material properties. Simulations were performed at tilting angles of volunteers between 0° and 45°.

FINDINGS

Subjects in a supine position or tilted have the highest strains in the muscle and fat. Tilting does affect the strain distribution, taking away the highest peak strains. There seems to exist an optimal tilting angle between 20° and 30°, which may vary depending on factors such as BMI of the subject and is in the current paper investigated only for the sacrum.

INTERPRETATION

The study shows that tilting indeed has a significant, positive influence on internal strains, which is important for the prevention of deep tissue injury. Additional studies are needed to draw conclusions about the greater trochanter area and the tissues around the shoulder.

Cell death induced by mechanical compression on engineered muscle results from a gradual physiological mechanism

Deep tissue injury (DTI), a type of pressure ulcer, arises in the muscle layers adjacent to bony prominences due to sustained mechanical loading. DTI presents a serious problem in the clinic, as it is often not visible until reaching an advanced stage. One of the causes can be direct mechanical deformation of the muscle tissue and cell. The mechanism of cell death induced by mechanical compression was studied using bio-artificial skeletal muscle tissues. Compression was applied by placing weights on top of the constructs. The morphological changes of the cytoskeleton and the phosphorylation of mitogen-activated protein kinases (MAPK) under compression were investigated. Moreover, inhibitors for each of the three major MAPK groups, p38, ERK, and JNK, were applied separately to look at their roles in the compression caused apoptosis and necrosis. The present study for the first time showed that direct mechanical compression activates MAPK phosphorylation. Compression also leads to a gradual destruction of the cytoskeleton. The percentage apoptosis is strongly reduced by p38 and JNK inhibitors down to the level of the unloaded group. This phenomenon could be observed up to 24h after initiation of compression. Therefore, cell death in bio-artificial muscle tissue caused by mechanical compression is primarily caused by a physiological mechanism, rather than through a physical mechanism which kills the cell directly. These findings reveal insight of muscle cell death under mechanical compression. Moreover, the result indicates a potential clinical solution to prevent DTI by pre-treating with p38 or/and JNK inhibitors.

The Effect of a Liner on the Dispersion of Sacral Interface Pressures During Spinal Immobilization

Sacral pressure ulcers are a significant problem following spinal cord injury and are felt to be in part due to the high interface-pressures generated while strapped to the spine board. The objective of this study was to determine sacral interface-pressure and sensing area in healthy volunteers on a spine board and the effects of a gel pressure dispersion liner. Thirty-seven volunteers were placed on a pressure-sensing mat between the subject and the spine board. Measurements were carried out with and without a gel liner. Pressures and sensing area were recorded every minute for 40 minutes. The highest pressure was generated at the sacral prominence of each subject. Mean interface-pressures were higher on the spine board alone than with the gel liner (p < .0001). Overall, mean sensing area was lower on the spine board than with the gel liner (p < .0001). Standard spinal immobilization causes high sacral interface-pressures. The addition of a gel liner on the spine board decreased overall mean sacral pressures and increased mean sensing area. Generation of sacral pressure ulcers may be related to the initial interface-pressures generated while the patient is strapped to the spine board. The addition of a gel liner may reduce the incidence of sacral pressure ulcers.

Deformation of the gluteal soft tissues during sitting

BACKGROUND

Excessive deformation of soft tissues is considered to be one of the major contributing factors to discomfort and injury for individuals who sit for long periods of time. Soft tissue deformation in research has been measured under the assumption that tissues deform uniaxially below the ischium, with very small or negligible deformations taking place in other directions. Therefore, this study describes the deformation of the gluteus maximus muscle and surrounding fat tissues in the buttock region for seated subjects.

METHODS

In vivo measurements of the deformation for the gluteal soft tissues were obtained from MRI scans of six seated subjects. Each subject was scanned in weight-bearing and non-weight-bearing sitting postures using a Positional MRI scanner (Fonar 0.6 Tesla Indomitable™). Deformations were measured below the ischium and the proximal femur. Deformation of the gluteus maximus was also measured in the distal direction along the thigh for each subject.

FINDINGS

Our data suggest that soft tissues undergo three-dimensional deformation with considerable components below the ischium (mean of 21.4mm) and in the distal direction along the thigh (mean of 20.3mm). Differences in muscle deformation below the ischium were also observed between obese (mean of 27.4mm) and non-obese subjects (mean of 16.5mm).

INTERPRETATION

Findings of this study demonstrate that tissue deformations in sitting include complex three-dimensional motions that are not well approximated by two-dimensional models.

Finite element analysis for evaluating liver tissue damage due to mechanical compression

The development of robotic-assisted minimally invasive surgery (RMIS) has resulted in increased research to improve surgeon training, proficiency and patient safety. Minimizing tissue damage is an essential consideration in RMIS. Various studies have reported the quantified tissue damage resulting from mechanical compression; however, most of them require bench work analysis, which limits their application in clinical conditions of RMIS. We present a new methodology based on nonlinear finite element (FE) analysis that can predict damage degree inside tissue. The effects of the boundary conditions and material property of the FE model on the simulated von Mises stress value and tissue damage were investigated. Four FE models were analyzed: two-dimensional (2D) plane strain model, 2D plane stress model, full three-dimensional (3D) model, and 3D thin membrane model. Nonlinear material properties of liver tissue used in the FEA were derived from previously reported in vivo and in vitro experiments. Our study showed that for integrated von Mises stress and tissue damage computations, the 3D thin membrane model yielded results closest to the full 3D analysis and required only 0.2% of the compute time. The results from 3D thin membrane and the full 3D models fell below plane-strain model and above the plane-stress model. Both stress and necrosis distributions were impacted by the material property of FE models. This study can guide engineers to design surgical instruments to improve patient safety. Additionally it is useful for improving the surgical simulator performance by reflecting more realistic tissue material property and displaying tissue damage severity.

On the genealogy of tissue engineering and regenerative medicine

In this article, we identify and discuss a timeline of historical events and scientific breakthroughs that shaped the principles of tissue engineering and regenerative medicine (TERM). We explore the origins of TERM concepts in myths, their application in the ancient era, their resurgence during Enlightenment, and, finally, their systematic codification into an emerging scientific and technological framework in recent past. The development of computational/mathematical approaches in TERM is also briefly discussed.

The effects of oxidative stress on the compressive damage thresholds of C2C12 mouse myoblasts: implications for deep tissue injury

Deep tissue injury (DTI) is a severe kind of pressure ulcers formed by sustained deformation of muscle tissues over bony prominences. As a major clinical issue, DTI affects people with physical disabilities, and is obviously related to the load-bearing capacity of muscle cells in various in vivo conditions. It has been hypothesized that oxidative stress, either induced by reperfusion immediately following tissue unloading or in chronic inflammatory conditions, may affect the cellular capacity against subsequent mechanical damages. In this study, we measured the compressive damage threshold of C2C12 mouse myoblasts with or without pre-treatment of hydrogen peroxide as an oxidative agent to understand how changes in the oxidative environment may contribute to the development of DTI. Spherical indentation was applied onto a layer of agarose gel (3 mm thick) covering a monolayer of C2C12 myoblasts. Cell damage was recognized by using a cell membrane damage assay, propidium iodide. The spatial profile of the measured percentage cell damage was correlated with the radially varying stress field as determined by finite element analysis to estimate the compressive stress threshold for cell damage. Results supported the hypothesis that chronic exposure to high-dosage oxidative stress could compromise the capability of muscle cells to withstand compressive damages, while short exposure to low-dosage oxidative stress could enhance such capability.

Prevention of Pressure Ulcers Among People With Spinal Cord Injury: A Systematic Review

OBJECTIVES

To evaluate the literature on the effectiveness of bed and wheelchair positioning and repositioning in the prevention of pressure ulcers (PUs) in both the spinal cord injury (SCI) and non-SCI populations.

DESIGN

Systematic review.

METHODS

PubMed, CINAHL, PsycINFO, and EMBASE were queried with the subject heading terms "pressure sore," "pressure ulcer," "position or turn in bed, wheelchair," "pressure relief," and "pressure release." All study design types that assessed the effectiveness of bed and wheelchair positioning and pressure relief maneuvers in any patient group and in any setting were sought. Three independent reviewers extracted and summarized details of eligible trials using a standardized method. Two independent reviewers assessed the methodological quality of each trial using the American Academy of Neurology guidelines. When reviewers were not able to reach consensus, a third independent reviewer served as tiebreaker.

RESULTS

We identified 2820 publications, of which 49 met inclusion criteria. Of these publications, the subject population was 2834 (923 persons with SCI, 717 persons without SCI, and 1194 healthy control subjects). Among studies examining pressure related to position or repositioning in bed or sitting, procedures for measuring skin pressure and metabolism were highly variable by anatomic location, measurement technique, outcome measure, study site, participant characteristics, and description of position/turning for bed and seated interventions. Numerous factors can influence tissue interface pressures, and no prospective studies had been performed to determine a causal relationship between interface pressure and skin breakdown. Several studies suggest that skin response to pressure differs between subjects with and without SCI. Conflicting results and insufficient evidence for optimal bed and seated positioning and turning and pressure relief maneuvers to prevent PUs in both SCI and non-SCI populations were limiting factors.

CONCLUSIONS

Although there is no clear optimal positioning or turning frequency in bed, the evidence suggests avoiding the 90° lateral position because of high pressures and PU risk over the trochanters. During sitting, pressures are linearly redistributed from the sitting area during recline and tilt; however, reclining carries with it an increased risk of shear forces on this skin. The evidence does not support conclusive guidelines on positioning or repositioning techniques for PU prevention in bed or during sitting. We conclude that PU risk is highly individualized, with the SCI population at a higher risk, which demands flexible PU prevention strategies for bed/seated positioning and pressure relief maneuvers. Education has and will remain our most powerful ally to thwart this pervasive public health problem.

Effects of wheelchair cushions and pressure relief maneuvers on ischial interface pressure and blood flow in people with spinal cord injury

OBJECTIVE

To investigate the effectiveness and interactions of 2 methods of pressure ulcer prevention, wheelchair cushions and pressure relief maneuvers, on interface pressure (IP) and blood flow of the buttocks.

DESIGN

Within-subject repeated measures.

SETTING

Rehabilitation center.

PARTICIPANTS

Wheelchair users with a spinal cord injury or disorder (N=17).

INTERVENTIONS

Participants performed 3 forward leans and 2 sideward leans with different degrees of lean while seated on each of 3 different wheelchair cushions.

MAIN OUTCOME MEASURES

IP measured with a custom sensor and blood flow measured with laser Doppler flowmetry were collected at the ischial tuberosity.

RESULTS

Pressure relief maneuvers had a significant main effect on the ischial IP (P<.001); all maneuvers except for the small frontward lean resulted in a significant reduction in IP compared with upright sitting. Blood flow significantly varied across postures (P<.001) with flow during upright sitting and small forward leans being significantly lower than during the full and intermediate leans in both the forward and sideward directions.

CONCLUSIONS

The results of the study highlight the importance of positioning wheelchair users in a manner that facilitates in-seat movement. Regardless of the cushion being used, the pressure relief maneuvers resulted in very large reductions in IPs and significant increases in buttock blood flow. Only the small frontward lean was shown to be ineffective in reducing pressure or increasing blood flow. Because these pressure relief maneuvers involved postural changes that can occur during functional activities, these pressure relief maneuvers can become a part of volitional pressure relief and functional weight shifts. Therefore, clinical instruction should cover both as a means to impart sitting behaviors that may lead to better tissue health.

A numerical study to analyse the risk for pressure ulcer development on a spine board

BACKGROUND

Spine boards are used to immobilise accident victims suspected of having spinal injury. Guidelines about the maximum time patients remain on the board are often exceeded and on occasions may lead to pressure ulcers. Etiological research has shown that two processes ultimately lead to pressure ulcers:"Ischemic damage" which takes several hours to initiate and "deformation damage" at high strains. The latter process is very quick and the first signs of cell damage are already evident within minutes. Thus in order to minimise the risk of pressure ulcer development during prolonged loading, a new soft-layered long spine board has been designed.

METHODS

A subject specific numerical approach has been adopted to evaluate the prototype spine board in comparison to a conventional spine board, with reference to the estimated strains in the soft tissues adjacent to the sacrum in the supine position. The model geometry is derived from magnetic resonance images of three human volunteers in an unloaded situation. The loaded images are used to "tune" the material parameters of skin, fat and muscle. The prediction of the deformed contours on the soft-layered board is used to validate the model.

FINDINGS

Comparison of the internal strains in muscle tissue near the spine showed that internal strains on the soft-layered board are reduced and maximum strains are considerably less than the threshold at which deformation damage is possible. By contrast, on the rigid spine board this threshold is exceeded in all cases.

INTERPRETATION

The prototype comfort board is able to reduce the risk for deformation damage and thus reduces the risk of developing pressure ulcers.

Thinking inside the box: keeping tissue-engineered constructs in vitro for use as preclinical models

Tissue engineers have made great strides toward the creation of living tissue replacements for a wide range of tissue types and applications, with eventual patient implantation as the primary goal. However, an alternate use of tissue-engineered constructs exists: as in vitro preclinical models for purposes such as drug screening and device testing. Tissue-engineered preclinical models have numerous potential advantages over existing models, including cultivation in three-dimensional geometries, decreased cost, increased reproducibility, precise control over cultivation conditions, and the incorporation of human cells. Over the past decade, a number of researchers have developed and used tissue-engineered constructs as preclinical models for testing pharmaceuticals, gene therapies, stents, and other technologies, with examples including blood vessels, skeletal muscle, bone, cartilage, skin, cardiac muscle, liver, cornea, reproductive tissues, adipose, small intestine, neural tissue, and kidney. The focus of this article is to review accomplishments toward the creation and use of tissue-engineered preclinical models of each of these different tissue types.

Mechanical stress meets autophagy: potential implications for physiology and pathology

Changes in the mechanical environment are a universal challenge for cells, and mechanical cues regulate tissue structure and cell physiology throughout life. Autophagy is an important degradative pathway, fulfilling a wide range of roles in survival, homeostasis and adaptation. The two are connected, and in vitro, autophagy is rapidly induced in cells exposed to mechanical compression. In vivo, autophagy is also induced in several medically relevant circumstances that are also under mechanical stress such as bone and muscle homeostasis and tissue injury. The induction of autophagy has wide-ranging effects on cells. In this article, I propose that the autophagic response to mechanical stress is an important factor in a wide range of both physiological and pathological settings.

Backrest position in prevention of pressure ulcers and ventilator-associated pneumonia: conflicting recommendations

Pressure ulcers and ventilator-associated pneumonia (VAP) are both common in acute and critical care settings and are considerable sources of morbidity, mortality, and health care costs. To prevent pressure ulcers, guidelines limit bed backrest elevation to less than 30 degrees, whereas recommendations to reduce VAP include use of backrest elevations of 30 degrees or more. Although a variety of risk factors beyond patient position have been identified for both pressure ulcers and VAP, this article will focus on summarizing the major evidence for each of these apparently conflicting positioning strategies and discuss implications for practice in managing mechanically ventilated patients with risk factors for both pressure ulcers and VAP.

Modelling the effect of repositioning on the evolution of skeletal muscle damage in deep tissue injury

Deep tissue injury (DTI) is a localized area of tissue necrosis that originates in the subcutaneous layers under an intact skin and tends to develop when soft tissue is compressed for a prolonged period of time. In clinical practice, DTI is particularly common in bedridden patients and remains a serious issue in todays health care. Repositioning is generally considered to be an effective preventive measure of pressure ulcers. However, limited experimental research and no computational studies have been undertaken on this method. In this study, a methodology was developed to evaluate the influence of different repositioning intervals on the location, size and severity of DTI in bedridden patients. The spatiotemporal evolution of compressive stresses and skeletal muscle viability during the first 48 h of DTI onset was simulated for repositioning schemes in which a patient is turned every 2, 3, 4 or 6 h. The model was able to reproduce important experimental findings, including the morphology and location of DTI in human patients as well as the discrepancy between the internal tissue loads and the contact pressure at the interface with the environment. In addition, the model indicated that the severity and size of DTI were reduced by shortening the repositioning intervals. In conclusion, the computational framework presented in this study provides a promising modelling approach that can help to objectively select the appropriate repositioning scheme that is effective and efficient in the prevention of DTI.

Single cell viability measurements in 3D scaffolds using in situ label free imaging by optical coherence microscopy

The focus on creating tissue engineered constructs of clinically relevant sizes requires new approaches for monitoring construct health during tissue development. A few key requirements are that the technology be in situ, non-invasive, and provide temporal and spatial information. In this work, we demonstrate that optical coherence microscopy (OCM) can be used to assess cell viability without the addition of exogenous probes in three-dimensional (3D) tissue scaffolds maintained under standard culture conditions. This is done by collecting time-lapse images of speckle generated by sub-cellular features. Image cross-correlation is used to calculate the number of features the final image has in common with the initial image. If the cells are live, the number of common features is low. The number of common features approaches 100% if the cells are dead. In control experiments, cell viability is verified by the addition of a two-photon fluorescence channel to the OCM. Green fluorescent protein transfected human bone marrow stromal cells cultured in a transparent poly(ethylene glycol) tetramethacrylate hydrogel scaffold is used as the control system. Then, the utility of this approach is demonstrated by determining L929 fibroblast cell viability in a more challenging matrix, collagen, an optical scatterer. These results demonstrate a new technique for in situ mapping of single cell viability without any exogenous probes that is capable of providing continuous monitoring of construct health.