Contoured Foam Cushions Cannot Provide Long-term Protection Against Pressure-Ulcers for Individuals with a Spinal Cord Injury: Modeling Studies

Strain-dependent shear properties of human adipose tissue in vivo

INTRODUCTION

Human adipose tissue (fat) deforms substantially under normal physiological loading and during impact. Thus, accurate data on strain-dependent stiffness of fat is essential for the creation of accurate biomechanical models. Previous studies on ex vivo samples reported human fat to be nonlinear and viscoelastic. When static compression is combined with magnetic resonance (MR) elastography (an imaging technique used to measure viscoelasticity in vivo), the large deformation properties of tissues can be determined. Here, we use magnetic resonance elastography to quantify fat shear modulus in vivo under increasing compressive strain and compare it to the underlying passive gluteal muscle.

METHODS

The right buttocks of ten female participants were incrementally compressed at four levels while MR elastography (50 Hz) and mDixon images were acquired. Maps of tissue shear modulus (G*) were reconstructed from the MR elastography phase images. Tissue strain was estimated from registration of deformed and undeformed mDixon images. Linear mixed models were fit to the natural logarithm of the compressive strain and shear modulus data for each tissue.

RESULTS

Shear modulus increased in an exponential relationship with compressive strain in fat: Gfat*=748.5*Cyy-1.18Pa, and to a lesser extent in muscle: Gmuscle*=956.4*Cyy-0.36Pa. The baseline (undeformed) stiffness of fat was significantly lower than that of muscle (mean G*fat = 752 Pa, mean G*muscle = 1000 Pa, paired samples t-test, t = -4.24, p = 0.001). However, fat exhibited a significantly higher degree of strain dependence (characterised by the exponent of the curve, t = -6.47, p = 0.0001).

CONCLUSION

Static compression of human adipose tissue results in an increase in apparent viscoelastic shear modulus (stiffness), in an exponentially increasing relationship. The relationships defined here can be used in the development of physiologically realistic computational models for impact, injury and biomechanical modelling.

Mechanical analysis of deep tissue injury during sitting in patients with spinal cord injury via parametric finite element model

Spinal cord injury patients are prone to develop deep tissue injury because of long-term mechanical load. However, there is a lack of statistical research on the influence of tissue characteristics on the internal mechanical state of soft tissue. This study aimed to investigate the influence of tissue characteristics on the internal mechanical state of buttock in spinal cord injury patients. A three-dimensional reference buttock model was established and a visualization program was generated to modify the parameter values. Through changing the muscle atrophy, body mass index and the radius of curvature of the ischial tuberosity, 96 different model variants were simulated and validated in this study. With body mass index increasing from 16 to 40, the principal shear stress was 10.4 times principal compressive stress, the maximum shear strain and the max cluster volume increased by 1.2 (P < 0.001) and 8.8 times (P < 0.001), respectively. The interaction between BMI and muscle atrophy was significant when BMI was greater than or equal to 22.5 kg/m². In all BMI stages, when the radius of curvature of the ischial tuberosity was 19 mm, the internal stress of the tissue always occupies the highest value. The results demonstrate that body mass index is the most important factor affecting the risk of buttock deep tissue injury. This study provides insights into investigation of inter-individual factors influencing the soft tissue response and assessment of deep tissue injury risk during sitting.

Computational studies of the biomechanical efficacy of a minimum tissue deformation mattress in protecting from sacral pressure ulcers in a supine position

Sustained soft tissue exposure to localised deformations is a trigger for the formation of pressure ulcers. Immersion and envelopment are critical benchmarks that determine comfort and the pressure ulcer risk mitigation, as they have considerable influence on tissue stress concentrations near bony prominences. In the present study, we developed a computer modelling framework for quantifying the extent by which optimal envelopment disperses tissue stress concentrations near the sacrum. To compare the risk of developing a sacral pressure ulcer while lying supine on a regular foam mattress with respect to lying on a specialised, minimum tissue deformation mattress (which closely conforms to the body contours), we used a three‐dimensional anatomically‐realistic model of the adult female buttocks. The strains and stresses in the subdermal soft tissues reached peak values of 65% and 2.4 kPa for the regular mattress, respectively, but always remained below 45% and 1.2 kPa for the minimum tissue deformation mattress, which indicates longer safe times for supine support on the latter mattress. Our work demonstrates that alleviation of localised, sustained stress concentrations through good immersion and envelopment of the support surface protects from pressure ulcers, and has the potential to relieve chronic pain which is associated with the pressure ulcer risk.

CLINICAL EFFECTIVENESS OF 3D-MODELING-BASED CUSTOMIZED OFF-LOADING PRESSURE-RELIEF CUSHIONS FOR SPINAL CORD INJURY

This study aimed to investigate the effectiveness of a 3D-modeling-based customized off-loading cushion to prevent pressure ulcers in people with spinal cord injury (SCI) using wheelchairs. The study included five people with SCI who use the traditionally manufactured customized off-loading cushions. As part of the test, each subject sat on three types of pressure-relief cushions, and the pressure between the seating surface and cushion was measured for 60[Formula: see text]min. The average measured pressure values were compared, and the change in pressure with time was analyzed to verify the clinical effect. The results showed that the CAD/CAM-based customized off-loading cushion exhibited a better decrease in pressure and pressure distribution effect on the ischial tuberosity and coccyx than the adjustable air cushion but did not differ much from the traditionally manufactured customized off-loading cushion. The clinical and economic effectiveness of the customized off-loading cushion based on the computer-aided design and manufacturing (CAD/CAM) technology was analyzed and tested on people with SCI. An occupational therapist evaluating the client followed by designing the customized off-loading strategy has no difference in terms of clinical effect compared to the traditional manufacturing method. However, time, effort, and cost should be considered when choosing an intervention strategy.

Key Components Related to Pressure Injury Formation: An Initial Investigation Into Pressure Distribution and Blood Perfusion Responses in Wheelchair Users

Soft tissue around bony prominences in the buttocks and back are high-risk areas prone to the development of pressure injuries. From a clinical perspective, prevention of pressure injuries all together is the ideal situation. Unfortunately, prevalence rates still reach 47% with recurrence rates even higher. The goals of this study were to evaluate the effects of a series of wheelchair movements, some that currently exist in commercial wheelchairs and some new, on interface pressures and perfusion under the buttocks. Twenty-seven chair positions were obtained by varying back recline, seat pan tilt, and articulation of two supports along the back. Although back recline is commonly taught by therapists to be used as a pressure relieving posture, results indicated an increase in pressures under the ischial tuberosities and sacral areas in reclined positions. Articulation of the back supports produced changes in posture moving from an "erect" to "slouched" position. These movements successfully shifted pressures across back regions. Seat pan tilt was effective in shifting pressures off the ischial tuberosity regions. Additionally, in a portion of the participants, seat pan tilt consistently increased perfusion under the ischial tuberosity region. The findings of this research suggest that movements other than back recline should be considered to more effectively alter interface pressures, particularly in high-risk regions like the sacrum and ischial tuberosities.

Tissue matters: In-vivo tissue properties of persons with spinal cord injuries to inform clinical models for pressure ulcer prevention

The prevalence of pressure ulcers in patients with spinal cord injuries has been estimated to be between 30% and 47%. Individuals with spinal cord injuries sit for a majority of the time, increasing the risk of developing pressure ulcers in the buttocks and thighs due to large internal stresses. Human body models have been developed to study the formation of pressure ulcers, yet a persistent limitation in these models has been the material properties used to represent the soft tissues in the buttocks and thighs. Specifically, soft tissue material property data have not included wheelchair users, such as those with spinal cord injuries. The goals of this research were 1) to determine the in-vivo material properties of soft tissue in the thighs and buttocks of individuals with spinal cord injuries and 2) compare these to properties obtained from able-bodied people. Results indicated that the proximal and middle thigh regions of those who had a spinal cord injury were softer than the same regions as able-bodied individuals, while the distal thigh regions were stiffer. These findings are vital because they indicate that models developed using properties from able-bodied individuals will not produce internal stress or strain magnitudes that represent individuals who have a spinal cord injury. This information suggests that models should obtain material property data sets from their desired population. Human body models must represent the population being studied if they are to inform clinical assessments and make accurate patient predictions.

Critical biomechanical and clinical insights concerning tissue protection when positioning patients in the operating room: A scoping review

An optimal position of the patient during operation may require a compromise between the best position for surgical access and the position a patient and his or her tissues can tolerate without sustaining injury. This scoping review analysed the existing, contemporary evidence regarding surgical positioning-related tissue damage risks, from both biomechanical and clinical perspectives, focusing on the challenges in preventing tissue damage in the constraining operating room environment, which does not allow repositioning and limits the use of dynamic or thick and soft support surfaces. Deep and multidisciplinary aetiological understanding is required for effective prevention of intraoperatively acquired tissue damage, primarily including pressure ulcers (injuries) and neural injuries. Lack of such understanding typically leads to misconceptions and increased risk to patients. This article therefore provides a comprehensive aetiological description concerning the types of potential tissue damage, vulnerable anatomical locations, the risk factors specific to the operative setting (eg, the effects of anaesthetics and instruments), the complex interactions between the tissue damage risk and the pathophysiology of the surgery itself (eg, the inflammatory response to the surgical incisions), risk assessments for surgical patients and their limitations, and available (including emerging) technologies for positioning. The present multidisciplinary and integrated approach, which holistically joins the bioengineering and clinical perspectives, is unique to this work and has not been taken before. Close collaboration between bioengineers and clinicians, such as demonstrated here, is required to revisit the design of operating tables, support surfaces for surgery, surgical instruments for patient stabilisation, and for surgical access. Each type of equipment and its combined use should be evaluated and improved where needed with regard to the two major threats to tissue health in the operative setting: pressure ulcers and neural damage.

Seated buttocks anatomy and its impact on biomechanical risk

AIM

The objective of this study was to describe the amount, types, and shapes of tissue present in the buttocks during sitting (i.e., seated buttocks soft tissue anatomy), and the impact of seated buttocks soft tissue anatomy on biomechanical risk.

MATERIALS AND METHODS

The buttocks of 35 people, including 29 full-time wheelchair users with and without a history of pelvic pressure ulcers were scanned sitting upright on 3" of flat HR45 foam in a FONAR Upright MRI. Multi-planar scans were analyzed to calculate bulk tissue thickness, tissue composition, gluteus maximus coverage at the ischium, the contour of the skin, and pelvic tilt.

RESULTS

Bulk tissue thickness varied from 5.6 to 32.1 mm, was composed mostly of adipose tissue, and was greatest in the able-bodied cohort. Skin contours varied significantly across status group, with wheelchair users with a history of pressure ulcers having tissue with a peaked contour with a radius of curvature of 65.9 mm that wrapped more closely to the ischium (thickness at the apex = 8.2 mm) as compared to wheelchair users with no pressure ulcer history (radius of curvature = 91.5 mm and apex thickness = 14.5 mm). Finally, the majority of participants presented with little to no gluteus coverage over their ischial tuberosity, regardless of status group.

CONCLUSIONS

This study provides quantitative evidence that Biomechanical Risk, or the intrinsic characteristic of an individual's soft tissues to deform in response to extrinsic applied forces, is greater in individuals at greater risk for pressure ulcers.

Skin stiffness determined from occlusion of a horizontally running microvessel in response to skin surface pressure: a finite element study of sacral pressure ulcers

Pressure ulcers occur following sustained occlusion of microvessels at bony prominences under skin surface pressure (SSP). However, the mechanical conditions of the surrounding soft tissue leading to microvascular occlusion are not fully understood. This study determined the stiffness of homogenized skin with microvasculature at the sacrum that occludes microvessels at an SSP of 10 kPa (consistent with a standard mattress) and recovers from occlusion at 5 kPa (consistent with a pressure-redistribution mattress). We conducted two-dimensional finite element analyses under plane stress and plane strain conditions to determine the stiffness of the skin. The results for plane stress conditions show that the microvessel was occluded with a Young's modulus of 23 kPa in response to an SSP of 10 kPa at the center of the sacrum and that the circulation recovered following a reduction in the SSP to 5 kPa. The resulting Young's modulus is consistent with reported data. Our study indicates that the critical value of the SSP for microvascular occlusion is determined not only by the stiffness of homogenized skin with microvasculature but also by the intraluminal pressure, microvascular wall stiffness, and body support conditions.