3D anatomy and deformation of the seated buttocks

A systematic review of the effectiveness of pressure relieving cushions in reducing pressure injury

This systematic review seeks to gather and analyze the evidence on wheelchair pressure relieving cushions, and report on the optimal materials and designs for reducing pressure injury risk. The following research question guides this study: Which wheelchair cushions best reduce pressure injury risk? PIs continue to impact the health and function of wheelchairs users with significant mobility impairments. Pressure relieving cushions are typically prescribed to provide pressure relief in the pre-wound, wound, and post-wound phases. Presently, no published reviews analyze all of the commonly available cushion materials. Most comparison studies typically address a specific population such as spinal cord injury, or only a few styles of cushion design/materials. Results suggest air-cell cushions provide optimal pressure relief and shear reduction. Furthermore, small sample single cohort studies suggest off-loading cushions provide superior pressure relief beyond that of air-celled cushions but require additional research for greater generalizability.

In vivo mechanical response of thigh soft tissues under compression: A two-layer model allows an improved representation of the local tissue kinematics

Biomechanical parameters have the potential to be used as physical markers for prevention and diagnosis. Finite Element Analysis (FEA) is a widely used tool to evaluate these parameters in vivo. However, the development of clinically relevant FEA requires personalisation of the geometry, boundary conditions, and constitutive parameters. This contribution focuses on the characterisation of mechanical properties in vivo which remains a significant challenge for the community. The aim of this retrospective study is to evaluate the sensitivity of the computed elastic parameters (shear modulus of fat and muscle tissues) derived by inverse analysis as a function of the geometrical modelling assumption (homogenised monolayer vs bilayer) and the formulation of the cost function. The methodology presented here proposes to extract the experimental force-displacement response for each tissue layer (muscle and fat) and construct the associated Finite Element Model for each volunteer, based on data previously collected in our group (N = 7 volunteers) as reported in (Fougeron et al., 2020). The sensitivity analysis indicates that the choice of the cost function has minimal impact on the topology of the response surface in the parametric space. Each surface displays a valley of parameters that minimises the cost function. The constitutive properties of the thigh (reported as median ± interquartile range) were determined to be (μ=198±322Pa,α=37) for the monolayer and (μmuscle=1675±1127Pa,αmuscle=22±14,μfat=537±1131Pa,αfat=32±7) for the bilayer. A comparison of the homogenised monolayer and bilayer models showed that adding a layer reduces the error on the local force displacement curves, increasing the accuracy of the local kinematics of soft tissues during indentation. This allows for an increased understanding of load transmission in soft tissue. The comparison of the two models in terms of strains indicates that the modelling choice significantly influences the localization of maximal compressive strains. These results support the idea that the biomechanical community should conduct further work to develop reliable methodologies for estimating in vivo strain in soft tissue.

Smart Cushions with Machine Learning-Enhanced Force Sensors for Pressure Injury Risk Assessment

Prolonged sitting can easily result in pressure injury (PI) for certain people who have had strokes or spinal cord injuries. There are not many methods available for tracking contact surface pressure and shear force to evaluate the PI risk. Here, we propose a smart cushion that uses two-dimensional force sensors (2D-FSs) to measure the pressure and shear force in the buttocks. A machine learning algorithm is then used to compute the shear stresses in the gluteal muscles, which helps to determine the PI risk. The 2D-FS consists of a ferroelectret coaxial sensor (FCS) unit placed atop a ferroelectret film sensor (FFS) unit, allowing it to detect both vertical and horizontal forces simultaneously. To characterize and calibrate, two experimental approaches are applied: one involves simultaneously applying two perpendicular forces, and one involves applying a single force. To separate the two forces, the 2D-FS is decoupled using a deep neural network technique. Multiple FCSs are embedded to form a smart cushion, and a genetic algorithm-optimized backpropagation neural network is proposed and trained to predict the shear strain in the buttocks to prevent PI. By tracking the danger of PI, the smart cushion based on 2D-FSs may be further connected with home-based intelligent care platforms to increase patient equality for spinal cord injury patients and lower the expense of nursing or rehabilitation care.

Prediction of tissue deformation based on mechanical and physiological factors in the prone position during surgery

AIM

This study aimed to predict tissue deformation based on the pressure applied while lying in the prone position and physiological factors.

METHODS

Healthy volunteers were instructed to lie on mattresses of four different hardness levels (50, 87.5, 175, and 262.5 N). The order in which the mattresses were used was randomized per participant. Pressure at the iliac crests was measured using a pressure mapping sensor sheet. Participants were placed in the prone position for 10 min, with pressure data used from the latter 5 min. For the tissue deformation at the iliac crests, our previous study data were used. Multiple regression analysis was used to identify predictive mechanical and physiological factors.

RESULTS

The distance between the left and right greater trochanters, maximum interface pressure and age were significant predictors for compression of the skin and soft tissue. Significant predictors of internal soft tissue displacement were the distances between the left and right anterior superior iliac spines and greater trochanters. No factors predicted skin surface displacement.

CONCLUSIONS

Our study provided predictive factors that may be measured easily in a clinical setting to reduce the risk of pressure ulcers during surgery in the prone position.

In vivo strain measurements in the human buttock during sitting using MR-based digital volume correlation

Advancements in systems for prevention and management of pressure ulcers require a more detailed understanding of the complex response of soft tissues to compressive loads. This study aimed at quantifying the progressive deformation of the buttock based on 3D measurements of soft tissue displacements from MR scans of 10 healthy subjects in a semi-recumbent position. Measurements were obtained using digital volume correlation (DVC) and released as a public dataset. A first parametric optimisation of the global registration step aimed at aligning skeletal elements showed acceptable values of Dice coefficient (around 80%). A second parametric optimisation on the deformable registration method showed errors of 0.99mm and 1.78mm against two simulated fields with magnitude 7.30±3.15mm and 19.37±9.58mm, respectively, generated with a finite element model of the buttock under sitting loads. Measurements allowed the quantification of the slide of the gluteus maximus away from the ischial tuberosity (IT, average 13.74 mm) that was only qualitatively identified in the literature, highlighting the importance of the ischial bursa in allowing sliding. Spatial evolution of the maximus shear strain on a path from the IT to the seating interface showed a peak of compression in the fat, close to the interface with the muscle. Obtained peak values were above the proposed damage threshold in the literature. Results in the study showed the complexity of the deformation of the soft tissues in the buttock and the need for further investigations aimed at isolating factors such as tissue geometry, duration and extent of load, sitting posture and tissue properties.

The theory and practice of pressure ulcer/injury risk assessment: a critical discussion

Pressure ulcer/injury (PU) risk assessment is widely considered an essential component in clinical practice. It is a complex and broad concept that includes different approaches, such as clinical judgement, using standardised risk assessment instruments, skin assessments, or using devices to measure skin or tissue properties. A distinction between PU risk assessment and early detection is important. PU risk measures the individual's susceptibility to developing a PU under a specific exposure (primary prevention), and early detection includes the assessment of early (sub)clinical signs and symptoms to prevent progression and to support healing (secondary prevention). PU risk is measured using prognostic/risk factors or prognostic models. Every risk estimate is a probability statement containing varying degrees of uncertainty. It therefore follows that every clinical decision based on risk estimates also contains uncertainty. PU risk assessment and prevention is a complex intervention, where delivery contains several interacting components. There is a huge body of evidence indicating that risk assessment and its outcomes, the selection of preventive interventions and PU incidence are not well connected. Methods for prognostic model development and testing in PU risk research must be improved and follow state-of-the-art methodological standards. Despite these challenges, we do have substantial knowledge about PU risk factors that helps us to make better clinical decisions. An important next step in the development of PU risk prediction might be the combination of clinical and other predictors for more individualised care. Any prognostic test or procedure must lead to better prevention at an acceptable cost.

PIPER adult comfort: an open-source full body human body model for seating comfort assessment and its validation under static loading conditions

Introduction: In this paper we introduce an adult-sized FE full-body HBM for seating comfort assessments and present its validation in different static seating conditions in terms of pressure distribution and contact forces. Methods: We morphed the PIPER Child model into a male adult-sized model with the help of different target sources including his body surface scans, and spinal and pelvic bone surfaces and an open sourced full body skeleton. We also introduced soft tissue sliding under the ischial tuberosities (ITs). The initial model was adapted for seating applications with low modulus soft tissue material property and mesh refinements for buttock regions, etc. We compared the contact forces and pressure-related parameters simulated using the adult HBM with those obtained experimentally from the person whose data was used for the model development. Four seat configurations, with the seat pan angle varying from 0° to 15° and seat-to-back angle fixed at 100°, were tested. Results: The adult HBM could correctly simulate the contact forces on the backrest, seat pan, and foot support with an average error of less than 22.3 N and 15.5 N in the horizontal and vertical directions, which is small considering the body weight (785 N). In terms of contact area, peak, and mean pressure, the simulation matched well with the experiment for the seat pan. With soft tissue sliding, higher soft tissue compression was obtained in agreement with the observations from recent MRI studies. Discussion: The present adult model could be used as a reference using a morphing tool as proposed in PIPER. The model will be published openly online as part of the PIPER open-source project (www.PIPER-project.org) to facilitate its reuse and improvement as well as its specific adaptation for different applications.

Upright vs. supine shape-capturing comparison for custom-contoured cushions

AIMS

This study aims to explore best practices of shape capturing methods for creating custom-contoured wheelchair cushions for the purpose of reducing pressure injury risk.

MATERIALS AND METHODS

Pressure redistribution qualities were measured by peak pressure index (PPI) and compared between custom-contoured seating systems molded in supine vs. upright shape-capturing positions. Subjects consisted of 14 able-bodied participants in a within-subjects comparison.

RESULTS

A Wilcoxon signed-rank test and paired t-test were calculated to compare the mean differences of the PPI between shape-capturing positions. Supine shape-capturing resulted in a statistically significant lower PPI compared to upright (z = 2.040, p < . 05) (t = -2.28, p < .05).

CONCLUSION

Shape-capturing in the supine position provided greater pressure distribution as opposed to an upright position. Creating custom-contoured cushions molded in a supine position can reduce the risk of pressure injury to the patient.

The design of a family of parametric anatomically-based compliant buttock models to evaluate wheelchair cushion performance

The evaluation of wheelchair cushion performance is of interest to a variety of stakeholders, including standards organizations, cushion manufacturers, clinicians, users and payers. The objective of this project was to develop a family of compliant buttock models that are based upon the anatomical parameters of persons with varying body sizes. The models are parametrically designed so can be scaled to evaluate different sized cushions. This paper will detail the designs, describe the anatomical basis for the design and provide the rationale for the design decisions. The manuscript also serves a secondary purpose to illustrate how anthropometric data can be applied to the design of anatomical phantoms that reflect both soft tissue and skeletal anthropometry. Supplemental material includes greater detail and the full CAD files and model fabrication instructions are available in an open access repository for persons who wish to fabricate the models.

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.

A parametric modeling of adult body shape in a supported seated posture including effects of age

Statistical body shape models (SBSM) provide compact, flexible representations of body shape that can be implemented in design software. However, few SBSMs have been created to represent adults in supported seated postures that are relevant for the design of seated environments, and none has incorporated the effects of age. This paper presents an SBSM based on surface laser-scan data from 155 U.S. adults. The data were processed to obtain homologous mesh structure and symmetric geometry, and the processed data were statistically analysed using principal component analysis to obtain a compact representation of the data variance. Regression analysis was conducted to predict body size and shape from stature, body mass index, ratio of sitting height to stature, sex, and age. The resulting model allows rapid generation of realistic body models for applications, including product design, accommodation assessment, and safety system optimisation. The model is publicly accessible at HumanShape.org. Practitioner summary: This paper presents a statistical model that represents adult body shapes in a supported seated posture based on 3 D anthropometric measurements. This model is the first whole-body parametric model known to incorporate age effects based on data extending beyond 65 years of age.

Modelling, design and control of a new seat-cushion for pressure ulcers prevention

Pressure ulcers are a frequent complication in patients having limited activity and mobility (e.g., elderly people, spinal cord injury patients, people with disabilities, etc.). The aim of this work is the conceptual design, modelling and control of a new seat cushion for pressure ulcers prevention. The whole system (constituted by the seat cushion equipped with a real-time pressure mapping with closed-loop control) is designed to identify the critical points on the human skin, before pressure ulcers creation, and to be able to distribute the contact pressure between the human and cushion avoiding wound creation. The seat cushion is constituted by soft air-cells actuated by air flow. To define the shape and size of the soft air-cells, finite element simulations are carried out, analysing the internal volume reduction with external loads application to reproduce the variable stiffness. The data obtained by finite element analysis are used to simulate inflation and deflation of the soft bubble air-cells. Finally, the control systems of a single air-cell and of the whole cushion are designed and simulated. The novelty of our work consists in the conception of a seat cushion able to recognise higher and lower risk zones of pressure ulcer generation on the human skin and to provide compensation automatically. This work can therefore be considered in line with the sustainable development goals recently launched by the EU Commission.

Subcutaneous Fat Thickness Remarkably Influences Contact Pressure and Load Distribution of Buttock in Seated Posture

Spinal cord injury patients are prone to develop deep tissue injury (DTI) as they may spend half their time per day in sitting postures, which produce excessive load in their buttocks. However, the impact of fat thickness on the biomechanical response of buttock in sitting posture remained unclear. This study aimed to investigate the influence of subcutaneous fat thickness on the interface pressure and load distribution of buttock of seated humans. To achieve this goal, a 3-dimensional finite element model of male buttock was constructed and the contact pressure on a rigid cushion was evaluated against experimental results. The modified models, which had various fat thicknesses under ischial tuberosity, were built and used to simulate the sitting conditions with different cushion stiffnesses. In the models simulating sitting on the rigid cushion, the peak contact pressure ranges from 0.052 MPa to 0.149 MPa. In the simulation of sitting on the soft cushion, the peak stress of muscle underneath ischial tuberosity in the model with the thickest fat tissue was slightly higher than that of the other models. The results demonstrate that the fat tissue in the buttock could reduce the contact pressure when sitting on the rigid seat. However, contact pressure solely could not be used to estimate the internal tissue stress of seated buttock, especially in subjects with thicker fat tissue.

Synchronous imaging of pelvic geometry and muscle morphometry: a pilot study of pelvic retroversion using upright MRI

This study investigated feasibility of imaging lumbopelvic musculature and geometry in tandem using upright magnetic resonance imaging (MRI) in asymptomatic adults, and explored the effect of pelvic retroversion on lumbopelvic musculature and geometry. Six asymptomatic volunteers were imaged (0.5 T upright MRI) in 4 postures: standing, standing pelvic retroversion, standing 30° flexion, and supine. Measures included muscle morphometry [cross-sectional area (CSA), circularity, radius, and angle] of the gluteus and iliopsoas, and pelvic geometry [pelvic tilt (PT), pelvic incidence (PI), sacral slope (SS), L3-S1 lumbar lordosis (LL)] L3-coccyx. With four volunteers repeating postures, and three raters assessing repeatability, there was generally good repeatability [ICC(3,1) 0.80-0.97]. Retroversion had level dependent effects on muscle measures, for example gluteus CSA and circularity increased (up to 22%). Retroversion increased PT, decreased SS, and decreased L3-S1 LL, but did not affect PI. Gluteus CSA and circularity also had level-specific correlations with PT, SS, and L3-S1 LL. Overall, upright MRI of the lumbopelvic musculature is feasible with good reproducibility, and the morphometry of the involved muscles significantly changes with posture. This finding has the potential to be used for clinical consideration in designing and performing future studies with greater number of healthy subjects and patients.

Changes in external ischial tuberosity width at varying trunk-thigh angles between sexes using two measurement methods

This study examined the influence of two methods and various trunk–thigh (TT) angles on external ischial tuberosity width (EITW) for 45 men and 45 women. In the experiment, the impress and seat pressure methods were applied at TT angles of 60°, 75°, 90°, and 105°. When the impress method was used, EITW remained highly consistent across the four measured TT angles with differences of 2.8 and 2.1 mm for men and women, respectively. Conversely, in the seated pressure method, EITW increased with TT angle such that differences in EITW across a full TT angle range were 11.5 and 11.7 mm for men and women, respectively. Irrespective of method, differences in EITW between genders measured approximately 12.6–13.7 mm across all TT angles. Correlation analyses revealed that hip circumference was positively related to EITW in all cases, whereas the relationship of hip width and depth with EITW varied by method and gender. Because of inherent differences in EITW between genders, these findings suggest that gender variability should be considered in seat cushion design.

A Case Study on the Effects of Foam and Seat Pan Inclination on the Deformation of Seated Buttocks Using MRI

OCCUPATIONAL APPLICATIONSWe investigated the effects of seat pan inclination and foam on the deformation of the seated buttocks using an upright MRI system. From observations among four healthy males, we found that soft tissue deformation under the ischial tuberosity (IT) could be reduced not only by using a soft cushion, but also by decreasing the shear force on the seat pan surface. These results suggest that soft tissue deformation could be used as an objective measure for assessing seating discomfort and injury risk, by accounting for the effects of both contact pressure and shear. We also confirmed that the gluteus maximus (GM) muscle displaced away from the IT once seated. As peak pressure and shear are most likely located below the IT, more realistic computational human body models in this region are needed that consider muscle sliding.

The relationship between pressure offloading and ischial tissue health in individuals with spinal cord injury: An exploratory study

Objectives: To compare thickness and texture measures of tissue overlying the ischial region in able-bodied (AB) individuals vs. individuals with spinal cord injury (SCI) and to determine if there is a relationship between pressure offloading of the ischial tuberosities (IT) and tissue health in individuals with SCI. Design: Exploratory cross-sectional study. Setting: University setting and rehabilitation hospital. Outcome Measures: Thickness and texture measurements from ultrasound images of tissues overlying the IT were obtained from AB individuals (n = 10) and individuals with complete or incomplete traumatic and non-traumatic SCI American Spinal Injury Association Impairment Scale (AIS) classification A-D (n = 15). Pressure offloading was measured in individuals with SCI and correlated with tissue health measurements. Results: The area overlying the IT occupied by the muscle was significantly greater in the SCI when compared with AB cohort. The area occupied by the muscle in individuals with SCI appeared to lose the striated appearance and was more echogenic than nearby skin and subcutaneous tissue (ST). There was no correlation between offloading times and thickness, echogenicity and contrast measurements of skin, ST and muscle in individuals with SCI. Conclusion: Changes in soft tissues overlying the ischial tuberosity occur following SCI corresponding to the loss of striated appearance of muscle and increased thickness of the area occupied by the muscle. Further studies using a larger sample size are recommended to establish if thickness and tissue texture differ between individuals with SCI who sustain pressure injuries vs. those who do not.

Visualizing Tissue Strain Under the Sacrum and Coccyx in Different Supine Postures: A Case Series

OBJECTIVE

To visually assess and report the influence of supine positioning and sacrum and coccyx anatomy on tissue deformation.

METHODS

A convenience sample of three participants was scanned using MRI. All participants were scanned in a supine position with a rig oriented in a flat or horizontal position and with the torso portion of the rig elevated to 30° to simulate head-of-bed elevation. Representative images were identified to visualize and depict (1) the differences in tissue thickness and deformation in response to changes in supine positioning (0° and 30°), (2) the relative displacement of the skeleton relative to the skin during 30° incline, and (3) differences in sacrococcygeal morphology.

RESULTS

The tissue thickness under the sacrum stayed the same or increased when torsos were elevated. Skeletons were displaced relative to the skin when the rig was elevated regardless of the pelvis location. Further, in the elevated position, coccyges flexed when pelvises were placed on the elevated segment but did not flex when pelvises were placed on the horizontal segment.

CONCLUSIONS

This case series is useful in defining new areas of research that can (1) identify the deformation induced by normal and frictional forces resulting from different positions of the bed chassis, (2) assess the impact of positioning the pelvis on elevated versus horizontal segments of the bed chassis, and (3) define the association between sacral and coccyx morphology and pressure ulcer occurrence in hospitalized patients.

Development and evaluation of a new methodology for the fast generation of patient-specific Finite Element models of the buttock for sitting-acquired deep tissue injury prevention

The occurrence and management of Pressure Ulcers remain a major issue for patients with reduced mobility and neurosensory loss despite significant improvement in the prevention methods. These injuries are caused by biological cascades leading from a given mechanical loading state in tissues to irreversible tissue damage. Estimating the internal mechanical conditions within loaded soft tissues has the potential of improving the management and prevention of PU. Several Finite Element models of the buttock have therefore been proposed based on either MRI or CT-Scan data. However, because of the limited availability of MRI or CT-Scan systems and of the long segmentation time, all studies in the literature include the data of only one individual. Yet the inter-individual variability can't be overlooked when dealing with patient specific estimation of internal tissue loading. As an alternative, this contribution focuses on the combined use of low-dose biplanar X-ray images, B-mode ultrasound images and optical scanner acquisitions in a non-weight-bearing sitting posture for the fast generation of patient-specific FE models of the buttock. Model calibration was performed based on Ischial Tuberosity sagging. Model evaluation was performed by comparing the simulated contact pressure with experimental observations on a population of 6 healthy subjects. Analysis of the models confirmed the high inter-individual variability of soft tissue response (maximum Green Lagrange shear strains of 213 ± 101% in the muscle). This methodology opens the way for investigating inter-individual factors influencing the soft tissue response during sitting and for providing tools to assess PU risk.

Feasibility of sub-dermal soft tissue deformation assessment using B-mode ultrasound for pressure ulcer prevention

Pressure Ulcer (PU) prevention remains a main public health issue. The physio-pathology of this injury is not fully understood, and a satisfactory therapy is currently not available. Recently, several works suggested that mechanical strains are responsible of deformation-induced damage involved in the initiation of Deep Tissue Injury (DTI). A better assessment of the internal behavior could allow to enhance the modeling of the transmission of loads into the different structures composing the buttock. A few studies focused on the experimental in vivo buttock deformation quantification using Magnetic Resonance Imaging (MRI), but its use has important drawbacks. In clinical practice, ultrasound imaging is an accessible, low cost, and real-time technic to study the soft tissue. The objective of the present work was to show the feasibility of using B-mode ultrasound imaging for the quantification of localised soft-tissue strains of buttock tissues during sitting. An original protocol was designed, and the intra-operator reliability of the method was assessed. Digital Image Correlation was used to compute the displacement field of the soft tissue of the buttock during a full realistic loading while sitting. Reference data of the strains in the frontal and sagittal planes under the ischium were reported for a population of 7 healthy subjects. The average of shear strains over the region of interest in the fat layer reached levels up to 117% higher than the damage thresholds previously quantified for the muscular tissue in rats. In addition, the observation of the muscles displacements seems to confirm previous results which already reported the absence of muscular tissue under the ischium in the seated position, questioning the assumption commonly made in Finite Element modeling that deep tissue injury initiates in the muscle underlying the bone.

Quantitative ultrasound imaging over the ischial tuberosity: An exploratory study to inform tissue health

BACKGROUND

Characterization of ischial tissue health using a standardized diagnostic ultrasound protocol capturing thickness and gray scale analysis has not been established.

OBJECTIVES

This study evaluates inter-participant and inter-trial reliability of thickness and gray scale analysis of ultrasound images of tissues overlying the ischial tuberosity. It provides recommendations for the number of images required to minimize the standard error of measurement (SEM) and determines the number of images required for thickness, gray scale and contrast values that exceed an a-priori minimal detectable change (MDC) for repeated tissue assessment.

METHODS

Brightness mode ultrasound images using a 12 MHz linear probe were collected on the dominant limb in the side lying position for ten healthy participants and partitioned into three regions of interest: skin, subcutaneous tissue and muscle. Thickness and gray scale measures of skin, muscle and subcutaneous tissue were calculated using a customized MATLAB program. Contrast of each region of interest was calculated using the Gray Scale Level Co-Occurrence Matrix. Generalizability theory was used to quantify indices of dependability and corresponding SEMs and MDCs with 90% Confidence Intervals.

RESULTS

Participants accounted for most of the total variance (75.56% to 94.78%). Coefficient of dependability (ϕ) for thickness, grey scale and contrast measures was greater than 0.80 when more than two images were averaged. In order to detect a MDC of 21% in thickness and echogenicity measures, at least three images are required, while at least 5 images are required for a MDC of 25% for contrast measures.

CONCLUSIONS

Obtaining reliable thickness, echogenicity and contrast measures of tissue overlying the ischial tuberosity can be achieved from two ultrasound images by a single therapist on an individual participant however three and five images are required to use a MDC of 21% for thickness measures and MDC of 25% for contrast measures respectively.

Technologies to monitor the health of loaded skin tissues

There are many situations where the skin and underlying soft tissues are compromised by mechanical loading in the form or pressure, or pressure in combination with shear. If sustained, this can lead to damage in the tissues particularly adjacent to bony prominences, resulting in chronic wounds. An array of bioengineering technologies have been adopted to assess the integrity of loaded soft tissues. This paper aims to review these approaches for the quantification, simulation and early detection of mechanically-induced skin damage. The review considers different measurements at the interface between the skin and support surface/medical device, involving pressure, shear, friction and the local microclimate. The potential of the techniques to monitor the physiological response of the skin to these external stimuli including biophysical measurement devices and sampling of biofluids are critically analysed. In addition, it includes an analysis of medical imaging technologies and computational modelling to provide a means by which tissue deformation can be quantified and thresholds for tissue damage defined. Bioengineering measurement and imaging technologies have provided an insight into the temporal status of loaded skin. Despite the advances in technology, to date, the translation to clinical tools which are robust and cost effective has been limited. There is a need to adapt existing technologies and simulation platforms to enable patients, carers and clinicians to employ appropriate intervention strategies to minimise soft tissue damage.

Buttock tissue response to loading in men with spinal cord injury

OBJECTIVE/BACKGROUND

Despite the fact that most people with a spinal cord injury who use a wheelchair for mobility are considered at-risk for pressure ulcer (PrU) development, there still exists a spectrum of risk amongst this group. Efforts to differentiate risk level would benefit from clinical tools that can measure or predict the buttocks response to loading. Therefore, the goal of this study was to identify how tissue compliance and blood flow were impacted by clinically-measurable risk factors in young men with SCI.

METHODS

Blood flow at the ischial tuberosity was measured using laser Doppler flowmetry while the seated buttock was unloaded, and loaded at lower (40-60 mmHg) and high (>200 mmHg) loads. Tissue compliance of the buttock was measured using the Myotonometer while subject were lifted in a Guldmann Net.

RESULTS

Across 28 participants, blood flow was significantly reduced at high loads, while no consistent, significant changes were found at lower loads. At 40-60 mmHg, blood flow decreased in participants with a pressure ulcer history and lower BMI, but stayed the same or increased in most other participants. The buttock displaced an average of 9.3 mm (2.7 mm) at 4.2 N, which represented 82% (7%) of maximum displacement. BMI was related to the amount of buttock tissue displacement while smoking status explained some of the variation in the percent of max displacement.

CONCLUSION

Wide variability in tissue compliance and blood flow responses across a relatively homogeneous population indicate that differences in biomechanical risk may provide an explanation for the spectrum of PrU risk among persons with SCI.

Quantifying the in vivo quasi-static response to loading of sub-dermal tissues in the human buttock using magnetic resonance imaging

BACKGROUND

The design of seating systems to improve comfort and reduce injury would benefit from improved understanding of the deformation and strain patterns in soft tissues, particularly in the gluteal region.

METHODS

Ten healthy men were positioned in a semi-recumbent posture while their pelvic and thigh region was scanned using a wide-bore magnetic resonance imaging (MRI) scanner. Independent measurements of deformation for muscles and fat were taken for the transition from non-weight-bearing to weight-bearing loads in three stages. A weight-bearing load was achieved through having the subject supported by a flat, rigid surface. A non-weight-bearing condition was achieved by removing the support under the left buttock, leaving all soft tissue layers undeformed. An intermediate condition partially relieved the subject's left buttock by lowering the support relative to the pelvis by 20mm, which left the buttock partially deformed. For each of these conditions, the thicknesses of muscle and fat tissues below the ischial tuberosity and the greater trochanter were measured from the MRI data.

FINDINGS

In this dataset, the greatest soft tissue deformation took place below the ischial tuberosity, with muscles (mean=17.7mm, SD=4.8mm) deforming more than fat tissues (mean=4.3mm, SD=5.6mm). Muscles deformed through both steps of the transition from weight-bearing to non-weight-bearing conditions, while subcutaneous fat deformed little after the first transition from non-weight-bearing to partial-weight-bearing. High inter-subject variability in muscle and fat tissue strains was observed.

INTERPRETATION

Our findings highlight the importance of considering inter-subject variability when designing seating systems.

Performance Assessment of a Humidity Measurement System and Its Use to Evaluate Moisture Characteristics of Wheelchair Cushions at the User-Seat Interface

Little is known about the changes in moisture that occur at the body-seat interface during sitting. However, as increased moisture can add to the risk of skin damage, we have developed an array of MEMS (Micro-Electro-Mechanical System) humidity sensors to measure at this interface. Sensors were first evaluated against traceable standards, followed by use in a cross-over field test (n = 11; 20 min duration) using different wheelchair cushions (foam and gel). Relative humidity (RH) was measured at the left mid-thigh, right mid-thigh and coccyx. Sensors were shown to be unaffected by loading and showed highly reliable responses to measured changes in humidity, varying little from the traceable standard (<5%). Field-test data, smoothed through a moving average filter, revealed significant differences between the three chosen locations and between the gel and foam cushions. Maximum RH was attained in less than five minutes regardless of cushion material (foam or gel). Importantly, RH does not appear to distribute uniformly over the body-seat interface; suggesting multiple sensor positions would appear essential for effectively monitoring moisture in this interface. Material properties of the cushions appear to have a significant effect on RH characteristics (profile) at the body-seat interface, but not necessarily the time to peak moisture.