Pelvic movement and interface pressure distribution during manual wheelchair propulsion

Studying mechanical load at body-seat interface during dynamic activities such as wheelchair propulsion: a scoping review

BACKGROUND

The increasing number of wheelchair users and their risk of medical complications such as pressure ulcers (PU) make it important to have a better understanding of their seating characteristics. However, while most studies tackling this issue are based on static measurements, wheelchair users are active in their wheelchairs when performing daily life activities. This suggests the need to assess the mechanical loads at the wheelchair user's body-seat interface during dynamic activities.

OBJECTIVES

A scoping review was conducted to explore the existing data (shear load and pressure) and highlight significant parameters, relevant conditions and methodological strategies when studying wheelchair users performing a dynamic task.

MATERIALS AND METHODS

The literature search was performed by applying the PRISMA methodology.

RESULTS

A total of 11 articles met the inclusion criteria. Differences between static and dynamic data were found in the literature for peak pressure values, pressure distribution and the location of peak pressure. None measured tangential load at the seat/body interface, although two studies measured the shift of the ischial region. A significant impact of the type of pathology has been quantified, showing the need to perform experimental studies on diverse populations. The protocol and the pressure parameters studied were very diverse.

CONCLUSION

Further studies carefully choosing interface pressure mapping parameters and investigating a broader range of pathologies are required. Additionally, researchers should focus on finding a way to measure seated tangential load.

Safety Assessment of Rehabilitation Robots: A Review Identifying Safety Skills and Current Knowledge Gaps

The assessment of rehabilitation robot safety is a vital aspect of the development process, which is often experienced as difficult. There are gaps in best practices and knowledge to ensure safe usage of rehabilitation robots. Currently, safety is commonly assessed by monitoring adverse events occurrence. The aim of this article is to explore how safety of rehabilitation robots can be assessed early in the development phase, before they are used with patients. We are suggesting a uniform approach for safety validation of robots closely interacting with humans, based on safety skills and validation protocols. Safety skills are an abstract representation of the ability of a robot to reduce a specific risk or deal with a specific hazard. They can be implemented in various ways, depending on the application requirements, which enables the use of a single safety skill across a wide range of applications and domains. Safety validation protocols have been developed that correspond to these skills and consider domain-specific conditions. This gives robot users and developers concise testing procedures to prove the mechanical safety of their robotic system, even when the applications are in domains with a lack of standards and best practices such as the healthcare domain. Based on knowledge about adverse events occurring in rehabilitation robot use, we identified multi-directional excessive forces on the soft tissue level and musculoskeletal level as most relevant hazards for rehabilitation robots and related them to four safety skills, providing a concrete starting point for safety assessment of rehabilitation robots. We further identified a number of gaps which need to be addressed in the future to pave the way for more comprehensive guidelines for rehabilitation robot safety assessments. Predominantly, besides new developments of safety by design features, there is a strong need for reliable measurement methods as well as acceptable limit values for human-robot interaction forces both on skin and joint level.

Exploring the Influence of Wheelchair-User Interface and Personal Characteristics on Ischial Tuberosity Peak Pressure Index and Gradient in Elite Wheelchair Basketball Players

This pilot study investigated the relationship between personal and wheelchair factors on skin pressures at the ischial tuberosity in wheelchair basketball players. Seventeen wheelchair basketball players (7 male and 10 female) were evaluated during static and dynamic propulsive conditions while peak pressure index and peak pressure gradient were recorded with an interface pressure mat. The results showed that greater seat dump angles and backrest heights were negatively associated with the peak pressure index. Therapeutic cushion use was moderately associated with a reduced peak pressure gradient. Higher-class players used chair configurations associated with augmented pressure; however, classification status alone was not associated with pressure magnitude. Body mass index was negatively correlated with the static peak pressure gradient at levels approaching significance (p < .10). In conclusion, greater seat dump angles and backrest heights may provide pressure relief, whereas greater body mass index and therapeutic cushion use may reduce pressure gradients.

A dual padding method for ischial pressure sore reconstruction with an inferior gluteal artery perforator fasciocutaneous flap and a split inferior gluteus maximus muscle flap

Background

Various surgical management methods have been proposed for ischial sore reconstruction, yet it has the highest recurrence rate of all pressure ulcer types. A novel approach combining the advantages of a perforator-based fasciocutaneous flap and a muscle flap is expected to resolve the disadvantages of previously introduced surgical methods.

Methods

Fifteen patients with ischial pressure ulcers with chronic osteomyelitis or bursitis, who underwent reconstructive procedures with an inferior gluteal artery perforator (IGAP) fasciocutaneous flap and a split inferior gluteus maximus muscle flap from January 2011 to June 2016, were analyzed retrospectively. The split muscle flap was rotated to obliterate the deep ischial defect, managing the osteomyelitis or bursitis, and the IGAP fasciocutaneous flap was rotated or advanced to cover the superficial layer. The patients’ age, sex, presence of bursitis or osteomyelitis, surgical details, complications, follow-up period, and ischial sore recurrence were reviewed.

Results

All ischial pressure ulcers were successfully reconstructed without any flap loss. The mean duration of follow-up was 12.9 months (range, 3–35 months). Of 15 patients, one had a recurrent ulcer 10 months postoperatively, which was repaired by re-advancing the previously elevated fasciocutaneous flap.

Conclusions

The dual-flap procedure with an IGAP fasciocutaneous flap and split inferior gluteus maximus muscle flap for ischial pressure ulcer reconstruction is a useful method that combines the useful characteristics of perforator and muscle flaps, providing thick dual padding with sufficient vascularization while minimizing donor morbidity and vascular pedicle injury.

Influence of wheelchair user interface and personal characteristics on static and dynamic pretibial skin pressures in elite wheelchair racers, a pilot study

Context/objective: To examine personal (athletic classification, age, sex, body mass index, duration of disability, tactile sensation of lower extremities) and wheelchair (kneeling plate angle) factors associated with increased pretibial skin pressures in elite wheelchair racers.

Design: Cross-sectional study.

Setting: University-based laboratory in Champaign, USA.

Participants: A convenience sample of elite wheelchair races with traumatic spinal cord injury and spinal disease were recruited for participation.

Interventions: Interface pressure mapping was used to examine athletes' average and peak pretibial skin pressures in their own racing wheelchairs during static and dynamic (propulsive) conditions on a dynamometer.

Outcome measures: The study examined associations between personal and wheelchair factors and pressure, differences in pressure between static and dynamic conditions, and the influence of athletic classification (T53 vs. T54) on kneeling plate angle preference and skin pressure magnitudes.

Results: Increased kneeling plate angle was moderately associated with dynamic pressures. T53 athletes utilized more vertical kneeling angles and experienced larger average and peak pressures during propulsion. Duration of disability was negatively associated with all measures of pressure. Overall, mean dynamic peak pressure was significantly greater than mean static peak pressure.

Conclusion: This pilot study represents a first step in understanding the influence of user interface on potentially injurious skin pressures in wheelchair racers. Vertical kneeling plate configurations were associated with increased pressures while increased years with disability was associated with lower pretibial pressures. In addition, T53 athletes with less trunk function may be at a greater risk for experiencing larger interface pressures than T54 athletes.

Dynamic changes in seating pressure gradient in wheelchair users with spinal cord injury

Pressure ulcer interventions are commonly assessed with measures of seating interface pressure, such as peak pressure gradients (PPGs). Decreases in PPG magnitudes may reduce pressure ulcer risk by decreasing tissue deformation and increasing tissue perfusion of at-risk weight-bearing tissues. Changes in PPG directions, which have previously been overlooked in the seating pressure literature, may provide a transient increase in blood flow to at-risk tissues, even if the PPG magnitude and location remain the same. The purpose of this study was to assess both PPG components in response to combinations of wheelchair tilt and recline angles. Thirteen power wheelchair users were recruited into the study. Six combinations of wheelchair tilt (15°, 25°, and 35°) and recline (10° and 30°) were tested in random order. Each combination was tested with 5-min upright sitting, 5-min tilt and recline, and 5-min maximal pressure relief recovery. Changes in PPG magnitudes and PPG directions under the left ischial tuberosity were computed for the six angle combinations. The findings in this study suggested that when combining wheelchair tilt and recline, the recline function may be particularly useful in reducing PPG magnitudes, while the tilt function may be particularly useful in manipulating PPG directions.

Blood perfusion changes during sacral nerve root stimulation versus surface gluteus electrical stimulation on in seated spinal cord injury

The objective of this article is to examine dynamic changes of ischial blood perfusion during sacral nerve root stimulation against surface functional electrical stimulation (FES). Fourteen adults with suprasacral spinal cord injury (SCI) were recruited. The gluteal maximus was activated by surface FES or stimulating sacral nerve roots by functional magnetic stimulation (FMS) or a sacral anterior root stimulator implant (SARS). Ischial skin index of hemoglobin (IHB) and oxygenation (IOX) was measured.Skin blood perfusion was significantly higher during FMS than the baseline (IHB 1.05 ± 0.21 before vs.1.08 ± 0.02 during stimulation, p = 0.03; IOX 0.18 ± 0.21 before vs. 0.46 ± 0.30, p = 0.01 during stimulation). Similarly, when using the SARS implant, we also observed blood perfusion significantly increased (IHB 1.01 ± 0.02 before vs.1.07 ± 0.02 during stimulation, p = 0.003; IOX 0.79 ± 0.81 before vs. 2.2 ± 1.21 during stimulation, p = 0.036). Among four participants who completed both the FMS and FES studies, the magnitude of increase in both parameters was significantly higher during FMS. This study demonstrates sacral nerve root stimulation confer better benefit on blood perfusion than applying traditional FES in SCI population.

Effects of Electrical Stimulation on Risk Factors for Developing Pressure Ulcers in People with a Spinal Cord Injury: A Focused Review of Literature

Pressure ulcers (PUs) are a common and serious problem for wheelchair users, such as individuals with a spinal cord injury (SCI), resulting in great discomfort, loss of quality of life, and significant medical care costs. Therefore, it is of utmost importance to prevent PUs. In this literature overview, the effects of electrical stimulation (ES) on the risk factors for developing PUs in people with an SCI are examined and synthesized from January 1980 to January 2015. Thirty-four relevant studies of PU prevention in SCI were identified. Four were randomized clinical trials, 24 were case series, 6 had other designs. Three types of ES modalities were identified. The methodological quality varied from poor to fairly strong, with a large variety in used ES parameters. Twenty-three studies were identified describing short-term effects of ES on interface pressure, oxygenation, and/or blood flow, and 24 studies described the long-term effects of ES on muscle volume, muscle strength, and histology. Whereas there is a lack of controlled studies on the effects of ES on PU incidence, which disallows definite conclusions, there is moderate evidence to suggest that ES-induced muscle activation has a positive influence on several risk factors for developing PUs in people with an SCI.

FINITE ELEMENT PREDICTION OF SUB-DERMAL TISSUE STRESSES OF THE BUTTOCKS DURING WHEELCHAIR PROPULSION

Pressure ulcers, involving sub-dermal tissue damage and originating in deep tissue injury (DTI), have attracted much attention of physicians and researchers for three decades. Finite element (FE) model is a very efficient tool to investigate internal stresses and strains in human body that induce pressure ulcers. However, there was scarce report available to explore stresses distribution in human buttocks during manual wheelchair propulsion. A three-dimensional (3D) comprehensive FE model, incorporating ischial tuberosities (ITs), muscle, fat, and custom-contoured cushion (CCC), was developed to investigate internal stress distribution in soft tissue of the buttocks. Based on the FE model, pressure distribution under ITs in static sitting and during different wheelchair propulsions is studied. Internal stresses in fat and muscle were about three times and five times higher than that on cushion surface in terms of static sitting and wheelchair propulsion. All peak pressures under wheelchair propulsion were higher than those of static sitting, and peak pressures went on increasing with increase of wheelchair movement speed. This method based on the comprehensive FE model allowed for the optimization of wheelchair seat cushion design.

Product limit estimation for capturing of pressure distribution dynamics

Measurement of contact pressures at the wheelchair-seating interface is a critically important approach for laboratory research and clinical application in monitoring risk for pressure ulceration. As yet, measures obtained from pressure mapping are static in nature: there is no accounting for changes in pressure distribution over time, despite the well-known interaction between time and pressure in risk estimation. Here, we introduce the first dynamic analysis for distribution of pressure data, based on the Kaplan-Meier (KM) Product Limit Estimator (PLE) a ubiquitous tool encountered in clinical trials and survival analysis. In this approach, the pressure array-over-time data set is sub-sampled two frames at a time (random pairing), and their similarity of pressure distribution is quantified via a correlation coefficient. A large number (here: 100) of these frame pairs is then sorted into descending order of correlation value, and visualized as a KM curve; we build confidence limits via a bootstrap computed over 1000 replications. PLEs and the KM have robust statistical support and extensive development: the opportunities for extended application are substantial. We propose that the KM-PLE in particular, and dynamic analysis in general, may provide key leverage on future development of seating technology, and valuable new insight into extant datasets.

Wheelchair seating: A study on the healthy elderly

BACKGROUND

Many frail elderly and disabled persons have to use a manual wheelchair to remain mobile and active. Apart from a well-fitted wheelchair, an appropriate wheelchair cushion is considered crucial for postural support and to prevent complications.

AIM

To examined the effect of two types of seat cushions in two types of wheelchairs with regard to pelvic rotation, respiratory function, and interface pressure.

MATERIAL AND METHODS

Forty healthy elderly between 67 and 85 years of age participated. Two types of commonly used manual wheelchairs and two types of seat cushions were tested. Interface pressure, pelvic position, and respiratory function were measured using a sensor array mat, a study-specific inclinometer, and a spirometer.

RESULTS

Differences in interface pressure and pelvic rotation between cushions were found in both wheelchairs. Compared with sitting on a standard cushion, sitting on the positioning cushion increased peak pressure. The posterior pelvic tilt increased with the positioning cushion in the Etac Cross wheelchair and for both cushions in the Etac Cross wheelchair compared with the HD Balance wheelchair. No difference was observed in respiratory function.

CONCLUSIONS

This study highlights the importance of a thorough evaluation of the effects of wheelchair interventions, especially for the elderly and disabled.

Proxy study on minimizing risk of sacral pressure ulcers while complying with ventilator-associated pneumonia risk reduction guidelines

Bed positioning poses a subtle, yet important, tradeoff in the competing needs of hospitalized patients, particularly those susceptible to lower respiratory tract infections and/or pressure ulcers. Although it is widely held that a minimum 30° incline is necessary to mitigate risk of ventilator-acquired pneumonia, it is unclear what effect semirecumbent positioning has on the risk of pressure ulcerations. The authors test several hypotheses with the objective of elucidating the relationship between bed incline, posture, and incline, pursuant to a more evidence-based recommendation for practice in clinical care. To this end, interfacial pressures from 40 healthy subjects were analyzed following observation in both supine and sidelying positions, at shallow (30°) and moderate (45°) bed-angle incline. Summarily, the authors report that supine postures reduce pressure signatures associated with pressure ulceration versus sidelying position: 15% increase area of contact (P = 1.3×10), and 17% decrease in ratio of peak to average pressure (P = 3.1×10). Within supine posture, the authors found significant increases in 4 measures of local pressure, including average pressure (10.4% decrease, P = .005) and coefficient of pressure variation (22.1%, P = 2.2×10) at moderate incline. The authors conclude that supine bed positionings at moderate incline appear to reduce predictors of pressure ulceration.

Prevalence of Sensor Saturation in Wheelchair Seat Interface Pressure Mapping

Pressure mapping is a frequently used tool with great power to provide information about the forces between a patient and a wheelchair seat. One widely recognized limitation to this paradigm is the possibility of data loss due to sensor saturation. In this study, we seek to quantify and describe the saturation observed in the measurement of interface pressures of wheelchair users. We recorded approximately two minutes of interface pressure data from 22 elderly wheelchair users (11M/11F, 80 ± 10 years) and found that 4.7% of data frames had 1 saturated sensor, and 9.0% had more than one saturated sensor, for a total of 13.7% of all frames of data. Data from three of the 22 subjects (13.6%) were substantially affected by the persistent presence of saturated sensors. We conclude that for this population of elderly wheelchair users, sensor saturation may be a concern and should be factored properly into study design a priori.

Evidence-Based Strategies for Preserving Mobility for Elderly and Aging Manual Wheelchair Users

Elderly and aging manual wheelchair (MWC) users have increased risk for accelerated loss of function and mobility that greatly limits independence and affects quality of life. This review paper addresses important issues for preserving function and mobility for elderly and aging individuals who use a MWC by presenting the current available evidence and recommendations. These include recommendations for maximizing function, by decreasing pain, improving the ability to self-propel, and prolonging mobility and endurance through ergonomics, individualized wheelchair selection and configuration, and adaptations for increasing the capacity to handle the daily mobility demands through training, strengthening, and exercise. Each recommendation is supported by current research in each relevant area.

Characterization of a pressure measuring system for the evaluation of medical devices

The purpose of this study is to evaluate the possible use of four “FSA” thin and flexible resistive pressure mapping systems, designed by Vista Medical (Winnipeg, Manitoba, Canada), for the measurement of interface pressure exerted by lumbar belts onto the trunk. These sensors were originally designed for the measurement of low pressure applied by medical devices on the skin. Two types of tests were performed: standard metrology tests such as linearity, hysteresis, repeatability, reproducibility and drift, and specific tests for this application such as curvature, surface condition and mapping system superposition. The linear regression coefficient is between 0.86 and 0.98; hysteresis is between 6.29% and 9.41%. Measurements are repeatable. The location, time and operator, measurement surface condition and mapping system superposition have a statistically significant influence on the results. A stable measure is verified over the period defined in the calibration procedure, but unacceptable drift is observed afterward. The measurement stays suitable on a curved surface for an applied pressure above 50 mmHg. To conclude, the sensor has acceptable linearity, hysteresis and repeatability. Calibration must be adapted to avoid drift. Moreover, when comparing different measurements with this sensor, the location, the time, the operator and the measurement surface condition should not change; the mapping system must not be superimposed.

A systematic review of electrical stimulation for pressure ulcer prevention and treatment in people with spinal cord injuries

CONTEXT

Electrical stimulation (ES) can confer benefit to pressure ulcer (PU) prevention and treatment in spinal cord injuries (SCIs). However, clinical guidelines regarding the use of ES for PU management in SCI remain limited.

OBJECTIVES

To critically appraise and synthesize the research evidence on ES for PU prevention and treatment in SCI.

METHOD

Review was limited to peer-reviewed studies published in English from 1970 to July 2013. Studies included randomized controlled trials (RCTs), non-RCTs, prospective cohort studies, case series, case control, and case report studies. Target population included adults with SCI. Interventions of any type of ES were accepted. Any outcome measuring effectiveness of PU prevention and treatment was included. Methodological quality was evaluated using established instruments.

RESULTS

Twenty-seven studies were included, 9 of 27 studies were RCTs. Six RCTs were therapeutic trials. ES enhanced PU healing in all 11 therapeutic studies. Two types of ES modalities were identified in therapeutic studies (surface electrodes, anal probe), four types of modalities in preventive studies (surface electrodes, ES shorts, sacral anterior nerve root implant, neuromuscular ES implant).

CONCLUSION

The methodological quality of the studies was poor, in particular for prevention studies. A significant effect of ES on enhancement of PU healing is shown in limited Grade I evidence. The great variability in ES parameters, stimulating locations, and outcome measure leads to an inability to advocate any one standard approach for PU therapy or prevention. Future research is suggested to improve the design of ES devices, standardize ES parameters, and conduct more rigorous trials.

Pressure changes under the ischial tuberosities during gluteal neuromuscular stimulation in spinal cord injury: a comparison of sacral nerve root stimulation with surface functional electrical stimulation

OBJECTIVE

To compare the magnitude of interface pressure changes during gluteal maximus contraction by stimulating sacral nerve roots with surface electrical stimulations in patients with spinal cord injuries (SCIs).

DESIGN

Pilot interventional study.

SETTING

Spinal injury research laboratory.

PARTICIPANTS

Adults (N=18) with suprasacral complete SCI.

INTERVENTIONS

Sacral nerve root stimulation (SNRS) via a functional magnetic stimulator (FMS) or a sacral anterior root stimulator (SARS) implant; and surface functional electrical stimulation (FES).

MAIN OUTCOME MEASURES

Interface pressure under the ischial tuberosity (IT) defined as peak pressure, gradient at peak pressure, and average pressure.

RESULTS

With optimal FMS, a 29% average reduction of IT peak pressure was achieved during FMS (mean ± SD: 160.1±24.3mmHg at rest vs 114.7±18.0mmHg during FMS, t5=6.3, P=.002). A 30% average reduction of peak pressure during stimulation via an SARS implant (143.2±31.7mmHg at rest vs 98.5±21.5mmHg during SARS, t5=4.4, P=.007) and a 22% average decrease of IT peak pressure during FES stimulation (153.7±34.8mmHg at rest vs 120.5±26.1mmHg during FES, t5=5.3, P=.003) were obtained. In 4 participants who completed both the FMS and FES studies, the percentage of peak pressure reduction with FMS was slightly greater than with FES (mean difference, 7.8%; 95% confidence interval, 1.6%-14.0; P=.04).

CONCLUSIONS

SNRS or surface FES can induce sufficient gluteus maximus contraction and significantly reduce ischial pressure. SNRS via an SARS implant may be more convenient and efficient for frequently activating the gluteus maximus.

A comparison of strategies for imputing saturated pressure array data with application to the wheelchair-seating interface

PURPOSE

The common responses to pressure sensor saturation are extreme: either discarding of data, or wholesale alteration of experimental protocol. Here, we test four simplistic strategies for restoring missing data due to sensor saturation, avoiding such drastic measures.

METHODS

We tested these algorithms on 62 pressure maps collected from 42 individuals (20 M/22 F, 54.1 ± 26.2 years, 1.7 ± 0.1 m, 71.9 ± 17.8 kg) under a variety of seating conditions. These strategies were tested via a cross-validation design, censoring the maximum pressure value in the datasets and measuring prediction error.

RESULTS

The four strategies showed various prediction error rates: ? = 0.43 ± 0.14 (simple substitution), ? = 0.16 ± 0.21 (scaled substitution), ? = 0.19 ± 0.21 (feature extraction), and ? = 0.24 ± 0.32 (extrapolation by non-linear modeling).

CONCLUSION

For single-sensor saturation, it may be possible to restore missing data using simple techniques. Implications for Rehabilitation We present a method for imputing missing data from pressure sensor arrays. The implications for rehabilitation are as follows. Improved flexibility in design of protocols concerning interfacial pressure measurement. Restoration of missing data from existing datasets. Reduction in recruitment burden for future studies. Reduction in exposure risk to study participants.

Investigation of peak pressure index parameters for people with spinal cord injury using wheelchair tilt-in-space and recline: methodology and preliminary report

The purpose of this study was to determine the effect of the sensel window's location and size when calculating the peak pressure index (PPI) of pressure mapping with varying degrees of wheelchair tilt-in-space (tilt) and recline in people with spinal cord injury (SCI). Thirteen power wheelchair users were recruited into this study. Six combinations of wheelchair tilt (15°, 25°, and 35°) and recline (10° and 30°) were used by the participants in random order. Displacements of peak pressure and center of pressure were extracted from the left side of the mapping system. Normalized PPI was computed for three sensel window dimensions (3 sensels × 3 sensels, 5 × 5, and 7 × 7). At least 3.33 cm of Euclidean displacement of peak pressures was observed in the tilt and recline. For every tilt angle, peak pressure displacement was not significantly different between 10° and 30° recline, while center of pressure displacement was significantly different (P < .05). For each recline angle, peak pressure displacement was not significantly different between pairs of 15°, 25°, and 35° tilt, while center of pressure displacement was significantly different between 15° versus 35° and 25° versus 35°. Our study showed that peak pressure displacement occurs in response to wheelchair tilt and recline, suggesting that the selected sensel window locations used to calculate PPI should be adjusted during changes in wheelchair configuration.

Effect of interpolation on parameters extracted from seating interface pressure arrays

Interpolation is a common data processing step in the study of interface pressure data collected at the wheelchair seating interface. However, there has been no focused study on the effect of interpolation on features extracted from these pressure maps, nor on whether these parameters are sensitive to the manner in which the interpolation is implemented. Here, two different interpolation paradigms, bilinear versus bicubic spline, are tested for their influence on parameters extracted from pressure array data and compared against a conventional low-pass filtering operation. Additionally, analysis of the effect of tandem filtering and interpolation, as well as the interpolation degree (interpolating to 2, 4, and 8 times sampling density), was undertaken. The following recommendations are made regarding approaches that minimized distortion of features extracted from the pressure maps: (1) filter prior to interpolate (strong effect); (2) use of cubic interpolation versus linear (slight effect); and (3) nominal difference between interpolation orders of 2, 4, and 8 times (negligible effect). We invite other investigators to perform similar benchmark analyses on their own data in the interest of establishing a community consensus of best practices in pressure array data processing.

A NEW CUSTOM-CONTOURED CUSHION SYSTEM BASED ON FINITE ELEMENT MODELING PREDICTION

High internal stress in the deep tissues adjacent to bony prominences can cause deep tissue injuries. Therefore, internal stress in the soft tissue should be considered when the performance of anti-decubitus cushions is evaluated during cushion design. This paper reports on a custom-contoured cushion (CCC) system incorporated with a three-dimensional (3D) slice subject-specific finite element (FE) model to investigate the internal stress distribution in the soft tissues. This stress distribution was used to transform the interface pressure into the carving depth of the fabricated cushions based on the biomechanical characteristics of the cushion materials. The internal stress in the soft tissues was investigated using an FE model of buttocks and cushion made from three cushion materials. The cushion design was optimized according to the properties of the material. The simulated interface stress between the buttocks and the cushion (18 kPa) was consistent with the measured interface pressure of the CCC (17.1 kPa). The 3D FE model predicted the internal stress and displacement of the soft tissues and cushion. Additionally, it efficiently optimized the selection of cushion material. Fifty subjects (25 subjects with spinal cord injuries (SCI) and 25 healthy subjects) were recruited to investigate the interface pressure and perform subjective comfort evaluation. The CCC decreased the interface pressure under the buttocks and simultaneously increased the subjective comfort and stability. The effectiveness of the cushion materials was predicted by the CCC system, which also validated the clinical performance of decreasing interface pressure.

Spinal cord injury and pressure ulcer prevention: using functional activity in pressure relief

Background. People with spinal cord injury (SCI) are at increased risk of pressure ulcers due to prolonged periods of sitting. Concordance with pressure relieving movements is poor amongst this population, and one potential alternative to improve this would be to integrate pressure relieving movements into everyday functional activities. Objectives. To investigate both the current pressure relieving behaviours of SCI individuals during computer use and the application of an ergonomically adapted computer-based activity to reduce interface pressure. Design. Observational and repeated measures design. Setting. Regional Spinal Cord Injury Unit. Participants. Fourteen subjects diagnosed with SCI (12 male, 2 female). Intervention.Comparing normal sitting to seated movements and induced forward reaching positions. Main Outcome Measures. Interface pressure measurements: dispersion index (DI), peak pressure index (PPI), and total contact area (CA). The angle of trunk tilt was also measured. Results. The majority of movements yielded less than 25% reduction in interface pressure compared to normal sitting. Reaching forward by 150% of arm length during an adapted computer activity significantly reduced DI (P < 0.05), angle of trunk tilt (p<0.05), and PPI for both ischial tuberosity regions (P < 0.001) compared to normal sitting. Conclusion. Reaching forward significantly redistributed pressure at the seating interface, as evidenced by the change in interface pressures compared to upright sitting.

A comparison of vertical reaction forces during propulsion of three different one-arm drive wheelchairs by hemiplegic users

PURPOSE

The aim of this pilot study was to compare the vertical reaction forces (N) generated in three different Action 3 manual one-arm drive wheelchairs: dual handrim, a lever drive and a Neater Uni-wheelchair (NUW). A CONFORmat® Pressure measurement mat, placed on top of the users' prescribed cushion, measured vertical force at the buttock/seat interface on both hemiplegic and non-hemiplegic sides in each wheelchair.

METHODS

Fifteen hemiplegic users were randomly assigned each wheelchair to drive around an indoor obstacle course. During propulsion of a multiple sensor, continuous measurement of force was recorded. Time taken to complete the circuit was recorded. Mean force and confidence intervals for each buttock were calculated per user per wheelchair.

RESULTS

The dual handrim produced the highest vertical force during propulsion under the right buttock (x= 484.43; SD = 55.4; p < 0.001) and the lever drive produced the least force (x= 368.05; SD = 53.55; p < 0.01). The NUW completed the course quickest (p < 0.01).

CONCLUSIONS

The dual-handrim wheelchair requires the greatest vertical force during propulsion. Since increases in this seat vertical reaction force may be related to the propulsive force. Further investigation is indicated as this may be a significant factor for clinicians when prescribing one-arm drive wheelchairs. Implications for Rehabilitation Review of clinical reasoning in prescribing wheelchairs. Addition of the Neater Uni-wheelchair to wheelchair services prescribing lists.

Effects of electrical stimulation-induced gluteal versus gluteal and hamstring muscles activation on sitting pressure distribution in persons with a spinal cord injury

STUDY DESIGN

Ten participants underwent two electrical stimulation (ES) protocols applied using a custom-made electrode garment with built-in electrodes. Interface pressure was measured using a force-sensitive area. In one protocol, both the gluteal and hamstring (g+h) muscles were activated, in the other gluteal (g) muscles only.

OBJECTIVES

To study and compare the effects of electrically induced activation of g+h muscles versus g muscles only on sitting pressure distribution in individuals with a spinal cord injury (SCI).

SETTING

Ischial tuberosities interface pressure (ITs pressure) and pressure gradient.

RESULTS

In all participants, both protocols of g and g+h ES-induced activation caused a significant decrease in IT pressure. IT pressure after g+h muscles activation was reduced significantly by 34.5% compared with rest pressure, whereas a significant reduction of 10.2% after activation of g muscles only was found. Pressure gradient reduced significantly only after stimulation of g+h muscles (49.3%). g+h muscles activation showed a decrease in pressure relief (Δ IT) over time compared with g muscles only.

CONCLUSION

Both protocols of surface ES-induced of g and g+h activation gave pressure relief from the ITs. Activation of both g+h muscles in SCI resulted in better IT pressure reduction in sitting individuals with a SCI than activation of g muscles only. ES might be a promising method in preventing pressure ulcers (PUs) on the ITs in people with SCI. Further research needs to show which pressure reduction is sufficient in preventing PUs.

The influence of backrest inclination on buttock pressure

Objective

To assess the effects of backrest inclination of a wheelchair on buttock pressures in spinal cord injured (SCI) patients and normal subjects.

Method

The participants were 22 healthy subjects and 22 SCI patients. Buttock pressures of the participants were measured by a Tekscan® pressure sensing mat and software while they were sitting in a reclining wheelchair. Buttock pressures were recorded for 90°, 100°, 110°, 120° and 130° seat-to-back angles at the ischial tuberosity (IT) and sacrococcygeal (SC) areas. Recordings were made at each angle over four seconds at a sampling rate of 10 Hz.

Results

The side-to-side buttock pressure differences in the IT area for the SCI patients was significantly greater than for the normal subjects. There was no significant difference between the SCI patients and the normal subjects in the buttock pressure change pattern of the IT area. Significant increases in pressure on the SC area were found as backrest inclination angle was changed to 90°, 100° and 110° in the normal subjects, but no significant differences were found in the SCI patients.

Conclusion

Most of the SCI patients have freeform posture in wheelchairs, and this leads to an uneven distribution of buttock pressure. In the SCI patients, the peak pressure in the IT area reduced as the backrest angle was increased, but peak pressure at the SC area remained relatively unchanged. To reduce buttock pressure and prevent pressure ulcers and enhance ulcer healing, it can be helpful for tetraplegic patients, to have wheelchair seat-to-back angles above 120°.

Biomechanics of Pressure Ulcer in Body Tissues Interacting with External Forces during Locomotion

Forces acting on the body via various external surfaces during locomotion are needed to support the body under gravity, control posture, and overcome inertia. Examples include the forces acting on the body via the seating surfaces during wheelchair propulsion, the forces acting on the plantar foot tissues via the insole during gait, and the forces acting on the residual-limb tissues via the prosthetic socket during various movement activities. Excessive exposure to unwarranted stresses at the body-support interfaces could lead to tissue breakdowns commonly known as pressure ulcers, often presented as deep-tissue injuries around bony prominences or as surface damage on the skin. In this article, we review the literature that describes how the involved tissues respond to epidermal loading, taking into account both experimental and computational findings from in vivo and in vitro studies. In particular, we discuss related literature about internal tissue deformation and stresses, microcirculatory responses, and histological, cellular, and molecular observations.

Comparative study of pressure distribution at the user-cushion interface with different cushions in a population with spinal cord injury

BACKGROUND

Few studies have offered comparative information on the mechanical characteristics of different wheelchair seat cushions. The objective of the present study was to compare the benefits of the wheelchair seat cushions most frequently used in a population of patients with spinal cord injury in terms of pressure distribution and contact surface at the user-cushion interface.

METHODS

Each one of 48 patients with spinal cord injury was seated in his or her own wheelchair on the four models of cushions analyzed (low-profile air, high-profile air, dual-compartment air, and gel and firm foam), which were presented in randomized order. The pressure distribution readings and support surface area of the user-cushion interface were obtained with a matrix of piezocapacitive sensors.

FINDINGS

The dual-compartment air cushion yielded lower readings for all pressure parameters analyzed (P(max), P(mean), P(sd), and P(isch)) than the other three cushion models (P<0.05). The best surface parameter results (S(tot), S>60 and %S>60) also were obtained with the dual-compartment air cushion (P<0.05).

INTERPRETATION

In the sample analyzed, the dual-compartment air cushion was the cushion with the best pressure distribution and largest contact surface of the user-cushion interface compared to the other three cushions studied.

Stress Analyses Coupled With Damage Laws to Determine Biomechanical Risk Factors for Deep Tissue Injury During Sitting

Deep tissue injury (DTI) is a potentially life-threatening form of pressure ulcer that onsets in muscle tissue overlying bony prominences and progresses unnoticeably to more superficial tissues. To minimize DTI, the efficacy of wheelchair cushions should be evaluated not only based on their performance in redistributing interface pressures but also according to their effects on stress concentrations in deep tissues, particularly muscles. However, a standard bioengineering approach for such analyses is missing in literature. The goals of this study were to develop an algorithm to couple finite element (FE) modeling of the buttocks with an injury threshold for skeletal muscle and with a damage-stiffening law for injured muscle tissue, from previous animal experiments, to predict DTI onset and progression for different patient anatomies and wheelchair cushions. The algorithm was also employed for identifying intrinsic (anatomical) biomechanical risk factors for DTI onset. A set of three-dimensional FE models of seated human buttocks was developed, representing different severities of pathoanatomical changes observed in chronically sitting patients: muscle atrophy and “flattening” of the ischial tuberosity (IT). These models were then tested with cushions of different stiffnesses representing products available on the market and semirigid supports. Outcome measures were the percentage of damaged muscle tissue volumes after 90min and 110min of simulated continuous immobilized sitting as well as muscle injury rates post-60min, -90min, and -110min of continuous sitting. Damaged muscle volumes grew exponentially with the level of muscle atrophy. For example, simulation of a subject with 70% muscle atrophy sitting on a soft cushion showed damage to 33% of the muscle volume after 90min of immobilized sitting, whereas a comparable simulation with a nonatrophied muscle yielded only 0.4% damaged tissue volume. The rates of DTI progression also increased substantially with increasing severities of muscle atrophy, e.g., 70% atrophy resulted in 8.9, 2.7, and 1.6 times greater injury rates compared with the “reference” muscle thickness cases, after 60min, 90min, and 110min of sitting, respectively. Across all simulation cases, muscle injury rate was higher when a “flatter” IT was simulated. Stiffer cushions increased both the extent and rate of DTI at times shorter than 90min of continuous sitting, but after 110min, volumes and rates of tissue damage converged to approximately similar values across the different cushion materials. The present methodology is a practical tool for evaluating the performances of cushions in reducing the risk for DTI in a manner that goes far beyond the commonly accepted measurements of sitting pressures.

Pressure ulcers and deep tissue injury in wheelchair users: A bioengineering perspective

Wheelchair users are highly susceptible to deep tissue injury. Interface pressures are unlikely to predict this, and an alternative assessment approach is needed that can easily monitor internal mechanical stresses and deformations.

This review aims to provide clinicians with a comprehensive perspective of the biomechanical aspects of pressure ulceration in wheelchair users.